This invention relates to a method for removing slime that deposits on packing material inside a tower. More particularly, the invention concerns removal of slime deposits on tower packing material in which the slime is created by air stripping of groundwater to remove organic contaminants.
A major problem today is the contamination of groundwater by hydrocarbons, such as gasoline, benzene, toluene, or xylene that leak out of underground storage tanks. Groundwater can also be contaminated by chlorinated solvents, such as trichloroethylene, trichloroethane, perchloroethylene, and methylene chloride, which are used in degreasing operations and then dumped into open pits in the ground. Another way that the water in underground aquifers becomes contaminated is from petroleum hydrocarbons that are dumped on top of the ground near oil and gas well sites.
One way to remove organic impurities from groundwater is by a technique referred to as air striping. In a typical air-stripping process, the contaminated water is passed through a tower containing a packing material. As the water moves through the packing material, it is contacted by air, at ambient temperature and pressure, that is moving counter-currently to the water. The air strips a major portion of the organics from the water and this stream may be passed through a catalytic incinerator, or other means which converts the organics to compounds that can be safely discharged into the atmosphere. The decontaminated water stream is discharged from the tower according to an acceptable procedure.
Although air stripping is a useful technique for removing organic contaminants from groundwater, it has a major drawback. The oxygen in the air provides an excellent environment for bacterial growth, which forms as a slime deposit on the packing material. In a fairly short time, usually only a few weeks, the slime deposit will increase the pressure drop through the system enough to make the process ineffective.
A common method for inhibiting the formation of the slime deposits is to chemically treat the contaminated water before it is introduced into the tower. For example, in one technique acidic or alkaline compounds are added to the water to control the pH level. Another technique involves adding oxidizing biocides to the water, such as chlorine, chlorine dioxide, chlorine donors, and ozone. Although the chemical treatment is reasonably effective, it has several disadvantages. These include the cost of equipment required to store and handle the chemicals safely, the necessity of adding chemicals to clean water, and the complex operations involved in pH control.