Impact of calcium on biofilm morphology, structure, detachment and performance in denitrifying fluidized bed bioreactors (DFBBRs)

摘要

The impact of calcium concentrations on the biofilm morphology, structure, detachment and denitrifica-tion efficiency in denitrifying fluidized bed bioreactors (DFBBRs) was investigated. The DFBBRs were operated on a synthetic municipal wastewater at five different calcium concentrations ranging from the typical Ca~(2+) concentration of the tap water (20 mg Ca~(2+)/L) to 240 mg Ca~(2+)/L at two different C/N ratios of 5 and 3.5 in phases I and 11, respectively for a period of 200 days. Extracellular polymeric substances (EPS), Ca~(2+) concentration, water quality parameters, and microscopic images were monitored regularly in both phases. Calcium concentrations played a significant role in biofilm morphology with the detachment rates for R_(120Ca) (bioreactor with a Ca~(2+) concentration of 120 mg/L), R_(18Ca). and R_(240Ca) 90% and 70% lower than for R_(20Ca) and R_(60Ca). respectively. The optimum influent calcium concentration at both organic and nitrogen loading rates was 120 mg Ca~(2+)/L, with higher concentrations exhibiting fractured and weak biofilms. Specific denitrifkation rates did not change with changing the C/N ratio at elevated Ca~(2+) concentration bioreactors while with lower Ca~(2+) concentrations, the specific denitrification rates dropped by 20-40%. Nutrients and Ca~(2+) mass balances were closed with reasonable accuracy.