The City of Celina, Ohio, supplies drinking water to 11,647 residents of the city and the East Jefferson District. Its source water, the 21-square-mile Grand Lake, contains high total organic carbon (TOC) and supports a high concentration of Planktothrix algae, causing severe taste and odor problems and fostering very high concentrations of disinfection byproducts (DBPs).

Much of the Grand Lake watershed is farmland, and the lake averages only seven feet deep. These conditions lead to massive algal blooms, along with TOC concentrations that average 12.5 mg/l and peak at more than 20 mg/l. In addition, pH fluctuates, and turbidity ranges from 10 to 300 NTU.

In 2007, the city faced a consent order to reduce total trihalomethane levels in the finished water. A granular activated carbon (GAC) adsorption treatment process that went online in 2009 resolved that issue while also eliminating taste and odor concerns.

 

Traditional treatment

For several years, the city supplied water through a series of treatment processes including lime slaking, upflow clarification, recarbonation, sand filtration, ozonation, and chlorination. These processes effectively removed solids along with taste and odor.

The city tried powdered activated carbon (PAC) treatment to improve taste and odor, but it was ineffective and ultimately discontinued. In 1995, the DBP levels became an issue. The four-quarter running average total trihalomethane (TTHM) level was measured at 221.5 µg/l, well above the 80 µg/l limit set by the U.S. and Ohio EPA. In 2003, the Ohio EPA placed the city water treatment plant under a Findings and Orders consent decree with a November 2007 compliance date for TTHM.

None of the treatment processes the city had used were effective in removing the organic DBP precursors. In fact, ozonation actually appeared to increase DBPs by breaking down some of the TOC into compounds that react readily with chlorine to produce TTHM and haloacetic acids (HAA). Therefore, the city looked into alternative treatments.

 

Careful investigation

In 2003 and 2004, the city considered finding a groundwater source of supply, but that proved unrealistic because the Great Lakes Water Compact of 1986 prohibits withdrawal of water from the Great Lakes watershed for discharge into another basin. (Celina discharges to the Gulf of Mexico watershed.)

The city then looked at new treatment technologies. A trial of sulfur modified iron (SMI) as a secondary coagulant produced no appreciable difference in water quality. A conventional water clarification system removed 67 to 69 percent of TOC and dissolved organic carbon (DOC) but reduced TTHM to only 170 µg/l, well above the limit. In addition, there were pH and stability issues that needed more control.

Trials of magnetic ion exchange technology achieved 38 to 48 percent DOC removal but could not produce TTHM levels below 100 µg/l, except when chloramine was substituted for free chlorine for final disinfection.

In September 2004, the Water Department engaged Floyd Brown and Metcalf & Eddy/AECOM to lead a plant improvement project. The consultants evaluated a short list of treatment technologies:

• Chloramine disinfection

• Reverse osmosis (RO) treatment

• Granular activated carbon (GAC) adsorption

 

Evaluating options

Chloramine disinfection, viewed as a short-term solution, involved adding ammonia to the chlorine chemical feed to form chloramines. Although chloramines can reduce the formation of currently regulated DBPs, the treatment can form other DBPs, including N-Nitrosodimethylamine and cyanogen chloride, suspected to be more toxic to humans. This, along with potential toxicity to fish and possible nitrification in distribution lines, led to rejection of that technology.

Reverse osmosis (RO) was an attractive option and, given the plant’s small scale, its cost was not prohibitive. However, pilot testing showed that feedwater pretreatment would be needed to prevent RO membrane fouling, and solving that problem would be complex and time consuming.

Ultimately, the city chose GAC adsorption, a technology proven effective on a wide variety of drinking water sources.

 

Pilot testing

A three-phase pilot study began in December 2005. Phase I evaluated different GAC products. Phase II simulated a two-vessel series system containing the selected GAC. Phase III studied the operation of two vessels in a lead/lag staged-bed operation.

The water plant operation was expanded to three shifts to accommodate pilot operation and testing. Calgon Carbon Corporation provided the pilot column system and various grades of GAC for testing. Individual pilot columns were filled to a four-foot depth with the selected products and run in various combinations to simulate beds with an eight-foot depth of media.

Pilot testing found that FILTRASORB 300 GAC from Calgon Carbon was best suited to the application. When used in series with staged replacement (spent carbon in the lead vessel exchanged with fresh activated carbon and valved to operate second in the series), there was a significant reduction in carbon usage over a single-bed operation. Staged filter operation is not common in drinking water treatment but the high TOC concentration at Celina made that approach an economical choice.

Piloting results showed that GAC adsorption could easily and consistently achieve the targeted TOC level of 2.5 mg/l. The annual GAC operating cost based on the use of virgin GAC only, was projected at $1.21 per 1,000 gallons treated at the ultimate design flow, assuming 10 mg/l of TOC fed to the GAC filters.

This cost is above the average for municipal GAC systems (generally $0.15 to $0.70 per 1,000 gallons treated), reflecting the extraordinarily high TOC present in the source water and the high removal rates needed to reach a TOC level where the DBP standards would not be exceeded.

 

System implementation

Based on the pilot testing, the engineers designed a full-scale GAC system of eight vessels, each containing 40,000 pounds of GAC, to be operated in four parallel trains. The adsorbers would operate in a staged sequence to maximize TOC loading on the GAC.

The design flow per train was set at 520 gpm, though the system now operates at less than half that volume (240 gpm) or roughly 1.5 mgd treated for the entire system. The current flow rate results in an empty bed contact time of 78 minutes per vessel.

The project timing was about one year from project award to startup. This included site concrete work, constructing the treatment building, setting equipment, and installing piping and wiring.

 

Operating data

The GAC system went online in July 2008. It reduced the finished water TOC to below 2.0 mg/l and consistently reduced TTHM and HAA5 to below the required levels of 80 µg/l and 60 µg/l.

The $7 million total capital cost of the plant upgrade covered building construction, new pumps, new wet well, automated controls, laboratory, replacement sand filter valves, replacement intake structure, piloting, and engineering, in addition to the GAC system, which cost $1.73 million, including the initial fill of GAC.

The plant recently switched to custom reactivated carbon, significantly reducing carbon operating costs with no measurable loss of performance. Operating costs include spent carbon reactivation, addition of make-up carbon, transportation and warehousing.

With the switch to custom reactivation, the annual GAC operating cost is $384,000 per year, or $0.35 per 1,000 gallons. Based on a 10-year life, the GAC system’s annual cost, including initial capital expense and annual operating costs, is projected at $0.51 per 1,000 gallons of installed capacity.

 

Meeting requirements

Since startup, the expanded and improved water plant has produced an average of 1.5 mgd of drinking water that consistently measures below the treatment goals for TTHM and HAA. In September 2009, the Findings and Orders decree was lifted.

If required, there is space on the site for four more GAC adsorbers. In case an alternative disinfection process should be required, the facility is equipped with risers on the finished waterlines where UV modules can be fitted.

GAC adsorption appears to have resolved the issues of DBP compliance while significantly improving the taste, odor, and appearance of water from Grand Lake.

 

ABOUT THE AUTHORS

T. Mike Sudman is the superintendent of water and distribution and Todd W. Hone is the assistant plant superintendent with the City of Celina, Ohio. Cheryl L. Green is the technical manager with AECOM.