Sludge stabilisation and conditioning

Alkaline stabilisation is a chemical method designed to reduce sludge odour and putrescence and the level of pathogenic organisms by raising the pH of the sludge using lime. The process may be supplemented by dosing with waste solid materials to reduce the amount of lime used.

A disadvantage of employing these processes is that they increase the total amount of sludge solids, and also incur an operating cost associated with the supply of the chemical reagent.

Sludge thickening and dewatering processes normally require pre-treatment to assist the separation of the water from the solids.

Pre-treatment is referred to as 'conditioning', which can take two forms − chemical or thermal. Chemical conditioning comprises dosing with inorganic coagulants (such as iron or aluminium) and/or organic flocculants (or polymers). In thermal conditioning, the sludge is heated under controlled conditions.

Chemical conditioning refers to the chemical pre-treatment of sludge in advance of thickening and dewatering processes. Conditioning coalesces the smaller sludge particles to produce larger ones and reduces the sludge colloidal content. This increases both particle settling and the product cake permeability, and so increases the efficiency of the downstream thickening and dewatering processes.

Aerobic digestion is the degradation of the organic sludge solids in the presence of oxygen. The oxygen is introduced as fine bubbles of air into the reactor.

The micro-organisms in the sludge convert the organic material to carbon dioxide and water, and the ammonia and amino species to nitrate.