Municipal solid waste (MSW) landfill operators are required to cover the landfill waste with at least six inches of daily cover to prevent odors, fires and other issues. For years, clay and soil we predominantly utilized as an effective cover, but more recently, operators have looked to alternative materials for daily landfill covers.
Waste360 recently sat down with two executives from Civil & Environmental Consultants Inc. (CEC) based in Pittsburgh, Pa., to discuss today’s most popular landfill covers and the benefits and challenges operators face in choosing an appropriate material.
CEC provides comprehensive market-oriented consulting services for the solid waste industry. Eric Chiado, is the vice president and head of CEC’s corporate waste management practice, and is based in Export, Pa. Anthony Eith, is the vice president and head of CEC’s Philadelphia, Pa. office.
Waste360: What are some of the most effective landfill covers?
Anthony Eith: These days, landfill final covers predominantly include geomembranes because geomembranes have proven to be the most impermeable of the barriers you can use. One of the most effective landfill cover systems is a geosynthetic cover system that would include a geomembrane, possibly a synthetic drainage layer above that, and then a soil cover for vegetation above that. Geomembranes avoid a lot of the problems that clay final covers had shown in the past—from cracking, settlement, desiccation, and erosion to problems during construction due to inclement weather, etc.
Another plus for geomembranes is that research has shown that they will last longer than several hundred years. You’d never get that with soil. As the landfill settles, the soil cap would crack. Geomembranes are flexible; they will stay in place and conform to the settlement. Geomembranes also are pretty much the main choice these days for temporary covers. Instead of waiting to put a final cover down, operators will put a geomembrane down for a short time to mitigate odor as well as rainwater infiltration, thus reducing leachate generation.
Waste360: Are there different types of covers for different locations or uses?
Anthony Eith: Yes. In arid climates, another type of cover system that can be evaluated is an evapotranspiration cover system. What that does is it utilizes the high evaporative potential for arid regions and, as opposed to the liquid flowing through the soil and getting into the trash to generate leachate, the liquid instead evapotranspirates from the soil and vegetation. In addition, depending on the region of the country, you can use different types of vegetation rooted in that soil layer to also pull out the moisture that would infiltrate the soil.
You could just have soil and some type of cover and look for evaporation to do its thing, but with a high evaporative potential, you can put in some kind of vegetation (i.e., shrubs, shallow-rooted trees) and utilize that vegetation to pull moisture away from the waste to mitigate the amount of moisture in the waste. If you are looking at alternates, that’s probably the best.
Eric Chiado: If your site lacks economic soil resources, you can also consider a geosynthetic cover system, consisting of either an exposed geomembrane by itself, or a composite system that contains a geomembrane, drainage layer and a durable synthetic turf.
Waste360: What are the biggest challenges in choosing the appropriate landfill cover?
Anthony Eith: The biggest challenges are initial cost for construction and the subsequent cost of long- and short-term maintenance. One of the big factors that enters into the equation on initial cost is usually related to the soil components of the selected final cover design. There are new products on the market that have been used and are being evaluated in certain states.
It’s a cover system where you put a geomembrane down, but instead of putting in a drainage layer and a soil layer on top, the drainage layer component is a structural component of the geomembrane itself. Another alternative is to put a geotextile on top of the geomembrane and then place soil. There also are turf products available. Instead of a geotextile and soil, you could put artificial turf down to avoid the cost of soil and the cost of long-term maintenance related to vegetation.
Waste360: What role do landfill covers play in greenhouse gas mitigation?
Eric Chiado: Landfill gas that is generated as waste decomposes primarily consists of two gases, carbon dioxide and methane, both of which are primary greenhouse gases. The geomembrane landfill cover creates an impermeable barrier over the landfill surface, and effectively blocks the ability of landfill gas to escape into the surrounding environment. In concert with the final cap, you need to have an active gas extraction system to collect methane generated from the site, which creates a vacuum to collect gas migrating from the waste.
By combining a geomembrane barrier and active gas extraction system, a modern landfill cover is very effective in reducing greenhouse gas emissions. Additionally, landfill gas that is not collected passes through the cover soil, and bacteria in the soil consume methane by reacting it with oxygen, which further reduces greenhouse gas emissions.
Waste360: How are landfill covers used to reduce odor/emissions and leachate seeps?
Anthony Eith: Similar to the above, the impermeable barrier blocks odor and gas emissions when combined with an active gas collection and control system. For odor and emissions control, you need to phase in your gas collection and control system early.
Eric Chiado: To enhance the collection of gas, some landfill operators will install temporary geomembrane covers over inactive landfill areas, which help to block gas emissions and makes active gas collection more effective.
Anthony Eith: If a landfill is not capped and the waste is exposed, rainwater infiltration may lead to leachate seeps as the rainwater will ultimately work its way through the waste and out to the slope due to differences in permeability of the waste and daily cover soil. And we all know liquid always takes the easiest route out. But with a final cover on, you’ve created a barrier that stops that infiltration of rainwater.
Waste360: What is the potential for landfill gas collection when a cover is used? What is the best cover for doing so?
Eric Chiado: The potential is substantially increased when either a permanent or temporary geomembrane cover is used, relative to simply covering waste with soil. Placement of a geomembrane cover increases the efficiency of a gas collection and control system by blocking the flow of gas into the environment and by allowing the operator to pull a larger vacuum on the active gas extraction system.
In other words, you get more (and better) collection with a geomembrane cover. If you don’t have a geomembrane cover, gas can more easily migrate into the atmosphere, and an operator cannot pull as hard on gas extraction wells because the absence of a geomembrane would allow more atmospheric oxygen to be collected, which is not desirable.
Anthony Eith: You can use a variety of covers to do so. Sometimes it comes down to economics. You can use soil for a temporary cover, or you can use a geomembrane. There are a variety of options for temporary covers associated with a gas collection and control system.