Final covers play a vital role in the closing chapter of a landfill’s life. Daily and interim covers, on the other hand, are minor characters in the plot, and their importance can go overlooked. However, both serve essential functions in landfill management as they work to keep waste under wraps and quell emissions.
With the proper planning and a little foresight, daily and interim covers can be optimized to improve landfill operations and save money in the process.
Tarek Abichou, who teaches a course on geosynthetics in landfill applications at the Florida Agricultural and Mechanical University-Florida State University College of Engineering, recently spoke during a webinar presented by the Environmental Research & Education Foundation (EREF) about the importance of these covers and how to rethink their traditional composition to further benefit landfill owners and the environment.
“Landfills are built over years and years, cell by cell,” Abichou says. “All of them will eventually get a final cover, but before getting there, we cover landfills with many other covers. … The system has a function to physically separate the waste from the environment, the waste from [the surrounding community] and the waste from air.”
Typically, each landfill cell experiences three different cover types in its life: a daily cover, an interim cover and a final cover. Daily covers are used to isolate waste overnight; temporary covers are used during intermediate filling; and a permanent cover is placed once the landfill reaches capacity.
Abichou says regulations surrounding landfill covers are limited. For daily covers, municipal solid waste (MSW) landfill operators must cover all disposed waste at the end of each day to control odors, vectors, fires, litter and scavenging. Federal regulations require landfill operators to use a minimum of six inches of earthen materials as daily cover.
While the scope of regulations pertaining to daily covers is insubstantial, “there are even fewer texts related to the interim cover,” Abichou says.
Interim covers are to be composed of at least a foot of compacted earthen material, such as soil, over surfaces where no additional solid waste will be deposited within 180 days. Some of these covers remain in place for years.
With such limited regulations in place, Abichou believes there is room to improve upon both the design and functionality of these covers to have more efficiency and a greater purpose.
Waste-derived daily covers
Abichou says daily covers can come with several drawbacks. They consume valuable airspace and impede the movement of landfill gas and leachate. Daily covers also require extra costs to excavate, load and haul to the site.
However, some of those costs can be alleviated with alternative daily covers (ADCs) made of waste-derived materials.
ADCs can be made either with waste-derived materials or synthetic products, such as reusable geosynthetic tarps, non-reusable geosynthetic fabric, spray-on foams and slurries. While all can be effective, landfills can use resources that are accessible and readily available to do the same thing at a fraction of the cost.
ADCs made with waste-derived materials can generate revenue, Abichou says, as landfill operators are paid to accept the waste and charge a tipping fee for materials they can turn around and use to their benefit. “It’s additional revenue instead of [having to] spend money on digging soil from elsewhere,” Abichou says.
Depending on state regulations, these materials can be used as a cover either on their own or after being blended with soil—the latter of which Abichou says is typically made up of a 1-1 ratio. In some states, like California, using ADCs can even help landfills meet diversion goals.
Some jurisdictions already have a list of waste-derived materials approved for ADC use. The California Department of Resources Recycling and Recovery (CalRecycle), for example, has 11 approved ADC materials, including ash and cement kiln dust, treated auto shredder waste, construction and demolition waste, compost, green material, contaminated sediment, sludge, and shredded tires.
These materials must be processed and applied so that no gaps exist in the exposed landfill face. Jurisdictions may also approve additional materials that aren’t listed on a case-by-case basis if landfill operators can prove their efficiency to the enforcement agency during a site visit or through a written report, depending on the state.
Some materials prove more beneficial than others when used as ADCs. Sludge, for example, is attractive as an ADC both because it can divert a dense material from being sent to landfill as waste and because it can be used to augment soil nutrients to grow vegetation and minimize erosion on the cover. Auto shredder fluff, on the other hand, needs to be blended with soil and may contain contamination that has the potential to seep into the ground, at times making it more harmful than helpful.
“One has to be careful when looking at the material to use as daily cover. [You need to consider materials’] long-term effects on leachate, stability, heat, landfill gas quality, odors, etc.,” Abichou says.
Abichou says it is ideal to have several types of covers approved for use at a given facility when pursuing approval for waste-derived ADCs, as no single type of cover, whether waste-derived or not, is ideal for all sites.
Biocovers for the interim
While daily covers can be composed of a variety of materials, landfill operators don’t have nearly as many choices when it comes to interim covers.
Some landfills use tarps and membranes as interim covers, though most use soil, which Abichou says is the preferable material.
“Rethinking interim covers … as biocovers is the way to go,” Abichou says.
Abichou says the most “action” takes place in a landfill cell when it’s covered by the interim cover, so operators should be mindful of choosing a solution that works best for their site.
“I believe that interim covers are the most important cover one should talk about,” Abichou says. “Landfills stay under interim covers for a long time. … Models show that 90 percent of percolation occurs before a final cover is placed. Most greenhouse gas emissions occur pre-final cover placement. Interim covers, in my opinion, should have more respect.”
Abichou has spent years studying how interim covers made of a variety of earthen materials reduce different types of emissions.
In one of his studies, Abichou experimented with soil, mulch and compost to determine which was most effective in minimizing hydrogen sulfide. The study found soil would attenuate 100 percent of incoming hydrogen sulfide for 75,000 days—more than 205 years.
On the other hand, mulch would do so for 4,400 days, while compost alone would attenuate that volume of hydrogen sulfide for just 462 days.
Another one of Abichou’s studies focused on the effect of different types of soils on methane reduction. The study showed that efficiency ultimately depends not just on a soil’s physical and biological properties, but also on environmental conditions, such as air and waste temperature, precipitation and barometric pressure.
The study concluded that the higher the porosity, vegetation and microbial levels of soil, the better.
By making a choice to use porous, healthy soil as an interim cover, operators can effectively minimize the level of gasses released at their landfill during the most critical period of emissions.
Whether it’s investing in higher quality resources or repurposing those already in a landfill’s possession, rethinking the covers that impact landfill performance on a day-to-day basis can help increase a site’s lifespan while preserving time, money and the environment.