Municipal wastewater contains numerous toxic pollutants and can seriously disturb ecological conditions of the receiving water bodies. As the urban population of most European cities is growing, the problem of adequate treatment of wastewater gains more topicality and requires more and more resources. Excessive activated sludge is one by-product of wastewater biotreatment technology. The sludge accumulates many inorganic and organic pollutants which can be released into the environment and cause serious secondary contamination of soils and groundwater as a result of improper collection and/or utilization of the separated sludge. This negative effect can be aggravated by natural decomposition of organic components of the sludge. This paper analyses the influence of routine and emergency secondary contamination of the treated and ready-to-discharge wastewater caused by storage of excessive activated sludge at the Chemivtsi wastewater treatment plant. Quantitative parameters of this influence have been determined and possible ways of mitigation are discussed.