SBBGR technology for reducing waste sludge production during plastic recycling process: Assessment of potential increase in sludge hazardousness

Sludge production in the wastewater treatment sector is consistently increasing and represents a critical environmental and economic issue. This study evaluated an unconventional approach for treating wastewater generated from the cleaning of non-hazardous plastic solid waste during the plastic recycling process. The proposed scheme was based on sequencing batch biofilter granular reactor (SBBGR) technology, which was compared with the activated sludgebased treatment currently in operation. These treatment technologies were compared regarding sludge quality, specific sludge production, and effluent quality to highlight whether the reduced sludge production shown by SBBGR corresponded to an increase in the concentration of hazardous compounds in the sludge. The SBBGR technology showed remarkable removal efficiencies (TSS, VSS, and NH3 > 99 %; COD >90 %; TN and TP > 80 %) and a sludge production six-fold lower than the conventional plant (in terms of kgTSS/kg CODremoved). Biomass from the SBBGR did not show a significant accumulation of organic micropollutants (i.e., long-chain hydrocarbons, chlorinated pesticides and chlorobenzenes, PCB, PCDD/F, PAH, chlorinated and brominated aliphatic compounds, and aromatic solvents), whereas a certain accumulation of heavy metals was observed. Furthermore, an initial attempt to compare the operating costs of the two treatment approaches revealed that the SBBGR technology would provide 38 % savings.