A gradual dosing of anaerobically digested effluent for cost-effective cultivation of Arthrospira platensis MUR 126

Anaerobically digested effluents (ADEs) are a promising low-cost alternative to Zarrouk medium for cultivating Arthrospira platensis, but their application is constrained by high turbidity and ammonia toxicity. This study investigated a periodic dosing strategy in which 20%, 40%, 60%, and 80% of the nitrogen supplied by Zarrouk medium was replaced with ammoniacal nitrogen from raw ADE, while the remaining Zarrouk nutrients were reduced proportionally. All treatments were initiated with the equivalent of 20% nitrogen replacement from raw ADE, followed by additional doses every two days until the target nitrogen replacement level was achieved. Average biomass yields (ash-free dry weight) followed the sequence: 40% ADE = 20% ADE > 60% ADE = Zarrouk medium > 80% ADE. The 40% raw ADE treatment achieved the highest performance, producing a biomass productivity of 0.358 ± 0.01 g L^-1 d^-1, and a maximum specific growth rate (μ_max) of 0.311 ± 0.03 (d^-1). Biomass composition was protein-dominant across treatments, with the highest level (∼67% ash-free dry weight) observed at 40% raw ADE, while maximum pigment (chlorophyll a and phycocyanin) yields occurred in Zarrouk medium. Nitrogen removal (nitrate + ammoniacal nitrogen) was highly efficient (72 - 100%) across treatments, whereas phosphate removal ranged from 19 to 49%. Overall, the gradual dosing strategy effectively mitigated the adverse effects of ADE turbidity and ammonia toxicity, enabling robust A. platensis growth and production of protein-rich biomass suitable for animal and aquaculture feed as well as food supplement applications.