An innovative membrane bioreactor process with iron dosing and side-stream acidogenic sludge co-fermentation for enhanced phosphorus removal and recovery in wastewater treatment

Abstract

Phosphorus (P) is one of main nutrients in wastewater that cause eutrophication and related water pollution problems. On the other hand, P is also an irreplaceable and non-renewable resource that supports global food security. In this research, an integrated wastewater treatment system with iron-dosing membrane bioreactor (MBR), acidogenic sludge co-fermentation, and side-stream denitrification units was developed for enhanced nutrient (phosphorus and nitrogen) removal and resource (phosphorus and volatile fatty acids (VFAs)) recovery. Phosphorus was removed by chemical precipitation and retained as Fe-P precipitates in the sludge of the aerobic MBR. By co-fermentation of the sludge with food wastes, VFAs was produced and phosphorus was extracted from the sludge as a result of acidogenesis and dissimilatory iron reduction. Phosphate in the supernatant was then recovered as vivianite by pH adjustment and re-precipitation. A side-stream denitrification process was applied using the VFAs in the sludge liquor for nitrogen removal. The influencing factors, including the iron dosage, food waste loading, fermentation time, sludge retention time, and sludge recirculation ratio were modeled and optimized for enhanced pollutant removal and resource recovery. The experimental results show that by adding FeCl3 at 20 mg Fe/L into the influent of domestic wastewater, about 95.6% of total P could be removed by the MBR. An overall P recovery efficiency of 62.1% from the wastewater influent was achieved, and the problem of inorganic build-up in the MBR was effectively alleviated. With a sufficient supply of soluble organics, mainly VFAs, in the supernatant of the fermented sludge, an overall 91.8% of TN in wastewater was removed by biological denitrification. The transition of iron and phosphorus species and related mechanisms and pathways of phosphorus removal and recovery through the iron-dosing MBR and acidogenic fermentation were revealed by X-ray absorption spectroscopy (XAS) analysis. The pH decrease and microbial iron reduction were identified as the crucial conditions for P extraction from sludge. The microbial community structure was also analyzed using the high-throughput sequencing technique on the sludge from the aerobic MBR, acidogenic fermenter and anoxic reactor, and thee functional microbial groups were identified. The research and development provided an environmentally-friendly process for advanced wastewater treatment together with effective resource recovery from the wastewater and food waste with no secondary pollution.