Biological nitrogen removal in activated sludge processes is dependent on sufficient supplies of easily metabolized carbon compounds for the denitrifying bacterial population. An external carbon source can increase denitrification rates and compensate for deficiencies in the influent C/N ratio. Plant performance and microbial adaptation were studied in a pre-denitrifying pilot-scale activated sludge plant with and without ethanol. Total nitrogen removal efficiency was 67 and 35% for the ethanol and reference line, respectively. The process responded rapidly to ethanol but one sludge age was necessary for full bacterial adaptation. An initial rapid increase suggests enzyme induction rather than alterations in bacterial species composition. Increased enzyme activity was explained by an increase in turn-over rate of biomass. Low effluent nitrate concentration was a result of the simultaneous use of influent COD and ethanol. Fluctuations in influent COD did not affect denitrification capacity with ethanol. Sludge settling properties were moderately better in the process without ethanol addition. An automatic control strategy for carbon dosage using feedforward from influent carbon and nitrate in the recirculated flow was simulated. Simulations with an adaptive linear quadratic controller demonstrated that the desired nitrate concentration at the end of the anoxic zone could be maintained despite relatively large disturbances.

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