The toxicity of municipal effluents depends on a variety of factors, including the size and characteristics of the sewer-shed, the type and efficiency of treatment and disinfection processes and the physical, chemical and biological characteristics of the receiving waters.
In many cases, the acute toxicity of municipal wastewater effluents is due to unionized ammonia, in the case of chlorinated effluents, it is because of total residual chlorine that can make wastewater toxic. Other causes of wastewater contamination including cyanide, sulfides, phenols, surfactants and heavy metals, such as copper, zinc and chromium, also contribute to acute or chronic toxicity.
Many factors including pH, hardness, dissolved organic carbon and temperature can moderate the toxicity in the water treatment plant effluent or receiving environment. Despite considerable investment in treatment systems, acute and chronic toxicity remains a concern in many sites receiving municipal effluents as well as toxic sewage.
Many chemicals detected in municipal effluents are hydrophobic and may tend to adsorb to particles in the effluent or sediments in the receiving environment, than remain in the water phase. The distribution of these chemicals may therefore differ considerably from more soluble compounds, which will tend to move with the effluent plume.
Hydrophobic wastewater toxic chemicals may also tend to bio-accumulate in organisms and move through food webs. The distribution and fate of contaminants in the environment is extremely complex. It is dependent on the physical and chemical characteristics of the chemicals as well as the physical, chemical and biological characteristics of the receiving environment.
Ammonia, chloramines, nonyl phenol and its ethoxylates and textile mill effluents are associated with municipal effluents and have been known to make wastewater toxic.