After the sedimentation and filtration process, the solid wastes get suspended, and then it is collected at the bottom of the tank which is known as primary sludge while the floating particles that is supernatant is known as effluent. The primary sludge which is formed during the sewage treatment have the capacity to trap many microbes and scattered pieces of rubbish which is further removed by the process of anaerobic digestion.

Some bacteria like Bacillus polymyxa, Pseudomonas striata, Aspergillus sp. has the power to release phosphate from bound or immobile form. These biofertilizers are called as microphos. These bacteria easily dissolve phosphate due to the secretion of some organic acid like lactic, formic, acetic, and fumarate. It also increases the efficiency of fertilizer use, and limits leaching losses and hence improve the crop yield.