Mary Cragun and Dr. William Hecker, Chemical Engineering
The treatment of wastewater or sewage is a multi-step process. One of the steps involves the treatment of the organic mass collected during the treatment process. There are several different methods of treating this organic mass or sludge. At the Springville Wastewater Treatment Facility, the sludge is decomposed under anaerobic conditions into simpler forms of organic solids for safe disposal. Two different types of bacteria are used to decompose the sludge. The products of the sludge decomposition are methane, carbon dioxide, water, and simpler organic solids. The amount of methane produced by the bacteria depends on the system used and the balance between these two bacteria. At Springville, approximately 9,000 to 20,000 cubic feet per day (SCFD) of digester gas are produced. The digester gas contains approximately 60% methane, 37% carbon dioxide, 2% water vapor, and the remaining liquid volume percentage is composed of hydrogen sulfide, nitrogen, and other gases.
At the Springville facility, the digester gas is destroyed by burning it. The Springville Electric Plant is located on the same area as the wastewater facility. This plant uses natural gas (consisting mostly of methane with a few other gases) to generate energy for the City of Springville. The project was to see if the electric plant could use the digester gas. If the electric plant could use the digester gas, then a more effective usage of energy would be possible for the City of Springville.
The original plans for the construction of the Springville Wastewater Treatment Facility and the Electric Plant included sharing of resources. The planned sharing included the hot water generated at the electric plant and the digester gas. The digester gas was to be burned in a boiler used to preheat the water before it is superheated in the steps required to produce electricity. A boiler, designed to handle the digester gas, was installed at the electric plant. The necessary system was installed for the transportation of the digester gas.
Upon completion of the digester gas transportation system, operation was attempted. However, all attempts to make the system operational failed. The reason for in-operability was not obtained. The system has since remained unused, but in good condition.
Trouble shooting was performed on the system to determine possible problem sources. The problem sources were found to be the variations in gas pressure and flow rate and the condition of the piping and pressure relief systems. Additionally, the electric plant requires that the digester gas be at a pressure slightly greater than 5.5 pounds per square inch (psi). This pressure is required to blend the digester gas with a stream of natural gas and to prevent backflow of gas into the wastewater facility.
The goal of the project was to develop a system to transport the digester gas to the electric plant at the lowest cost possible. To obtain this goal, the original system was investigated to see if it could be made operational. Use of the original system would reduce the amount of new equipment to be purchased and installation cost. It was discovered that the original pump used could not generate the required pressure or handle the flowrate of the digester gas and was the main source of system 63 in-operability. Another pump was recommended to replace this pump. The new pump is capable of handling the current flow of the digester gas and can increase the pressure of the gas up to 6 psi. A major concern in changing the pump was the need for a new motor. Fortunately, the new pump can use the existing motor to provide the needed horsepower required for operation. Additional requirements for smooth operation of the original system include resetting the pressure sensor and controller, checking the pressure relief system, and checking all pipes for corrosion.
The use of this knowledge to correct the existing system is dependent on the profitability of making such a correction. Using the digester gas represents a potential source of savings for the electric plant. The 20,000 SCFD of digester gas has a heating potential of 1.2 x 107 BTUs (12.5 decatherms) per day. At the current price of natural gas, this totals approximately $8,240 per year. The costs for making the system operational include the new pump and labor. The pump cost is approximately $2,500. The exact cost depends on the distributor used and the method of installation. It was calculated that there is a potential saving of $50,000 after 10 years of use for the electric plant. This potential saving makes the corrections to the system both desirable and recommendable.