Is the process of removing contaminants from waste water, including household sewage and runoff effluents. It includes physical, chemical, and biological processes to remove physical, chemical and biological contaminants. Its objective is to produce an environmentally safe fluid waste stream or treated effluent and a solid waste or treated sludge suitable for disposal or reuse usually as farm fertilizer. With suitable technology

• Pretreatment:

Pretreatment removes all materials that can be easily collected from the raw sewage before they damage or clog the pumps and sewage lines Objects that are commonly removed during pretreatment include trash, tree limbs, leaves, branches, and other large objects. The influent in sewage water passes through a bar screen to remove all large objects , This is most commonly done with an automated mechanically raked bar screen in modern plants serving large populations while in smaller or less modern plants, a manually cleaned screen may be used

• Grit removal:

Pretreatment include sand or grit channel or chamber, where the velocity of the incoming sewage is adjusted to allow the settlement of sand, grit, stones, and broken glass. These particles are removed because they may damage pumps and other equipment
Grit chambers come in 3 types: horizontal grit chambers, aerated grit chambers and vortex grit chambers

• Primary Sedimentation tanks (Primary clarifiers):

In the primary sedimentation stage, sewage flows through large tanks, commonly called primary sedimentation tanks” or “primary clarifiers. The tanks are used to settle sludge while grease and oils rise to the surface and are skimmed off. Primary settling tanks are always 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

• Secondary treatment:

Secondary treatment is designed to substantially degrade the biological content of the sewage which are derived from human waste, food waste, soaps and detergent. The majority of municipal plants treat the settled sewage liquor using aerobic biological processes (Aeration process). To be effective, the biota require both oxygen and food to live. The bacteria andprotozoa consume biodegradable soluble organic contaminants Activated sludge systems can be transformed into aerobic granular sludge systems (aerobic granulation) which enhance the benefits of activated sludge, like increased biomass retention due to high sludge settlability.

• Secondary sedimentation:

The final step in the secondary treatment stage is to settle out the biological floc or filter material through a secondary clarifier and to produce sewage water containing low levels of organic material and suspended matter.

• Filtration:

Sand filtration removes much of the residual suspended matter. Filtration over activated carbon, also called carbon adsorption, removes residual toxins.
The most common type of filter is a rapid gravity sand filter. Water moves vertically through sand which often has a layer of activated carbon or anthracite coal above the sand. The top layer removes organic compounds, which contribute to taste and odor. The space between sand particles is larger than the smallest suspended particles, so simple filtration is not enough. Most particles pass through surface layers but are trapped in pore spaces or adhere to sand particles. Effective filtration extends into the depth of the filter.
To clean the filter, water is passed quickly upward through the filter, opposite the normal direction (called backwashing) to remove embedded particles. Prior to this step, compressed air may be blown up through the bottom of the filter to break up the compacted filter media to aid the backwashing process; this is known as air scouring. This contaminated water can be disposed of, along with the sludge from the sedimentation basin.

• Nozzles system:

-Backwash nozzles differ in type and size. The main parts are the nozzle and the pipe. The width of the slits in the nozzle is chosen based on the filter material and must be smaller than the smallest particle of the filter material.
In order to create an even distribution of air over the entire filter bottom during backwashing, resistance is necessary over the filter bottom. This resistance is created by the pipe that is attached
to the nozzle. This pipe causes an air cushion, which in its turn causes resistance, to form underneath the bottom. This cushion can be varied in thickness by changing the position of the hole in pipe. The diameter of the hole can be accurately calculated to fit the amount of necessary
backwash air. As this is a difficult calculation and sometimes different amounts of air are used during backwashes, a second hole can be made in the pipe. When a larger amount of air is used it can also escape through this second hole. The distribution of air over the filter will remain even.
A pipe with a groove can be used as an alternative. For this, however, it is necessary that the filter bottom is very accurately horizontal, in order to get an even distribution of air over the filter. This alternative is therefore not preferred.

• Disinfection:

The purpose of disinfection in the treatment of waste water is to substantially reduce the number of microorganisms in the water to be discharged back into the environment for the later use of drinking, bathing, irrigation, etc. The effectiveness of disinfection depends on the quality of the water being treated, the type of disinfection being used, the disinfectant dosage (concentration and time), and other environmental variables. Cloudy water will be treated less successfully, since solid matter can shield organisms, especially from ultraviolet light or if contact times are low. Generally, short contact times, low doses and high flows all militate against effective disinfection. Common methods of disinfection include ozone, chlorine, ultraviolet light, or sodium hypochlorite. Chloramines, which is used for drinking water, is not used in the treatment of waste water because of its persistence. After multiple steps of disinfection, the treated water is ready to be released back into the water cycle by means of the nearest body of water or agriculture. Afterwards, the water can be transferred to reserves for everyday human uses.

• Chlorination system
Chlorination remains the most common form of waste water disinfection due to its low cost and long-term history of effectiveness.

Chlorination system is consisting of:

Chlorine Container & Handling Equipment.

Electric crane

Suspended Scale

Lifting beam

Chlorine Headers.

container weighing scale

Vacuum Regulator.

Automatic Changeover system for gas supply



Residual Chlorine analyzer


Injection & Sampling Points

Booster pumps and sample pumps

Chlorine leak detection system and associated warning lights etc.

Safety equipment

Emergency tool kit

Testing and Commissioning

Chlorine gas scrubber

• Ultra violet :

Ultraviolet (UV) light can be used instead of chlorine, iodine, or other chemicals. Because no chemicals are used, the treated water has no adverse effect on organisms that later consume it, as may be the case with other methods. UV radiation causes damage to the genetic structure of bacteria, viruses, and other pathogens, making them incapable of reproduction. UV light is becoming the most common means of disinfection because of the concerns about the impacts of chlorine in chlorinating residual organics in the wastewater and in chlorinating organics in the receiving water. Some sewage treatment systems use UV light for their effluent water disinfection.

• Odor control:

Odors emitted by sewage treatment are typically an indication of an anaerobic or “septic” condition. Early stages of processing will tend to produce foul smelling gases, with hydrogen sulfide being most common in generating complaints.

• Active carbon:

Activated carbon is commonly used for odor control as it has an amazing ability to adsorb contaminants from a variety of waste streams. Carbons can also be impregnated with a range of other chemicals which help increase the removal efficiency of certain compounds, for example, caustic impregnated activated carbon works very well on H2S and mercaptans in moist air streams.

• Biological

The biological odor control system can be used to remove H2S and reduced sulfur compound (RSC) odors generated during the treatment of municipal wastewater at pump stations, headworks and dewatering applications. The system uses the patented media technology, a unique combination of synthetic and organic medias that provide the proper environment for promoting bacteria growth to eliminate odorous compounds.