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Converting high-strength waste proves economical & environmentally friendly
Transport and application of high-strength waste to distant land application sites can be time-consuming and costly; however, this may be the only option if adequate, suitable application sites are not within reasonable proximity of the waste source. Soil conditions and odor ordinances also limit available sites for land application of wastes. As regulations protecting soil and water quality continue to become more restrictive, land application becomes a less sustainable practice. All of these factors frequently make it challenging and costly to obtain approvals for, and implement and operate a land application program for managing high-strength wastes.
As companies strive to become more efficient and cultivate more sustainable business practices, waste-to-energy projects have become more viable. Converting high-strength waste to energy is an effective means of generating clean energy while significantly reducing the costs and liabilities associated with managing these wastes. Modern waste-to-energy technologies have proven to be safe, environmentally friendly and economical.
A large number of projects to convert waste into electricity are underway throughout the U.S. These projects typically utilize anaerobic bacteria to break down organic matter in the waste stream to produce biogas (principally methane), which is subsequently burned to generate electrical power. Common project sites include landfills and manufacturers that produce a continuous flow of high-strength organic wastes. Sustainable practices and green energy funding incentives have provided the opportunity for development of “regional” projects, i.e., projects that secure wastes from multiple sources to develop a critical volume of wastes, sufficient to anaerobically treat the wastes and generate electrical power.
There are many new waste-to-energy projects being developed and implemented in Wisconsin. Green Whey Energy Inc. in Turtle Lake, for example, collects more than 300,000 gal per day (gpd) of high-strength wastes from six food processing facilities. Two 2-million-gal high-rate anaerobic digesters biologically will treat the waste stream to produce about 1,000 cu ft per minutes (cfm) of biogas. The biogas will be treated (filtered, dehumidified and compressed) to meet the fuel specifications of the engine generator. The treated biogas will be burned in two internal combustion engines to generate 3.2 MW of green energy. Heat captured from the engines will be transferred to an adjacent food processing facility, reducing its need for natural gas. Effluent from the digester will be further treated with a moving bed aerobic bioreactor and via dissolved air floatation prior to discharge to the city sewer.
FCPC Renewable Energy LLC, owned by the Forest County Potawatomi Community (FCPC) and located in Milwaukee, is implementing a project that will collect 120,000 gpd of high-strength wastes from multiple food-processing facilities located in the metropolitan Milwaukee area. Wastes will be trucked to the regional waste-to-energy facility, located in the Menomonee River Valley, immediately west of downtown Milwaukee. Two 1-million-gal high-rate anaerobic digesters biologically will treat the organics to generate about 700 cfm of biogas, which will be treated and used in two internal combustion engines to generate 2 MW of green energy. The project supports FCPC initiatives for creation of renewable energy.
Grande Cheese Co. saw the opportunity to reduce the cost and liability associated with land application of the high-strength wastes generated at its Brownsville, Wis., facility by implementing a waste-to-energy project utilizing a two-stage digestion process that maximizes loading rates while protecting the specific anaerobic bacteria colonies. Acid-forming bacteria (for waste breakdown) are isolated from the methanogenenic bacteria (for generation of biogas) in two separate reactors that operate under different conditions. Biogas generated by the anaerobic process will be burned in microturbines. Effluent from the anaerobic digester is treated in Grande’s existing aerobic treatment system prior to being discharged to a local receiving stream.
The primary objective of the project was to utilize the biogas as a fuel source to produce electricity for Grande’s cheese and whey plants. After drying and compressing, the biogas will be fired in two 200-kW microturbines. The electricity produced by the microturbines will displace electricity being purchased from the utility.
The system treats more than 60,000 gpd of high-strength wastewater that otherwise would have to be trucked to a municipality or land applied. Trucking costs, fuel savings, and reduced emissions and land management efforts are significant—savings amount to more than $1.5 million per year.
The anaerobic treatment system currently is producing approximately 120,000 standard cu ft of biogas daily, averaging more than 68% methane with less than 700 ppm hydrogen sulfide and no siloxanes. The energy value of the biogas is about 82 million BTU per day.
The addition of the anaerobic digestion process has allowed the facility to treat less biochemical oxygen demand in the existing aerobic treatment system (some waste streams have been redirected from the aerobic system to the more efficient anaerobic process). As a result, less sludge has been generated by the aerobic system and, consequently, the facility has benefited from lower costs for chemical addition and disposal of waste-activated sludge. The potential of the treatment process to achieve biological phosphorus removal could result in additional savings, as the quantity of phosphorus-precipitating chemicals will be reduced by 50%.
With the emphasis on renewable energy, there are several funding mechanisms available to help projects move from concept to realization. On the federal level, the Investment Tax Credit and the New Markets Tax Credit are potential mechanisms to help make projects economically attractive. Many states have similar incentive programs available to stimulate these types of projects. Utilities, most with goals for increasing power from renewable energy sources, typically are interested in evaluating and potentially purchasing power from these projects. Finally, private investors that participate in the energy sector also are potential sources of funding.
Assessing the Benefits
These projects demonstrate the economic and environmental benefits of waste-to-energy projects. Anaerobic treatment technologies are cost-effective means of managing high-strength organic wastes and avoiding expensive land application programs or high treatment costs imposed by municipal wastewater treatment plants. The biogas generated serves as a renewable energy source that can be converted to electrical power and heat, reducing an owner’s utility costs and carbon footprint. Financial incentives to generate more renewable energy and rising energy costs over the long term are driving many owners and developers to evaluate the viability of these projects.