Landfill Gas Collection System
Landfill gas facility conditions conditions and operational goals both influence the design of a gas collection system (GCS). Site conditions such as landfill geometry, moisture, compaction rates, waste types, waste depths, cover soils permeability and final cover all affect GCS design. The greater the moisture within the waste mass, the faster landfill gas (LFG gas) will be generated and the higher the peak landfill methane generation rate. A more rapid biogas generation rate also leads to a landfill waste mass that tends to settle faster, which may cause damage to collectors that will need to be inspected and potentially replaced. Liquids within the waste mass may decrease the pore space within the waste mass, decreasing the ability of landfill gas to move to the LFG gas extraction wells. Thus, landfills with higher moisture content may have a smaller effective zone (radius) of influence for individual collectors and may require more collectors for the same area of coverage. Also some facilities choose to add moisture to facilitate decomposition, which increases landfill biogas generation but may increase GCS operational costs due to additional wells, increased settlement and larger header sizing.
Physical properties of the waste mass such as waste density (compaction), type and depth vary by location and affect the moisture level and methane generation potential of the landfill. Many facilities accept special waste streams such as sludges, ash, construction and demolition (C&D) and liquids, which affect the gas collection system design, landfill gas generation rates and the suitability of the LFG gas for beneficial use. For example, gypsum wall board and onions are known to elevate hydrogen sulfide (H2S) within landfill methane, which may need to be removed.
The materials used for daily, intermediate and final cover also change depending on local availability of soils, climate and approvals for alternate cover methods. Daily cover prevents blowing litter and odors and is usually not considered part of the gas collection system design. Sites that use a low-permeability soil such as clay for daily and intermediate cover can greatly reduce the influence of the LFG gas collectors and the effectiveness of the GCS. If this low-permeability soil cover is not completely stripped between placement of waste lifts, the landfill waste mass can be isolated from other landfill components, which adversely affects the ability to collect LFG and drain leachate. It also increases the potential of landfill biogas emissions and perched leachate (pooling of leachate on top of an impermeable layer) within the waste mass.
At the landfill surface, intermediate and final cover are designed to give a seal between the landfill and the atmosphere. A more impermeable seal on the surface of the landfill allows more vacuum to be applied to landfill methane collectors while minimizing the potential for atmospheric air and water to seep into the waste mass and ultimately into the LFG gas collectors. The more impermeable the intermediate and final cover, the greater the potential well spacing and the better the landfill gas wells are likely to operate.
Gas collection system (GCS) design can change dramatically due to local climatic conditions. The two most critical elements are temperature and precipitation. Accounting for temperature involves considering how GCS components will respond both during typical and extreme weather events. For example, facilities in areas that experience prolonged temperatures below 0 C (32 F) require winterization of equipment and vessels, and all header pipes and laterals should be buried to prevent freezing. Alternately, sites in very warm, sunny areas can have exposed gas collection system components experience significant thermal movement as they expand during the day and then contract overnight.