BioCycle May 2007, Vol. 48, No. 5, p. 39
With electricity costing about 7 cents per kilowatt hour, energy saved by using digester gas in the engines is worth about $675,000 per year.
Paul Nehm

THE MADISON Metropolitan Sewerage District collects and treats the sanitary wastewater generated in a 176 square mile area surrounding Madison, Wisconsin. About 40 million gallons of wastewater are treated each day at the Nine Springs Wastewater Treatment Plant. Settleable solids are removed from the wastewater in primary settling tanks. The water then travels to an activated sludge system where organic carbon, ammonia and phosphorus are removed by a biological culture. Before being released to the environment, the treated water is disinfected by ultraviolet light. Most of the treated water is pumped five miles through a 54-inch pipeline to a stream in the Yahara River watershed. Because about three million gallons of wastewater are generated each day in the Sugar River watershed, that amount of treated water is pumped through a 16 inch diameter pipeline to a stream in that watershed.
The processes used to clean the water so it can be safely returned to the environment result in the production of materials that require further treatment. One of these is the solids that settle in the primary settling tanks. Known as primary sludge, this material has additional water removed from the solids in gravity thickeners. The thickened material is then pumped to anaerobic digesters.
The microorganisms that provide the treatment in the activated sludge system continuously reproduce. To keep the population at the necessary level, ten percent of the biological culture is removed from the system each day. This material is referred to as waste activated sludge. It also has water removed from it by floating the solids in flotation thickeners. The thickened waste activated sludge is also pumped to the anaerobic digesters.
The purpose of the anaerobic digesters is to allow anaerobic microorganisms to further decompose the primary sludge and waste activated sludge so the resulting material can meet Environmental Protection Agency (EPA) and Wisconsin Department of Natural Resources (WDNR) requirements for land application as a fertilizer and soil conditioner.
As the anaerobic microorganisms digest the sludge, gas is produced. This gas contains methane, carbon dioxide, and a trace of hydrogen sulfide. It has a fuel value of about 540 BTU per cubic foot. The value of the gas was realized shortly after the treatment plant was built in 1928. A newspaper article from 1929 reported that a group of power plant engineers who toured the treatment plant were cooked a German lunch on a gas stove. The fuel for the stove was gas which had been collected from Imhoff tanks, a predecessor of the anaerobic digesters. Although the District no longer provides visitors with meals prepared with digester gas, the gas continues to be a valuable resource.
USING COMBUSTION ENGINES TO GENERATE GAS
Each day about 660,000 cubic feet of digester gas are produced. Three Waukesha internal combustion engines use the gas as their fuel. Two of the engines turn generators which provide electric power to the treatment plant. Each generator can produce 450 kilowatts of energy. Although this is a substantial amount of energy, the District still needs to purchase $120,000-$145,000 worth of electricity each month from the local electric utility. Since the cost of utility power is higher during the “on-peak” times of 10:00 am to 9:00 pm on weekdays, the generators are operated at full capacity during these times. During “off-peak” hours the engines operate at full capacity if adequate digester gas is available. If the gas supply is less than what is needed to operate at peak capacity, the engine speed is reduced to appropriate levels.
The third engine powers a blower which produces compressed air for the activated sludge system. The oxygen in the air is used by the microorganisms in the aeration tanks as they remove pollutants from the water. Dissolved oxygen probes in the aeration tanks measure the amount of oxygen available to the microorganisms. As the oxygen levels in the tanks vary, the speed of the engine is adjusted by the plant’s process control system to produce more or less air as necessary. If the engine is not available, an electric motor driven blower must be used.
Heat is recovered from all three of the engines. Heat exchangers on the jacket water, lube oil, and exhaust transfer the heat to an in-plant hot water system. The hot water is used to heat the anaerobic digesters, to provide building heat in many of the plant buildings, and to provide hot water for an absorption chiller which air conditions the plant’s Administrative Building in the summer. Since the engines can not supply all of the heat to meet these needs, some of the digester gas is also burned in boilers. The hot water produced in the boilers supplements the heat provided by the engines.
On average, the engine generators produce about 760 KW of electric energy and the blower engine saves the equivalent of 340 KW of purchased energy. With electric energy costing about seven cents per kilowatt hour, the energy produced and saved by using the digester gas in the engines is worth about $675,000 per year.
By using the gas as fuel for the boilers and by recovering heat from the engines, an annual savings in the purchase of natural gas equivalent to about $250,000 is realized.
As would be expected, there are costs involved with the maintenance of the engines and boilers. Annual maintenance costs for each engine range from $9,000 to $12,000 on average. Depending on the amount of work done, an overhaul of an engine costs $48,000 to $95,000. Overhauls occur at about 40,000 hours of run time. Annual boiler maintenance costs are $1,900 to $3,900.
ANAEROBIC MICROORGANISMS DECOMPOSE ORGANIC MATTER
The anaerobic digestion process can also generate costs that may not be incurred if an alternate means of stabilizing sludge was used. High concentrations of ammonia and phosphorus are generated in the digesters as anaerobic microorganisms decompose the organic matter in the sludge. Since the water in the Madison area is fairly hard, there is a high concentration of magnesium in it. This creates the perfect conditions for formation of a magnesium ammonium phosphate crystal known as struvite. Under turbulent conditions, the struvite can form significant deposits attached to the sides of pipes, mixers, and pumps. Flow can be restricted to the point that the piping and equipment has to be taken apart to physically remove the struvite. The use of glass-lined pipes and valves has helped to reduce the tendency for struvite to attach to the sides of pipes and valves and to make removal easier. In some cases the potential for struvite formation has been controlled by diluting the flow stream having high concentrations of the precipitating compounds. At other times ferric chloride has been added to tie up phosphorus to reduce its availability for the struvite forming reaction.
A recent problem for the wastewater treatment industry has been the increased use of siloxanes in products. Siloxanes are silicon-based compounds that are used in cosmetic and similar products. In a wastewater treatment plant, most of the siloxanes partition to the sludge streams. When they enter the digesters, the siloxanes volatilize and move to the digester gas. Upon being used in high temperature equipment such as engines and boilers, the siloxanes form silicon oxide compounds. These abrasive materials significantly increase the wear on moving parts within engines and act as insulators in boilers.
Recently, the District began digesting solids at thermophilic temperatures of 127 – 129 degrees F. This is being done to prepare to produce a biosolids product that will meet the EPA requirements of a Class A biosolids. By producing Class A biosolids, the options that will be available for beneficial reuse of the biosolids will be significantly increased. However, operation of the digesters at the higher temperatures increases the amount of siloxanes that volatilize and also increases the amount of moisture in the digester gas. The poorer quality gas has resulted in a major failure of one of the generator engines. It is being totally rebuilt at a cost of over $125,000.
As shown above, the energy cost savings from operation of the engines and boilers is significantly more than the costs involved in their operation. The District has been investigating processes to remove the moisture and siloxanes from the digester gas. By the end of 2007, it is hoped that equipment will be installed to provide a higher quality gas for use in the engines and boilers. Although quite expensive, preliminary cost estimates for the gas treatment equipment can be justified by the energy savings.
Paul Nehm is Director of Operations and Maintenance with the Madison, Wisconsin Metro Sewerage District.