Suitability of pre-digested dairy effluent for mixotrophic cultivation of the hydrogen-producing microalgae Tetraselmis subcordiformis.

The costs associated with microalgal biomass production can be reduced by leveraging alternative and cheap growth media. Digestate from fermentation reactors is a particularly interesting candidate for use in cultivating mixotrophic species. The aim of the present study was to assess whether pre-digested milk-industry effluent can be harnessed to grow Tetraselmis subcordiformis and produce hydrogen. The experimental series with 25% and 50% effluent in the growth medium performed the best, producing more than 2000 mgVS biomass/dm3. The biogas produced in these variants contained over 60% hydrogen. Increasing the effluent in the medium to 75% led to significant deterioration of performance, both in terms of T. subcordiformis biomass growth and biohydrogen production. The highest efficiency of nitrogen and phosphorus removal, respectively 98.1 ± 1.9% and 97.1 ± 1.4%, was observed in the system to which 25% of sewage was introduced. Increasing the share of fermented wastewater directly reduced the efficiency of removing biogenic compounds. A very strong negative correlation was found between initial N-NH4 in the growth medium and T. subcordiformis biomass production rates (R2 = 0.9177).