Effluent Treatment Plant covers the mechanisms and processes used to treat such waters that have been contaminated in some way by
anthropogenic industrial or commercial activities prior to its release into the environment or its re-use. This effluent contains several pollutants, which can be removed with the help of an effluent treatment plant. ETP treat water and make free from all objectionable impurities present in suspension, colloidal or dissolved form.

Why do we need to treat Effluent?

To prevent groundwater pollution
To prevent sea shore
To prevent soil
To prevent marine life
Protection of public health
To reuse the treated effluent
For agriculture
For groundwater recharge
For industrial recycle
Solving social problems caused by the accumulation of wastewater

Need of ETP in Industry

Manufacturers face strict regulations on discharge and waste. Non-compliance can lead to expensive fees and operations interference. A wastewater treatment helps them to: Stay in compliance Reduce transportation and off-site treatment costs Reduce supply costs by recovering production materials out of the waste-stream for re-use Eliminate municipal fees Eliminate unnecessary water usage during processing

Preliminary Treatment

Preliminary treatment removes gross solids and materials that can be easily collected from the effluent and can damage or clog the
pumps and skimmers of primary treatment clarifiers. These are in-organic materials and insoluble organic pollutants (i.e. large floating and suspended solid matter, grit, oil & grease) which
are inert and cause problems to further chemical and biological treatments. The presence and sequence of preliminary treatment units are totally depend upon the characteristics of effluent that is to be treated. All preliminary treatment consists of physical separation techniques by controlling flow rate of effluent.

Major Treatment units in ETP

Preliminary Treatment-Screens

Are used for removal of large floating and submerged material such as plastics, paper pieces, rubber, etc. from effluent. Major objectives are: They prevent clogging in pump, pipes and valves, etc. Prevent to interfere large material in primary and secondary treatment units. Screens are classified in number of ways as follows: Based on the size of opening – such as coarse, medium and fine screens. Depending upon shape – such as disc, drum, band, etc. Based on method of cleaning – such as mechanical or manual. Preliminary Treatment Detractors/ Scrappers Detractors: Are shallow circular ponds used in pre treatment plants with the purpose of grit removal? Chain or blade type detractors/scrappers are utilized in an adjacent channel into which the collected grit is transferred. Used when the soft and sticky substances, sticks on fixed bars and cannot get through the screen. Grits with a 6 m diameter are removed in the detractor.

Primary Treatment:

Consist of temporarily holding the sewage in a inert basin where heavy solids can settle to the bottom while oil, grease and lighter solids float to the surface. The settled material go for sludge treatment and floating materials are skimmed off the remaining liquid is passed to secondary treatment. Tanks are usually equipped with mechanically driven scrapers that continually drive the collected sludge towards a hopper in the base of the tank where it is pumped to sludge treatment facilities. Primary treatment can be either only physical or can be combined with chemical methods depending upon the effluent characteristics. At this stage, over 70% of the suspended solids and 40% of the BOD is removed from effluent. 

Sedimentation Tank (Clarifier)

Are designed to remove suspended particles that are heavier than water through gravitational force by reducing water velocity. So it’s a solid liquid separation process in which a suspension Is separated into two phases- Clarified supernatant leaving the top of the sedimentation tank (underflow). Concentrated sludge leaving the bottom of the sedimentation tank (overflow). An efficient sedimentation tank can remove about 90% of SS and 40% organic matter with two hour of detention time.


To remove coarse dispersed phase. To remove coagulated and flocculated impurities. To remove precipitated impurities after chemical treatment.

Factors affecting sedimentation

Characteristics of solid particles -size, shape, specific gravity, concentration, etc. Characteristics of liquid  temp., viscosity, specific gravity, etc. Physical characteristics of clarifiers- detention period, shape and depth of basins, flow rate, etc. Types of

Sedimentation Tanks:

According to shape rectangular tank, circular tank. According to direction of flow longitudinal and radial flow, vertical flow. According to nature of working-Fill and draw, Continuous flow. According to method of sludge collection-flat bottom tank with or without scrapper, hopper bottom tank. The most common are horizontal flow sedimentation tank and center -feed circular clarifiers.


Clariflocculator consist of coagulation (chemical) and flocculation (mechanical) process. Employed when finely divided SS and colloidal particles nan not be efficiently removed by simple sedimentation. In such cases chemical assistance is taken. The chemical used for coagulation are called coagulants. The most common coagulants are alum, hydrated lime, ferric chloride, ferrous sulfate and PAC (Poly aluminum Chloride).

Coagulation, Flocculation

Coagulation is the conversation of finely dispersed colloids into a small flock with the addition of coagulates. Coagulation can either be followed by the process of settling or flotation. The purpose of flocculation is to bring coagulated particles together by mechanically inducing a velocity gradient within a liquid. Organic polyelectrolyte flocculation aids are effective in promoting soluble solids removals. Flocculation improves the removal of finely divided solids by gravity settling.

