Chlorinated Wastewater Effluents - PLS1

2.4 Effects-related Information

Several studies on fish have shown that free or combined chlorine can adversely affect both the structure of the gill (Servizi and Martens, 1974; Bass and Heath, 1975a;b; Wiley, 1983) and the ability of hemoglobin in the blood to transport oxygen (Buckley, 1976; Grothe and Eaton, 1975). The resulting damage to gill membranes and oxidation of hemoglobin to methemoglobin suggests that the ultimate cause of mortality from exposure to free or combined chlorine is asphyxiation (Rosenberger, 1971; Dandy, 1972; Bass and Heath, 1975a;b; Cairns et al., 1975). In addition, several studies have also suggested that free and combined chlorine may have other sites of toxicity, including the nervous system (Fobes, 1971; Wolf et al., 1975).

A study by Katz (1979) indicated that exposure to residual chlorine increased gill permeability which in turn may lead to increased accumulation and hence toxicity of other chemical substances found in CWWE.

There are three categories of studies on the effects of CWWE on the environment:

1. in situ studies of community structure, population survival, or other biological endpoints;
2. whole effluent tests; and
3. chemical tests for chlorinated substances known to occur in CWWE (e.g., total residual chlorine, monochloramine).

This section will focus on the first two categories, since they provide the most direct evidence of effects to the Canadian environment.

Much of the available information on in situ and effluent testing was generated in the 1970s and earlier. Only those studies applicable to current practices in Canada are discussed in the following subsections.

2.4.1 In situ Surveys and Field Tests

In situ studies using caged sockeye salmon (Oncorhynchus nerka) and pink salmon (O. gorbuscha) were conducted in 1972 and 1973 in three tributaries of the Fraser River in British Columbia downstream from MWTPs (Servizi and Martens, 1974). In one tributary, during February and March 1972, 100% mortality of caged sockeye salmon fingerlings was observed at each of five stations located between 9 and 92 m downstream (control mortality = 0%). The total residual chlorine (TRC) level at the 92 m station was 0.07 mg/L. Twenty percent mortality occurred at the sixth station, 185 m downstream, where the TRC concentration was 0.02 mg/L. Higher levels of TRC occurred in October and November, 1972 and 100% mortality of sockeye salmon fingerlings was observed at the furthest station, 277 m downstream from the effluent outfall (TRC = 0.24 mg/L). When the chlorinator was not operating at the MWTPs on tributaries I and III, mortality of sockeye and pink salmon fingerlings and alevins either did not occur or declined significantly. Undiluted effluent from the MWTP on tributary II, which had been dechlorinated by storage in a lagoon (TRC was below the detection limit of 0.02 mg/L), caused no mortality to juvenile salmon.

The Ontario Ministry of the Environment has conducted in situ toxicity testing downstream from several MWTPs using chlorination for disinfection. Mortality of caged juvenile rainbow trout (Oncorhynchus mykiss) after acute exposures was observed in the Grand River downstream from the Waterloo MWTP (100% mortality at the 100 m station)(OMOE, 1992b), the Avon River downstream from the Stratford MWTP (66% mortality at the 475 m station)(Flood et al., 1984a), and Otter Creek downstream from the Tillsonburg MWTP (100% mortality at the 53 m station) (Flood et al., 1984b). No mortality was observed at the control stations located above the MWTP outfall at each of these sites. Chemical characterization of the receiving waters at each site indicated that TRC levels above approximately 0.04 mg/L were associated with 100% mortality during acute exposures (Figure 1). The available data on levels of ammonia, metals, and other substances discharged by the MWTPs indicated that these were below the levels normally associated with toxic effects to rainbow trout. Disinfection of the effluents from the Tillsonburg MWTP using ultraviolet light instead of chlorination led to a reduction in observed mortality of rainbow trout (100% to ≤6% at the 53 m station).

In a recent study, Szal et al. (1991) examined the effects of CWWE from three sites near MWTPs discharging effluents to streams in Massachusetts. At each site, caged fathead minnows (Pimephales promelas) experienced 100% mortality after a 24-h exposure at distances ranging from 21 to 91 m downstream and 25 to 85% mortality at distances ranging from 183 to 427 m downstream. Chlorination of wastewater effluents was determined to be a major cause of mortality because: (i) survival at control stations was >85%; and (ii) survival in whole effluents that had not been chlorinated was >90% in laboratory tests. The relationship between TRC levels and minnow mortality varied among sites. Minnows exposed to between 0.05 and 0.35 mg/L of TRC experienced no mortality at one site, while at another site where TRC levels ranged from <0.03 to 0.14 mg/L, minnows experienced 100% mortality. Variations in apparent TRC toxicity levels at different sites may have been associated with interactive effects among TRC, ammonia, and oxygen saturation (Szal et al, 1991).

Figure 1 Total Residual Chlorine Versus Mortality of Rainbow Trout Downstream from Three Municipal Wastewater Treatment Plants in Ontario (Periods for the studies ranged from 48 h to 120 h.)

In a study by Osborne et al. (1981), 40% mortality of caged rainbow trout (Oncorhynchus mykiss) was observed 100 m downstream from a MWTP on the Sheep River in Alberta (mean TRC = 0.35 mg/L). No mortality occurred when caged fish were exposed to non-chlorinated whole effluent for 24 hours. A biological survey conducted downstream from the Sheep River MWTP demonstrated a shift in structure from a community dominated by mayflies, chironomids, caddisflies, and stoneflies, to one dominated by oligochaete worms, after chlorine disinfection was begun in 1977 (Osborne, 1985). This shift in structure was apparent up to 500 m downstream in 1979. Osborne (1985) hypothesized that the shift in community structure occurred because oligochaete taxa are more tolerant of exposure to TRC than are other aquatic insect taxa (Arthur et al., 1975; Brooks and Seegert, 1977).

