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University of Guyana
Faculty of Technology
Department of Civil with Environmental Engineering
Septic Tanks Course: CIV 3202 Lecturer: Mrs. S. Eastman Date: 14th April, 2016
Table of Contents TABLE OF CONTENTS............................................................................................................................................... I GROUP ................................................................................................................................................. 1
LIST OF FIGURES..................................................................................................................................................... 2 LIST OF TABLES ...................................................................................................................................................... 3
INTRODUCTION ..................................................................................................................................................... 4 1.0
SEPTIC SYSTEMS ........................................................................................................................................... 5
2.0
STANDARDS AVAILABLE IN GUYANA FOR THE DESIGN OF SEPTIC TANKS ................................................... 13
3.0
SEPTIC TANK DESIGN .................................................................................................................................. 22
1.1 PRIMARY TREATMENT (SEPTIC TANKS) .....................................................................................................................5 1.1.1 What is a septic tank? ............................................................................................................................5 1.1.2 Processes Involved in the Treatment of Waste In the Septic Tank (Physical, Chemical & biological) ....5 1.1.3 Design Considerations............................................................................................................................7 1.1.4 Usage of the Septic Tank/Appropriateness ............................................................................................7 1.1.5 Health Aspects/Acceptance ...................................................................................................................8 1.1.6 Operation & Maintenance of Septic Tanks ............................................................................................8 1.1.7 Advantages & Disadvantages of Septic Tanks .......................................................................................9 1.2 SECONDARY TREATMENT & DISPOSAL SYSTEMS .........................................................................................................9 1.2.1 Soak Pits ...............................................................................................................................................10 1.2.2 Leach Fields ..........................................................................................................................................11 1.2.3 Solids Free Sewer .................................................................................................................................12 2.1 GENERAL CONSIDERATIONS OF THE STANDARD ........................................................................................................13 2.2 CALCULATIONS...................................................................................................................................................13 2.2.1 Septic Tank Capacity (Volume) ............................................................................................................13 2.2.2 Septic Tank Length ...............................................................................................................................17 2.2.3 Design Considerations..........................................................................................................................17 2.2.4 Construction .........................................................................................................................................18 2.3 SECONDARY TREATMENT & DISPOSAL SYSTEMS ........................................................................................................19 2.3.1 Filter Box ..............................................................................................................................................19 Methods of Effluent Disposal .............................................................................................................................20 3.1 PRELIMINARY DATA FOR DESIGN ...........................................................................................................................22 3.2 CALCULATIONS FOR SEPTIC TANK DESIGN................................................................................................................22 3.2.1 Capacity of Septic Tank ........................................................................................................................22 3.2.2 Length of Septic Tank ...........................................................................................................................23 3.2.3 Two Chambers Septic Tank ..................................................................................................................24 3.2.4 Pipes & Fittings ....................................................................................................................................24 3.2.5 Tank Cover ........................................................................................................................................... 24 3.2.6 Construction .........................................................................................................................................25 3.3 SECONDARY TREATMENT & DISPOSAL SYSTEMS ....................................................................................................... 25 3.3.1 Filter Box ..............................................................................................................................................25
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4.0
ADEQUACY CHECK OF EXISTING SEPTIC TANK............................................................................................. 27
4.1 4.2
CHECK FOR LENGTH TO WIDTH RATIO......................................................................................................................27 CAPACITY OF THE PRESENT TANK............................................................................................................................28
SKETCH OF A TWO COMPARTMENT SEPTIC TANK……………………………………………………………………………………………..29
CONCLUSION ....................................................................................................................................................... 30
BIBLIOGRAPHY ..................................................................................................................................................... 31
ii
Group Name
USI
Afazal Baksh
1012318
Jitendra Bridgemohan
1011884
Mahendra Mentore
1016205
Neil Beeraspat
1012008
Chandradeo Ghansham 1011529 Stephano Seecharan
1012360
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List of Figures FIGURE 1SHOWING A SECTIONAL VIEW OF A SEPTIC TANK ............................................................................................................7 FIGURE 2 SHOWING MOTORIZED EMPTYING AND TRANSPORT OF SEPTIC TANK SLUDGE ......................................................................8 FIGURE 3 SHOWING HUMAN-POWERED EMPTYING AND TRANSPORT OF SEPTIC TANK SLUDGE ............................................................ 9 FIGURE 4 SHOWING A SOAK PIT ...........................................................................................................................................10 FIGURE 5 SHOWING A LEACH FIELD/DRAINAGE FIELD ................................................................................................................11 FIGURE 6 SHOWING A SOLIDS-FREE SEWER NETWORK ..............................................................................................................12 FIGURE 7 SHOWING A SECTIONAL ELEVATION OF ATYPICAL TWO COMPARTMENT SEPTIC TANK DESIGN AS ILLUSTRATED IN G 26:2007 . 18 FIGURE 8 SHOWING A FILTER BOX DESIGN AS ILLUSTRATED IN THE G 26: 2007 .........................................................................19 FIGURE 9 SHOWING A SECTIONAL ELEVATION OF A TYPICAL SEEPAGE PIT AS ILLUSTRATED IN G 26: 2007. ......................................20 FIGURE 10 SHOWING A LAYOUT & SECTIONAL VIEW OF AN ABSORPTION TRENCH AS ILLUSTRATED IN G 26: 2007. .......................... 21 FIGURE 11 SHOWING THE SECTIONAL ELEVATION OF A TYPICAL SAND FILTER TRENCH AS ILLUSTRATED IN G 26: 2007 ......................21 FIGURE 12 SHOWING THE EXISTING SEPTIC TANK FOR THE FAMILY OF 5........................................................................................27
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List of Tables TABLE 1 RATE OF TOTAL SLUDGE ACCUMULATION (K), ON DAYS, PER INTERVAL BETWEEN CLEANINGS AND TEMPERATURE OF THE COLDEST MONTH .................................................................................................................................................................14 TABLE 2 SHOWING THE PERIOD OF DETENTION (T) OF THE DISPOSSESSIONS, BY DAILY CONTRIBUTION ................................................ 14 TABLE 3 SHOWING THE DAILY CONTRIBUTION OF COOL SEWER (C) AND OF SLUDGE (LF) BY TYPE OF PROPERTY AND OF OCCUPANT...........15 TABLE 4 SHOWING SEPTIC TANK DESIGNED DIMENSIONS THAT ARE RECOMMENDED FOR USE IN AREAS WHERE IMPERMEABLE SOIL STRATA PREVAIL, THAT IS, WHERE THE PERCOLATION RATE EXCEEDS 60 MINUTES. ...........................................................................15 TABLE 5 SHOWING DESIGN DIMENSIONS FOR SEPTIC TANK DESIGNED FOR ALL WASTES INCLUDING LAUNDRY WASTES ............................16 TABLE 6 SHOWING DESIGN DIMENSIONS FOR SEPTIC TANK DESIGNED FOR ALL WASTES EXCLUDING LAUNDRY WASTES ...........................16 TABLE 7 SHOWING RECOMMENDED METHODS OF DISPOSAL OF SEPTIC TANK EFFLUENT ...................................................................20
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Introduction Sewage treatment is the process of removing physical, chemical and biological contaminants from wastewater. It utilizes physical, chemical and biological processes to convert wastewater into fluids and solids that are environmentally safe and suitable for disposal or possibly reuse.
Septic systems are small scale on-site sewage treatment systems that are used in areas where there are no sewerage networks. Septic systems usually consist of a septic tank which performs primary treatment and a secondary treatment system. Septic system utilises of anaerobic bacteria to decompose or mineralize the waste within the tank. The solid materials then sink to the bottom of the tank as sludge and the fluid drains out. The drained fluid may contain bacteria that could be harmful to the immediate environment thus there is a need for a secondary treatment system to treat this drained fluid. Also, the sludge that is not decomposed by the bacteria will build up over time and as such the tank will require periodic cleaning. To determine the capacity of the tank, the type of secondary treatment system and the frequency at which the tanks need to be cleaned; a comprehensive analysis of area needs to be done; data regarding the number of permanent/temporary s need to be gathered and calculations need to be performed based on the prevailing standards. The design of the septic system will comply with the local regulations, that is, The Code of Practice for the design & construction of septic tanks and associated secondary treatment and disposal system (G 26: 2007).
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1.0 Septic Systems A typical septic system comprises of a septic tank-which performs primary treatment of the waste; and a secondary treatment system, for example, a drain-field.
1.1
Primary Treatment (Septic Tanks)
1.1.1 What is a septic tank?
The septic tank is an enclosed receptacle designed to: collect excreta & flushwater from flush toilets and other domestic sullage (collectively known as wastewater); segregate settleable and floatable solids (sludge & scum); accumulate, consolidate and store solids; digest organic matter and discharge partially or primarily treated effluent. A septic tank is a watertight chamber made of concrete, fiberglass, PVC or plastic, through which blackwater and greywater flows for primary treatment. Settling and anaerobic processes reduce solids and organics, but the treatment is only moderate.
1.1.2 Processes Involved in the Treatment of Waste In the Septic Tank (Physical, Chemical & biological)
The septic tank has an inlet and an outlet pipe. Wastewater flows from the home to the septic tank through the sewer pipe. The septic tank treats the wastewater naturally by holding it in the tank long enough for solids and liquids to separate. The wastewater forms three layers inside the tank. Solids lighter than water (such as greases and oils) float to the top forming a layer of scum. Solids heavier than water settle at the bottom of the tank forming a layer of sludge. This leaves a middle layer of partially clarified wastewater (effluent).
