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

The impact of calcium concentrations on the biofilm morphology, structure, detachment and denitrification 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 Ca²⁺ concentration of the tap water (20mg Ca²⁺/L) to 240mg Ca²⁺/L at two different C/N ratios of 5 and 3.5 in phases I and II, respectively for a period of 200days. Extracellular polymeric substances (EPS), Ca²⁺ 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 Ca²⁺ concentration of 120mg/L), R_180Ca, 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 120mg Ca²⁺/L, with higher concentrations exhibiting fractured and weak biofilms. Specific denitrification rates did not change with changing the C/N ratio at elevated Ca²⁺ concentration bioreactors while with lower Ca²⁺ concentrations, the specific denitrification rates dropped by 20-40%. Nutrients and Ca²⁺ mass balances were closed with reasonable accuracy.