When the Cobb County-Marietta Water Authority (CCMWA) anticipated the need to upgrade the Hugh A. Wyckoff water treatment plant, its operators turned to granular activated carbon (GAC) technology after vetting several alternatives. The plant, a wholesaler in a two-plant system, processes up to 72 million gallons per day and serves about 350,000 people. Comprised of Wyckoff and the James E. Quarles treatment plants, CCMWA is the second largest water provider in Georgia.
The Wyckoff plant draws its water from Lake Allatoona. Because of the plant’s location and setup, water treated there can remain in transit for up to 10 days, leading to higher disinfection byproduct (DBP) levels. CCMWA needed to upgrade its plant to comply with the U.S. Environmental Protection Agency’s (EPA) Stage 2 regulations for DBPs.
In 2002, the CCMWA began piloting and testing different technologies including:
- Continuous ion exchange
- Carbon adsorption
In 2009, the plan to pursue activated carbon technology was set in place, and the CCMWA and CH2M HILL engineers began the final design work. A GAC contained in pressure vessels was the best solution for the plant’s DBP requirements. The installation of 28 vessels made it the largest installation of GAC pressure vessels at a single facility.
To keep DBPs low and prepare for future compliance, the CCMWA looked at several options when selecting a technology. The five guiding criteria were:
- Effectiveness for lowering DBPs
- Operational fit of the technology to the plant
- Flexibility of operations
- Capital costs
- Operation and maintenance (O&M) costs
- Limitation of wastewater discharges
Before this decision, CCMWA investigated continuous ion exchange technology, which removes specific molecular weight organics. It removes naturally occurring organic matter (NOM), measured as total organic carbon (TOC), but not pharmaceutical and personal care products (PPCPs), which the CCMWA felt would likely be regulated in the future. Wastewater disposal from this process would be an issue at Wyckoff given the small capacity of the local wastewater plant. Continuous ion exchange systems also often require a large footprint, which was unavailable at Wyckoff. In addition, the continuous ion exchange tested by CCMWA uses a resin that, globally, is only manufactured in one plant, causing supply/cost concerns, said Patrick Pherson, project engineer for the CCMWA.
Ozone and biofiltration provided a marginal improvement in the TOC reduction. The capital and O&M costs would be high for the incremental improvement in water quality, Pherson said. Additionally, existing multimedia filters would need to be replaced with deeper beds to accommodate biofiltration media, which would be difficult given the hydraulics of the plant.
CCMWA then examined nanofiltration. Though plant operators said removals were excellent, the membranes were fouled quickly, and the process generated a waste stream with high TOC content. The flow from this waste stream was estimated at approximately 3 million gallons per day for the CCMWA. The plant judged this would be too much to be sent to the local wastewater treatment plant or to be easily treated on site. Nanofiltration was, therefore, deemed infeasible for this site.
Despite the accepted use of chloramines in neighboring states, Georgia discourages their use. Chloramines may eliminate regulated DBPs, but they also can create their own set of DBPs, which are currently unregulated but may be in the future. The CCMWA decided that chloramines could only be a short-term solution.
After eliminating these options, the CCMWA decided that a GAC made from select grades of bituminous coal best met the criteria because it provided solutions to current and future regulations, had operational flexibility, was maintainable within the Wyckoff context and did not produce too much wastewater. Operation of GAC filters was anticipated to be relatively easy to accommodate because the plant already had traditional multimedia filters in place, and GAC filtration is similar in terms of operation.
"We are anticipating that over the next 20 years, there will be more regulations on contaminants that come from pharmaceutical and personal care products," said Pherson. "We will already have a technology that will take many of these emerging contaminants out. This installation was not only designed with current regulations in mind, but with an eye to the future ones also."
The CCMWA chose to competitively bid the pressure vessels (see Image 1) and the GAC separately. The manufacturer of the GAC was also chosen for the reactivation supply contract based on its experience with potable reactivation and experience working with CCMWA on the design and supply of the GAC facility.
The vessels hold an adsorption system that uses GAC to remove dissolved organic contaminants, such as DBPs and natural organic contaminants, from liquids. These vessels can hold up to 40,000 pounds of GAC, providing the additional contact time to remove either compounds at low concentrations or poorly adsorbing compounds.
Provided with two GAC discharge lines positioned to extract 20,000 pounds of spent carbon each, the system minimizes the time required for GAC exchanges by eliminating the guesswork involved when removing spent carbon from the vessels. In addition, three nozzles along the straight side of the vessel can be fitted with in-bed sample assemblies to allow the operator to monitor the progress of the adsorbate as it flows through the bed.
The standard design consists of two vessels combined with a centralized pipe manifold to allow for series or parallel operation, but it also can come as a single vessel if preferred.
Meeting regulations in a cost-effective way
CCMWA operates GAC filters seasonally from May to October to remain under DBP limits. This limited operation extends the life of the GAC beds. The GAC filters are able to treat partial flow of the plant, and its treated water is blended back with filtered water. This reduces the amount of flow to GAC vessels and extends the life of the GAC.
Continuous monitoring of the GAC facility’s effluent ensures the system is optimizing the GAC usage. Monitoring points on each GAC filter, sampled weekly, determine when each filter is approaching exhaustion. CCMWA tracks how much TOC the carbon is able to process. Once the carbon runs down to 30 percent removal of the TOC coming through the plant, it is sent back for regeneration.
Usually, Pherson says, the carbon becomes exhausted at the end of the five-month season when the GAC normally operates and must only be reactivated once per year. The vessels can be turned on and shut down individually, which provides flexibility for the plant operators to determine how much carbon is used during the summer.
For its exchanges, CCMWA uses custom municipal reactivated (CMR) carbon. During this process, spent activated carbon is removed from the filters and transported to one of the GAC manufacturer’s NSF-certified, exclusively potable reactivation facilities where it is reactivated thermally at high temperature. The manufacturer has a dedicated NSF-approved reactivation plant. During the reactivation process, organic compounds that have been captured by GAC are destroyed when subjected to high temperatures that, at the same time, restore the GAC to a usable state. This results in a cost savings for the customer over the use of virgin carbon. The reactivation/recycling process also is better for the environment, with a reduced carbon dioxide footprint as compared to the manufacture of virgin activated carbon. The custom reactivation process reduces the cost of using virgin GAC and the carbon footprint associated with GAC use.
The reactivated carbon, along with a small amount of virgin makeup GAC, is then returned to CCMWA for reuse at its Wyckoff facility. Custom reactivation, in comparison to yearly replacement with virgin GAC, saves CCMWA about 30 percent of the cost of using activated carbon.
Calgon Carbon is a producer of granular activated carbon and supplies more than 100 types of activated carbon products — in granular, powdered, pelletized and cloth form — for more than 700 applications. The company provides purification solutions for drinking water, wastewater, pollution abatement, and industrial and commercial manufacturing processes. For more information about activated carbon solutions from Calgon Carbon, visit calgoncarbon.com.