APRIL 2010

The modern landfill: more than just a pit

The design of American landfills underwent a major change when the United States Environmental Protection Agency (EPS) established new national standards for landfills in 1991 with Subtitle D. Subtitle D applies to municipal solid waste landfills, and requires them to have a composite liner system consisting of natural materials and plastics to prevent groundwater contamination.

Since then, engineers continue to make improvements on the national standards to ensure landfills are environmentally safe and provide assurances to ever increasing public opposition to the expansion of existing landfills or the creation of new facilities.

“We are seeing more use of electrical leak detection tests on the membrane portion to provide that extra security for landfill construction,” said Curtis Hartog, senior technology manager with Foth Infrastructure and Environment. “In Minnesota we have 19 landfills and none of the newer Subtitle D landfills are showing groundwater contamination according to the Minnesota Pollution Control Agency, with the youngest one built in 1993.

“Most of the changes have come in quality control and assurance, with the emphasis on detecting holes in the membrane,” he added. “Most landfills go through a pretty rigorous quality control process when they are constructed – that the clay is of the right density and is installed correctly and that the plastic membrane is sealed appropriately. The question is how long will the plastics last? There have been various reports that are in the neighborhood of 100 years. If that is the case, the goal is to get the leachate to a point where in 100 years it is not a threat of contamination.”

All new landfills have leachate collection systems that pump out liquids for treatment or containment. The minimum thickness for clay liners is 2 feet and for areas where there is insufficient clay, a geo-synthetic clay is utilized. The plastic membrane is considered to be the primary containment system.

“If you have a leak,” said Hartog, “it still has to penetrate the clay liner, which is very tight and it takes a long time for the liquids to penetrate it.”

Dr. Tim Townsend, a professor of Environmental Engineering at the University of Florida, is a leading researcher on landfill design, technologies and processes to clean up and extend the lifespan of landfills. He is conducting research on bioreactor technology to help speed up the decomposition process.

“A big component of the waste in municipal solid waste landfills is biodegradable and if you operate it in a manner where you enclose it, the waste won’t decompose rapidly,” he said, “but there are techniques that operators can use to treat a landfill more as a treatment system than as a storage system. It is similar to composting or anaerobic digestion.

“When waste undergoes biological decomposition, it converts to biogas and a stabilized residue,” he added. “You would much rather have all of these biological processes occur when the landfill is relatively new and in prime shape, as opposed to decades or centuries from now. We haven’t been using lined landfill technology long enough to truly know how these systems will perform in the distant future. So it is much better to control and treat the waste soon after it is placed. When this process is undertaken and you hopefully have captured all the methane and converted it into some energy product, you have a stable landfill that should pose much less environmental harm and risk in the future and you opened up more airspace to fit in more waste.”

Townsend noted that while the technology has been studied, “it needs full-scale practice and implementation to work out the all the bugs to apply this technology on the large scale of an operating landfill, to control the process in the most environmentally safe manner, and so that an operator can afford to do it as safely and economically as possible.”

Hartog has been testing bioreactor technology in landfills in Iowa to help reduce the threat of possible contamination, a concern that is shared by state officials. While dry entombing the waste is an option, Hartog said re-circulating leachate at landfills is becoming more common to help degrade the waste.

“By purposely putting liquid into the waste – the waste degrades and the leachate becomes less of a threat because the leachate ‘strength’ is reduced by re-circulating it back through the waste,” he said. “In 100 years, if there is a hole, and the leachate is weak, the threat of groundwater contamination is reduced.”

It may be possible to retrofit individual landfills to apply bioreactor technology. Hartog said that landfills, which his firm designed nine years ago, were planned with bioreactor treatments in mind. Foth tested a bioreactor process in Linn County, Iowa from 1998 to 2008, a county where it just completed a landfill gas project to and hopes to generate green power.

“Not all state regulators are comfortable with this process yet,” he said.

Hartog said that some states like Wisconsin are more proactive than others in terms of landfill design, but each state has concerns unique to their situations. Minnesota, for example, currently has a moratorium on new landfills, with existing landfills being allowed to expand only if the application is approved, purportedly in order to protect their water sources. And while Minnesota is concerned about the affect of landfills on its water resources, in California, siting concerns include seismic conditions.

