HDR Inc., Omaha, Nebraska, has been supporting municipal and private solid waste clients for over 40 years with the planning, permitting and construction of secure solid waste landfills across the U.S.
Since first stepping onto the solid waste scene in the 1970s, the company has grown its waste sector operations to include over 150 professionals dedicated to solid waste landfill, solid waste facility, waste planning, waste-to-energy, landfill gas and environmental compliance projects.
In addition to these projects, HDR is often involved in the development of clients’ post-closure use for their landfills. In many instances, the post-closure plan is a component of a site’s master plan, where the solid waste department, agency or private operator makes a commitment to the community on what the closed facility will look like and how it will be managed in the future.
“In the last 10 to 15 years, communities have started taking more interest in redevelopment of their closed landfill facilities for beneficial use,” says Jeffrey Murray, senior solid waste project manager at HDR. “Instead of just perpetually mowing and maintaining the final cover system, forward-thinking leaders are creating innovative uses for [these] spaces, such as walking trails, scenic overlooks, mountain bike trails ... and many other alternatives.”
MAKING A PLAN
According to Murray, many of HDR’s clients elect to initially permit and construct landfill closure projects using conventional geosynthetic and soil final cover systems, which is known as a Subtitle D closure.
“Typically, the closure planning process includes analysis of existing and planned waste acceptance rates, filling plans, determination of appropriate size for closure areas … and timing for the phased closure construction,” says Murray. “The planning also includes evaluation of operational and construction soil consumption rates and the availability and suitability of on-site cover soil and topsoil material for the planned closure events.”
When planning for closure and the post-closure period, Murray says HDR prefers to keep it simple and manageable, where practical, to reduce long-term maintenance costs. Important considerations of a site’s redevelopment can include:
- Type of grass seed mix
- Soil material quality above the barrier layer to support vegetative growth
- Design of the stormwater management system to reduce erosion potential
- Layout of the stormwater management system for maintenance of the vegetation
- Development of effective means to discharge water above the final cover system geomembrane
- Steepness of slopes and transitions from stormwater benches
- Swales for maintainability
“It’s important to identify post-closure uses prior to development of the closure design and construction to incorporate the use into programming and avoid having to retrofit systems after the fact,” Murray says.
Once a use has been identified, the development of the post-closure plan can begin.
The first step Murray recommends when developing a post-closure site is to identify the minimum inspections and reporting frequencies required by local regulations. This can determine, based on operational experience, if more frequent inspections and maintenance of the leachate collection, groundwater monitoring, or stormwater retention and control system are necessary.
Next, operators should review the frequency needed for inspections of the closure system, which can be based on site-specific conditions to ensure the final cover system remains functional as designed and reduces potential for having to make large-scale repairs.
Following this review, developers can identify any unique details of closure construction that will require attention to ensure cover system function is not affected. This will help determine if inspection frequency of certain parameters can be reduced over time as part of the post-closure plan, as the reduction of landfill waste settlement and landfill gas production following closure is anticipated.
In addition, operators will need to assess the type of data (and data collection frequency) to be recorded to demonstrate whether the final cover system is meeting performance standards and that functional or organic stability is being achieved.
Lastly, it is recommended to characterize the criteria the regulatory agency and host community would require to determine if a closed facility could be transitioned from post-closure care to long-term management.
FINDING A USE
Some pioneering clients of HDR have seized on the opportunity to design and construct alternative long-term intermediate or final cover systems with an exposed geomembrane or synthetic turf system.
These alternatives can be beneficial to some landfill owners based on their location, availability of on-site soil and other factors as they can result in a reduction in closure construction costs and long-term post-closure maintenance costs.
“During the post-closure period, we have assisted clients with plans to redevelop the closed site for beneficial use. These projects have included adding walking and bike paths, picnic areas and solar arrays,” says Murray.
Of HDR’s beneficial use projects, one of the company’s most reputable post-closure developments is the Hickory Ridge Landfill in Atlanta. In 2010, HDR utilized a new exposed geomembrane solar cap technology to transform the landfill into the largest solar energy-generating facility in Georgia.
According to the company, it is the world’s largest solar energy cap and the first use of the technology as part of a fully permitted landfill final closure system.
Utilizing over 7,000 solar panels to convert sunlight into more than 1 megawatt of clean, renewable electricity, the new technology essentially takes a durable, high-strength geomembrane material made for outdoor exposure on roofs and secures it to the landfill like a bedsheet through the use of vertical anchor trenches.
HDR says the Hickory Ridge Landfill closure represents a milestone in the solid waste industry because it replaces a traditional Subtitle D closure with an alternative cap system that provides many environmental and economic advantages.
“The solar energy cover helps avoid thousands of tons of greenhouse gases that would be emitted from the mowing and soil replacement activities needed for long-term care of a grass-covered cap,” the company says. “Because rainwater runs off the geomembrane liner like water coming off a roof into a gutter, reusable water can be harvested without the need for sedimentation and cleaning.”
The installation costs of an exposed geomembrane closure are substantially less than a traditional closure, HDR says. By reinvesting construction and maintenance savings into solar panels, long-term maintenance costs can be replaced with a positive revenue stream.
“HDR’s interest in supporting our clients to find the best solution for their post-closure use is driven by our philosophy of looking at waste as a resource through its entire life cycle, and ultimately, using the landscape created from the disposal of waste and residue in a beneficial manner,” says Murray. “For post-closure plans to be successful, they require innovative thinking, being aware of technological advancements and taking a collaborative approach with the client as you work to gain political and economic support.”