Effluent Treatment Plant (ETP)
Effluent Treatment Plants are another form of wastewater treatment plant which are designed and manufactured specially for treating water that contains effluents and works towards making the water effluent-free. Effluent treatment plant convert wastewater – which is water no longer needed or suitable for its most recent use – into an effluent that can be either returned to the water cycle with minimal environmental issues or reused. If an industry has a right to use water, then it becomes their duty to make it clean or re-usable. Industries consume clean water and in outcome they generate polluted water which includes harmful chemicals such as oils, grease, solids etc. water produced through the industrial process is called effluent. Kelvin’s duty is to clean the pollutants through the effective water treatment methods. So, plants are established for this process and these plants are called “Effluent Treatment plants“.
Treatment Methods used by us for treating Effluent
Chemical Unit Processes
Chemical unit processes are used in combination with physical operations and may also with biological treatment processes. In chemical process addition of special chemicals is done to the water for creating change in parameters/quality of waste water. They include pH control, coagulation, chemical precipitation and oxidation.
It is necessary to adjust the pH in the treatment process to make the wastewater pH neutral. A neutral pH is required particularly when biological treatment is being used, as the microorganisms used in biological treatment need a pH within the range of 6-8 and will be destroyed by highly acidic or alkali wastewater. Various chemicals are used for pH control. For acidic wastes (with low pH), sodium hydroxide, sodium carbonate, calcium carbonate or calcium hydroxide, can be used. For alkali wastes (with high pH), Sulphur acid or hydrochloric acid can be used. Acids can cause corrosion of equipment and care must be taken while choosing. Hydrochloric acid is probably better from an environment’s point of view but can corrode stainless steel. Therefore, plastic or appropriately coated pumps and pipes must be used.
Chemical coagulation is an important unit process in water treatment for the removal of turbidity. Chemical coagulants such as aluminum sulphate (alum) or ferrous sulphate may be added to wastewater to improve the attraction of fine particles so that they attach to form larger particles called flocs.
Flocculation is aided by gentle mixing of flocculant chemical which causes the particles to collide. The floc may then float to the top of the liquid (creaming), settle to the bottom of the liquid (sedimentation), or be readily filtered from the liquid.
Biological Unit Processes:
Biological treatment process generally aims at removal or reduction of the concentration of organic and inorganic compounds from the industrial wastewater. Biological treatment process can take many forms but all are based around microorganisms, mainly bacteria. These microorganisms consume the effluent components for their survival and breaks them down to less complex and less hazardous compounds. In this process the microorganisms increase in number.
There are two main types of processes, these involve suspended microbial growth (e.g. activated sludge) and attached microbial growth (e.g. fixed film). With both approaches large populations of microorganisms are brought into contact with effluent in the presence of an excess oxygen. In both systems the microbial population has to be retained in a tank called as reactor.
With suspended growth systems microbes grow in small aggregates or “flocs” (this is known as activated sludge). Activated sludge (AS) leaves the reactor with the treated effluent but is settled out in a clarifier and returned to the aeration unit to recycle the bacteria. If the amount of AS is excessive some amount of it can be disposed off.
Physical Unit Processes
Some of the common physical unit operations include among processes such as screening, flow equalization, sedimentation, clarification and aeration.
Screening process includes the use of a screen with uniform sized openings to remove larger solid particles that can damage the equipment, reduce the effectiveness of the ETP or pollute the waterways.
Equalization tanks are used to collect and store the wastes that allows to mix and become a regular quality before it is pumped to the treatment units at a constant rate. ETPs are designed to treat wastewater that normally has a constant flow and a quality that fluctuates only within a narrow range. The equalization tank overcomes this. The hourly variation of flow needs to be determined so that the required volume of the tank is determined.
Sedimentation is used to remove grit and suspended solids, to produce clarified effluent, and to thicken the sludge produced in biological treatment. A sedimentation tank (settling tank or clarifier) is used for this process. Flocculation and sedimentation should remove most of the suspended solids and a portion of the BOD.
This involves aerating the wastewater. The water is aerated (or shaken up) to release dissolved gases such as hydrogen sulfide. Aeration also replenishes oxygen, which is consumed by decaying organic matter found in wastewater. Bacteria are also used at this stage to consume nutrients and organic materials. Aeration at times can remove up to 90 percent of all solids and organic material. Two main methods are used for this, either mechanical agitation of the water so that air from the atmosphere enters the water, or by introducing air into the tank through diffusers.
Combined Effluent Treatment Plants ( C-ETP )
CETP’s are set up in the industrial estates where there are clusters of small scale industrial units and where many polluting industries are located. Many of the Small Scale Industries (SSI) are unable to install the individual treatment systems. Hence, the concept of CETP’s (Common Effluent Treatment Plants) is envisaged to benefit such industries in treating its effluent before disposal whether it is in stream, land, sewerage system or in rivers and seas. The Ministry of Environment & Forest, Government of India has launched a centrally sponsored scheme, namely, Common Effluent Treatment Plant (CETP) in order to make a co-operative movement of pollution control especially to treat the effluent, emanating from group of compatible Small-Scale Industries. The CETP therefore, reduces the treatment cost to be borne by an individual member unit while protecting the water environment to a maximum.
There are two basic classification types for CETPs –
Homogenous – When the various industries in the estate are of similar nature.
Heterogeneous – When the various industries produce different products, due to which the quality of the wastewater differs.
Since it becomes difficult to classify the characteristics of the wastewater due to the different industries involved, heterogeneous type of CETP becomes a better option.