Effects of nutrient load on microalgal productivity and community composition grown in anaerobically digested food-waste centrate

Abstract

Anaerobic digestion of food waste has many environmental benefits over traditional landfilling and is a promising technology to convert food waste to energy. However, the centrate, a liquid by-product of anaerobic digestion, is high in total ammonia, with concentrations ~20,000 g m−3, which requires treatment being discharged to the environment. Microalgae offer a promising and cost-effective treatment solution for this centrate but some dilution is required to prevent free-ammonia inhibition. In this study, we investigate the performance of microalgae, grown outdoors in high rate algal mesocosms, under three different total ammonia loads i) 20 g m−3 (20N), ii) 60 g m−3 (60 N) and iii) 100 g m−3 (100N). Both total suspended solids (TSS) and volatile suspended solids (VSS) increased with increasing nutrient load, with 100 N significantly higher (p < .01) than 60 N and 20 N biomass. The percentage nitrogen uptake was significantly higher in 20 N compared to 60 N (p < .05) and 100 N (p < .01). In contrast, the percentage biological uptake of phosphorus (P) did not differ significantly between treatments. Total microalgal biovolume increased with increasing nutrient load with nine species of chlorophytes (green algae) observed across all treatments throughout the experiment. Bray–Curtis percentage similarities between the microalgal community relative abundance, showed that the community in the 100 N treatment was at least 50% dissimilar to 20 N and 60 N, which were at least 75% similar to each other throughout the course of the experiment. These results indicate the capability of microalgae to bioremediate centrate from anaerobically digested food waste with high ammonia loading. Coupled centrate treatment and resource recovery could help support the circular bioeconomy.