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Migration of heavy metals in the environment is a serious problem for wastewater treatment plants (WWTP) and the environment as a whole. The combined effect of eight heavy metals on the biological wastewater treatment process was analysed in this research. The heavy metals examined were Cd, Pb, Zn, Cu, Ni, Cr, Co, and Mn. In order to evaluate their effect, mathematical models were created, taking into account the hydraulic retention time, the load of heavy metals (HeM), temperature, and the air consumption in aeration tanks in biological treatment. The modelling demonstrated that a 1 kg/d increase in the HeM reduced the nitrogen removal efficiency by 1.05% and the nitrification efficiency by 1.04%. Taking into account the variability of the HeM, this constituted a 5.68% change in the nitrogen removal efficiency for the examined WWTP. The air consumption in aeration tanks was taken as a basis for the assessment of the effect of the HeM on the entire biological treatment process, as a substantial part of the oxygen used for biological treatment is consumed by microorganisms and the inhibitory effect is observed as a decrease in the air consumption. Oxygen is consumed for the degradation of organic matter and nitrification. The modelling results showed that a 1 kg/d increase in the HeM reduced the air consumption by 9300 m/d in the aeration tanks due to the inhibition, causing a decrease in treatment efficiency.

Combined effect of heavy metals on the activated sludge process