Activated Sludge

An activated sludge process refers to a multi-chamber reactor wastewater treatment unit to produce a high-quality effluent. Flocs of microorganism are suspended and mixed with wastewater in an aerated tank. As microorganism grow, they degrade organics and remove nutrients from the sludge and transform it to  water, gas, energy and new cell material. To maintain aerobic conditions and to keep the activated sludge suspended, a continuous and well-timed supply of oxygen is required. An activated sludge unit is normally combined to a primary and tertiary treatment unit. Excess sludge produced through bacteria growth needs also to be removed regularly and treated appropriately.

Aerated Pond

Aerated ponds are aerobic wastewater treatment ponds (also called waste stabilisation ponds) in which the natural oxygenation is enhanced by mechanical or diffused air injection to achieve high rates of organic degradation and nutrient removal. Aerated ponds can treat almost all wastewater as long as they are biodegradable. Dependent on the type of treatment, a pre-treatment unit is required (e.g. settling, screening, etc.). If completely mixed, the aerated ponds also require a sedimentation step at the output. The process set-up is then in essence similar to activated sludge systems without sludge return.


Describes biological processes that require oxygen to drive cellular respiration and store energy.

Aerobic Pond

A lagoon that forms the third treatment stage in waste stabilization ponds (WSP). It is a shallow pond with large surface area to enable light penetration and oxygenation by wind mixing. The effluent is generally free of pathogens but rich in nutrients and can be reused for fertigation or aquaculture.

Synonyms: Polishing Pond, Maturation Pond, MP
Abbreviation: AP MP

Describes biological processes that that occur in the absence of any form of oxygen available for metabolic activity. Anaerobic processes are either hindered, or halted by the presence of oxygen.

Anaerobic Baffled Reactor

An anaerobic baffled reactor (ABR) is an improved Septic Tank with a series of baffles under which the wastewater is forced to flow. The increased contact time with the active biomass (sludge) results in increased retention and anaerobic degradation of suspended and dissolved organic pollutants. ABRs are robust and can treat a wide range of wastewater, but both remaining sludge and effluents still need further treatment in order to be reused or discharged properly.

Anaerobic Biogas Digester

A biogas reactor or anaerobic digester is an anaerobic treatment technology that uses organic material (e.g. wastewater, kitchen wastes, etc.) and produces (a) a digested slurry (digestate) that can be used as a fertilizer and (b) biogas that can be used for energy.

Synonyms: Biogas Reactor, Anaerobic Digester
Anaerobic Digestion

The degradation and stabilization of organic compounds by microorganisms in the absence of oxygen, leading to production of biogas.

Anaerobic Filter

An anaerobic filter is a fixed-bed biological reactor with one or more filtration chambers in series. As wastewater flows through the filter, particles are trapped and organic matter is degraded by the active biomass that is attached to the surface of the filter material. Anaerobic filters are similar to septic tank, with the difference, that wastewater, once entered into the tank must flow downwards (anaerobic downflow filter) or upwards through a filter media (anaerobic upflow filter). Anaerobic filters are used as secondary treatment in household-level or decentralised wastewater treatment systems. As for any anaerobic digestion process, biogas can be recovered and transformed to energy or light.

Anaerobic Pond

A lagoon that forms the first treatment stage in waste stabilization ponds. It is a rather deep pond (3 to 5 m) with small surface area. As wastewater flows in, suspended solids settle to the ground and degraded by anaerobic microorganism. As for any anaerobic digestion process, biogas can be recovered and transformed to energy or light.

Anal Cleansing Water

Anal Cleansing Water is water used to cleanse oneself after defecating and/or urinating; it is generated by those who use water, rather than dry material, for anal cleansing. The volume of water used per cleaning typically ranges from 0.5 L to 3 L.


Anoxic describes processes, which occur in the absence of molecular gaseous oxygen, but other forms of oxygen are still present (in opposite to anaerobic, which means that there is no oxygen at all) and used for the respiration of microorganism. Anoxic conditions are often found at the interface between aerobic and anaerobic environments (e.g. in trickling filters or in facultative ponds).The biological conversion of nitrate is to nitrogen gas is anoxic. This process is also known as denitrification.

Application of Pit Humus and Compost

Compost is the soil-like substance resulting from the controlled aerobic degradation of organics. Pit humus is the term used to describe the material removed from a double pit technology (S.4, S.5 or S.6) because it is produced passively underground and has a slightly different composition than compost. Both products can be used as soil conditioners.

Application of Sludge

Depending on the treatment type and quality, digested or stabilized sludge can be applied to public or private lands for landscaping or agriculture.


Aquaculture refers to the controlled cultivation of aquatic plants and animals such as fish, crustaceans and aquatic plants. Aquaculture can be part of an integrated sanitation system making use of nutrients and heat from pre-treated wastewater.


An aquifer is a wet underground layer of water-bearing permeable rock or unconsolidated materials (gravel, sand, or silt) from which groundwater can usefully be extracted using a water well.


Arborloo is a type of composting toilet. Combining a user interface and a storage and composting pit. The shallow pit is filled with excreta and soil/ash and then covered with soil. The decommissioned full pit poses no immediate health risk and the contents will degrade naturally over time. Alternatively, a tree planted on top of the nutrient-rich pit will vigorously grow.


Simple, single cell organisms that are found everywhere on earth. They are essential for maintaining life and performing essential “services”, such as composting, aerobic degradation of waste, and digesting food in our intestines. Some types, however, can be pathogenic and cause mild to severe illnesses. Bacteria obtain nutrients from their environment by excreting enzymes that dissolve complex molecules into more simple ones, which can then pass through the cell membrane.


Biodegradable means that a substance is contained of molecules which can be broken down by biological processes (e.g. by bacteria, fungi, and other microorganisms) without causing any harm to them.


Biological transformation of organic material into more basic compounds and elements (e.g., carbon dioxide, water) by bacteria, fungi, and other microorganisms.


Biogas is the common name for the mixture of gases released from anaerobic digestion. Biogas is comprised of methane (50 to 75%), carbon dioxide (25 to 50%) and varying quantities of nitrogen, hydrogen sulphide, water vapour and other components. Biogas can be collected and burned for fuel (like propane).

