A Septic Tank is a watertight chamber made of concrete, fibreglass, PVC or plastic, through which blackwater and greywater flows for primary treatment, before further treatment or infiltration. Settling and anaerobic processes reduce solids and organics. The liquid effluent is commonly disposed of in a Leach Field D.9 or Soak Pit D.10 which provides further treatment.Describes biological processes that occur in the presence of oxygen.
Describes biological processes that
occur in the absence of oxygen.
Mixture of urine, faeces and flushwater along with anal cleansing water (if water is used for cleansing) and/or dry cleansing materials. Blackwater contains the pathogens, nutrients and organic matter of faeces and the nutrients of urine that are diluted in the flushwater.General term for a liquid that leaves a technology, typically after blackwater or sludge has undergone solids separation or some other type of treatment. Effluent originates at either a collection and storage or a (semi-) centralised treatment technology. Depending on the type of treatment, the effluent may be completely sanitised or may require further treatment before it can be used or disposed of.Refers to (semi-solid) excrement that is not mixed with urine or water. Depending on diet, each person produces approximately 50–150 L per year of faecal matter of which about 80 % is water and the remaining solid fraction is mostly composed of organic material. Of the total essential plant nutrients excreted by the human body, faeces contain around 39 % of the phosphorus (P), 26 % of the potassium (K) and 12 % of the nitrogen (N). Faeces also contain the vast majority of the pathogens excreted by the body, as well as energy and carbon rich, fibrous material.Total volume of water generated from washing food, clothes and dishware, as well as from bathing, but not from toilets (see blackwater). It may also contain traces of excreta (e.g. from washing diapers) and, therefore, some pathogens. Greywater accounts for approximately 65 % of the wastewater produced in households with flush toilets.Refer to biodegradable plant material (organic waste) that must be added to some technologies in order for them to function properly. Organic degradable material can include, but is not limited to, leaves, grass and food market waste. Although other products in this compendium contain organic matter, the term organics is used to refer to undigested plant material.Mixture of solids and liquids, containing mostly excreta and water, in combination with sand, grit, metals, trash and/or various chemical compounds. A distinction can be made between faecal sludge and wastewater sludge. Faecal sludge comes from on-site sanitation technologies, i.e. it has not been transported through a sewer. It can be raw or partially digested, a slurry or semisolid, and results from the collection and storage/treatment of excreta or blackwater, with or without greywater. Wastewater sludge (also referred to as sewage sludge) originates from sewer-based wastewater collection and (semi-)centralised treatment processes. The sludge composition will determine the type of treatment that is required and the end-use possibilities.The liquid produced by the body to rid itself of nitrogen in the form of urea and other waste products. In this context, the urine product refers to pure urine that is not mixed with faeces or water. Depending on diet, human urine collected from one person during one year (approx. 300 to 550 L) contains 2 to 4 kg of nitrogen. The urine of healthy individuals is sterile when it leaves the body but is often immediately contaminated by coming into contact with faeces.Describes technologies for on-site collection, storage, and sometimes (pre-) treatment of the products generated at the user interface. The treatment provided by these technologies is often a function of storage and is usually passive (i.e. requires no energy input), except a few emerging technologies where additives are needed. Thus, products that are ‘treated’ by these technologies often require subsequent treatment before use and/or disposal. In the technology overview graphic, this functional group is subdivided into the two subgroups: “Collection/Storage” and “(Pre-)Treatment”. This allows a further classification for each of the listed technologies with regard to their function: collection and storage, (pre-) treatment only or both.Refers to the methods through which products are returned to the environment, either as useful resources or reduced-risk materials. Some products can also be cycled back into a system (e.g. by using treated greywater for flushing).A functional group is a grouping of technologies that have similar functions. The compendium proposes five different functional groups from which technologies can be chosen to build a sanitation system:
User interface (U), Collection and Storage/Treatment (S), Conveyance (C), (Semi-) Centralised Treatment (T), Use and/or Disposal (U).
A sanitation system is a multi-step process in which sanitation products such as human excreta and wastewater are managed from the point of generation to the point of use or ultimate disposal. It is a context-specific series of technologies and services for the management of these sanitation products, i.e. for their collection, containment, transport, treatment, transformation, use or disposal. A sanitation system comprises functional groups of technologies that can be selected according to context. By selecting technologies from each applicable functional group, considering the incoming and outgoing products, and the suitability of the technologies in a particular context, a logical, modular sanitation system can be designed. A sanitation system also includes the management and operation and maintenance (O & M) required to ensure that the system functions safely and sustainably. The utilisation of products derived from a sanitation system.
A mechanical separation process using a porous medium (e.g., cloth, paper, sand bed, or mixed media bed) that captures particulate material and permits the liquid or gaseous fraction to pass through. The size of the pores of the medium determines what is captured and what passes through.Any substance that is used for growth. Nitrogen (N), phosphorus (P) and potassium (K) are the main nutrients contained in agricultural fertilisers. N and P are also primarily responsible for the eutrophication of water bodies.
A sanitation system in which excreta and wastewater are collected and stored or treated on the plot where they are generated.
An organism or other agent that causes disease.The first major stage in wastewater treatment that removes solids and organic matter mostly by the process of sedimentation or flotation.
The means of safely collecting and hygienically disposing of excreta and liquid
wastes for the protection of public health and the preservation of the quality of public water bodies and, more generally, of the environment.

