Co-digestion of microalgae and primary sludge: Effect on biogas production and microcontaminants removal

Microalgal-based wastewater treatment plants are conceived as low cost and low energy consuming systems. The operation of these plants involves the management of primary sludge and microalgal biomass. The aim of this study is to analyse the anaerobic co-digestion of both by-products in terms of biogas production and contaminants of emerging concern removal. The co-digestion of microalgae and primary sludge (25/75% on a volatile solids basis) was investigated in continuous reactors and compared to microalgae mono-digestion at a hydraulic retention time of 20days. Results showed how the co-digestion enhanced the anaerobic digestion of microalgal biomass, since primary sludge is a more readily biodegradable substrate, which increased the methane production by 65% and reduced the risk of ammonia toxicity. Regarding the contaminants, musk fragrances (galaxolide and tonalide) and triclosan showed the highest abundance on primary sludge (0.5-25μg/g TS), whereas caffeine, methyl dihydrojasmonate and triphenyl phosphate were barely detected in both substrates (<0.1μg/g TS). The removal of these contaminants was compound-depending and ranged from no removal to up to 90%. On the whole, microalgae mono-digestion resulted in a higher removal of selected contaminants than the co-digestion with primary sludge.