Challenges in biological treatment plants: the approach of zero sludge waste

The current trend in operation of biological Wastewater Treatment Plants (WWTPs) is the minimization of sludge wastage due to its labor and cost intensive management process. Minimization of sludge can be achieved by increasing the solids retention time (SRT) in a WWTP, up to an almost complete retention of solids. This operational modification can induce problems that compromise effluent quality, such as: (a) excessive accumulation of sludge, (b) Dissolved Oxygen availability, (c) changes of C:N:P nutrient ratio, (d) changes of microbiological and morphological characteristics of biomass and (e) insufficient treatment. By imposing specific WWTP design and operational conditions, the SRT related problems can be resolved. The current study presents results based on monitoring five full scale industrial and municipal WWTPs, operating towards “complete solids retention”. The results showed that under high SRT and after the modification in the design and operation of a WWTP, successful microbial manipulation can be achieved. This leads to (i) excess sludge minimization, up to 95%, (ii) good sludge settling characteristics, with SVI < 120 ml/g, (iii) sufficient wastewater treatment, with removal efficiencies, up to 99%, 98% and 99% for COD, TN and TP respectively.