After the Flush
|
|
 |
 |
| |
 |
|
Safe disposal of our wastewater is a valued and complex, not-for-profit municipal service.
|
| |
 |
|
Wastewater collected from 77 southeast Michigan communities is transported to the Detroit WWTP.
|
| |
 |
|
The final effluent discharged from the Detroit WWTP meets state and federal treatment requirements for the protection of public health.
|
Sewage or wastewater is something we rarely think or talk about. The wastewater from our toilets, showers, washing machines and dishwashers quickly departs our homes through drains that lead to a network of sewer pipes where it becomes somebody else’s responsibility. Nobody wants to smell it, see it or worry about it – and usually we don’t have to.
Sewage collection and treatment is a complex and technically-sophisticated public service. It costs roughly twice as much to provide sewer service as water service because the treatment process is more expensive and there are more solids to dispose of. Understanding our sewer system, how it is impacted by wet weather and how it is operated and maintained provides a greater appreciation of the value of this service to public health and the environment. We all have a responsibility to protect our waterways.
From toilets to the wastewater treatment plant – protecting our waterways
It comes as a surprise to many that most US cities did not start treating their wastewater until the 1940s. As people moved to urban areas, the volume of wastewater increased and the problems of discharging without treatment became evident. Real progress came with the Clean Water Act in 1972 that required a higher level of pollutant removal in the treatment process and created a grant program to help upgrade wastewater treatment plants and sewer systems. The vast majority of plants in operation today discharge a cleaner effluent, or treated wastewater, as a result of treatment process improvements or expansions made with those grant funds.
In southeast Michigan, approximately three million residents and thousands of businesses send wastewater down their drains each day to a network of sewer pipes that lead to the Detroit Wastewater Treatment Plant (WWTP). During the most active day in 2010, the WWTP treated 1.68 billion gallons of wastewater helping to keep 77 communities operating smoothly. The collection and treatment of wastewater is a coordinated effort between suburban communities, the counties and the City of Detroit. Wastewater is collected in 28,500 miles of sewer pipe leading to three major interceptor pipes that transport flows directly into the WWTP.
Detroit has one of the largest, single-sited WWTPs in the US. When it was constructed in the 1930s, the engineers planning the system selected a site that could easily accommodate expansion and receive flows from a very large land area. Two large interceptors were constructed to transport flows to the site. As the population grew so did the plant and collection system. The third major interceptor was added in the 1970s to service areas northeast of Detroit.
Today, two enormous pumping stations lift wastewater flows from the interceptors into the treatment plant. Wastewater then flows through a 5-step treatment process to meet National Pollutant Discharge Elimination System (NPDES) permit requirements before being discharged to the Rouge and Detroit Rivers. This process removes harmful pollutants and disinfects flows to kill bacteria. Analytical testing is performed on final effluent prior to discharging to confirm it meets a variety of water quality standards including fecal coliform limits.
“The treatment process successively removes more solids in each step using physical, chemical and biological treatment methods to remove pollutants,” states Sam Smalley, Assistant Director of Wastewater Operations for the Detroit Water and Sewerage Department (DWSD). “On average, it takes about 8 hours to complete the treatment process and deliver a treated effluent that meets permit requirements.”
How the weather impacts sewer service
While flushing toilets and heavy rain storms are unrelated events, they both impact our sewer system. Combined sewers that transport both storm water and sewage in a single pipe make up 30% of the sewer system land area (or 26 of the 77 communities) that sends flows to the Detroit WWTP. These older sewers were constructed when development was much lower. During rain storms, combined sewers collect the storm water that runs off our streets and houses in addition to sewage flows, receiving up to three times the volume of flow that is normally transported on a dry day. This places an enormous burden on the collection and treatment system, forcing equipment to operate at higher capacities. If flows exceed the capacity of the system, combined sewer overflows, or CSOs, occur where excess flows are discharged through emergency outfalls without treatment to a lake or river to prevent basement flooding.
Many of the conventional sanitary sewers in the remaining 70% of the service area also receive additional flows during storms from footing drains that are connected to the sanitary sewer. Up until the 1970s, the footing drains on homes were connected to the sanitary sewer rather than to a sump in the basement. Like combined sewers, these “wet” sanitary sewers can become overloaded during storms requiring excess flows to be discharged at pump stations into lakes or rivers creating unwanted sanitary sewer overflows, or SSOs, to prevent basement flooding.
Thus, as sewer flow volume increases during storms, the need to manage and balance flows within the system becomes critical to avoid overloading the WWTP or to prevent sewer backups in people’s homes or sewer overflows into the rivers. Additional facilities have been constructed to address this. The collection and treatment process becomes more complex as storage basins and satellite treatment facilities begin operations to handle these additional storm water flows in the collection system and the WWTP ramps up to maximum capacity.
The sewer collection system tributary to the Detroit WWTP contains a variety of devices to control and manage variable flows. Pump stations lift flows to a higher elevation so they can continue to flow by gravity. Gates and inflatable dams are used in combined sewers to slow down and temporarily store wastewater in sewer pipes when the system is nearing capacity during heavy rains. Retention treatment basins, or RTBs, temporarily store, and when necessary, treat combined sewage from combined sewers. Screening and disinfection facilities treat combined sewage without storing it and discharge the effluent directly to a river. Outfalls located along the Rouge and Detroit Rivers are used as a last resort to discharge CSO and prevent sewer backups. Regulators, small structures in the sewer, modify the direction of flows toward outfalls when the sewer pipe is in danger of being overloaded. Equalization basins are used to temporarily store wastewater flows in separate systems.
During storms, the WWTP can literally have to work twice as hard. Capable of treating 930 million gallons per day during dry weather, the Detroit WWTP kicks into high gear and can treat up to 1.8 billion gallons per day during wet weather.
“When a storm is approaching, we will begin pumping more flow into the WWTP, drawing down the collection system to maximize storage capacity,” explains Smalley. “When the storm water gets in the pipes, we can see flows triple from .5 billion gallons to 1.7 billion gallons per day. We have to modify our operations quickly to handle the increased flow. It can take 3 to 4 days to dewater the sewers down to dry weather levels so the plant has to keep operating at full speed even when it quits raining. Wet weather can be extremely challenging to manage.”
Article continued on page 2
|