Wastewater Treatment Glossary

Aerobic Wastewater Treatment

It is a wastewater treatment process where the microbes involved in the treatment process require the presence of oxygen to survive, thrive and treat the wastewater by breaking down the organic load in the process.

Anaerobic Wastewater Treatment

It is a wastewater treatment process where the microbes involved in the treatment process do not require the presence and/or require the complete absence of oxygen to survive, thrive and treat the wastewater by breaking down the organic load in the process.

Activated Sludge Process

It is a process used in wastewater treatment where suspended partially degraded organic solids floc together with active microbes present in the treatment system and settle with time. A calculated portion of the settled ‘sludge’ serves as seed to inoculate and treat incoming raw wastewater. The major benefit of doing so is the microbial ecosystem in the activated sludge primes the secondary treatment system for a more efficient and faster treatment.

Biochemical Oxygen Demand (BOD)

It is the quantum of dissolved oxygen required by microbes to degrade organic matter in a unit quantity of water at a specific temperature and time period. It is measured in terms of milligrams of oxygen consumed per litre of the water during 5 days of incubation at 20°C and is the most commonly used parameter to analyse the degree of organic pollution of water.

Biogas Production

Biogas production involves the generation of a mixture of gases as a consequence of the breakdown of organic matter by microbes in the absence of oxygen. The biogas gas thus generated is a variable mixture of methane and carbon dioxide. The substrates used for anaerobic digestion to produce biogas are agricultural, agro-industrial, food processing waste, animal manure or municipal sewage waste. Read more

Chemical Oxygen Demand (COD)

It is the quantum of dissolved oxygen that can be consumed in a reaction to completely oxidise organic matter present in a unit quantity of water at a specific temperature and time period. It is expressed in SI units as milligrams per litre (mg/L).

Denitrifying Bacteria

Wastewater often contains large amounts of nitrogen in various forms which could be deleterious to the environment if left untreated. As part of the biological treatment process, wastewater goes through phases of nitrification and/or denitrification. Denitrifying bacteria are heterotrophic in nature. When an organic carbon source is supplied to denitrifying bacteria in anoxic conditions, they utilise the oxygen present in the nitrate to oxidize the carbon containing substrate. This leads to the evolution of nitrogen gas from nitrate, which is then released from the wastewater.

Dispersed Growth

In secondary treatment units (in both aerobic and anaerobic treatment units), when the microbial population is not attached to any substrate / media, the growth is known to be as dispersed or suspended growth.

Effluent Treatment Plant

Effluent Treatment Plants or (ETPs) are used by industries to treat waste water and remove any toxic and non- toxic contaminating materials or chemicals from it, ultimately making it acceptable to be reused or discharged into the environment.

Equalization Basins

Industrial processes in manufacturing units create different types of wastewater depending on the procduct being manufactured or process being carried out. The quality and quantum of the wastewater created therefore varies accordingly. Extreme variations in these parameters could cause collapse of the microbial population in the secondary treatment system which is crucial to the treatment process. The equalization basins are created in order to overcome these issues as they help in maintaining the overall quality and flow rate of wastewater to the treatment unit.

Fixed Growth

In secondary treatment units (in both aerobic and anaerobic treatment units), when the microbial population is attached to any substrate / media, the growth is known to be as fixed growth.

Food Microorganism Ratio (F:M ratio)

The F:M ratio describes the ratio of biodegradable organic matter versus the microbial biomass present in a wastewater system. It is a great indicator of the biological health of the secondary treatment unit. Any skewing of the F:M ratio may spell problems for the secondary treatment unit. For example : low F:M ratio encourages the excessive growth of filamentous bacteria. High F:M ratio may cause the formation of pin flocs and discourage proper settling of sludge. Either way, it also means that the treatment of the secondary wastewater would be inefficient.


Flocculation is the physico-chemical separation of a dissolved solids from a solution mediated by a flocculating agent which causes the creation of ‘flocs’ which are large, heavier and settle down. It is a critical step in waste water treatment and is used as part of the primary treatment process.

Hydraulic Retention Time (HRT)

The time duration for which the influent raw untreated wastewater is available for microbial interaction and subsequent degradation after entering the system upto its discharge is known as hydraulic retention time. It is a factor of the flow rate and geometry of the treatment system.