The layers of sludge and scum remain in the septic tank and overtime, ae degraded anaerobically by the bacteria found naturally in the wastewater. However, the rate of accumulation is faster than the rate of decomposition, and the accumulated sludge and scum must be periodically removed. The effluent of the septic tank must undertake secondary treatment & disposal. Septic Tank Biology Septic tanks are ive low-rate anaerobic digesters, with their own ecosystem, in which facultative and anaerobic organisms perform complex biochemical processes. The tank operates as a plug-flow type of reactor (fluid and particles enter and exit the tank in progressive sequence), so there is usually no mixing or heating, particles ascend or descend and stratification develops. Effluent quality suffers when this stratification doesn’t develop. The environment within the tank’s clear zone is generally anoxic, or inadequate in oxygen, while sites within the sludge and scum layers may be completely free of oxygen, or anaerobic. The inflowing wastewater directed into the clear zone (just beneath the scum layer) by the inlet fixture normally contains high levels of dissolved oxygen. The microbial population,
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however, rapidly depletes the dissolved oxygen as the flow disperses in the tank and moves towards the outlet. The bacteria found in residential wastewater are enteric, the same as those found in the gut (Ziebell et al. 1974). These organisms are primarily heterotrophic bacteria which oxidize and solubilize organic matter. Facultative microbes (organisms that can function in either aerobic or anaerobic conditions) solubilize complex organic material to volatile organic acids, while strict anaerobes ferment the volatile organic acids to gases (methane, carbon dioxide, hydrogen sulfide, etc.). The microbes use the solubilized nutrients in the wastewater for cell growth and energy. The microbes are enteric, therefore, natural habitants of the wastewater, but it takes years to develop volatile organic acid and metabolite concentrations sufficient for colonization of methane formers and optimum digestion. Their population, growth and effectiveness are dependent on the characteristics of the wastewater (e.g., temperature, organic load, inorganic trash, toxic chemicals or cleaners, excessive fats, oils, grease, detergents, high hydraulic loads, etc.) as well as the sizing and design features of the tank. Consequently, a tank must be adequately sized for the occupancy usage in order to ensure a long-term quiescent environment for the organisms to colonize. When long-term storage is allowed, the effectiveness of digestion within the layers of stored volatile solids can be as great as 80 percent (Metcalf and Eddy, Inc., 1972), and the microbial population (biomass) required to accomplish the feat may range from one-fifth to only one-twentieth of that generated in an equivalent aerobic treatment process (Bitton, 1994).
The dominant bacterial groups measured in the septic tanks by Ziebell et al. in 1974, were total and fecal coliform, fecal streptococci, lactic acid bacteria, anaerobes, and others. The total bacteria population can range up to 230,000,000 per ml (Tyler et al. 1978). Taber (1976) divided the bacteria into two groups, separating the methanogenic bacteria (e.g.Methano bacterium), or methane formers, from the non-methanogenic bacteria (e.g. Bacillus, E.coli, etc.). The digestion that takes place in the tank is performed predominately by bacteria. The most common bacteria shapes are spheres (coccus), rods (bacillus) and spirals (spirillum). These shapes can be observed as individual cells, or they may be seen grouped or linked together. Each organism is encapsulated by a slime layer of extracellular enzymes. These extracellular enzymes hydrolyze organic material by adding water to the organic molecules, reducing them to simple soluble organic compounds small enough to be absorbed through the cell wall. Inside the cell, intracellular enzymes further metabolize and oxidize the volatile organic molecules creating the energy required for cell growth. Enzymes are complex proteins and can be precipitated, or have their enzyme reactive points tied up, by excessive amounts of salts and heavy metals. Either of these contaminants will inhibit the ability of the microbes to adequately produce their soluble organic nutrition, in effect, retarding the tank’s performance. Taking precautions to reduce excessive disposal of household products containing large concentrations of zinc, copper, calcium, magnesium, iron, ammonium sulfate, sodium sulfate, sodium chlorides, etc., is an important first step in assuring natural biochemical processes. Normal or conservative residential uses of salts, bleaches and detergents, however, are not detrimental to the microbial population.
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1.1.3 Design Considerations
Figure 1showing a sectional view of a septic tank
Source: (Tilley, Ulrich, Reymond, Luthi, 2014)
A septic tank should have at least two chambers. The first chamber should be at least 50% of the total length, and when there are only two chambers, it should be two thirds of the total length. Most of the solids settle out in the first chamber. The baffle, or the separation between the chambers, is to prevent scum and solids from escaping with the effluent. A T-shaped outlet pipe further reduces the scum and solids that are discharged. Accessibility to all chambers (through access ports) is necessary for maintenance. Septic tanks should be vented for controlled release of odorous and potentially harmful gases. The design of a septic tank depends on the number of s, the amount of water used per capita, the average annual temperature, the desludging frequency and the characteristics of the wastewater. The retention time should be 48 hours to achieve moderate treatment. 1.1.4 Usage of the Septic Tank/Appropriateness
This technology is most commonly applied at the household level. Larger, multi-chamber septic tanks can be designed for groups of houses and/ or public buildings (e.g., schools). A septic tank is appropriate where there is a way of dispersing or transporting the effluent. If septic tanks are used in densely populated areas, onsite infiltration should not be used, otherwise, the ground will become oversaturated and contaminated, and wastewater may rise up to the surface, posing a serious health risk. Instead, the septic tanks should be connected to some type of Conveyance technology, through which the effluent is transported to a subsequent Treatment or Disposal site. Even though septic tanks are watertight, it is not recommended to construct them in areas with high groundwater tables or where there is frequent flooding. Because the septic tank must be regularly desludged, a vacuum truck should be able to access the location. Often, septic tanks are installed in the home, under the kitchen or bathroom, which makes emptying difficult. Septic 7|Page
tanks can be installed in every type of climate, although the efficiency will be lower in colder climates. They are not efficient at removing nutrients and pathogens. 1.1.5 Health Aspects/Acceptance
Under normal operating conditions, s do not come in with the influent or effluent. Effluent, scum and sludge must be handled with care as they contain high levels of pathogenic organisms. s should be careful when opening the tank because noxious and flammable gases may be released. 1.1.6 Operation & Maintenance of Septic Tanks
Because of the delicate ecology, care should be taken not to discharge harsh chemicals into the septic tank. Scum and sludge levels need to be monitored to ensure that the tank is functioning well. Generally, septic tanks should be emptied every 2 to 5 years. This is best done by using a Motorized Emptying and Transport technology1, but Human-Powered Emptying2 can also be an option. Septic tanks should be checked from time to time to ensure that they are watertight. - Motorized emptying and transport refers to a vehicle equipped with a motorized pump and a storage tank for emptying and transporting faecal sludge and urine. Humans are required to operate the pump and manoeuvre the hose, but sludge is not manually lifted or transported. 1
Figure 2 showing motorized emptying and transport of septic tank sludge
Source: (Tilley, Ulrich, Reymond, Luthi, 2014)
- Human-powered emptying and transport refers to the different ways by which people can manually empty and/or transport sludge and solid products generated in onsite sanitation facilities. 2
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Figure 3 showing human-powered emptying and transport of septic tank sludge
Source: (Tilley, Ulrich, Reymond, Luthi, 2014)
1.1.7 Advantages & Disadvantages of Septic Tanks PROS Simple & robust technology No electrical energy is required Small land area required (can be built underground) Long service life Low operating costs
1.2
CONS Low reduction in pathogens, solids & organics Regular desludging must be ensured Effluent and sludge require further treatment and/or appropriate discharge
Secondary Treatment & Disposal Systems
The clarified liquid in the middle layer of the septic tank has to undergo secondary treatment which can be done in many different ways, for example; using a soak pit, leach field or some other treatment technology which involves transporting the effluent to that technology through Solids-Free Sewer systems.
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1.2.1 Soak Pits
Figure 4 showing a soak pit
Source: (Tilley, Ulrich, Reymond, Luthi, 2014)
A soak pit, also known as a soakaway or leach pit, is a covered, porous-walled chamber that allows water to slowly soak into the ground. Pre-settled effluent from a Collection and Storage/Treatment or (Semi-) Centralized Treatment technology is discharged to the underground chamber from which it infiltrates into the surrounding soil.
As wastewater (greywater or blackwater after primary treatment) percolates through the soil from the soak pit, small particles are filtered out by the soil matrix and organics are digested by microorganisms. Thus, soak pits are best suited for soil with good absorptive properties; clay, hard packed or rocky soil is not appropriate. Pros & Cons + Can be built and repaired with locally available materials + Technique simple to apply for all s + Small land area required + Low capital and operating costs - Primary treatment is required to prevent clogging - May negatively affect soil and groundwater properties
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1.2.2 Leach Fields
Figure 5 showing a leach field/drainage field
Source: (Tilley, Ulrich, Reymond, Luthi, 2014)
A leach field, or drainage field, is a network of perforated pipes that are laid in underground gravel-filled trenches to dissipate the effluent from a water-based Collection and Storage/Treatment or (Semi-) Centralized Treatment technology.
Pre-settled effluent is fed into a piping system (distribution box and several parallel channels) that distributes the flow into the subsurface soil for absorption and subsequent treatment. A dosing or pressurized distribution system may be installed to ensure that the whole length of the leach field is utilized and that aerobic conditions are allowed to recover between dosings. Such a dosing system releases the pressurized effluent into the leach field with a timer (usually 3 to 4 times a day). Pros & Cons
+ Can be used for the combined treatment and disposal of effluent + Has a long lifespan (depending on conditions) + Low maintenance requirements if operating without mechanical equipment + Relatively low capital costs; low operating costs - Requires expert design and construction - Not all parts and materials may be locally available - Requires a large area - Primary treatment is required to prevent clogging - May negatively affect soil and groundwater properties
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1.2.3 Solids Free Sewer
Figure 6 showing a Solids-free Sewer network
Source: (Tilley, Ulrich, Reymond, Luthi, 2014)
A solids-free sewer is a network of small-diameter pipes that transports pre-treated and solidsfree wastewater (such as Septic Tank effluent). It can be installed at a shallow depth and does not require a minimum wastewater flow or slope to function.
