Microbial population dynamics in biowaste mesophilic anaerobic digestion: the impact of sewage sludge addition

Complex organic materials such as biowaste range from highly recalcitrant material to extremely biodegradable compounds, so the best option to improve the AD performances is a case-specific and properly designed strategy. In the case of food waste (FW), mainly composed by prompt biodegradable carbohydrates, the high load long-term digestion leads to inhibition phenomena due to intermediate accumulation suggesting the need of co-digestion with other substrates to stabilize the entire process. The addition of waste activated sludge (WAS), typically recalcitrant due to the floc structure, may be a feasible way to increase the organic loading rate utilizing the free digestion capacity at existing sewage treatment facilities. Currently, general knowledge on AD microbial community compositions is well established, while only a few studies have investigated correlations between microbial community composition, feedstock and process parameters. Hence, main scope of this research was to study changes in the microbial population community as a response to variation of feedstock and organic load in long- term semi-continuous systems. Compared to the feeding with FW alone, the co-digestion with WAS permitted the establishment of a stable microbial population by reducing the unbalance between fermentative bacteria and methanogens. The availability of a slow degrading substrate, in fact, allowed to limit VFA accumulation and pH drop favoring the activity of the archaeal population, as confirmed by the improved process stability and methane yield. Remarkably, the beneficial effect of co-digestion increased with the increase of OLR.