The Roche Harbor Water Treatment Plant is located on the northwest side of San Juan Island in San Juan County, Washington state. Although a significant part of the land in the county is privately owned, the watersheds there provide a public benefit for both residents and visitors. San Juan Island’s economy is tourist-driven; the island was voted #2 on the New York Times list of “41 Places to go in 2011.” Its marina has been a top boating destination for more than fifty years. Incorporated in 1886, the community has seen slow but steady population growth in recent decades. Roche Harbor Water System Inc. has been in operation since 1968 when PVC piping replaced the island’s existing wooden pipe system.
Roche Harbor serves approximately 500 homes and one resort. The facility’s capacity is rated at 0.5 million gallons per day (MGD), but is normally operated at 0.3 MGD. The Washington Department of Heath regulates the area’s drinking water. The plant draws its water from Briggs Lake, a small surface impoundment. High color and total organic carbon (TOC) were water filtration challenges associated with the lake water source. Suspended matter – comprised of a wide variety of organic particles such as zooplankton, algal cells, threadlike organisms, and bacterial cells combine – leads to high TOC. Roche Harbor’s filtered water had high levels of tastes and odors that generated numerous customer complaints.
Raw Water Quality and Filter Plant Performance
Roche Harbor source water is low in turbidity and high in both TOC and color as shown in Table 1. Current filter plant performance removes approximately 56 percent of the TOC.
Raw Water Quality and Filter TOC Performance
Meeting Regulatory Requirements
In 1998, the U.S. Environmental Protection Agency (EPA) established two drinking water regulations with which the Roche Harbor Water System needed to comply: Stage 1 Disinfectant and Disinfection Byproducts Rule (DBPR), which is designed to protect water consumers from disinfectant byproducts (DBPs), and the Interim Enhanced Surface Water Treatment Rule (IESWTR), which targets the reduction of microbial contamination. The EPA introduced the Stage 2 DBPR in 2006 to build upon earlier rules that addressed DBPs to improve drinking water quality and provide additional public health protection from DBPs.
Roche Harbor turned to Gray and Osborne consultant Russell Porter to help them identify a technology that would treat the high TOC levels in its water and bring them into compliance with DBPR. Roche Harbor’s plant manager David Gibbs found that the plant’s water exceeded the DBPR compliance levels for both trihalomethane (THM) and haleoacetic acid (HAA).
Historical DBP Monitoring
Roche Harbor uniformly failed to meet the 80 μg/L THM standards and often failed to meet the 60 μg/L HAA5 standard.
Table 2 Historical DBP Data, 2004-2009
Porter suggested the use of granular activated carbon (GAC) to remove the organic precursors from the water that, when combined with the chlorine used to disinfect the water, formed the regulated DBPs. Porter performed a bench scale GAC test in September of 2007 and installed a GAC column pilot test that December, which has been in operation for a little over a year.
Bench – Scale Pilot Study
GAC Column Pilot Study
Comparison of Pilot and Full-Scale Parameters
After both of these test programs confirmed the effectiveness of activated carbon for this application, Porter made the recommendation to install a full-scale GAC system.
In order to aid in specifying the optimal carbon for this project, Porter designed the GAC column pilot test to compare activated carbon products from two suppliers. Porter made the decision to sole-source the GAC based on the results of this testing. The specifications for the two tested GAC products were technically the same (i.e. both had the same mesh size, iodine number, abrasion number, etc). The selected product – Calgon Carbon’s Filtrasorb 400 – was made from low ash, metallurgical-grade bituminous coal furnished from North American mines, and manufactured via a reagglomeration method that increases both the adsorptive capacity (as confirmed by the pilot study) and durability of the product as compared to the direct-activated, imported GAC (an important consideration for custom reactivation).
Comparison of Direct Activated and Reagglomerated GAC
Two 10,000 lb carbon adsorber vessels and the Filtrasorb 400 GAC were then specified based on the pilot data. The vessels, known as “post-filter contractors,” were added to the existing treatment plant after the coagulation, clarification and filtration steps, just prior to the final chlorination step. Roche Harbor purchased 20,000 pounds of Filtrasorb 400 and a Model 8 dual vessel system from the selected manufacturer and installed them at their filter plant in June of 2009.
Full Scale Implementation
The pilot program had shown that the selected GAC lasted twice as long as the alternative carbon, and the full-scale actual system performance even exceeded what was predicted by the pilot work. The activated carbon system allowed the water plant to come into compliance with the DBP Stage 2 regulation.