Solids-free sewers are also referred to as settled, smallbore, variable-grade gravity, or septic tank effluent gravity sewers. A precondition for solids-free sewers is efficient primary treatment at the household level. An interceptor, typically a single-chamber Septic Tank, captures settleable particles that could clog small pipes. The solids interceptor also functions to attenuate peak discharges. Because there is little risk of depositions and clogging, solids-free sewers do not have to be self-cleansing, i.e., no minimum flow velocity or tractive tension is needed. They require few inspection points, can have inflective gradients (i.e., negative slopes) and follow the topography. When the sewer roughly follows the ground contours, the flow is allowed to vary between open channel and pressure (full-bore) flow. Pros & Cons + Does not require a minimum gradient or flow velocity + Can be used where water supply is limited + Lower capital costs than conventional gravity sewers; low operating costs + Can be extended as a community grows + Greywater can be managed concurrently - Space for interceptors is required - Interceptors require regular desludging to prevent clogging - Requires training and acceptance to be used correctly - Requires repairs and removals of blockages more frequently than a conventional gravity sewer - Requires expert design and construction - Leakages pose a risk of wastewater exfiltration and groundwater infiltration and are difficult to identify
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2.0 Standards Available in Guyana for the Design of Septic Tanks The standard used in Guyana for the design of septic tanks is called:
Code of Practice for the design & construction of septic tanks and associated secondary treatment and disposal systems G 26: 2007
This Guyana Standard was adapted in 2007 by the GNBS, after the draft was finalized by the Sub-Committee – Septic tanks sewerage systems and the National Buildings Code Committee and approved by the National Bureau of Standards.
The standard basically gives recommendations for the design, location, construction and maintenance of septic tanks. It includes methods of treatment and disposal of septic tank effluent from domestic sewage. Lastly, the standard is applicable to individual housing units and institutions where the number of s does not exceed 20 persons
2.1
General Considerations of the Standard
2.2
Calculations
a) b) c) d)
the position and nature of outfall ditches and small streams; the position of any boreholes, wells or water storage tanks; the invert level of the drain at its point of entry into the proposed septic tank; the effect of seepage and surface water from surrounding areas at higher levels than the proposed absorption area; and e) the effect of seepage from the proposed absorption area on surrounding inhabited areas.
2.2.1 Septic Tank Capacity (Volume)
V = 1000 + N (C.T + K.Lf)……………………………..Equation 1. V = chamber volume, in litres N = number of s C = contribution of dispossessions, in litres per person x day (obtained from Table 3) T = detention period, in days (obtained from Table 2) K = Rate of accumulation of sludge (obtained from Table 1) Lf = contribution of sludge fresh, in litres per person x day (obtained from Table 3)
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Interval between cleaning (Years)
Value of K by room temperature band (t), in 0C t 10
10 t
t 20
2
134
105
97
4
214
185
1 3 5
94
174 254
65
57
145
137
225
217
177
Table 1 Rate of total sludge accumulation (K), on days, per interval between cleanings and temperature of the coldest month
Source: table 1, page 4 of G 26: 2007
Daily contribution in litres
Detention (T)/days
Time/Hours
From 1,501 to 3,000
0.92
22
From 4,501 to 6,000
0.75
18
Even 1,500
From 3,001 to 4,500 From 6,001 to 7,500 From 7,501 to 9,000 More than 9,000
1.00
0.83 0.67 0.58
0.50
24
20 16 14
12
Table 2 showing the period of detention (T) of the dispossessions, by daily contribution
Source: table 2, page 4 of G 26: 2007
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Unit
Sewer contribution ( C)
Contribution of sludge fresh (Lf)
person/litres person/litres person/litres person/litres
160 130 100 80
1 1 1 1
person/litres person/litres
70 50
0.3 0.2
person/litres person/litres repasts place
50 6 25 2
0.2 0.1 0.1 0.02
vase
480
4
Property 1. Permanent occupants Residents: - High standard : - Average standard: - Provisional lodging. 2. Temporary occupants Factory in general; - Desk - Public or commercial buildings - School (half day) and long permanence sites; - Bars; - Restaurants and similar;
- Cinema theatres and sites of short permanence;
person/litres
50
0.2
Public sanitary
Table 3 showing the daily contribution of cool sewer (C) and of sludge (L f) by type of property and of occupant
Source: table 3, page 5 of G 26: 2007 Number of s Up to & including 9 10
11 to 15 16 to 20
Nominal capacity (L) 1450
1610
1820 2420
Length 1.50
1.75
1.90 2.20
Recommended Dimensions (m) Width
Liquid
Total Depth
0.75
1.30
1.60
0.75
0.75 0.75
1.20
1.30 1.40
1.50
1.70 1.70
Table 4 showing septic tank designed dimensions that are recommended for use in areas where impermeable soil strata prevail, that is, where the percolation rate exceeds 60 minutes.
Source: table 5, page 6 of G 26: 2007
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Table 5 showing design dimensions for septic tank designed for all wastes including laundry wastes
Note: The above table was calculated using the following variables: C = 130 (above standard), T = 1, K = 2, Lf = 1 Source: table 4, page 6 of G 26: 2007
Table 6 showing design dimensions for septic tank designed for all wastes excluding laundry wastes
Source: table 6, page 7 of G 26: 2007
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2.2.2 Septic Tank Length
L = V/ (1000 x W x HD)………………………………Equation 2. Where:V = Chamber volume, in litres
L = Length of Chamber, in metres
W = Width, in meters (in accordance with clause 4.4).
HD = Liquid depth, in meters (in accordance with clause 4.4). 2.2.3 Design Considerations
Location Septic tank should be designed such that it is:
not within 1.5 m of a building or property boundary accessible for desludging and maintenance
Minimum Dimensions Septic tanks shall have a minimum width of 0.75 m and a liquid depth ranging from 1.2 to 1.5 m. The clearance above water level should not be less than 0.3m. The length to width ratio for rectangular septic tanks shall not be less than 2 nor shall it be greater than 3.
Recommended Design Septic tanks may be designed with one compartment or with two compartments Double compartment tanks are recommended as they have a greater capability for suspended solids removal. In double compartment tanks the inlet compartment should have a capacity of ½ to 2/3s of the total tank capacity, and the inlet compartment should have a capacity of not less than 1450 L. Pipes & fittings A minimum pipe diameter of 100mm is recommended.
Inlet Invert The invert level of the inlet fitting shall be at a distance not less than 75mm above water level and the clearance between the top of the vertical leg of the inlet fitting and the underside roof of the tank shall be not less than 75 mm.
Outlet Invert The invert level of the outlet fitting shall not be less than 75 mm below the invert level of the inlet fitting and the vertical leg of the outlet fitting shall extend downward for a distance of not less than 330 mm.
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Tank Cover Each tank shall be provided with a cover of adequate strength. Access openings shall be provided for the purposes of desludging and inspection.
Minimum Cover to Reinforcement The minimum cover to reinforcement in reinforced concrete tank sections shall be 43 mm (50% more than the ordinary concrete). 2.2.4 Construction
The wall of a septic tank may be constructed of blocks, concrete or other suitable material approved by the responsible authority. The walls and floors shall be rendered with mortar of adequate strength to make the septic tank assembly completely water-tight to surface and sub-surface water. Access opening covers – the cover to access openings shall be of reinforced concrete, cast iron or any other material approved by the responsible authority. A cover shall incorporate a suitable lifting device, shall be in one piece, and shall fit neatly and be seated to prevent the ingress of water. Thickness of floors & walls – the minimum thickness of floors and walls shall be as follows, unless otherwise approved by the responsible authority: a) For plain concrete walls, 150 mm, but the floor slab shall be 100 mm thick and shall be provided with anti-crack reinforcement at the top and bottom; b) For reinforced concrete floors & walls, 100 mm thick; c) For walls constructed of blocks, 175 mm thick, but the floor shall be of reinforced concrete 100 mm thick and provided with anti-crack reinforcement at the top & bottom; and d) The floor shall be level.
Figure 7 showing a sectional elevation of atypical two compartment septic tank design as illustrated in G 26:2007
Source: (Sankies, Munroe, Williams, Seecharan, et al., 2007)
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2.3
Secondary treatment & disposal systems
2.3.1 Filter Box
The filter box shall be used wherever the water table is below 1.8 m or where impermeable soil strata prevail. They are basically used to provide additional treatment of septic tank effluent. Filter boxes contain gravel or crushed rocks as the filter media. Minimum volume of the filter is 1250 L. The pipes and fittings between the septic tank and the filter shall have a minimum diameter of 0.1 m. It shall be placed in the center of the filter and is allowed a steady level of effluent, 30 cm from top of the filter bed.
Figure 8 showing a filter box design as illustrated in the G 26: 2007
Source: (Sankies, Munroe, Williams, Seecharan, et al., 2007)
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Calculations for filter box The volume of the filter (V) in litres should be calculated using the following equation:
V = 1.6 x N.C.T
Where:V = volume in litres; and S = horizontal section
Methods of Effluent Disposal
The effluent from a septic tank shall be disposed of by one of the methods listed below. Recommended methods of disposal for various soil and subsoil conditions are given in Table 7 below.
