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

• Impact of Ca2+ on biofilm morphology and stability was evaluated in CFBBR at low C/N.
• Role of calcium in EPS formation and biofilm attached was evaluated.
• Correlations between denitrification performance and biofilm were established.
• Optimization of biofilm was achieved at 120 mg Ca2+/L in the wastewater influent.
• Calcium fate closure in biofilm processes was presented and evaluated.

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 (20 mg Ca2+/L) to 240 mg Ca2+/L at two different C/N ratios of 5 and 3.5 in phases I and II, respectively for a period of 200 days. 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 R120Ca (bioreactor with a Ca2+ concentration of 120 mg/L), R180Ca, and R240Ca 90% and 70% lower than for R20Ca and R60Ca, respectively. The optimum influent calcium concentration at both organic and nitrogen loading rates was 120 mg 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.