BIOREACTORS AND BEYOND

Michael Fickes

THE ENVIRONMENTAL PROTECtion Agency (EPA) has finalized a long-anticipated rule permitting the transformation of qualifying landfills into bioreactors. While not all states will make use of the EPA's new flexibility, industry observers view the rule as an opportunity to refine emerging bioreactor technology, which promises to extend landfill life and produce landfill gas (LFG) as an alternative source of revenue and energy.

The new EPA rule took effect in late April, allowing state directors of federally approved municipal solid waste landfill programs to issue research, development and demonstration (RD&D) landfill permits that alter existing requirements for run-on control systems, liquids restrictions and final cover. To receive an RD&D permit, applicants must demonstrate that their plans will not increase risks to human health and the environment when compared to complying with the standard operating permit.

According to SCS Engineers, Long Beach, Calif., key to the RD&D rule is providing states flexibility to allow landfill operators to add bulk liquids to landfills constructed with approved alternative liner designs and leachate collection systems. Approved systems must limit leachate depths at the liner to no more than 30 centimeters (cm).

RD&D permits, which carry no restriction related to landfill size, will last for three years and may be renewed three times, making the effective duration of prospective projects 12 years. The new rule does not reduce requirements related to post-closure care and may even increase post-closure financial assurances required of landfill owners.

Wait And See

Despite the new rule, some states will probably not authorize bioreactors for the time being, says David Hansen, president of Landfill Service Corp., Apalachin, N.Y., and chairman of SWANA's Landfill Bioreactor Committee.

“You can't make a general statement about what the states will do,” Hansen says. “Some states like New York strongly support biostabilization of waste based on good experience. Others, such as Massachusetts, prohibit bioreactor landfills. Still other states are in between the two extremes.”

States that are skeptical about bioreactors want to make sure there are benefits to recirculating leachate. Over the past decade, studies have indicated that leachate recirculation changes leachate for the better. “Recirculation actually decreases the toxicity of the leachate significantly,” says B. Todd Watermolen, vice president of engineering with Onyx Waste Services in Milwaukee. “Once the anaerobic process begins, the pH and the occurrence of organic acids fall. In addition, the amount of biological oxygen demand (BOD) and chemical oxygen demand (COD) decrease significantly. These are common measurements that describe the strength of sewage at wastewater treatment plants.”

Some state regulators have found these counter-intuitive results difficult to believe. “They want more confirmatory data on what the leachate constituents look like,” Watermolen continues. “It will take five to 10 years to generate this data.”

States hesitant to approve RD&D permits will nevertheless watch the results of new bioreactors, hoping that leachate recirculation will indeed prove to reduce groundwater contamination risks.

Industry Reaction

According to Watermolen, the RD&D rule provides the regulatory flexibility necessary to enable the fledgling bioreactor industry to progress in three key areas. First, landfill owners with RD&D permits will be able to ascertain how much moisture it will take to fuel practical bioreactors in their regions. Second, RD&D landfills will provide engineers with the opportunity to develop operational technologies capable of circulating liquids in ways that will enhance bioreactivity. Finally, the rule will spur the development of LFG collection systems tailored to higher rates of LFG production and new, potentially profitable LFG-to-energy projects.

“Those are the big issues,” agrees Robert B. Gardner, senior vice president with SCS. “I think the main issue is how do I get moisture into this landfill.”

Finding Moisture

As many as 80 percent of the nation's landfills may eventually find bioreactor landfills to be beneficial, Watermolen says. The remaining 20 percent of landfills are located in climates that lack the moisture necessary to stimulate bioreactor activity.

Although it is not always done, current regulations generally permit landfills to recirculate leachate produced within the landfills themselves. But few landfills generate enough leachate to stimulate the strong biological reactions required of a true bioreactor. For these facilities, leachate recirculation is nothing more than an economic method for disposing of leachate.

The new RD&D rule permits the addition of moisture obtained from other sources in quantities large enough to stimulate powerful bioreactions. But where will this additional moisture come from? The new RD&D rule will allow landfill operators to experiment with a variety of potential sources including storm water run-on and run-off, sewage sludge, gray water, water discharges from food processing operations and certain industrial wastewater streams.

“But landfills will have to be careful,” Gardner warns. “Different liquids will affect the biology in different ways. You probably wouldn't want to use waste streams that are highly acidic or highly basic, because they might inhibit biological reactions.”

Moisture & Air Management

Once adequate sources of moisture have been found, operators must determine how much or how little moisture to add. “In the United States, most garbage comes into a landfill too dry for bioreactions to occur,” Hansen says. “Typically you have to add between 20 and 60 gallons of water per ton of waste. The goal is to raise moisture content above 20 percent. There is debate about the correct moisture content. The range engineers have focused on is 35 percent to 45 percent.”

Bioreactor opponents claim that engineers plan to flood landfills with liquids equaling as much as 70 percent of waste volume. Such virtual liquification of landfills will create structural dangers, the critics say. Hansen and others working out bioreactor designs dispute the claim. “We do not want a completely flooded condition for reasons related to efficient decay as well as geo-technical stability,” Hansen says. “While too little moisture will not allow the garbage to fully decay, too much will inhibit decay. You need the right balance of solids, liquids and gases for a bioreactor landfill to operate properly. The right balance is a moisture content somewhere between 35 percent and 45 percent.”

