Characterization of microalgae cultivated in continuous operation combined with anaerobic co-digestion of sewage sludge and microalgae

The present study reveals characteristics of microalgae cultivated in continuous laboratory-scale experiments combined with anaerobic co-digestion of sewage sludge and microalgae for as long as 10 months. Filtrate from dewatering of anaerobically digested sludge was used for continuous cultivation of microalgae. The effect of hydraulic retention time (HRT) on continuous microalgal cultivation was investigated, and HRT 7.8 d was selected for the continuous experiment. 16S/18S rRNA gene analysis confirmed that indigenous wild type Chlorella was stably cultivated without microalgal species control, and no cyanobacteria were cultivated. Average volatile solids (VS) per total solids and chemical oxygen demand per VS mass ratios of the cultivated microalgal mixture were 0.918 g g−1 and 1.39 g g−1, respectively. A measured higher heating value of the cultivated Chlorella mixture was around 22 MJ kg−1. Fatty acid methyl ester analysis showed that the obtained microalgal mixture cultivated without controlling species is useful for anaerobic digestion rather than biodiesel production. A biochemical methane potential assay for 28 d showed that methane conversion from the cultivated Chlorella mixture was approximately 0.26 ± 0.02 m3 kg−1 of VS-fed at standard conditions of 101.3 kPa and 273.15 K. This indicates that approximately 46% of the energy was recovered by anaerobic digestion as methane. No specific elemental accumulation was observed during the continuous operation. Combined anaerobic co-digestion of sewage sludge and microalgae, and microalgal cultivation is a promising energy recovery approach.