Bioaugmentation of low C/N ratio wastewater: Effect of acetate and propionate on nutrient removal, substrate transformation, and microbial community behavior.

The effect of various acetate/propionate ratios (1:0, 2:1, 1:1, 1:2, and 0:1) in a two-sludge A2/O - MBBR process was investigated. Results showed that the increased propionic/acetic ratios exerted indistinctive impact on COD (91.21-93.44%) and P (92.23-93.87%) removals, but high P content (7.42%) accelerated sludge granulation proved by SEM and EDS. Acetate favored N removal (79.52%-82.92%) with higher PURA (3.53-4.06 mgP/(gVSS·h)), while the removal declined (75.14%) due to lower PHB/PHA ratio (52.3-57.8%) with propionate as sole carbon source. Based on the stoichiometry-based quantifications, PAOs were the major contributors to nutrient removal although certain GAOs and OHO participated. The mixture ratio of 1:1 facilitated microbial diversity (995 OTUs), Rhodobacteraceae (25.63%) was responsible for high-efficient denitrifying phosphorus removal, while Defluviicoccus (15.23%) contributed to nitrite accumulation was the main competitiveness with PAOs. Nitrospira, Nitrosomonas, and Nitrosomonadaceae responsible for nitrification accounted for 7.73%, 27.11%, and 38.76% in MBBR, but the biodiversity decreased owing to the enrichment and purification.