Production of algal biomass on electrochemically recovered nutrients from anaerobic digestion centrate

Abstract

Microalgae cultivation on the nutrients in anaerobic digestion (AD) centrate represents a promising strategy for circular economy by converting wastes to valuable products. However, direct cultivation of microalgae in AD centrate would introduce some toxic compounds, bacteria, high turbidity, and coloration that could hamper biomass production. Herein, nutrients were extracted from AD centrate by an electrochemical nutrient recovery cell (ENRC) and then used to support microalgal growth. At an applied current density of 4 A m−2, the ENRC recovered 67.5 ± 1.2 % of NH4+-N and 93.1 ± 2.1 % of PO4−3-P from AD centrate at energy consumption of 16.49 ± 1.97 kWh kgN−1 or 177.49 ± 13.11 kWh kgP−1. The high free ammonia concentration of 214.46 mg L−1 in the nutrient recovery solution inhibited microalgal growth, and this effect could be reduced by dilution, which led to a maximum biomass concentration of 0.68 ± 0.04 g L−1with chlorophyll a concentration of 10.10 ± 0.20 mg L−1 at 35 times of dilution. The second mitigation strategy was to control the pH of nutrient recovery solution below the pK value of NH4+/NH3. At pH 8, the initial NH3-N concentration in the 5× solution was 12.14 mg L−1, resulting in an enhanced growth of Chlorella vulgaris with a biomass concentration of 1.02 ± 0.02 g L−1 and chlorophyll a concentration of 13.30 ± 0.34 mg L−1. To improve microalgal growth in the ENRC-based nutrient recovery, one should also consider the tradeoff between inorganic carbon supply and free ammonia formation under the influence of pH.