Filtration of disinfection by-products | Summary of water and waste
Christy Bostardi is Calgon Carbon’s Drinking Water Solutions Marketing Manager. Bostardi can be contacted at firstname.lastname@example.org.
In 2010, Shelby County Water Services (SCWS) was planning for the future. With new regulations on the horizon, SCWS determined that the Talladega/Shelby Water Treatment Plant (WTP) in Shelby County, Alabama needed more efficient sub-base removal. disinfection products (SPD). Specifically, the treatment plant needed help complying with the new US EPA Stage 2 Disinfection Byproduct Rule (DBPR).
SCWS serves both retail and wholesale customers. Active wholesale customers, Alabaster, Pelham and Sterrett-Vandiver, sell significant quantities of SCWS water to their customers. The number of SCWS retail customers, primarily located in Westover, Chelsea and the Eagle Point, Greystone, Forrest Park, Forrest Lakes, Mt. Laurel, Regent Park, Belvedere Villas and Highland Village subdivisions, grew from approximately 3,200 in 2001 to over 10,500 in 2010. Part of this increase was attributed to the acquisition of customers from the former Westover Water Authority, which merged with SCWS in 2007.
SCWS draws water from Lay Lake on the Coosa River and treats the water at two facilities: Talladega/Shelby WTP and Shelby County South WTP. The Shelby County South plant near Wilsonville is owned and operated by Shelby County and began production of drinking water treated with granular activated carbon (GAC) for DBP removal in 2008.
Over the next year, SCWS considered the use of GAC and ion exchange at the Talladega/Shelby plant to ensure compliance with Stage 2 DBPR regulations. Based on its analysis , the county concluded that the cost and performance of a GAC system would be similar to the other options, assuming the carbon lasted at least a year before requiring reactivation.
Another consideration in the analysis was the location of the Talladega/Shelby plant because the remote location and lack of septic sewers favored a technology that resulted in minimal waste. Waste from the plant flows through a series of settling lagoons and is ultimately discharged into the Coosa River. These waste lagoons were not designed to handle high levels of color, total dissolved solids or salt, which may be typical of ion exchange waste. In addition, the Alabama Department of Environmental Management indicated that a discharge of concentrated waste would require special permits and extensive compliance testing. For these reasons, SCWS was drawn to the GAC system, which generally results in little waste.
The county cost analysis showed that the GAC filter design was required to effectively remove DBP for a full year before requiring reactivation. Calgon Carbon offered their newest and largest GAC adsorption system, the Model-14. This system, equipped with two vessels each containing 60,000 pounds of GAC, was developed while SCWS was evaluating how best to modernize the plant. Birmingham-based Municipal Consultants Inc., which played an important role in the plant’s design process, learned about the new system.
“When Calgon Carbon made the 60,000 pound vessels available, we were able to reduce the number of vessels we needed to four,” said Chris Cousins, president of Municipal Consultants. “It saves money because with the larger size, we only have to reactivate two of them each fiscal year.”
Ultimately, the plant installed four of the 14-foot-diameter Model 14 pressure vessels over a 10-month period. Filtered water is pumped through the GAC to remove Natural Organic Matter (NOM) from the water source prior to disinfection, preventing the formation of SPD. The system is designed to allow operators to pump all or part of the filtered water through the tanks in parallel or series operation.
Approximately every year, the county sends the depleted GAC of two ships to Calgon Carbon for custom reactivation. Spent GAC is transported to one of Calgon Carbon’s custom reactivation facilities, where it is thermally reactivated to remove adsorbed contaminants and restore adsorption capacity. The filtration media company then returns and installs the reactivated carbon (including a small amount of virgin GAC to compensate for reactivation losses) in the Talladega/Shelby vessels. The entire reactivation process is performed in accordance with the latest NSF and AWWA standards governing the reactivation of GAC used for drinking water treatment.
Ultimately, GAC was a wise selection of treatment options that were considered because GAC is an effective removal technology that accomplishes more than just DBP compliance. GAC not only removes targeted contaminants of concern, such as Natural Organic Matter (NOM) and DBPs, but it also acts as a defense barrier against accidental contamination from unregulated compounds such as Perfluorinated Compounds (PFCs), pesticides and a number of other contaminants listed on the US EPA’s Contaminant Candidate List 4 (CCL4).
Since the installation of the Model-14 vessels with GAC at the Talladega/Shelby plant, DBP levels throughout the distribution system have remained compliant. The installation of GAC systems anticipated the impact of Stage 2 DBPR, ensuring Shelby County water was in continued compliance during the transition from Stage 1 to the more stringent Stage 2.
The affordability of the proposed solution, the availability of custom reactivation services and the large vessel design were all factors in SCWS choosing to work with Calgon Carbon, according to Michael Cain, County Water Services Manager. from Shelby.
“This was supplied as a complete system: carbon, vessels, pipes, pressure vessels and everything,” Cain said. “Calgon Carbon turned out to be the right choice.”
What are DBPs and why are they regulated?
Disinfection of water using chemical agents, such as chlorine, is an essential element of public health, as it protects consumers against disease-carrying microorganisms. However, disinfectants react with NOM in water to form compounds called SPD. DBPs have been linked to a number of human health issues and have been regulated by the US EPA.
Some alternative disinfectants, such as chloramines and ozonation, reduce the level of regulated DBPs, but result in the creation of new, currently unregulated and still toxic DBPs. Many municipal water suppliers take the initiative to remove NOM from the water before adding disinfectant chemicals, thus preventing the formation of regulated and unregulated DBPs. Granular Activated Carbon (GAC) is one of the most common technologies used to remove NOM from water.
The US EPA Stage 2 DBPR requires water systems to meet maximum levels of disinfection contaminants at each monitoring site of a distribution system.
What is Custom Reactivation?
Calgon Carbon has an NSF-approved reactivation plant in North Tonawanda, New York, which serves customers east of the Mississippi River.
This facility is used for custom municipal reactivation, which means that each customer’s carbon is segregated and treated separately from the carbon of other customers. During the reactivation process, the organic compounds captured by the GAC are destroyed when subjected to high temperatures which, at the same time, restores the GAC to a near-pristine state. This resulting reactivated product results in a cost saving. The reactivation/recycling process is also better for the environment, with a reduced CO2 footprint compared to making virgin activated carbon.
Model 14 Adsorption Vessels
Calgon Carbon’s Model-14 Adsorption System uses GAC to remove dissolved organic contaminants, such as DBS and NOM, from liquids. These containers can hold up to 60,000 pounds of GAC, providing the extra contact time to remove low concentration compounds or poorly adsorbing compounds.
It is designed with one GAC fill line and three GAC dump lines positioned to extract 20,000 pounds of spent coal each. Discharge line layout facilitates efficient GAC exchanges in three easy-to-remove increments. Additionally, three nozzles along the right side of the vessel can be fitted with in-bed sample assemblies to allow the operator to monitor the mass transfer area of adsorbate through the bed.
The standard system is a single vessel, and typical designs include multiple single vessels operated in parallel. However, two tank systems can also be provided for lead-lag operation.