What is sludge?

Hint:It is separated from domestic sewage waste. It is a semisolid residual produced by primary treatment.

Complete answer:
Sludge is a residual, semi-solid slurry that is produced by domestic sewage, industrial or wastewater treatment. Sludge is also obtained by sanitation systems like sanitation of septic tanks. The slurry that settles down during primary treatment, wastewater treatment is called sludge. This sludge is removed from the primary tank by using a scraper. After removing, it is transferred to another tank which contains anaerobic bacteria. These anaerobic bacteria decompose the sludge to produce biogas. This biogas has various uses. The biogas is used in cooking, heating, etc. Biogas is also used for the production of electricity and low-cost fuel.
Dried sludge is used as organic manure (the substance that provides nourishment to the soil and makes it productive and fertile). Dried sludge has a lot of nutrient-rich organic materials that can boost plant growth.

Note:Dried sludge should be used as manure rather than disposing it into landfills or other disposing facilities. To achieve this there should be the separation of dry and wet waste. As wet waste will take part in the formation of sludge.