• When Greater Cincinnati Water Works added granular-activated carbon as a postfiltration treatment, the water-service provider achieved a more than 50% total organic carbon (TOC) reduction. GCWW is now maintaining a TOC level of less than 1 mg/L.

    Credit: Greater Cincinnati Water Works

    When Greater Cincinnati Water Works added granular-activated carbon as a postfiltration treatment, the water-service provider achieved a more than 50% total organic carbon (TOC) reduction. GCWW is now maintaining a TOC level of less than 1 mg/L.
In the illustration above, the red line  labeled with GAC  shows the average UVT of the utilitys GAC-treated effluent water when combined with UV disinfection. The blue line  labeled w/o GAC  is sand filter effluent water that has not been treated with GAC. The average UVT of UV-disinfected GAC effluent water is 7% higher than that of UV-disinfected water that hasnt been treated with GAC.

Credit: Greater Cincinnati Water Works

In the illustration above, the red line — labeled “with GAC” — shows the average UVT of the utility’s GAC-treated effluent water when combined with UV disinfection. The blue line — labeled “w/o GAC” — is sand filter effluent water that has not been treated with GAC. The average UVT of UV-disinfected GAC effluent water is 7% higher than that of UV-disinfected water that hasn’t been treated with GAC.
 

As the U.S. EPA’s dates for compliance with the Disinfectants and Disinfection Byproducts Rule (DBPR) Stage 2 loom, municipalities must decide how to meet the requirements of this regulation. Water utilities have several options, each with a particular price tag, and each with pros and cons in terms of efficiency.

Disinfection byproducts (DBPs) are formed when disinfectant chemicals, such as chlorine, used to kill harmful organisms in drinking water react with natural organic matter (NOM; e.g., decaying vegetation) present in the water. There are two primary methods of reducing these byproducts: switch to a different disinfectant chemical that forms lower levels of the currently regulated DBPs, or remove organic matter from the water altogether prior to disinfection, thereby reducing the formation potential for DBPs.

Among the technologies available to remove organic matter is granular-activated carbon (GAC). The technology has been used for decades in municipal drinking water treatment, primarily to remove taste- and odor-causing compounds such as 2-methylisoborneol and geosmin.

When considering GAC as a potential solution for DBPR compliance, you should at minimum look at:

  • Effectiveness
  • Cost
  • Additional benefits and drawbacks.

You should also compare these factors to those of other applicable approaches. We’ll explore all three factors and then take a brief look at other approaches.

Effectiveness: Is GAC right for your system?

Municipalities should run tests with granular-activated carbon and their own water to gauge how effective the treatment will be for reducing disinfection byproducts formation in their systems. The reason being, although GAC is effective in removing organic material from water, not all organic matter is created equal. Some NOM components are more likely than others to form DBPs when exposed to a disinfectant chemical. Each source water may have different species of organic matter in different concentrations.

One of the earliest examples of the successful application of GAC for disinfection byproduct control dates back to the late 1980s, when Greater Cincinnati Water Works (GCWW) recognized that conventional treatment alone wouldn’t permit compliance with then-proposed DBPR Stage 1. The regulation aimed to reduce the maximum contaminant level for total trihalomethanes (TTHM) from 100 parts per billion (ppb; also referred to as micrograms/liter or µg/L) to 80 ppb. This presented a challenge to the water utility since its current treatment strategy obtained a finished water TTHM level of about 80 ppb, which was too close to the proposed maximum contaminant level than was comfortable for the utility.

By adding postfiltration GAC treatment, the utility achieved more than 50% total organic carbon reduction (see graphic at top right). This reduced the TTHM formation to less than 50 ppb, achieving compliance with not only DBPR Stage 1, but also with the impending Stage 2 rule — which also has a TTHM maximum contaminant level of 80 ppb. Similar results were achieved with reducing haloacetic acids.

Cost: initial investment vs. life-cycle value

As compared to switching to alternate disinfectants such as chloramines, adopting granular-activated carbon is an undeniably greater expense — perhaps by as much as a factor of 10. Converting to chloramines for most municipalities may amount to little more than adding an ammonia feed system to the existing chlorine feed system and making some relatively minor changes to the distribution system. In contrast, adding GAC involves not only the purchase (and periodic replacement) of the activated carbon itself, but also the installation of a structure to contain it. The carbon is generally contained either in a concrete filter bed (similar to the sand/gravel/anthracite filters used by most municipal water treatment plants) or in a series of steel pressure vessels.

However, switching to GAC still amounts to a minimal overall life-cycle cost, especially when compared to membranes and ozone + bio GAC treatments (more on these later). For a moderately sized community, GAC can generally be installed and maintained for less than $1/month per person.

