Retrospective of ecological approaches to excess sludge reduction

The problem of excess sludge handling produced during wastewater treatment is undeniable reality of grave concern with increasingly stringent legislations. The sludge synthesis yield being 0.4-0.6 kgVSS/kgCOD (0.57-0.8 kgCODcell/kgCOD), results in high power consumption on its digestion and therefore taken considerable attention to achieve sustainable strategies. Solids reduction by physico-chemical methods results in buildup of chemicals. This may present risk to the environment and may require further treatment to remove the chemicals of concern in future. Wastewater sludge reduction upto 100% by biological, sustainable, non-hazardous, and environment friendly methods has been successfully tested at different levels. Therefore, above reasons were sufficient driving forces to confine this review to non-chemically assisted processes. Similarly, the thermally assisted processes result in high carbon footprint and excluded from the scope of this review. Enough has been reviewed on sludge reduction, as numbers of articles on the same subject with different angles have been reported, still the progress in the last few years is missing; hence, special emphasis is given herewith to highlight the efforts of the last five years.