Editor's Note: This article is part of a profile on the Cudahy (Wisconsin) Water Utility, which was published in the July 2015 issue of Treatment Plant Operator. Read the rest of the profile here, and watch a video of the plant here. And while you're at it, make sure you're getting every issue delivered monthly to your mailbox by subscribing to the print version of TPO.

Lake Michigan is an excellent source for drinking water — clear, abundant and consistent — at least for most of the year. Since the late 1990s, the lake has been subject to seasonal algae growths that die off and release organic compounds such as geosmin and 2-methylisoborneol (MIB). Even at levels as low as 10 parts per billion, these can impart a musty, moldy odor and a bad taste to drinking water.

The Cudahy (Wisconsin) Water Utility, just south of Milwaukee, had to deal with these compounds at its 6 mgd water treatment plant. Plant superintendent Frank Miller and his team found a remedy in granular activated carbon caps for the plant’s eight 750,000 gpd sand filters. The odor and taste events, which typically occurred in late summer or early fall, are no longer an issue. The utility has the option to also add powdered activated carbon to its rapid-mix tank at the head end of the treatment process, if it appears a seasonal event will be especially severe.

Responsible recycling
Calgon Carbon supplies granular activated carbon in a 0.7 mm sieve size. The filter beds now include 12 inches of the carbon on top of 15 inches of sand. In place of virgin carbon, Cudahy chose Calgon’s reactivation service, which saves the expense of spent carbon disposal. Each year, the carbon is changed out in two of the eight filter beds. Calgon takes it to a reactivation facility and returns it.

Reactivation is a relatively simple process — trouble-free for the carbon user. After activated carbon has exhausted its capacity to adsorb organic compounds, it is removed and subjected to thermal reactivation. During the process, the spent carbon is heated in furnaces devoid of oxygen, using steam as a selective oxidant. Organics adsorbed on the carbon are either volatilized away or pyrolysed (broken down by heat) into a carbon char. Volatilized organics are destroyed in the furnace's afterburner, and acid gases are removed by a chemical scrubber.

The high-temperature reaction with steam restores the carbon’s adsorptive capacity so that in most applications it will perform nearly as well as virgin carbon, according to Calgon.

Earth-friendly
Calgon notes that a benefit of carbon reactivation and recycling is a lower CO2 footprint than for the production, supply and use of virgin activated carbon. In addition, the spent carbon can be restored at a lower cost than the manufacture of activated carbon from raw material.

Calgon can use computer modeling to provide an assessment of standard reactivated carbon products against virgin carbon for most applications. If necessary, laboratory evaluation by accelerated column testing or pilot scale trials can be conducted.