Algae-Based Remediation of Nitrocellulose Alkaline Hydrolysis Liquors: Nitrogen Recovery and Ecotoxicity Insights

The production of Nitrocellulose (NC) generates wastewater containing insoluble fines which can be solubilized via alkaline hydrolysis (AH). However, this process yields effluents with high NO2-N and NO3-N concentrations (3:1 ratio), opening the possibility of further treatment and nutrient recovery. This study investigated the ecotoxicity and algae-based treatment potential of post-AH liquor as a proof of concept for wastewater management. Ecotoxicity assessments such as microalgal and Microtox® bioassays showed 30% toxicity (corresponding to 240–270 mg NO2-N/L and 100 mg NO3-N/L) and a 15 min EC50 of 331–399 mg NO2-N/L and 133–146 mg NO3-N/L, respectively. Additional studies on toxicity identified nitrite (NO2−) as the primary toxicant, inhibiting the freshwater microalga Scenedesmus obliquus at concentrations higher than 60 ± 5 mg N/L. Furthermore, higher toxicity was observed in the presence of sodium and nitrate. Consequently, growth screening tests using synthetic liquors (20–300 mg TN/L, with NO2-N:NO3-N ratio 3:1) compared S. obliquus against the marine microalga Nannochloropsis salina, revealing that S. obliquus thrived at concentrations below 160 mg N/L, whereas N. salina performed poorly. System efficiency was shown to be highly dependent on the initial nitrogen load; S. obliquus achieved 60% removal at 200 mg/L total nitrogen (3:1 NO2-N:NO3-N), whereas efficiency reached 99% at concentrations less than or equal to 100 mg/L within seven days. Nitrogen removal rates peaked at 15 mg/(L·day) (at <160 mg TN/L), a result validated through scale-up experiments using both monoculture and a consortium with post-AH liquor. These preliminary findings demonstrate a promising two-step chemical and biological treatment strategy for NC wastewater, which assimilates hazardous inorganic nitrogen into valuable algal biomass for potential energy production.