Integrated partial nitrification and Tribonema minus cultivation for cost-effective ammonia recovery and lipid production from slaughterhouse wastewater

High ammonium content in untreated anaerobic digestion (AD) wastewater can inhibit microalgal growth, hindering both bioremediation and biomass/lipid productivity. This study investigated the potential of partial nitrification (PN) as a pretreatment step for AD effluent before cultivating the oleaginous microalga Tribonema minus for bioremediation and lipid production. The PN process transforms inhibitory ammonium into nitrate, a nitrogen form that microalgae can effectively utilize, thereby reducing ammonia toxicity. The results showed remarkable resilience in T. minus, with the species being able to tolerate high ammonium levels (≤120 mg/L) when combined with nitrate in synthetic wastewater. However, when ammonium was the sole nitrogen source (>30 mg/L), its inhibitory effect on T. minus growth became evident. Nevertheless, successful cultivation of T. minus was achieved in PN effluent from slaughterhouse AD using a Sequencing Batch Reactor (SBR) (25 °C and hydraulic retention time (HRT) of 1.5 h) containing 190 mg/L ammonium, achieving 51.25 % lipid accumulation. Additionally, cultivation with 125 mg/L NH4+-N in the PN effluent resulted in removal rates exceeding 95 % for NH4+-N. This approach, therefore, delivered the dual advantage of not only mitigating the environmental risks associated with wastewater but also fostering renewable biomass production for cleaner energy.