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About UNEP
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United Nations Environment Programme
Division of Technology, Industry and Economics
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Newsletter and Technical Publications
<International Source Book On Environmentally Sound Technologies
for Wastewater and Stormwater Management>

BOD5

Biochemical oxygen demand domestic loads can be increased by 30 percent if garburators are in use in a community. Stormwater runoff only accounts for about 5 percent of total BOD5 load in combined sewer systems. Septage pumpouts have very high BOD5 concentrations, typically ranging from 2,000 to 50,000 mg/L. Restaurants in resort areas can also be major contributors to plant BOD5 loads, and oil and grease from such operations can be a nuisance in the treatment plant downstream.

Nitrogen and phosphorus

Besides human and culinary waste, other sources of nitrogen and phosphorus are agricultural runoff into storm sewers, food processing industries and fertilizer industries. These nutrients are a major problem in many areas of North America, particularly when discharging to inland water bodies. Phosphorus in laundry detergents was banned in some areas of the U.S. and Canada, and as a consequence few detergents sold in North America now contain phosphates. Some resort lakes, for example Okanagan Lake in British Columbia, surrounded by increasing development began developing problems with nuisance weed and algal growth. Stringent local requirements to limit nitrogen and phosphorus levels in wastewater effluent were implemented to control the growth and maintain water quality, requiring tertiary level wastewater treatment.

pH and Alkalinity

pH and Alkalinity are mainly a function of local geography and whether the local water source is groundwater or surface water, with surface water tending to be less mineralized. Although acid rain over much of the continent results in many surface drinking water sources having pH as low as 5 or less, wastewater tends to be roughly neutral or slightly basic pH. Industrial wastewater can have extreme acid or basic pH. In the event nitrification is required to eliminate effluent ammonia toxicity to fish, low alkalinity wastewater can result in low pH levels. Consequently, it is often advisable to denitrify to recover some of the alkalinity and energy used for nitrification. During denitrification, bacteria use nitrate and nitrite as an electron source, reducing the overall energy required for aeration.

Temperature

Temperature of wastewater in North America varies widely, with larger seasonal variations in Canada and the northern United States. Some low technology options which would otherwise be appropriate, such as wetlands, cannot be used in some northern areas because of freezing and thus only seasonal treatment. Biological nitrogen removal is also affected by low wastewater temperatures, although nitrification has been observed at plants with liquid temperatures as low as 4 oC. Higher temperatures in the south, which can be as high as 30 - 35 oC, result in year-round odour control issues.

Chlorides and sulphates

Wastewater chloride concentration may be high in communities with high groundwater tables, and saltwater intrusion into groundwater or near shore sewage pumping stations. Chlorides may also be high in communities with hard water, where individual water softeners are in common use. The presence of chlorides, in sufficiently high concentrations, can interfere with sewage treatment.

Sulphates formed in long, low slope sewers in warm climates have proven to be a corrosion problem in older plants with concrete tanks, and in older sewers. They can also contribute to severe odour problems. This condition is commonly controlled by injecting ferric chloride upstream of the odour release point to control hydrogen sulphide formation.

Toxic inorganics and priority pollutants

The EPA has identified a number of priority pollutants based on known or suspected carcinogenicity, mutagenicity, teratogenicity or high acute toxicity. Examples are heavy metals, halogenated organic compounds, and pesticides. The main controllable sources of such contaminants are industrial users of the sewer system. The implementation of Sewer Discharge Bylaws that levy charges for contaminant loading is used to reduce this loading, as the bylaw makes it more cost effective for industries to initiate source control measures and treat the wastewater before discharge to sewer.

Contaminant loading from residential and commercial sources is more difficult to control and regulate. When combined with low mineralization, low pH water tends to leach metals from the water distribution system, resulting in relatively high copper and iron loads at wastewater treatment plants. As well, there are still some homeowners and small industries who find it easier to dispose of hazardous wastes such as cleaning solutions to the sewer rather than pay for proper disposal or take the hazardous waste to a collection site. Public information and education programs have been used to reduce contaminants from residential and commercial sources by educating people as to the environmental effects of discharging materials such as latex paints and pesticides to the sewer.

Stormwater road runoff is another source of trace metals and petroleum products. Public education programs have been effectively used to reduce the discharge of toxic contaminants and hydrocarbons to storm drains. One of the more effective programs is a community initiative to paint fish symbols next to catchbasins to remind people that materials discharged to the storm drain affect fish.

Many contaminants, especially ionic metals, tend to become adsorbed to fine solid particles in the sewage and to bacteria in the waste treatment process, and become part of the waste sludge stream. In some cases, the contaminant concentrations in the sludge can be sufficiently high enough that they limit disposal options for the biosolids generated by the wastewater treatment process, and may even inhibit sludge digestion.

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