Biofilm morphology, structure, and detachment in denitrifying fluidized bed bioreactors (DFBBRs)

Divalent cations, such as magnesium (Mg²⁺) and calcium (Ca²⁺), have been shown to change the biofilm structure and detachment rate. 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. The results showed that calcium concentrations played a significant role in biofilm morphology with the detachment rates for R_120Ca (bioreactor with a Ca²⁺ concentration of 120 mg/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 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. Some of the data presented here is published in Eldyasti et al.