Gobi Kanadasan

Graphical abstract showing how increasing NH4+ -N drives microbial adaptation and VFA response in an anaerobic sequencing batch reactor. Moderate nitrogen enriches efficient VFA-producing bacteria and maximizes VFA production at 3,104 mg/L, while excess free ammonia inhibits microbial activity. Anaerobic digestion is extensively utilized for high-strength organic wastewater stabilization; however, the precise role of ammoniacal nitrogen in regulating microbial activity and volatile fatty acid (VFA) generation remains poorly understood. This study investigates ammoniacal nitrogen as a selective driving force governing microbial adaptation, VFA accumulation, and speciation dynamics. A mesophilic anaerobic acidogenic sequencing batch reactor was operated across controlled ammoniacal nitrogen concentrations (0–5,000 mg/L) and varying carbon-to-nitrogen ratios (COD:N). Moderate nitrogen enhanced VFA accumulation, reaching a maximum observed concentration of 15,742 mg/L at 3,104 mg/L NH4+ -N (COD:N = 84:5), a 110.6% net increase relative to the initial VFA concentration. This optimal threshold dynamically altered the VFA fingerprint by stimulating chain-elongation pathways, expanding medium-chain fractions (caproic and heptanoic acids), while short-chain components remained stable. Conversely, excessive loading (>4,000 mg/L) suppressed VFA yields due to free ammonia toxicity at high pH. During long-term continuous operation, the microbial consortium successfully acclimated to nitrogen stress; although biomass expanded from 1,180 to 9,240 mg/L, cell-specific VFA yields dropped from 15.72 to 1.42 mg VFA/mg volatile suspended solids, indicating a metabolic shift toward survival-driven maintenance. High-throughput profiling demonstrated that moderate nitrogen selectively cultivated competitive, core acidogenic taxa, predominantly the phylum Firmicutes (57%) and genera Lactobacillus and Aeriscardovia, whereas extreme thresholds triggered a community collapse favoring less-efficient, ammonia-tolerant Propionibacteriaceae. Collectively, ammoniacal nitrogen acts as a potent selective pressure, stimulating acidogenic communities at intermediate levels while inducing severe inhibition at excessive thresholds, providing a strategic framework to maximize anaerobic VFA recovery.