Municipal wastewater status

Municipal Wastewater Treatment Plants

In 1992, there were approximately 2800 MWTPs in Canada that discharged approximately 1 x 107  m3/d (300 L/capita/d) of municipal wastewater to the environment. These ranged from primary wastewater treatment facilities (employing physical processes to decrease influent biochemical oxygen demand (BOD) by 20 to 30% and total suspended solids (TSS) by about 60%), to secondary treatment facilities (employing additional biological treatment, such as anaerobic and/or aerobic microoganisms, to decrease influent BOD and TSS by 80 to 95%), to tertiary treatment facilities (employing advanced chemical or biological treatment to remove specific compounds or materials that remain after secondary treatment). In addition to that, some MWTPs chlorinate their effluent on a seasonal or continuous basis. Of all MWTPs in Canada, approximately 14% discharge chlorinated effluent for a total chlorinated discharge of 6.11 x 106  m3/day (Government of Canada 1993). The volume of effluent discharged from municipal sources in Canada is large in comparison with wastewater discharge from many industrial sectors (Table 1). In 1986, domestic wastewater discharge in Canada totaled 5.2 km3 and was the single largest source of wastewater, with the exception of electricity production (United Nations 1992b).

Table 1. Wastewater generation in Canada by sector for 1986 (United Nations 1992b)
Sector Wastewater Generation (km3)
1 Industrial sector classifications are based on the International Standard Industrial Classification (ISIC) of all economic activities.
Domestic
   non-treated
   treated
 
1.5 (estimate)
 3.7
Industrial1
   Total mining sector
   Manufacture of paper and paper products
   Manufacture of chemical and chemical products
   Manufacture of metallic mineral products
   Other manufacturing
   Electricity production
   Other
 
0.4
2.8
1.6
1.8
1.4
25.1
1.5

The percentage of Canadians served by wastewater treatment has increased in recent years. As of 1994,81% of Canadians were served by some level of treatment (Table 2), whereas < 56% of Canadians were served in 1980.  In Quebec alone, the population served by municipal wastewater treatment increased from 2 to 75% between 1980 and 1994 (MEFQ 1995).  The proportion of the population served by wastewater collection and treatment varies across the country.  Municipal wastewater discharges to inland waters in Canada generally receive secondary or tertiary treatment, whereas discharges to coastal waters often receive only primary or no treatment.  As of 1986, approximately 5 x 105 m3/day of municipal waste in Atlantic Canada were discharged into coastal waters with only 30% receiving wastewater treatment (Environment Canada 1986). On the Pacific coast, the British Columbia government registered over 250 marine sewage discharges in 1986.  In 1990, there were over 300 permits or applications forsewage discharge to coastal waters, accounting for about 20% of the effluent discharged to British Columbia coastal waters from all sources (Wells and Rolston 1991).  In the Arctic, Wells and Rolston (1991) reported that in the 1980’s untreated or primary treated municipal wastewater from nine communities was discharged directly to the marine environment or by percolation through lagoon substrates with subsequent leaching into surface drainage system.  In the Northwest Territories, alone, 35 of the 59 organized communities (or 31% of the 56808 population in these communities) have no wastewater treatment, with approx. 14 communities (approx. 15% of total population) discharging to coastal waters (UMA 1993).

Table 2. Percentage of Canadian population served by wastewater treatment in 1994. (Data obtained from D. Tate, Environment Canada, Hull, Quebec,  pers. com.)
Region None1br />(no sewer) None (sewers) Primary Secondary Tertiary
1May have individual on-site systems.
Atlantic 22 40 11 26 <1
Quebec 11 14 36 30 9
Ontario 10 <0.1 5.7 15 69
Prairies 5 <1 7 61 27
BC  & Territories 18 2 54 20 6
Total  12 7 20 27 34

Stormwater and Combined Sewer Overflows

Surface runoff from urban areas is usually transported in separate storm sewer systems (i.e., separated from sewage) or combined sewer systems (i.e., combined with sewage). The volume of surface runoff varies in proportion to the degree of imperviousness of the ground area. For an entire urban area, 30 to 50% of the total rainwater volume may be converted into surface runoff that is then fed into a separate or combined sewer system (Falk 1983). Stormwater from a separate sewer system is either discharged directly to the receiving water or passed through stormwater management facilities which may reduce flows and improve water quality (Marsalek and Kok 1997).  In the case of a combined sewer system, however, the whole flow is conveyed to a wastewater treatment facility under low flows but, during high flows that would otherwise exceed the sewer system or MWTP capacity (i.e., during heavy rainstorms), some flow is diverted out of the sewer system to receiving waters at overflow structures. These overflows are referred to as combined sewer overflows (CSOs) and contain both surface runoff and municipal sewage.  CSOs may be highly polluted and typically are discharged to nearby receiving waters without treatment. In relatively few locations, however, special facilities for storing and treating CSOs have been built (Marsalek and Kok 1997).  MWTP bypasses are also found in some cities. As these divert flow immediately upstream of the MWTP, bypass characteristics are similar to those of CSOs.

Detailed data are not available on the proportion of the Canadian population served by storm sewer versus combined sewer systems.  However, most urban areas developed prior to the early 1940s are served by CSOs.  Waller (1969) estimated that about 6.7 million Canadians were served by combined sewers in 1969.  In large cities, tens of CSO outfalls exist and contribute to a great spatial extent of CSO impacts on a single receiving waterbody.  For example, in Greater Vancouver, British Columbia, 252 stormwater outfalls and 53 CSOs discharge to the lower Fraser River and estuary (UMA 1994, 1995). Given that combined sewers were used mostly in the older sections of cities where populations are now declining and that sewer separation programs have been undertaken by some communities during the past 25 years, the current Canadian population served by combined sewers is likely smaller than in 1969.

The quantities of both stormwater and CSOs vary temporally and with location, and depend on local climate, sewer design and drainage practices.  CSO or stormwater discharges are not routinely monitored in Canada; therefore, estimates of their volume and impact on receiving waters are rare.  However, detailed measurements of stormwater or CSO quality can be integrated over a large drainage area to give approximate discharges and loads.  Thus, large scale estimates gave an average annual stormwater discharge of about 760 L/capita/d for the Great Lakes basin (Marsalek and Schroeter 1988).  However, the stormwater discharges would be 2000 to 3000 L/capita/d if the averaging was done just for the wet-weather days.  The average annual discharge was about 473 L/capita/d for urban runoff and CSOs in the Greater Vancouver Regional District (Environment Canada 1992 from GVRD 1988). These flows greatly exceed the average municipal sewage flow of 300 L/capita/d. 

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