Table 7 showing recommended methods of disposal of septic tank effluent
I.
Seepage Pit
Figure 9 showing a sectional elevation of a typical seepage pit as illustrated in G 26: 2007.
Source: (Sankies, Munroe, Williams, Seecharan, et al., 2007)
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II.
Absorption Trench
Figure 10 showing a layout & sectional view of an absorption trench as illustrated in G 26: 2007.
III.
Source: (Sankies, Munroe, Williams, Seecharan, et al., 2007) Sand Filter Trench
Figure 11 showing the sectional elevation of a typical sand filter trench as illustrated in G 26: 2007
Source: (Sankies, Munroe, Williams, Seecharan, et al., 2007)
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3.0 Septic Tank Design 3.1
Preliminary Data for Design
As highlighted in the Code of Practice for the design & construction of septic tanks and
associated secondary treatment and disposal systems, the soil data is a relative factor in the
design of a septic tank. Assumption is being made that the soil type is best suited for the design
of the septic tank. The soil is assumed to be of dense clays and soil with a percolation rate greater than sixty minutes. The water table is approximately 1.8m below ground level. The number of permanent s for the septic tank design is six (6).
3.2
Calculations for Septic Tank Design
3.2.1 Capacity of Septic Tank
The capacity of the septic tank is to be determined using equation 1 (Section 4.1.1 G 26: 2007)
V = 1000 + N (C.T + K.Lf)…………………………………….……………………..Equation 1. V = chamber volume, in litres N = number of s =6 C = contribution of dispossessions, in litres per person x day (obtained from Table 3) = 80 for average standard permanent occupants T = detention period, in days (obtained from Table 2) = 1.0 K = Rate of accumulation of sludge (obtained from Table 1) = 177 (assuming 3 year interval between cleanings and room temperature, t > 20 oC) Lf = contribution of sludge fresh, in litres per person x day (obtained from Table 3) = 1 (since we are dealing with average standard permanent occupants) V = 1000 + N (C.T + K.Lf) V = 1000 + 6 [(80 x 1) + (177 x1)] V = 2542 L
Since the volume calculated is between 1,501 to 3000 litres in daily contribution, the volume will change using the appropriate and corresponding values from table 2. From table 2, T = 0.92 22 | P a g e
Therefore,
V = 1000 + 6 [(80 x 0.92) + (177 x 1)] V = 2503.6 L
3.2.2 Length of Septic Tank
The length of the septic tank was calculated according to equation two (2) (Section 4.1.2 G 26: 2007).
L = V/ (1000 x W x HD)…………………………………….…………………Equation 2. Where:
V = chamber volume, in litres = 2503.6 L
L = length of chamber, in meters
W = width of chamber, in meters = 1.00 m according to table 5
HD= Liquid depth, in meters
= 1.25 m according to table 5
L = V/(1000 x W x HD)
= 2503.6/(1000 x 1.00 x 1.25)
L = 2.003 m
According to the design regulations, (section 4.4.1 of G 26:2007), the length to width ratio for a rectangular septic tank should not be less than 2 nor greater than 3. Since the design tank is rectangular in shape a check should be performed. Check: Ratio = Length/Width = 2.003 m / 1.00 m Ratio = 2.003 Since the ratio is 2.003, the dimensions for the proposed septic tank are acceptable.
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3.2.3 Two Chambers Septic Tank
The septic tank being designed has two chambers instead of one. According to the regulations, section 4.5.1 of G 26: 2007, the inlet compartment should have a capacity of no less than 1450 L and should have a capacity of one – half to two – thirds of the total tank capacity. Capacity of chamber one
= 2/3 x 2503.6 L = 1669.07 L…………………………………………..Acceptable
Therefore, capacity of chamber two = (2503.6 – 1669.07) L = 824.53 L Width of inlet chamber Using:
= 1.00 m
L = V/(1000 x W x HD)
Length of chamber one (inlet)
= 1509.07 / (1000 x 1.00 x 1.25) = 1.335 m
Width of outlet chamber
= 1.00 m
Length of chamber two (outlet)
= total length – length of chamber one = 2.003 m – 1.335 m = 0.668 m
Total depth of entire septic tank
= 1.55 m according to table 5 for 6 s
3.2.4 Pipes & Fittings
According to the regulations, section 4.6 of G 26: 2007, the pipes should be placed in straight lines and avoid bends with a minimum diameter of 0.1m. Thus, all pipes and fitting shall be of 0.1m diameter.
3.2.5 Tank Cover
According to regulations, section 4.6.6 of G 26: 2007, each tank should be provided with a cover of adequate strength and a minimum cross section of 650 mm x 450 mm. The
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recommended design dimensions for this tank would be 650 mm x 500 mm which will be
constructed directly above the interior opening of the outlet pipe and carry a lifting device. 3.2.6 Construction
According to the regulations, sections 4.8, 4.8.1, 4.8.2 of G 26: 2007, the septic tank may be constructed of blocks, concrete or any suitable material. Concrete with a 1:2:3 mix ratio is
preferable, as well as high strength blocks for the walls of the tank to ensure the tank is water –
tight to surface and sub-surface water. The walls shall have a thickness of 175 mm and the floor slab a thickness of 100 mm with adequate reinforcement to comply with the regulations in standards.
3.3
Secondary Treatment & Disposal Systems
Taking into consideration the soil data and the regulations, the most suitable method for the secondary treatment and disposal of effluent will be the filter box. The regulations that will govern this design comply with section 5.1.3 of the code of practice. 3.3.1 Filter Box
Using equation three (section 5.1.3 of G 26: 2007) from the code of practice the volume of the filter box (Vf) can be calculated. Equation Three :
Vf =1.6 x N.C.T
Where:
V = volume, in litres N = number of s =6
C = contribution of dispossessions, in litres per person x day (obtained from Table 3) = 80 (according table 3)
T = detention period, in days (obtained from Table 2 in Appendix) = 0.92 (according to table 2)
Therefore;
Vf = 1.6 x (6 x 80 x 0.92) Vf= 706.56 L
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But according to the regulations, section 5.1.3 of G 26: 2007, the minimum volume of filter shall be 1250 L, thus the recommended volume of the filter will be 1250 L. Filter median is
gravel ranging from 3.8 – 6.4 cm. Following regulations the depth of the media filter will be 1.8m with a total recommended depth of 2.5 m and a width of 0.90 m (same width of septic tank). Using equation two to determine the length: L = V/(1000 x W x HD) Length of filter box
= 1250/(1000 x 0.90 x 1.8) = 0.772 m
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4.0 Adequacy Check of Existing Septic Tank
Figure 12 showing the existing septic tank for the family of 5
Length of current septic tank
= 2.75 m
Width of current septic tank
= 1.36 m
HD of current septic tank
= 1.25 m
Depth of current septic tank
4.1
= 1.75 m
Check for length to width ratio
Length to width ratio =
= 2.75 m/1.36 m = 2.02 m
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4.2
Capacity of the present tank V = L x 1000 x W x HD
V = 2.75 x 1000 x 1.36 x 1.25 V = 4,675 L The current septic tank system is located at a distance of 1.65 m from building boundaries. Its
width is greater than 0.75 m and a liquid depth that lies between the boundaries 1.2 to 1.5. It has
a cover with dimensions 0.60 m x 0.45 m. From calculations, it has a capacity of 4.675 litres and a length to width ratio of 2.02. These figures and dimensions, except for the cover dimensions and capacity, fall within the regulations found within the code of practice. The capacity is
significantly greater than the required nominal capacity. This therefore means that the interval
between cleanings can be increased from three to probably four or five years. With consideration of the nominal cross section of the tank cover as indicated by the Code of Practice, the existing tank is inadequate but to a small extent.
The filter box, however, has deteriorated over time and due to this, measurements could not have been made and as such the parameters necessary for the checking the adequacy of the filter box could not have been deduced.
Hence, with consideration of the checks performed, it can be concluded that the existing tank was designed within the regulations and overall, is considered adequate for a family of 6.
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Conclusion Septic tanks require little maintenance, they provide partial treatment of excreta, have long
service lives, low operating costs, require no electrical energy and most of all they allow safe
disposal of waste water, particularly in rural areas where it would be impractical to make use of sewer systems.
The design process of septic tanks is a relatively simple one and is outlined in the Code of Practice for the design & construction of septic tanks and associated secondary treatment and disposal system (G 26: 2007).
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Bibliography Guyana Standards G 26: 2007 Code of Practice for the design and construction of septic tanks and associated secondary treatment and disposal systems Guyana National Bureau of Standards. (2008). Retrieved April 9, 2016, from Powershow.com: http://www.powershow.com/view4/42efb5OTY4Y/Guyana_Standards_G_26_2007_Code_of_Practice_for_the_design_and_cons truction_of_septic_tanks_and_associated_secondary_treatment_and_disposal_systems_G uyana_National_Bureau_of_Standards_powerpoint_ppt_presentati How a Septic Tank and Septic System Works. (2010). Retrieved April 10, 2016, from Septic System Service: http://www.jgallagherseptic.com/how-a-septic-system-works.asp
Sankies, Munroe, Williams, Seecharan, et al. (2007). Code of Practice for The Design & Construction of Septic Tanks and associated Secondary Treatment and Disposal Systems (G 26: 2007). Georgetown: Guyana National Bureau of Standards. Tilley, Ulrich, Reymond, Luthi. (2014). Compendium of Sanitation Systems and Technology. Switzerland.
