Rochelle Phillips and Drs. William Hecker and Vincent Wilding, Chemical Engineering

During the treatment of municipal wastewater, several means are used to accomplish the breakdown of the wastewater and its solid components into innocuous substances. At one point, the filtered solids are combined in a digester with various bacteria. These bacteria convert the waste into carbon dioxide, methane gas, water, and small amounts of hydrogen sulfide.

At the Springville Wastewater Treatment facility the gaseous products of the digester are collected, vented and flared. The gas from the digester is approximately sixty percent methane, on a dry basis. Methane is the main component of natural gas. The wastewater treatment facility and the municipal power plant are on the same property and the power plant burns methane to generate the electricity for the city of Springville. The original intent of the engineers who designed the system was to transport the digester gas to the power plant where it would be burned as part of the electricity generating process. This plan was not put into action, in part, due to the corrosive nature of the gas.

Methane gas alone is not corrosive. However, the carbon dioxide and hydrogen sulfide present in the digester increase the acidity of the gas and combine with the moisture in the gas to create a highly corrosive environment. The operators at the power plant could not use the digester gas in their normal equipment due to this corrosiveness. There are capabilities for the gas to be burned in a converted diesel burner but this is less efficient and not the optimal use for the gas.

There are many systems available for removing hydrogen sulfide or carbon dioxide from gases and liquids. Large oil and natural gas refineries use the Claus process for removing hydrogen sulfide. This process requires at least four reactors to obtain the purity required by the power plant. The process is generally economically feasible for systems over 50 long tons per day. The Springville system produces less than two pounds per day.

Research of other removal systems showed a similar tendency to be economically feasible only for large facilities. Oxidants, caustic scrubbers, alkanolamine absorption and many of the other available systems were not adjustable to the small quantities of waste produced at Springville. Two processes that seemed more likely were the Lo-Cat iron complexing/reducing system available from US Filter and a carbon absorption system from Calgon Carbon.

Lo-Cat is an iron chelating complex. Small particles of an iron compound conjugate with the hydrogen sulfide to keep it in solution. The hydrogen sulfide is then reduced to sulfur. The conjugated reduced complex is then removed and disposed. The chelating mixture must be replaced periodically. A cylindrical vessel is also required. To use Calgon Carbon, a cylindrical vessel must also be installed. The vessel contains activated carbon granules that react with the hydrogen sulfide, binding it to the carbon. When all of the sites for absorption are filled, the carbon is replaced.

Calculations for these projects showed that the advantages to the environment and the equipment do not offset the projected costs of installation and design. Fortunately, the gas can be burned and used in the diesel burner now that some other problems have been worked out. As technologies for removing hydrogen sulfide become less expensive, and as the restrictions on release of sulfide compounds to the atmosphere become more stringent, the addition of cleaning technologies may become more advantageous.