A major issue affecting landfills is what to do with coal combustion ash from power generating facilities. “They either dispose of the waste or reuse it to make cement and bricks. But some of it is disposed of in landfills because the economics of reuse are just not there and because of that, it’s a lot tougher to site those landfills,” said Hartog.

“If not into a landfill, some facilities place the ash into ponds,” he added. “The Tennessee Valley Authority had an ash impoundment that burst and caused a lot of problems. In response, the EPA has promised to come out with new rules on how to manage ash (coal combustion residue).”

The ash problem is just one facet of the NIMBY problem that is making it difficult to secure public approval to build new landfills or expand existing facilities.

“It’s a really huge issue in most of the areas we work in,” said Hartog. This is not just an American problem. Several years ago in Ontario, the residents of Kirkland Lake rejected a proposal to convert a former open pit mine into a landfill for the City of Toronto’s solid waste.

Hartog noted that some cities do allow expansions of existing landfills, but that they must close by a specified date, and that many restrictions are imposed. He added that while residents can point to past examples of landfills that have caused environmental problems, having new technology and designs does not always win over minds.

“We have seen at meetings that the public does not distinguish between new and old landfills. Rather, they lump them all together which isn’t fair to the newer landfills,” said Hartog. “But it really isn’t about safety, risk or engineering issues. It’s an emotional issue. People say, ‘I don’t want it here, build it somewhere else.’ We just ran into that with a solid waste transfer station. It didn’t matter how safe it was. Political careers have ended over landfill sitings.”

Some opposition to new landfills is based on the facility being constructed to accept out-of-state trash. But such shipments cannot be stopped by states due to the federal inter-state commerce clause. New York City exports its trash and even Minnesota sends trash to Iowa and Wisconsin.

According to Hartog, the solid waste industry does take landfill safety seriously, and that best practices and technological developments are generally shared by companies and engineers, with research conducted by Dr. Townsend and Dr. Craig Benson at the University of Wisconsin playing a critical role in providing information needed to design state-of-the-art landfills. National, regional and state conferences held by the Solid Waste Association of North America help to keep regulators, engineers and companies abreast of the latest developments.

“Waste Management and other private companies do a good job at engineering their facilities to protect groundwater,” he said. “Each landfill is surrounded by groundwater monitoring wells to make sure that the containment integrity is maintained and if there is contamination, they have to do something to remediate it. We try to do state-of-the-art design, but sometimes we are constrained by the location of the landfill.”

Foth recently engineered the transfer of the material from an old landfill in Lacrosse, Wisconsin to a new landfill to mitigate a potential environmental risk.

“It was costly, but it’s pay me now or pay me later,” he said. “In some cases we don’t know what is inside a landfill, but the important thing is that the material needs to be in a lined facility. We’ll be seeing more waste relocations in the future. A lot of those older facilities were a lot closer to towns and development occurred around them. It’s really a risk management issue for counties and cities because the investment of removing the trash can be less compared to providing water to citizens, or trying to collect and clean up groundwater.”

Incinerators help to reduce the amount of waste going to landfills, as do waste-to-energy plants, but Hartog said these solutions also creates the problematic ash waste that needs to be disposed of, which requires strict quality control and checks for engineers and construction companies that design and build landfills.

“From a design standpoint,” he said, “there are prescribed components that must be in the design. During construction there are a series of checklists, forms and data that we have to gather. After Subtitle D went into effect, the checklist system got better. It’s more standardized in terms of the sequence, what you have to check and how frequently you have to check the materials, but it takes a pretty experienced field person to get it right because there is a lot of data you need to gather on these projects.”

Townsend and his students are also studying what happens to the soil beneath the clay liners of landfills and how the change over time compares to what the engineers predict during the design phase.

“It’s very important from an engineering and design perspective,” he said. “There are standard engineering procedures that we go through when a landfill is constructed to determine how the subsurface soils are going to respond. At one site we have installed instruments beneath the liner system to monitor the changes over time.”