Innovative Technologies of Detroit’s Large CSO Control Facility
- M. Rabbaig, Detroit Water and Sewerage Department
- K. A. Al-Omari, Hazen and Sawyer
- A. J. Varas, Hazen and Sawyer
- C. D. Courter, Hazen and Sawyer
The Detroit Water and Sewerage Department (DWSD) developed a Long-Term Combined Sewer Overflow (CSO) Control Plan (Plan). The Plan addresses control of discharges from CSO outfalls to the Detroit River and the Rouge River. One component of the Plan is the Conner Creek CSO Control Facility, which discharges to Conner Creek before entering the Detroit River.
The $187 million, 30-million-gallon Conner Creek CSO screening and disinfection facility, the largest in Michigan, is designed with five minutes of contact time at the ten-year, one-hour peak flow of 13,262 cubic feet per second (cfs). In lieu of following the State of Michigan Department of Environmental Quality’s (MDEQ’s) presumptive criteria of 30 minutes of retention time for the disinfection of CSOs for the 10-year, 1-hour storm, DWSD negotiated with MDEQ to study, design, and construct a pilot CSO treatment facility to achieve daily and monthly fecal coliform limits of 400 and 200 colony forming units per 100 milliliters (cfu/100 ml), respectively, with five minutes of contact time at the afore-mentioned peak flow.
The retention basin, which is partially constructed in Connor Creek, is approximately 250 feet wide by 550 feet long by 40 feet high with four parallel compartments, each measuring 57.5 feet wide. The nominal water depth is 30 feet. Upstream of the basin, there is a 250 feet wide by 174 feet long screening, disinfection and flow distribution facility. Settling, skimming and high rate disinfection is required to meet water quality standards. The major process systems of the Conner Creek CSO Control Facility include: mechanically cleaned bar screens, sodium hypochlorite storage and feed system, high-rate induction-type mixers, flushing gates, dewatering, and odor control. Construction of the CSO facility began in March 2001 and is scheduled for completion in December 2004.
The objective of this paper is to present the innovative technologies implemented with this project. The paper will focus on the following:
• 5-minute contact time,
• Basin flushing,
• Re-use of existing infrastructure,
• Storage Optimization/Effluent Launders,
• Swirl Concentrators.
5-Minute Contact Time
To properly design the CSO disinfection system, an extensive study program was developed to determine the appropriate design criteria for storage, feeding, and mixing of the disinfectant to achieve the effluent fecal coliform limits. This study program included the following distinct components:
• Disinfection Study – The purpose of the disinfection study was to evaluate bacterial reduction at various combinations of disinfectant dose and contact time.
• Degradation Study – The purpose of the degradation study was to evaluate sodium hypochlorite degradation at various starting concentrations and to calibrate a sodium hypochlorite degradation model.
• Feed Control Study – The purpose of the feed control study was to evaluate the ability of the proposed feed control system to achieve the required turndown and to calibrate a hydraulic model of the system.
• Mixing Study – The purpose of the mixing study was to evaluate various mixing technologies and to optimize the sizing of the selected mixing technology.
Flushing gates were selected after a thorough evaluation and site visits were conducted, based on their cost-effectiveness, operational simplicity, and low maintenance requirements. Three technologies were considered: tipping buckets, flushing nozzles, and flushing gates. The flushing gates used retained CSO to provide significant energy via wave action to scour the retained solids. The flushing gates do not have bearings, which require greasing, and are operated by a low-pressure hydraulic system.
Other area CSO basins currently utilize flushing nozzles or tipping buckets. The majority use some form of flushing nozzles. Some basins incorporated submersible mixers in the basins to augment the flushing nozzles by stirring up the settled solids during dewatering. In all cases, the flushing nozzle systems proved inadequate in cleaning the basins. Tipping buckets are a good technology, however they did not lend themselves to the depth of the Conner Facility. Additionally, the layout and use of City water made the life cycle cost prohibitive.
Re-use of Existing Infrastructure
Dewatering, inspection, rehabilitation, modification, and utilization of existing CSO conveyance structures including outfalls that were built in the 1920’s. Some of the modifications included the construction of a passive pressure relief and underdrain system alongside the structures to improve the structural capacity to resist uplift forces, the construction of overlay slabs to resist buoyancy, and raising the discharge weirs at the pump stations due to changed hydraulic conditions. The existing conveyance structures provide an additional 32 mg of CSO storage. Reusing the exiting infrastructure included the Conner Creek widening from 150 to 200 feet to improve hydraulics along the approximate length of 0.5 mile before entering the Detroit River.
Storage Optimization/Effluent Launders
To maximize the storage in the basin and conveyance structures, effluent launders were added to the basin. The effluent launders provide nearly 3,800 linear feet of weir to be installed in the 250-foot wide basin, which allows the discharge of up to 4,100 cfs prior to diverting flow below the weirs. Low leakage sluice gates were also added at the effluent end to allow for overflow discharge and relief.
Although not regulated by the DWSD’s permit, deposition of suspended solids from the untreated CSO discharges within the Conner Creek has been and continues to be a major source of contamination and poor aesthetics. For this reason the DWSD evaluated the use of a vortex settling facility in combination with the rectangular tank to maximize solids removal while meeting the storage and treatment goals. It was determined as part of the evaluation that due to site constraints a larger rectangular tank provided a minor increase in annual average solids removal over the combination vortex settling/rectangular tank. In addition, the swirl concentrator/rectangular tank option was more costly and required additional head for gravity conveyance.
For a copy of the full paper, please contact the author at firstname.lastname@example.org
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