In Greater Cincinnati Water Works’ case, the utility added 175-mgd postfiltration GAC facilities with onsite GAC reactivation in January 1992. The $63.9 million price tag included engineering, construction, initial GAC fill, and a reactivation facility. The average cost of amortized construction and operation is estimated at 6 cents/day, or $5.40/quarter, for the average single-family household.

GAC can be made even more affordable, and more environmentally friendly, by adopting “custom reactivation.” Traditionally, once the carbon becomes exhausted, it’s removed, disposed of, and replaced by a new, fresh carbon. In recent years, however, a new approach has been introduced that recycles the spent carbon, restores its capabilities, and returns the carbon to the drinking water plant for continued use. This custom reactivation approach has dramatically reduced the cost of GAC use by as much as 40% in some cases, while also reducing the carbon footprint associated with GAC manufacture by up to 80%.

Depending on plant location, carbon reactivation can become expensive due to transportation costs. But even with these expenses, the overall life-cycle value of GAC remains lower than that of its direct precursor treatment process competitors.

There are also ways to save future reactivation costs. Because GCWW built its own reactivation facility when it added GAC facilities in 1992, the utility controls when the carbon is reactivated — which has further reduced DBP formation dramatically. The initial capital investment for the reactivation facility itself was quite steep — $6.13 million dollars — but it has reduced the cost of replacing the GAC by about 47% — by cutting out transportation fees and the high expense of purchasing all-new virgin carbon.

Some activated carbon suppliers may also offer an alternative financing option to make GAC more affordable. They provide leasing/payment plans in exchange for a contracted supply of activated carbon and the associated services for a predetermined period of time. This allows municipalities to spread out the cost of the acquisition over a number of months or years, paying in monthly or quarterly installments and assigning the costs to annual operating budgets rather than capital budgets.

Additional benefits and drawbacks

Granular-activated carbon is a multitasking technology. When a utility installs GAC for DBP regulatory compliance, it is also installing a technology that improves the taste of the water, removes offensive odors, and protects against any industrial volatile organic compounds that might have been spilled into the source water. Additionally, this same carbon provides a barrier against the variety of endocrine-disrupting and pharmaceutical compounds that make their way into our water supply.

Cincinnati noticed significant success in this area when it participated in a 2007 EPA/U.S. Geological Survey study of water utilities using surface water for the occurrence of pharmaceutical compounds in the Ohio River and its tributaries. Seventeen different pharmaceuticals and personal care products were initially detected in GCWW intake source water. After conventional treatment, all 17 remained at a reduced concentration. After GAC treatment, only trace levels of caffeine remained.

GAC does have several disadvantages to keep in mind, including the relatively high initial investment and its large footprint. The space requirements needed to accommodate GAC facilities, however, can be minimized by employing steel pressure vessels in lieu of conventional concrete gravity filters.

Other treatments

Membrane filtration (ultra/nanofiltration) and ozone + bio GAC combination systems are also effective at removing organic material (see table at the top of this page).

Membrane treatment is relatively simple from an operational perspective and can be quite effective for particle and microbial removal. However, it poses a number of disadvantages including increasing pressure drops, increased biological growth, requiring additional prefiltration treatments to prevent clogging, and requiring disposal of concentrated waste. Its high initial capital cost and ongoing operating costs may also be difficult for smaller operations to swallow (see table on page 39).

Ozone + bio GAC combined systems provide reliable removal of organic precursors with extended run times due to biological activity. However, as with the installation of post-filter GAC beds, capital costs of the system and GAC bed can be high. Operation of the ozone system can be somewhat complicated and costly as well, based on energy demand. In addition, ozone may promote the formation of halonitromethanes, an emerging class of DBPs not yet regulated by EPA but still potentially harmful.

You can also combine GAC with complementary technologies such as enhanced coagulation, pH control, and even postfiltration UV disinfection. These approaches can boost the matter-removal performance of the carbon as well as remove some organic matter constituents that are not as effectively removed by GAC. (See sidebar at right.)

Meeting future regulations

With ever-evolving advancements in water-science analysis and health studies, many cities can expect to face increasing federal and state drinking water regulations. Utility managers must find a water treatment solution that both complies with pending regulations like DBPR Stage 2 and minimizes cost.

No two water systems (or their budgets) are the same. But most municipalities will find that by adding GAC to water treatment processes, they can significantly improve their overall approach at protecting the public health of the communities they serve while still operating in an affordable manner for years to come.

— Zappa (lzappa@calgoncarbon-us.com) is director, municipal water market, for Calgon Carbon Corp. (www.calgoncarbon.com); Pohlman (rpohlman2@cinci.rr.com) is retired from the City of Cincinnati where he was a water treatment supervisor.

Using UV to increase organic matter removal

Greater Cincinnati Water Works is installing UV technology to enhance the water disinfection process at its Richard Miller Treatment Plant.

By placing eight 48-inch medium-pressure UV reactors after the existing granular-activated carbon (GAC) contactors in its postfiltration process, Greater Cincinnati Water Works (GCWW) expects to provide even cleaner water while saving about $250,000/year in operational expenses.

