Simply put, municipal wastewater effluents are the liquid wastes that come out of a community’s sewer system and municipal wastewater treatment plants (MWTPs). These wastes are of two types: sanitary sewage, which comes from homes, businesses, institutions, and industries, and stormwater, which comes from rain or melting snow that drains off rooftops, lawns, roads, and other urban surfaces. Sanitary sewage usually receives some level of treatment before being discharged into a receiving body of water. Stormwater, on the other hand, is usually discharged without treatment, although stormwater treatment capabilities have improved in many communities over the past decade.
Since the mid-1950s, most communities in Canada have built separate sewer systems for sanitary sewage and stormwater, but in older neighbourhoods both sanitary sewage and stormwater are often carried together in a combined sewer system. If the combined sewer is connected to a sewage treatment plant, as it usually is, the stormwater can be treated along with the sanitary sewage. However, heavy storms can overload the treatment facilities, causing raw sewage to overflow from the system directly into the receiving water body before it reaches the treatment plant.
Municipal wastewater effluents are of concern not only because of the many pollutants that they normally contain, but also because of their sheer volume. In fact, municipal wastewater discharges represent one of the largest single effluent discharges, by volume, in the country — some 14.4 million cubic metres per day of treated wastewater alone from 1 118 municipalities, according to estimates made in 1999 (Environment Canada 1999b).
Municipal wastewater effluents can contain:
Social and economic costs associated with municipal wastewater effluents, resulting from the closure of fisheries and beaches, the loss of tourism revenue, or the need to adopt extra treatment measures before water can be used for domestic or industrial purposes, can be considerable.
Figure 1 shows the various paths that can be taken by municipal wastewater, from its origins as sanitary sewage or stormwater to its final discharge into surface waters or the ground. What goes into the wastewater stream and the treatment it receives before discharge have an important influence on the type and magnitude of the stresses that these effluents will place on the environment.
Sanitary sewage
Sanitary sewers receive everything that is flushed down the toilets or rinsed down the drains of
households, commercial establishments, institutional facilities, and factories. Raw sewage contains
a variety of substances in addition to human wastes — dirt particles, food fragments, oil and
grease, soaps, detergents, bleaches, other cleaning agents, solvents, paint, pharmaceuticals, and
cosmetics. Even human wastes can contain surprising amounts of trace metals, such as copper,
zinc, iron, cobalt, manganese, and molybdenum, because they are essential elements in human
nutrition. Although most metals and chemicals in municipal wastewater come from industries,
businesses, and institutions, the contribution from domestic sources is also important. Regardless
of its origins, the largest single constituent of raw sewage is water, which comprises about 99.9%.
In many communities, wastewater from industries, businesses, and institutions has a significant effect on the volume and composition of the sewage stream. Process wastes from these sources can include silver from photo-finishing outlets, solvents from dry-cleaning services, and inks and dyes from printing plants, to name a few examples. Many municipalities have sewer use by-laws that either prohibit the discharge of certain hazardous substances in sewer systems or establish allowable limits for the levels that can be discharged. Many large industries have wastewater management systems to collect, treat, and reuse (where feasible) their own process or cooling waters, while using public sewers to discharge the human component of their wastewater.
Municipal wastewater effluents are typically a mix of biological, chemical, and physical constituents (Appendix 1). The specific composition of these effluents will vary from one municipality to another, however, depending on the level of treatment they receive and the number and type of households, businesses, industries, and public facilities discharging into the system. The presence of combined sewers conveying stormwater is also an important determinant of sewage quality.
Raw sewage can be either treated in a septic tank or a MWTP or discharged directly into a body of water. About 26% of Canadians, mostly living in rural areas, rely on septic tanks with tile fields for sewage treatment, according to 1999 figures. The remaining 74%, living in some 1200 municipalities, are serviced by municipal sewers. In 1999, 97% of the Canadian population on sewers was served by some level of sewage treatment (Environment Canada 1999b). This coverage compares favourably with that of other developed countries, such as the United Kingdom (96%), Denmark (94%), and the Netherlands (92%).
