Septic Tank

The septic tank is the most common small-scale decentralised treatment unit for grey water and blackwater from cistern or pour-flush toilets. It is basically a sedimentation tank. Its shape can be rectangular or cylindrical.

Septic tanks are used for wastewater with a high content of settleable solids, typically for effluent from domestic sources, but they are also suitable for other wastewater of similar properties (SASSE 1998). Liquid flows through the tank and heavy particles sink to the bottom, while scum (mostly oil and grease) floats to the top. Over time, the solids that settle to the bottom are degraded anaerobically. 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 be dispersed by using a Soak Pit , evapo-transpiration mound or Leach Field, or transported to another treatment technology via a Solids-Free Sewer , simplified sewer or solids-free sewer. For secondary treatment e.g. surface flow, horizontal or vertical flow constructed wetlands) are suitable. Sludge must be emptied regularly (see also human-powered or motorized emptying) and treated for safe disposal or reuse. It can be dried in planted or unplanted drying beds, settling or thickening ponds. If the sludge is dried or composted, it can be applied in agriculture as valuable nutrient-rich soil amendment (see also pplication of pit humus and compost or application of sludge). There exist also several new processes to produce fertilizer from sludge.

When septic tanks are used as a primary settling treatment in DEWATS systems, they are generally followed by anaerobic filters, anaerobic baffled reactors ABRs, horizontal, surface flow or vertical flow constructed wetlands (planted gravel filters) and maturation ponds. In any case, water is needed to pour and bring the wastes to the septic tank (5 to 40 L of water per day per person, DFID 2003) .

Generally, the removal of 50% of solids, 30 to 40% of BOD and a 1-log removal of E. coli can be expected in a well-designed and maintained septic tank, although efficiencies vary greatly depending on operation and maintenance and climatic conditions.

A septic tank should have at least two chambers made out of concrete or brick work . Pre-fabricated concrete rings, PVC or fibreglass septic tanks are also available and may be less expensive in some contexts (WSP 2008). The first chamber should be at least 50% of the total length (SASSE 1998) , 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 , the lower arm of which dives 30 cm below water level (SASSE 1998), further reduces the scum and solids that are discharged.  

Over time, anaerobic bacteria and microorganisms start to digest the settled sludge anaerobically, transforming it into CO_² and CH_⁴ (biogas) and some heat. Optimal physical treatment by sedimentation takes place when the flow is smooth and undisturbed. Biological treatment by anaerobic digestion is optimised by a quick and intensive contact between the new inflow and old sludge, particularly when the flow is turbulent. Depending on the way the new influent flows through the tank, different treatment effect predominate. With a turbulent flow, the degradation of suspended and dissolved solids starts more quickly; however, more suspended solids are discharged with the effluent. This leads to bad odours, as active solids that are not completely fermented leave the tank (SASSE 1998). The contact and hence degradation is slower when the flow is less turbulent, but also less suspended solids leave the tank. The gases produced during anaerobic digestion must be allowed to 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 (WHO 1992).

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 users, the amount of water used per capita, the average annual temperature, the desludging frequency and the characteristics of the wastewater (SASSE 1998) . Normally, the chambers are all of the same depth (between 1.5 to 2.5 m), but sometimes the first chamber is made deeper as the others. Approximately 80 to 100 L should be provided per domestic user (SASSE 1998), but most countries provide a national standard for tank volume per domestic user. For help on dimensioning of septic tank, an exercise is given in Eawag Sandec 2008 and Excel spreadsheets are available in SASSE 1998.

The retention time should be 48 hours to achieve moderate treatment.[no-ecompendium

A septic tank will remove 30 to 50% of BOD (Biological Oxygen Demand), 40 to 60% of TSS (Total Suspended Solids) (UNEP 2004) and result in an abatement of 1 log units E. coli (a faecal indicator bacteria) (TILLEY et al. 2008) although efficiencies vary greatly depending on the influent concentrations and climatic conditions. The retention time should be 48 hours to achieve moderate treatment.

Aquaprivy

A variation of the septic tank is called an Aquaprivy. This is a simple storage and settling tank that is located directly below the toilet so that the excreta fall into it through a pipe. The bottom of the pipe is submerged in a liquid in the tank, forming a water seal to prevent escape of flies, mosquitoes and smell (WHO 1992). The tank functions like a septic tank. The effluent usually infiltrates into the ground through a soak pit and accumulated solids (sludge) must be removed frequently (WHO 1992). In any case, the accumulating sludge must be treated . The Aquaprivy has a low treatment efficiency.

Under normal operating conditions, users do not come in contact with the influent or effluent. Many of the problems with septic tank systems arise because no adequate consideration is given to the disposal of the tank effluent. Since the effluent from septic tanks is anaerobic, it is likely to contain large numbers of pathogens, which can be a potential source of infection (WHO 1992). Effluent, scum and sludge must be handled with care as they contain high levels of pathogenic organisms. Motorized emptying in a vacuum truck or a manual technology like a sludge gulper can decrease the health risks.

Users should be careful when opening the tank because noxious and flammable gases may be released. Thus, open fire should be avoided when opening the septic tank.

Construction costs of septic tanks are relatively low compared to other water based systems. However, they are much more expensive than for dry or composting toilets and unlikely to be affordable for poorer people in society. They also require sufficient piped water to flush all the wastes through the drains and manual or mechanical (vacuum or gulper) de-sludging needs to be done periodically. Engineers must prepare design and layout, while unskilled labourers can carry out construction if a mason supervises the work.

To start up a septic tank it should be "seeded" with sludge from a tank that has been operating for some time to ensure that the necessary microorganisms responsible for anaerobic digestion are present (WHO 1992). 

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. De-sludging is needed when 1/2 to 2/3 of the total depth between the water level and the bottom of the tank are occupied by sludge and scum (WHO 1992). One of the difficulties with septic tanks is that when the tank is almost full of solids, the inflow scours a channel through the sludge and passes through the tank in a matter of minutes rather than remaining in the tank for the required retention time (SASSE 1998). Generally, septic tanks should be emptied every 2 to 5 years. This is best done by using a Motorized Emptying and Transport technology, but Human-Powered Emptying can also be an option. This is an unpleasant work and care must be taken to ensure that sludge is not spilled around the tank during emptying, as the removed sludge from a septic tank includes fresh sludge and presents a risk of transmission of diseases of faecal origin.

The faecal sludge needs to be dehydrated (see also planted or unplanted drying beds, settling or thickening ponds) and further treated (e.g. small or large scale composting, anaerobic digestion). Before that, the faecal sludge can also be further separated from the liquid in drying beds or settling. The separated effluents from these systems should be treated in waste stabilisation ponds (WSP) or constructed wetlands (surface flow, horizontal or vertical flow). Regular de-sludging activities require well-organised community or public/private service provider.

Septic tanks should be checked from time to time to ensure that they are watertight. Routine inspection is also necessary to remove floating debris such as coarse materials and grease, to ensure that there are no blockages at the inlet or outlet and to check whether de-sludging is needed.