Municipal Sludge And What Can Be Done With It
Municipal sludge is a product of wastewater treatment.
The sewage system gets both wastewater and stormwater which flow into water treatment facilities where solid wastes are separated through settling. By this point, the solid wastes or municipal sludge are decomposed by bacteria.
Two major factors in the design of water treatment plants are disposal and treatment of the municipal sludge.
The volume of the sludge must be reduced and the offensive odor is required to be removed before it is disposed.
Treatment Methods
A combination of digestion, thickening and dewatering constitute the treatment of municipal sludge.
Thickening
This is the first step in the treatment of municipal sludge as it is not feasible to handle watery sludge. Thickening is done in a gravity thickener tank.
Through this process the total volume of the sludge is reduced to less than half its original volume.
Another option is dissolved floatation in the air, where the solids are carried to the surface by air bubbles. Eventually, the sludge becomes thick.
Digestion
Decomposition is the second step in the treatment process. Decomposition or digestion dries the sludge by reducing the total volume of the solids, destroys the pathogens and makes dewatering easy. Such digested municipal sludge is odorless and looks like soil in appearance.
Sludge digestion can also be achieved aerobically in the presence of oxygen. For around 20 days, municipal sludge is aerated in an open tank. Though it is easier to operate aerobic systems, the cost factor is high due to the power needed.
Dewatering
The sludge digested in the previous method is dewatered before final disposal, but still contains about 70% of water.
Even with so much moisture, no municipal sludge behaves like a liquid and cannot be treated as a solid waste.
The right technique for enough dewatering is by sludge drying beds where the digested municipal sludge is laid open on a bed of sand and remains until it becomes dry.
A combination of drainage through sand via gravity and evaporation completes the drying process.
Disposal
The last step in the municipal sludge is land. It can be buried under a landfill or spread out on agricultural areas to be used as a fertilizer or soil conditioner.
As the sludge might carry harmful industrial chemicals it is not laid on land where crops are cultivated.
Finally, if no suitable site is available, the sludge can be incinerated, which is the process of completely evaporating the moisture content and converting it into ash.
