SUMMARY
In order to avoid possible toxic and harmful effects on domestic wastewater treatment plants (WWTPs) as well as to meet influent criteria for the sewer network, industrial facilities are generally obliged to pre-treat their process effluents before discharging. Although industrial inflows may indeed cause operational nuisances (particularly in case of hidden shock-loads and toxic agents), possible effects are highly depending on the origin and quality of the effluent discharged. On the other hand, however, readily biodegradable carbon shortage of domestic wastewater has been commonly becoming the bottleneck of cost-effective biological nutrient (N and P) removal worldwide. The paper draws attention to the promising novel approach of combined wastewater treatment which may offer an optimal solution by shifting the inlet C:N ratio into a favorable range for efficient denitrification and biological excess P-removal. At the start of the study the temporarily decreased treatment efficiency of the domestic WWTP investigated had been basically attributed to the dairy wastewater discharged from an ice cream factory of the catchment area. However, both on-site profile measurements and mathematical simulations suggested that instead of a direct harming effect, aeration was insufficient, and dissolved oxygen concentration had to be increased in the aerobic reactors for efficient nitrification, especially in winter season. The studies also confirmed that with adequate aeration, efficiency of denitrification would have considerably decreased without using the dairy effluent as complementary carbon source or expensive external C-source would have been required. However, in order to save unnecessary aeration costs, overdosing of dairy effluent has to be carefully avoided.