Bringing wastewater resources back to life - transforming anaerobic digestate into single cell protein

Food shortages currently represent a great global challenge, and they may become even worse if we take no action to upgrade agricultural practices and improve our dietary habits. Traditional agriculture produces food in a way that is heavily dependent on natural resources, consumes large amounts of water and arable land, leads to global warming and harms the welfare of animals. As an alternative, cellular agriculture uses cell cultures to sustainably produce food and ingredients; for example, microorganisms can provide us with a novel dietary protein called single cell protein (SCP). Additionally, it is possible to raise SCP microorganisms from waste streams, which contributes further to sustainable manufacturing.

In this context, this thesis presents a win-win solution in which waste streams were recycled by electrochemical-membrane methods, and brewer's yeast Saccharomyces cerevisiae was used to produce SCP on these recovered substrates.

This thesis begins by screening different volatile fatty acids (VFAs) to determine potential substrates for S. cerevisiae. Inorganic ions were also investigated in order to determine their impact on SCP production when substrates are derived from waste streams. According to the results, waste streams containing lactate, oleate or linoleate, together with acetate, as the main components are desirable substrates for S. cerevisiae. Only NH₄⁺, as an inorganic nitrogen source, was edible for S. cerevisiae. S. cerevisiae required 8.2 times more Na⁺ than K⁺ as an essential element. In response to increasing NO₃^-, SO₄^2- and Cl^- concentrations, S. cerevisiae's growth phase was prolonged, but SCP production was not affected.

In the second part of this thesis, acetate-rich anaerobic digestate was selected as a substrate for yeast SCP production based on an understanding of yeast physiology grown on VFA-based substrates. Using a single electrodialysis (ED) or a hybrid electrodialysis-forward osmosis (ED-FO) system, acetic acid and ammonium were simultaneously recovered from anaerobic digestate. Thereafter, S. cerevisiae used the recovered nutrients as a substrate to produce SCP. In the ED system, 60.1% ammonium and 42.4% acetic acid were recovered at 2 V, resulting in a final concentration of 0.76 g/L and 4.5 g/L, respectively. The FO system enriched ED recovery 14-fold in acetate and 10-fold in ammonium. S. cerevisiae’s SCP products demonstrated a well-balanced amino acid profile, using either ED recoveries or FO concentrate derived from anaerobic digestate. From an economic perspective, a single ED system would recover nutrients more cost effectively than a hybrid ED-FO system, if the nutrients were immediately used to produce SCPs.

In the final part of this thesis, lactate-rich anaerobic digestate was chosen as another substrate for the production of yeast SCP. ED was first used to recover lactate, acetate and ammonium from anaerobic digestate, followed by yeast fermentation for SCP production. The results show that the recovery efficiencies of three target ions improved in line with higher voltage, but current efficiencies declined because of side redox reactions and lower membrane selectivity. During the ED processes, targeted ions migrated in the following order: ammonium, lactate and acetate. Lactate generally passed the membrane slower than acetate, but it took precedence in a shallow acetate concentration. With a higher anolyte pH, there was less competition between H⁺ and NH₄⁺, thus facilitating ammonium recovery. Recoveries containing lactate yielded an average protein of 0.62±0.25 g/g-C. All essential amino acids were 1.7-3.3 times higher than the FAO recommendation as a high-quality protein for adults.

Overall, this Ph.D. project succeeded in simultaneously recovering VFAs and ammonium from acetate-rich and lactate-rich anaerobic digestate via ED and FO systems. S. cerevisiae was able to produce SCP products from these recoveries. This combination of anaerobic digestate resource recycling and yeast SCP production will therefore contribute to a green transition to sustainable cellular agriculture.