Most of the factors that control the final gas production rate of any particular landfill cell, such as soil compaction or liner selection, are chosen long before it’s closed. However, well spacing and placement is one particular feature of a gas collection system that is determined after the closing of most cells. It’s also an essential factor in determining the total capture rate for the cell. Even with careful compaction and the use of only permeable cover materials, it’s all too easy for a landfill cell to fail to perform to expectations if too many or too few wells are placed. Too few wells allow for vacuum loss that leaves gas lingering at the bottom, while too many wells lead to moisture and leakage issues. Aim for just the right balance of well spacing for the best gas control and most efficient capture rates.
General Spacing
The exact spacing for wells in a landfill gas collection system depends on many factors, starting with the exact size and type of borehole or horizontal trench. However, general spacing tends to fall between 150 and 300 feet between wells. That measurement must be calculated from the center measurement of each well or placement will be skewed. However, there’s no reason to make well placement completely even or regular over the surface of a large landfill cell of 100 acres or more. Since conditions vary from one part of the cell to another, well spacing may be heavier in one area than another. Maximizing gas extraction rates, without risking environmental damage or losses, requires a comprehensive and creative approach to well placement.
Edge of the Cell
Even with the most impermeable cover and liner materials for good pressurization, wells along the edges of the cell experience a greater level of pressure than the wells in the center. This means that using a standardized placement rate will cause the edge wells to overpull on gas, resulting in releases to the atmosphere that may violate both state and federal clean air laws. It’s also a waste of valuable gas. Placing wells closer together along the edges of a landfill cell compensates for this effect by reducing the amount of pressure spread along these areas. This also helps protect both the liner and cover materials from damage over time.
Other Considerations
A number of other considerations go into both well placement and distance. Only an experienced waste management engineer can narrow down the perfect arrangement of wells, but some general factors that go into the process include:
- The total amount of vacuum being applied over the entire cell and where the wellheads are located
- Density of the compacted waste inside the cell and its composition
- The total amount of liquid in the waste, how well it is distributed through the pile, and at what level it begins to accumulate at the bottom of the cell
- Total height of the cell and depth of the waste
- Width and length of the cell, which determines the length of each edge as well
- Goals for the finished gas, including volume and composition
- Nearby residents that are concerned about odor or exposure to irritants.
With landfill cells often stretching over hundreds of acres at a time, it’s no surprise that each cell can support dozens to hundreds of wells in turn. Make sure your cover materials can handle the number of penetrations needed, to connect them all to the wellheads, by choosing a durable geomembrane from BTL Liners. Our products can live up to the harsh environment of the gas producing landfill cell with minimal maintenance requirements over the years.
