In This Department
Wastewater plant is turning GREASE and food SCRAPS into energy.
"Green" waste-to-energy equipment uses grease and food scraps to create electricity to run plant processes.
West Lafayette Indiana, October 2010.
The City of West Lafayette has completed a major renovation project at its wastewater treatment plant (WWTP) and it has been in operation for over one year. The onsite equipment allows the city to convert fats, oils, and grease (called FOG) and food scrap waste from Purdue University to produce energy that is being used to augment the plant's electricity usage. This innovative waste-to-energy treatment system reduces the plant's overall operating and maintenance costs, has decreased the overall carbon footprint of the community, and reduces waste going to landfills.
Kennedy/Jenks Consultants, as an engineering sub-consultant to M.D. Wessler & Associates, designed innovative digester upgrades to replace two 50-year-old anaerobic digesters with a new, efficient, externally-pumped mixing system housed inside a new digester building. The building also incorporates a new sludge heating system, with heat exchangers and a boiler, and a co-generation system that uses digester gas (methane) to generate electricity and heat. The heat is used to raise the temperature of the contents of the digesters in order for the microorganisms to break down the waste. The old digesters had a full time boiler which used a lot of natural gas. This new system relies on the heat generated by the microturbines, and only has a natural gas boiler for backup purposes. Not having to use natural gas to heat the digesters is saving the city a substantial amount of money.
Converting Waste to Energy
"Our renovations allow us to capture the methane gas produced at the plant and use it to produce electricity through two new microturbines," says Dave Henderson, Utility Director for the City of West Lafayette, Indiana. "To augment methane production, we also have a new fats, oils, and grease (FOG) receiving station that allows us to introduce waste grease from restaurants into the digester to boost our methane production. The methane gas, burned in two microturbines, generates 15% of the total electricity needed to operate the plant processes. An added plus is that grease haulers are paying tipping fees for dumping their grease, and this is generating revenue for the plant and help pay for the costs of our renovation."
An exciting outgrowth of the original project is the chance to use food waste as an energy source. "We are taking food waste from Purdue university's dining courts and are adding that to the digesters through the central grease-receiving station," relates Henderson. The university was already separating out the food waste from other waste for a composting project. Although the composting project ended, they were still separating the food from other waste. It was a perfect opportunity to take this preprocessed pulp matter and use it in the digesters to increase energy production. There was only minimal added expense to make this work."We are very pleased with the initial results," added Henderson. "We worked out the logistics of receiving this new source and the digesters responded very well to the food waste. We're excited about this new waste stream going into the digesters.
The normal digestion process produces 37 cu. ft. of gas per minute. Adding FOG doubles the amount of digester gas. By adding the food waste from Purdue, the plant could get 10% more energy. Purdue can avoid paying fees for disposing of the food waste at the landfill while we will get the benefit of converting that food waste to energy.
"Although the use of food waste improved gas production, the additional cost to allow the plant to treat the food waste was minimal," adds Henderson. "We bought a portable cart lifter to tip the digested food waste into the grease-receiving station. The university is just across the street from the plant, so the cost of transporting the waste was far less than for hauling it to the landfill. Both the city and university staffs are excited about this project and are pleased with the results so far. Although we still have a few details to work out, we are in the process of installing our own food grinder in order take even more food waste. Food waste that hasn't been ground up like what we already receive from Purdue will come to the plant soon. The possibilities for diverting this waste to the plant for energy production instead of sending it to a landfill are extremely exciting.
Another sustainable feature also grew out the original project. "We added a couple of vehicle charging stations at the plant," states Henderson. "The City plans to purchase electric vehicles to use at the plant and we will be able to charge them overnight using energy produced by the microturbines."