A TRUE BIOREACTOR LANDFILL has a lot to live up to. When operated properly, bioreactors can reduce environmental risks, increase landfill capacity, control closure expenses and elevate energy production by producing additional landfill gas. In theory, bioreactors are able to pull off these feats by accelerating waste decomposition by recirculating leachate. In practice, however, recirculation technology has not yet allowed bioreactors to become an everyday possibility. Nevertheless, according to the Delaware Solid Waste Authority (DSWA), Dover, evolving recirculation methods that speed decomposition are moving bioreactor landfills closer to becoming an everyday occurrence.
The DSWA has been experimenting with recirculation systems and bioreactors since the early 1980s. The authority's experience suggests that for a landfill to become a true bioreactor, all benefits must be achieved “in excess.” For example, the increase in landfill capacity caused by decomposition must be substantial enough to produce an economic effect by extending the facility's operational life.
There are four ways landfill operators can recirculate leachate, says Logan V. Miller, DSWA facility manager. 1) An operator can spray leachate on the landfill's working face or on an intermediate lift. 2) The facility can construct ponds on top of the landfill and allow leachate to percolate down and through the waste mass. 3) Pressure created by gravity can cause leachate to flow out into the waste from the bottom of wells. 4) And the newest and most productive recirculation method, according to the DSWA, is to inject leachate under pressure into wells or trenches, then force it out into wide expanses of waste.
Spray application and ponds are the least expensive methods of managing landfill leachate, according to the DSWA, which has experimented with the method since 1983. Spray application only requires a water truck with a spray bar to disperse the liquid, so there are little costs in capital investment, training and personnel, Miller says. Training is necessary to protect employees doing the spraying from exposure to leachate.
However, spray application has its drawbacks. According to the DSWA, the method recirculates less leachate than the alternatives — a problem exacerbated by evaporation. Bad weather also can limit leachate spraying. For example, wind can blow the leachate beyond the landfill's boundaries, while rain and snow can cause runoff, Miller says. Recently, regulators have begun to reduce leachate spraying because of odors and run-off risks.
Percolation ponds provide a solution to the problems associated with spray applications. Additionally, the DSWA has found ponds to be the easiest way to recirculate leachate, Miller says. Following its experiments with spray applications, the authority excavated several 50-foot square ponds that were approximately 1.5 feet deep. There were costs for excavation, which requires labor and heavy equipment. However, once the ponds have been constructed, Miller says one worker with a 6,000-gallon tanker truck can regularly and inexpensively fill the ponds with leachate.
DSWA reports that unlike spray application, ponds reduce risks to personnel and the environment. “The liquid [goes] right down into the waste,” Miller says. “You don't see the liquid, and it doesn't evaporate. It also reduces odor problems.” Ponds also help to eliminate the run-off problem because the liquid that goes into the ponds stays in the landfill, he adds.
To some extent, ponds also encourage decomposition. “Wherever we happen to get the proper moisture content, we probably create mini-bioreactors,” Miller says. However, coverage is limited to landfill areas beneath the ponds.
According to Miller, spray applications cover wide surface areas; ponds send leachate deep into the waste. Gravity wells also push leachate deep into the landfill and out into wide circles around the wells. In the early 1990s, the DSWA tested 15 gravity wells in a 22.5-acre area of its facility. The design called for well installation in new cells on top of two 10-foot lifts of refuse. Each well covered 1.5 acres.
To construct the wells, the DSWA installed manhole sections with perforated sides on top of a 1-foot stone foundation. Three 4-foot-tall manhole sections were placed on top of another to form 12-foot-deep wells. Each well accommodated a 10-foot lift of waste. As new lifts were added, new manhole sections built-up the wells. By the time the cells were finished, each well was 80 feet tall.
“We filled the manholes with stone,” Miller says. “It wouldn't be safe to leave deep holes in the landfill. At first we used small stones, but those wells tended to clog. Now we use rip-rap or baseball-sized stone[s].”
Once leachate is poured into the wells, it spreads out into the waste. In the DSWA's test, the broadest coverage occurred at the bottom of each 80-foot well, where leachate pressurized by gravity seeped out to a radius of 200 feet.
However, conical coverage can be problematic with gravity wells, Miller says, explaining that higher pressure at the bottom of the wells pushes the leachate out the farthest, and lower pressure above produces less coverage. The good news is that gravity systems can eliminate problems related to odor, runoff and weather, he says. Gravity wells also expand coverage and move closer to the goal of swift decomposition.
Trenches and Leachate Injection
According to the DSWA, the newest method of leachate recirculation employs vertical wells or horizontal trenches, and injects the leachate under pressure. Four years ago, the DSWA built a pressurized leachate injection system using horizontal trenches. Operators must work around vertical wellheads when spreading waste, but horizontal trenches with perforated piping could be added and covered as the landfill builds up.
The DSWA system pumps leachate under 20 pounds per square inch (psi) of pressure, which provides lateral coverage at each trenching level.
“We're still working to get complete coverage,” Miller says. “Given the rate of decomposition and methane production, however, injection looks like the big step.”
According to Miller, the DSWA believes spray application, ponds and gravity wells probably cause similar waste decomposition rates, but injection systems produce methane much faster. The authority is working on a data collection system to quantify the differences. Nevertheless, Miller says injection systems are beginning to produce the first true, powerful bioreactors.