The 13-month pilot data indicated that GAC would provide effective THM treatment for approximately one year. A life-cycle cost analysis indicated that one-year GAC life was cost effective (<$1/1,000 gallons for GAC replacement). Full-scale installation occurred in June of 2009.
Figure 2 compares the normalized UV absorbance data with THM and HAA5 data for the first 14 months of full-scale GAC treatment. As the data indicates, UV absorbance provided a good qualitative indicator of THM and HAA5 levels with the expectation of the final THM value in August of 2010. THM values were higher than predicted by pilot SDS testing.
Figure 3 compares the normalized UV absorbance data for both the pilot and full-scale installations as a function of full-scale performance for removal of UV absorbing material.
Comparison of Full-Scale UV Absorbance Data and Compliance DBP Averages, Comparison of UV Absorbance Data for Pilot and Full-Scale installations
In addition to achieving compliance with the USEPA DBP rule, the number of taste and odor complaints that Roche Harbor received decreased substantially after installation. Another benefit resulting from the installation of the GAC system was the reduction in chlorine demand for disinfection. When free chlorine reacts with natural organic matter (NOM), it forms THMs, HAAs and other DBPs. Chlorine that has reacted with NOM in this way is no longer present to disinfect. By reducing the amount of NOM present in the untreated water before chlorination – through the introduction of both enhanced coagulation and GAC filtration – there was less NOM present to react with the chlorine. That outcome enabled the plant to use less chlorine for disinfection while maintaining an effective dose for its distribution residual.
Optimizing GAC Operations
During the first year of usage, Gibbs discovered ways to optimize the operation of the GAC and the plant’s coagulation dosing system in order to extend the carbon life. The GAC originally installed with Model 8 units lasted approximately 15 months (June 2009 through September 2010) before the carbon was spent and required replacement. Because of the lessons learned during the first year of operation, Roche Harbor was able to make the second installment of carbon last two full years before replacement was necessary in September of 2012. This improvement was due to increased operator attention to post-filter, pre-GAC conditions such as minimizing organic carryover as measured by UV254 absorbance. The first batch filtered 46,952,562 of water and the second batch filtered 60,493,248 gallons. Roche Harbor’s increased run-time was also attributed to more effective coagulation monitoring and dosing, as indicated by the UV254 (see Figure 4).
Gibbs commented, “Carbon has worked extremely well for Roche Harbor water systems. The taste of our water has greatly improved. People like to drink their coffee now.” He noted that changeover could be completed in one day and that the manufacturer’s extremely knowledgeable field service employees put the client’s needs first. Nearby Friday Harbor followed suit after Roche Harbor, installing the same carbon system.
The next step for Roche Harbor is to have their GAC custom reactivated once it becomes spent. Custom reactivated GAC offers many of the same advantages for water treatment applications as new GAC does, including the ability to perform multiple processes simultaneously, such as the enhancement of taste, the elimination of odor, and the removal of DBPs. Custom reactivated GAC achieves these aims at a significant cost savings, while producing only a fraction of the greenhouse gas emissions associated with the manufacture of virgin GAC. Custom reactivation transforms GAC into a sustainable technology, conserving both energy and raw materials.
The Calgon Carbon Model 8 is an adsorption system designed for the removal of dissolved organic compounds from water or other liquids using granular activated carbon. The modular design concept allows for selection of options or alternate materials to best meet the requirements of the site and treatment application.
The Model 8 system is delivered as two adsorbers: a separate compact center piping network and interconnecting piping requiring minimal space and field assembly. The pre-engineered Model 8 design ensures that adsorption system functions can be performed with the system as provided – due in part to its superior production capabilities.
The process piping network for the Model 8 offers operation of the two adsorbers in parallel or two-stage series flow, with either adsorber in the lead position. The piping can also isolate either adsorber for carbon exchange or backwash operations, while maintaining flow through the other adsorber. In addition, the underdrain design provides for efficient use of the carbon through uniform collection of water at the bottom of the bed, and even distribution of backwash water to minimize carbon bed disturbance.
The Model 8 system is designed for use with the closed loop carbon transport trailers. The spent carbon can be removed from the adsorber via pressurized carbon-water slurry, and fresh carbon refilled in the same manner. This closed loop transfer is accomplished without exposure of personnel to either spent or fresh carbon. The spent carbon is then returned to a facility for custom reactivation. The reactivated carbon is returned to the customer using the same carbon transport trailers.