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The effectiveness of an advanced treatment of wastewater generated by non-hazardous plastic solid waste (PSW) washing, based on the Sequencing Batch Biofilter Granular Reactor (SBBGR), was assessed in terms of gross parameters, removal efficiencies and sludge production. The proposed treatment was also compared with the conventional treatment, which was based on primary and secondary treatments, using the activated sludge process, performed by Recuperi Pugliesi, a leading company in the plastic recycling industry located in Bari, Italy. The company produces low-density polyethylene (LDPE) regenerated granules from PSW used in agricultural and floricultural greenhouse activities and industrial packaging after a washing stage in the aqueous phase. The latter generates large volumes of wastewater, the conventional treatment of which is characterised by large quantities of sludge and the associated disposal problems. Under steady-state conditions, the SBBGR provided impressive removal efficiencies regarding the main gross parameters (over 90% for COD and TKN, over 99% for BOD5, TSS, VSS and NH3, and over 80% for TN) with a statistically better effluent quality than that of the conventional treatment. The SBBGR effluent quality was modelled in terms of washing water characteristics by using generalized additive models (GAMs). The SBBGR treatment was characterised by a specific sludge production five times lower than that of the conventional treatment (0.21 kg TSS vs. 1.0 kg TSS per m3 of wastewater treated). Compared with the conventional treatment, the proposed process showed a five-fold reduction in the cost of sludge disposal, which saved 50% of the operating cost.

Nowadays, sludge management represents one of the most critical challenges in the field of sewage treatment for economic and environmental impacts. Therefore, the reduction of sludge has become a major issue for the operators of municipal wastewater treatment plants. In the present paper, a new system, whose acronym is MULESL (MUch LEss SLudge), is proposed and tested at full scale for reducing the quantity of sludge in the water line of the sewage treatment plant. MULESL system takes the advantage of maintenance metabolism to significantly reduce the sludge production. The effectiveness of MULESL system in removing the typical pollutants and reducing sludge production was evaluated at full scale by using 3500 PE unit located in Putignano's WWTP (Puglia, Italy). This unit was obtained by retrofitting an existing activated sludge basin. The results obtained over 1-year period, during which MULESL unit treated the effluent of the preliminary treatment step, have indicated that it was characterized by a specific sludge production as low as 0.13 kg of dry sludge per kg of COD removed; 77% lower than that recorded for primary and secondary treatments of the conventional plant during the same period. This sludge reduction was obtained with a plant volume 27% smaller than that of the conventional water line. Furthermore, the organic matter of the sludge was already stabilized, thus allowing to save investment costs for digestion process facilities. Finally, MULESL unit guaranteed a mean removal efficiency higher than 95% for COD, BOD5, TSS, TKN, NH3 and TN.