Coupled high-rate anaerobic treatment and microalgae growth for digestate valorization

Overall sustainability of anaerobic digestion processes is based on the good management practices developed for digestates. The proposed digestate management or treatment practices offered so far such as membrane filtration, vacuum evaporation, struvite precipitation, ammonia stripping have been costly or low-yielded[1] which limited their application. Therefore, new cost-efficient management practices for digestates should be developed within the scope of reuse and recovery approaches. Digestates accommodate residual biogas which can be extracted by further anaerobic decomposition. However, studies on residual biogas production from digestates demonstrated that decomposition of residual organics required long hydraulic retention times (HRTs) in the range of 28 [2] - 136 [3] days. The use of a high-rate anaerobic digester has a potential to reduce HRT which in turn reduces the footprint of the digesters[4]. The effluent of the high-rate anaerobic reactor treating digestate can also be used as a feedstock for microalgae. This effluent can still be expected to have high nutrient concentrations and a pH level convenient for the growth of freshwater and alkaliphilic microalgal species. This study investigated the residual biogas potential of liquid digestate obtained from a full-scale anaerobic digester operated with 90% laying hen and 10% cattle manure in an anaerobic fixed-film reactor (AFFR) and the subsequent microalgal processing of its effluent (Figure 1). The results indicated 0.277-0.494 Lbiogas/g VSadded could be obtained from the liquid digestate in AFFR. Microalgal biomass concentration reached to 15.48 ± 0.83 mg/L and could be concentrated to 327 mg/L by simple gravity settling.