Biogas Combustion

In principal, biogas can be used like other fuel gas. When produced in household-level biogas reactors, it is most suitable for cooking. Additionally, electricity generation is a valuable option when the biogas is produced in large anaerobic digesters.

Biological Oxygen Demand

A measure of the amount of oxygen used by microorganisms to degrade organic matter over time (expressed in mg/L and normally measured over five days as BOD5). It is an indirect measure of the amount of biodegradable organic material present in water or wastewater: the more the organic content, the more oxygen is required to degrade it (high BOD). A high BOD can be caused by high levels of organic pollution or high nitrate levels, which trigger high plant growth.


Biomass refers to plants or animals cultivated using the water and/or nutrients flowing through a sanitation system. It may include fish, insects, vegetables, fruit, forage or other beneficial crops, which can be utilized for food, feed, fibre and fuel production.


Blackwater is the 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 of faeces and the nutrients of urine that are diluted in the flushwater.


Brownwater is the mixture of faeces and flushwater, and does not contain urine. It is generated by urine-diverting flush toilets and, therefore, the volume depends on the volume of the flushwater used. The pathogen and nutrient load of faeces is not reduced, only diluted by the flushwater. Brownwater may also include Anal Cleansing Water (if water is used for cleansing) and/or Dry Cleansing Materials.

Capital Cost

Funds spent for the acquisition of a fixed asset, such as sanitation infrastructure.

Carbon to Nitrogen Ratio

The ratio of the mass of carbon to the mass of nitrogen in a substrate. It is often expressed as a single number representing the carbon content. A balanced carbon – nitrogen ratio is relevant for composting and anaerobic digestion. If the digestion process is hampered by a lack of carbon (low C/N ration) organic material (e.g. household waste) can be added. To prevent a lack of nitrogen urine can be added.


A cesspit is an ambiguous term either used to describe an underground holding tank or a soak pit. An underground pool (also called cesspool) is a tank pit used for the temporary collection and storage of faeces, excreta or faecal sludge. The pit is lined with bricks or concrete, covered with a slab and needs to be emptied frequently. When soil conditions allow, the pit is not constructed watertight (similar to a soak pit) allowing liquid to leach out, while solids decay and collect in the base. A soak pit requires less frequent emptying but leachate infiltration may cause groundwater pollution.

Synonyms: Underground Holding Tank, Cesspool, Cess Pit
Chemical Oxygen Demand

A measure of the amount of oxygen required for chemical oxidation of organic material in water by a strong chemical oxidant (expressed in mg/L). COD is always equal to or higher than BOD since it is the total oxygen required for complete oxidation. It is an indirect measure of the amount of organic material present in water or wastewater: the more the organic content, the more oxygen is required to chemically oxidise it (high COD). A high organic content is generally an indictor for water pollution. The COD subtracted by the BOD gives an indication of the non-biodegradable chemicals and thus the toxicity of the water. See also: COD/BOD5 Ratio

Cistern Flush Toilet

The cistern flush toilet is usually made of porcelain and is a mass-produced, factory-made user interface. The flush toilet consists of a water tank that supplies the water for flushing the excreta and a bowl into which the excreta are deposited. Excreta are flushed away with water stored in the cistern (depending on the type between 6 to 20 litres per flush). Cistern-flush toilets provide a high level of convenience for the user but their installation can result in a significantly increase of the fresh water consumption and increase of wastewater to be collected and treated. Dual flush toilets (with a smaller flush-volume for urine), low-flush toilet and urine diverting toilets are available in order to reduce the amount of generated blackwater.


Co-composting is the controlled aerobic degradation of organics, using more than one feedstock (faecal sludge, excreta, organic solid waste, etc.). Faecal sludge has a high moisture and nitrogen content, while biodegradable solid waste is high in organic carbon and has good bulking properties (i.e., it allows air to flow and circulate). By combining the two, the benefits of each can be used to optimize the process and the product. Other organic materials, which can be used or subjected to co-composting, comprise animal manure, sawdust, wood chips, bark, and slaughterhouse waste, sludge or solid residues from food and beverage industry. See also compost or composting.


The destabilization of particles in water by adding chemicals (e.g., aluminium sulphate or ferric chloride) so that they can aggregate and form larger flocs. Coagulation is often used in combination with flocculation.

Collection and Storage/Treatment

Collection and storage/treatment describes the ways of collecting, storing, and sometimes treating the products that are generated at the User Interface. Treatment that is provided by these technologies is often a function of storage and usually passive (e.g. no energy inputs). Thus, products that are ‘treated’ by these technologies often require subsequent treatment before Use and/or Disposal.


Compost is decomposed organic matter that results from a controlled aerobic degradation process. In this biological process, microorganisms (mainly bacteria and fungi) decompose the biodegradable waste components and produce an earth-like, odourless, brown/black material. Compost has excellent soil-conditioning properties and a variable nutrient content. Because of leaching and volatilization, some of the nutrients maybe lost, but the material is still rich in nutrients and organic matter. Generally, Excreta or Sludge should be composted long enough (2 to 4 months) under thermophilic conditions (55 to 60 °C) in order to be sanitized sufficiently for safe agricultural use. This temperature is not guaranteed in most Composting Chambers, but considerable pathogen reduction can normally be achieved.

Composting Chamber

A composting chamber is designed to convert excreta and organics into compost. In composting toilets, faeces or excreta fall into a composting chamber together with cleansing material. Dry organic material such as sawdust is added to adjust moisture content and C/N ratio in order to obtain optimum conditions for thermophilic composting. Organic household waste can also be added for co-composting. Depending on the process, shorter or longer maturation periods and maybe also secondary treatment are required. Urine might be diverted to decrease humidity of the compost and to be reused separately.

Condominial Sewer

Condominial sewerage is the application of simplified sewerage coupled with consultations and on-going interactions between users and agencies during planning and implementation. The term is used primarily in Latin America and is derived from the term condominio/condominium (housing block).

