Leachate Collection Layer
Regardless of whether a landfill has an engineered landfill liner system installed, leachate collection is still an important issue. If a bottom liner system is installed, it is usually covered with a protective layer of sand or stone that serves as a drainage medium. This protective layer also protects the bottom liner system from being punctured by equipment or protruding objects on the waste pile. Sometimes a network of perforated pipes is installed within this protective covering to facilitate better drainage of landfill leachate.
Optimizing Leachate Collection
In order to optimize leachate drainage and collection, the bottom of the landfill should be gently sloped, channelling leachate to centralized locations from where it can be effectively removed. Landfills situated on flat sites can be expensive to construct, as a slope needs to be created by excavating and/or filling. Consequently, landfills that are located on sites with a gentle slope can reduce costs.
A landfill that is equipped with a bottom liner system, but which has no functioning leachate collection/management system in place, will rapidly fill up with fluids as there is no way for liquid to escape from the landfill. This is undesirable as it can cause problems in terms of slope stability, and can negatively impact the functionality of the landfill gas management system.
Leachate pipes are typically spaced 45-60 meters apart, with the bottom contours depending largely on the geometry of the site and on groundwater depth. Leachate drains from the landfill are funneled to a sump, from where it is extracted by a submersible pump
. In certain scenarios, such as side slope landfills, canyon fills, and above-ground mounds that require no excavation, gravity systems may offer a feasible alternative to the sump/pump extraction method.
Landfill Leachate Management
Once the leachate has been drained and collected, it needs to be disposed of appropriately. This can be achieve in a number of ways:
- Leachate can be stored in evaporation ponds
- It can be treated and discharged into sewage wastewater streams
- Treated leachate can be also discharged into local waterways
- It can be directly discharged, or hauled, to a wastewater treatment facility
- Leachate can be vaporized in LFG-fueled leachate evaporators
- It can be recirculated
Recirculating leachate results in the formation of a bioreactor, and due to concerns regarding slope stability, should only be used as a leachate management solution at landfills that are well managed and stable. However, when conditions are suitable, leachate recirculation offers some benefits: It provides a method of managing landfill leachate; and it can improve landfill density, resulting in an increase in the waste disposal capacity of a landfill.
Leachate can either be recirculated using surface or subsurface methods. Examples of surface methods of recirculation include application by tanker truck or spray irrigation. Examples of subsurface methods of recirculation include horizontal trenches or vertical injection wells. In most cases, pumping liquids into vertical landfill gas extraction wells is less effective.
No matter which landfill leachate management system is utilized, all equipment related to this task, including piping, collection/storage tanks and pumps, need to be adequate for the volume of liquid anticipated in order to prevent leachate from being released into the surrounding environment.
Common Problems Associated with Leachate Collection
In some instances, leachate can breakout from the slope sides or from the base of closed cell slopes covered with soil and a plastic cover. In these cases, the rogue leachate needs to be captured and the offending method of escape sealed. According to Steven Viny, CEO of Envision Waste Services, this problem can arise as a result of hydraulic pressure and due to ponding water tables typically found in landfills. He recommends taking the following action to rectify the problem:
â€œIf the pop outs occur regularly in this given area, you can install some well points to pump down the ponded water ((leachate) table. Drill a nominal 2' bore down to or below the elevation of the pop out. Install a perforated pipe in gravel pack to an elevation 10' below and 10' above the pop out elevation give or take. Solid pipe above that level with a bentonite plug and in-situ clay above the perforated pipe section elevation. Install a submersible water pump in the pipe. You can use a portable generator to power it or you can hook it up to permanent electrical service. Pump the leachate into your leachate collection pipe network. You will be amazed at the amount of water that you can remove! You can leave the pump in the pipe permanently and set it on a float switch or you can simply check the water level occasionally and pump it down as necessary. Once the initial head pressure is relieved with the pumping action, pack powdered bentonite into the area of the former seep,â€ he suggests. â€œAlso check your gas wells for water level. Sometimes you can simply pump the gas wells down which will increase the gas flow and decrease the head pressure on the landfill which causes your leachate pop outs.â€
Image Credit: "Leachate Pond" by Victor787 [CC BY 3.0] via Commons