Growth of microalgal biomass on supernatant from biosolids dewatering.

The paper reports the results of a preliminary work focused on the feasibility of including a photobioreactor within the sketch a wastewater treatment plant, growing microalgae on the centrate from anaerobic sludge dewatering. The growth of algal biomass would allow nitrogen removal and provide substrate for biogas production by anaerobic digestion. Tests have been carried out in continuous by feeding a photobioreactor with a centrate/effluent blend and by increasing the fraction of centrate. The experimental results show that the centrate does not induce any toxicity and, on the contrary, can be well utilized by algae, whose average specific growth rate (µ), on centrate as such, was 0.080 d-1. The maximum biomass production was 0.68 g/L d-1 at 10 days HRT. Methane production tests lead to BMP values, referred to normal conditions of temperature and pressure conditions, of 335±39 mL CH4/g VS, and 284±68 mL CH4/g VS for the two tested samples, in agreement with literature values. Settling tests show that the settling capacity of microalgae, yet satisfactory, could be effectively improved after mixing with activated sludge. These results confirm the potential to use the existing primary settler for microalgae thickening in order to feed them to anaerobic digestion with primary/secondary sludge.