In Nova Scotia, biological surveys conducted near three MWTPs discharging CWWE revealed that benthic community structure was significantly altered at the furthest station sampled for each MWTP (Figure 2) (Rutherford, 1992). For example, in Fales Brook, taxa evenness was significantly depressed 180 m from the Greenwood MWTP effluent outfall. This was due to an increase in the number of chironomids found below the effluent outfall and a corresponding decrease in the number of invertebrates belonging to other taxa. In Halifax Harbour, the total number of invertebrates was significantly reduced 100 m from the effluent outfall when compared to a control station 2.5 km from the outfall. This resulted from decreases in the abundance of several annelid, crustacea, and mollusc species. It is not possible to determine whether residual chlorine or other substances produced by effluent chlorination contributed to the observed effects on benthic community structure at these three sites. For example, the Eastern Passage MWTP is a primary treatment facility that discharges effluent with ammonia levels in the range of 19 to 20 mg/L. Ammonia may be a greater factor than TRC in causing the observed changes in community structure at this facility (OMOE, 1990b).

Tsai (1970, 1971) examined species diversity and abundance changes of fish downstream from over 150 MWTPs in the eastern United States. Fish species, particularly salmonids, were adversely affected (e.g., reductions in diversity, shifts in community structure) downstream from MWTPs in which chlorination was the final process before discharge. Brook trout (Salvelinus fontinalis) and brown trout (Salmo trutta) were absent from receiving waters in which mean TRC concentrations exceeded 0.02 mg/L. No effects on species diversity or community structure were observed downstream from MWTPs that dechlorinated effluents by storage in open lagoons.

Figure 2 Effects on Benthic Communities of Chlorinated Wastewater Efluents Discharged by Three Municipal Wastewater Treatment Plants in Nova Scotia

2.4.2 Whole Effluents

A study of ten Ontario MWTPs was conducted in the summer and winter of 1989/1990 by Beak Consultants Limited (OMOE, 1991b). Eight of these plants practiced seasonal or year-round chlorination for disinfection. Of a total of 123 grab and composite samples of effluent collected (both chlorinated and non-chlorinated), 43 contained 0.1 mg/L of TRC or greater. Of these, 33 (77%) were acutely lethal to rainbow trout (Oncorhynchus mykiss) before dechlorination, while 18 (42%) were lethal after dechlorination (TRC levels were less than the detection limit of 0.01 mg/L following dechlorination of the samples by addition of sodium sulphite). When Daphnia magna was used as the test species, 28 (65%) of the 43 samples with TRC levels at or greater than 0.1 mg/L were acutely lethal, while 8 (19%) were lethal following dechlorination. Furthermore, the LC₅₀ s for samples tested as received were distributed toward lower effluent concentrations (greater toxicity) than were dechlorinated samples for both rainbow trout and Daphnia magna. These results occurred despite declines in TRC levels between time of collection and time of testing and the rapid dissipation of TRC during testing.

A 1991 study of four MWTPs in Nova Scotia (CFB Cornwallis, Eastern Passage, Greenwood, and Lakeside) indicated that CWWE effluents from all four plants were not toxic to the algal species, Selenastrum capricomutum (Rutherford, 1992). For the remaining species tested in laboratory bioassays (rainbow trout, threespine stickleback, and oyster) acute lethal effects were more pronounced with effluents from the CFB Cornwallis and Eastern Passage treatment plants, which had mean TRC levels slightly above 0.2 mg/L. Mean TRC levels in effluents from the Greenwood and Lakeside treatment plants were ≤ 0.005 mg/L and 0.12 mg/L, respectively. For example, the 96-h LC₅₀ (expressed as percent of whole effluent) for rainbow trout was 32.9% when exposed to effluents from the Eastern Passage treatment plant, 35.4% from the CFB Cornwallis treatment plant, >100% for the Lakeside treatment plant, and 70.7% for the Greenwood treatment plant. Threespine stickleback (Gasterosteus aculeatus) and a luminescent bacterium (Photobacterium phosphoreum) were found to be less tolerant of exposure to effluents from the Eastern Passage plant with an observed mean 96-h LC₅₀ of 8.8 to 11.4% and a 15-minute inhibition concentration (IC₅₀ ) of 8.0 to 9.7%, respectively. Abnormal shell development in larval oysters (Crassostrea gigas) was observed at levels as low as 0.2% after a 48-h exposure to effluents from the CFB Cornwallis plant. Dechlorination using sodium thiosulphate was found to detoxify one sample from CFB Cornwallis, but only partially detoxified other samples in the study, indicating that TRC was responsible for some but not all of the observed effluent toxicity.

2.4.3 Effects of Substances Found in Chlorinated Wastewater Effluents

The evidence presented in the preceding subsections indicates that total residual chlorine and other chlorinated compounds produced by chlorination are important contributors to the observed toxicity of treated wastewaters from some of the MWTPs tested. Toxicity may also be caused or enhanced by a number of other factors, including high concentrations of ammonia, metals, surfactants and other compounds, high biological oxygen demand, or extremes of pH and temperature (for a review, see OMOE, 1990b).

Toxicity studies are available for substances produced by chlorination of wastewater effluents and are briefly reviewed in the Supporting Document for CWWE. For in-depth reviews, the reader is referred to documents such as the Canadian Water Quality Guidelines (CCREM, 1987; and updates). For this report, the toxicity data on single substances found in CWWE were not deemed critical to the assessment because of the availability of in situ studies and whole effluent test results.