What is a septic system? How do I maintain one? (2016). Retrieved April 9, 2016, from Septic Systems - National Environmental Services Center: http://www.nesc.wvu.edu/subpages/septic_defined.cfm
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Faculty of Technology
Department of Civil with Environmental Engineering
Septic Tanks Course: CIV 3202 Lecturer: Mrs. S. Eastman Date: 14th April, 2016
Table of Contents TABLE OF CONTENTS............................................................................................................................................... I GROUP ................................................................................................................................................. 1
LIST OF FIGURES..................................................................................................................................................... 2 LIST OF TABLES ...................................................................................................................................................... 3
INTRODUCTION ..................................................................................................................................................... 4 1.0
SEPTIC SYSTEMS ........................................................................................................................................... 5
2.0
STANDARDS AVAILABLE IN GUYANA FOR THE DESIGN OF SEPTIC TANKS ................................................... 13
3.0
SEPTIC TANK DESIGN .................................................................................................................................. 22
1.1 PRIMARY TREATMENT (SEPTIC TANKS) .....................................................................................................................5 1.1.1 What is a septic tank? ............................................................................................................................5 1.1.2 Processes Involved in the Treatment of Waste In the Septic Tank (Physical, Chemical & biological) ....5 1.1.3 Design Considerations............................................................................................................................7 1.1.4 Usage of the Septic Tank/Appropriateness ............................................................................................7 1.1.5 Health Aspects/Acceptance ...................................................................................................................8 1.1.6 Operation & Maintenance of Septic Tanks ............................................................................................8 1.1.7 Advantages & Disadvantages of Septic Tanks .......................................................................................9 1.2 SECONDARY TREATMENT & DISPOSAL SYSTEMS .........................................................................................................9 1.2.1 Soak Pits ...............................................................................................................................................10 1.2.2 Leach Fields ..........................................................................................................................................11 1.2.3 Solids Free Sewer .................................................................................................................................12 2.1 GENERAL CONSIDERATIONS OF THE STANDARD ........................................................................................................13 2.2 CALCULATIONS...................................................................................................................................................13 2.2.1 Septic Tank Capacity (Volume) ............................................................................................................13 2.2.2 Septic Tank Length ...............................................................................................................................17 2.2.3 Design Considerations..........................................................................................................................17 2.2.4 Construction .........................................................................................................................................18 2.3 SECONDARY TREATMENT & DISPOSAL SYSTEMS ........................................................................................................19 2.3.1 Filter Box ..............................................................................................................................................19 Methods of Effluent Disposal .............................................................................................................................20 3.1 PRELIMINARY DATA FOR DESIGN ...........................................................................................................................22 3.2 CALCULATIONS FOR SEPTIC TANK DESIGN................................................................................................................22 3.2.1 Capacity of Septic Tank ........................................................................................................................22 3.2.2 Length of Septic Tank ...........................................................................................................................23 3.2.3 Two Chambers Septic Tank ..................................................................................................................24 3.2.4 Pipes & Fittings ....................................................................................................................................24 3.2.5 Tank Cover ........................................................................................................................................... 24 3.2.6 Construction .........................................................................................................................................25 3.3 SECONDARY TREATMENT & DISPOSAL SYSTEMS ....................................................................................................... 25 3.3.1 Filter Box ..............................................................................................................................................25
i
4.0
ADEQUACY CHECK OF EXISTING SEPTIC TANK............................................................................................. 27
4.1 4.2
CHECK FOR LENGTH TO WIDTH RATIO......................................................................................................................27 CAPACITY OF THE PRESENT TANK............................................................................................................................28
SKETCH OF A TWO COMPARTMENT SEPTIC TANK……………………………………………………………………………………………..29
CONCLUSION ....................................................................................................................................................... 30
BIBLIOGRAPHY ..................................................................................................................................................... 31
ii
Group Name
USI
Afazal Baksh
1012318
Jitendra Bridgemohan
1011884
Mahendra Mentore
1016205
Neil Beeraspat
1012008
Chandradeo Ghansham 1011529 Stephano Seecharan
1012360
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List of Figures FIGURE 1SHOWING A SECTIONAL VIEW OF A SEPTIC TANK ............................................................................................................7 FIGURE 2 SHOWING MOTORIZED EMPTYING AND TRANSPORT OF SEPTIC TANK SLUDGE ......................................................................8 FIGURE 3 SHOWING HUMAN-POWERED EMPTYING AND TRANSPORT OF SEPTIC TANK SLUDGE ............................................................ 9 FIGURE 4 SHOWING A SOAK PIT ...........................................................................................................................................10 FIGURE 5 SHOWING A LEACH FIELD/DRAINAGE FIELD ................................................................................................................11 FIGURE 6 SHOWING A SOLIDS-FREE SEWER NETWORK ..............................................................................................................12 FIGURE 7 SHOWING A SECTIONAL ELEVATION OF ATYPICAL TWO COMPARTMENT SEPTIC TANK DESIGN AS ILLUSTRATED IN G 26:2007 . 18 FIGURE 8 SHOWING A FILTER BOX DESIGN AS ILLUSTRATED IN THE G 26: 2007 .........................................................................19 FIGURE 9 SHOWING A SECTIONAL ELEVATION OF A TYPICAL SEEPAGE PIT AS ILLUSTRATED IN G 26: 2007. ......................................20 FIGURE 10 SHOWING A LAYOUT & SECTIONAL VIEW OF AN ABSORPTION TRENCH AS ILLUSTRATED IN G 26: 2007. .......................... 21 FIGURE 11 SHOWING THE SECTIONAL ELEVATION OF A TYPICAL SAND FILTER TRENCH AS ILLUSTRATED IN G 26: 2007 ......................21 FIGURE 12 SHOWING THE EXISTING SEPTIC TANK FOR THE FAMILY OF 5........................................................................................27
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List of Tables TABLE 1 RATE OF TOTAL SLUDGE ACCUMULATION (K), ON DAYS, PER INTERVAL BETWEEN CLEANINGS AND TEMPERATURE OF THE COLDEST MONTH .................................................................................................................................................................14 TABLE 2 SHOWING THE PERIOD OF DETENTION (T) OF THE DISPOSSESSIONS, BY DAILY CONTRIBUTION ................................................ 14 TABLE 3 SHOWING THE DAILY CONTRIBUTION OF COOL SEWER (C) AND OF SLUDGE (LF) BY TYPE OF PROPERTY AND OF OCCUPANT...........15 TABLE 4 SHOWING SEPTIC TANK DESIGNED DIMENSIONS THAT ARE RECOMMENDED FOR USE IN AREAS WHERE IMPERMEABLE SOIL STRATA PREVAIL, THAT IS, WHERE THE PERCOLATION RATE EXCEEDS 60 MINUTES. ...........................................................................15 TABLE 5 SHOWING DESIGN DIMENSIONS FOR SEPTIC TANK DESIGNED FOR ALL WASTES INCLUDING LAUNDRY WASTES ............................16 TABLE 6 SHOWING DESIGN DIMENSIONS FOR SEPTIC TANK DESIGNED FOR ALL WASTES EXCLUDING LAUNDRY WASTES ...........................16 TABLE 7 SHOWING RECOMMENDED METHODS OF DISPOSAL OF SEPTIC TANK EFFLUENT ...................................................................20
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Introduction Sewage treatment is the process of removing physical, chemical and biological contaminants from wastewater. It utilizes physical, chemical and biological processes to convert wastewater into fluids and solids that are environmentally safe and suitable for disposal or possibly reuse.
Septic systems are small scale on-site sewage treatment systems that are used in areas where there are no sewerage networks. Septic systems usually consist of a septic tank which performs primary treatment and a secondary treatment system. Septic system utilises of anaerobic bacteria to decompose or mineralize the waste within the tank. The solid materials then sink to the bottom of the tank as sludge and the fluid drains out. The drained fluid may contain bacteria that could be harmful to the immediate environment thus there is a need for a secondary treatment system to treat this drained fluid. Also, the sludge that is not decomposed by the bacteria will build up over time and as such the tank will require periodic cleaning. To determine the capacity of the tank, the type of secondary treatment system and the frequency at which the tanks need to be cleaned; a comprehensive analysis of area needs to be done; data regarding the number of permanent/temporary s need to be gathered and calculations need to be performed based on the prevailing standards. The design of the septic system will comply with the local regulations, that is, The Code of Practice for the design & construction of septic tanks and associated secondary treatment and disposal system (G 26: 2007).
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1.0 Septic Systems A typical septic system comprises of a septic tank-which performs primary treatment of the waste; and a secondary treatment system, for example, a drain-field.
1.1
Primary Treatment (Septic Tanks)
1.1.1 What is a septic tank?
The septic tank is an enclosed receptacle designed to: collect excreta & flushwater from flush toilets and other domestic sullage (collectively known as wastewater); segregate settleable and floatable solids (sludge & scum); accumulate, consolidate and store solids; digest organic matter and discharge partially or primarily treated effluent. A septic tank is a watertight chamber made of concrete, fiberglass, PVC or plastic, through which blackwater and greywater flows for primary treatment. Settling and anaerobic processes reduce solids and organics, but the treatment is only moderate.
1.1.2 Processes Involved in the Treatment of Waste In the Septic Tank (Physical, Chemical & biological)
The septic tank has an inlet and an outlet pipe. Wastewater flows from the home to the septic tank through the sewer pipe. The septic tank treats the wastewater naturally by holding it in the tank long enough for solids and liquids to separate. The wastewater forms three layers inside the tank. Solids lighter than water (such as greases and oils) float to the top forming a layer of scum. Solids heavier than water settle at the bottom of the tank forming a layer of sludge. This leaves a middle layer of partially clarified wastewater (effluent).