Synonyms: Condominial Sewer System, Condominial Sewerage
Constructed Wetland

A treatment technology for wastewater that aims to replicate the naturally occurring processes in wetlands. Constructed shallow ponds are lined and filled with some sort of filter material (substrate), usually sand, gravel, rock or soil, and planted with vegetation tolerant of saturated conditions (e.g. reeds). As wastewater flows through the ponds, the filter material filters out particles and microorganisms degrade the organics. See also free-water surface constructed wetland, horizontal subsurface flow constructed wetland, Vertical flow constructed wetland.

Synonyms: CW, Wetpark, Reed Bed, Planted Filter, Constructed Treatment Wetland
Abbreviation: CW
Conventional Gravity Sewer

Conventional gravity sewers are large networks of underground pipes that convey blackwater, greywater and, in many cases, stormwater from individual households to a (Semi-) Centralized Treatment facility, using gravity (and pumps when necessary). When blackwater and stormwater are collected in the same pipe, conventional sewers are also called combined sewers.

Synonyms: Combined Sewerage, Combined Sewer, Combined Sewer System, Conventional Sewer System, Conventional Sewer, Conventional Sewerage

Conveyance describes the transport of products from one functional group of a sanitation system to another. Although sanitation products may need to be transferred in various ways between functional groups, the longest, and most important gap is between User Interface or Collection and Storage/Treatment and (Semi-) Centralized Treatment. Therefore, for the sake of simplicity, Conveyance only describes the technologies used to transport products between these functional groups.

Decentralised Wastewater Treatment System

A small-scale system used to collect, treat, discharge, and/or reclaim wastewater from a small community or service area. The treatment unit of DEWATS includes generally sedimentation and floatation (e.g. in septic tanks or biogas settlers); anaerobic treatment (e.g. in upflow baffled reactors or anaerobic filters); aerobic treatment (in sub-surface flow constructed wetlands or unplanted sand- or gravel filters); and a final aerobic treatment in maturation ponds.

Synonyms: DEWATS
Abbreviation: DEWATS
Dehydration of Faeces

When faeces are stored in the absence of moisture (i.e., urine), they dehydrate into a crumbly, white-beige coarse, flaky material or powder. The moisture naturally present in the faeces evaporates and/or is absorbed by the drying material (e.g., ash, sawdust, lime) that is added to them. Dehydration by adding dry organic material and long-term storage at high ambient temperature is the simplest treatment in order to transform faeces into a product that is safe for reuse as soil conditioner or disposal.

Synonyms: Application of Dehydrated Faeces
Dehydration Vaults

Dehydration vaults are used to collect, store and dry (dehydrate) faeces. Faeces will only dehydrate when the vaults are well ventilated, watertight to prevent external moisture from entering, and when urine and anal cleansing water are diverted away from the vaults. See also dehydration of faeces.


The process of removing the accumulated sludge from a sanitation storage or treatment facility (e.g. pit toilet, septic tank).


The process of reducing the water content of a sludge or slurry. Dewatered sludge may still have significant moisture content, but it typically is dry enough to be conveyed as a solid (e.g., shovelled).


The solid and/or liquid material remaining after undergoing anaerobic digestion.


The elimination of (pathogenic) microorganisms by inactivation (using chemical agents, radiation or heat) or by physical separation processes (e.g., membranes). The process is used for water purification or for advanced or tertiary wastewater treatment.

Double Ventilated Improved Pit

The double VIP has almost the same design as the single VIP with the added advantage of a second pit that allows it to be used continuously and permits safer and easier emptying.

Synonyms: Double VIP
Abbreviation: Double VIP
Dried Faeces

Dried Faeces are Faeces that have been dehydrated until they become a dry, crumbly material. Dehydration takes place by storing Faeces in a dry environment with good ventilation, high temperatures and/or the presence of absorbent material. Very little degradation occurs during dehydration and this means that the Dried Faeces are still rich in organic matter. However, Faeces reduce by around 75% in volume during dehydration and most pathogens die off. There is a small risk that some pathogenic organisms can be reactivatedunder the right conditions, particularly, in humid environments.

Dry Cleansing Material

Dry Cleansing Materials are solid materials used to cleanse one-self after defecating and/or urinating(e.g., paper, leaves, corncobs, rags or stones). Depending on the system, Dry Cleansing Materials may be collected and separately disposed of. Although extremely important, a separate menstrual hygiene products like sanitary napkins and tampons is often forgotten. In general (though not always), they should be treated along with the solid waste generated in the household.

Dry Toilet

A dry toilet is a toilet that operates without flushwater. The dry toilet may be a raised pedestal on which the user can sit, or a squat pan over which the user squats. In both cases, excreta (both urine and faeces) fall through a drop hole.

Drying Bed

Drying beds are either planted or unplanted sealed shallow ponds filled with several drainage layers and designed for the separation of the solid from the liquid fraction of (faecal) sludge from latrines, septic tanks, biogas reactors, trickling filters, etc. Sludge is dried naturally by a combination of percolation and evaporation and evapotranspiration. When drying beds are covered with plants, evaporation is enhanced by transpiration (evapotranspiration). See also planted drying beds and unplanted drying bed.

Ecological Sanitation

An approach that aims to safely recycle nutrients, water and/or energy contained in excreta and wastewater in such a way that the use of non-renewable resources is minimized.

Synonyms: Resources-Oriented Sanitation, Re-use Oriented Sanitation, Ecosan
Abbreviation: Ecosan

Effluent is the 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-) centralized treatment technology. Depending on the type of treatment, the effluent maybe completely sanitized or may require further treatment before it can be used or disposed of.

Emerging Technology

A technology that has moved beyond the laboratory and small-pilot phase and is being implemented at a scale that indicates that expansion is possible.


The utilisation of products derived from a sanitation system.

Environmental Sanitation

Interventions that reduce peoples’ exposure to disease by providing a clean environment in which to live, with measures to break the cycle of disease. This usually includes hygienic management of human and animal excreta, solid waste, wastewater, and stormwater; the control of disease vectors; and the provision of washing facilities for personal and domestic hygiene. Environmental Sanitation involves both behaviours and facilities that work together to form a hygienic environment.

