Venting trench

Because gas migration cannot be easily prevented, removal of the landfill gas, to reduce the driving force for gas movement is often the preferred option. This is normally undertaken by the use of within waste vents (wells) through which the gas can be extracted either actively or passively, or by the use of stone-filled vent trenches, often placed around the periphery of landfill sites. The design of a typical gas vent system is shown in Figure 9. 

Stone-filled vent trenches represent a second form of passive gas control. These trenches are often used on site boundaries where gas migration has been identified, but such systems can only be effectively used when the landfill depth does not exceed 8m (DoE, 1992a), and they are seldom used at depths greater than 5m. The limit on the effective depth of trench is controlled by the depth to which an excavator may dig. To excavate deeper than this requires specialised equipment or techniques and is not cost-effective. The effectiveness of vent trenches (Figure 8) increases when they are used in conjunction with a geomembrane, placed within the trench along the wall furthest from the gas source. The vent trenches again... 

Audenshaw, Manchester, UK, landfill gas migrated from a former clay pit which was being used as a landfill site. Gas alarms were fitted to four properties after gas was detected during a routine inspection. A vent trench was also installed (House of Commons debate, 1 November 2005)... 

Widespread migration to existing housing outside iandfiii site Unlikely Severe Moderate/low risk Provide venting trench... 

Quick installation compared to vent trenches and impermeable barriers in trenches... 

All gas protection measures require maintenance. Even passive systems require regular inspections to ensure that vents are not blocked and are still working. The importance of maintenance is highlighted by the example in Chapter 1 where a vent trench became blocked, thus causing an explosion in a building. 

Inspection of vent trenches to ensure that the vents are not blocked... 

Control of ga.s movement by recovery. The movement of gases in landfills can also be controlled by instadhng gas-recovery wells in completed landfills (see Fig. 25-74b). This is considered an active venting system. Clay and other hners are used when landfill gas is to be recovered. In some gas-recovery systems, leachate is collected and recycled to the top of the landfill and reinjected through perforated lines located in drainage trenches. Typically, the rate of gas production is greater in leachate-recirculation systems. 

Passive perimeter gas control systems are designed to alter the path of contaminant flow through the use of trenches or wells, and typically include synthetic flexible membrane liners (FMLs) and/or natural clays as containment materials. The membrane is held in place by a backfilled trench, the depth of which is determined by the distance to a limiting structure, such as groundwater or bedrock. A permeable trench installation functions to direct lateral migration to the surface, where the gases can be vented (if acceptable) or collected and conveyed to a treatment system (Figure 10a and 10b). 

Where there is a landfill gas-generation problem, active (i.e. pumped) or passive venting, by way of stone-filled trenches at least 3 feet deep, may be needed. Where active... 

The applications and limitations of passive gas control systems must also be understood. They can be used at virtually any site where there is the capability to trench or drill and excavate to at least the same depth as the landfill. Limiting factors could include the presence of a perched water table or rock strata. Passive vents should generally be expected to be less effective in areas of high rainfall or prolonged freezing temperatures. 

Note Grating type used must have a minimum of 50% open area to prevent flammable concentrations inside the trench and to provide venting. 

Kato C, Li L, Nogi Y, Nakamura Y, Tamaoka J, and Horikoshi K (1998) Extremely barophilic bacteria isolated from the Mariana Trench, Challenger Deep, at a depth of 11,000 meters. Appl Environ Microbiol 64 1510-1513 Kaye JZ, Baross JA (2000) High incidence of halotolerant bacteria in Pacific hydrothermal-vent and pelagic environments. FEMS Microbiology Ecol 32 249-260... 

Soil Type Measures Trench Venting Excavation Wells... 

Natural ventilation can be adequate provided there are sufficient vents or louvers at the ceiling or floor level. An effective design will ensure that the airflow either sweeps across the floor or ceiling continuously and adequate outside make-up air is provided. The discharge should be directed outside and away from any air inlets, other openings, and equipment such as compressors. If adequate natural ventilation cannot be provided a mechanical ventilation system should be used. Mechanical ventilation systems for applications involving transfer of flammable liquids typically have a capacity of 0.25 to 2 cfm/ft2 (0.075-0.60 m3/min/m2) and higher in trenches, sumps, or other collection areas. 

DISPOSAL AND STORAGE METHODS neutralize small quantities in trenches and bum vent fumes through absorption equipment and add to soda ash-slaked lime solution add neutralized solution to excess running water and route to sewage plant store in a cool, dry location separate from silica, concrete, glass, ceramics, and incompatible metals glass containers are not recommended even for dilute solutions. 

Vent and drain openings associated with combustible fluids or gases Drainage ditches, gulleys, trenches, and associated remote impounding basins... 

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