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About UNEP
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United Nations Environment Programme
Division of Technology, Industry and Economics
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Newsletter and Technical Publications
<International Source Book On Environmentally Sound Technologies
for Wastewater and Stormwater Management>

Glossary

Activated Carbon Activated carbon is produced by heating organic materials (e.g. coconut shells) in the absence of oxygen to a very high temperature (above 450 oC). The carbon that is produced has active surfaces; the latter can remove many pollutants from water.
Activated sludge Activated sludge is the assemblage of micro-organisms, dead and alive, together with adsorbed organic and inert materials present in aerated wastewater.
Activated sludge process In an activated sludge process settled wastewater is contacted with recirculated activated sludge in an aerated chamber. In this way a substantial amount of micro-organisms is kept in the treatment process to consume the organic materials in the wastewater. Excess sludge is removed after sedimenting the aerated mixture.
Aerobic With or in the presence of air (oxygen)
Aerobic lagoon Wastewater treatment lagoon that has a high dissolved oxygen level. This is usually a result of a combination of low organic waste input and sufficient oxygen transfer from the air, and occurs in the last few in a series of lagoons. See Lagoon.
Alkaninity A measure of the amount of acid that a given volume of water can absorb without substantially changing its pH.
Anaerobic Without air (oxygen)
Anaerobic digestion Decomposition of organic substances in the sludge by micro-organisms under anaerobic conditions.
Anaerobic lagoon Wastewater lagoon which has very little dissolved oxygen. This condition is the result of oxygen uptake by micro-organisms in the lagoon exceeding oxygen transfer from the air, and occurs in the first lagoon in a series of lagoons. See Lagoon.
Anoxic Without oxygen but with the presence of nitrate (chemically bound oxygen)
Aquaculture (wastewater fed) Rearing of fish in ponds relying on the nutrients (nitrogen and phosphorus) contained in wastewater. Guidelines for wastewater fed aquaculture should be followed to ensure protection of public health.
Attached growth system An attached growth system retains micro-organisms on the surfaces of solids (e.g. a bed of gravel or coarse sand) for the purpose of treating wastewater. As wastewater flows through the surfaces of the solids organic materials are consumed by the micro-organisms. See Trickling Filter.
   
Biochemical oxygen demand (BOD) A measure of the strength or concentration of wastewater. This is so because micro-organisms consume the organic substances in the wastewater for energy and growth. In the process of microbial respiration dissolved oxygen is taken up from the water by the micro-organisms. Because the oxygen uptake is measured over a period of 5 days, it is also denoted as BOD5
Biofilter A biofilter is used for the treatment of wastewater by passing the wastewater through a bed of gravel, coarse sand or other solid surfaces. Organic substances are removed by micro-organisms attached to the solids surfaces (See Attached Growth system).
Biofilter for odour A biofilter used for the treatment of odorous gases consists of a bed of mature compost. As the odorous gas passes through the moist compost bed the odorous substances are adsorbed to the compost and degraded by compost micro-organisms.
Biogas Biogas is the gases that are produced from the anaerobic decomposition of organic materials (e.g. sewage sludge). The gases are mainly methane and carbondioxide, and hence have energy value from its methane content.
Biogeochemical cycles Natural cycles of elements which constitute living materials of plants and animals. These elements (e.g. carbon, nitrogen, phosphorus) occur in various forms in the living organisms, in the dead organisms, in the soil and atmosphere and when they are taken up again by organisms.
Biological filter See Trickling Filter
Biological nutrient removal (BNR) Nutrients in wastewater (nitrogen and phosphorus) can be removed by the use of specific groups of micro-organisms by providing suitable conditions for their growth. Systems that are designed and operated to achieve this purpose are called BNR systems.
Biosolids The term used for sludge removed from the treatment of sewage that has undergone processing and with the intention of reusing the sludge.
Blackwater Waste from the toilet. See Greywater.
Bucket latrine Latrine where faeces and urine are collected in a bucket. See Night Soil.
   
