Impact of the co-treatment of food waste and municipal wastewater on the performance of an aerobic granular sludge system

Abstract

Due to the improvement in urbanization and industrialization, the assimilation capacity of the environment is decreasing. Municipal wastewater includes a significant amount of organics and nutrients that can be hazardous to the ecosystem. Hence, the treatment of wastewater is vital for public health. Biological treatment is the most frequent method in wastewater treatment to remove pollutants discharged into the aquatic environment. Biological nutrient removal (BNR) systems are widely used to remove nutrients from wastewater. Among the BNR processes, the anaerobic/anoxic/aerobic (A2O) process has a wide range of uses in wastewater treatment to remove nitrogen and phosphorus. However, due to the high footprint and energy requirement, the need for novel technologies become important. The main objective of wastewater treatment plants (WWTPs) is to maintain sustainability by reaching a neutral/positive energy balance without compromising discharge standards. In this regard, aerobic granular sludge (AGS) technology provides an efficient treatment performance with low energy consumption and less footprint requirement compared to the A2O process. AGS process is designed to produce granules that contain both autotrophic and heterotrophic microorganisms to simultaneously remove chemical oxygen demand (COD), nitrogen, and phosphorus in a single granule. The purpose of this study was to investigate the impact of co-treatment of municipal wastewater and food waste (FW) on the treatment efficiency, settling behavior of the granules, and granule morphology. Additionally, the energy recovery potential of excess sludges from the AGS process, both fed solely with municipal wastewater and fed with a mixture of municipal wastewater and FW, was investigated. Two different stages were applied in this study. At Stage 1, the reactor was fed with sole municipal wastewater taken from a full-scale municipal WWTP having a daily capacity of 600000 m3/day, whereas a mixture of municipal wastewater and FW was fed to the reactor at Stage 2. In this study, the reactor was operated with a cycle time of 4 h, consisting of 3 min of feeding, 30 min of settling, 120 min of aeration, 65 min of anaerobic phase, 2 min of idle phase, and 20 min of decanting. At each stage, more than 80% of COD removal efficiencies were achieved. Higher total nitrogen (TN) and total phosphorus (TP) removal efficiencies were achieved at Stage 2 in comparison to Stage 1. An environmental scanning electron microscopy (ESEM) combined with an energy dispersive X-ray (EDX) spectroscopy was applied to examine the morphology of granules. It is observed that aerobic granules have a more uniform and compact structure at Stage 2 compared to Stage 1. According to the EDX results, aerobic granules at each stage contain a higher percentage of carbon and nitrogen, indicating a higher organic content of granular sludge. Similar peaks were observed in fourier transform infrared spectroscopy (FTIR), which explained the similar functional groups on the surface of aerobic granules. Additionally, excess sludge from AGS process fed with the mixture of municipal wastewater and FW demonstrated a slightly higher methane (CH4) yield than excess sludge from the AGS process fed with solely municipal wastewater. The findings obtained from this study highlighted the importance of integration of FW as a co-substrate in the AGS process in enhancing the energy recovery and quality of effluent.