Home page  |  About this library  |  Help  |  Clear       English  |  French  |  Spanish  
Expand Document
Expand Chapter
Full TOC
Preferences
to previous section to next section

close this bookA Guide to the Development of on-site Sanitation (WHO; 1992; 246 pages)
View the documentPreface
open this folder and view contentsPart I. Foundations of sanitary practice
close this folderPart II. Detailed design, construction, operation and maintenance
open this folder and view contentsChapter 5. Technical factors affecting excreta disposal
close this folderChapter 6. Operation and maintenance of on-site sanitation
View the documentPit latrines
View the documentSimple pit latrines
View the documentVentilated pit latrines
View the documentVentilated double-pit latrines
View the documentPour-flush latrines
View the documentOffset pour-flush latrines
View the documentDouble-pit offset pour-flush latrines
View the documentRaised pit latrines
View the documentBorehole latrines
View the documentSeptic tanks
View the documentAqua-privies
View the documentDisposal of effluent from septic tanks and aqua-privies
View the documentComposting latrines
View the documentMultiple latrines
View the documentOther latrines
open this folder and view contentsChapter 7. Components and construction of latrines
open this folder and view contentsChapter. 8 Design examples
open this folder and view contentsPart III. Planning and development of on-site sanitation projects
View the documentReferences
View the documentSelected further reading
View the documentGlossary of terms used in this book
View the documentAnnex 1. Reuse of excreta
View the documentAnnex 2. Sullage
View the documentAnnex 3. Reviewers
View the documentSelected WHO publications of related interest
View the documentBack Cover
 

Septic tanks

Septic tanks are commonly used for wastewater treatment for individual households in low-density residential areas, for institutions such as schools and hospitals, and for small housing estates. The wastewater may be waste from toilets only, or may also include sullage.

The septic tank, in conjunction with its effluent disposal system, offers many of the advantages of conventional sewerage. However, septic tank systems are more expensive than most other on-site sanitation systems and are unlikely to be affordable by the poorer people in society. They also require sufficient piped water to flush all the wastes through the drains to the tanks.

Treatment processes

Wastes from the toilet, and possibly kitchens and bathrooms, pass through drains into a sealed, watertight tank, where they are partially treated. After a period - usually 1-3 days - the partially treated liquid passes out of the tank and is disposed of, often to the ground through soakpits or tile drains in trenches (Fig. 6.19). Many of the problems with septic tank systems arise because inadequate consideration is given to the disposal of the tank effluent.


Fig. 6.19. Septic tank disposal system

 

WHO 91438

Settlement

A principal aim of septic tank design is to achieve hydraulically quiescent conditions within the tank to assist the settlement by gravity of heavy solid particles. The settled material forms a layer of sludge on the bottom of the tank which must be removed periodically. The efficiency of removal of solids by settlement can be high. Majumder et al. (1960) reported removal of 80% of suspended solids in three tanks in West Bengal; similar removal rates were reported in a single tank near Bombay (Phadke et al., undated). However, much depends upon the retention time, the inlet and outlet arrangements, and the frequency of desludging. Large surges of flow entering the tank may cause a temporarily high concentration of suspended solids in the effluent owing to disturbance of the solids which have already settled out.

Flotation

Grease, oil, and other materials that are less dense than water float up to the liquid surface, forming a layer of scum which can become quite hard. The liquid moves through the tank sandwiched between the scum and sludge.

Sludge digestion and consolidation

Organic matter in the sludge and scum layers is broken down by anaerobic bacteria with a considerable amount of organic matter being converted into water and gases. Sludge at the bottom of the tank is consolidated owing to the weight of liquid and solids above. Hence the volume of sludge is considerably less than that of raw sewage solids entering the tank. Rising bubbles of gas cause a certain amount of disturbance to the liquid flow. The rate at which the digestion process proceeds increases with temperature, a maximum rate being achieved at about 35°C. The use of ordinary household soap in normal amounts is unlikely to affect the digestion process (Truesdale & Mann, 1968). The use of abnormally large amounts of disinfectant causes bacteria to be killed off and thereby inhibits the digestion process.