The layers of sludge and scum remain in the septic tank and overtime, ae degraded anaerobically by the bacteria found naturally in the wastewater. However, the rate of accumulation is faster than the rate of decomposition, and the accumulated sludge and scum must be periodically removed. The effluent of the septic tank must undertake secondary treatment & disposal. Septic Tank Biology Septic tanks are ive low-rate anaerobic digesters, with their own ecosystem, in which facultative and anaerobic organisms perform complex biochemical processes. The tank operates as a plug-flow type of reactor (fluid and particles enter and exit the tank in progressive sequence), so there is usually no mixing or heating, particles ascend or descend and stratification develops. Effluent quality suffers when this stratification doesn’t develop. The environment within the tank’s clear zone is generally anoxic, or inadequate in oxygen, while sites within the sludge and scum layers may be completely free of oxygen, or anaerobic. The inflowing wastewater directed into the clear zone (just beneath the scum layer) by the inlet fixture normally contains high levels of dissolved oxygen. The microbial population,
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however, rapidly depletes the dissolved oxygen as the flow disperses in the tank and moves towards the outlet. The bacteria found in residential wastewater are enteric, the same as those found in the gut (Ziebell et al. 1974). These organisms are primarily heterotrophic bacteria which oxidize and solubilize organic matter. Facultative microbes (organisms that can function in either aerobic or anaerobic conditions) solubilize complex organic material to volatile organic acids, while strict anaerobes ferment the volatile organic acids to gases (methane, carbon dioxide, hydrogen sulfide, etc.). The microbes use the solubilized nutrients in the wastewater for cell growth and energy. The microbes are enteric, therefore, natural habitants of the wastewater, but it takes years to develop volatile organic acid and metabolite concentrations sufficient for colonization of methane formers and optimum digestion. Their population, growth and effectiveness are dependent on the characteristics of the wastewater (e.g., temperature, organic load, inorganic trash, toxic chemicals or cleaners, excessive fats, oils, grease, detergents, high hydraulic loads, etc.) as well as the sizing and design features of the tank. Consequently, a tank must be adequately sized for the occupancy usage in order to ensure a long-term quiescent environment for the organisms to colonize. When long-term storage is allowed, the effectiveness of digestion within the layers of stored volatile solids can be as great as 80 percent (Metcalf and Eddy, Inc., 1972), and the microbial population (biomass) required to accomplish the feat may range from one-fifth to only one-twentieth of that generated in an equivalent aerobic treatment process (Bitton, 1994).
The dominant bacterial groups measured in the septic tanks by Ziebell et al. in 1974, were total and fecal coliform, fecal streptococci, lactic acid bacteria, anaerobes, and others. The total bacteria population can range up to 230,000,000 per ml (Tyler et al. 1978). Taber (1976) divided the bacteria into two groups, separating the methanogenic bacteria (e.g.Methano bacterium), or methane formers, from the non-methanogenic bacteria (e.g. Bacillus, E.coli, etc.). The digestion that takes place in the tank is performed predominately by bacteria. The most common bacteria shapes are spheres (coccus), rods (bacillus) and spirals (spirillum). These shapes can be observed as individual cells, or they may be seen grouped or linked together. Each organism is encapsulated by a slime layer of extracellular enzymes. These extracellular enzymes hydrolyze organic material by adding water to the organic molecules, reducing them to simple soluble organic compounds small enough to be absorbed through the cell wall. Inside the cell, intracellular enzymes further metabolize and oxidize the volatile organic molecules creating the energy required for cell growth. Enzymes are complex proteins and can be precipitated, or have their enzyme reactive points tied up, by excessive amounts of salts and heavy metals. Either of these contaminants will inhibit the ability of the microbes to adequately produce their soluble organic nutrition, in effect, retarding the tank’s performance. Taking precautions to reduce excessive disposal of household products containing large concentrations of zinc, copper, calcium, magnesium, iron, ammonium sulfate, sodium sulfate, sodium chlorides, etc., is an important first step in assuring natural biochemical processes. Normal or conservative residential uses of salts, bleaches and detergents, however, are not detrimental to the microbial population.
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1.1.3 Design Considerations
Figure 1showing a sectional view of a septic tank
Source: (Tilley, Ulrich, Reymond, Luthi, 2014)
A septic tank should have at least two chambers. The first chamber should be at least 50% of the total length, and when there are only two chambers, it should be two thirds of the total length. Most of the solids settle out in the first chamber. The baffle, or the separation between the chambers, is to prevent scum and solids from escaping with the effluent. A T-shaped outlet pipe further reduces the scum and solids that are discharged. Accessibility to all chambers (through access ports) is necessary for maintenance. Septic tanks should be vented for controlled release of odorous and potentially harmful gases. The design of a septic tank depends on the number of s, the amount of water used per capita, the average annual temperature, the desludging frequency and the characteristics of the wastewater. The retention time should be 48 hours to achieve moderate treatment. 1.1.4 Usage of the Septic Tank/Appropriateness
This technology is most commonly applied at the household level. Larger, multi-chamber septic tanks can be designed for groups of houses and/ or public buildings (e.g., schools). A septic tank is appropriate where there is a way of dispersing or transporting the effluent. If septic tanks are used in densely populated areas, onsite infiltration should not be used, otherwise, the ground will become oversaturated and contaminated, and wastewater may rise up to the surface, posing a serious health risk. Instead, the septic tanks should be connected to some type of Conveyance technology, through which the effluent is transported to a subsequent Treatment or Disposal site. Even though septic tanks are watertight, it is not recommended to construct them in areas with high groundwater tables or where there is frequent flooding. Because the septic tank must be regularly desludged, a vacuum truck should be able to access the location. Often, septic tanks are installed in the home, under the kitchen or bathroom, which makes emptying difficult. Septic 7|Page
tanks can be installed in every type of climate, although the efficiency will be lower in colder climates. They are not efficient at removing nutrients and pathogens. 1.1.5 Health Aspects/Acceptance
Under normal operating conditions, s do not come in with the influent or effluent. Effluent, scum and sludge must be handled with care as they contain high levels of pathogenic organisms. s should be careful when opening the tank because noxious and flammable gases may be released. 1.1.6 Operation & Maintenance of Septic Tanks
Because of the delicate ecology, care should be taken not to discharge harsh chemicals into the septic tank. Scum and sludge levels need to be monitored to ensure that the tank is functioning well. Generally, septic tanks should be emptied every 2 to 5 years. This is best done by using a Motorized Emptying and Transport technology1, but Human-Powered Emptying2 can also be an option. Septic tanks should be checked from time to time to ensure that they are watertight. - Motorized emptying and transport refers to a vehicle equipped with a motorized pump and a storage tank for emptying and transporting faecal sludge and urine. Humans are required to operate the pump and manoeuvre the hose, but sludge is not manually lifted or transported. 1
Figure 2 showing motorized emptying and transport of septic tank sludge
Source: (Tilley, Ulrich, Reymond, Luthi, 2014)
- Human-powered emptying and transport refers to the different ways by which people can manually empty and/or transport sludge and solid products generated in onsite sanitation facilities. 2
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Figure 3 showing human-powered emptying and transport of septic tank sludge
Source: (Tilley, Ulrich, Reymond, Luthi, 2014)
1.1.7 Advantages & Disadvantages of Septic Tanks PROS Simple & robust technology No electrical energy is required Small land area required (can be built underground) Long service life Low operating costs
1.2
CONS Low reduction in pathogens, solids & organics Regular desludging must be ensured Effluent and sludge require further treatment and/or appropriate discharge
Secondary Treatment & Disposal Systems
The clarified liquid in the middle layer of the septic tank has to undergo secondary treatment which can be done in many different ways, for example; using a soak pit, leach field or some other treatment technology which involves transporting the effluent to that technology through Solids-Free Sewer systems.
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1.2.1 Soak Pits
Figure 4 showing a soak pit
Source: (Tilley, Ulrich, Reymond, Luthi, 2014)
A soak pit, also known as a soakaway or leach pit, is a covered, porous-walled chamber that allows water to slowly soak into the ground. Pre-settled effluent from a Collection and Storage/Treatment or (Semi-) Centralized Treatment technology is discharged to the underground chamber from which it infiltrates into the surrounding soil.
As wastewater (greywater or blackwater after primary treatment) percolates through the soil from the soak pit, small particles are filtered out by the soil matrix and organics are digested by microorganisms. Thus, soak pits are best suited for soil with good absorptive properties; clay, hard packed or rocky soil is not appropriate. Pros & Cons + Can be built and repaired with locally available materials + Technique simple to apply for all s + Small land area required + Low capital and operating costs - Primary treatment is required to prevent clogging - May negatively affect soil and groundwater properties
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1.2.2 Leach Fields
Figure 5 showing a leach field/drainage field
Source: (Tilley, Ulrich, Reymond, Luthi, 2014)
A leach field, or drainage field, is a network of perforated pipes that are laid in underground gravel-filled trenches to dissipate the effluent from a water-based Collection and Storage/Treatment or (Semi-) Centralized Treatment technology.
Pre-settled effluent is fed into a piping system (distribution box and several parallel channels) that distributes the flow into the subsurface soil for absorption and subsequent treatment. A dosing or pressurized distribution system may be installed to ensure that the whole length of the leach field is utilized and that aerobic conditions are allowed to recover between dosings. Such a dosing system releases the pressurized effluent into the leach field with a timer (usually 3 to 4 times a day). Pros & Cons
+ Can be used for the combined treatment and disposal of effluent + Has a long lifespan (depending on conditions) + Low maintenance requirements if operating without mechanical equipment + Relatively low capital costs; low operating costs - Requires expert design and construction - Not all parts and materials may be locally available - Requires a large area - Primary treatment is required to prevent clogging - May negatively affect soil and groundwater properties
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1.2.3 Solids Free Sewer
Figure 6 showing a Solids-free Sewer network
Source: (Tilley, Ulrich, Reymond, Luthi, 2014)
A solids-free sewer is a network of small-diameter pipes that transports pre-treated and solidsfree wastewater (such as Septic Tank effluent). It can be installed at a shallow depth and does not require a minimum wastewater flow or slope to function.
