Modelling combined effect of chloramine and copper on ammonia-oxidizing microbial activity using a biostability approach

• Copper lowers the disinfectant level needed to prevent the onset of nitrification.
• A copper dosing ≥0.25 mg/L is needed for complete inhibition of nitrification.
• A copper dosing ≥0.044 mg-Cu L−1 is needed before inhibition starts.
• A non-competitive inhibition model fits the experimental data well.
• A biostability approach works well for the combined effect of metal and disinfectant.

Continuous and batch laboratory experiments were used to evaluate the combined effects of copper and chloramine on ammonia oxidizing microbes present in otherwise high nitrifying water samples. The experimental data were analyzed using a biostability concept and quantified with the biostable residual concentratrion (BRC) of monochloramine, or the concentration that prevents the onset of nitrification. In the batch experiments, copper dosing ≥0.25 mg-Cu L−1 resulted in complete inhibition of nitrification, and a lower copper dosing (0.1 mg-Cu L−1) delayed nitrification. The BRC was systematically lowered with the addition of copper. For example, a free-ammonium concentration of 0.1 mg-N L−1 had a BRC of 0.73 mg-Cl2 L−1 with no Cu, but addition of 0.1 mg-Cu L−1 lowered the BRC to 0.16 mg-Cl2 L−1, while addition of 0.25 mg-Cu L−1 eliminated the need to add chloramine (BRC = 0). A non-competitive inhibition model fit the experimental data well with a copper threshold of 0.044 mg-Cu L−1 and can be used to estimate Cu doses needed to prevent nitrification based on the chloramine concentration. Full scale systems applications need further study.

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