In addition to establishing an appropriate moisture mix, engineers must figure out the best way to inject moisture into landfills. There is general agreement that circulation systems should add moisture at various levels of the landfill to help promote even distribution.

Another issue concerning engineers is the amount of air used to promote decay. Bioreactor sites can use three different designs: aerobic, anaerobic and hybrid.

An aerobic landfill pumps air and liquid into the waste mass. The oxygen in the air causes swift decay of organic garbage and does not produce methane. Aerobic bioreactors may be useful for composting, but most engineers doubt their utility for bioreactor landfills designed to produce methane.

Engineers generally favor anaerobic bioreactors, which operate without air or oxygen, to slow the process of decay and allow greater control of the biological reactions. Anaerobic reactions also produce LFG, which is considered an important byproduct of bioreactor landfills because it can be used as energy.

A third kind of bioreactor landfill, called a hybrid, takes advantage of both aerobic and anaerobic decay. “When you first place garbage on top of a landfill's working face, it is exposed to air and the first reactions are aerobic,” Hansen says. “If you prolong the aerobic phase for say, 30 days, by pushing air into the garbage, you can kick-start the decay at very low cost,” he says.

Most landfill engineers agree that controlling LFG requires installing conduit pipes as waste goes into a landfill. In a hybrid bioreactor, operators can push a slightly negative air pressure into the uppermost pipes. This fosters aerobic decay of the garbage.

“There is an art to this,” Hansen cautions. “You don't want to suck large amounts of air through the garbage — just enough to create a semi-aerobic condition in the upper areas. This not only provides a head-start on biostabilization, it also reduces atmospheric emissions from the garbage by more than 90 percent, according to peer-reviewed research that I've seen,” he says.

Controlling and Using LFG

Reflecting the importance attached to the new RD&D rule by regulators, an EPA representative reportedly warned a group of landfill operators to be careful “not to blow this.” The representative was referring to the need for adequate LFG collection and control systems. Anaerobic decay produces LFG, which is composed of methane, carbon dioxide and trace gases responsible for odors characteristic of landfills. In the past, poor LFG controls at landfills experimenting with the addition of air and leachate to garbage have produced public outrage over the odor of decaying garbage. “Gas control equals odor control equals public acceptance,” says another observer.

Bioreactor landfills produce much more gas than conventional landfills. According to Watermolen, an Onyx bioreactor landfill at Seven Mile Creek in Au Clair, Wis., has produced about 10 times the amount of LFG as would be expected from a conventional landfill.

It's better gas, too, he adds. Methane is the valuable component of LFG. Conventional landfills in the United States produce LFG with methane concentrations ranging from 20 percent to 50 percent. The difference has to do with moisture concentration in the trash going to the landfills, Watermolen explains. Garbage collected from regions with more rainfall produces LFG with higher methane concentrations. At the Seven Mile Creek bioreactor landfill, Watermolen reports LFG with methane concentrations ranging from 48 percent to 54 percent.

Seven Mile Creek now produces enough methane- rich LFG to support a gas-to-energy system. “For years, the landfill didn't produce enough gas to sell, but the bioreactor has gotten us over that hump,” Watermolen says. “Right now we're negotiating an agreement to sell electricity to Dairyland Power, the local electric cooperative. We plan to install three Waukesha engines that will produce three megawatts of electricity — green power.”

Three engines rather than one will reduce labor costs for the installation, because one person can maintain three engines as easily as one engine.

Additionally, bioreactor landfills produce gas at higher, more predictable rates for predictable periods of time. “The data shows that LFG generation increases significantly in a bioreactor landfill over a 10 to 15 year period, and then drops lower to a steady production rate,” Watermolen says. “This enables you to select LFG engines that better fit the LFG production cycle. Overall, a bioreactor enables you to keep economies of scale working with you.”

Landfills Last Longer

The primary benefit landfill managers hope to gain from bioreactors is longer lasting landfills. Studies have shown that the bioreactor process may increase landfill capacity by 20 percent to 30 percent. “If this is true, it will have a tremendous impact on the industry,” Watermolen says. “If a bioreactor landfill increases revenue-generating airspace by 20 percent, there is the potential to reduce fixed costs by that much.”

The potential for increased airspace goes beyond new cells in existing landfills. Some companies have begun to toy with the idea of bioreacting closed cells with the goal of selling gas, mining the bioreacted trash for recyclable materials such as metals, and then re-opening the space for new trash.

Will bioreactor technology really make landfills last longer? Cost less? Allow profitable LFG sales?

Over the next few years, a host of bioreactor landfills operating with RD&D permits will answer these questions.

Mike Fickes is Waste Age's business editor based in Cockeysville, Md.

BIOREACTOR BREAKDOWN

According to the Solid Waste Association of North America's Bioreactor Committee, a bioreactor landfill is a controlled landfill or landfill cell where liquid and gas conditions are actively managed in order to accelerate or enhance biostabilization of the waste.

Biostabilization is the process of biologically decaying organic materials subsequently reducing leachate strength and gas generation.
— MF