Escherichia Coli

Escherichia Coli is a type of bacteria that inhabits the intestinal tract of humans and other mammals. It is not necessarily harmful, but it is used as an indicator of faecal contamination of water.

Synonyms: E. Coli, E.Coli
Abbreviation: E. Coli

The enrichment of water, both fresh and saline, by nutrients (especially the compounds of nitrogen and phosphorus) that accelerate the growth of algae and higher forms of plant life and lead to the depletion of oxygen, blockage of sunlight and increasing temperatures. These changes may harm the original ecosystem. Eutrophication may occur naturally or as the result of anthropogenic influences (e.g. water pollution). Eutrophic comes from the Greek word eutrophos meaning well nourished. Eutrophic waters are distinguished from logographic waters, characterised by a nutrient deficiency, and mesotrophic waters with an intermediate level of productivity.


The phase change from liquid to gas that takes place below the boiling temperature and normally occurs on the surface of a liquid.


The combined loss of water from a surface by evaporation and plant transpiration. It is the process by which water is transferred from vegetated soil to the atmosphere.


Excreta consists of urine and faeces that is not mixed with any flushwater. Excreta is small in volume, but concentrated in both nutrients and pathogens. Depending on the quality of the faeces, it has a soft or runny consistency.

Facultative Pond

A lagoon that forms the second treatment stage in waste stabilisation ponds (WSPs). It is a shallow pond (1 to 2m) consisting of an aerobic zone close to the surface and a deeper, anaerobic zone. AS algae grow on the surface they produce oxygen, which is consumed by aerobic bacteria in the middle of the pond degrading the BOD. In the lower zones of the pond, anaerobic digestion takes place.

Faecal Sludge

Faecal sludge comes from onsite sanitation technologies, and has not been transported through a sewer. It is raw or partially digested, a slurry or semisolid, and results from the collection, storage or treatment of combinations of excreta and blackwater, with or without greywater. Examples of onsite technologies include pit latrines, unsewered public ablution blocks, septic tanks, aqua privies, and dry toilets. Faecal sludge management includes the storage, collection, transport, treatment and safe enduse or disposal of faecal sludge. Faecal sludge is highly variable in consistency, quantity, and concentration.

Synonyms: Fecal Sludge
Faecal Sludge Management

Faecal sludge management includes the storage, collection, transport, treatment and safe enduse or disposal of faecal sludge.

Synonyms: Fecal Sludge Management, FSM
Abbreviation: FSM

Faeces refer to (semi-solid) excrement that is not mixed with urine or water. Depending on diet, each person produces approximately 50 L per year of faecal matter. Fresh faeces contain about 80% water. Of the total nutrients excreted, faeces contain about 12% N, 39% P, 26% K and have 107 to 109 faecal coliforms in100 mL.


The liquid that has passed through a filter.


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.

Fish Pond

Fish can be grown in ponds that receive effluent or sludge where they can feed on algae and other organisms that grow in the nutrient-rich water. The fish, thereby, remove the nutrients from the wastewater and are eventually harvested for consumption. See also aquaculture.

Floating Plant Ponds

A floating plant pond is a modified maturation pond with floating (macrophyte) plants. Plants such as water hyacinths or duckweed float on the surface while the roots hang down into the water to uptake nutrients and filter the water that flows by.


The process by which the size of particles increases as a result of particle collision. Particles form aggregates or flocs from finely divided particles and from chemically destabilized particles and can then be removed by settling or filtration. Flocculation is often used in combination with coagulation.


The process whereby lighter fractions of a wastewater, including oil, grease, soaps, etc., rise to the surface, and thereby can be separated.


Flushwater is the water discharged into the User Interface to transport the content and/or clean it. Freshwater, rainwater, recycled Greywater, or any combination of the three can be used as a flushwater source.

Fossa Alterna

The Fossa Alterna is a short cycle alternating, waterless (dry) double pit technology. Compared to the double VIP which is just designed to collect, store and partially treat excreta, the Fossa Alterna is designed to make an earth-like product that can be used as a nutrient-rich soil conditioner. The Fossa Alterna is dug to a maximum depth of 1.5 m and requires a constant input of cover material (soil, ash, and/or leaves).

Free-Water Surface Constructed Wetland

A free-water surface constructed wetland aims to replicate the naturally occurring processes of a natural wetland, marsh or swamp. As water slowly flows through the wetland, particles settle, pathogens are destroyed, and organisms and plants utilize the nutrients. This type of constructed wetland is commonly used as an advanced treatment after secondary or tertiary treatment processes.

Synonyms: Free Water Surface Flow Constructed Wetland, Free-Surface Constructed Wetland, Free Water Surface Flow CW, Free Water Constructed Wetland, FWS CW, Free-Surface CW, Surface Flow Constructed Wetland
Abbreviation: FWSCW
Functional Group

A functional group is a grouping of technologies that have similar functions. There are five different functional groups from which technologies can be chosen to build a system.

Grease trap

The goal of the grease trap is to trap oil and grease so that it can be easily collected and removed from water. Grease traps are chambers made out of brickwork, concrete or plastic, with an odour-tight cover. Baffles or tees at the inlet and outlet prevent turbulence at the water surface and separate floating components from the effluent. A grease trap can either be located directly under the sink, or, for larger amounts of oil and grease, a bigger grease interceptor can be installed outdoors. Generally grease traps are fitted at garages, restaurants and other commercial premises where large quantities of oil or grease are discharged into wastewater.


Greywater is the total volume of water generated from washing food, clothes and dishware, as well as from bathing, but not from toilets. It may contain traces of Excreta (e.g., from washing diapers) and, therefore, also pathogens. Greywater accounts for approximately 65% of the wastewater produced in households with flush toilets.

Grit Chamber

Where subsequent water or wastewater treatment technologies could be hindered or damaged by the presence of sand, grit chambers (or sand traps) allow for the removal of heavy inorganic fractions by settling. There are three general types of grit chambers: horizontal-flow, aerated, or vortex chambers. All of these designs allow heavy grit particles to settle out, while lighter, principally organic particles remain in suspension.


