Polishing ponds as a necessary technological element of wastewater treatment in the case of discharge into environmentally sensitive water bodies

The problem of wastewater treatment under modern conditions is becoming increasingly acute due to growing anthropogenic pressure on water bodies, deterioration of wastewater treatment facilities, and stricter environmental regulations. Small rivers and other environmentally sensitive water bodies are particularly vulnerable, as they are characterized by limited assimilative capacity and low self-purification potential. Under such conditions, even formal compliance with discharge standards does not guarantee environmental safety. One of the key technological problems of biological wastewater treatment is the washout of activated sludge from secondary clarifiers. This phenomenon is caused by sludge bulking, hydraulic overloading of treatment facilities, instability of wastewater composition, presence of toxic components, and disturbances in operational regimes. The washout of activated sludge leads to deterioration of effluent quality, increased concentrations of suspended solids, organic matter and nutrients, as well as the discharge of microbial biomass and toxic substances accumulated in sludge into natural water bodies. This paper substantiates the necessity of using polishing ponds as an additional technological element of wastewater treatment systems in cases of discharge into environmentally sensitive water bodies. Polishing ponds are considered as a technological barrier between treatment facilities and natural water bodies, capable of stabilizing effluent quality, retaining washed-out activated sludge, reducing concentrations of organic substances and nutrients, and decreasing effluent toxicity. The main indicators of wastewater quality (BOD, COD, suspended solids, nitrogen and phosphorus compounds, toxicity) and environmental risks associated with the discharge of insufficiently treated wastewater into small rivers are analyzed. Using the example of the wastewater treatment facilities of the municipal enterprise «Brovaryteplovodoenergiya», it is shown that operation of treatment facilities close to regulatory limits creates prerequisites for instability of treatment processes and increases the risk of negative impacts on aquatic ecosystems. It is concluded that for wastewater treatment facilities discharging effluents into small rivers and other environmentally sensitive water bodies, polishing ponds should be considered not as an optional element, but as an integral component of the technological treatment scheme. The conclusions and recommendations formulated in the paper can also be applied to wastewater treatment systems of pulp and paper industry enterprises