Enhancing algal growth and nutrient removal from nitrified anaerobic digestion piggery effluent through process optimization

Anaerobic digestion effluent (ADE) with a large amount of ammonium (NH4+) generally needs high dilution before microalgae cultivation, owing to its toxicity to algal growth. As high nitrite concentration is also toxic to algal growth, complete nitrification of ADE, i.e., nitrate only as an inorganic nitrogen source, can not only reduce NH4+ toxicity but be a feasible option in most places with limitations of freshwater. Therefore, complete nitrification of ADE and subsequent microalgae cultivation in the nitrified ADE (NADE) may be a more reliable procedure for algal growth and nutrients recovery from ADE. In this study, we determined the dilution effect of AD piggery effluent (ADPE) and nitrified ADPE (NADPE) on the growth of Chlorella vulgaris, and further confirmed the suitable conditions in undiluted NADPE with additional inputs of magnesium (Mg) and dissolved inorganic carbon (DIC) for enhancing algal yield and nutrient removal. Then, continuous algal cultivation was adopted to determine optimal hydraulic retention time (HRT) in NADPE enriched with Mg and suitable DIC concentrations (NADPE + Mg + DIC), thereby achieving maximum algal biomass productivity and nutrient removal. The results showed that C. vulgaris could be successfully cultured in the undiluted NADPE for algal growth, due to scarce NH4+. NADPE + Mg enriched with DIC of 100 mg L−1 (NADPE + Mg + DIC (100)) as culture medium and 12 days of HRT provided suitable conditions for maximizing algal biomass productivity (226.8 mg L−1 d−1) and nutrient removal rate (30.3 and 3.5 mg L−1 d−1 for nitrate nitrogen and phosphate phosphorus, respectively) in the continuous cultivation system. These results indicated that ADPE pre-treated with nitrification and subsequently continuous microalgae cultivation can be a promising approach to achieving algal biomass recovery and nutrient removal from ADPE.