Water that is located beneath the earth’s surface. The groundwater level may be located several centimetres, or up to a hundred metres below the surface. It is generally of good quality and can be used as drinking water. Therefore, care should be taken to not contaminate groundwater with leaching sewage.

Groundwater Recharge

Treated effluent and/or stormwater can be directly discharged into receiving water bodies (such as rivers, lakes, etc.) or into the ground to recharge aquifers. Surface groundwater recharge is the planned, man-made increase of groundwater levels. By improving its natural replenishment capacities and percolation from surface waters into aquifers, the amount of groundwater available for abstraction is increased. This is particularly useful in areas where water and groundwater resources are heavily utilised and acute problems with dropping watersheds, soil salinization, saltwater intrusion in coastal areas or water scarcity in general exist. Subsurface groundwater recharge refers to the different recharge techniques (generally injection or recharge wells) that release treated effluent and/or collected stormwater underground and directly replenish groundwater aquifers (without soil percolation).

Synonyms: Groundwater Recharge, Surface Groundwater Recharge, Sub-Surface Groundwater Recharge, Subsurface Groundwater Recharge

A parasitic worm, i.e. one that lives in or on its host, causing damage. Some examples that infect humans are roundworms (e.g., Ascaris and hookworm) and tapeworms. The infective eggs of helminths can be found in excreta, wastewater and sludge. They are very resistant to inactivation and may remain viable in faeces and sludge for several years.

Horizontal Subsurface Flow Constructed Wetland

A horizontal subsurface flow constructed wetland is a large gravel and sand-filled basin that is planted with wetland vegetation. As wastewater flows horizontally through the basin, the filter material filters out particles and microorganisms degrade the organics. Compared to free-water surface flow constructed wetlands, horizontal sub-surface flow constructed wetlands have the advantage of better oxygen transmission reduced risk of mosquito breeding.

Synonyms: Sub-surface Flow Constructed Wetland, Horizontal Flow Constructed Wetland, HF CW, Sub-surface Horizontal Flow Constructed Wetland, HFCW
Abbreviation: HFCW
Human-Powered Emptying and Transport

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.


The stable remnant of decomposed organic material. It is a dark-brown or black earthen material. Used as a soil amendment in agriculture, it improves soil structure and increases water retention.

Hydraulic Retention Time

The average amount of time that liquid and soluble compounds stay in a reactor or tank. It is calculated by dividing the volume of a reactor (e.g. m3) by the influent flow rate (e.t. m3/day). In wastewater treatment systems the HRT influence the treatment efficiency and is therefore an important design parameter.

Synonyms: Liquid Retention Time, HRT, Detention Time
Abbreviation: HRT
Imhoff Tank

The Imhoff tank is a primary treatment technology for raw wastewater, designed for solid-liquid separation and digestion of the settled sludge. It consists of a V-shaped settling compartment above a tapering sludge anaerobic digestion chamber with gas vents.

Improved Sanitation

According to the WHO & UNICEF Joint Monitoring Programme (JMP), improved sanitation facilities are those that ensure hygienic separation of human excreta from human contact. As defined by JMP, IMPROVED sanitation facilities include: Flush or pour-flush toilets to piped sewer systems; septic tanks, pit latrines, VIP latrines, pit latrine with slab, or composting toilets. UNIMPROVED Sanitation facilities include: Shared facilities of any type, no facilities (bush or field); flush or pour-flush to elsewhere (that is, not to piped sewer system, septic tank or pit latrine); pit latrines without slab / open pits, bucket systems; hanging toilet or hanging latrine. This definition is received not without criticism, for instance because there is no reference to treatment of the toilet contents.


The general name for the liquid that enters into a system or process (e.g., wastewater). The effluent of one process is the influent of the next.


Irrigation is the artificial application of water to land or soil especially to assist in growing agricultural crops in dry areas or during periods of inadequate rainfall. To reduce dependence on freshwater and maintain a constant source of water for irrigation throughout the year, wastewater that has had secondary treatment (i.e., physical and biological treatment) can be used for irrigation.


Jerrycans are light, plastic containers that are readily available and can be easily carried by one person.

Leach Field

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. They allow for a further treatment of the effluent and to recharge groundwater bodies.


The liquid fraction that is separated from the solid component by gravity filtration through media (e.g., liquid that drains from drying beds).


The common name for calcium oxide (quicklime, CaO) or calcium hydroxide (slaked or hydrated lime, Ca(OH)2). It is a white, caustic and alkaline powder produced by heating limestone. Slaked lime is less caustic than quicklime and is widely used in water/wastewater treatment and construction (for mortars and plasters).

Log Reduction

Organism removal efficiencies. 1 log unit = 90%, 2 log units = 99%, 3 log units = 99.9%, and so on.


An aquatic plant large enough to be readily visible to the naked eye. Its roots and differentiated tissues may be emergent (reeds, cattails, bulrushes, wild rice), submergent (water milfoil, bladderwort) or floating (duckweed, lily pads).


A colourless, odourless, flammable, gaseous hydrocarbon with the chemical formula CH4. Methane is present in natural gas and is the main component (50-75%) of biogas that is formed by the anaerobic decomposition of organic matter.


Any cellular or non-cellular microbiological entity capable of replication or of transferring genetic material (e.g., bacteria, viruses, protozoa, algae or fungi).

Synonyms: Micro-organism, Micro Organism, Microbe

Pollutant that despite their extremely low concentration have an significant effect (e.g., trace organic compounds) Pharmaceuticals and hormones are two groups of micropollutants which are causing increasing concern for their effects on the endocrine system and sexual development of aquatic animals.

Motorised Emptying and Transport

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.

Synonyms: Motorized Emptying and Transport
Night Soil

A historical term for faecal sludge.