MWTPs in Canada, especially those in larger municipalities, each have unique engineering designs with various combinations of treatment processes. The design and volume capacity of treatment systems depend on such things as the specific needs or objectives of municipalities, the source and quantity of the wastewater, and financial constraints. Treatment plants can be roughly categorized as having up to three levels of treatment, depending on their particular design — primary, secondary, and advanced or tertiary treatment (see Box 1 for detailed descriptions).
Primary treatment
To prevent damage to pumps and clogging of pipes, raw wastewater passes through
mechanically raked bar screens to remove large debris, such as rags, plastics, sticks, and cans.
Smaller inorganic material, such as sand and gravel, is removed by a grit removal system. The
lighter organic solids remain suspended in the water and flow into large tanks, called primary
clarifiers. Here, the heavier organic solids settle by gravity. These settled solids, called primary
sludge, are removed along with floating scum and grease and pumped to anaerobic digesters
for further treatment.
Secondary treatment
The primary effluent is then transferred to the biological or secondary stage. Here, the
wastewater is mixed with a controlled population of bacteria and an ample supply of oxygen.
The microorganisms digest the fine suspended and soluble organic materials, thereby
removing them from the wastewater. The effluent is then transferred to secondary clarifiers,
where the biological solids or sludges are settled by gravity. As with the primary clarifier, these
sludges are pumped to anaerobic digesters, and the clear secondary effluent may flow directly
to the receiving environment or to a disinfection facility prior to release.
Advanced treatment (tertiary treatment)
Advanced wastewater treatment is the term applied to additional treatment that is needed to
remove suspended and dissolved substances remaining after conventional secondary
treatment. This may be accomplished using a variety of physical, chemical, or biological
treatment processes to remove the targeted pollutants. Advanced treatment may be used to
remove such things as colour, metals, organic chemicals, and nutrients (phosphorus and
nitrogen).
Disinfection
Before the final effluent is released into the receiving waters, it may be disinfected to reduce
the disease-causing microorganisms that remain in it. The most common processes use
chlorine gas or a chlorine-based disinfectant such as sodium hypochlorite. To avoid excess
chlorine escaping to the environment, the effluent may be dechlorinated prior to discharge.
Other disinfection options include ultraviolet light and ozone.
The degree of wastewater treatment varies greatly across Canada, according to data collected by the Municipal Water Use Database (MUD) survey.3 In British Columbia, about 1.9 million people or 63% of the population served by sewers had secondary or advanced treatment in 1999, up significantly from 1996 (Figure 2). In both Ontario and the Prairie provinces, over 94% of the sewered populations had secondary or advanced treatment in 1999. Quebec had an even mix in 1999, with about 43% of the sewered population with primary treatment and 49% with secondary and advanced treatment. In the Atlantic provinces, nearly half of the population served by sewer systems released untreated wastewater directly into inland and coastal waters, unfortunately relying on the dilution capability of the receiving waters to reduce environmental impacts. Insufficient data exist to adequately assess the degree of wastewater treatment in the Northwest Territories, Yukon, or Nunavut.
The level of wastewater treatment in Canada also differs greatly between municipalities discharging to coastal versus inland (fresh) waters (Figure 3). In 1999, about 84% of the inland municipal population served by sewers received secondary or advanced wastewater treatment, while 15% received primary treatment. By contrast, many coastal municipalities served by sewers had only primary or secondary treatment, while some had no treatment at all. Of the municipalities discharging directly into Pacific coastal waters, about 80% of the population served by sewers received primary treatment and 15% received secondary treatment. Among municipalities discharging to Atlantic coastal waters and the St. Lawrence estuary, about 18% of the population served by sewers received primary treatment, about 34% received secondary treatment, while 48% had no treatment (adapted from Environment Canada 1999b).
Discharge into coastal versus inland (fresh) waters is largely self-reported. The Atlantic coastal waters include municipalities discharging into the St. Lawrence estuary.
(Source: Adapted from Environment Canada 1999b)
Municipal wastewater treatment plant effluents
After treatment, the concentrations of many pollutants that were present in the raw sewage are
reduced, but smaller amounts of most of these pollutants still remain in the effluent. In many cases, the concentrations of the remaining pollutants may still be high enough to cause serious
environmental damage.
