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Chlorinated Wastewater Effluents - PLS1

3.0 Assessment of "Toxic" under CEPA

In Canada, there are approximately 400 municipal wastewater treatment plants (MWTPs) discharging chlorinated wastewater effluents (CWWE) to aquatic systems. Total discharges of CWWE effluents are estimated to be approximately 6.11 x 10⁶ m³/day. Based on an estimated total residual chlorine (TRC) concentration of 0.95 mg/L in final effluents, it is estimated that approximately 5800 kg of TRC are discharged to the Canadian environment from MWTPs daily.

Most of the 11 major power utilities in Canada employ chlorination of cooling waters at some or all of their thermal power generating stations to control biofouling. Effluent flows from these power stations vary from a mean of 10 000 m³/day to a mean of 7.8 x 10⁶ m³/day. Levels of TRC in cooling waters generally vary from 0.002 to 0.4 mg/L and chlorination may take place on an irregular, daily, weekly, seasonal, or continuous basis.

Little information is available on discharges of CWWE effluents from industry. There is evidence that some industrial plants employ chlorination for disinfection or control of biofouling and discharge their effluents directly to the Canadian environment.

Insufficient information was available to determine if the practice of chlorinating cooling waters in power generating stations and other industries causes harmful effects to aquatic biota in Canada. Therefore, the remainder of this assessment will focus on the available evidence for MWTPs that chlorinate for disinfection.

3.1 CEPA 11(a) Environment

A variety of substances are discharged to aquatic systems in Canada as a result of wastewater chlorination. These substances can be broadly classified as total residual chlorine, which includes free chlorine (HOC1, OC1^-, C1₂) and the organic and inorganic chloramines (NH_xC1_3-x), chloralkyl and chloralkene substances, chloraryl substances, and other halogenated substances.

Levels of free chlorine and the organic and inorganic chloramines are rapidly reduced in most aquatic systems due to dilution, volatilization, chemical degradation, and reaction with other substances.

Chloralkyl, chloralkene substances, and the lower chlorinated chloraryl substances discharged in CWWE effluents are generally non-persistent in aquatic systems.

It is important to note that substances not produced by chlorination (e.g., ammonia, metals) and other characteristics of wastewater effluents (e.g., temperature, pH, biological oxygen demand) may cause harmful effects to aquatic biota. The evidence presented in this subsection addresses only the issue of whether chlorination of wastewater effluents produces compounds that at least partially contribute to the observed toxicity of these effluents following discharge to Canadian aquatic ecosystems.

In situ testing and biological surveys downstream from Canadian MWTPs demonstrated that CWWE effluents have caused acute lethality to fish (Servizi and Martens, 1974; Osborne et al., 1981; Flood et al., 1984a; b; OMOE, 1992b), and changes in community structure (e.g., reductions in diversity, shifts in species composition) to benthic invertebrate communities (Osborne, 1985; Rutherford, 1992). These effects were evident up to 500 m from the outfalls of several MWTPs. In most cases where testing was conducted, the observed effects were reduced or eliminated by dechlorinating effluents before discharge or by switching to discharges of non-chlorinated effluents (Servizi and Martens, 1974; Osborne et al., 1981; Flood et al., 1984b; Osborne, 1985). For example, caged rainbow trout (Oncorhynchus mykiss) experienced 40% mortality 100 m downstream from an MWTP located on the Sheep River, Alberta. No mortality occurred when fish were exposed to whole effluents that had not been chlorinated (Osborne et al., 1981). In situ tests and biological surveys of receiving waters near MWTPs in Nova Scotia, Ontario, British Columbia, Alberta, and the eastern United States have shown that harmful effects to aquatic biota frequently occur when TRC levels exceed 0.02 mg/L.

Laboratory tests with whole effluent samples demonstrated that CWWE effluents from MWTPs in Nova Scotia and Ontario were acutely lethal to fish and invertebrate species, particularly in effluents where TRC levels exceeded 0.1 mg/L (OMOE, 1991b; Rutherford, 1992). When the CWWE whole effluent samples were dechlorinated in the study involving eight Ontario MWTPs, the acute lethality to rainbow trout (Oncorhynchus mykiss) declined from 77% to 42% of the samples tested, and for Daphnia magna, acute lethality declined from 64% to 18% of the samples tested. The results from two MWTPs discharging to marine waters in Nova Scotia indicated that CWWE effluents may be acutely lethal at levels of <10% effluent. Marine bivalve larvae were found to be particularly sensitive, with effects observed at 0.2% CWWE.

Little information was available to compare the toxic effects of chlorinated and dechlorinated or non-chlorinated wastewater effluents discharged to marine ecosystems. For two of the MWTPs in Nova Scotia that discharge to marine waters, however, TRC levels in the effluent were slightly above 0.2 mg/L (Rutherford, 1992). This level is higher than the levels of TRC associated with effects to freshwater biota in in situ and whole effluent tests.

Therefore, on the basis of available data, which demonstrate that chlorinated wastewater effluents discharged to the Canadian environment by municipal wastewater treatment plants have caused harmful effects to freshwater aquatic biota, chlorinated wastewater effluents are considered to be "toxic" as defined under Paragraph 11(a) of CEPA. There is insufficient information to make an assessment of "toxic" as defined under Paragraphs 11(b) and 11(c) of CEPA.