Nitrification is an aerobic process carried out by a series of bacterial population that sequentially oxidize ammonium (NH4+) and organic nitrogen to nitrate (NO3-) with intermediate formation of nitrite (NO2-). The first step of nitrification is carried out by bacteria called nitrosomonas : 2NH4+ + 3O2 -> 2NO2- + 4H+ + 2H2O. The second step is carried out by nitrobacter bacteria: 2NO2-+O2 -> 2NO3-. The two steps can also be described in a single reaction: NH4+ + 2O2 -> NO3- + H2O + H+. Once nitrate has formed, the wastewater can undergo a denitrification process in order to reduce nitrate to nitrogen gas, that is released into the atmosphere. Since complete nitrification is a sequential reaction treatment process, systems must be designed to provide an environment suitable for the growth of both groups of nitrifying bacteria.


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.

Offsite Sanitation

A sanitation system in which excreta and wastewater are collected and conveyed away from the plot where they are generated. An off-site sanitation system relies on a sewer technology (e.g. simplified sewer, solids free sewer or conventional sewer) for conveyance.

Onsite Sanitation

A sanitation system in which excreta and wastewater are collected, stored and/or treated on the plot where they are generated. There are two main categories of on-site sanitation technologies: ‘wet’ which require water for flushing; and ‘dry’ which do not require any water for flushing.

Operation and Maintenance

Routine or periodic tasks required to keep a process or system functioning according to performance requirements and to prevent delays, repairs or downtime.


Organics refers to biodegradable plant material (organic waste) that must be added to some technologies in order for them to function properly (e.g., composting chambers). Organic degradable material can include, but is not limited to, leaves, grass and market waste.

Synonyms: Organic Solid Waste, Organic Waste
Pit Humus

Pit Humus is the term used to describe the nutrient-rich, hygienically improved, humic material that is generated in double pit technologies (Double Ventilated Improved Pit (VIP), Fossa Alterna, Twin Pits for Pour Flush). through dewatering and degradation. This earth-like product is also referred to as EcoHumus, a term conceived by Peter Morgan in Zimbabwe. The various natural decomposition processes taking place in alternating pits can be both aerobic and anaerobic in nature, depending on the technology and operating conditions. The main difference between Pit Humus and Compost is that the degradation processes are passive and are not subjected to a controlled oxygen supply, C:N ratio, humidity and temperature. Therefore, the rate of pathogen reduction is generally slower and the quality of the product, including its nutrient and organic matter content, can vary considerably. Pit Humus can look very similar to Compost and have good soil conditioning properties, although pathogens may still be present.

Planted Drying Beds

A planted drying bed is similar to an Unplanted Drying Bed, but has the added benefit of transpiration and enhanced sludge treatment due to the plants. The key improvement of the planted bed over the unplanted bed is that the filters do not need to be desludged after each feeding/drying cycle. Fresh sludge can be directly applied onto the previous layer; the plants and their root systems maintain the porosity of the filter. The percolate or leachate needs to be collected and treated before reuse or disposal.

Pour Flush Toilet

A pour flush toilet is like a regular cistern flush toilet except that the water is poured in by the user, instead of coming from the cistern above. When the water supply is not continuous, any cistern flush toilet can become a pour flush toilet.


Pre-treatment is the preliminary removal of wastewater or sludge constituents, such as oil, grease, and various solids (e.g., sand, fibres and trash). Built before a conveyance or treatment technology, pre-treatment units can retard the accumulation of solids and minimize subsequent blockages. They can also help reduce abrasion of mechanical parts and extend the life of the sanitation infrastructure. See also primary treatment.

Pre-Treatment Products

Pre-treatment products are materials separated from blackwater, brownwater, greywater or sludge in preliminary treatment units, such as screens, grease traps or grit chambers. Substances like fats, oil, grease, and various solids (e.g. sand, fibres and trash), can impair transport and/or treatment efficiency through clogging and wear. Therefore, early removal of these substances is crucial for the durability of a sanitation system.

Primary Treatment

The first major stage in wastewater treatment that removes solids and organic matter mostly by the process of sedimentation or flotation. Examples of primary treatment technologies include septic tanks, biogas settlers, etc. See also pre-treatment.

Synonyms: Primary Wastewater Treatment

Products are materials that are also called ‘wastes’ or ‘resources’. Some products are generated directly by humans (e.g. urine and faeces), others are required in the functioning of technologies (e.g. flushwater to move excreta through sewers) and some are generated as a function of storage or treatment (e.g. Sludge). For the design of a robust sanitation system, it is necessary to define all of the products that are flowing into (inputs) and out of (outputs) each of the sanitation technologies in the system. The products referenced within this text are described below.


A diverse group of unicellular eukaryotic organisms, including amoeba, ciliates, and flagellates. Some can be pathogenic and cause mild to severe illnesses.


Use of recycled water.


The portion of precipitation that does not infiltrate the ground and runs overland.

Sanitation Systems

Sanitation is as a multi-step process in which human excreta and wastewater are managed from the point of generation to the point of use or ultimate disposal. A Sanitation System is a context-specific series of technologies and services for the management of these wastes (or resources), i.e., for their collection, containment, transport, transformation, utilisation or disposal. A sanitation system is comprised of Products (i.e. waste, organic wastewater, excreta, blackwater, greywater, etc.) that travel through functional groups, which contain Technologies that can be selected according to the context. By selecting a Technology for each Product from each applicable Functional Group, one can design a logical Sanitation System. A sanitation system also includes the management, operation and maintenance (O&M) required to ensure that the system functions safely and sustainably.

Sanitation Technology

Sanitation technologies are defined as the specific infrastructure, methods, or services that are designed to contain and transform sanitation products, or to transport them to another functional group (i.e. user interface, conveyance, storage, treatment and final disposal or reuse).


Screening is a wastewater pre-treatment, which aims to prevent coarse solids, such as plastics, rags and other trash, from entering a sewage system or treatment plant. Solids get trapped by inclined screens or bar racks. The spacing between the bars usually is 15 to 40 mm, depending on cleaning patterns. Screens can be cleaned by hand or mechanically raked. The latter allows for a more frequent solids removal and, correspondingly, a smaller design.


Scum is the general name given to the floating layer of dirt or froth on the surface of a liquid. It is also used to describe the layer of solids formed by wastewater constituents that float to the surface of a tank or reactor (e.g., oil and grease).

