Since 1979, methyl tertiary-butyl ether (MTBE) has been used as an octane-enhancing replacement for lead in gasoline. In the 1990s its use became more widespread under the Environmental Protection Agency’s (EPA) Oxygenated Fuel and Reformulated Gasoline Programs. As a gasoline oxygenate, MTBE helps gasoline burn cleaner. Unfortunately, MTBE-contaminated groundwater has become a major problem in many areas throughout the U.S. because of leaking underground storage tanks.
MTBE has been banned or restricted as a gasoline additive and mainly replaced by ethanol. Because of its high solubility in water, when it escapes into the environment through gasoline releases, MTBE is capable of traveling much farther from the source of the original spill than other petroleum constituents. Also, because MTBE is poorly broken down in the soil by natural biodegradation, it can persist as a source of groundwater contamination for many years. The EPA has recommended MTBE levels in drinking water should be less than 20 to 40 parts per billion (ppb). Many states have adopted much lower levels.
Activated Carbon (AC) Treatment
The physical and chemical characteristics of MTBE make it more difficult to remove from water than other common organic contaminants. Treatment options like air stripping, biological treatment and oxidation may work to some degree but are generally cost prohibitive for small point-of-use (POU) and point-of-entry (POE) systems. AC can be effective in removing MTBE, but its removal can be difficult because of the high solubility of MTBE in water. Therefore, it is critical that the correct type of AC be utilized for cost-effective removal.
Traditional AC used in POU and POE water treatment applications are coconut shell-based and bituminous coal-based. Coconut shell-based AC has a greater number of micropores. Micropores are defined as pores less than 20 angstrom units (2 nm) in diameter. AC produced from bituminous coal has fewer micropores but larger pores called mesopores and marcopores. The quantity of micropores present in coconut shell-based AC is about 50% higher than bituminous coal-based AC. The greater amount of micropores in coconut shell-based AC allows it to have a higher capacity to adsorb MTBE. In addition, coconut shell-based AC has a higher retentivity. High retentivity ensures MTBE will not come off or desorb off the AC when the influent MTBE concentration fluctuates. Coconut shell-based AC is also purer, containing 97% to 98% carbon, whereas bituminous coal-based AC contains about 88% to 94% carbon.
To partially overcome some of the deficiencies of coal-based AC, some producers have optimized their coal-based AC for better MTBE adsorption. The optimized AC is about 30% heavier, requiring 30% more AC be purchased to fill a given volume compared to coconut shell-based AC. To compare the different ACs, the saturation capacities of the ACs were measured for MTBE in water. In a comparison of coconut shell-based AC, bituminous coal-based AC that is considered an industry standard for water treatment and an optimized coal-based AC for MTBE, the data shows that at low MTBE concentrations in water, the coconut shell-based AC has a MTBE capacity 2.5 times higher than the bituminous coal-based AC. Also, the coconut shell-based AC has a MTBE capacity 1.8 times higher than the optimized coal-based AC.
The AC cost was calculated to treat 1,000 gal of water. It was assumed that the cost of all AC was the same at $1.50 per pound. This most likely will not be the case, because coconut shell-based AC is usually lower cost. Also, optimized bituminous coal-based AC sells at a premium. The cost savings using coconut shell-based AC with less frequent AC change-outs is significant.
For POE systems, the smallest mesh size AC should be used that has acceptable pressure drop characteristics. Usually a 12-by-30 (1.7-by-0.6 mm) or 12-by-40 (1.7-by-0.4 mm) mesh size AC is preferred. An 8-by-30 (2.4-by-0.6 mm) mesh size would be too large of a particle size. For acceptable MTBE removal performance, flow rates between 0.75 to 1 gal per minute (gpm)/cu ft of AC should be used. With a 20-by-50 (0.8-by-0.3 mm) mesh size AC, a flow rate up to 1.5 gpm/cu ft of AC could be utilized. But with the smaller size AC, you can expect pressure drop to be three to four times higher compared to the 12-by-30 or 12-by-40 mesh size AC. For POU systems, carbon block systems are normally utilized.
Numerous systems have been certified to NSF/ANSI Standard 53 for MTBE removal. Carbon block systems have been rated for 0.5 to 0.9 gpm to treat up to 1,300 gal before the carbon block requires replacement. The majority of the carbon blocks for MTBE removal are coconut shell-based AC. Coconut shell-based AC provides cost-effective treatment to remove MTBE from contaminated water.