Diversion of food waste into sewer system : characterization, transformation and implications

Abstract

Food waste (FW) management is a worldwide environmental issue. Diversion of FW by using Food Waste Disposers (FWDs) into the sewer system is considered potentially viable for relieving the burden of municipal solid waste (MSW) management. However, the feasibility of such practice is still under debate due to significant concerns over the impacts of adding FW into the sewer system and the downstream wastewater treatment plants (WWTPs). In this context, the present study has systematically investigated local FW characterization, FW transformation during wastewater treatment, and potential implications of the FW addition on sewer processes and WWTP operation. Main contents and results are as follows: Firstly, FW characterization is a key issue and needed to be conducted in advance to lay a foundation for this study. A one-year physicochemical analysis of representative samples suggested that one gram wet weight of FW contains 160 mg solids, 230 mg chemical oxygen demand (COD), 3 mg nitrogen and 1.2 mg phosphorus on average. Moreover, Hong Kong household FW typically consists of 50% fruit, 20% vegetables, 20% starchy food and 10% meat. Theoretical estimations indicate the addition of FW into wastewater would certainly increase the pollutant loading on the WWTPs, but also has energy recovery potential. Secondly, the impacts of the FW addition on the sewer biofilm were evaluated via pilot-scale gravity sewers. In a gravity sewer system, the sewer biofilm properties change with long-term FW addition, resulting in a greater thickness (by 32%), an increased dry density (by 13%), and more extracellular polymeric substance (by 141%). The thicker and denser biofilm limits oxygen diffusion, enlarges the anaerobic area in the sewer biofilm, promotes an increase in the sulfate-reducing bacteria (SRB) population, and enhances the sulfide production potential. Moreover, the impacts of the FW addition on the sewer processes were examined by a lab-scale sewer reactor system under low and high sulfate conditions (40 and 160 mg S/L). The long-term monitoring of the system revealed that the FW addition has little impact on sulfide production, but methane production was enhanced by up to 62% under a low sulfate condition, whereas improvements of sulfide and methane production were around 39% and 44% by dosing FW under a high sulfate condition. The addition of FW significantly promoted the accumulation of methanogenic archaea (MA) and altered the spatial distributions of SRB and MA in the sewer biofilms. Further, a plant-wide COD-based transformation model was established by using local data to evaluate the conversion of FW, the energy balance and the overall operational cost in WWTPs, in sludge treatment and in MSW management. The WWTPs can remove about 78% of solids and 58% of COD in FW. The FW addition however has limited impact on the treatment capacity, effluent quality and sludge production in WWTPs in the short term. The increases in energy consumption and operational cost are dependent on the treatment processes and the FWDs penetration rates, while the MSW treatment can benefit from the use of FWDs. This research has shed light on the overall impacts of diverting FW into the sewer system, and the results provides important information for future research and policy decision-making in FW management.

Date of Award	2020
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology