1-year, 1-hour storm - The intensity of a storm that statistically occurs at least once every year is called the 1-year, 24-hour storm. In Michigan, this storm produces 2.2 inches of rain. The 1-year, 1-hour storm is the hour during that storm with the heaviest rainfall. In Michigan, this would equal about 1 inch of rain.
10-year, 1-hour storm - The intensity of a storm that statistically occurs at least once every ten years for a 24-hour period. In Michigan, this storm produces about 3.6 inches of rain in a 24-hour period. The 10-year, 1-hour storm is the hour during that storm with the heaviest rainfall. In Michigan, this would equal about 1.8 inches of rain.
Acid rain - Rain with a pH of less than 5.6 that has mixed with sulfur and nitrogen oxides in the atmosphere as a result of burning fossil fuel. Acid rain can damage buildings, wildlife and aquatic life.
Best Management Practice (BMP) - A practice or combination of relatively low cost practices that prevents or controls the discharge of pollutants to receiving waters.
Biochemical Oxygen Demand (BOD) - A laboratory measurement of wastewater that is one of the main indicators of the quantity of pollutants present. BOD measures the amount of oxygen that will be consumed by microorganisms when oxygen in wastewater biologically reacts with organic material in the wastewater. A decrease in BOD indicates that water quality is improving.
Bioengineering - The science that uses living plant materials as a main structural component to control erosion, sedimentation, and flooding. Also referred to as soil bioengineering, it is used for land stabilization and habitat restoration.
Catch basin - A structure designed to remove debris from storm water runoff that is collected from streets. A catch basin includes a small, underground storage area to remove sediment and a cover with 8 to 36 holes to prevent sticks and debris from entering the storm sewer.
Clean Water Act (CWA) - A federal law that dates back to 1948, the 1972 CWA amendments set the basic structure for regulating discharges of pollutants to waters in the United States. The law gave the EPA authority to set effluent standards on an industry basis and continued the requirements to set water quality standards for all contaminants in surface waters. The CWA makes it unlawful to discharge any pollutant into a navigable water unless a NPDES permit is obtained under the Act.
Cistern - An underground tank or pipe to collect storm water runoff from catch basins prior to discharge into sewer systems. Cisterns are used to store and slowly release storm water from residential areas into the combined sewer system until the threat of CSOs has passed.
Collection system - A network of sewer pipes used to collect wastewater and/or storm water and transport it to a wastewater treatment plant or sewer outfall.
Combined sewer - A sewer that carries both wastewater and storm water. During dry weather, all flows are sent to a WWTP. During wet weather, the sewers fill and flows that do not reach a WWTP overflow to nearby streams or rivers. Combined sewers are found primarily in older, urban systems in the northeast and upper Midwest of the United States. Combined sewer systems were the primary type of sewer system constructed prior to the 1950s.
Combined sewer overflow (CSO) - The overflow of a mixture of wastewater and storm water into a river when heavy rainfalls overload a combined sewer. CSOs pose a health and safety hazard.
Commingling - The mixing of flows from two different sources. An example would be discharging sanitary sewer flows into a combined sewer system.
Cubic feet per second (cfs) - A measurement of flow rate. It represents the number of cubic feet of volume passing by a stationary point in one second.
Decanting/Dewatering - The process of draining or removing water from a storage structure like a basin or tunnel after a storm.
Demonstrative approach - An alternative treatment and control approach proposed by a community to meet regulatory requirements for CSO or SSO control. Under this approach, the community tests the completed facility to demonstrate that it is achieving required treatment levels. If it cannot meet requirements, improvements need to be made to achieve the required level of treatment. Demonstrative approaches are undertaken to save money. Detroit used a demonstrative approach to shave $2-3 billion off original cost estimates to control CSOs based on MDEQ’s presumptive approach for adequate treatment.
Design storm capture - The size of storm a collection and treatment system has been designed to capture and hold. Design storm sizes are expressed in terms of their frequency of occurrence.
Detention - The temporary storage of storm water runoff to control peak discharge rates and provide gravity settling of pollutants.
Detroit Water and Sewerage Department (DWSD) - A City of Detroit Department that provides water and sewer service for the southeast Michigan area. Sewage service is provided to three million people in 78 communities. DWSD operates and maintains 3,500 miles of sewer that carry rain water and wastewater.
Diffuser -A porous tube or other device that air is forced through and divided into very small bubbles for interaction with organic pollutants found in liquids.
Discharge - Treated and untreated water released from an outfall into a surface water. Discharges classified as intermittent include CSO, SSO and storm water. Continuous discharges include treated effluent from WWTPs.
Disinfection - Disinfection devices are used in CSO basins and screening facilities to kill pathogens in the discharge. Current practice is disinfection by means of chlorination using sodium hypochlorite at a low concentration (similar to laundry bleach).
Dissolved oxygen - The oxygen freely available in water, vital to fish and other aquatic life and for the prevention of odors. Dissolved oxygen (DO) levels are considered the most important indicator of a water body’s ability to support desirable aquatic life. An increasing DO means water quality is improving. Secondary and advanced waste treatment are generally designed to ensure adequate DO in waste-receiving waters.
Downspout disconnection - Downspouts are connected into many combined sewer systems increasing the amount of rain water that gets into a system. Downspouts are disconnected at ground level to divert rain water onto lawns where it will filter into the ground. This requires capping the existing outlet pipe, installing a concrete splash pad at the downspout outlet and diverting the flow away from the home or building.
Dry weather flow - Flow in a combined or sanitary sewer that is not influenced by a rain storm or snowmelt.
Effluent - The treated discharge from wastewater treatment and manufacturing plants discharged into a surface or ground water. NPDES permits outline the water quality requirements of effluent.
Enclosed storm drainage system - A system of buried sewer pipes to collect and transport storm water to an outlet on a river, stream or lake. Most urban areas have enclosed storm drainage systems.
EPA - The United States Environmental Protection Agency is the federal regulator responsible for administering the Clean Water Act.
First flush - During the initial part of a storm, rainfall washes accumulated materials (grit, paper, oil, salt, lawn chemicals) off impervious surfaces. This first flush of storm water runoff contains the highest level of pollutants. If the runoff discharges directly to lakes and streams it can cause a shock load. As a minimum, basins are generally capable of capturing the first flush.
First Tier Customers - Wastewater customers that have direct contracts with DWSD. There are 15 First Tier Customers including Wayne, Oakland and Macomb Counties.
Floatables - Materials found in sewers and storage tanks that are lighter than water.
Flushing gates - Devices (gates) in a sewer or tank that can store flow. The flow is then released to flush sediments deposited in sewer and tank sections below the gates.
Footing drain - Drain tiles around a home’s foundation that collect water from around the home and prevent it from leaking into the basement.
Footing drain disconnection - The removal of storm water footing drain flow from a combined or sanitary sewer system. This typically requires the installation of a sump pump to direct flows onto a lawn or into a nearby storm sewer system.
Heavy metals - Metals that can be precipitated by hydrogen sulfide in acid solution, including lead, silver, gold, mercury, bismuth, and copper. Heavy metals are considered harmful to humans when ingested.
Hydraulics - The branch of engineering that deals with water or other fluid in motion.
Hydraulic modeling - Development of a computer model to represent the flow of wastewater in a collection system to determine how the system will react under different flow conditions.
Illicit connection - An illegal connection of a sanitary sewer into a storm sewer that allows human waste to go directly into streams and rivers. Illegal connections also include illegal storm water connections to sanitary systems, such as sump pumps that homeowners have connected into the sanitary sewer system instead of discharging into their yard or storm sewer.
Impervious areas - Surfaces that cannot absorb rain water including streets, sidewalks, roofs, parking lots and driveways. The larger the impervious area, the greater the runoff volume.
Infiltration - The absorption of water into the ground, expressed in terms of inches per hour. It is also the penetration of water from the soil into sewer or other pipes through defective joints, connections, or manhole walls.
Inflow - The discharge of storm water into a sanitary sewer system through footing drains, sump pumps, inappropriate catch basin connections, leaking manhole covers or other sources.
Influent - Flows into a treatment facility, storage facility, or sewer system. Influent characteristics, flow rates and volume are used to determine the size and treatment requirements of a facility.
In-system storage - The use of existing sewer pipes and structures to store excess flows during a wet weather event.
Interceptor - A large sewer that collects flow from a number of trunk sewers and transports the flow to the WWTP. These sewers do not connect to homes, buildings or streets. Detroit’s system drains to three main interceptors: the Detroit River Interceptor (DRI), the North Interceptor - East Arm (NI-EA) and the Oakwood Northwest Interceptor (ONWI).
Lateral sewer - A sewer that collects flows from homes and businesses for discharge into trunk sewers. There are thousands of lateral sewers throughout the collection system tributary to Detroit.
Long Term Control Plan (LTCP) - A plan that outlines a program to control combined sewer overflows through a variety of ways, such as sewer separation, construction of facilities that treat overflows, storage facilities to store them until they can be sent to a WWTP, etc.
Manhole - A structure designed to provide access to a sewer system for cleaning and other maintenance activities.
Michigan Department of Environmental Quality (MDEQ) - Formerly known as the Michigan Department of Natural Resources, the MDEQ has regulatory oversight and issues all NPDES permits in the state of Michigan.
MDEQ presumptive level of control - For CSO control, this requires complete capture of the 1-year, 1-hour storm and screening, settling and 30 minutes of disinfection contact time for discharge of the 10-year, 1-hour storm.
Million gallons per day (mgd) - A measurement of flow. It represents the number of million gallon increments of volume passing by a stationary point in a 24-hour period.
Mitigation - An activity or project to reduce the impact of a pollutant or replace lost land features such as wetlands and surface water.
National Pollutant Discharge Elimination System (NPDES) - A provision of the Clean Water Act that prohibits discharge of pollutants into waters of the United States unless a special permit is issued by EPA, a state, or, where delegated, a tribal government on an Indian reservation.
Negative treatment - The remixing of settled solids in a basin that creates an effluent that is more polluted than the influent.
Non-point source - Sources of pollution that cannot always be traced to an exact point of entry. Non-point sources of pollution include land runoff that goes directly in the river, illicit sanitary sewer connections to a storm sewer and streambank erosion.
Nutrient - An element or compound such as nitrogen, phosphorus or potassium that is necessary for plant growth. Fertilizers contain nutrients.
Open drainage system - A system of ditches and open channels that collect and transport storm water to an outlet on a river or stream. Open drainage systems are typically found in rural and industrial areas.
Organic material - Material derived from organic or living things.
Outfall - The sewer pipe where a combined sewer or storm sewer discharges into a lake or river.
Overland flow - The flow of storm water across the land surface that ultimately reaches a stream, river or lake.
Oxbow - A loop formed by a horseshoe-shaped bend in a river.
Pervious surfaces - Surfaces that are permeable and absorb storm water. Grass is a pervious surface.
Pilot project - A project conducted on a small scale to demonstrate the effectiveness of an approach, such as to treat wet weather pollution or limit storm water from getting into a collection system.
Point source - Discharges from stationary locations such as WWTPs, storm sewer outlets and factories. Point source discharges include combined sewer outfalls and storm sewer outfalls that are regulated by the MDEQ.
Primary treatment - The first stage of wastewater treatment that removes settleable or floating solids only. Generally, 40% of suspended solids are removed and 30-40% of the BOD is removed in the wastewater.
Pumping station - A structure containing pumps and the associated piping, valves and other mechanical and electrical equipment for pumping wastewater, storm water or combined sewage. Also called a lift station when it is used to lift flows from a low point to a higher pipe so that it can be transported by gravity.
Regulator - A device installed in combined systems to control the amount of flow into the sewer system during periods of wet weather. Excess flows are routed to an outfall.
Relief sewer - Sewers constructed to relieve capacity deficiencies on existing sewer systems.
Retention Treatment Basin - A concrete tank to capture and treat overflows from a combined sewer system. RTB's are typically about 30 feet deep, covered, and buried 3 to 5 feet below grade. The captured flow is returned to the interceptor sewer when there is sufficient capacity to transport it to the WWTP. Basins are equipped with screening and disinfection devices.
Sanitary sewer - A sewer that carries wastewater only.
Sanitary sewer overflow (SSO) - The discharge of untreated sanitary sewage into a waterway as a result of operational problems, undersized pipes, heavy inflow and infiltration or sewer pipe breaks, blockages or failures. Excessive inflow from footing drain connections are believed to be responsible for much of the SSO problem in southeast Michigan.
Screening devices - Devices, such as bar screens, used in basins and other screening facilities to remove larger solids and floatables. In the case of basins, the screening devices remove material about ?-inch in diameter and larger.
Scum baffle - A plate that extends below the surface of wastewater in a tank to prevent floating matter from passing through the tank.
Second Tier Customers - Customers within the DWSD system who contract for wastewater disposal from Wayne, Oakland and Macomb Counties. There are 52 Second Tier Customers in the Detroit wastewater system.
Secondary treatment standards - Minimum requirements of the Clean Water Act to remove 85% of the BOD and total suspended solids in wastewater. Secondary treatment normally uses biological treatment processes followed by settling tanks.
Sedimentation - See Settling.
Septic system - A domestic wastewater treatment system that treats household waste through a septic tank and a soil absorption system. Bacteria decomposes the waste, sludge settles to the bottom of the tank, and treated effluent flows out into the ground through drainage pipes. Failing septic systems can contaminate ditches, creeks and shallow drinking water supplies.
Settling - The process of subsidence and deposition of suspended matter carried by water, wastewater, or other liquids. It is usually accomplished by reducing the velocity of the liquid below the point that it can transport the suspended material. Also called sedimentation.
Sewer separation - Replacing a combined sewer with a separate sanitary sewer pipe and a storm sewer pipe. The sanitary sewer pipe flow is transported to a wastewater treatment plant and storm sewer flow is discharged directly to a drain or river, without treatment.
Sewer siphon - An inverted siphon that transports sewer flows under a river or stream, eliminating the need for pumping.
Shunt channel - A channel used to route flows around a storage basin when flow rates result in negative treatment.
Skimming - The removal of floatables from combined or sanitary sewage.
Snowmelt - Runoff created when snow melts and the resulting water enters sewer systems.
Sodium hypochlorite - A water solution of sodium hydroxide and chlorine where sodium hypochlorite is the essential ingredient. It is similar to laundry bleach and is used as a disinfectant in CSO basins.
State Revolving Fund (SRF) - A federal and state program offering low interest loans to municipalities for the construction of publicly-owned water pollution control facilities.
Storm sewer - A system of sewer pipes that carry only storm water runoff from buildings and land surfaces.
Storm water runoff - Water that runs off streets, roofs and land during rain storms, washing pollutants off these surfaces into the sewer system. Typical pollutants of storm water runoff include chlorides, coliform bacteria, heavy metals, nutrients, oil and grease, and suspended solids.
Stream bank erosion - The movement of sediment and soil material from the banks and bottom within a stream or river. The higher the flow, the greater the erosion.
Subwatershed - A drainage area within a watershed.
Suspended solids - Solid organic or inorganic particles physically held in suspension in wastewater by agitation or flow.
Swirl concentrator -A treatment device that uses centrifugal force to remove pollutants from wastewater.
Trunk sewer - A sewer that receives flow from many lateral and other trunk sewers, serving a large territory. Trunk sewers feed into interceptors.
Tunnel - A large, deep, underground pipe used to store and transport wastewater or combined sewage during rain storms. Tunnels typically have higher storage capacities than basins and are capable of transporting flows directly to a wastewater treatment plant or interceptor.
Wastewater Treatment Plant (WWTP) - A facility that treats wastewater, industrial waste and sludge. Detroit’s WWTP uses three main processes: primary treatment, secondary treatment and disinfection. More than 90% of all incoming pollutants are removed through treatment. This exceeds the compliance standards set by federal and state regulators.
Watershed - The complete area or region draining into a river, river system, or body of water.
Wet sanitary system - A sanitary sewer system that experiences a higher volume of flow during wet weather due to storm water inflow and infiltration.
Wet weather - Weather that creates precipitation including rain, snow, sleet and hail.
Wet weather pollution - Pollution that occurs as the result of storm water entering a sewer system or running off impervious surfaces. Types of wet weather pollution include CSO, SSO and storm water runoff.
Wetland - An area that periodically has water logged soils or is covered with a shallow layer of water resulting in reduced soil conditions. A wetland area typically supports plant life adapted to wet environments.