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Bio Verte Environnement, Mayenne

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Definitions of Words or Terms:

Activated Sludge
Flocculent sludge produced by the growth of bacteria and other organisms in raw or settled sewage, when it is continuously aerated.


Activated Sludge Process
A biological treatment process in which a mixture of sewage and activated sludge is agitated and aerated. The activated sludge is subsequently separated from the treated sewage by settlement and may be re-used.


Aerobic action
A biological process promoted by action of bacteria in the presence of dissolved oxygen.


Ammoniacal Nitrogen (N or NH3)

The nitrogen in sewage is assessed as ammoniacal nitrogen. This indicates the amount of nitrogenous organic matter which has been converted to ammonia. The average strength of crude domestic sewage will have a combined nitrogen content of 40 mg/l to 60 mg/l. pH VALUES. The pH of a liquid is a measurement of its acidity ( low figures of 3 to 5.) or alkalinity (high figures of 9 to 12.) with a neutral of 7.0. The average strength of crude domestic sewage is slightly alkaline at 7.2.


Anaerobic action
A biological process promoted by the action of bacteria in the absence of dissolved oxygen.


Biochemical Oxygen Demand (BOD)
This test is the measurement of the weight of dissolved oxygen consumed by chemical and micro-biological action during the incubation of a sample for a definite period at a defined temperature. In the UK the period is 5 days and the temperature is 20 degrees C. The average 5 day BOD strength of domestic crude sewage is about 300 mg/l. to 400 mg/l., although this can very easily vary between 250 mg/l. (weak sewage) to 500 mg/l. (strong sewage).


This is a generic term used for a packaged type of complete sewage treatment system using an Rotating Biological Contactor (R.B.C.)


Biological Filter
A bed of relatively inert material to promote or assist natural aerobic degradation of sewage.


Biological Treatment
This is a stage in the treatment of sewage or other effluents which biologically treats the pollution frequently by the natural aerobic degradation of the pollutant.


Centrifugal Pumps
A lot of sewage pumps are of the centrifugal type, where an electrically driven impellor rotates at a high speed and discharges the sewage through a volute into the pumping main. (see also
submersible pumps.)

A covered watertight tank used for receiving and storing sewage from premises which cannot be connected to the public sewer and where conditions prevent the use of a small sewage treatment works, including a septic tank.


Chemical Oxygen Demand
The amount of oxygen used in the chemical oxidation of the matter present in a sample by a specified oxidising agent under standard conditions.


Chlorides. (C1)

Chlorides are always present in domestic sewage and natural unpolluted water (at levels averaging 10 to 20 mg/l. ) The average strength of chlorides in crude domestic sewage is about 70 mg/l to 100 mg/l.


Colloidal Material
The finely divided solids which will not settle but which may be removed by coagulation.


Crude Sewage
Sewage which has received no treatment.


The biochemical decomposition of organic matter using anaerobic bacteria, which results in the formation of simpler and less offensive compounds.


A device for spreading settled sewage over the surface of a biological filter.


Dosing Chamber
A small tank which receives settled sewage until the desired quantity has accumulated, when it is discharged automatically to the distributor of a biological filter.


These are usually the smaller pipes that serve a single property, and they join up to form a Drainage System. There may be foul water, and surface water drains, which are usually in separate systems. Many queries are caused by drainage problems, and odours

or smells from these systems.


Dry Weather Flow ( DWF )
When the sewage flow is mainly domestic in character, the average daily flow to the treatment works during seven consecutive days without rain following 7 days during which the rainfall did not exceed 0.25mm on any one day. Usually taken as 200 litres per head per day on domestic properties. (Note - this figure will vary according to your location and specialist help must be sought or see our table of loadings.)

Effluent Polishing or Tertiary Treatment
A further stage of treating sewage or effluents, by removing suspended solids and or pollutants. Consequential removal of suspended solids may also remove residual BOD or other pollutants.


Environmental Laws
In many parts of the world there are now Environmental laws which investigate, protect, legislate to ensure our environment is protected.


The enrichment of water in watercourses and lakes by chemical substances, especially compounds of nitrogen and phosphorous.


Filter medium
The material of which the biological filter is formed and on which a biological film (or biomass) containing bacteria and fungi develops.


Final Effluent
The effluent discharged from a sewage treatment plant.


Foul Sewage
This is the sewage that is derived from all sinks, baths, and water closets in a property.


Humus Tank (or final settlement tank )
A secondary settlement tank through which biologically treated effluent flows, and is for the purpose of separating settleable solids.


Hydraulic Gradient
The surface slope of a liquid in a pipeline. This is generally taken as parallel to the invert in a smaller gravity sewer, but it must be checked. In a treatment works this will be the overall head losses.


The lowest point of the internal cross section of a channel or sewer.


Inverted Syphon
A portion of pipe or conduit in which the sewage flows under pressure, due to the sewer dropping below the hydraulic gradient and then rising again.


All systems will necessitate some form of maintenance, varying from just emptying from tanks the residual sludge's in simple systems, to considerably more involved maintenance of the system and process in more complicated plants.


New technology has been developed whereby the pollutants in liquids can be removed by the use of ultra fine membranes, such as the Reverse Osmosis method in the treatment of fresh potable water, or membranes in the treatment of polluted water or effluents.


