An advanced engineered closure system has proven that a landfill can be aesthetically pleasing, erosion-free, produce clean water runoff, and effectively contain odors and gases.
By Krista Willey
Albert Einstein once said, “The world as we have created it is a process of our thinking. It cannot be changed without changing our thinking.” He obviously changed history with his brilliant inventions but just as significantly, Einstein inspired others to rethink their approach to solving issues across many different industries. As a society, it has forced us to embrace change in products, ideas and methods that, ultimately, make things faster, more economical, better performing and sustainable. Innovation is an inevitable process, and such is the case in the solid waste industry.
Thirty years ago, Subtitle D was introduced with a prescriptive cap to seal contaminants from affecting the surrounding environment. While these structures have co-existed with us, it has not been without sacrifice. The topsoil layer may have been borrowed from another site and in some cases, may have disturbed virgin land. It may not always be of good quality, which could not only hinder vegetation growth but could also introduce other contamination from the previous site’s exposure. It has to weather erosive forces such as excessive saturation as well as severe drought. It is not to say that a prescriptive cap cannot successfully be achieved, but in many cases, the rate of failure has led to an investigation of advances in landfill closure technology to do things better.
Technology that Disrupted the Industry
In 2007, a new Subtitle D, hybrid technology was introduced into the marketplace. The three-pronged geomembrane, engineered turf and sand infill system, referred to as an advanced engineered closure system, promised new hope in seeking to eliminate both the ongoing plague of erosion and the headaches of those who consistently fought to rebuild. Eight years later with more than 40 million square feet installed, doubts have been laid to rest and innovation has once again created a paradigm shift in a stagnant industry in need of drastic improvement.
This advanced engineered closure system has repeatedly proven that a landfill can be aesthetically pleasing, erosion-free, produce clean water runoff, and effectively contain odors and gases. But, most importantly, for the ones who are birthing, raising and laying to rest these large giants, it can provide a predictable platform that is cheaper and better controlled.
Although it has fundamentally maintained the same design elements as a traditional soil cap landfill, there are some major differences in performance factors. It removes any and all risk of slope erosion. In fact, it eliminates issues with slope veneer failure and can be used to close much more aggressive slope profiles. In the traditional soil cover profile, saturation can contribute to veneer failures due to landfill gas buildup underneath the liner. Since it does not have a heavy soil veneer to fail, any gas that comes to the surface can easily be collected by a patented surficial gas collection system.
What about deep gas wells that constantly need maintenance due to watering-in issues? Deep gas wells extending to within feet of the base liner were designed so that their expected radius of influence to collect gas would overlap. One aspect of a gas design that was not often discussed was the ‘radius of influence’ of each gas well was not a 2D object as drawn, but rather it was a 3D area of influence. The gas that migrated above the well screen and beyond the vertical radius of influence would certainly cause pressure to build on the final cover system. Deep gas wells also introduced many new problems. The chain in temperature at ~200 feet deep versus the surface temperature caused large quantities of condensate. Condensate, simply stated, is instant leachate. Since the wells must be designed to allow the free flow of gas, they are prone to watering in both from the percolation of leachate and the creation of leachate as the deep well produces condensate. This is a major reason why deep wells must be replaced from time to time. With the patented surficial gas system used with the advanced engineered system, landfill gas is collected without the expense of these deep wells, eliminating the need all together.
Another advantage of the advanced engineered system is that water runoff from the site is naturally clean and filtered. With prescriptive soil caps, the stormwater has to travel almost the complete distance to the retention area being forced to dissipate energy created along the way. A pond full of liquefied sediment is left to deal with. With the advanced engineered system, typical NTU values for storm water measure at around 11. The water is almost as clean as it fell from the sky and much of that water is filtered through the ½ inch of sand. Thus, storage requirements are decreased because there is no sediment runoff to hold and settle out.
Post-closure care has been a topic of debate since Subtitle D was enacted. Every design had to show how the landfill could be financially managed through the post closure care period. With the advanced engineered system, post-closure costs have been shown to be reduced by more than 90 percent. This has allowed owners to shift their financial outlook in a positive way, not just focusing on perpetual liabilities.
A Game-Changer in Hydraulic Methodology
As with any successful product innovation, the progression of the technology should keep progressing. The creators of the advanced engineered system technology initially found that by replacing sand infill with a cementitious infill in downslope channels, storm water was safely conveyed down the slope with no future maintenance issues. (It is common knowledge that revetments for these chutes are notorious for being under designed and, therefore, in constant need of maintenance). With this technology in place, a new exploration began that would challenge the entire landfill drainage footprint.
In the typical landfill design, slope lengths must be limited or soil will begin to migrate due to shear forces. Historically, landfill designs have incorporated diversion berms (benches) and down slope channels to move the water off the cover as quickly as required. With the introduction of a specially designed binder that is sprayed into the sand infill component of the advanced engineered system, landfill designers can use a rarely used concept: benchless design. This technology binds the sand infill within the engineered turf fibers, coats the sand particles, yet is porous and allows storm water to flow through to the drain liner without eroding the sand infill. The bound sand will now have no drainage length limitation. This is a game-changing event for landfill design. It means no requirement for diversion berms or down slope channels.
It is also important to note that with this system, storm water is ‘de’tained rather than ‘re’tained. Since there is not a significant sediment contributor that would degrade water quality, the storm water is detained only long enough to safely convey peak storm events. This allows the pond size to be smaller and more efficient. It could also allow for natural vegetation in the pond area that will not be killed off by sediment buildup since there will be none.
The advanced engineered system with the environmentally friendly binding agent design has been used on two sites to date, both successfully. It was most recently installed on a 22-acre landfill closure in Milledgeville, GA, where it eliminated the standard EPA costly, low-permeability clay layer, as well as deep-drilled in-waste gas vents. Additionally, the use of the system eliminated the need for typical final closure diversion berms and down chutes, to allow the entire cap to sheet flow to the perimeter ditches. It is currently being deployed on coal combustion residual sites in the southeastern U.S.
Einstein also said, “Insanity is doing the same thing over and over again and expecting different results.” This begs a few questions for everyone involved. Should we not expect that technological improvements would develop that would allow us to accomplish the primary objective of a long-lasting, environmentally sound encapsulation and cover method? How much risk and costs could be mitigated by making a few design changes based on a proven technology? When it comes to managing our risks (and our sanity), we need predictable results, and the advanced engineered system is predictable.
Krista Willey is the Vice President of Marketing for Watershed Geo (Alpharetta, GA), a geosynthetics company specializing in engineered products that solve earth’s greatest challenges with solutions that improve the environment, reduce costs and provide long-term performance and predictability. Krista can be reached at (770) 777-0386, e-mail [email protected] or visit www.watershedgeo.com.