Application of physico-chemical pretreatment methods to enhance the sludge disintegration and subsequent anaerobic digestion: an up to date review

Anaerobic digestion of waste activated sludge (WAS) is most commonly used at wastewater treatment facilities to stabilize the sludge. However, poor biodegradation efficiency of sludge and longer retention times (20–30 days) are the major limitations of anaerobic digestion method, which can be overcome by disintegrate the sludge and make intracellular material readily available to anaerobic biodegradation. Several sludge disintegration techniques (Thermal, chemical and mechanical) and their combinations were studied previously in order to improve the anaerobic digestibility of sludge, reduce the sludge retention time and enhance the production of biogas. However, none of the pretreatment technique shows a real breakthrough in enhancement of anaerobic digestibility of WAS due to associated drawbacks, such as high operational and capital cost, poor sanitization and energy intensive. On the other hand, hybrid pretreatment techniques endow with promising outcomes through neutralizing the associated drawbacks with individual pretreatment techniques. Nevertheless, the research on the application of hybrid pretreatment techniques is in its infant stage, presently. Therefore, the satisfactory research in this direction can lead to policy maker and environmental protection agency to select more robust and sustainable solution for sludge treatment. This review encompasses the up-to-date information about the thermal, chemical and mechanical sludge disintegration techniques (principle, performance at lab, pilot and full scale, advantages and disadvantages). A comparative evaluation of these techniques are also presented in terms of COD solubilization, dewaterability, pathogens removal, lab/pilot/full scale application, capital and, operation and maintenance cost involved and energy requirement.