Stabilization of liquids

The liquid in the septic tank undergoes biochemical changes, but there are few data on the removal of pathogens. Both Majumder et al. (1960) and Phadke et al. (undated) found that although 80-90% of hookworm and Ascaris eggs were removed by the septic tanks studied, in absolute terms very large numbers of viable eggs were contained in the effluent, with 90% of effluent samples containing viable eggs.

Since the effluent from septic tanks is anaerobic and likely to contain large numbers of pathogens which can be a potential source of infection, it should not be used for crop irrigation nor should it be discharged to canals or surface-water drains without the permission of the local health authority.

Design principles

The guiding principles in designing a septic tank are:

 

- to provide sufficient retention time for the sewage in the tank to allow separation of solids and stabilization of liquid;

- to provide stable quiescent hydraulic conditions for efficient settlement and flotation of solids;

- to ensure that the tank is large enough to store accumulated sludge and scum;

- to ensure that no blockages are likely to occur and that there is adequate ventilation of gases.

Factors affecting design

The design method outlined below provides sufficient volume for both retention of liquid and storage of sludge and scum. The volume required for liquid retention depends upon the number of users, the amount of wastewater passed to the tank and whether sullage is accepted as well as waste from WCs. The volume for sludge and scum storage depends on the frequency with which the tank is desludged, the method of anal cleaning of the users and the temperature.

Estimating the volume of a septic tank

Retention time

A sewage retention time of 24 hours is assumed to be sufficient. This should correspond to the situation immediately before the tank is desludged. After desludging the effective liquid retention time is greater because liquid then occupies the regions previously full of sludge and scum.

Codes of practice vary in their recommendations from a retention time of just less than 24 hours to about 72 hours. In theory, improved settlement results from a longer retention time, although the maximum rate of settlement is usually achieved within the first few hours. Settlement is impeded by flow disturbances caused by the inlet and outlet arrangements. The problem is likely to be greater in small tanks than large ones (whose hydraulic capacity is better able to damp out disturbances) and it is reasonable to assume that in large tanks correspondingly lower retention times can be used (Mara & Sinnatamby, 1986). The Brazilian code of practice (Associação Brasileira de Normas Técnicas, 1982) allows for reduced retention time in large tanks, such as those serving institutions or small communities. In summary, if the wastewater flowrate is Q m3 per day, it recommends that the retention time should be T hours, as follows:

If Q is less than 6

T = 24

If Q is between 6 and 14

T = 33-1.5 Q

If Q is greater than 14

T = 12

Liquid retention volume

If the septic tank accepts sullage as well as toilet waste, the sewage flow from a house or institution usually represents a high proportion of the water supplied. If the water supply per person is known, the sewage flow may be taken as 90% of the water supply. If the water supply exceeds about 250 litres per person per day, the excess is likely to be used for watering gardens. In most developing countries, the maximum sewage flow may be assumed to be between 100 and 200 litres per person per day.

If only WCs are connected to the septic tank, the sewage flow is estimated from an assumption about the number of times each user is likely to flush the WC. For example, each person may flush a 10-litre cistern four times a day.

The minimum capacity required for 24 hours' liquid retention is:

A = P × q litres

where

A = required volume for 24 hours' liquid retention;

 

P = number of people served by the tank;

 

q = sewage flow per person (litres per person per day).

Volume for sludge and scum storage

The volume required for the accumulation of sludge and scum depends upon the factors discussed in Chapter 5. Pickford (1980) suggested the formula:

B = P × N × F × S

where

B = the required sludge and scum storage capacity in litres;

 

N = the number of years between desludging (often 2-5 years; more frequent desludging may be assumed where there is a cheap and reliable emptying service);

 

F = a factor which relates the sludge digestion rate to temperature and the desludging interval, as shown in Table 6.2;

 

S = the rate of sludge and scum accumulation which may be taken as 25 litres per person per year for tanks receiving WC waste only, and 40 litres per person per year for tanks receiving WC waste and sullage.