Certain constituents, mostly associated with human waste, are present in all sewage effluent. These include:
BOD and TSS4 are the single largest constituents of municipal wastewater effluents. A litre of effluent that has received primary treatment typically contains between 100 and 200 milligrams of each of these effluent components, although these amounts drop off sharply with higher levels of treatment. Nevertheless, even with advanced treatment, the amounts discharged to the environment by large treatment plants can be substantial. For example, a sewage treatment plant in Montreal that uses primary treatment enhanced by additional physical and chemical processes produces TSS and BOD concentrations of about 20 and 40 milligrams per litre, respectively, and discharged an average of nearly 23 tonnes of TSS and 43 tonnes of BOD per day into the St. Lawrence River in 1993 (CUM 1994). In 1999, releases of BOD and TSS from all Canadian MWTPs were estimated at 101 950 tonnes and 121 619 tonnes, respectively (OMOE 1993; Environment Canada 1999b).
Nitrogen and phosphorus5 concentrations are an order of magnitude lower, with typical nitrogen concentrations in the 20–40 milligrams per litre range and phosphorus concentrations in the 7–15 milligrams per litre range for primary treatment. In inland areas where eutrophication problems from phosphorus discharges have been widespread, tertiary treatment is often needed to reduce phosphorus concentrations to more benign levels (typically 3 milligrams per litre or less, depending on the characteristics of the ecosystem that is exposed to the discharges).
Although microorganisms6 are found in huge numbers in raw sewage (e.g., anywhere from 1 million to 1 billion fecal coliforms per 100 millilitres), wastewater treatment is effective at reducing their numbers in the effluent. Septic tanks typically remove 25–75% of all microorganisms, primary treatment removes 5–40%, and more advanced treatments remove over 90% of microorganisms (Droste 1997). Beyond the removal efficiency of standard wastewater treatment, facilities with well-functioning disinfection processes can achieve a nearly 100% reduction in the number of microorganisms present in the final effluent. However, even with a 99% removal rate, 10 000–100 000 organisms may still remain in the treated effluent. This causes problems when the receiving water is used for an activity, such as swimming or shellfish harvesting, that requires a very low number of microorganisms per 100 millilitres in order for the activity to be safe. Microorganisms are of even greater concern in stormwater and CSOs, where effluent is generally released untreated.
In comparison, metals7 are present only in very small quantities. Aluminum, strontium, and iron are the most abundant, as salts of these metals are often used in the sewage treatment process. Their concentrations are typically in the area of a few milligrams per litre. However, concentrations of other metals, such as cadmium, copper, lead, zinc, manganese, molybdenum, and nickel, are generally in the low microgram (i.e., billionths of a gram) per litre range. Mercury, which is a metal of considerable environmental concern, is usually present only in trace quantities, measured in nanograms (trillionths of a gram) per litre. A study of 37 Ontario treatment plants serving a total of 5.1 million people, published in 1988, gives some idea of the relative proportions of these substances that are being released into the environment. The study reported that the combined yearly discharges of metals from the 37 plants averaged as high as 450 tonnes for strontium and 284 tonnes for aluminum and as low as 48 kilograms for mercury. Zinc loadings from the 37 plants averaged 89 tonnes per year, while loadings of cadmium, copper, chromium, lead, nickel, and five other metals were each less than 150 kilograms per year (OMOE 1988).
Concentrations of organic chemicals8 tend to be even lower than those of the metals, with most falling in the very low microgram per litre range. Concentrations of PCBs and of dioxins and furans are lower still and fall in the nanogram per litre range. Together, the 37 Ontario treatment plants in the 1988 study discharged an average of 30 kilograms of PCBs per year, 1.2 kilograms of dioxins and furans per year, and 1.6 and 2.5 tonnes of the solvents tetrachloroethylene and trichloroethylene, respectively, per year (OMOE 1988).
In spite of their very low concentrations in wastewater effluent, organic chemicals and metals do not have to be discharged in large quantities to result in environmental degradation. That is because many of these chemicals can be toxic at low levels and can remain in the environment for very long periods. Consequently, large amounts of these substances can build up in sediments over time or be transported by water and air currents to other environments far from the original point of discharge. Some of these substances also tend to accumulate in living tissue and be passed up the food chain. As a result, concentrations in top predators such as fish-eating birds can reach very high levels, even though ambient concentrations in the water are very low.
