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Back Pressure

Pressure that can cause water to backflow into the water supply when a user's waste water system is at a higher pressure than the public system.

Back siphonage

Reverse seepage of water in a distribution system.


The flow of water in a medium in a direction opposite to normal flow. Flow is often returned into the system by backflow, if the wastewater in a purification system is severely contaminated.


Reversing the flow of water back through the filter media to remove entrapped solids.

Bag filter

Textile or sintered polymer filters used to remove dust and fume particles from gas streams. Used also on liquid waste to provide a final polish or remove floc particles.

Balancing tank

Provides sufficient storage volume to permit a non-uniform flow of waste water to be collected, mixed and pumped forward to a treatment system at a uniform rate.

Bathing waters

Areas designated under EC directive 76/160/EEC. Number of bathers is main criterion.

Bed load

Sediment particles resting on or near the channel bottom that are pushed or rolled along by the flow of water.

Benthic zone

The lower region of a body of water including the bottom.


Salts containing the anion HCO3-. When acid is added, this ion breaks into H2O and CO2, and acts as a buffer.


Chemicals that hold short fibres together in a cartridge filter.


The increase in concentration of a substance in living organisms, as they take in contaminated air, water, or food, due to slow metabolization and excretion.

Biochemical oxygen demand (BOD)

The quantity of dissolved oxygen in water (mg/l) consumed under test conditions during a given period (5 days) through the microbiological oxidation of biodegradable organic matter present in wastewaters. One of the standard tests used to characterise effluent quality.


A chemical that is toxic to microrganisms. Biocides are often used to eliminate bacteria and other single-cell organisms from water.


Ability of substance to decompose without adding chemicals.

Biodegradable pollutants

Pollutants that are capable of decomposing under natural conditions.


Population of various microrganisms, trapped in a layer of slime and excretion products, attached to a surface.

Biological contaminants

Living organisms such as viruses, bacteria, fungi and mammal and bird antigens that can cause harmful health effects to humans.

Biological filtration

A process in which settled sewage uniformly trickles downward through a bed of inert material such as slag, moulded plastics or clinker, thus permitting contact with the biological film with which the surfaces of the medium are coated so that oxidation and clarification take place.

Biological oxidation

Decomposition of complex organic materials by microrganisms through oxidation.

Biologically activated carbon

Activated carbon that supports active microbial growth, in order to aid in the degradation of organics that have been absorbed on its surface and in its pores.


The use of living organisms to test the suitability of effluents for discharge into receiving waters and to test the quality of such waters downstream from the discharge.


The biological treatment of wastewater and sludge, by inducing the breakdown of organics and hydrocarbons to carbon dioxide and water.

Biosolids (sewage sludge)

Produced from the treatment of sewage. Biosolids have to comply with EC and UK legal standards and the Safe Sludge Matrix for agricultural use.


All living organisms in a region or ecosystem.


Conversion of a substance into other compounds by organisms; including biodegradation.


Water that contains waste of humans, animals or food.

Blind spots

Any place on a filter medium where fluids cannot flow through.


A build-up of particles in a filter medium, that prevents fluids from flowing through.

Blue flag beaches

Beaches which meet a number of detailed criteria for cleanliness and facilities including compliance with the EC directive's guideline bathing water standards.


Biodegradable Municipal Waste.


Biological Oxygen Demand.


The amount of dissolved oxygen consumed in five days by bacteria that perform biological degradation of organic matter.


Best Practicable Environmental Option.

Brackish water

Water that is neither falls in the category of salt water, nor in the category of fresh water. It holds the middle between either one of the categories.

Breakpoint chlorination

Addition of chlorine to water until there is enough chlorine present for disinfection of water.


Crack or break in a filter bed that allows the passage of floc or particulate matter through a filter.


Highly salty and heavily mineralised water, containing heavy metal and organic contaminants.


A substance that reacts with hydrogen or hydroxyl ions in a solution, in order to prevent a change in pH.


Bathing Water Directive.


Bathing Water Regulations.

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