Table 6.2. Value of the sizing factor F in determining volume for sludge and scum storage

Number of years between desludging

Value of F

 

Ambient temperature

 

>20°C
throughout year

>10°C
throughout year

<10°C
during winter

1

1.3

1.5

2.5

2

1.0

1.15

1.5

3

1.0

1.0

1.27

4

1.0

1.0

1.15

5

1.0

1.0

1.06

6 or more

1.0

1.0

1.0

Total tank volume

The total capacity of the tank (C) is:

C = A + B litres

In practice, there are limitations on the minimum size of tank that can be built; the guidelines described below are illustrated in the design examples given in Chapter 8.

Shape and dimensions of septic tanks

Having determined the overall capacity of the septic tank it is necessary to determine the depth, width and length. The aim is to achieve even distribution of flow so that there are no dead areas and no "short-circuiting" (that is, incoming flow shooting through the tank in less than the design retention time).

A tank may be divided into two or more compartments by baffle walls. Most settlement and digestion may occur in the first compartment with some suspended materials carried forward to the second. Surges of sewage entering the tank reduce the efficiency of settlement but have less effect in the second compartment. Laak (1980) reported a number of studies in which septic tanks with more than one compartment performed more effectively than single-compartment tanks. His survey also indicated that the first compartment should be twice as long as the second. Any advantage of more than two compartments has not been quantified.

The following guidelines can be used to determine the internal dimensions of a rectangular tank.

 

1. The depth of liquid from the tank floor to the outlet pipe invert should be not less than 1.2 m; a depth of at least 1.5 m is preferable. In addition a clear space of at least 300 mm should be left between the water level and the under-surface of the cover slab.

2. The width should be at least 600 mm as this is the minimum space in which a person can work when building or cleaning the tank. Some codes of practice recommend that the length should be 2 or 3 times the width.

3. For a tank of width W, the length of the first compartment should be 2W and the length of the second compartment should be W (Fig. 6.20). In general, the depth should be not greater than the total length.


Fig. 6.20. Tank dimensions

 

WHO 91439

These guidelines give the minimum size of tank. There is no disadvantage in making a tank bigger than the minimum capacity. It may be cheaper to build larger tanks using whole blocks, rather than cutting blocks. Examples of septic tank design are given in Chapter 8.

Construction

The construction of a septic tank usually requires the assistance and supervision of an engineer or at least an experienced construction foreman. The design of the inlet and outlet is critical to the performance of the tank. Careful checking of levels is particularly important for large tanks that include complicated inlet, outlet and baffle-board arrangements.

For small household tanks, the floor is usually made of unreinforced concrete thick enough to withstand uplift pressure when the tank is empty. If the ground conditions are poor or the tank is large, the floor may have to be reinforced. The walls are commonly built of bricks, blocks or stone and should be rendered on the inside with cement mortar to make them watertight. Large reinforced concrete tanks serving groups of houses or institutions must be designed by a qualified engineer to ensure that they are structurally sound.

The tank cover or roof, which usually consists of one or more concrete slabs, must be strong enough to withstand any load that will be imposed.

Removable cover slabs should be provided over the inlet and outlet. Circular covers, rather than rectangular ones, have the advantage that they cannot fall into the tank when removed.

Septic tanks have been constructed from a variety of prefabricated sections, including large-diameter pipes. Experience has shown that the problems involved in fixing the inlet and outlet outweigh the advantages of using pipes. A number of proprietary designs of tank are manufactured from asbestos cement, glass-reinforced plastic and other materials and are sold commercially.

Inlet

The sewage must enter the tank with the minimum possible disturbance to the liquid and solids already in the tank. Surges and turbulence reduce the efficiency of settlement and can cause large amounts of solid matter to be carried out in the tank effluent. Suitable inlet arrangements are shown in Fig. 6.21 and 6.24.


Fig. 6.21. Septic tank inlet pipe

 

WHO 91440

Surges are caused by flushing of the WC and emptying of sinks and baths. Their effect can be minimized by using drainpipes of not less than 100 mm in diameter and ensuring that the gradient of the pipe approaching the septic tank is flatter than about 1 in 66. Sizes and gradients of pipes between the building and the septic tank may be specified in local building regulations.

