Project Focus: CMF technology strengthens wet weather resilience in Ohio
Overflow treatment facilities using compressible media filtration have proved effective at increasing wet weather resilience at a wastewater treatment plant in Springfield, Ohio
- Untreated CSO flows needed to be reduced to the Mad River, an important watercourse for cold water fishing
- Extra capacity was required to accommodate and deal with additional flows during wet weather events
- Remote operation was preferable to allow the solution to be available 24/7 without being reliant on operators
by Tony Koodie, European Process Director, Black & Veatch
Changing rainfall patterns and increasingly stringent environmental protection standards are challenging the way water utilities the world over manage storm flows. An interesting approach can be found in Springfield, Ohio, U.S.A. The city recently added an enhanced high-rate treatment (EHRT) facility featuring the world’s largest compressible media filtration (CMF) system.
The innovative project has added significant peak wet-weather flow capacity – 380 million litres a day (ML/d) – to Springfield’s publicly owned treatment works and serves to reduce untreated overflows into the local river. Moreover, the CMF technology is fully automated, does not require clarification chemicals, and its capital and operating costs are a fraction of those associated with an expansion using conventional storage, conveyance and treatment technologies. The lesson from Springfield: overflow treatment facilities that incorporate best-fit advances in technology can offer a cost-effective alternative to new storage and conveyance infrastructure.
The Mad River flows through Springfield and is the largest cold water fishery in the state of Ohio. Its watershed drains more than 1,550 square kilometers, with about two-thirds of it used as crop and pasture land and one-fifth for urban-residential use. Approximately 170,000 people reside in the area.
Water quality in the Mad River watershed is impaired by a number of sources. They include excessive nutrients such as nitrogen and phosphorus from fertilisers, animal waste and sewage, which enter the watershed during wet-weather events. Polluted runoff from urban and agricultural areas is a culprit as is excess siltation resulting from stream channelization.
Wet and dry capability
Springfield is served by a wastewater treatment plant that has an average design flow of 95 million litres per day and provides treatment of dry-weather flows through conventional preliminary, primary, trickling filters and nitrifying activated sludge processes. The plant has overflow weirs that discharged excess wet-weather flows largely untreated to the Mad River. Most of Springfield’s collection system is separate sanitary sewers, but about 22 percent of the system consists of combined sanitary and storm sewers, including 57 combined sewer overflow (CSO) outfalls. During wet weather events when infiltration and inflow through the combined sewers exceeds capacity, discharges through the CSO outfalls go untreated into the river. According to Ohio’s Environmental Protection Agency, CSOs impacting the Mad River were approximately 3.8 billion litres a year in 2000.
Under federal and state regulations, the city of Springfield developed a long-term control plan, to increase its wet-weather flow capacity and its control of untreated CSOs to the Mad River. The plan recommended the addition of EHRT facilities as part of a range of treatment plant improvements.
Dynamic influent characterisation and full-plant dynamic process modelling was conducted to predict performance and evaluate impacts to the existing liquid and biosolids treatment facilities during wet-weather events. Alternatives were evaluated for the new EHRT facilities, and equipment bids were solicited for solids contact high-rate clarification, ballasted flocculation and CMF.
Conceptual facility designs for the technology alternatives were developed and evaluated for economic and non-economic factors. It was determined that lifecycle costs were within 7 percent of each other. As a result, CMF technology was selected largely because of non-economic factors, including:
• Operations can be monitored and controlled remotely without additional staff at the treatment facility during an event
• Simplicity of process, equipment and maintenance
• No need for clarification coagulants or specialty polymers and associated makeup and feed equipment
• No constraints concerning process start-up time or turn-down
• Potential for dual function during dry weather conditions
An on-site demonstration pilot unit was operated from October 2010 to June 2011. Approximately 150 tests were run on dry and wet-weather flows. In addition to filtration performance, effluent disinfection dose response was tested, using E. coli as an indicator. Results of the pilot confirmed the influent characteristics, process design criteria, and process performance during a variety of dry- and wet-weather conditions.
CMF FlexFilter choice
CMF uses a bed of synthetic fibre balls to capture influent suspended solids and colloidal particles. Two types are available in the United States: the Fuzzy Filter and the FlexFilter. The Fuzzy Filter is usually configured with influent flowing up through the media bed. While in filtration mode, a perforated plate compresses the media from the top using an electrically actuated screw drive. The FlexFilter operates similarly except that it uses a down-flow configuration, and the media bed is compressed transversely to the liquid flow through sidewall bladders using influent hydrostatic pressure instead of external mechanical or electrical actuators. The CMF system selected for Springfield was the WWETCO FlexFilter.
