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Wastewater. One Name, Many Terms. | A Wastewater Glossary | YASA ET Tips

Wastewater, also referred to as sewage, includes water from household or building use (such as toilets, showers, and sinks) that can contain human faecal waste, as well as water from non-household sources (such as rain and industrial use) which for this reason contain organic and inorganic substances that can damage health and the environment.

These types of water, after their use, cannot therefore be discharged directly into the environment (into the soil, rivers, lakes and seas) without first being subjected to constantly monitored purification interventions.

This process must be carried out in accordance with local regulations, which should provide for actions necessary to achieve or maintain the good quality of surface or deep waters, protecting them from pollution.

In the following paragraphs we introduce a Wastewater Glossary which includes the most common terms used in the wastewater industry.

Wastewater Treatment Plant COD Reduction YASA ET
Wastewater Treatment Plant

Wastewater Glossary

Industrial Wastewater

Any type of wastewater from buildings or installations in which commercial activities or the production of goods take place, qualitatively different from domestic wastewater and from rainwater runoff, including those that have come into contact with substances or materials, including polluting materials, not connected with the activities carried out in the establishment;

Domestic Wastewater

Wastewater originating from residential settlements and services and deriving mainly from human metabolism and domestic activities;

Urban Wastewater

The mixture of domestic wastewater, industrial wastewater, and / or run-off rainwater conveyed into sewer networks, even separate ones.

BOD - Biochemical Oxygen Demand

Biochemical oxygen demand is the amount of oxygen required by aerobic microorganisms, in order to proceed with the assimilation and degradation of the organic substances present in the sewage. This value is higher the greater the organic substance present in the wastewater. The measurement of the oxygen present in the measuring cells carried out after five days of incubation provides the BOD5 while after twenty days the BOD20;

BOD5 - Five-day Biochemical Oxygen Demand

BOD5 Refers to the five-day biochemical oxygen demand. The total amount of oxygen used by microorganisms decomposing organic matter increases each day until the ultimate BOD is reached, usually in 50 to 70 days.

COD - Chemical Oxygen Demand

Chemical oxygen demand (COD) is an index used to measure the amount of oxygen required to chemically oxidize the oxidizable substances present in the sewage;

SS - Suspended Solids

All those undissolved substances, present in the water sample to be examined, which are retained by a membrane filter, of certain porosity, when the sample itself is subjected to filtration. The filter to be used, to obtain a separation of all suspended solids (including colloidal ones), must have pores with an average diameter of 0.45 µm;

TDS - Total Dissolved Solids

Total dissolved solids (TDS) are the amount of organic and inorganic materials, such as metals, minerals, salts, and ions, dissolved in a particular volume of water measured in ppm;


Nitrate is a compound that is formed naturally when nitrogen combines with oxygen or ozone. Nitrogen is essential for all living things, but high levels of nitrate in drinking water can be dangerous to health, especially for infants and pregnant women.


Phosphorus is a common constituent of agricultural fertilizers, manure, and organic wastes in sewage and industrial effluent. It is an essential element for plant life, but when there is too much of it in water, it can speed up eutrophication, a reduction in dissolved oxygen in water bodies caused by an increase of mineral and organic nutrients.


Conductivity is a measure of the ability of water to pass an electrical current. Because dissolved salts and other inorganic chemicals conduct electrical current, conductivity increases as salinity increases. Conductivity is useful as a general measure of water quality and high levels are an indicator that a discharge or some other source of pollution has entered the aquatic resource.


The term discharge describes any introduction of wastewater into surface waters, on the ground, in the subsoil and in the sewerage system, regardless of their polluting nature, also subjected to preventive purification treatment;

Wastewater Treatment

Treatment of wastewater by means of a process which, after discharge, should guarantee the conformity of the receiving water bodies with the relative quality objectives or complies with the provisions of the local regulators. Some of these processes can be performed using Vacuum Evaporators, Electrocoagulation, Reverse Osmosis, and other technologies.


Flocculant is a type of substance that can reduce or eliminate the precipitation stability and polymerization stability of dispersed particles in water, and make dispersed particles coagulate and flocculate into aggregates. They can be divided into three categories: inorganic, organic and microbial. Polyacrylamide (PAM) and Poly-aluminium Chloride (PAC) are the most used flocculants in wastewater treatment;

ELV - Emission Limit Value

In the Emission Limit Value (ELV) approach, focus is on the maximum allowed quantities of pollutants that may be discharged from a particular source into waters, especially at the end of a process (such as from urban or industrial waste water treatment).


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