Estimating the impact of inhibitory substances on activated sludge denitrification process

Industrial wastewater frequently contains substances which inhibit activated sludge treatment processes. Inhibitory characteristics of different substances are usually evaluated based on testing the impact of respective substance on activated sludge nitrification or oxygen uptake rates. However, denitrification is always before aerobic processes in conventional activated sludge treatment plants and thereby more exposed to inhibitory compounds. There is no easily applicable and validated method available for determination of denitrification process efficiency and inhibition. In this study, a method for evaluation of inhibition on the activated sludge denitrification process was developed and validated using 3,5-dichlorophenol (3,5-DCP) as a model inhibitory compound and additionally controlled with real wastewater produced in the shale oil industry. Average IC₅₀ value (5.5 ± 2.2 mg L⁻¹) for 3,5-DCP showed that denitrifiers were less sensitive than nitrifiers (IC₅₀ = 2.9 ± 0.7 mg L⁻¹) and more sensitive than aerobic heterotrophs (IC₅₀ = 7.2 ± 2.4 mg L⁻¹). Methodological aspects like accumulation of nitrite nitrogen, acclimatization of biomass and technical issues were discussed. Achieved validation characteristics were similar with ISO Standards estimating activated sludge nitrification and oxygen uptake rates, which proves the reliability of the method: standard deviation, 95.4% confidence level, relative standard deviation were calculated to be 2.2 mg L⁻¹, 1.2 … 9.8 mg L⁻¹ and 39.2%, respectively.