Filtration of disinfection by-products | WWD
In 2010, Shelby County Water Services (SCWS) was planning for the future. With new regulations on the horizon, SCWS has determined that the Talladega / Shelby Water Treatment Plant (UTE) in Shelby County, Alabama needs more efficient disposal of disinfection byproducts. (SPD). Specifically, the processing plant needed help complying with the US EPA’s new Disinfection By-Product (DBPR) rule.
SCWS serves both retail and wholesale customers. The 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 subdivisions of Eagle Point, Greystone, Forrest Park, Forrest Lakes, Mt. Laurel, Regent Park, Villas Belvedere and Highland Village, increased 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 obtains 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 the removal of DBP in 2008.
Over the next year, SCWS considered using GAC and ion exchange at the Talladega / Shelby plant to ensure compliance with DBPR Stage 2 regulations. Based on its analysis, the county concluded that the cost and performance of an AGC 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 technology that resulted in minimal waste. Waste from the plant flows through a series of settling lagoons and is ultimately dumped 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. Additionally, the Alabama Department of Environmental Management has indicated that a concentrated discharge of 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.
County cost analysis showed that the GAC filter design was required to effectively remove SPDs for a full year before requiring reactivation. Calgon Carbon offered its newest and largest GAC adsorption system, the Model-14. This system, fitted with two vessels each containing 60,000 pounds of GAC, was developed as SCWS assessed how best to modernize the plant. Birmingham-based Municipal Consultants Inc., which played an important role in the plant’s design process, took note of the new system.
“When Calgon Carbon brought out the 60,000 pound vessels, 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 per 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 CAG to remove natural organic matter (NOM) from the water source prior to disinfection, thus preventing the formation of DBPs. The system is designed to allow plant operators to pump all or part of the filtered water through the vessels in parallel or series operation.
Approximately every year, the county sends the depleted CAG of two ships to Calgon Carbon for a personalized reactivation. The spent GAC is transported to one of Calgon Carbon’s custom reactivation facilities, where it is thermally reactivated to remove adsorbed contaminants and restore its adsorption capacity. The filter media company then returns and installs the reactivated carbon (including a small amount of virgin GAC to compensate for losses during reactivation) in the Talladega / Shelby vessels. The entire reactivation process is carried out in accordance with the latest NSF and AWWA standards governing the reactivation of CAG used for drinking water treatment.
Ultimately, the GAC was a wise selection of the treatment options being considered, as GAC is an effective disposal technology that accomplishes more than just compliance with DBP. 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 by unregulated compounds such as perfluorinated compounds (PFCs), pesticides and a number of other contaminants listed on the US EPA 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 the GAC systems anticipated the impact of the Stage 2 DBPR, ensuring that Shelby County water was in continuous compliance during the transition from Stage 1 to the more stringent Stage 2.
The affordability of the proposed solution, the availability of customized reactivation services and the design of large vessels were all factors in SCWS’s choice to work with Calgon Carbon, according to Michael Cain, head of water services for the county. by Shelby.
“This was delivered as a complete system: carbon, vessels, pipes, pressure vessels and everything,” Cain said. “Calgon Carbon turned out to be the right choice.
What are SPDs and why are they regulated?
Disinfection of water with chemical agents, such as chlorine, is an essential part of public health because it protects consumers from disease-causing microorganisms. However, disinfectants react with NOM in water to form compounds called SPDs. 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 new DBPs that are currently unregulated and still toxic. Many municipal water providers take the step of removing NOM from the water before adding disinfectant chemicals, thereby 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 supply systems to meet maximum contaminant levels for disinfection at each monitoring site of a distribution system.
What is personalized reactivation?
Calgon Carbon has a dedicated NSF-approved reactivation plant in North Tonawanda, New York, which serves customers east of the Mississippi River.
This facility is used for personalized municipal reactivation, which means that each customer’s carbon is segregated and processed separately from the carbon of other customers. During the reactivation process, the organic compounds captured by the CAG are destroyed when subjected to high temperatures which, at the same time, restore the CAG to an almost pristine state. This resulting reactivated product results in cost savings. The reactivation / recycling process is also better for the environment, with a reduced CO2 footprint compared to the manufacture of virgin activated carbon.
Adsorption vessels model 14
Calgon Carbon’s Model 14 adsorption system uses GAC to remove dissolved organic contaminants, such as SPDs and NOM, from liquids. These containers can hold up to 60,000 pounds of GAC, providing additional contact time to remove low concentration compounds or weakly adsorbent compounds.
It is designed with one GAC fill line and three GAC unload lines positioned to extract 20,000 pounds of spent carbon each. The arrangement of the discharge lines facilitates efficient GAC exchanges in three easily removable increments. Additionally, three nozzles along the right side of the vessel can be fitted with in-bed sample sets to allow the operator to monitor the adsorbate mass transfer area through the bed.
The standard system is a single vessel, and typical designs include several single vessels operated in parallel. However, two vessel systems can also be provided for lead-lag operation.