Stormwater and combined sewer overflows
Since urban lands are covered largely by deforested areas and impervious surfaces such as asphalt
or concrete, they absorb much less water than natural landscapes. As a result, about 30–50% of
stormwater or snowmelt in urban areas is converted into surface runoff, and in downtown areas
the amount may be 90% or higher. Urban runoff flushes debris and contaminants from roads,
parking lots, sidewalks, rooftops, lawns, and other surfaces into the sewer system as well as into
other drainage channels, such as ditches and creeks.
Stormwater contains suspended solids, nutrients, bacteria and other microorganisms, and most of the other constituents found in sanitary sewage; however, because much of it comes from road surfaces, it also contains substantial amounts of oil and grease, chlorides from road salting, toxic metals, and organic chemicals, such as PAHs (a group of combustion by-products, some of which are carcinogenic). In addition, runoff from lawns and gardens is likely to contain residues of fertilizers, insecticides, and herbicides. Other common constituents of stormwater include debris, sand and eroded soil, and air pollutants that have settled on the ground or been washed out of the atmosphere by rain.
If stormwater is carried in a combined sewer system, it is usually treated in a MWTP, unless flows are too high, in which case the system is commonly allowed to overflow into receiving waters at various points upstream of the treatment plant. In most municipalities, though, stormwater is carried in separate storm sewers and discharged directly into a receiving water body without treatment. That situation has been changing gradually over the past 10–20 years, however, as communities have begun to realize that stormwater is an important pollution source.
Loadings of stormwater and CSO contaminants are difficult to measure because of the episodic and highly variable nature of wet weather events. In addition, contaminant concentrations in CSOs are much greater in the early phases of these events (known as the first flush) and drop off considerably during their later stages. Stormwater and CSO discharges, unlike those from sewage treatment plants, also occur at many points within an urban area. The Greater Vancouver Regional District, for example, has 50 CSO outfalls within the region’s boundaries. In general, however, loadings to the environment depend on the extent and type of urban development in the watershed, the level of treatment (if any) that the stormwater might receive, and, in the case of CSOs, the source of the sewage that overflows (e.g., the amount and type of industries discharging to the sewer system). Consequently, the mix of discharged contaminants can vary considerably between watersheds and even between different locations within watersheds. There is also often considerable variability from one season to another and from one runoff event to another.
The approximate quantity and quality of stormwater entering aquatic ecosystems in Canada have not been very well documented. However, a recent review of 140 studies from the United States, Europe, and Canada (Makepeace et al. 1995) provides a useful indication of the contaminants that are commonly present. The review identified 28 pollutants with the potential to affect aquatic life and human health (mainly through the drinking water supply). The list included total solids, TSS, chloride, oxygen-depleting substances, 3 types of microorganisms, 12 heavy metals, and 9 organic chemicals.
CSO constituents have been studied even less than those of stormwater, in part because the design of combined sewers makes them more difficult to monitor than storm sewers. During the first flush, however, the CSO constituent levels resemble or even exceed those of raw sanitary sewage (especially if sewage sludge is scoured from the sewer bottom by high flows). The main pollutants of concern in CSOs are suspended solids, oxygen-depleting substances, nutrients (nitrogen and phosphorus), fecal bacteria, and toxic chemicals originating from local municipal and industrial sources (Environment Canada 1997).
Municipalities with combined sewer systems usually experience tens of CSO events in the course of a year. In the Greater Vancouver area, which experiences more CSOs than any other Canadian city, some of the major outfalls have 100–150 discharge events annually, with most of them occurring in the winter months (Hall et al. 1998a). Surface runoff volumes during an average stormwater discharge in the Great Lakes basin have been estimated at about 760 litres per capita per day (Marsalek and Schroeter 1988). If the average is taken for wet weather days only, however, the discharges are in the range of 2 000–3 000 litres per capita per day — considerably higher than the average municipal sewage flow of 300 litres per capita per day.
