A complete leachate collection system begins under the landfill and extends from under the cells of trash. The liners and pipes underneath the mound of waste direct the liquid to leachate ponds located around the perimeter of the facility. Without all the various parts of the system working together, leachate will only accumulate and overflow somewhere to cause serious environmental impact. It’s not enough to simply accumulate leachate in large sumps because pumping is unlikely to keep up with sudden and unexpected surges in the amount of liquid coming out of the landfill. Make sure you’re designing a complete approach to handling leachate, and not a piecemeal solution, by addressing all of the following concerns.
Who Regulates Landfill Leachate?
Despite municipal solid waste, and its resulting wastewater being designated as non-hazardous waste, the Environmental Protection Agency (EPA) still has the right to regulate it. This was established on the federal level through the Resource Conservation and Recovery Act in 1976. When the environmental impact of leachate was discovered in the 1950s and 1960s, most states also began to implement strict controls on the design of new landfills. It’s also possible for local water protection agencies and natural resource departments to have input on the placement and design of new landfills. This is especially likely for areas with wetlands and other protected areas that have a high chance of affecting public waters.
The Main Parts of the System
A leachate collection system is complex and few of them are identical to each other. Each design must be adjusted and adapted to the exact site conditions and state regulations. However, most of them share similar construction methods and layers of materials.
Basin Slope:
The first part of the collection system is a slight slope to the base of a waste cell or collection basin. Sloping the base of the compacted soil towards the collection pipes ensures that gravity directs the wastewater where it can flow into the rest of the system. Without this small amount of slope, leachate will build up in the corners and sit so long it’s likely to escape any kind of liner or barrier. Most designs call for a 2% maximum slope on the base and indicate multiple angles of slope to direct water to multiple runs of perforated collection pipe.
Underlayments and Base Layers:
Above the graded and carefully sloped compacted soil, most landfill collection systems start with a layer of compacted clay. This won’t serve as the only, or final, liner because it’s still permeable. However, it does work well as a smooth and non-compressive base for applying a geomembrane liner. Another option is adding a cushion-like underlayment that has a texture similar to felt. While it’s also not impermeable or designed to stop seepage, it’s meant to protect the flexible liner material and keep it from shifting or tearing.
Impermeable Barrier:
The first required layer of any leachate collection system above the soil is the impermeable barrier. This is usually a flexible geomembrane that is anywhere from 25mils to over 100 mils thick. It must have as high of a functional impermeability rating as possible to keep water from seeping out over time since leachate ponds are built as long-term storage structures. If the material used is too permeable or reacts to the chemicals in the leachate, it will allow the waste to seep out over time.
Perforated Pipes:
Above the impermeable barrier of the landfill leachate collection system are the perforated pipes responsible for collecting the leachate so it can be directed to the storage ponds. These pipes lead to well-like depressions known as sumps, so they must slope slightly as well to direct fluid where it’s needed. Most designs call for two layers of perforated pipes known as the primary and secondary collection stages. These layers are separated by coarse gravel, sand, and possibly the addition of permeable geotextiles to stabilize the construction.
Drainage Layer:
Geotextiles and other permeable fabrics are used above the perforated pipes to keep trash, silt, and other material from settling downward and mixing with the leachate below. Without an appropriate and strong drainage layer at the very top, the best collection systems are inevitably clogged with bits of trash and organic material. The material needs just the right amount of permeability to let liquid in quickly enough so that the mound doesn’t overflow, but also not so fast that it overwhelms the collection system.
Sump Pumps, Transfer Pipes, and Trenches:
Sump pumps, transfer pipes, and open trenches are all used to connect the collection layers under the landfill to the larger storage system. Without these features, leachate ponds tend to sit empty because only a limited amount of liquid can be drained to a pond without direct connections and powered pumps. These features must be designed for easy access since they tend to need the most maintenance out of the entire collection system.
Leachate Ponds:
Finally, the leachate ponds are the end of the on-site collection and handling system. Even if they’re drained for later disposal, they serve as the final stage of storage in most designs. Leachate ponds must be lined, as well as the collection system under the landfill that supply them, but they don’t need the secondary collection layers and permeable layers found in the rest of the system. Another major difference is exposure. Leachate ponds tend to have exposed banks where the liner material receives constant UV degradation. In contrast, the liners in the rest of the collection system should be buried at all times if they’re working properly. This means slightly different liners are needed for the storage ponds than for the rest of the system. BTL Liners has the RPE geomembranes to handle both exposed and fully buried installation.
What Do Leachate Ponds Need?
Aside from lining, leachate ponds need a few basic design features to ensure they’re easy to clean and maintain. First, these ponds tend to feature large surface areas and minor slopes along their banks to minimize wave action and speed evaporation. Unless there is a concern with gas production or odor from evaporation, most ponds will aim to reduce total volume by concentrating the leachate. Second, make sure the soil can be stabilized to the point where the banks won’t collapse during routine dredging. Sludge accumulation is rapid in these ponds due to the high concentration of organic waste, so it’s likely the pond will need multiple dredges before it’s finally closed off. Some banks may need concrete or asphalt reinforcement to make them easier to maintain. Finally, the total desired volume of holding capacity should be divided into two or more individual ponds. This allows for easier cleaning and reduces the damage caused by a sudden release of water if an embankment is compromised.
Adjusting to Varying Leachate Levels
Landfills do not produce steady amounts of leachate over the course of their lifespan. This makes it tricky to predict exactly how much leachate may come from a particular cell or unit, especially before taking into account weather variations and changes in the actual waste stream. This means that leachate collection systems and ponds must be built with extra storage in mind, to absorb large and unsteady flows. For example, many states like Ohio require engineers to use 100-year flood records to predict the largest possible amount of rainwater to percolate through the system. This sizing should guide any clean water runoff systems as well, so that water doesn’t overwhelm those drainage areas and interfere with the leachate instead.
Dealing with Weight
Some states allow for up to 300 feet of compacted waste before a landfill unit must be closed and capped, making a staggering amount of compacted waste that each drop of water trickles through. That’s also a lot of weight pressing down on the trusty leachate collection system at the very bottom of it all. Without proper support, time and weight slowly compresses the system until it’s unable to function because the perforated pipes have snapped or become filled with dirt. Make sure to include the extra weight of the leachate working its way through the system, so a sudden influx of water doesn’t raise the weight enough to crush the collection layers.
The complete landfill leachate system begins with a sound engineering plan and soil testing. Once you’re ready to select a liner for the impermeable barrier above the base, turn to BTL Liners. We also have the cover materials you need for both temporary and permanent landfill caps and pond covers.
