Sludges are treated using a variety of digestion techniques, purpose of which is to reduce amount of organic matter and number of disease-causing micro-organisms present in solids. The most common treatment options include anaerobic digestion, aerobic digestion and composting.

The unthickened primary sludge combined with trickling filter humus is 2-6 %. The thickened sludge is around 5-9%. In this case, trickling filter humus is recycled back to primary clarifier which gets combined with primary sludge formed.

The volumetric organic loading in Kg COD/ m³ day for an anaerobic contact digestion process is 1-8. This is loading rate obtained at 30 degree Celsius. This value holds good for suspended growth anaerobic digestion.

Anaerobic digestion is a bacterial process that is carried out in absence of oxygen. Sludge is fermented in tanks at a temperature of 55 °C in rmophilic digestion and in mesophilic digestion it is fermented at a temperature of around 36 °C.

The pressure exerted by rising gas can be used to transport gas to collection vessel or directly to where it is going to be sued. The digestate is rich in organics and nutrients, almost odourless and pathogens are partly inactivated.

Water removal is primary means of weight and volume reduction, while pathogen destruction is frequently accomplished through heating during rmophilic digestion, composting or incineration. The choice of a sludge treatment method depends on volume of sludge generated and comparison of treatment costs required for available disposal options.

Centrifugation may be a preliminary step to reduce sludge volume for subsequent filtration or evaporation. In a sand drying bed or as a separate mechanical process filtration may occur through underdrain in a belt filter press.

To remove 1% primary sludge usually a 150 sized mesh of cyclone degritter is used. In se cyclone degritters sludge is fed tangentially. This is most effective way to degrit sludge.

Around 30-50% of gas is produced. This carbon dioxide produced reduces pH. Hence alkaline is required to be added.

The unthickened primary sludge combined with rotating biological contractor is 2-6 %. The thickened sludge is around 5-8%. In this case, sludge from rotating biological contractor is recycled back to primary clarifier which gets combined with primary sludge formed.