Feasibility of sewage sludge and food waste aerobic co-composting: Physicochemical properties, microbial community structures, and contradiction between microbial metabolic activity and safety risks

Abstract

Co-composting of sludge and food waste eliminates the disadvantages of composting these waste products separately. Specifically, co-composing neutralizes the pollutants and improves the organic matter that occur in sewage sludge, and solves the problem of the low pH values and high moisture content of food waste. However, little is known about the functional microorganisms, microbial metabolic capacity, and biosecurity risks involved in sewage sludge and food waste co-composting. Therefore, this study established four lab-scale composting reactors [T1 (separate composting of food waste), T2 (separate composting of sewage sludge), T3 (sewage sludge and food waste co-composting at a C/N ratio of 25), and T4 (equal proportions composting of sewage sludge and food waste)] to assess the feasibility of sewage sludge and food waste aerobic co-composting. Our findings indicated that polysaccharides and proteins in T3 could be effectively degraded, and the total nutrient levels in T3 were higher than those in the other groups. After composting, the microbial diversity and richness of T3 were higher than that of T1. In later composting stages, the functional microorganisms in T1 maintained higher metabolic activity, however, it also had a higher biosecurity risk than T3 due to the presence of pathogenic bacteria such as Enterococcus_faecalis and Bacillus_circulan. Although the product of T3 could not be used as a microbial fertilizer, its biosecurity risk was lower than that of T1 and could therefore be used as an organic fertilizer. Redundancy analysis (RDA) results indicated that changing the microbial community structure by adjusting key environmental factors could improve composting quality and reduce microbial safety risks. Collectively, our results provide a theoretical basis for the development of co-composting strategies for the biodegradation of perishable solid organic waste, in addition to proposing the risk of pathogenic bacteria exposure that could endanger human and animal health.