The Real Cost of Treating Drinking Water with Granular Activated Carbon

What's the cheapest way to get a glass of clean water? The answer may surprise you. Especially if, like many people, you are under the impression that powdered activated carbon (PAC) is more cost-effective than granular activated carbon (GAC) in a typical water treatment process .

Over the last decade a number of articles have been published discussing the cost of installed GAC treatment systems. The conclusion reached by the majority of these studies is that GAC is cost prohibitive and that PAC represents a clear savings. A study by Adams and Clark, for instance, indicates that the total cost estimate for GAC systems ranges from 10 cents to $1.00 per 1,000 gallons of water, depending on the size of the system (specifically 150 mgd to 0.1 mgd respectively).

Those numbers probably are enough to make the managers of most water treatment plant think twice about implementing GAC, no matter if the GAC-treated water may taste and smell better. But a recent survey of drinking water facilities across the U.S. which use GAC revealed some interesting economic results. The study was conducted by Calgon Carbon Corporation, a company which makes and markets both PAC and GAC.

Not So Fast
What isn't immediately apparent is that research studies like this have been based on GAC treatment facilities which had been constructed from the ground up. They lump the cost of GAC with high construction expenses-excavation and site work, manufactured equipment, concrete, steel, labor, pipes and valves, electrical components, and instrumentation. Even the U.S. EPA, in a recent study, estimated capital and operations and maintenance (O&M) costs for complete treatment facilities without considering existing filtration systems (green-field installations).

In reality, however, today very few water treatment plants are built from the ground up with the activated carbon process in the design. The fact is, most U.S. plants using activated carbon have retrofitted existing facilities. Retrofitting puts cost comparisons between GAC and PAC in a more realistic light.

Real Numbers from Real Plants
Water facilities across the country which have been retrofitted to incorporate GAC were recently surveyed regarding the actual costs they have experienced. The results show most facilities incurred little, if any, capital costs to retrofit their plants. The majority of sites required only a replacement of the current media (typically sand/anthracite) with GAC. While the height of the filter troughs usually had to be increased to accommodate the proper quantity of carbon and achieve the necessary contact time, the cost for this was reported to be insignificant.

Comparing Apples with Apples
Since retrofitting eliminates or drastically reduces the capital cost associated with GAC, how does the technology stack up against PAC in terms of day-to-day operating expenses?

Consider that the typical PAC water treatment facility, using an average of 3, 5 or 7 mg/l of powdered carbon annually, spends between $31,000 and $75,000 per year to produce 10 mgd of treated water. Since GAC continues to provide taste and odor control for three years, it is necessary to first annualize the cost for GAC treatment, then compare those numbers with the PAC numbers.

The cost comparison results are shown in the figure. At a typical empty bed contact time (EBCT) of 5 minutes, the cost of GAC is actually comparable to PAC at standard dosages. Obviously, the numbers vary based on the EBCT and PAC dosage level, but-for the majority of the water plants responding to the survey-the additional cost calculated per customer family lies between 1/4 to 1/2 cent per day for either PAC or GAC.

If your process currently includes powdered carbon treatment, a quick rule of thumb can be applied to estimate the GAC conversion quantities. Granular carbon is roughly three times more effective on a weight-basis than PAC for taste and odor control. That means if you currently use 90,000 pounds of PAC, you can achieve the same quality (or better) of water with 30,000 pounds of GAC.

Go with the Flow
Many water plant operators assume that, even with GAC in place, they will still need some level of PAC dosage. This is not the case. At typical dosages, a granular carbon bed can completely replace powdered carbon, producing a comparable, if not superior, quality of water during the life of the carbon.

To determine an accurate PAC vs. GAC cost comparison for a particular facility, you must also consider the influent conditions of the water and the ultimate treatment objective. A report prepared by Environmental Engineering & Technology (EE&T), a consulting firm, for Erie County Water Authority in Pennsylvania concludes that the use of GAC filter media is cost-effective. The engineers based their findings on comparing GAC filter usage with PAC dosages ranging from 15 mg/l to 30 mg/l, for a 70 mgd plant flow at 2.0 and 1.0 year replacement frequency.

EE&T's EBCT values in these studies averaged two minutes. In normal circumstances, at least a five minute EBCT is recommended for effective treatment over several years. It is better to operate at the five-minute contact time and have the carbon on-line three years, instead of for only one year at a two-minute EBCT.

Other Differences
As a result of installing GAC, retrofitted plants surveyed reported a minor savings in sludge disposal costs and utility costs (for running the mixer for the PAC slurry).

Their most significant issue in terms of savings, however, centered on chemical cost changes. After installing GAC, the use of chemicals-including disinfectants (primarily chlorine), coagulants/polymers (aluminum sulfate, sodium sulfate, ferric sulfate) for turbidity removal, and potassium permanganate for taste and odor control-declined substantially.

For example, the City of Cincinnati reduced chlorine costs by two thirds when a switch was made to GAC. Staff estimated an incremental cost per family of $22 annually, or six cents per day. Another city reported that the decrease in chemical costs was so significant that the move to GAC actually resulted in a net savings.

Beyond Budget
So, if the cost of GAC and PAC are comparable, why choose granular carbon over powdered?

• Passive technology-Once a GAC system is installed, an operator does not have to determine dosages on a day-to-day basis as influent conditions change. This is especially important if the source water comes from a highly traveled river where spills or contamination occur frequently.
• More convenient-If PAC is used year-round, replacement with GAC will save time and money. Most PAC users dose only at particular times, roughly four months a year, but if PAC is used on a year-round basis, the advantages of GAC increase dramatically, not only in terms of cost-effectiveness, but also convenience.
• Less waste-Reducing powered carbon cuts the quantity of sludge that requires disposal.
• Less contamination-GAC eliminates the problem of the fine powdered carbon bleeding through the filters and being carried out into the distribution system.
• Better taste, better smell-Many water plants have been motivated to switch to GAC from PAC by the need to reduce taste and odor complaints. Survey results suggest that after installation of GAC, complaints disappear. Customers notice the improvement and comment on it. After the carbon is on-line for several years, the return of undesirable tastes and odors can be detected, prompting the water plant to change out the material.
• More widely effective-Given the direction of future regulations, particularly the disinfection/disinfection by-products (D/DBP) rule, and GAC's undisputed effectiveness for organics removal, the advantages of implementing a granular activated carbon system are clear.

The Bottom Line
In the real world, water utilities do not generally build new GAC facilities, they retrofit existing sites-at minimum expense. Data shows that the incremental cost of installing GAC is usually less than 1/2 cent per day per customer family, with the high end at 6 cents per day per family. (The high end cost, as in the Cincinnati case, factors in the construction costs for an on-site carbon reactivation facility.) After the installation of a GAC system, not only is PAC dosage eliminated or significantly reduced, the consumption of other treatment chemicals also decreases measurably. And the water is said to taste and smell better too.

What's the cheapest way to get a glass of clean water? When the actual numbers are annualized and compared, and related issues of waste and maintenance, and individual site specifics and objectives, are all factored in, granular activated carbon could very well be the cost-effective alternative.

About the Authors: This article was written by Annette Vickers, Marketing Manager-Municipal and Gary Van Stone, Business Director-Municipal, both with Calgon Carbon Corporation in Pittsburgh, Pennsylvania; Tel. 412-787-6851; Fax. 412-787-6676.