Wastewater Treatment Process

The objective of municipal wastewater treatment is to treat water collected from households and industries such that the wastewater meets the quality standards specified by the government before discharge back into the rivers or further treated for reclamation. The typical steps of a wastewater treatment process are collection, primary sedimentation, biological treatment, secondary sedimentation, disinfection and discharge.

(1) Collection
The wastewater of a city is channelled from the various sewage water collection points through a pipeline network system to the treatment plant. In the treatment plant, the wastewater first passes through bar screens whereby large objects that could damage or interfere with the process and equipment are removed. The wastewater then enters grit removal tanks, whereby heavy inorganics such as sand, gravel and rocks, collectively known as grit is allowed to settle to the bottom of the tank and removed.

(2) Primary sedimentation



The wastewater is allowed to pass slowly through large tanks known as primary clarifiers, where suspended solids and other non-soluble particles can settle and floating material such as grease
and oils can rise to the surface and be skimmed off. The main purpose of this stage is to produce a generally homogeneous liquid capable of being treated biologically. Primary clarifiers are usually equipped with mechanically driven scrapers that continually drive the collected sludge towards a hopper in the base of the tank from where it can be pumped for further sludge treatment.
(3) Biological treatment
The wastewater flows into large aeration basins, where it is mixed with air to aid the growth of
micro-organisms that naturally live in wastewater. The micro-organisms, called “activated sludge”clean the wastewater by digesting organic materials and other soluble contaminants. Wastewater remains in the aeration basins for a few hours to allow time for the micro-organisms to grow and consume the bacteria.
(4) Secondary sedimentation
The wastewater is then pumped into secondary clarifiers, where the “activated sludge” is allowed to settle out of the water.
(5) Disinfection
The purpose of disinfection in the treatment of wastewater is to substantially reduce the number of micro-organisms in the water and to kill any remaining pathogenic (disease producing organisms before the water is discharged back into the environment. Common methods of disinfection include adding chlorine or using ultraviolet light.
(6) Discharge
After disinfection, the treated water should be able to satisfy the national standard of sewage
discharge stipulated by the PRC government and can be discharged to the river directly or undergo more advance treatment process for reclamation purposes.