Wet or dry? For many years, dry tombing -- which minimizes the water that filtrates through landfilled waste -- was the preferred method of cell design.
In fact, it became mandated in landfill rules, including the famed Subtitle D. This technique's leng-thy process, however, along with other limitations, has sparked in-terest in alternative designs.
For example, wet cell technology allows wastes to decompose rapidly, as well as increases post-landfill closure opportunities. On the other hand, dry tombed wastes decompose slowly over a long period, which results in long-term care and monitoring requirements. Also, dry tomb landfills settle slowly, limiting end use possibilities such as parks and golf courses, and require long-term leachate management.
Wet cells have been investigated and tested for a number of years. In this method, water is infused in-to the landfill, speeding up decomposition and stabilizing sites in a shorter time period. In addition, leachate recirculation has been used to stabilize leachate quantities and characteristics.
Several new landfill design projects, including one in Alachua County, Fla., are being designed to potentially incorporate a wet cell.
Wet cell technology may be beneficial when used for: leachate pretreatment/stabilization, more rapid site stabilization for specific end-use requirements, gas recovery, mining and reuse applications.
Rather than being used to retrofit older sites, wet cell technology is easier to apply to a new site de-signed for this type of operation. In addition, regulatory requirements often make it difficult to obtain permits for wet cell facilities if only the minimum design requirements are proposed.
If the landfill system's primary goal is to use a bio-reactor to recover compost, several questions must be addressed. For example, if additional facilities will be used to further treat the mined materials, the landfill design may be simpler be-cause a wider range of results may be acceptable.
On the other hand, if a higher quality product is desired directly, and if gas recovery is economically important, then a more comprehensive system using wet cell technology may be justified.
How do you determine the best design for a landfill closure? You should consider if the landfill plans intend to:
* use a vertical or horizontal distribution system;
* maintain an even leachate distribution, settlement and stability of pipes and facilities; and
* manage leachate and gas si-multaneously.
In addition, you should consider the landfill's operation and maintenance costs.
The first step, however, is to make sure that the program's goals are clearly delineated and attainable. Remember, the reasons for using dry tombing versus wet cell technology are not mutually exclusive.
Although still in its developmental stages, use of wet cell technology is likely to be more used in the future. Keep in mind, however, that evaluating the risks and benefits of using either method, as well as un-derstanding the design, operation and technical issues, will create the most effective landfill system possible.