City of Saginaw MI -
Wet Weather Treatment
The City of Saginaw operates and maintains seven Retention Treatment Basins, five lift stations and twenty seven sewer regulators. The seven Retention and Treatment Basins (RTB's), Hancock, Weiss, Fitzhugh, Emerson, 14th Street, Webber and Salt/Fraser, can store a combined total of 60 million gallons of sewage and stormwater.
These basins, along with collection system modifications, prevent the discharge of any raw sewage to the river. The RTBs provide primary settling and disinfection. Overflow to the river of the treated effluent occurs only during severe and/or sustained rainstorms. Retained storm water and settled solids are sent to the Wastewater Plant for treatment.
Retention & Treatment Basins Process
Retention and Treatment Basins Locations
Saginaw Retention Treatment Basin & Sewerage System Description
The majority of the City of Saginaw's sewerage is received and carried in combined sewers, which includes sanitary and storm water in a common pipe. All collector sewers flow toward the River and all dry weather and light wet weather flows are intercepted at regulator chambers and directed into an interceptor sewer for gravity transport to the WWTP. During heavier wet weather flows, the regulator chambers divert excess combined flow to the Combined Sewer Overflow (CSO) Control Facilities, which include collector sewers, pumping stations and Retention and treatment Basins (RTB's).
The storage provided by the collector sewers and RTBs can capture smaller storm events. Combined flows from larger storm events are skimmed, settled, and disinfected in the RTBs prior to discharge to the Saginaw River. After a storm event has subsided, all stored flows are dewatered to the interceptor and receive treatment at the WWTP. The RTBs are then flushed and accumulated solids are dewatered to the interceptor. There are no untreated CSO discharges to the River. A map of the City's sewerage facilities is provided in Figure 2.
The Weiss, Emerson, and Salt-Fraser RTBs are designed for gravity flow. However, the Weiss and Emerson facilities can also receive pumped flow from the interceptor if necessary to relieve the system during flood conditions, thus maintaining the original system's design concept. The Webber and 14th Street RTBs utilize both gravity and pumped flow during operation. All flow to the Fitzhugh RTB is pumped during flow-through conditions.
The Hancock RTB was originally constructed in 1976 and operated as a solely pumped facility, but it was modified as part of the Phase B work to receive gravity flows. The facility can still receive pumped flow, if necessary, and operates similarly to the Weiss and Emerson facilities. All RTBs are designed to protect against flood stage either with backwater gates (Weiss, Emerson, Salt-Fraser, Hancock, and 14th Street) or by setting the effluent weir above record high water (Fitzhugh and Webber).
After an event, flow is stored in the collector sewers and all of the RTBs until capacity at the WWTP is available for dewatering. The entire system is typically dewatered in two to three days after the end of a runoff event. Dewatering is based on the available WWTP capacity and is controlled using meter and rate controllers (Weiss, Emerson, Hancock, and Salt-Fraser) or vortex valves (14th Street, Fitzhugh, and Webber).
Each RTB is equipped with an automated flushing system designed to flush sediment and debris from the tank bottom to the interceptor sewer. Weiss, an open RTB, and the Hancock RTB have high-pressure, system that pumps strained river water to a series of sequentially operated nozzle headers. The remaining RTBs all use city water in automatic, sequential pivoting flushing troughs to clean the basins.
The City has also implemented a catch basin restrictor plate program for its entire combined sewer system. The restrictor plates are used to restrict flow from entering the sewer system during heavy rainstorms. The restricted flow is temporarily stored in the streets and parking lots until the storm subsides and it can drain back into the system. This program has virtually eliminated all basement flooding in the City.
Retention & Treatment Basin Basis of Design
Two of the seven RTBs, Weiss and 14th Street, were designed for the total capture of an approximate 0.90 and 0.76-inch in one hour rainfall event, respectively. The two basins both receive gravity flow and pumped flow from their respective pumping stations. The pumped flow discharges to vortex separators (swirl chambers) designed for solids removal ahead of the RTBs. Weiss has one 34-foot diameter Vortex Solids Separator (VSS) and 14th Street has three 36-foot diameter VSS along with 3 MG of first flush storage. Both facilities have high-energy disinfection systems and were designed to meet the MDEQ's DAT for CSO control.
The remaining four new RTBs were designed to meet the 1989 NPDES permit requirement of "total storage for the retention treatment of the one-half inch, one hour rainfall event, two thirds of which volume will be provided for settling, skimming and disinfection." These facilities, along with the Hancock RTB, will be evaluated to determine the ability to meet the MDEQ demonstrative criteria.
Table II-1 provides a summary of pertinent systems data for the City's Phase A and B CSO Control Facilities:
|CSO Area||Service Area||Storage Provided (MG)
per 1000 acres
|2. 14th St.||1,200||
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