PILOT THE PROCESS
When considering a GAC system, a pilot plant study can determine if the technology will meet discharge permit requirements. Pilot plant testing of actual streams is the most reliable means to predict performance. Pilots should match the full-scale project equipment as closely as possible as far as superficial velocity, bed depth and empty bed contact time. For example, you can conduct an organic contaminant removal trial that uses a portable liquid-treatment unit and a liquid-phase GAC. Organics readily adsorbed by GAC include:
• aromatic solvents (benzene, toluene and nitrobenzenes);
• chlorinated aromatics (polychlorinated biphenyls, chlorobenzenes and chloronaphthalene);
• phenols and chlorophenols;
• fuels (gasoline, kerosene and oil);
• polynuclear aromatics, e.g., acenaphthene and benzopyrenes; and
• pesticides and herbicides, e.g., DDT, aldrin, chlordane and hepthaclor.
The pilot study also should quantify optimum flow rate, bed depth and operating capacity for a particular liquid or gas. This information is needed to determine the dimensions and number of carbon contactors required for continuous treatment. Other options also might be possible. For example, point source treatment of lower flows may provide a more-economical alternative than whole effluent treatment. Through use of computer predictive modeling or treatability studies, a supplier can determine if carbon adsorption technology can effectively reduce the concentration of the pollutants to levels that would allow discharge into the total wastewater stream — thus eliminating the need for more-expensive treatment methods for the total wastewater flow. By using these various studies and analyses, activated carbon manufacturers accurately can predict the viability as well as capital and operating costs of applying adsorption treatment, allowing you to compare these costs to those of other applicable technologies.
KNOW WHEN TO REPLACE THE GAC
During the carbon adsorption process, the available surface and pores of the GAC fill up with chemicals. At some point, the system no longer can meet the required performance criteria — often this is determined when the effluent quality from the carbon treatment vessels begins to approach the quality of the influent. The carbon is said to be "spent" and must be replaced. The spent carbon then either is discarded or recycled for reuse.
OPT FOR REACTIVATION
Three alternatives exist for dealing with spent carbon. The first is shipping it to a landfill or incinerator. However, this approach necessitates the purchase of new carbon and isn't the most environmentally friendly.
Regeneration via either a chemical or steam process may offer advantages over disposal in a landfill. However, this option generally is reserved for recovering and reusing a valuable adsorbate. It also is less efficient than reactivation.
The third option, high-temperature thermal reactivation, usually makes the most sense. The process destroys the adsorbed organic compounds and restores the GAC's adsorptive capacity. Reactivation can achieve up to 95% recovery of the virgin activated carbon's capacity. The reactivated material then can be blended with a small amount of virgin carbon to make up for the minor loss of volume.
Over the past few years, reactivation and reuse have surged in popularity at process plants for several reasons. From an environmental standpoint, reactivated carbon is considered an environmentally friendly product because reactivation produces only about 20% of the greenhouse gases generated in making virgin activated carbon. Moreover, GAC has a nearly infinite reactivation capability, so it rarely ends up in a landfill or incinerator. Reactivation is a logical choice for companies that incorporate sustainability in their long-term strategy.
Reactivation also delivers significant cost savings — it typically costs 20–40% less than purchasing virgin GAC. In addition, it ends the chain of custody for adsorbed contaminants, eliminating spent carbon handling and disposal liabilities. Some facilities may qualify to receive environmental credits issued by regulatory agencies for waste minimization because reactivated carbon is considered a recovered resource.
The profiling and testing processes to identify reactivation as an option are very straightforward. Depending upon the economics and volume of spent carbon produced, some plants may opt for onsite reactivation facilities. Those deciding to contract for off-site reactivation services should look for a vendor with the following field capabilities:
• spent carbon analyses;
• spent carbon removal and packaging;
• appropriate waste handling (hazardous or non-hazardous);
• transportation to the reactivation plant;
• carbon vessel inspection with minor repair; and
• vessel reloading with reactivated carbon.
TACKLING EMERGING APPLICATIONS
Carbon adsorption has treated some organic contaminants for more than four decades and is considered a mature technology. However, its role promises to expand as the EPA regulates additional chemicals. The agency maintains a contaminant candidate list of chemicals of emerging concern (CECs) that the EPA may consider for future regulation. Some carbon manufacturers like Calgon Carbon provide forward-looking assistance to chemical makers by monitoring the CEC list, offering a preview of what federal and state rules may require for treatment technologies, and conducting research and development to advance the use of activated carbon and treatment methods for removing CECs.
Every chemical manufacturer must contend with the ongoing demands of achieving regulatory compliance while maintaining operational profitability and creating high-quality products. For organic contaminant removal from liquids and gases in process applications, GAC remains a proven, reliable way to satisfy environmental management demands and product purification needs. Furthermore, use of reactivated carbon instead of virgin carbon offers additional cost efficiencies and environmental benefits.