Impacts of influent COD/N ratio on floc physicochemical characteristics and microbial community of nitrifying sludge under high-strength ammonia conditions

The impacts of influent COD/N ratios on the floc properties and microbial communities of nitrifying sludge under high-strength ammonia conditions were investigated. Excessive organic loads severely deteriorated the nitrification rate and biomass-liquid separation. Moreover, both deficient and ample organic substrates affected the bioflocculation. Changes in COD/N ratios resulted in considerable variations in the functional groups and compositions of extracellular polymeric substances (EPS). The EPS contents almost unchanged as decreasing the COD/N ratio from 2 to 0; however, the polysaccharides (PS) contents significantly surpassed that of proteins when COD/N ratio increased from 2 to 10. Tryptophan proteins-like substances were the prominent constituents in EPS, but the intensities tapered off with the increment of COD/N ratio; while, more humic acid-like substances generated under both extremely low and high COD/N ratio situations. Structural changes of EPS constituents were dominated by COD/N ratio varying. Mass generation of hydrophilic functional groups related to polysaccharides significantly deteriorated flocculation. Furthermore, the elevated level of COD/N ratios favoured the proliferation of denitrifying bacteria and polysaccharide secreting bacteria but inhibited nitrifying bacteria. Nevertheless, the presence of certain amounts of organic substrates were beneficial to maintain microbial species diversity and proliferation of Nitrospira.