Solids-free sewers are also referred to as settled, smallbore, variable-grade gravity, or septic tank effluent gravity sewers. A precondition for solids-free sewers is efficient primary treatment at the household level. An interceptor, typically a single-chamber Septic Tank, captures settleable particles that could clog small pipes. The solids interceptor also functions to attenuate peak discharges. Because there is little risk of depositions and clogging, solids-free sewers do not have to be self-cleansing, i.e., no minimum flow velocity or tractive tension is needed. They require few inspection points, can have inflective gradients (i.e., negative slopes) and follow the topography. When the sewer roughly follows the ground contours, the flow is allowed to vary between open channel and pressure (full-bore) flow. Pros & Cons + Does not require a minimum gradient or flow velocity + Can be used where water supply is limited + Lower capital costs than conventional gravity sewers; low operating costs + Can be extended as a community grows + Greywater can be managed concurrently - Space for interceptors is required - Interceptors require regular desludging to prevent clogging - Requires training and acceptance to be used correctly - Requires repairs and removals of blockages more frequently than a conventional gravity sewer - Requires expert design and construction - Leakages pose a risk of wastewater exfiltration and groundwater infiltration and are difficult to identify
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2.0 Standards Available in Guyana for the Design of Septic Tanks The standard used in Guyana for the design of septic tanks is called:
Code of Practice for the design & construction of septic tanks and associated secondary treatment and disposal systems G 26: 2007
This Guyana Standard was adapted in 2007 by the GNBS, after the draft was finalized by the Sub-Committee – Septic tanks sewerage systems and the National Buildings Code Committee and approved by the National Bureau of Standards.
The standard basically gives recommendations for the design, location, construction and maintenance of septic tanks. It includes methods of treatment and disposal of septic tank effluent from domestic sewage. Lastly, the standard is applicable to individual housing units and institutions where the number of s does not exceed 20 persons
2.1
General Considerations of the Standard
2.2
Calculations
a) b) c) d)
the position and nature of outfall ditches and small streams; the position of any boreholes, wells or water storage tanks; the invert level of the drain at its point of entry into the proposed septic tank; the effect of seepage and surface water from surrounding areas at higher levels than the proposed absorption area; and e) the effect of seepage from the proposed absorption area on surrounding inhabited areas.
2.2.1 Septic Tank Capacity (Volume)
V = 1000 + N (C.T + K.Lf)……………………………..Equation 1. V = chamber volume, in litres N = number of s C = contribution of dispossessions, in litres per person x day (obtained from Table 3) T = detention period, in days (obtained from Table 2) K = Rate of accumulation of sludge (obtained from Table 1) Lf = contribution of sludge fresh, in litres per person x day (obtained from Table 3)
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Interval between cleaning (Years)
Value of K by room temperature band (t), in 0C t 10
10 t
t 20
2
134
105
97
4
214
185
1 3 5
94
174 254
65
57
145
137
225
217
177
Table 1 Rate of total sludge accumulation (K), on days, per interval between cleanings and temperature of the coldest month
Source: table 1, page 4 of G 26: 2007
Daily contribution in litres
Detention (T)/days
Time/Hours
From 1,501 to 3,000
0.92
22
From 4,501 to 6,000
0.75
18
Even 1,500
From 3,001 to 4,500 From 6,001 to 7,500 From 7,501 to 9,000 More than 9,000
1.00
0.83 0.67 0.58
0.50
24
20 16 14
12
Table 2 showing the period of detention (T) of the dispossessions, by daily contribution
Source: table 2, page 4 of G 26: 2007
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Unit
Sewer contribution ( C)
Contribution of sludge fresh (Lf)
person/litres person/litres person/litres person/litres
160 130 100 80
1 1 1 1
person/litres person/litres
70 50
0.3 0.2
person/litres person/litres repasts place
50 6 25 2
0.2 0.1 0.1 0.02
vase
480
4
Property 1. Permanent occupants Residents: - High standard : - Average standard: - Provisional lodging. 2. Temporary occupants Factory in general; - Desk - Public or commercial buildings - School (half day) and long permanence sites; - Bars; - Restaurants and similar;
- Cinema theatres and sites of short permanence;
person/litres
50
0.2
Public sanitary
Table 3 showing the daily contribution of cool sewer (C) and of sludge (L f) by type of property and of occupant
Source: table 3, page 5 of G 26: 2007 Number of s Up to & including 9 10
11 to 15 16 to 20
Nominal capacity (L) 1450
1610
1820 2420
Length 1.50
1.75
1.90 2.20
Recommended Dimensions (m) Width
Liquid
Total Depth
0.75
1.30
1.60
0.75
0.75 0.75
1.20
1.30 1.40
1.50
1.70 1.70
Table 4 showing septic tank designed dimensions that are recommended for use in areas where impermeable soil strata prevail, that is, where the percolation rate exceeds 60 minutes.
Source: table 5, page 6 of G 26: 2007
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Table 5 showing design dimensions for septic tank designed for all wastes including laundry wastes
Note: The above table was calculated using the following variables: C = 130 (above standard), T = 1, K = 2, Lf = 1 Source: table 4, page 6 of G 26: 2007
Table 6 showing design dimensions for septic tank designed for all wastes excluding laundry wastes
Source: table 6, page 7 of G 26: 2007
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2.2.2 Septic Tank Length
L = V/ (1000 x W x HD)………………………………Equation 2. Where:V = Chamber volume, in litres
L = Length of Chamber, in metres
W = Width, in meters (in accordance with clause 4.4).
HD = Liquid depth, in meters (in accordance with clause 4.4). 2.2.3 Design Considerations
Location Septic tank should be designed such that it is:
not within 1.5 m of a building or property boundary accessible for desludging and maintenance
Minimum Dimensions Septic tanks shall have a minimum width of 0.75 m and a liquid depth ranging from 1.2 to 1.5 m. The clearance above water level should not be less than 0.3m. The length to width ratio for rectangular septic tanks shall not be less than 2 nor shall it be greater than 3.
Recommended Design Septic tanks may be designed with one compartment or with two compartments Double compartment tanks are recommended as they have a greater capability for suspended solids removal. In double compartment tanks the inlet compartment should have a capacity of ½ to 2/3s of the total tank capacity, and the inlet compartment should have a capacity of not less than 1450 L. Pipes & fittings A minimum pipe diameter of 100mm is recommended.
Inlet Invert The invert level of the inlet fitting shall be at a distance not less than 75mm above water level and the clearance between the top of the vertical leg of the inlet fitting and the underside roof of the tank shall be not less than 75 mm.
Outlet Invert The invert level of the outlet fitting shall not be less than 75 mm below the invert level of the inlet fitting and the vertical leg of the outlet fitting shall extend downward for a distance of not less than 330 mm.
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Tank Cover Each tank shall be provided with a cover of adequate strength. Access openings shall be provided for the purposes of desludging and inspection.
Minimum Cover to Reinforcement The minimum cover to reinforcement in reinforced concrete tank sections shall be 43 mm (50% more than the ordinary concrete). 2.2.4 Construction
The wall of a septic tank may be constructed of blocks, concrete or other suitable material approved by the responsible authority. The walls and floors shall be rendered with mortar of adequate strength to make the septic tank assembly completely water-tight to surface and sub-surface water. Access opening covers – the cover to access openings shall be of reinforced concrete, cast iron or any other material approved by the responsible authority. A cover shall incorporate a suitable lifting device, shall be in one piece, and shall fit neatly and be seated to prevent the ingress of water. Thickness of floors & walls – the minimum thickness of floors and walls shall be as follows, unless otherwise approved by the responsible authority: a) For plain concrete walls, 150 mm, but the floor slab shall be 100 mm thick and shall be provided with anti-crack reinforcement at the top and bottom; b) For reinforced concrete floors & walls, 100 mm thick; c) For walls constructed of blocks, 175 mm thick, but the floor shall be of reinforced concrete 100 mm thick and provided with anti-crack reinforcement at the top & bottom; and d) The floor shall be level.
Figure 7 showing a sectional elevation of atypical two compartment septic tank design as illustrated in G 26:2007
Source: (Sankies, Munroe, Williams, Seecharan, et al., 2007)
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2.3
Secondary treatment & disposal systems
2.3.1 Filter Box
The filter box shall be used wherever the water table is below 1.8 m or where impermeable soil strata prevail. They are basically used to provide additional treatment of septic tank effluent. Filter boxes contain gravel or crushed rocks as the filter media. Minimum volume of the filter is 1250 L. The pipes and fittings between the septic tank and the filter shall have a minimum diameter of 0.1 m. It shall be placed in the center of the filter and is allowed a steady level of effluent, 30 cm from top of the filter bed.
Figure 8 showing a filter box design as illustrated in the G 26: 2007
Source: (Sankies, Munroe, Williams, Seecharan, et al., 2007)
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Calculations for filter box The volume of the filter (V) in litres should be calculated using the following equation:
V = 1.6 x N.C.T
Where:V = volume in litres; and S = horizontal section
Methods of Effluent Disposal
The effluent from a septic tank shall be disposed of by one of the methods listed below. Recommended methods of disposal for various soil and subsoil conditions are given in Table 7 below.
Table 7 showing recommended methods of disposal of septic tank effluent
I.