Sewage by its very nature will produce odours, and these can sometimes be a problem at sewage pumping stations or sewage treatment works. At a good treatment works there will be very little odour, but a lot will depend on the sludge's produced at the works. It is therefore vital in the siting of a pumping station or treatment works that these considerations be considered.


The chemical change which a substance undergoes when it takes up oxygen.


Percolating Filter
Biological Filter.


Phosphorous (P)

Phosphorous occurs in domestic sewage as organic phosphorous derived from food. The average strength of phosphorous in crude domestic sewage is between 3 mg/l to 22 mg/l.

Population Equivalent
The equivalent, in terms of a fixed population, of a varying or transient population (eg. of a hospital or restaurant ) based upon a figure of 60 g. BOD per head per day and/or 200 l. per head per day.


Positive Displacement (or PD.) Pumps
These types of pumps invariably sit at ground level, and pump the liquid through an archimedian screw system into a pumping main.


Primary Settlement Tank
A tank, in which the majority of settleable solids are removed from the crude sewage that will flow into it.


Pumping Main (or Rising Main.)
This is a continuous main through which sewage or effluent is pumped and running full and at a pressure greater than atmospheric, to a final destination.


Pumping Station
This is usually an underground structure that the foul (or surface water) sewage is discharged into. The types vary but in smaller systems these comprise of a wet well, into which the sewage is discharged, and the wet well also houses submersible pumps which pump the sewage to its destination. In a larger station there may be a separate dry well, adjacent to the wet well, which houses the pumps. On some pumping stations the pumps may be housed above ground near the wet well.


Reed Beds
These are usually specially constructed beds which contain reeds (frequently the Phragmites Australis reed ) to biologically treat sewage and other effluents. Many natural reed beds exist in wetland areas.


Rotating Biological Contactor (RBC)
This is a system of sewage or effluent treatment, that uses closely spaced parallel discs mounted on a horizontal shaft, which rotate about a horizontal axis, and the discs are alternatively exposed to polluted liquors and air as the shaft rotates to biologically treat the sewage.


Secondary Settlement tank
A tank in which settleable solids or humus is separated from the effluent flowing through it, from biological filters or other biological treatment units.


Separate System
A drainage system in which foul and surface water are conveyed by separate pipes.


Septic Tank
A type of settlement tank in which the sludge is retained for sufficient time for the organic matter to undergo anaerobic decomposition.


Settlement (or Sediment) Tank
This is a tank which is used in the treatment of sewage or effluents to settle out the suspended solids contained in the liquids. The tank may be sited near the works inlet as a 'primary settlement tank' or alternatively after biological treatment as a final (humus) settlement tank.


The water-borne wastes of a house or community.


Sewage Treatment Works
The site which contains all the necessary plant for the treatment of sewage.


Sewage Treatment Plant
The items of equipment or structures which treat the sewage.


This term is used to describe such items as a 'sewerage system' or 'sewerage network'. ( for example - sewage runs in a sewerage system.)


A mixture of solids and water produced during the treatment of waste water or sewage. This will frequently have to be removed from the treatment system by de-sludging.


Sludge Removal or De-Sludging
This is the process of removing sludge from treatment systems or tanks and can be carried out manually or automatically. All sludge's removed from tanks or systems should be transported with care and in accordance with current legislation.


see Odours.


Submersible Pumps
These are usually electrically driven centrifugal pumps, housed in a water proof casing, which can be submerged in a liquid and pump the liquid through a pumping main.


Surface Water
This is rain water which falls on Roof's, Drives, Roads, Car Parks which should be discharged into a surface water sewerage system. It should not be discharged into a foul sewerage system, (although in many urban areas it is already connected to the foul system) and should be discharged into nearby watercourses, streams, rivers, lakes or to sea.


Supernatant Liquor
The liquor in a settlement tank, lying between the deposited solids and any floating scum.


Suspended Solids (SS )
Solids in suspension in sewage liquors as measured by filtration through a filter paper followed by washing and drying at 105 deg. C.


Tertiary Treatment
See also
Effluent Polishing.- There are many differing types of tertiary treatment of effluents, the most common being, Grass Plots, Reed Beds, Upward flow Clarifier. Rapid Gravity Sand Filter, Microstrainer, Sand Filter, Drum Filter, Lagoons, Nitrifying Filter.


Top Water Level ( TWL )
The maximum water level in a settlement tank, an aeration tank or a sludge storage tank.


This usually refers to the toxic element of waterborne wastes, and the toxic elements may comprise of metals, pesticides, or other chemicals which cause pollution of streams, watercourses, rivers, beach's, or ground water.


See Suspended Solids.


There are many types of valves that are used to control sewerage or effluent systems. These can vary from - Non Return Valves, Gate Valves, Sluice Valves, Penstock Valves, each of which has a specific purpose.


Water Quality
This term is used to describe the quality of water in rivers, lakes, streams or watercourses, as well as the quality of Potable or Drinking Water


Water Table
The level below which the ground is saturated with water.


Many natural wetland areas exist throughout the world and these are frequently found where this is some form of pollution that they use as a nutrient feed. In some areas these natural wetlands have been constructed by man to treat sewage or other forms of effluent pollution.

- This list is not exhaustive - If you have any suggestions or additions please contact us