Since leachate treatment and disposal can be a significant part of the operating cost for sanitary landfills, it makes sense to consider cost-savings options for treatment as well as minimization. There are a few approaches that aid in this.
Reduce: Controlling Stormwater Incursion
In areas where rainfall is not uncommon, stormwater can contribute considerably to the volume of leachate produced on an annual basis. So, one possible strategy for minimizing leachate is careful management of stormwater. Strategic placement of stormwater ditches and management ponds is a tried-and-true strategy. Typically, stormwater ponds are located just outside the berm of an existing landfill cell, meaning the freeboard is no higher than the adjacent berm. As additional cells are opened as the level of the landfill rises, the drainage field for the original pond increases substantially, meaning that overtopping is an increasingly likely event unless the volume of the pond is adjusted for the changes. If precipitation escapes the drainage field, uncontrolled stormwater will flow almost directly into the leachate drainage layer.
In most cases, then, a landfill should be constructed with swales around the perimeter on the upslope and sides to direct surface water around and away from the landfill without significant loss of velocity. The goal in this case is to encourage the extra water to pass by quickly, rather than infiltrating daily covers. Cells may be finished to provide a slope on the ADC to help the water drain away efficiently as well.
Other structures in a landfill, including roads and daily covers should be constructed with low grades that promote runoff away from cells toward swales or other stormwater features. Ensure that the directed runoff does not cause erosion alongside slopes, roads, etc.
It’s easy to overlook it, but the size of a landfill’s working face contributes significantly to the volume of leachate produced in the landfill. Since the open area of a cell is also open to rainfall, it must be included when calculating the effect of precipitation. For example, one inch of rain over a single day is hardly worth noticing in some parts of the country, but even with a very small working face of only 1 acre, that level of precipitation will produce about 27,000 gallons of leachate.
Recycle: Treating Leachate
In today’s improved landfills, leachate collection systems are mandatory. These systems include installation of impermeable liners to provide a barrier between the collected waste and the soil below. Typically, another layer of liner is installed above the barrier to act as a collection point, where leachate is automatically removed for reuse or further treatment. Automatic pumps ensure that the leachate doesn’t overtop the barrier layer and escape into the environment even during periods of intense rainfall.
Once leachate has been collected, it may be diverted to a facility for treatment, either on-site or more often to a local wastewater treatment facility. The ultimate goal in treatment is to remove contaminants and release the treated water back to the environment, once it’s been proven to meet federal, state, and local standards for safety.
In a waste treatment facility, activated sludge with live aerobic bacteria may be used to promote uptake of nutrients and digestion of organic matter in an aerated lagoon. Physicochemical processes can prepare metals, ammonia and dissolved solids for removal by flocculation or filtration, for example. If those methods are impractical, membrane separation and reverse osmosis are common, though expensive, options. In the latter cases, the remaining dewatered waste can often be disposed of in the same landfill that produced the leachate to begin with.
Reuse
A newer approach to managing solid waste in landfills involves collecting the leachate and repeatedly passing it through the landfill cells. This continuous addition of moisture helps the organic waste decompose much faster, while the leachate has more opportunity to remove heavy metals and other dangerous contaminants. Options for reusing leachate are discussed throughout this series.