Seepage Pit
Figure 9 showing a sectional elevation of a typical seepage pit as illustrated in G 26: 2007.
Source: (Sankies, Munroe, Williams, Seecharan, et al., 2007)
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II.
Absorption Trench
Figure 10 showing a layout & sectional view of an absorption trench as illustrated in G 26: 2007.
III.
Source: (Sankies, Munroe, Williams, Seecharan, et al., 2007) Sand Filter Trench
Figure 11 showing the sectional elevation of a typical sand filter trench as illustrated in G 26: 2007
Source: (Sankies, Munroe, Williams, Seecharan, et al., 2007)
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3.0 Septic Tank Design 3.1
Preliminary Data for Design
As highlighted in the Code of Practice for the design & construction of septic tanks and
associated secondary treatment and disposal systems, the soil data is a relative factor in the
design of a septic tank. Assumption is being made that the soil type is best suited for the design
of the septic tank. The soil is assumed to be of dense clays and soil with a percolation rate greater than sixty minutes. The water table is approximately 1.8m below ground level. The number of permanent s for the septic tank design is six (6).
3.2
Calculations for Septic Tank Design
3.2.1 Capacity of Septic Tank
The capacity of the septic tank is to be determined using equation 1 (Section 4.1.1 G 26: 2007)
V = 1000 + N (C.T + K.Lf)…………………………………….……………………..Equation 1. V = chamber volume, in litres N = number of s =6 C = contribution of dispossessions, in litres per person x day (obtained from Table 3) = 80 for average standard permanent occupants T = detention period, in days (obtained from Table 2) = 1.0 K = Rate of accumulation of sludge (obtained from Table 1) = 177 (assuming 3 year interval between cleanings and room temperature, t > 20 oC) Lf = contribution of sludge fresh, in litres per person x day (obtained from Table 3) = 1 (since we are dealing with average standard permanent occupants) V = 1000 + N (C.T + K.Lf) V = 1000 + 6 [(80 x 1) + (177 x1)] V = 2542 L
Since the volume calculated is between 1,501 to 3000 litres in daily contribution, the volume will change using the appropriate and corresponding values from table 2. From table 2, T = 0.92 22 | P a g e
Therefore,
V = 1000 + 6 [(80 x 0.92) + (177 x 1)] V = 2503.6 L
3.2.2 Length of Septic Tank
The length of the septic tank was calculated according to equation two (2) (Section 4.1.2 G 26: 2007).
L = V/ (1000 x W x HD)…………………………………….…………………Equation 2. Where:
V = chamber volume, in litres = 2503.6 L
L = length of chamber, in meters
W = width of chamber, in meters = 1.00 m according to table 5
HD= Liquid depth, in meters
= 1.25 m according to table 5
L = V/(1000 x W x HD)
= 2503.6/(1000 x 1.00 x 1.25)
L = 2.003 m
According to the design regulations, (section 4.4.1 of G 26:2007), the length to width ratio for a rectangular septic tank should not be less than 2 nor greater than 3. Since the design tank is rectangular in shape a check should be performed. Check: Ratio = Length/Width = 2.003 m / 1.00 m Ratio = 2.003 Since the ratio is 2.003, the dimensions for the proposed septic tank are acceptable.
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3.2.3 Two Chambers Septic Tank
The septic tank being designed has two chambers instead of one. According to the regulations, section 4.5.1 of G 26: 2007, the inlet compartment should have a capacity of no less than 1450 L and should have a capacity of one – half to two – thirds of the total tank capacity. Capacity of chamber one
= 2/3 x 2503.6 L = 1669.07 L…………………………………………..Acceptable
Therefore, capacity of chamber two = (2503.6 – 1669.07) L = 824.53 L Width of inlet chamber Using:
= 1.00 m
L = V/(1000 x W x HD)
Length of chamber one (inlet)
= 1509.07 / (1000 x 1.00 x 1.25) = 1.335 m
Width of outlet chamber
= 1.00 m
Length of chamber two (outlet)
= total length – length of chamber one = 2.003 m – 1.335 m = 0.668 m
Total depth of entire septic tank
= 1.55 m according to table 5 for 6 s
3.2.4 Pipes & Fittings
According to the regulations, section 4.6 of G 26: 2007, the pipes should be placed in straight lines and avoid bends with a minimum diameter of 0.1m. Thus, all pipes and fitting shall be of 0.1m diameter.
3.2.5 Tank Cover
According to regulations, section 4.6.6 of G 26: 2007, each tank should be provided with a cover of adequate strength and a minimum cross section of 650 mm x 450 mm. The
24 | P a g e
recommended design dimensions for this tank would be 650 mm x 500 mm which will be
constructed directly above the interior opening of the outlet pipe and carry a lifting device. 3.2.6 Construction
According to the regulations, sections 4.8, 4.8.1, 4.8.2 of G 26: 2007, the septic tank may be constructed of blocks, concrete or any suitable material. Concrete with a 1:2:3 mix ratio is
preferable, as well as high strength blocks for the walls of the tank to ensure the tank is water –
tight to surface and sub-surface water. The walls shall have a thickness of 175 mm and the floor slab a thickness of 100 mm with adequate reinforcement to comply with the regulations in standards.
3.3
Secondary Treatment & Disposal Systems
Taking into consideration the soil data and the regulations, the most suitable method for the secondary treatment and disposal of effluent will be the filter box. The regulations that will govern this design comply with section 5.1.3 of the code of practice. 3.3.1 Filter Box
Using equation three (section 5.1.3 of G 26: 2007) from the code of practice the volume of the filter box (Vf) can be calculated. Equation Three :
Vf =1.6 x N.C.T
Where:
V = volume, in litres N = number of s =6
C = contribution of dispossessions, in litres per person x day (obtained from Table 3) = 80 (according table 3)
T = detention period, in days (obtained from Table 2 in Appendix) = 0.92 (according to table 2)
Therefore;
Vf = 1.6 x (6 x 80 x 0.92) Vf= 706.56 L
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But according to the regulations, section 5.1.3 of G 26: 2007, the minimum volume of filter shall be 1250 L, thus the recommended volume of the filter will be 1250 L. Filter median is
gravel ranging from 3.8 – 6.4 cm. Following regulations the depth of the media filter will be 1.8m with a total recommended depth of 2.5 m and a width of 0.90 m (same width of septic tank). Using equation two to determine the length: L = V/(1000 x W x HD) Length of filter box
= 1250/(1000 x 0.90 x 1.8) = 0.772 m
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4.0 Adequacy Check of Existing Septic Tank
Figure 12 showing the existing septic tank for the family of 5
Length of current septic tank
= 2.75 m
Width of current septic tank
= 1.36 m
HD of current septic tank
= 1.25 m
Depth of current septic tank
4.1
= 1.75 m
Check for length to width ratio
Length to width ratio =
= 2.75 m/1.36 m = 2.02 m
27 | P a g e
4.2
Capacity of the present tank V = L x 1000 x W x HD
V = 2.75 x 1000 x 1.36 x 1.25 V = 4,675 L The current septic tank system is located at a distance of 1.65 m from building boundaries. Its
width is greater than 0.75 m and a liquid depth that lies between the boundaries 1.2 to 1.5. It has
a cover with dimensions 0.60 m x 0.45 m. From calculations, it has a capacity of 4.675 litres and a length to width ratio of 2.02. These figures and dimensions, except for the cover dimensions and capacity, fall within the regulations found within the code of practice. The capacity is
significantly greater than the required nominal capacity. This therefore means that the interval
between cleanings can be increased from three to probably four or five years. With consideration of the nominal cross section of the tank cover as indicated by the Code of Practice, the existing tank is inadequate but to a small extent.
The filter box, however, has deteriorated over time and due to this, measurements could not have been made and as such the parameters necessary for the checking the adequacy of the filter box could not have been deduced.
Hence, with consideration of the checks performed, it can be concluded that the existing tank was designed within the regulations and overall, is considered adequate for a family of 6.
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Conclusion Septic tanks require little maintenance, they provide partial treatment of excreta, have long
service lives, low operating costs, require no electrical energy and most of all they allow safe
disposal of waste water, particularly in rural areas where it would be impractical to make use of sewer systems.
The design process of septic tanks is a relatively simple one and is outlined in the Code of Practice for the design & construction of septic tanks and associated secondary treatment and disposal system (G 26: 2007).
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Bibliography Guyana Standards G 26: 2007 Code of Practice for the design and construction of septic tanks and associated secondary treatment and disposal systems Guyana National Bureau of Standards. (2008). Retrieved April 9, 2016, from Powershow.com: http://www.powershow.com/view4/42efb5OTY4Y/Guyana_Standards_G_26_2007_Code_of_Practice_for_the_design_and_cons truction_of_septic_tanks_and_associated_secondary_treatment_and_disposal_systems_G uyana_National_Bureau_of_Standards_powerpoint_ppt_presentati How a Septic Tank and Septic System Works. (2010). Retrieved April 10, 2016, from Septic System Service: http://www.jgallagherseptic.com/how-a-septic-system-works.asp
Sankies, Munroe, Williams, Seecharan, et al. (2007). Code of Practice for The Design & Construction of Septic Tanks and associated Secondary Treatment and Disposal Systems (G 26: 2007). Georgetown: Guyana National Bureau of Standards. Tilley, Ulrich, Reymond, Luthi. (2014). Compendium of Sanitation Systems and Technology. Switzerland.
What is a septic system? How do I maintain one? (2016). Retrieved April 9, 2016, from Septic Systems - National Environmental Services Center: http://www.nesc.wvu.edu/subpages/septic_defined.cfm
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