Screening, acclimation and ammonia tolerance of microalgae grown in food waste digestate

Anaerobic digestate of food waste as a waste product of anaerobic digestion contains a significant amount of nutrients making its direct disposal prohibitive due to environmental regulations. However, the nutrients in this waste are a valuable feedstock for waste-to-product endeavours such as microalgae cultivation coupled to the treatment of the digestate. A limitation to this path is the high toxic concentration of ammonia nitrogen in the digestate which limits microalgae growth, leading to the requirement for significant dilution before use. This study focused on the bioprospecting and sourcing of species capable of sustained growth in very high concentrations of ammonia nitrogen. Ten local strains of microalgae were isolated, comprising mainly of unicellular species, a colonial species, and a filamentous species. Three unicellular species were chosen (Chlorella sp., MUR 271; Scenedesmus obliquus (Tetradesmus obliquus), MUR 272; and Oocystis sp., MUR 273) and screened alongside previously isolated strains (Scenedesmus quadricauda, MUR 268; Chlorella sp., MUR 269; and Scenedesmus dimorphus (Tetradesmus dimorphus) MUR 270) which had undergone long-term acclimation in digestate. The most tolerant of the newly isolated strains was MUR 273 (Oocystis sp.), capable of proliferation in up to 600 mg L−1 NH3-N concentration in digestate. The maximum specific growth rate, μmax, of MUR 273 was 0.36 ± 0.01 day−1 at 150 mg L−1 NH3-N. The results indicate that MUR 273 displayed tolerance levels similar to that obtained with MUR 268 which had previously undergone long-term acclimation in digestate and could potentially be used in the treatment and valorization of the anaerobic digestate of food waste with significantly less dilution