The technology originated in Japan and has been in use for more than a decade in full-scale wet-weather treatment applications. Considered an innovative technology by the U.S. Environmental Protection Agency, CMF can operate at higher flow rates and provide cleaner effluent compared to conventional clarification technologies. It can be used to treat storm sewer, combined sewer and sanitary sewer overflows, and provide polishing of plant effluent during normal, dry-weather conditions.
A key advantage of CMF is automation. Process control uses robust flow and level meters and timers without needing additional human operators. It also requires no chemicals for most applications.
Another advantage of CMF, and most high-rate filtration technologies, is that chemicals are generally not required for wet-weather treatment applications. High-rate filters like CMF also can serve as a safety net downstream of secondary clarifiers, to allow existing biological facilities to be maximised during smaller wet-weather events with less risk of losing biomass.
Springfield’s EHRT facilities have two structures: a wet-weather headworks for preliminary treatment, and a separate common wall structure for the CMF process, high-rate disinfection and effluent pumping. The structures boast a number of innovative design features.
• Wet-weather headworks: The system consists of a velocity control channel and “rock box” to remove large settleable solids such as grit, sand, gravel, and stones. Modulating gates at the end of the channel control the flow split between the new EHRT train and the existing treatment plant’s conventional process trains. Horizontal rake bar screens located on the influent channel’s overflow weir prevent screened material from flowing to the downstream CMF process. Placing the screens above the normal dry-weather influent channel serves to minimize screenings and grit handling while providing appropriate preliminary treatment for downstream processes.
• Compressible media filters: The project represents the first full-scale wet-weather installation of this type of CMF technology located on the same site as the dry-weather wastewater treatment facility. The technology features a unique cell filter design that maximizes the solids loading capacity of the filter bed and reduces backwash requirements.
• High-rate disinfection: Sodium hypochlorite is mixed rapidly into the CMF effluent, which then flows through a contact basin that is 33 percent smaller than conventional design standards. This provides significant capital cost savings.
Construction began in August 2013, with commissioning in October 2014. Process control programming and functionality testing of the CMF system started with secondary effluent then switched to raw influent for process performance testing.
Challenges inevitably arise when emerging technologies are integrated into existing facilities and Springfield was no exception. Issues included odour control, which was resolved by recirculating influent in the rock-box chamber, to keep lighter organics and solids suspended and flowing to the main headworks. The CMF cells in the facility are the largest of their kind, which required construction materials, fabrication details and installation procedures to be carefully scrutinized by the equipment supplier and contractor prior to installation. Adjustments to control programme settings and override protections were suggested after a significant first-flush load overwhelmed the initial system programming.
By June 2015, wet-weather events were sizeable enough to require operation of the EHRT facilities on 21 days. Flows from five of those events were small enough in magnitude and duration to be completely captured by the new facilities. Performance was consistent with design criteria and pilot results. The CMF system has achieved excellent turbidity and removal of suspended solids and associated organics.
The Springfield facility is unique, being among the first of its kind and the largest capacity CMF installation to date. Findings from its design and operation demonstrate its effectiveness for treatment of wet-weather flows and its relevance to communities around the world. For example:
• The EHRT facilities are co-located at Springfield’s main wastewater treatment plant. Yet the facility design and technologies are also suitable for decentralized facilities in the collection system. The wet-weather headworks design and equipment are more commonly used in collection system overflow structures and remote wet-weather facilities.
• The automated CMF process is integrated into a supervisory control and data acquisition (SCADA) system for remote process control and does not need staff at the facility for startup or operations.
• For treatment scenarios that require tertiary filtration, a dual-use CMF facility can be designed to provide secondary effluent filtration during dry-weather and then switched to a parallel auxiliary mode to treat wet-weather flows.
Advanced clarification processes such as CMF serve as a viable parallel auxiliary treatment strategy. They offer effluent quality significantly better than primary equivalency, and are generally more cost effective than conventional secondary treatment alternatives in instances of peak wet-weather flows. As such, Springfield’s new EHRT facilities should help to advance auxiliary treatment practices.
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