Optimize Water Cleanup with Activated Carbon

Follow a few pointers to make the most of your adsorption system

By Robert Deithorn, Calgon Carbon Corp.

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Treating and reusing process water is a multidimensional challenge for process plants. Compliance with regulatory requirements to prevent and mitigate industrial pollution can require significant capital investment as well as ongoing maintenance outlays. The increasing scarcity and cost of fresh water for production processes also compounds the problem. (For insights on how major chemical manufacturers view water issues, see "The Tide is Turning.") Ultimately, equally compelling pressures to address product purification needs, reduce the carbon footprint, and operate efficiently and profitably ratchet up the challenges.

The hard truth is that process plants need a practical solution that's economical and regulatory-compliant. For more than 40 years, no other method has offered better results for control of organic chemicals in liquids and gases than activated carbon adsorption. However, some plants undermine their treatment efforts. So, let's go over a few pointers.

DETERMINE THE BEST METHOD
Don't presume that one process can handle everything. Instead, put in time to identify the most appropriate technology for the job(s) at hand. A wide range of treatment technologies, e.g., reverse osmosis (RO), ion exchange and granular activated carbon (GAC), exist and can be used alone or in combination for industrial water treatment. The most-appropriate technology depends upon the feed water quality and effluent water purity required for a given application.

RO systems typically remove or reduce dissolved mineral salts, organics and other particles; they may require water pretreatment to protect the RO membranes against fouling, scaling or chemical degradation. Such systems usually incur higher investment and operating costs than a GAC system.

Systems with ion exchange resins can produce high-purity deionized water for reuse by exchanging the ions present in the water. The choice of resin depends upon the specific ions present. These systems typically aren't used to remove soluble organic species as GAC does.

GAC is a highly porous, high-surface-area adsorbent onto which contaminant molecules collect. It has an excellent track record as a cost-effective material for removing organic contaminants from liquids and gases. At process plants, GAC finds wide use in liquid and gas purification and to purify and reuse process water. GAC also meets regulatory requirements in wastewater treatment, groundwater remediation and for volatile organic compound (VOC) abatement in vapor-phase applications. GAC technology can help plants maintain emissions permit levels, meet state and local environmental requirements, and adhere to U.S. Environmental Protection Agency (EPA) guidelines and regulations such as the Resource Conservation and Recovery Act, the Clean Water Act and the Clean Air Act, particularly its National Emission Standard for Hazardous Air Pollutants program and benzene regulations.

Recycling or thermally reactivating carbon gives process plants the opportunity to reduce cost and waste, save energy, lower carbon-dioxide emissions and conserve natural resources while decreasing the long-term liability of spent-carbon disposal.

In fact, GAC has been classified as an EPA Best Available Technology (BAT) for removal of many organic contaminants. As defined by the EPA, "BAT effluent limitations guidelines, in general, represent the best existing performance of treatment technologies that are economically achievable within an industrial point source category or subcategory." That being said, how does a chemical company determine if GAC adsorption is the best technology to meet its organic contaminant removal needs?

SELECT THE RIGHT GAC`
A fundamental consideration is choosing the type of activated carbon that will deliver on your water purification and reuse goals. Virgin GAC is best reserved for initial system startup and reactivation of spent GAC (which we'll discuss later).

A standard, unimpregnated, bituminous-coal-based material made by the re-agglomeration method is used most often for adsorption of organic contaminants in industrial applications because it has a wide range of pore sizes to adsorb a broad variety of organic chemicals.

Re-agglomerated GAC is produced by grinding the raw material to a powder, adding a suitable binder for hardness, recompacting and then crushing to the specified size. Next, the material is thermally activated in a furnace using a controlled atmosphere and high heat. The resultant product has an incredibly large surface area per unit volume and network of submicroscopic pores where adsorption takes place. GAC has the highest volume of adsorbing porosity of any known material. Amazingly, five grams of re-agglomerated carbon have the surface area of one football field. Re-agglomerated carbon is generally preferred over direct activated because it's a more-robust material with a fully developed porosity, and at the same time has the necessary strength to withstand use and reuse.

To ensure optimal GAC adsorption operations, process plant installations typically include carbon adsorption equipment with the associated transfer piping. These systems can be operated with single- or multi-stage vessels, depending upon the desired treatment objective. The adsorption system generally follows chemical clarification and filtration and precedes disinfection, if these steps are required.

Activated carbon can remove a variety of VOCs and semi-volatile organic compounds in one unit operation. It's important to fully characterize a stream prior to analyzing it for activated carbon purification. Information on a vapor-phase stream should include all VOCs and gases present, humidity concentration, temperature and pressure. All these factors will affect activated carbon performance. Similarly, characterization of a liquid-phase stream, including its ionic content and profile, types and concentrations of suspended solids, and pH, is crucial. Capacity tests that measure the mass of adsorbate removed per unit weight or unit volume of activated carbon then can measure adsorption effectiveness.

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