Question

What is sludge? Explain how it is treated.

Answer 1

Hint: Sludge can be defined as a residual semi-solid material left from industrial, water treatment or waste-water treatment processes. It’s a sediment of accumulated minerals in a steam boiler. They are also defined as solids separated from suspension in a liquid. It's brown or green.

Complete answer:
After a wastewater treatment, we can get two end products, the first product is effluent which is the wastewater. And the second product is sludge. Sludge is the settled suspended particles in a primary tank which gets settled at the bottom. Sludge has a high amount of organic matter and moisture content. As it is highly organic, it has to be treated well before disposal.
Whenever we consider the treatment of sludge, it goes through different stages of treatment starting from primary treatment to secondary treatment and then moving to the final or tertiary treatment. The sludge is of three types based on the sources of sludge:- Raw Sludge, Secondary Sludge and Activated Sludge. The sludge taken from the primary tank is raw, when taken from a secondary sludge it's known as secondary Sludge and the sludge received from the activated sludge treatment is known as activated sludge. The raw sludge contains more Organic matter than the other two.
The sludge treatment occurs in four steps:-
I. Thickening- In this step, the sewage sludge is thickened in a gravity thickener to reduce its overall volume, thus enabling the easy handling of the sludge. Dissolved air flotation is also used to thicken the sludge.
II. Digestion:- This is a biological process in which the organic solid present within the sludge gets decomposed into stable substances. This sludge digestion process is a two-step process. In the initial stage, the dry solid sludge is heated and mixed in a closed tank to enable anaerobic digestion by acid-forming bacteria. The large molecules are then converted into smaller molecules by the help of different proteins and bacteria. The mixture is then transferred to a second tank where they are converted into a mixture of methane and carbon dioxide.
III. Dewatering:- Dewatered sludge generally contains a major amount of water, as much as seventy percent, despite its solidified state. Therefore, it is necessary to dry and dewater the sludge. Centrifugation and use of rotary drum vacuum filter and belt filter are slowly becoming one of the most preferred methods to dewater sludge.
IV. Disposal:- Once the sludge has been effectively dewatered, it can be buried underground in a sanitary landfill or can be used as a fertilizer, depending on its chemical composition.

Note:
 Sludge has components such as radioactive material, lead, mercury, arsenic and also microbes that can cause hepatitis and food poisoning. Although it also has a useful source of phosphorus, nitrogen, and also an organic matter which can be used as a liming material.