Estimates of annual stormwater and CSO pollutant loadings for the whole country are not available. However, loadings for the Canadian Great Lakes basin, an area that is home to over 9.2 million Canadians, have been calculated. For stormwater runoff, loadings were highest for TSS (91 000 tonnes per year), followed by oil and grease (100–1 000 tonnes per year), metals (420 tonnes per year), PAHs (0.73 tonnes per year), PCBs (0.08 tonnes per year), chlorinated benzenes (0.06 tonnes per year), and organochlorine pesticides (0.03 tonnes per year) (Marsalek and Schroeter 1988; Marsalek, unpublished data). Typical concentrations of fecal coliforms and E. coli in Ontario stormwater have been measured at 1 200–5 100 cells per 100 millilitres and 800–6 100 cells per 100 millilitres, respectively (Marsalek et al. 1992). For CSOs, estimated loadings of conventional pollutants were 17 400 tonnes per year for TSS, 3700 tonnes per year for BOD, 760 tonnes per year for total nitrogen, and 130 tonnes per year for total phosphorus (Waller and Novak 1981). Fecal coliforms measured in Ontario CSOs have measured as high as 1 million cells per 100 millilitres, probably during the first flush of contaminants (Waller and Novak 1981).
Municipal wastewater effluents are a leading source of the BOD, TSS, nutrients, organic chemicals, and metals that are discharged into Canadian waters. Table 1, for example, shows that loadings of phosphorus from stormwater and CSOs are roughly comparable to those from industries that do not use municipal sewer systems, while loadings from municipal treatment plants are between two and three times higher. In the case of nitrogen, loadings from municipal treatment plants may be seven times higher than those from industries that discharge directly to the environment. Unfortunately, there is insufficient information available for comparison of the loadings of nutrients through runoff or leaching from agricultural fields in Canada.
| |||||||||||||||||||||||||||
Municipal wastewater effluents also overshadow direct industrial discharges as the dominant source of waterborne PCBs and mercury entering lakes Superior and Ontario, according to estimates for 1991 and 1992 (Table 2). The significance of municipal wastewater effluents as a source of water pollution, especially in heavily populated areas, is underscored by the fact that municipal wastewater pollution was identified as a major problem in 10 of the 17 Canadian Great Lakes localities originally identified as Areas of Concern in 1985 by the International Joint Commission.
| ||||||||||||||||||||||||||||||||||
Some data may refer to earlier years.
(Sources: Lake Ontario: Thompson 1992; Lake Superior: Dolan et al. 1993)
The relative contribution of MWTPs, storm sewers, and CSOs to total municipal wastewater discharges varies considerably from place to place and is very much influenced by the population and development patterns of each area. There is also considerable variability between seasons and, of course, between wet and dry weather. Some idea of the relative importance of the different types of discharges can be gleaned, however, from a recent study that compared the estimated contaminant loadings and discharge volumes of wastewater treatment plants, storm sewers, and CSOs for 17 Canadian Areas of Concern in the Great Lakes area (Schroeter 1997). The results showed that stormwater runoff contributed 17–65% of annual wastewater volume in these areas, CSOs 1–6%, and wastewater treatment plants 35–80%. The wide range in these numbers reflects factors such as population density and the extent and type of development in each of the 17 areas. During wet weather events, however, these relative contributions changed dramatically, as stormwater, CSOs, and treatment plants discharged about 80%, 7%, and 13% of total wastewater volume, respectively. Over half of the TSS was discharged by storm sewers. During wet weather, however, loadings of suspended solids came almost entirely from stormwater and CSOs. On the other hand, treatment plants contributed the highest annual loadings of toxic contaminants and CSOs the lowest.
The stresses that municipal wastewater effluents place on aquatic environments depend on several principal factors: the amount of effluent discharged, the quality of the effluent (i.e., the kinds and quantities of contaminants it contains), the characteristics of the receiving environment, the assimilative capacity of the receiving water, and climate and season.
