The US Environmental Protection Agency and Research Triangle Institute municipal solid waste—decision support tool is still at the forefront of full-cost and life cycle environmental assessment for integrated MSW management.

By Keith Weitz and Susan Thorneloe

The EPA/Research Triangle Institute (RTI) municipal solid waste—decision support tool (MSW-DST) was created to help solid waste planners in the evaluation of costs and environmental issues and make informed decisions about MSW management. Extensive and detailed databases have been assembled and incorporated into the sophisticated software program that provides a comprehensive and standard method to screen MSW management strategies and ultimately bridge the gap between economics and environmental science. Included in the MSW-DST are multiple design options for waste collection, transfer stations, materials recovery facilities, mixed MSW and yardwaste composting, combustion, refuse-derived fuel combustion, and disposal.

The MSW-DST has been made available to communities and other users on a case-by-case basis. For example, RTI has been working with the Cities of Seattle and Spokane, WA, to build tailored versions of the MSW-DST for use in those cities. This exercise has been successful, and the tools are currently being transferred to the cities for their use. RTI also continued to provide its services in applying the MSW-DST for a number of different clients. For example, a national-scale study of greenhouse gas emissions and MSW management conducted for the Integrated Waste Services Association and the US Conference of Mayors has been completed, and results will be released in summer 2002 in a journal article.

Ultimately RTI would like to develop an easy-to-use Internet-based version of the MSW-DST. In this version, users would submit their data via the Internet using specially designed data questionnaires, the MSW-DST would run the scenarios through a server system, and results would be e-mailed back to the user. It was hoped that the development of the Internet-based version of the MSW-DST would have taken place in 2002. However, funding to implement this plan has not yet materialized. It is still possible that the process will get underway within the next year.

In addition to the MSW-DST, RTI has produced a one-of-a-kind database that contains extensive data on energy consumption and environmental releases for various waste management operations, as well as equipment used in those operations. Life cycle inventory data for the production of electrical energy and materials are also included. The database is currently in the final round of quality-assurance reviews at EPA, and it is anticipated that the database will be released in summer 2002.

Evaluation of Composting at EPA's New Facility in North Carolina

The MSW-DST was recently applied at one of EPA's own facilities in the Research Triangle Park (RTP), NC. RTI worked with the Triangle J Council of Governments to use the MSW-DST to analyze options for implementing an onsite composting operation at EPA's new RTP facility. Currently EPA sends its organic material to a composting site 60 mi. away, so it was interested in evaluating the relative environmental aspects of implementing an onsite composting program. A baseline scenario of landfilling the materials versus options for onsite and offsite composting was evaluated. The results are being used to design the composting operation at the facility to maximize the environmental benefit of composting.

It was estimated that the EPA facility would produce approximately 175 tpy of organic materials, including:

• 100 tons of foodwaste
• 9 tons of mixed paper
• 4 tons of animal bedding (wood shavings)
• 62 tons of yard trimmings (grass, leaves, and branches)

This same 175-tpy mixture of organic materials was modeled in three different management scenarios:

• Landfilling at an offsite facility (90 mi. from EPA)
• Composting at an offsite facility (60 mi. from EPA)
• Composting at an onsite facility

Scenario 1: Landfilling

In this scenario, EPA has its foodwaste and mixed paper collected and sent to the landfill. The 66 tons of yardwaste and animal bedding are collected and instead sent to the transfer station for mulching. It is assumed that the yardwaste and the animal bedding waste are collected three times per week, transported to a transfer station 15 mi. away, and mulched on-site at the transfer station using a tub grinder.

The 9 tons of mixed paper and 100 tons of foodwaste are assumed to be collected three times per week at the facility, transported to a transfer station 15 mi. away, and then packed in a semi-tractor trailer and hauled 90 mi. to a landfill. The landfill has been operating for five years and is designed and operated to meet all applicable federal, state, and local regulatory requirements, including the Resource Conservation and Recovery Act Subtitle D and the New Source Performance Standards of the Clean Air Act. Landfill gas (LFG) is collected and flared. The landfill module in the MSW-DST includes landfill design, operation, closure, and postclosure. In addition, the time scale for estimating LFG emissions can be set at 20, 100, and 500 years. For this study, we used a 100-year time scale and assumed that any LFG generated is vented for the first two years following waste placement.

Scenario 2: Offsite Composting

In this scenario, EPA has its animal bedding and mixed paper collected three times per week, sent to the transfer station, and then long hauled to the landfill. The yardwaste is also collected three times per week and sent to the transfer station, but it is mulched on-site at the station. The 100 tons of foodwaste are collected three times per week at the facility and transported to an offsite composting facility 60 mi. away. It is assumed that at the compost facility, the foodwaste is placed in windrows and turned 10 times during a nine-week composting and curing process using a windrow turner. There is no pregrinding or shredding of the foodwaste at this facility. However, the final compost product is screened using a front-end loader and a trommel screen.

Scenario 3: Onsite Composting

In this scenario, EPA collects its mixed paper, animal bedding, foodwaste, and yardwaste and delivers the materials for composting to an onsite location. It is assumed that all materials are collected three times per week and transported 1 mi. to the compost site in a light-duty diesel truck. Once at the compost site, the 175 tons of materials are composted using the methods described for the second scenario.

Note that this scenario assumes that the 62 tons of yardwaste have been chipped in a shredder chipper by the landscaping firm prior to delivery to the compost site. The energy use and emissions associated with the shredder have been included through an offline calculation.

Results

The results of the three scenarios are summarized in Table 1. As one might expect, the onsite composting option (Scenario 3) is the clear winner environmentally. Why? The very short haul distance in this scenario avoids significant collection and transportation burdens associated with the landfilling and offsite composting scenarios.

A surprise to many, however, was that the landfill option (Scenario 1) beat out the offsite composting option (Scenario 2), which is EPA's current practice. After some investigation of the results, the significant factor between the two options turned out to be the collection and transportation activities. The landfill option had much lower emissions associated with collection and transportation activities because all of the waste first passes through a transfer station and then is long-hauled in a semi-tractor trailer to the landfill 90 mi. away. Conversely, in the offsite collection option, the waste is long-hauled 60 mi. away to the compost site in a traditional collection vehicle. Because the collection vehicle is less fuel-efficient than a semi-tractor trailer for hauling, the collection and transportation emissions are much higher despite the shorter travelling distance.

Conclusions

Do these results mean that EPA shouldn't compost its organic waste if it can't implement an onsite composting program? No. Although onsite composting has clear advantages, the results highlight that EPA's current approach (including the vehicle used) to collect and transport the organic materials is inefficient. By developing a more efficient collection and transportation system, the offsite composting option can greatly improve its environmental profile.

The results of this study are being presented and discussed with EPA and other companies in the RTP. A centrally located compost facility at EPA's new location, or at another site within the RTP area, could be used by a consortium of companies in the RTP to minimize collection and the burdens of collection and transportation and to maximize the benefits of composting.

The MSW-DST has helped in this study to elicit tradeoffs between organic fraction management alternatives and to pinpoint inefficiencies that could be improved. The benefit of using the MSW-DST in this case will be informed decision-making to balance the cost and environmental aspects of waste management to provide a win-win solution.

Keith Weitz is a research scientist at RTI's Center for Environmental Analysis, and Susan Thorneloe is a senior environmental engineer with EPA's Office of Research and Development/National Risk Management Research Laboratory/Air Pollution Prevention and Control Division, both in Research Triangle Park, NC.