Secondary Treatment

Follows primary treatment to achieve the removal of biodegradable organic matter and suspended solids from wastewater effluent. Nutrient removal (e.g., phosphorus) and disinfection can be included in the definition of secondary treatment or tertiary treatment, depending on the configuration.

Synonyms: Secondary Wastewater Treatment
Sedimentation/Thickening Ponds

Sedimentation or thickening ponds are settling ponds that allow sludge to thicken and dewater. The effluent is removed and treated, while the thickened sludge can be further treated in a subsequent technology.

Synonyms: Sedimentation Ponds, Thickening Ponds

A historical term to define sludge removed from septic tanks.


Describes the conditions under which putrefaction and anaerobic digestion take place.

Septic Tank

A septic tank is a watertight chamber made of concrete, fibreglass, 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.


A settler or clarifier is a primary treatment technology for wastewater; it is designed to remove suspended solids by sedimentation. It may also be referred to as a sedimentation or settling basin or tank, or clarifier. The low flow velocity in a settler allows settleable particles to sink to the bottom by gravity, while constituents lighter than water float to the surface.


Waste matter that is transported through the sewer. It contains generally excreta (urine or faeces) and water (blackwater), grey water and may also contain other wastes (e.g. kitchen waste).


An open channel or closed pipe used to convey sewage.

Sewer Discharge Stations

A variation of the transfer station, but it is directly connected to a conventional gravity sewer main. Sludge emptied into the sewer discharge station is released into the sewer main either directly or at timed intervals (e.g., by pumping) to optimize the performance of the sewer and of the wastewater treatment plant, and/or reduce peak loads. See also transfer station.


The physical sewer infrastructure (sometimes used interchangeably with sewage). A sewerage system includes all the components of a system used for collecting and transporting (including pipes, pumps, tanks, etc.).

Simplified Sewer

A simplified sewer describes a sewerage network that is constructed using smaller diameter pipes laid at a shallower depth and at a flatter gradient than conventional sewers. The simplified sewer allows for a more flexible design at lower costs.


Someone who prefers to sit on the toilet, rather than squat over it.


Sludge is a 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 onsite 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. For a more detailed characterization of faecal sludge refer to Strande et al., 2014. Wastewater sludge (also referred to as sewage Sludge) is sludge that originates from sewer-based wastewater collection and (semi-) centralized treatment processes. The sludge composition will determine the type of treatment that is required and the end-use possibilities.

Small-bore Sewer

A solids-free sewer is a network of small-diameter pipes that transports pre-treated and solids-free 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. This can significantly lower construction costs.

Synonyms: Solid-free Sewer System, Small-bore Sewer, Settled Sewer, Solids-free Sewer
Soak Pit

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.

Soil Conditioner

A product that enhances the physical properties of soil, such water and nutrient retention, permeability and infiltration capacity and aeration. The goal is to provide a better environment for roots.

Specific Surface Area

The ratio of the surface area to the volume of a solid material (e.g., filter media).

Specific Surface Area

Specific Surface Area

Synonyms: SSA

Someone who prefers to squat over the toilet, rather than sit directly on it.


The degradation of organic matter with the goal of reducing readily biodegradable compounds to lessen environmental impacts (e.g., oxygen depletion, nutrient leaching).

Stabilisation Phase

The stabilisation phase describes the second period following an event. Estimation of the duration of the stabilisation emergency phase varies: while Davis & Lambert (2002) define it to typically start two to four weeks after the event and last for two to six months, Harvey (2007) estimates that the stabilisation and recovery phase together last for several months or years after the event, depending on the type and severity of the emergency. The exact duration depends on the event and the context of the emergency. Duration is not time-bound but rather depends on the achievement of set targets (indicators).

Stored Urine

Stored Urine is Urine that has been hydrolysed naturally over time, i.e., the urea has been converted by enzymes into ammonia and bicarbonate. Stored Urine has a pH of approximately 9. Most pathogens cannot survive at this pH. After 6 months of storage, the risk of pathogen transmission is considerably reduced.


Stormwater is the general term for the rainfall runoff collected from roofs, roads and other surfaces before flowing towards low-lying land. It is the portion of rainfall that does not infiltrate into the soil.


A historical term for greywater.


The liquid lying above a solid residue after settling, sedimentation, or other processes.

Surface Disposal and Storage

Surface disposal refers to the stockpiling of sludge, faeces or other materials that cannot be used elsewhere. Once the material has been taken to a surface disposal site, it is not used later. Storage refers to temporary stockpiling. It can be done when there is no immediate need for the material and a future use is anticipated, or when further hygienization and drying is desired before application.

Surface Water

A natural or man-made water body that appears on the surface, such as a stream, river, lake, pond, or reservoir. Surface water, unlike groundwater is generally of bad quality and need treatment before human consumption because it accumulates pathogens, metals, nutrients and chemicals as it flows across contaminated surfaces.

System Template

A system template defines a suite of compatible technology combinations from which a sanitation system can be designed.

Tertiary Filtration

Application of filtration processes for tertiary treatment of effluent.

Tertiary Treatment

Follows secondary treatment to achieve enhanced removal of residual suspended solids and other pollutants from effluent. Nutrient removal (e.g., phosphorus) and disinfection can be included in the definition of secondary treatment or tertiary treatment, depending on the configuration.


Toilets are sanitation facilities at the user interface that allow the safe and convenient urination and defecation. Toilets can be combined with onsite storage and/or treatment and are then referred to as toilet systems such pit latrines, VIPs, UDDTs, composting toilet systems or terra preta toilet systems.

Total Solid

The residue that remains after filtering a water or sludge sample and drying it at 105°C (expressed in mg/L). It is the sum of Total Dissolved Solids (TDS) and Total Suspended Solids (TSS).

Transfer Station

Transfer stations or underground holding tanks act as intermediate dumping points for faecal sludge when it cannot be easily transported to a (semi-) centralized treatment facility. A vacuum truck is required to empty transfer stations when they are full.

Trickling Filter

A trickling filter is a fixed-bed, biological reactor that operates under (mostly) aerobic conditions. Pre-settled wastewater is continuously ‘trickled’ or sprayed over the filter made out of rocks, gravel, plastic modules. As the water migrates through the pores of the filter, organics are degraded by the biofilm covering the filter material.

