Alternative daily covers that are applied at the end of the day but not retrieved the following day become a permanent part of the waste heap, similar to soil. All single-use ADCs take up less airspace than the original soil cover material, but there is a lot of variation beyond that.
Waste-Derived Materials
Waste-derived ADCs are made of materials that are bound for a landfill anyway, so there’s no “real” loss of airspace when these are employed. If a landfill operator can charge to accept waste that can be turned around to create ADC, then that’s even better. In some states, you may even be able to count this toward diversion, recycling or beneficial use goals.
Common Waste-derived ADCs include
- Ash
- Auto shredder fluff
- Construction and demolition (C&D) waste fines
- MSW derived compost
- Digested waste
- Processed C&D waste
- Composted green waste such as vegetation and leaves
- Contaminated sediment and soil
- Composted sewage sludge
- Shredded tires and paper
Non-Waste Derived Materials
Single-use, non-waste derived materials for ADCs are typically sprayed on products or extremely lightweight polyethylene film applied with a tarping machine.
Polyethylene Film
This extremely thin plastic (think Saran Wrap) is applied from a roll using a deployer, so no mixing or cleanup is necessary. The film is overlapped as it is deployed in order to create a continuous seal; it’s also best practice to deploy ballast along with the film as it’s being applied. Since the film remains in place, there’s no morning release of accumulated odors and VOCs. Instead, methane gas remains trapped within the cell and can be diverted to collection points where it can be diverted to energy production. The film is mechanically destroyed by placing the next day’s waste on top of it, and over time the film breaks down, although some companies offer additives that speed up the rate of decomposition.
Some film products based on cellulose can absorb moisture to create a barrier that reduces intrusion of water as well as escaping gas. This improved film also does a better job of odor control.
Polyethylene film typically requires only 2 crew members to apply, and the work goes quickly so the landfill can remain open later. Film ADCs can be applied even in high-wind conditions, which can be a problem for other ADCs.
Slurries
Slurries are made by mixing solids like wood fiber, newspaper, cement kiln dust and fly ash, with water and other materials that function as an adhesive to help the mixture adhere to the waste surface. One type of slurry uses recycled paper and wood dust combined with polymers. When sprayed on a landfill’s working face, the slurry forms a cement-like crust. At a minimum thickness of only ¼”, the advantage over soil just in saved airspace is clear. In addition, ADC slurries are typically non-flammable, biodegradable, and non-toxic. Other popular slurry products use fiber and polymers to create a film.
Slurries do require pre-mixing and cleanup, which adds labor time to the cost. Rain can wash the slurry away before it hardens, and high winds can carry it off before it even touches the ground. They also are vulnerable to freezing and it can be difficult to get consistent coverage on all sides, especially in windy conditions. It may take several additional applications from different directions to ensure even coverage of the waste.
Foam
Foams are also applied using specialized spraying equipment. They’re a mixture of soap, starch, or resin and water, and can be hardening or non-hardening. When waste is deposited on top of foams on the following working day, the barrier is effectively broken up to permit free movement of gases and leachate throughout the cells. Non-hardening foams are particularly effective at discouraging insects, birds, and rodents from interacting.
The biggest advantage of non-reusable ADCs is that they do not need to be removed. Some spray-ons may also include odor control agents, but they can be difficult to apply during heavy rain and windy conditions. In freezing conditions, the materials may be impossible to apply without special equipment to keep the dispersing tools and the mixture from freezing. Finally, the overall cost of spray and foam ADCs is fairly high. Material cost, specialized application equipment and the labor required to prepare the mixture and clean it up afterwards can take away some of the cost savings compared to soil.
