Some landfills are moving away from conventional caps and installing phytocaps—vegetative soil covers. They typically do it for two main reasons: These natural caps may help reduce leachate by uptaking water through their roots, and they improve leachate quality by uptaking chemicals and contaminants.
Phytocaps are relatively cheap to install and maintain, with no mowing needed. And some developers say they can help manage leachate onsite, cutting hauling-related costs and providing an alternative to having to depend on wastewater treatment plants that are imposing stricter load limits.
How effectively they reduce leachate is still in question by some environmental engineers, and some regulatory agencies are skeptical about this fairly new concept. But project developers say phytocaps do well with the right design and plant type, among other factors. There is ongoing research to answer the leachate management questions.
North Liberty, Iowa-based Ecolotree has installed several phytocaps on U.S. landfills. Lou Licht, Ecolotree president and founder, says his customers are successfully managing leachate onsite with these caps, minimizing or eliminating hauling and translating to savings on fuel, wear and tear on trucks and insurance.
With about 4 acres, the right soil, the right location and mature trees, operators can manage about 1.5 million gallons of leachate per year onsite by applying it to the cover, he says. He irrigates by dripping water through tubes near the trees. During the winter, he “drips” with tubes buried 30 inches, enabling year-round operation.
A Chicago landfill customer of Ecolotree is replacing six truckloads of leachate a day (30,000 gallons a day) by not requiring offsite transport, says Licht. A client in Milan, Ill., is applying 1.4 million gallons of leachate over 4 acres per year.
But phytocaps may be a temporary alternative to offsite management, as some leachate will eventually drain to the bottom of the landfill and have to be removed, according to Milind Khire, an environmental engineer at the University of North Carolina, Charlotte (UNCC).
Delaware Solid Waste Authority installed a phytocap on a closed, 25-acre section of a landfill. The vegetative cap was coupled with a sprinkler system, similar to a golf course irrigation system. Material is treated and reapplied on the phytocap in warm weather.
“The irrigation component facilitated delivery of nutrients into the ground that are beneficial to vegetation. It also enabled us to control how much treated leachate was distributed on which portion of the landfill and at what time,” says Mike Lenkiewicz, an engineer at Delaware Solid Waste Authority.
But this particular irrigation system coupled with phytocap tends to work best in regions without heavy rainfall and adds significant expense in energy and mechanical systems.
Phytocaps may need a few years to become established enough to perform at their optimum. Delaware Authority had a challenge to contend with as it waited for trees to mature: deer were eating the plants. Installing a wire horse farm fence with a charge helped, and the authority discovered plant selection mattered. Deer like hybrid poplars, but evergreen species and American sycamore did well, notes Lenkiewicz.
Trees with shallow roots are desirable as they will not penetrate trash and start dying off. They must be able to tolerate both heavy rainfall and droughts since minimal soil is used and dirt can easily get too wet or dry. Additionally, it’s important to plant trees that mature fairly quickly. Hybrid poplars often work well as they grow fast and in many regions.
UNCC is looking into plant covers’ ability to reduce leachate.
“While most caps’ purpose is to reduce the quantity of leachate only, phytocaps have the benefit of improving quality by uptaking ammonia and other chemicals at high concentrations. But more work is needed to determine their effect on quantity,” explains Khire, the lead investigator on the UNCC study.
He is studying vetiver plants at a municipal solid waste landfill in the southern region of the country, where leachate is slowly applied to the plant cover.
“We are looking to quantify how much water the plant transpires (takes up) and measure the effect of plants themselves on quantity of rainwater and ultimately leachate. This will have implications tied to permitting these caps because there are landfill cover regulations around quantity of leachate,” says Khire.
If research shows the plant-based system reduces leachate, phytocaps may be an easier sell to regulators.