Integrated sewage sludge treatment for the sustainable recovery of fine-chemicals: Technical and economic analysis

Abstract

A sustainable procedure for effectively isolating grease from urban thickened sewage sludge was developed and examined. Thickened sludge underwent centrifugation, was treated with HCl and H2O2, heated to 353 K and then centrifuged again. Besides a pure lipid current with a FFA content of 90–92 % (with a grease yield of up to 73.3 %), an aqueous acidic current and a wet residual solid cake (TS: 31.7 ± 3.4 %) were also generated. The technical integration of this procedure in conventional sludge-treatment lines were then investigated under pilot scale. The cogenerated streams were found to be digestible under anaerobic conditions, leading to a production of biomethane higher than that foreseeable based on the biomethanogenic potential of the starting sludge. This improvement of biomethane production demonstrated that the reactive step of the grease recovery process has a favorable effect on the digestibility of the residual sludge. Concomitantly, a reduction in the volumes of final sludge to be disposed of by 20 % was also determined. Alternatively, a different scenario of integration of the grease recovery was studied for WWTP not equipped with anaerobic digesters. The residual solid cake obtained from the lipid recovery process can be efficiently dried through a low-temperature desiccator. The desiccated cake (TS: 86.0 ± 6.0 %) had a high calorific value (17 MJ/Kg) capable of producing the electric and thermal energy needed for running the lipid recovery process. For both cases, the proposed integrated sewage sludge treatments could produce economic benefits capable of making the costs of investments recoverable within three years. These results pose the proposed grease recovery as a sustainable approach for the treatment of municipal sewage sludge, while facilitating the recovery of valuable resources.