Treatment of wet weather flows to prevent combined sewer overflows (CSOs) has evolved over the past two decades as operators, engineers, scientists and regulators continue to build upon past experience and refine the capture and treatment process.
The Detroit Water and Sewerage Department (DWSD) recently brought its ninth CSO treatment facility online increasing the volume of wet weather flows that are captured and treated each year. The Oakwood Pumping Station and CSO Treatment Facility includes a new sanitary and storm water pumping station to replace an older, smaller pumping station, and a retention treatment basin (RTB) to treat and store combined sewage.
Located near the Rouge River and I-75, the Oakwood Sewer District has historically experienced frequent basement flooding and discharged untreated CSO to the O’Brien Drain that leads to the Rouge River. This new facility triples pumping capacity and provides a 9-million-gallon treatment and storage basin to increase the level of service. The pumping station has four sanitary pumps that are used 24 hours a day to pump the district’s sanitary sewage to the Detroit Wastewater Treatment Plant (WWTP). During heavy rain events, the eight storm water pumps lift increased flows from the combined sewers into the RTB.
Like other RTBs, the facility was designed to treat anticipated pollutant loadings from the tributary area and protect the quality of the receiving water. “Strict design criteria were followed with the intent to meet water quality standards,” explains Phil Argiroff, Permit Section Chief of the Water Resources Division at the Michigan Department of Environmental Quality (MDEQ). “Because the Oakwood RTB is in an industrial area where oil sometimes accumulates in the sewer system, some additional features were incorporated into the design. Baffles were included to trap oil in the RTB and permanent attachments were constructed in the O’Brien Drain for booms to trap oil. Combined sewage is treated and an oil concern along the Rouge River is addressed.”
During the design process, features were also identified to help operators better manage the treatment process based on experience gained at other DWSD facilities. Automation was increased where feasible and the use of mechanical equipment like mixers and pumps was reduced. The result is an above-ground structure that dewaters by gravity and features the latest technology. The performance of the facility and these enhancements will be monitored by DWSD during the first year of operation and reported to MDEQ.
“The Oakwood RTB includes a high level of automation to assist the operator in decision making during an event,” explains Kerry Rudolph, DWSD CSO Supervisor. “Having the ability to monitor the flow rate, and control pumps, gates and screens from a single computer work station allows the operator to react to changing conditions and quickly make informed decisions. Sampling and testing flow for water quality indicators is still a manual task so the easier it is to make adjustments, the better the process works.”
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| Charni Calvert, Wastewater Technician, checks the oil level sensor as part of her daily routine to make sure the large pumps are ready to go when a storm hits. |
Sodium hypochlorite is injected into the flow as it is pumped into the RTB eliminating the need for mixers to distribute the disinfectant in the flow. The black panel to the right controls the chlorine feed rate into the pump as the flow begins its ascent into the RTB. |
As flow enters the Oakwood RTB, sodium hypochlorite (NaOCl) is injected through the pumps uniformly dispersing the disinfectant through the flow. This eliminates the need for mixers to circulate flow in the basin, reducing the amount of mechanical equipment the operator must maintain and repair if problems arise. Flow is then sent through a series of fine screens to remove debris including leaves, twigs and sanitary trash. The screens are equipped with larger motors than used in previous facilities and can be bypassed if they become clogged. Debris collected from the screens is transported through an enclosed conveyor to a grinder that delivers the ground screenings to a dumpster for transport to a landfill.
With debris removed, the flow is then diverted to one of two basin compartments selected by the operator. Like other DWSD RTBs, one compartment is used as a first flush capture tank to store flow from the first part of the storm that is known to contain the highest level of pollutants. Once this compartment is filled, flow is directed to the second compartment for storage and potential discharge. Any remaining solids settle out in the compartments and adequate contact time is provided for the NaOCl to kill bacteria. If the storm generates more than 9 million gallons of runoff, the treated effluent begins discharging to the O’Brien Drain, and into the Rouge River. The effluent is sampled and tested for a variety of parameters and to confirm fecal counts are within permit limits.
After the storm is over and capacity is available in the sewer system, stored flows in the basin compartments are sent to the Detroit WWTP and potable water is flushed through the basin compartments to clean them. Because the Oakwood RTB is above ground, flows can be dewatered or drained from the RTB by gravity eliminating the need for dewatering pumps and their yearly maintenance cost to remove grit.
Another feature to help the operators included locating sample pumps close to the floor so it is easier to collect required samples when the RTB is in operation. Operators need to collect influent samples every 2 hours during the first 8 hours of operation and then every 4 hours after that. Most operators sample more frequently than this in the beginning of an event to confirm adequate disinfection is occurring and residual chlorine is not too high. Effluent samples are taken on the same schedule if discharge is necessary.
While new facilities like the Oakwood RTB continue to incorporate technology and features to support the treatment process, operator judgment remains critical to successful outcomes. Kerry Rudolph emphasizes the need for his operators to be flexible problem solvers. “Every storm is different in terms of pollutant loadings and duration, and each one of our CSO treatment facilities operates a little differently during the treatment process. The treatment process moves a lot faster than traditional wastewater treatment – adjustments need to be made quickly. Closely managing the treatment process to meet permit requirements is our number one priority.”