The estimated savings is due in part to the high UV transmittance (UVT) of the GAC-treated water. UV transmittance is related to the organics, colloidal solids, and other material in water that absorb and scatter UV light. In a UV disinfection system, if the UVT of the water is too low then the UV light can’t penetrate the water as efficiently, thereby reducing the effectiveness of a UV dose delivered by the system. Because the utility uses GAC to remove organic matter, making the water more clear, the water will have a higher UVT as it flows through UV radiation for treatment. A higher UVT means less energy is needed to apply the proper UV dose to the water.

The project, which broke ground in 2010, also includes installation of 160 solar panels on the roof of the new UV facility to help reduce the utility’s carbon footprint.

Why upgrade with UV

The U.S. EPA has identified UV disinfection as one of the best technologies to protect against certain contaminants in drinking water such as the chlorine-tolerant Cryptosporidium and Giardia. The utility’s GAC-treated water is currently chlorinated.

“UV disinfection treatment adds an additional level of protection to ensure public health,” says biologist Edna S. Kaneshiro, PhD, distinguished research professor with the University of Cincinnati’s Department of Biological Sciences. Kaneshiro also serves on the GCWW Advisory Committee on Quality of Drinking Water. “One reason we need this additional level of protection is because we have sources of contamination, including wastewater treatment plants, upstream of our intakes, and we don’t know what is being dumped into the Ohio River.” The wastewater treatment plant, located near Alexandria, Ky., discharges just 11 miles upstream of Cincinnati’s drinking water intakes. The Ohio River is the utility’s primary raw water source.

The $30 million project was funded by selling municipal bonds. When the 19,600-square-foot UV Disinfection Treatment Facility is operational, GCWW will be the largest water utility in North America to use UV following sand filtration and GAC adsorption. The project is scheduled to be finished in 2013.


Using UV to increase organic matter removal

Greater Cincinnati Water Works is installing UV technology to enhance the water disinfection process at its Richard Miller Treatment Plant.

By placing eight 48-inch medium-pressure UV reactors after the existing granular-activated carbon (GAC) contactors in its postfiltration process, Greater Cincinnati Water Works (GCWW) expects to provide even cleaner water while saving about $250,000/year in operational expenses.

The estimated savings is due in part to the high UV transmittance (UVT) of the GAC-treated water. UV transmittance is related to the organics, colloidal solids, and other material in water that absorb and scatter UV light. In a UV disinfection system, if the UVT of the water is too low then the UV light can’t penetrate the water as efficiently, thereby reducing the effectiveness of a UV dose delivered by the system. Because the utility uses GAC to remove organic matter, making the water more clear, the water will have a higher UVT as it flows through UV radiation for treatment. A higher UVT means less energy is needed to apply the proper UV dose to the water.

The project, which broke ground in 2010, also includes installation of 160 solar panels on the roof of the new UV facility to help reduce the utility’s carbon footprint.

Why upgrade with UV

The U.S. EPA has identified UV disinfection as one of the best technologies to protect against certain contaminants in drinking water such as the chlorine-tolerant Cryptosporidium and Giardia. The utility’s GAC-treated water is currently chlorinated.

In the illustration above, the red line  labeled with GAC  shows the average UVT of the utilitys GAC-treated effluent water when combined with UV disinfection. The blue line  labeled w/o GAC  is sand filter effluent water that has not been treated with GAC. The average UVT of UV-disinfected GAC effluent water is 7% higher than that of UV-disinfected water that hasnt been treated with GAC.

In the illustration above, the red line — labeled “with GAC” — shows the average UVT of the utility’s GAC-treated effluent water when combined with UV disinfection. The blue line — labeled “w/o GAC” — is sand filter effluent water that has not been treated with GAC. The average UVT of UV-disinfected GAC effluent water is 7% higher than that of UV-disinfected water that hasn’t been treated with GAC.

Credit: Greater Cincinnati Water Works

“UV disinfection treatment adds an additional level of protection to ensure public health,” says biologist Edna S. Kaneshiro, PhD, distinguished research professor with the University of Cincinnati’s Department of Biological Sciences. Kaneshiro also serves on the GCWW Advisory Committee on Quality of Drinking Water. “One reason we need this additional level of protection is because we have sources of contamination, including wastewater treatment plants, upstream of our intakes, and we don’t know what is being dumped into the Ohio River.” The wastewater treatment plant, located near Alexandria, Ky., discharges just 11 miles upstream of Cincinnati’s drinking water intakes. The Ohio River is the utility’s primary raw water source.

The $30 million project was funded by selling municipal bonds. When the 19,600-square-foot UV Disinfection Treatment Facility is operational, GCWW will be the largest water utility in North America to use UV following sand filtration and GAC adsorption. The project is scheduled to be finished in 2013.