February 1, 2016
Using a nearly $400,000 grant from the City of Denton, Texas, civil engineers at The University of Texas at Arlington (UTA) have created a plan to use moisture to speed up methane gas production at the city’s landfill.
The project’s objective is to increase the efficiency of enhanced leachate recirculation (ELR) or bioreactor landfill operation by monitoring the moisture content and moisture flow within the solid waste due to leachate recirculation and moisture addition. This has been successfully conducted at the City of Denton’s landfill and is the first such operation in the state of Texas.
“Under current Texas regulations, municipal solid waste (MSW) landfills are not allowed to operate as bioreactors. However, landfills are allowed to operate as ELR landfills as long as the average moisture content is maintained below 40 percent at all times,” says Professor Sahadat Hossain, director of the UTA Solid Waste Institute for Sustainability and professor of civil engineering. “Therefore, operating landfill as ELR is same as operating the landfill as bioreactor landfill with maximum allowable moisture content as 40 percent.”
However, Hossain says that several studies have pointed out the potential benefits of the bioreactor landfill approach, including:
Increased rate of settlement of MSW which results in increasing the landfill’s capacity,
Increased rate of landfill gas production for energy recovery projects,
Stabilization of waste occurs in a shorter time, reducing post closure monitoring cost, and
Reduced leachate treatment and disposal costs.
“As a result of these benefits, there has been an increasing trend to operate landfills as bioreactor landfills, particularly in areas where landfill space is crucial,” he says.
In conventional landfills, efforts are typically made to minimize the moisture entering the landfill. The reason behind that is to minimize the generation of leachate and reduce the risk of groundwater contamination. However, the time required for the decomposition of waste in a dry tomb landfill ranges typically from 30 to 100 years. Also, landfill gas is produced at a slower rate over that long period of time.
What makes this project unique is that UTA and the City of Denton are the first group in the U.S. to have successfully utilized resistivity imaging (RI) methods for an effective bioreactor landfill operation.
“During the last six years, there was no potential seeping of water or leachate through the side slopes, which is a major concern for the landfill operators during the ELR/bioreactor landfill operation,” Hossain says.
The results of his group’s research have the potential to change the way landfills operate and help build a case for operating the landfill as bioreactor landfill, according to Hossain.
“In the state of Texas, the Texas Council on Environmental Quality is encouraging potential landfill operators to operate their landfill as ELR landfill, based on our research findings,” he says.
The immediate benefits of the study include tripling the City of Denton’s gas production since it started operating its landfill as an ELR landfill. That landfill gas is now converted into electricity, with a 1.6 MW capacity engine however, they have the potential to add two engines of similar size based on the accelerated gas production.
“Dr. Hossain's proposed method of increasing the efficiency of generating gas from landfills by use of non-invasive moisture monitoring techniques will pave the way for more municipalities’ participation, and that, in due course, will make realization of a global sustainable waste management system more feasible,” says Khosrow Behbehani, dean of the College of Engineering at UTA.