Effluent volume
Other than precipitation, the amount of effluent discharged from a municipal sewage system
depends mostly on the size of the population and the area served by the system, the nature of land
use within the area, and the amount of water used by the population. Urban population growth has
been a major factor in increasing the amount of municipal effluent discharged, through the increase
in total water used and land development. In the quarter century since 1971, Canada’s urban
population grew by 37% to a total of 22.5 million people, or 76% of the total population. Because
this growth has been accommodated mostly by the development of low-density suburbs, urban land
area has actually increased at a much greater rate than urban population. Between 1971 and 1996,
Canada’s urban land area grew by 77%, or an additional 12 250 square kilometres, an amount
equivalent to twice the area of Prince Edward Island (Statistics Canada 1997) (Figure 4). This growth
is occurring within a relatively small area — the narrow band, no more than a few hundred kilometres
wide, that runs adjacent to the border with the United States and contains 90% of the Canadian
population. Many water bodies within this area are already stressed by human activities and
competing land uses. The expansion of urban land use within this area only serves to intensify these
pressures. The resulting increase in developed area has meant a corresponding increase in urban
runoff and in the pollutants (such as oil and road salt) that it typically carries.
These stresses have been partially offset by an overall decline in municipal per capita water usage in the 1990s, reducing the per capita volume of sanitary sewage generated. However, total water usage in Canada is still increasing as a result of increasing urban populations. After peaking in 1989, municipal per capita water usage in Canada declined during the early 1990s by over 10%. Nevertheless, Canadian water usage is still exceptionally extravagant by international standards, and has recently increased slightly (2%) to an average municipal per capita consumption in 1999 of 638 litres per day — a level of usage second only to that of the United States. Slightly more than half of that water is used for household purposes such as cooking, cleaning, bathing, watering lawns, filling pools, and flushing toilets. The rest is used for commercial and industrial purposes and for other uses such as firefighting (Figure 5). Water lost through leaks in water mains can also account for a significant portion of municipal water use, ranging from 10 to 30% in some municipalities. This heavy water use, in combination with current land use patterns, is resulting in unnecessarily high volumes of municipal wastewater effluent.
Effluent quality
The kinds of contaminants in sanitary sewage depend initially on what is released into the sewer
system. Industrial and commercial discharges, in particular, have an important impact on sewage
characteristics, with the difference between one community’s sewage and another’s often being
determined by the number and types of businesses and industries connected to each municipality’s
sewers. Household sewage is more consistent from place to place, but the extent to which
households dump motor oil, oil-based paints, solvents, and other toxic substances down their drains
may also affect a community’s sewage quality.
The level of treatment that wastewater receives determines the final concentrations of the major constituents in the effluent that is discharged to the environment. However, plants providing the same level of treatment may vary considerably in the quality of their effluent, and even individual plants will show variations in their effluent quality. These differences can be due to a wide variety of factors, including the plant’s design, the skill of its operators, fluctuations in the flow level, and the season of the year. Local water consumption is also a significant factor, because heavy water usage dilutes the raw sewage and makes it more difficult to process effectively. Treatment plants operate more efficiently when processing relatively undiluted sewage in which the contaminants are more concentrated.
About 3% of the Canadian population served by sewer systems lives in communities that provide no treatment whatsoever for their sanitary sewage. Even in communities that have treatment facilities, significant discharges of untreated sewage can also occur, sometimes frequently, as a result of CSOs and sanitary sewer overflows or bypasses.
In the case of stormwater, land use is the major factor determining effluent quality. Heavily developed areas with high traffic volume, for example, tend to contribute higher levels of suspended solids, metals, and PAHs to stormwater and CSOs than do residential areas. Since most stormwater in Canada is discharged without treatment, stormwater discharges can have a significant impact on local water pollution characteristics.
Receiving environment characteristics
The physical and chemical characteristics of the receiving waters are important factors influencing the
impacts of municipal wastewater on aquatic environments. These characteristics include water
hardness, temperature, acidity or alkalinity, background concentrations of nutrients and metals, and
the physical nature of the receiving water body (e.g., whether it is a stream, lake, or estuary; whether
it contains fresh water or salt water). The toxic effects of ammonia, for example, are related to the pH
and temperature of the receiving waters. Un-ionized ammonia, which is highly toxic to fish, exists in
equilibrium in water with its non-toxic counterpart, ammonium (or ionized ammonia). When the
water becomes warmer and more alkaline, however, more ammonium is converted back to unionized
ammonia, and the concentrations of un-ionized ammonia rise. Thus, quite significant
amounts of ammonia can form merely as the result of a change in water temperature and pH.
