Waste water treatment in Pakistan stages in Detail

waste water treatment in Pakistan

Sewage treatment plants, also known as waste water treatment in Pakistan plants, are technical systems that are used to purify industrial and household wastewater. Depending on the quality of the wastewater, the construction, and the efficiency of the wastewater treatment plant, wastewater treatment takes place in three stages.

1. The mechanical cleaning stage
2. The biological cleaning stage and
3. The chemical cleaning stage

A further stage may be required for special cleaning requirements.

4th purification stage (filtration)

Ponndorf peristaltic pumps in mechanical-biological wastewater treatment plants

1st cleaning stage (mechanical wastewater treatment)

(A) Rake
The wastewater first passes through a rake that holds back coarse debris (paper, bottles, branches, etc.) and removes it with an automatic scraper. The rubbish is then pressed and taken away by the garbage disposal.

(B) Sand trap
In the sand trap, the drainage channel widens, as a result of which the speed of the sewage flowing on decreases, and coarse mineral sediments such as gravel and sand are deposited on the ground and can be removed.

(C) Primary clarifier
In the primary clarifier, the water is retained for about two hours. In these large rectangular or round basins, the fine suspended matter can settle as sludge on the bottom. This raw sludge is sucked off, thickened and conveyed into a digestion tower.

In the mechanical wastewater treatment stage, around 30% of the pollutants are removed from the wastewater through rakes, sand traps, and primary clarifiers.

2nd purification stage (biological waste water treatment in Pakistan )

(D) Aeration basins (nitrification and denitrification basins)
The pre-treated wastewater from the primary treatment is introduced into the so-called second treatment stage, also known as biological treatment. In this combined nitrification and densification basin, the remaining pollutants are mixed with activated sludge. Compressed air is also blown in during the nitrification phase. The microorganisms and bacteria present in the activated sludge now convert ammonium into nitrate. During the denitrification phase, the wastewater is “only stirred”. No air is blown in. The bacteria and protozoa are alternately brought into “shortness of breath” between the nitrification (ventilated) and denitrification phase (not ventilated) and thus put into “stress”.

As a result of this “stress phase”, the microorganisms store, among other things, more phosphate as a “reserve material”. Thus, phosphate is removed in a biological way. The bacteria now “get” the oxygen they need to live from the nitrate as well. This is how the nitrate is removed from the wastewater. The nitrogen escapes into the atmosphere as a gas. After a certain time, the sludge-water mixture is fed into the secondary clarifier.

(E) Secondary clarification tank
The sludge flakes sink to the bottom in the subsequent secondary clarification tank and are either pumped back into the activated sludge tank (D) again (return sludge) or conveyed for sludge removal (excess sludge).

After passing through the mechanical and biological cleaning stages, the wastewater is now about 90% clean of biodegradable substances.

3rd cleaning stage (chemical wastewater treatment)

Since the wastewater treatment in Pakistan contains the plant nutrient (phosphate), which leads to secondary pollution in the waters, a further wastewater treatment stage is to be aimed for. A chemical solution is added to the wastewater from a precipitant dosing station with vigorous mixing.

The phosphate-containing wastewater “flocculates” and can settle in a secondary clarifier as sludge, which – thickened – is fed to the digestion tower. The chemical cleaning process described here is called “precipitation”. Today wastewater technology is increasingly switching to biological processes to reduce the nutrients in wastewater. This can severely limit the addition of precipitation chemicals.

After passing through the third purification stage, around 95% of the wastewater is now cleaned of degradable substances.

4th purification stage (filtration)
Since the finest suspended matter and bacterial flakes can still remain in the treated wastewater, it is still passed through filter basins in some sewage treatment plants. These are mostly filled with pumice or sand. With the filtration, the wastewater treatment has come to an end.

(F) Discharge point
The purified water can now be discharged into natural water bodies.