Outlet

For septic tanks less than 1.2 m wide, a simple T-pipe arrangement can be used for the outlet. A removable cover above the T-pipe should be provided to permit clearance of any blockage. An alternative to the T-pipe is a baffle plate made of galvanized sheet, ferrocement or asbestos cement fitted round the outlet pipe (Fig. 6.22). A deflector may be provided below the outlet to reduce the possibility of settled sludge being resuspended and carried out of the tank. For tanks wider than 1.2 m, a full-width weir can be used to draw off the flow evenly across the tank. A scumboard should be fitted to prevent the scum washing over the weir (Fig. 6.23).


Fig. 6.22. Septic tank out-let baffle plate

 

WHO 91441


Fig. 6.23. Septic tank outlet using full width weir (Plan)

 

WHO 91442


Fig. 6.23. Septic tank outlet using full width weir (Section)

 

WHO 91442

Dividing wall

If a tank is divided into two or more compartments, slots or a short length of pipe should be provided above the sludge level and below the scum level, as shown in Fig. 6.24. At least two should be installed to maintain uniform flow distribution across the tank.


Fig. 6.24. Septic tanks showing options for connections between compartments (A)

 

WHO 91443


Fig. 6.24. Septic tanks showing options for connections between compartments (B)

 

WHO 91443

Ventilation of the tank

The anaerobic processes that occur in the tank produce gases which must be allowed a means of escape. If the drainage system of the house or other building has a ventilation pipe at the upper end, gases can escape from the septic tank along the drains. If the drainage system is not ventilated, a screened vent pipe should be provided from the septic tank itself.

The tank floor

Some codes of practice recommend that the floor of a septic tank should slope downwards towards the inlet. There are two reasons: firstly, more sludge accumulates near the inlet, so a greater depth is desirable; secondly, the slope assists movement of sludge towards the inlet during desludging. For a two-compartment tank, the second compartment should have a horizontal floor and the first compartment may slope at a gradient of 1 in 4 towards the inlet. When calculating the tank volume, it should be assumed that the floor is horizontal at the higher level. The effect of sloping the floor provides extra volume. The disadvantages of providing a sloping floor are that additional depth of excavation is required, the construction is made more complicated, and the cost of construction is increased.

Operation and maintenance

Starting up the tank

The process of anaerobic digestion of the sewage solids entering the tank can be slow in starting and it is a good idea to "seed" a new tank with sludge from a tank that has been operating for some time. This ensures that the necessary microorganisms are present in the tank to allow the digestion process to take place in a short time (McCarty, 1964).

Maintenance

Routine inspection is necessary to check whether desludging is needed, and to ensure that there are no blockages at the inlet or outlet. A tank needs to be desludged when the sludge and scum occupy the volume specified in the design. A simple rule is to desludge when solids occupy between one-half and two-thirds of the total depth between the water level and the bottom of the tank. One of the difficulties with septic tanks is that they continue to operate even when the tank is almost full of solids. In this situation the inflow scours a channel through the sludge and may pass through the tank in a matter of minutes rather than remaining in the tank for the required retention time.

The most satisfactory method of sludge removal is by vacuum tanker. The sludge is pumped out of the tank through a flexible hose connected to a vacuum pump, which lifts the sludge into the tanker. If the bottom layers of sludge have cemented together they can be jetted with a water hose (which may be fitted to the tanker lorry) or broken up with a long-handled spade before being pumped out.

If a vacuum tanker is not available, the sludge must be bailed out manually using buckets. This is unpleasant work which exposes the operatives to health hazards.

Care must be taken to ensure that sludge is not spilled around the tank during emptying. Sludge removed from a septic tank includes fresh excreta and presents a risk of transmission of diseases of faecal origin. Careful disposal is therefore necessary.

When a septic tank is desludged it should not be fully washed out or disinfected. A small amount of sludge should be left in the tank to ensure continuing rapid digestion.

to previous section to next section

Please provide your feedback   English  |  French  |  Spanish