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4.1 Wastewater characteristics (Topic a)

4.1.1 Cultural and technical practices

In general, human waste in North America is collected using water as the transport medium. High water use flush toilets, showers, appliances (e.g. clothes washing machines), dishwashers, and garburators result in relatively dilute wastewater in comparison to that generated in other parts of the world. To save water and wastewater treatment costs, as well as to save water in arid areas, there is now a growing trend towards the legislated use of water saving devices such as water meters and low flow toilets, shower heads and other fixtures, especially in new developments, which can reduce wastewater flows up to 30 percent.

Dry washing after urination and defecation using toilet paper (which is disposed of to the wastewater stream) is standard practice and is the cause of the relatively high amounts of cellulose fiber found in North American wastewater.

Most large cities, except those where the bulk of development occurred in the latter half of the twentieth century, have combined stormwater and wastewater sewers which affect wastewater collection and treatment. Since the use of the automobile is widespread, runoff from large paving areas often contributes to wastewater flow in urban areas. Trace contaminants from automobile exhaust, oil and fuel, and tires then become part of the wastewater stream.

Wastewater from large industries is generally treated on-site and then discharged into nearby water courses. Smaller industries, especially when located within or very near to a city boundary, may discharge into municipal sewers and contribute to municipal wastewater flows. In such cases, pre-treatment is generally required to adjust pH, reduce corrosiveness, reduce high solids content (to prevent obstruction of flow), reduce temperatures to below 40 degrees C, and prevent discharge of flammable materials that could create a fire and explosion hazard. Volatile organic compounds are also now largely regulated due to concern for air pollution, and the health of collection system and treatment plant workers. The release of volatile and toxic contaminants, and high strength conventional contaminants is often controlled through the use of Sewer Discharge Bylaws which restricts such discharges or applies heavy charges to discourage them.

4.1.2 Wastewater flows

Domestic wastewater

Domestic wastewater is generated by: dwellings, commercial facilities (e.g. stores, office buildings, airports, hotels, laundries, restaurants and shopping centres), institutions such as hospitals, prisons, rest homes and schools, and recreational facilities (e.g. pools, camps, resorts, golf clubs and parks). It is that part of the waste stream resulting from washing, bathing, culinary use, and human waste disposal.

There are generally two peaks in wastewater flow each day corresponding to sewage generated in the early morning as people get ready for the day, and early evening when they return home. A mid-day peak in wastewater flow may also occur. The number of peaks often depends on the size of the sewage collection system and the time it takes for sewage to reach the treatment facility. Lowest wastewater flows typically occur in the very late night to very early morning hours. Seasonal flow variations also occur due to inflow and infiltration of stormwater and groundwater into the sewer or due to changes in population (i.e. tourist and resort communities such as Banff, Alberta, and Aspen, Colorado, and in small communities serving colleges or universities).

The amount of domestic wastewater generated from residential dwellings ranges from 100 to 600 litres/capita/day with a typical average value of 260 litres/capita/day. Lower per capita volumes typically occur in areas with higher population densities, and areas with water use restrictions and controls. Areas with unreliable or limited water resources such as resorts with individual holding tank systems, also have characteristically low per capita sewage volumes. Higher per capita volumes are generated by lower density high cost housing with reliable water supplies, which tend to make heavy use of water using luxury appliances. Geography, such as water conservation in arid regions, does affect overall wastewater generation rates, but property value is a better indicator of expected per capita wastewater flow in individual cases.

Industrial wastewater

Large industries such as pulp mills, refineries, and power plants tend to treat (and often dispose of) their wastewater separately. This may be due to being situated in a remote location, or the prohibition of discharge to the municipal sewers due to the adverse impacts upon wastewater treatment systems of the associated high flows, high organic loads, high temperatures, acid or caustic solutions, or the presence of metals and other toxic contaminants. Smaller industries such as metal plating operations, or food processing industries that are located in urban areas often may discharge to municipal sewers. Flows from such sources may be highly variable, including batch operations and seasonal operations. Peak flows can be strongly affected in small communities with large industrial contributors. Many municipalities levy charges based on flow and contaminant loading from industrial users, which gives these industries economic incentives to investigate options for internal reuse and recycling of process water.

Infiltration

Sewers built in North America prior to 1970, using mortar or mastic joining materials, are particularly susceptible to infiltration from leaking joints, cracks and breaks in the collection system. Newer plastic and cast iron piping with elastomer gaskets have greatly reduced the amount of infiltration. Infiltration rates can thus range from 93 to 92,590 L/d/cm/km, with the higher rates usually in areas with high groundwater levels (Metcalf and Eddy Inc. 1991). An infiltration rate of 185 L/d/cm/km is considered to be reasonably acceptable for new systems.

Inflow

Inflow resulting from direct connection to storm drains, roof leaders, foundation and basement drains, and leaking manhole covers, is highly seasonal and geographical. In areas with combined sewer systems, all such flow is directed to the treatment plant, resulting in higher peak flows than expected from sanitary sewage flow alone. Wet to dry ratios in combined sewers (sewage to stormwater) range from 1:1 to 8:1, with median 4:1 (Metcalf and Eddy Inc. 1991). Although newer systems are separate, illegal connection of roof leaders, foundation and basement drains is common in small communities where building inspection may not be rigorous. A typical per capita allowance for infiltration and inflow in a separate sewer is about 150 L/capita/d .

A common design tool in use in North America is the Storm Water Management Model (SWMM), developed in 1971 and updated since by the U.S. Environmental Protection Agency (Athens, Georgia) [http://www.ccee.orst.edu/swmm]. This and other computer models are in general use, and require calibration with the system being modelled.

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