Pollution Control: Wastewater

Ever wonder what happens to water you flush down the toilet or the rain that runs down the street? Most people do not. However, keeping homes and streets from flooding and sewage from polluting land and waterways takes more than just knowing that water flows downhill. It takes sophisticated engineering skill and expertise to transport, treat and dispose of millions of gallons of wastewater each day.

What is Wastewater

Wastewater is the flow of used water from a community. The characteristics of the wastewater discharges will vary from location to location depending upon the population and industrial sector served, land uses, groundwater levels, and degree of separation between storm water and sanitary wastes. Domestic wastewater includes typical wastes from the kitchen, bathroom, and laundry, as well as any other wastes that people may accidentally or intentionally pour down the drain. Sanitary wastewater consists of domestic wastewater as well as those discharged from commercial, institutional, and similar facilities. Industrial wastes will be as varied as the industries that generate the wastes. The quantities of storm water that combines with the domestic wastewater will vary with the degree of separation that exists between the storm sewers and the sanitary sewers. Most new sewerage systems are separate, collect sanitary wastewater and storm wastes, whereas older combined systems collect both sanitary wastewater and storm water.

Stormwater and Combined Wastewater

Stormwater runoff is precipitation that finds its way across and surfaces into receiving waters. Urban storm runoff is collected and transported in storm or combined sewers. Storm sewers carry storm water only; combined sewers also carry sanitary wastewater.

Combined wastewater in dry weather is composed of sanitary wastewater. Since the pipes are sized to carry high flows in wet weather, in dry weather they may allow solids to settle out. In wet weather combined wastewater composition at any time depends on the extent to which sanitary wastewater is diluted by stormwater, and is augmented by contaminants in stormwater and in the solids deposited in dry weather and scoured in wet weather flows.

The drains (catch basins) along the curbs of our city streets are connected by sewer pipes directly to creeks, rivers and Lakes. Whatever goes down the catch basins will flow directly out into the natural environment. Every property in the city contributes to this stormwater flow.

Types of Sewers within the City

There are (3) main types of sewers:

_55126746_wastewateradjSanitary Sewers – Sanitary sewers transport wastewater that we release from a drain, toilet, sink or appliance such as a clothes or dishwasher. This wastewater from residences and businesses flows to treatment plants where it is cleaned before being released into Lakes or Rivers.
Storm Sewers – Storm sewers capture rainwater or snowmelt from residential and commercial properties. This water flows into nearby watercourses or the lake. Watercourses include creeks, streams and rivers – natural, concrete channels or underground pipes – that carry water, including storm water and snow melt from catch basins into Lakes.
Combined Sewers – If your home is located in one of the city’s older areas, the sewer serving your property may be a combined sewer. In a combined sewer, there is only one pipe, which carries both sanitary and storm drainage. During dry weather, combined sewers carry all contents to treatment plants. However, during periods of the heavy rain or snow melting there are occasions when the combined storm water and sewage flow is greater than the combined system can handle. In this case, rather than allow the flow to flood streets or back-up into basements, the flow is released into the river or lake, being treated with only chlorination. These occasions have been reduced and will continue to improve.

What is stormwater pollution?

Most of our rainwater travels through gutters, storm drains, channels, washes and eventually into the major source of our drinking water. The largest source of stormwater pollution in USA results from every day activities.  The most common pollutants are:

  • Trash (fast-food wrappers, cigarette butts, Styrofoam cups, etc.)
  • Contaminants (used motor oil, antifreeze, fertilizer, pesticides, sewage overflow, pet droppings, etc.)

These pollutants are picked up as water (from rain, hoses, sprinklers, etc.) drains from streets, parking lots, and lawns and enters the thousands of catch basins throughout the USA. From there, these pollutants flow through a massive system of pipes and open channels straight into Lakes or Rivers. Anything dumped or dropped on the ground or in the gutter may contribute to stormwater pollution

Combined Sewer Overflows – What is The Problem?


As we mentioned above, it is often difficult to determine any one thing that is causing water pollution. Environmentalist have made great strides over the past 50 years however we are continuously learning more about the impact of our actions on our natural environment and what we need to do about it. During heavy rainstorms and rapid snowmelt, extra flow from stormwater runoff into these combined sewers is greater than the interceptor pipes and treatment plant can accommodate. At these times the combined wastewater (including the stormwater runoff) overtop flow regulator structures, resulting in combined sewer overflows to a Lake and River.

When CSOs occur, they discharge untreated sanitary wastewater and runoff from rainfall and snowmelt to the Lake and River. The combination of raw sewage and stormwater can carry a variety of human bacteria and viruses. In addition, combined sewer overflows contain a variety of chemicals, oils and other wastes. Although the untreated overflow is typically diluted by rain and river water, it still poses a potential health and environmental hazard. Those most likely to be affected by these overflows include people involved in water contact sports (i.e., boaters, swimmers, people who fish, etc.).