Twin Pits for Pour Flush

A sanitation technology consisting of two alternating pits connected to a pour flush toilet. The blackwater (and in some cases greywater) is collected in the pits and allowed to slowly infiltrate into the surrounding soil. Over time, the solids are sufficiently dewatered and can be manually removed with a shovel.

Unplanted Drying Beds

An unplanted drying bed is a simple, permeable bed that, when loaded with sludge, collects percolated leachate and allows the sludge to dry by evaporation. Approximately 50% to 80% of the sludge volume drains off as liquid or evaporates. The sludge, however, is not effectively stabilized or sanitized. Unplanted drying beds need to be desludged regularly. The percolate or leachate needs to be collected and treated before reuse or disposal. See also planted drying beds.

Upflow Anaerobic Sludge Blanket Reactor

The upflow anaerobic sludge blanket reactor (UASB) is a single tank process used for the biological treatment of wastewater. Wastewater enters the reactor from the bottom, and flows upward. A suspended sludge blanket filters and treats the wastewater as the wastewater flows through it. Biogas, which is produced during the process can be collected and reused. Effluent requires a further treatment step to remove pathogens. Sludge can be composted and/or dried before safe disposal or reuse in agriculture as soil amendment.


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.


A urinal is used only for collecting urine. Urinals are generally for men, although models for women have also been developed. Most urinals use water for flushing, but waterless urinals are becoming increasingly popular.


Urine is the liquid produced by the body to rid itself 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. With the exception of some rare cases, urine is sterile when it leaves the body.

Urine Diverting Dry Toilet

A Urine Diverting Dry Toilet (UDDT) is a toilet that operates without water and has a divider so that the user, with little effort can divert the urine away from the faeces.

Synonyms: Urine Diverting Dry Toilet, Urine Diversion Dehydration Toilet, UDDT
Abbreviation: UDDT
Urine Storage

Urine storage is the simplest and cheapest way to hygienise separately collected urine in order to reuse it safely in agriculture. Urine is generally sterile but contains most of the plant nutrients excreted by humans. To hygienise urine, it is stored for one up to six or more month, depending on estimated risk of cross-contamination with faeces. The sanitising effect comes from a combination of the rise of pH (due to the transformation of urea to ammonium by the naturally present enzyme urease), the raise of temperature and time.

Urine Storage Tank

When urine cannot be used immediately or transported using a conveyance technology (e.g. jerrycans), it can be stored onsite in containers or tanks. The storage tank must then be moved or emptied into another container for transport.

Urine-Diverting Flush Toilet

The urine-diverting flush toilet is similar in appearance to a cistern flush toilet except for the diversion in the bowl. The toilet bowl has two sections so that the urine can be separated from the faeces. Both sitting and squatting models exist. See also urine diversion components.

Synonyms: UDFT, Urine Diversion Flush Toilet, Urine Diverting Flush Toilet
Abbreviation: UDFT
Use and/or Disposal

Refers to the sanitation methods by which products are ultimately returned to the environment, either as useful resources or reduced-risk materials. Furthermore, sanitation products can also be cycled back into a system (e.g., by using treated greywater for flushing).

User Interface

User Interface describes the ways of collecting, storing, and sometimes treating the products generated at the User Interface. The treatment provided by these technologies is often a function of storage and is usually passive (e.g., requiring no energy input). Thus, products that are ‘treated’ by these technologies often require subsequent treatment before Use and/or Disposal.


An organism (most commonly an insect) that transmits a disease to a host. For example, flies are vectors as they can carry and transmit pathogens from faeces to humans.

Ventilated Improved Pit

The single VIP is a ventilated improved pit. It is an improvement over the single pit because continuous airflow through the ventilation pipe vents odours and acts as a trap for flies as they escape towards the light. See also double VIP

Synonyms: Single Ventilated Improved Pit, Blair Toilet, Ventilated Improved Pit Latrine, Single VIP, VIP
Abbreviation: VIP, Single VIP
Vertical Flow Constructed Wetland

A vertical flow constructed wetland is a planted filter bed that is drained at the bottom. Wastewater is poured or dosed onto the surface from above using a mechanical dosing system. The water flows vertically down through the filter matrix to the bottom of the basin where it is collected in a drainage pipe. The important difference between a vertical and horizontal flow constructed wetland is not simply the direction of the flow path, but rather the aerobic conditions. See also constructed wetland.

Synonyms: Vertical-Flow Constructed Wetland, Sub-surface Vertical Flow Constructed Wetland, Sub-surface Flow Constructed Wetland, VFCW
Abbreviation: VFCW

An infectious agent consisting of a nucleic acid (DNA or RNA) and a protein coat. Viruses can only replicate in the cells of a living host. Some pathogenic viruses are known to be waterborne (e.g., the rotavirus that can cause diarrheal disease).


Someone who prefers to use water to cleanse after defecating, rather than wipe with dry material.

Waste Stabilisation Pond

Waste stabilisation ponds are large, manmade water bodies used for the treatment of wastewater by the means of naturally occurring processes, such as solar light, wind, and microbial activity. There are three types of ponds, anaerobic ponds, facultative ponds and aerobic or maturation ponds, each with different treatment and design characteristics. The ponds can be used individually, or linked in a series for improved wastewater treatment. Investment and operation costs are relatively low, but large surface areas far from housings are required.

Synonyms: Waste Stabilization Pond, WSP
Abbreviation: WSP

Used water from any combination of domestic, industrial, commercial or agricultural activities, surface runoff/stormwater, and any sewer inflow/infiltration. Wastewater from sanitation contains one or more of the following products: excreta, faeces, urine, blackwater, brownwater, flushwater, dry cleansing material, cleansing water.

Water Table

The level below the earth’s surface, which is saturated with water. It corresponds to the level where water is found when a hole is dug or drilled. A groundwater table is not static and can vary by season, year or usage.

Synonyms: Groundwater level

Someone who prefers to use dry material (e.g., toilet paper or newspapers) to cleanse after defecating, rather than wash with water.