It looks like the Northeast Ohio Regional Sewer District might have found a cost-effective solution to combined sewer overflow issues that led to a consent decree with EPA in 2010.

It’s a process called Chemically Enhanced High Rate Treatment. NEORSD has been pilot-testing the process for the last year and a half. Greg Binder, program manager for treatment plants at NEORSD and Nick Bucurel and Dan Davis, engineers with Brown & Caldwell, reported favorable results with the new process in recent presentations to the Ohio WEA and the 2014 WEFTEC conference in New Orleans.

“It’s working,” says Binder. “We’re quite pleased with the results so far.”

Consent decree
In the consent decree, NEORSD agreed to pilot test and demonstrate CEHRT as a way to treat CSOs and reach these effluent requirements:

• TSS of less than 40 mg/L (flow-weighted arithmetic mean)
• Bacteria of 126 CFU/100 mL E. coli (daily geomean, May to October) and 248 CFU per 100 mL e-coli (rolling geomean for seven consecutive days)
• Residual chlorine level of 0.038 mg/L (daily basis).

The consent decree also requires NEORSD to have wet weather facilities in place and operational by 2020-21, and capture and treat 98 percent of CSOs by 2036. Should CEHRT not achieve the effluent requirements, the district will be required to build ballasted flocculation facilities.

Pilot plants were commissioned at all three of the district’s treatment plants — Southerly (average treatment flow of 125 mgd), Easterly (94 mgd) and Westerly (26 mgd.)

Although the pilots differ in configuration based on available capacity and equipment at the three plants, the basic flow scheme is the same: a metal coagulant neutralizes particle charge and a polymer flocculant aggregates the coagulated solids. Then, high doses and rapid dispersion of  chlorine (up to 6 mg/L) achieve high-rate disinfection of the flow in less than 10 minutes detention time.

“It’s primary settling on steroids,” Binder explains.”And we’ve been able to achieve the required disinfection at a faster rate than required — less than 5 minutes.”

Pilot plants
The pilot plants range in flow rate from 60 to 150 gallons per minute at Easterly to 18 and 70 mgd at Westerly and Southerly, where the testing is being accomplished in partial-scale and full-scale facilities. Pilot treatment began in July 2013 and will continue into 2015.

Binder and Bucurel note that wet weather flows exhibit specific characteristics that must be taken into account: low pH rain water dilutes and lowers alkalinity, and certain coagulants can further lower the pH.

“This impacts coagulation if the pH gets below the optimal range,” Binder says.

The CEHRT team at the district used jar testing to determine the most effective coagulant and flocculant, reporting that it simulated coagulation and flocculation “very well.”

There are other challenges, too, including the weather itself. In order to have staff on hand at the three pilot facilities during storms, the team watches weather forecasts like a hawk. When wet weather threatens — any time day or night — crews of five to six operators are deployed to each pilot testing site, and the laboratory and operations staff at the plant are alerted. Distances and the time it takes pilot plant operators to reach the site have to be taken into account.

“The inherent nature of rainfall prediction and the size of our service area can lead to false starts,” Binder says.

Mobile app
Because more than 200 samples are collected at each wet weather event, the district has developed a tablet-based mobile app to facilitate sample analysis and chain of custody. Sample information is electronically sent to the laboratory ahead of sample transport, Binder explains, and the chain of custody form is printed out in the field, avoiding hand-written forms.

“This approach reduces the chance for errors in the chain of custody forms and expedites sample processing,” he says.

Although the district is also building a deep tunnel to contain and control wet weather overflows, the CEHRT process might prove to be the biggest bang for the buck. The district estimates it could save as much as $200 million over the ballasted flocculation alternative.

“We’ve seen chemical addition to primary tanks to reduce phosphorus,” Binder says, “but we’re not aware of too many for this application … certainly not at this scale.”