Activated Sludge System & Sludge Treatment
- Aug 7, 2015
- 2 min read
One of the most common way to treat wastewater is by the used of Activated Sludge System. It is used to treat sewage water and industrial wastewater, with bacteria/sludge.
During the secondary treatment, oxygen is supplied to the aeration tank. Aerobic microbes in flocs oxidize organic matter by catabolic oxidation. Nitrogenous matter and Carbogenous matter (mainly nitrogens and ammoium) is removed.
Aerobic reactions:
-Nitrification: NH3-> NO2- (Nitrosomonas)-> NO3-( Nitrobacter)
-Biological removal of phosphates and metals: Cell of microbes take in (PO4)3- and metals
-Removal of Carbon: C-> CO2
Anaerobic reactions:
-Denitrification: NO3->N2 (paracoccus)
-Removal of Carbon: C-> CH4
Microbes that are suspended in the sludge feed on the pollutants from the wastewater producing supernatant. Effluent can now be discharged into the reservoir or further proceed to teritiary treatment. Some of the sludge that settled is returned back into the aeration tank as activated sludge (recirculated). Remaining sludge is removed from the system.
WHAT HAPPENED TO THE SLUDGE AFTER IT IS DONE DIGESTING THE POLLUTANT?
After every aerobic process where the sludge digest the pollutant, the sludge would
settle down in the settling tank. We would then tap out the sludge that is at the bottom of settling tank, and reuse them for the next batch of water treatment.
However, when sludge goes through aeration process and digest the pollutant from the wastewater, it would multiply. Causing more sludge to be tapped out every single time it finish treating the water. The number of microbes in the activated sludge doubles everytime it is recycled. Hence the extra sludge are called "surplass activated sludge" or excess sludge, which would be treated differently
HOW DO WE TREAT EXCESS SLUDGE?
For excess sludge we will make it go through anaerobic sludge digestion which will dewater the sludge, thickened it, and compost it in landfill.
The reason why we need to treat excess sludge is to
1) Stabilisation of sludge (denote the process of BOD production)
2) Reduce odour from the sludge
3) Reduce water volume (dewatering)
4) Kill pathogens and disinfect them
We have to treat the sludge before we can compost in landfill such that we don't pollute the soil in the ground causing more pollution which would contradicts the whole process of treating wastewater.
Credits:
GIF extracted from : https://www.youtube.com/watch?v=8isr9nSDCK4&spfreload=10
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