Carbon cycle See Biogeochemical cycle.
Catchment (basin) Area of land from which stormwater run-off flows to a common stream or river.
Central(ised) systems Centralised systems are systems where wastewater and/or stormwater are collected from individual houses and brought to a central treatment plant. A system of pipes (sewer) is used for this purpose. Gravity is relied upon for flow, but pumping may be required.
Centrifuge (sludge) Mechanical equipment used to remove moisture from sludge by giving the sludge a rotating motion in a drum forcing the mixture of water and solids towards the cylindrical surface of the drum where a filter medium is placed. Water passes through while the solids are retained on the filter medium.
Chamber soakway A chamber where stormwater run-off is directed to. Water percolates through the sides and bottom of the chamber into the surrounding soil to groundwater.
Chlorination The use of chlorine for the purpose of disinfection of water. Chlorine is available in the form of chlorine gas or solid (calcium hypochlorite, e.g. powder or tablet) or liquid (sodium hypochlorite).
Clarifier A clarifier separates solids from wastewater by settling the solids out in a settling pond or tank, and in this way the wastewater is clarified.
Coliforms Bacteria which are abundant in the blood of warm blooded animals including people. They are used as an indicator of faecal contamination of water.
Combined sewer Combined sewer carries both wastewater and stormwater. Separate sewers collect only wastewater or stromwater.
Combined sewer overflow (CSO) Combined sewer (see above) invariably results in an overflow during heavy rain storm. This overflow is the volume that exceeds the capacity of the treatment plant and is termed combined sewer overflow.
Composting Composting is the process that converts organic materials into humus. This process is carried out by micro-organisms under aerobic conditions.
Composting toilet A composting toilet collects human excreta in an aerated chamber for the purpose of producing humus by composting. Air is drawn from the toilet chamber into the collection chamber and out through a vent, thus odour is controlled.
Conditioning (sludge) Sludge may need to be conditioned to assist the process to reduce its moisture. This is usually done by adding chemicals that help coagulate or flocculate the particles in the sludge.
Condominium sewerage Pipes which collect wastewater in a condominium sewer pass through neighbouring properties. This saves pipes compared to conventional sewer where pipes from each property are connected to mains running in front of the properties. See Simplified Sewerage.
Constructed wetlands See Wetlands
Conventional sewerage In conventional sewerage pipes are laid relatively deep in the ground to prevent interference from traffic, and allowance is made for self-cleansing flow in the pipes on a daily basis. As a result pipes are larger in size and are laid deeper compared to shallow sewerage. Costs are correspondingly higher.
Cross-cutting issues Cross-cutting issues in the implementation of environmentally sound technology refers to the broader issues of planning, involvement of the community, methods for cost-recovery for capital cost and for operating cost, and other social and political factors to ensure successful and long term sustainability of the technology.
   
Dechlorination The removal of excess chlorine once the objective of chlorination (disinfection) has been achieved.
Denitrifiers Bacteria which carry out the denitrification process.
Denitrification The conversion of nitrate into nitrogen gas. This process is carried out by denitrifiers and requires an organic carbon source and anaerobic conditions.
Digestion (aerobic, anaerobic) Digestion is the process of biodegradation of organic materials by micro-organisms. It can take place under aerobic conditions (in the presence of air) or anaerobic conditions (in the absence of air).
Disinfection The removal of pathogens. This can be done in a number of ways (e.g. boiling water, ultra-violet radiation, chlorination).
Dissolved air flotation (DAF) The process of removing suspended solids (such as oils and grease particles) by floating them using very small air bubbles and skimming them. Pressurised air is dissolved in the water, and when the pressure is reduced to atmospheric very small bubbles are produced.
Dissolved oxygen (DO) Dissolved oxygen is the oxygen that is dissolved in water. At an ambient temperature of 20 oC the amount is about 9 mg/L (parts per million). The small amount of oxygen that can be dissolved in water limits its availability to aquatic organisms, and can be readily depleted by wastewater. See Biochemical Oxygen Demand.
Drying bed (sludge) A bed of sand on which wet sludge is placed for drying. Moisture is removed by evaporation and percolation through the sand bed. Excess water is removed from the base of the bed for further treatment.
   
Environmentally sensitive zones Environmentally sensitive zones refer to water bodies, such as rivers, lakes and seas, that are sensitive to addition of nutrients (nitrogen and phosphorus). See Eutrophication.
Eutrophication Eutrophication refers to the process or conditions of a water body that receives and contains excessive amounts of nutrients. These conditions result in algal bloom. Toxins may be produced by some algae. The decay of the algae can result in depletion of oxygen from the water body.
Evapotranspiration In wastewater treatment it refers to the removal of the water by use of plants. The plants act a biological pump by absorbing moisture through their roots and transpiring it to the atmosphere. Water can be distributed in a trench filled with gravel and topped with sand or soil (Evapotranspiration trench) or when the water table is high, into a bed of sand above ground (Evapotranspiration bed).
   
Facultative lagoon Wastewater treatment lagoon which contains a high level of oxygen during the day but a low level at night. Algae in the lagoon produce the oxygen during sunlight hours by photosynthesis, and taking carbon dioxide produced by the respiration of bacteria consuming the organic waste. See Lagoon.
Filter drains Permeable materials located below ground to store stormwater run-off. Run-off flows to the storage area via a permeable surface.
Filter press (sludge) Pressure filter consisting of plates and frame. Sludge slurry is pumped under pressure onto a filter medium placed on the plates. Solids are retained on the filter and water passes through, thus dewatering the sludge.
Filter strips (stormwater) Vegetated surface that allows stormwater run-off to flow evenly to collection areas while at the same time filters solids out of the water.
   
Garburator Garburator is usually installed beneath a kitchen sink to shred kitchen waste and dispose it with wastewater. Treatment of the wastewater involves removing the kitchen waste from the wastewater, so it may not be considered as a desirable method for disposing of kitchen waste.
Grass filtration See Overland Flow
Grease trap Grease trap is a device to remove grease from wastewater, so that the grease does not interfere with downstream treatment of the wastewater (e.g. blockage).
Greywater Wastewater from bathroom, laundry and kitchen. See Blackwater.
Grit removal The removal of solids that settle relatively quickly from wastewater (e.g. sand particles). This is usually carried out in a settling time with a short residence time.

 

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