Leachate is water (principally rainwater) that comes into contact with waste and is potentially contaminated by nutrients, metals, salts and other constituents. Groundwater and surface water can be contaminated by untreated leachate from landfill sites. Leachate has the potential to cause serious water pollution if not managed properly. Surface water may also be adversely affected by sediment or contaminants in uncontrolled storm water flows.
The effective management of leachate within a site continues to present a challenge to landfill operators. Every site is different and demands a unique solution. At Biogas we have gained experience over a wide range of sites and installations and have developed effective solutions to the regular problems encountered.
Where the quantity of leachate to be pumped is variable and low, and perhaps distributed over a large area, then pneumatic cylinder pumps can offer particular benefits – and still fit within established gas wells for ease of installation. Similarly where there is an efficient leachate drainage system within the site which collects the leachate at one or two deep locations, then conventional submersible pumps can be effective.
Output arrangements from the extraction system can be supplied to suit specific site requirements. Discharge can be pumped or by gravity, and the system supplied with automatic sampling and monitoring or linked to treatment plant. In some cases leachate recirculation may be used to optimise gas production, or spray irrigation for on-site disposal.
Within many sites the management of leachate is directly linked to the management of landfill gas, since there is a recognised interaction between the two. Where a gas utilisation scheme is in place and leachate levels are high, an effective leachate extraction system may increase gas yields. And the potential for re-circulation of the leachate within the landfill mass as part of a progressive management system may create further opportunities for an increase in gas production.
It is therefore highly beneficial to approach landfill environmental management in a coordinated way, considering the tasks of leachate extraction and treatment along with gas management and a clear understanding of the other elements of the landfill management process in the site.
Sites must include leachate collection pipes within the drainage blanket. The spacing and sizing of leachate collection pipes must be designed to control the potential depth of leachate on top of the base liner to less than 0.3 m. The design must consider the base liner gradients, drainage aggregate, estimated leachate generation and long-term performance. Typical pipe spacing is between 25 to 40 m. Pipe sizing must consider potential leachate flow, strength, inspection and maintenance and the issues presented above. Pipes must be resistant to degradation by leachate and landfill gas and must be manufactured from HDPE or MDPE.
The sizing of leachate pipes is based on leachate flow rates within the pipe and the diameter required for the passage of remote inspection and cleaning equipment. This equipment typically required pipe diameters greater than 150-200mm. Manning’s equation should be used to derive the required pipe size based on leachate flow rates and pipe slopes. Leachate flow rates are derived from a water balance model. Pipe perforations should include 12 mm diameter holes. The hole locations should be alternated in pairs at 150 mm intervals along the pipe. Each alternate pair of holes should be located at 45 and 225 degrees to the vertical axis (pair 1) and 135 and 315 degrees to the vertical axis (pair 2). Leachate collection pipes must drain at a minimum grade of 1% to a sump. The minimum slope of the surface of the underlying liner is 2% towards drainage lines. The pipes must extend across the base and up the sides of the landfill cell and be able to be inspected and maintained.
The system needs to maintain its integrity under the vertical loads and stresses from the overlying waste and operating equipment. This will require design of access for vehicle traffic to the cell and procedures for placement of the initial layer of waste. Class MB+ and L sites must have a geotextile separation layer between the overlying waste and drainage layer to prevent migration of solids from the waste and clogging of the drainage layer.
Leachate sump must be located at the lowest point of the cell to facilitate monitoring and removal of leachate so that the maximum head of leachate on top of the base liner is less than 0.3 m. Design considerations include access for monitoring and inspection, leachate generation volumes, operation of pumping equipment (including the depth and storage volume for leachate), connection to the leachate storage and treatment facilities and maintenance of integrity during landfill operation. Leachate storage and treatment to prevent pollution of surface water and groundwater, odour and to minimise human contact.
Assessment of options for leachate storage and treatment will need to consider the quantity and composition of leachate. Possible treatment options include evaporation, degradation by aerobic bacteria or chemical or physical treatment. Design of the leachate storage capacity will need to consider the potential leachate generation, rainfall, climate conditions, the risk of overtopping and treatment options. Other design considerations include odour management and control of access.