Industrial Wastewater

Industrial wastewater originates from production, industrial and commercial activities, and the composition of industrial wastewater is much more highly variable than sewage water due to the wide range of industrial processes and raw materials.

Mixed Liquor (ML)

Mixed liquor refers to a mixture of activated sludge and raw untreated wastewater for as long as it is in the aeration or secondary treatment tank.

Mixed Liquor Suspended Solids (MLSS)

Undissolved particulate matter found wastewater of an aeration tank containing mixed liquor is known as Mixed Liquor Suspended Solids.

Mixed Liquor Volatile Suspended Solids (MLVSS)

The volatile suspended solids (primarily microbial biomass) present in mixed liquor of a biological treatment tank. It is used as a quantitative indicator of the microbial population present in the mixed liquor of a secondary wastewater treatment unit.

Municipal Wastewater

It is a varying amalgamation of sewage water, industrial wastewater and unabsorbed rain water that enter public sewers.

Nitrifying Bacteria

Bacteria that convert ammonia and organic nitrogen in wastewater into nitrates are known as nitrifying bacteria. These bacteria are instrumental in oxidising the ammonia into nitrates via a chemolithotrophic pathway and belong to the genera Nitrosomonas, Nitrosococcus, Nitrobacter and Nitrococcus They are key to stripping wastewater of ammonia which could otherwise be toxic if released into the environment.

Organic Loading Rate

It is the measure of influent substrate that enters digester per unit time for microbial treatment.


Microorganisms such as bacteria, fungi, virus that possess the potential to infect and cause disease in higher living organisms are known as pathogens.


When anaerobic microbes become predominant in wastewater due to lack of dissolved oxygen and noxious gases including hydrogen sulphide, darken the wastewater, it is said that the wastewater has turned septic.


Sedimentation is the physical deposition of suspended particulate matter in a wastewater treatment unit. It occurs physically by the action of gravity ending in the separation of solids from the liquid phase. It is an important phenomenon that occurs and is used in wastewater treatment.

Sewage Water

Sewage water is the water used and discarded from residences and offices. This water contains water from kitchens and bathrooms.

Solids Retention Time (SRT)

It is the average time that the combination of organic solids undergoing degradation as well microbial biomass i.e. the activated sludge solids are retained in the secondary waste water treatment unit.

Total Suspended Solids (TSS)

Total suspended solids (TSS) is the absolute weight of suspended particles, that are derived from a pre-defined quantity of water sample and dried until a constant weight is achieved for three consecutive readings indicating the complete absence of water.

Volatile Suspended Solids (VSS)

Volatile solids are undissolved suspended particulate matter present in water that are incinerated at 550°C. It is a water quality measurement obtained from the loss on ignition of total suspended solids including microbial biomass.

Waste Water Treatment (WWT)

Wastewater treatment is strategy devised to treat wastewater with the ultimate aim of reducing/eliminating the harm it may cause to the environment and/or with the aim of reusing it for industrial processes.

Preliminary Wastewater Treatment

This requires physical removal of large debris in the wastewater that may impede the treatment process.

Primary Wastewater Treatment

Partial removal of suspended solids and dissolved organic and inorganic solids in wastewater using sedimentation and flocculation. This is an important step in wastewater treatment as it reduces the pollution load in the wastewater upto 60%.

Returned Activated Sludge (RAS)

The portion of activated sludge that is returned back from the clarifier to aeration tank which acts as microbial seed for new wastewater entering the tank.

Secondary Wastewater Treatment

Microbe mediated breakdown and subsequent reduction of dissolved and suspended organic nutrients along with other nutrients resulting in the overall reduction of organic load in wastewater is known as secondary wastewater treatment. It is also known as biological wastewater treatment.

Sludge Bulking

Sludge bulking is a condition in an activated sludge tank characterized by poor compaction and settling of the sludge. The condition usually arises due to secretion of slime by microbes present in the system as reaction to extreme environmental conditions such as starvation or high BOD with high organic loading condition or dominance of filamentous bacteria in activated sludge occurring due to a skewed F:M ratio, low D.O etc.

Tertiary Wastewater Treatment

Tertiary treatment is the final treatment process involves removal of inorganic compounds, and substances, such as the nitrogen and phosphorus, making the water reusable and/or safe to discharge.