The toxicity of many substances, in fact, tends to be affected by elevated temperatures, such as are common near municipal wastewater outfalls. For most chemicals, acute toxicity increases by an average of 3.1 times for every 10°C rise in temperature (Mayer and Ellersieck 1988). The effects of water hardness and pH, on the other hand, tend to vary with the type of substance involved. Water hardness, for instance, affects the toxicity of most inorganic chemicals, such as chlorides, but has little effect on the toxicity of organic chemicals (Pickering and Henderson 1964; Inglis and Davis 1972). The relative acidity or alkalinity of the water can also alter the toxicity of metals and weak organic and inorganic acids and bases (Mayer et al. 1994). As the water becomes more alkaline, the toxicity of bases, such as ammonia, increases, and the toxicity of acids, such as sulphuric acid, decreases.
In addition, in the case of organic chemicals, their bioavailability (i.e., the portion of the total amount of chemical that is available for uptake by an organism) can be reduced by the presence of particles of organic matter. This is because organic chemicals tend to form complexes with particulate matter, and these complexes are too large to pass through gill membranes, for example (Gobas and Zhang 1994). Since the amount of particulate matter can differ between aquatic ecosystems, the bioavailability and hence the toxicity of a given concentration of a contaminant can differ substantially from one ecosystem to another. Similarly, the bioavailability and toxicity of a substance can be different in marine and freshwater ecosystems, although these differences have not been widely studied.
Assimilative capacity of the receiving water
The volume and flow of receiving water will determine its ability to dilute or assimilate effluent discharges
and, hence, the extent of toxic effects occurring in the vicinity of the discharge. Although a concentrated
effluent may be highly lethal in laboratory tests, receiving systems with a large assimilative capacity may
dilute the effluent to the point where it is no longer deadly. However, in small watercourses, intertidal
areas, or receiving waters that are subject to periodically low seasonal flows, the water volume may be
insufficient to dilute the effluent to non-toxic levels (OMOE 1990). In addition, a large assimilative capacity
may have little effect on the long-term impact of persistent chemicals that tend to accumulate in
sediments or the tissues of aquatic organisms over long periods of time.
The dilution capacity of a receiving water body also varies with time and depends on the volume of the discharge and the flow of the receiving water at the point of discharge. Receiving water flow is determined by precipitation, surface runoff, groundwater discharge, and the area, slope, soils, and vegetation of the drainage basin. Tidal patterns can also influence the dilution capacity of estuarine and marine receiving waters.
Climate and season
Climatic conditions and seasonal variations can act upon a number of factors that influence the toxicity
of municipal wastewater and its effects in the receiving environment. The factors affected include
dissolved oxygen concentrations in receiving waters, temperature of the wastewater and the receiving
environment, water levels and assimilative capacity, the types of contaminants that accumulate on
urban surfaces (e.g., road salt), and the efficiency of sewage treatment plants. In the Fraser River Valley,
for example, a study of stormwater contaminants showed that concentrations were higher in the
summer months. This was because summer rainfall events in the area were on average less frequent
but more intense than winter rainfall events. Not only were the more intense summer rains more
effective at flushing contaminants from the streets, but the longer intervals between rainstorms left
more time for contaminants to accumulate (Hall et al. 1998b). In Ontario, on the other hand,
stormwater runoff, especially from highways, showed the highest levels of toxicity during the winter,
because of the use of road salt, the accumulation of contaminants in snow, and the higher mobility
of metals in chlorine-laden runoff (Marsalek et al. 1999).
 |
 |
 |
||||
|
||||||
|
|
 |
 |
 |
 |
 |
|
|
  |
 |
 |
 |
 |
|
||||||
 View in print format |
 Previous
| Next 
|
|
|
| | What's New | About Us | Your Environment | Information/Publications | Weather | Home | |
|
|
| Français
| Contact
Us | Help | Search
| Canada Site
|
|
|
| The Green LaneTM, Environment Canada's World Wide Web site | ||
| Last updated: 2005-04-11 | Important Notices and Disclaimers | |