Equalization Tank

Some industries produce different types of wastes, having different characteristics at different intervals of time. Hence, equalization tank is used to mixed different type of effluent thoroughly to produce homogenous and equalized effluent for uniform treatment. This are large holding Thanks with aeration or mechanical agitation with paddles manually to give better mixing of the different unit volumes of effluents. The effluent is hold and mixed for specified period of time.

Neutralization Tank

Neutralization tank is used to provide treatment to highly acidic or highly alkaline effluent through neutralization PH by addition of chemicals. Industry requires neutralization of discharged effluent when it has as highly acidic and /or highly alkaline effluent. Acidic effluent is\ usually treated with lime stone or lime-slurry or caustic soda, depending upon the type and quantity of effluent. Alkaline effluent is neutralized by sulphuric acid or CO2 or waste boiler flue gas.

Secondary treatment

Is a biological treatment of effluent which is typically performed by indigenous, water-borne micro-organisms in a managed habitat. Secondary treatment removes dissolved and suspended organic (bio-degradable) matter by consuming it as a food and converts it into new cell mass, energy and CO2. The most common micro organisms are bacteria (aerobic or anaerobic), protozoa and rotifers; least common are fungi and algae. After secondary treatment almost 70-90% of BOD and 80-90% of dissolved solids are removed from effluent.

Trickling Filter

In trickling filter treatment, wastewater is sprayed through sprinkling rotating are circular beds consist of 3-10 ft. deep coarse media (of crushed stones, gravels or synthetic material). Microorganisms get attach and grows on the media and results in formation of film of micro-organismal called zoogloea film. This zoogloea film consists of millions of micro-organism breakdowns organic material of effluent into simple and soluble matter. Treated effluent is drain at bottom from where it is collected then undergoes for sedimentation (SST). It is simple to operate and give almost 80-90% of BOD removal with high quality effluent.

Activated Sludge Process (ASP)

This is most versatile biological oxidation method employed for the treatment of effluent containing of dissolved solids, coarse and colloidal organic matter. In this method, effluent is aerated in a reaction tank consist of microbial population in suspension form. Aerobic bacteria degrade effluent into CO2 and H2O for which oxygen is supplied through mechanical aeration or by diffused aeration system. The bacterial flora grows and remains suspended in the form of a flock called activated sludge. A part of sludge is recycled for the same tank to provide an effective microbial population for a fresh treatment cycle. Industrial require 6 to 24 hours of aeration though which almost 90-95% of BOD can be removed.

UP-flow Anaerobic Sludge Blanket Reactor (UASB)

UASB system is a three phase settler which separates sludge, liquid and biogas in same tank under high turbulence without occupying large space. Organic matter is degraded under anaerobic conditions by microbes producing methane and CO2. More effective than conventional aerobic process, produces only 5 10% of sludge. Hence, more popular in anaerobic treatment methods.

Tertiary Treatment

Tertiary treatment is the final treatment, meant for ‘polishing’ the effluent and removal of pollutants not removed in primary and secondary treatment. These pollutants may include soluble inorganic compounds such as phosphorous or nitrogen which may support algal growth in receiving waters. Also removes organic materials contributing BOD, COD, color, taste, odor; bacteria, viruses, colloidal solids contributing turbidity; or soluble minerals which may interfere with subsequent re-use of the wastewater. Preferred when treated water is need to be reuse or discharge is into a highly sensitive or fragile ecosystem (estuaries, low-flow rivers, coral reefs, etc.) 

Tertiary Treatment

Tertiary treatment add additional cost to the treatment process but produce high quality effluent which can be reuse further for commercial and industrial applications. Treated water can be reuse for the irrigation of a golf course, green way or park, construction work, industrial process, etc. If it is suffic Treated water is sometimes disinfected chemically or physically depending upon the discharging location.


Filtration is process of removing particulates and bacterial impurities that could not be removed in earlier treatment, from water by passing it through a porous medium. It is used to remove colloidal and other impurities which impart turbidity to water. Also for disinfection of water by reducing 90% of bacterial load. It is also used to reduce odor and color by arresting them in filter media. There is various type of filter available to treat certain type of wastewater. Selection is totally depending upon characteristics of ww, efficiency of absorbent, flow rate and pollutant to be removed from ww.

Activated Carbon Filters

ACF consist of activated carbon granules supported by very fine quartz filter media. Various grades of carbon are available for specialized treatment of wastewater. ACF are used to remove free chlorine, organic residues, toxic heavy metal ions and color from ww. Most common adsorbents used are activated carbon, peat moss, brown coal and other cellulose materials. This treatment not only improves taste of water but also protects other water treatment units such as reverse osmosis membranes and ion exchange resins from possible damage due to oxidation or organic fouling.


This treatment is used for removal of toxic materials and recovery of valuable materials from effluent. Ion-exchange is only economical when recovered materials are reused. So not economical when objectives is only removal of pollutants. Majorly used for recovery of Cr, Ni, phosphate and H2SO4, Cu, Pb, Hg and removal of cyanides from wastewater after Cr recovery.