Gravity settling of particles in a liquid such that they accumulate. Waste matter that is transported through the sewer.
An open channel or closed pipe used to convey sewage. See C.3 and C.4
User interface used for urination and defecation. The organic molecule (NH2)2CO that is excreted in urine and that contains the nutrient nitrogen. Over time, urea breaks down into carbon dioxide and ammonium, which is readily used by organisms in soil. It can also be used for on-site faecal sludge treatment. See. S.18Used water from any combination of domestic, industrial, commercial or agricultural activities, surface runoff/stormwater, and any sewer inflow/infiltration.

Wastewater enters the first chamber of the tank, allowing solids to settle and scum (mostly oil and grease) to float to the top. Over time, solids that settle are degraded anaerobically. Generally, the removal of 50 % of solids, 30–40 % of the biochemical oxygen demand and a 10-fold reduction 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.

Design Considerations

A Septic Tank should have at least two chambers. The first chamber needs to be at least 50 % of the total length. Most of the solids settle out in the first chamber. The baffle, or the separation between the chambers, prevents scum and solids from escaping with the effluent, as well as reduces short circuiting through the tanks. A T-shaped outlet pipe further reduces 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 expected number of users, the water used per capita, average annual temperature, desludging frequency and wastewater characteristics. The minimum recommended retention time for small tanks is 24 hours, decreasing to 12 hours in very large tanks. The volume must be large enough to avoid turbulent flow. An “aqua privy” is a variation of the Septic Tank where the storage and settling tank is located directly below the toilet so that the excreta fall into it. The aqua privy can be smaller than a Septic Tank because no flushing water is required to transport excreta to the tank.

Materials

A Septic Tank can be made of local bricks, cement blocks or stone and thus can be constructed on site using local materials. Prefabricated tanks are available in fibreglass, PVC or plastic.

Applicability

This technology is appropriate at the household level as well as for institutions such as hospitals and schools. A Septic Tank is appropriate where the volume of wastewater produced is too large for disposal in pit latrines, and when there is sufficient water for flushing solids from the toilet to the tank. This depends on the distance between toilet and tank. If Septic Tanks are used in densely populated areas, on-site soil infiltration should not be used, because the ground may become saturated and contaminated, posing a serious health risk. Instead, Septic Tanks should be connected to a 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. As the Septic Tank must be regularly desludged, a vacuum truck should be able to access the location C.2 . They can be implemented in every type of climate, although the efficiency will be lower in colder climates (as anaerobic digestion occurs more efficiently at higher temperatures).

Operation and Maintenance

Desludging is required for Septic Tanks and frequency will depend on the volume of the tank relative to the input of solids, the amount of indigestible solids, and the ambient temperature, as well as usage, system characteristics and the requirements of the relevant authority. Well-functioning systems will require emptying every two to five years. Scum and sludge levels need to be monitored to ensure that the tank is functioning well. Emptying is best done by using a Motorised Emptying and Transport technology C.2 , but Manual Emptying and Transport C.1 can also be an option. The effluent and faecal sludge require further treatment prior to reuse or disposal. The most common cause of failure of Septic Tanks is the failure of the infiltration system. Tanks connected to under-designed disposal systems will require emptying more frequently.

Health and Safety

Under normal operating conditions, users do not come in contact with the influent or effluent. Effluent, scum and sludge must be handled with care as they contain high levels of pathogens. During sludge and scum removal, workers should be equipped with personal protective equipment. Users should be careful when opening the tank because noxious and flammable gases may be released. If effluent is to infiltrate the ground, it is important to evaluate the contamination risk to groundwater, as well as the infiltration capacity of the soil.

Costs

This is a low to medium cost option, both in terms of capital and operational costs. However, additional costs for subsequent regular desludging, transport, treatment and disposal need to be taken into consideration.

Social Considerations

The Septic Tank is a very common and well-accepted technology among people who use flush toilets. Because of the delicate ecology in the system, awareness raising on eliminating the use of harsh chemicals for the users is necessary.

Case Studies & Related Content

Anaerobic Baffled Reactor - maturation ponds
Suggested by: (Save the Children International) at 20.03.2021