In order to assess the toxicity of compounds in wastewater and activities in reactors a precise, reproducible, fast and simple batch test is used. This research was focused on the development of such a test to determine denitrifying activity. The test is based on measuring the rate of gas production during denitrification, by tracking both the pressure and the gas composition in the headspace of a sealed bottle. The optimum pH for batch activity was found to equal the pH in the lab-scale reactor (9.5) from which the sludge originated. The minimum COD/N ratio required for linear gas production curves with acetate/nitrate was 3.5 g COD/g N. A significant increase in specific activity at low solids concentrations (<2 g VSS/l) was observed. No clear relation between activity, DNA and RNA content of the sludge after the test and initial substrate to biomass (S0/X0) ratio could be established, indicating that other factors play a role in determining the activity, possibly adsorption and substrate mass transfer. Standardization of batch tests should therefore include the solids concentration.

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