Effluent/Wastewater Treatment Plant | ETP Process

What is Wastewater Treatment Plant?

Effluent or wastewater treatment is the process of converting Effluent – water that is no longer needed or is no longer suitable for use – into bilge water that can be discharged back into the environment. It’s formed by a number of activities performed in factories including manufacturing, washing and using the toilet. Effluent is full of contaminants including bacteria, chemicals and other toxins. Its treatment aims at reducing the contaminants to acceptable levels to make the water safe for discharge back into the environment.

Effluent treatment plant

When in a factory, we are converting the raw materials into good finished products, exactly at the same time we are converting the good input water into wastewater. Public welcomes the product but who will welcome to the waste? However, if the wastewater is once again subjected to a conversion of water, then such effort will be again welcome. This responsibility is on the operator of ETP. Water is a universal solvent and many solids get dissolved into it. Purification is nothing but an attempt to remove solids from liquid. The solids are divided in four classes as per their size such as Coarse, suspended, colloidal and dissolved solids. These four types of solids are required to be removed in descending order which is a train of treatment.

The effluent treated in many stages namely preliminary, primary, secondary, tertiary and sludge management. All the treatment unit mentioned are based on physical, chemical or biological principles or a combination thereof. The units based on physical are relatively easy, those employing chemistry are also easy if proper dose is administrated, and the biological are difficult to manage but less costly if once we are accustomed to it. The physical and biological units are subtractive in nature while the chemical unit process of treatment is additive and more energy consuming. When the water becomes wastewater, it means that some solids, liquids or gasses are mixed into it. Thus when we asked to make water out of wastewater, what we have to do is to take away the solids liquids or gaseous pollutants.

Wastewater Treatment Process

The following is a step by step process of how wastewater is treated:

1. Wastewater Collection

Waste water is required to be taken from the factory to the ETP site. This can be done by closed pipe lines or by open gutters. The shorter is the distance or near is the ETP, the better it is. If the travel distance is too high, then the properties or nature of the effluent (call it characteristics) gets changed. The oil-grease gets mixed due to travel turbulence the solids become smaller in size. Due to grinding action or attrition while flowing, thereby both becoming difficult for separation. One more point during this collection transport. The following waste water should not leak and go outside.

2. Screening

The screen fitted in ETP is like a watchman. If a watchman is alert, further mishap can be averted. Likewise if the screen is OK, it will not allow the gunny bags, plastics, branches, rubbers, packing materials, gaskets to pass further and damage pump, agitator, mixture, aerators. Thus screen is an important unit.

bar-screen

3. Grit Chamber

Wastewater brings sand, ash, stone particles, road sweeping, grit, building materials, packing nails, earth, coconut shells pieces etc. This too is required to be removed. The peculiarity of this pollutant is that it is largely inorganic and inert in nature, heavier in weight and easily, quickly settleable. If these are not removed here itself, it creates problem further. By friction it erodes many parts of ETP, which is a slow death of efficiency.

4. Oil-Grease Trap

Oil and grease should be removed from the effluent at least up to a certain state value. Oil-grease removing unit is therefore necessary. Oil is of two types, edible and mineral. Generally in an industrial effluent, we come across the mineral one, which again maybe either used for machines lubrication or as coolant to save machine or tools or the job from heating up. Oil being lighter than water, floats. This property is used to separate it out.

5. Primary Treatment

In primary treatment the effluent is collected in a tank called equalization tank or neutralization tank. Where pH is adjusted by sodium hydroxide and HCL. The effluent then treated with chemicals called Aluminium sulfate and polyelectrolyte. The settled sludge than transferred with help of pump to the sludge drying beds (SDB)

6. Secondary Treatment

In Secondary Treatment the bacteria who are capable of purifying the effluent are classified in three groups. Aerobic, anaerobic and facultative. Bacteria need oxygen for the slow combustion. If this oxygen is lifted from water (having it in dissolved oxygen) those bacteria are aerobic. The anaerobic bacteria pickup oxygen from the bound formed by decomposing carbonates, sulfates, nitrate etc.; and the facultative class can use either of the source of oxygen i.e. work both in presence or absence of oxygen.

6.1 Anaerobic Lagoons

The anaerobic bacteria too needs oxygen but there is no need of fixing aerator. Their need of urea and superphosphate is limited, they get less disturbed by shock load and normally there is no arrangement needed for returning the sludge. The anaerobic bacteria are slow workers, the hydraulic retention time is large (HRT) and the lagoons are very wide and long.

Primary clarifier is used to store the culture temporary. In the duration of few days the culture is recycled in the tank again.

6.2 Activated sludge process

In anaerobic lagoon bacteria work. They are slow workers. In their place if fast working aerobic bacteria are employed, the size of ETP and time of treatment can be attempted to be reduced. As the availability of oxygen is assured, the efficiency is also expected to be higher. Various techniques like trickling filter, oxidation pond, aerated lagoons, activated sludge etc. Where aerobes will work can be designated and operated. Activated sludge is one such process, where lakhs of tiny microorganism stay, grow, live, multiply and work. They eat constantly and increase the population as much as possible!! For the growth they use the dissolved or even un-dissolved organic solids as food. This food is transformed in the growth of microbial community. When these growing bacteria floc together the lump becomes bigger in size and heavier in weight and as a result will show a tendency of setting out from the remaining water. This settled bacteria could now be termed as “activated sludge” because it is active to consume the Biochemical oxygen demand (BOD) as food and looks collectively as sludge.

 Same as the primary clarifier, secondary clarifier is also used to store the activated sludge. Where the clear effluent called supernatant is forwarded and the activated sludge recycled back to the aeration tank to maintain the MLSS.

7. Tertiary treatment

This stage is similar to the one used by drinking water treatment plants which clean raw water for drinking purposes. The tertiary treatment stage has the ability to remove up to 99 percent of the impurities from the wastewater. This produces effluent water that is close to drinking water quality. Unfortunately, this process tends to be a bit expensive as it requires special equipment, well trained and highly skilled equipment operators, chemicals and a steady energy supply. All these are not readily available.

8. Sludge Treatment

Sludge settles in the clarifier. We remove daily. Some portion of it is sent back into the aeration tank as recycled activated sludge (RAS) as per requirement to maintain the MLSS or F:M ratio therein. This leaves the remaining portion of which we have to take care of. This is a wasted activated sludge (WAS). Before this wasted sludge enters into the environment for further disposal, it has to be made amenable for handling. Dewatering is to be made atleast to an extent that a spadable cake is formed. A number of modern gadgets are available for this purpose. For a small or medium size ETP, however the dewatering is done by placing the sludge on sand. Water bound with solids percolated through the layered sand drying bed SDB. The separated water is called as sludge liquor and is high in BOD and sometimes high in the nutrient Nitrogen and Phosphorus. It therefore cannot be sent outside. The sludge liquor should be sent back for treatment in ETP.

In the final, the treated effluent send to the common effluent treatment plant (CETP) or used for irrigation within factory premises. But the pollution parameters as assigned by pollution control board should be within limits.

 Wastewater treatment has a number of benefits. For example, wastewater treatment ensures that the environment is kept clean, there is no water pollution, makes use of the most important natural resource; water, the treated water can be used for cooling machines in factories and industries, prevents the outbreak of waterborne diseases and most importantly, it ensures that there is adequate water for other purposes like irrigation.

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