Every plumber knows that finding a leak in an old, leaky faucet might mean that the entire plumbing system needs to be fixed. This unpleasant surprise is similar to what Des Moines, Iowa-based Metro Waste Authority (MWA), experienced after installing a leachate collection system in 1992 in its municipal solid waste landfill 10 miles east of the city.
Designed to handle commercial and residential waste, as well as construction waste from 500,000 people, MWA's landfill is equipped with a leachate collection system in a portion of the completed landfill, as well as in the active cell. In the completed landfill area, this system is linked to 10 extraction wells drilled into the refuse, plus seven gas and leachate extraction wells installed by Waste Management Inc. (WMI), Houston. Well depths vary from 15 to 125 feet deep.
At the heart of the leachate extraction system is a compressed air power source. Pressurized air is supplied to each well to power leachate ejector pumps.
Leachate is pumped to gravity leachate collector lines that drain to a common sump. Six observation wells also were drilled in this area to monitor the zone of influence from the leachate extraction wells.
The collection system is designed to produce on average approximately 12,400 gallons of leachate per day — approximately 20 percent from the well system and 80 percent from the perimeter drains and landfill expansion area gravity drains.
However, once the leachate collection system was installed, MWA began having difficulty bringing the leachate extraction system on-line and producing the expected levels. It was eventually discovered that construction and post-construction activities and a lack of as-built documentation created many of the problems.
MWA suspected leaks in the air and leachate collection lines, and possible plugging of the gravity underdrain system in the expansion area. If that wasn't enough, the Iowa Department of Natural Resources (IDNR), Des Moines, Iowa, required the agency to submit an operation and maintenance (O&M) plan to satisfy the landfill's permitting requirements.
With the help of Stanley Consultants Inc.'s Des Moines office, MWA began investigating and troubleshooting the leachate collection system, as well as developing corrective strategies and the O&M plan.
Well Leachate Extraction System
The troubleshooting team discovered that the leachate extraction system was performing poorly at best. Construction of the leachate drain lines and air lines to power the extraction well pumps had just been completed, but the system was leaking. MWA wanted its leachate extraction system operational as soon as possible, so the team had to locate the leaks quickly.
The well leachate extraction system and the expansion area gravity underdrain system became the initial focus. Equipped with a backhoe and a set of design drawings, the team isolated the air lines to locate the leaks. Workers primarily worked with the lines serving extraction well #2 (EW#2) because it did not work at all. All three air lines serving this well were broken, causing the air to leak out.
Investigation confirmed that air lines had been installed shallower than desired due to a thin cover layer — only six to 24 inches in depth in some areas. The original design had anticipated a thicker cover layer over the refuse and called for 36-inch bury depth.
It also became obvious why air lines were breaking and some of the extraction wells were not working. As heavy equipment moved over the site, the vehicle weight had crushed or cracked the extraction system's plastic lines.
The team decided to completely replace all three air lines: a total of 1,650 feet of ¾-inch piping. This would ensure that no hidden cracks would be present in the pipe and verify pipe integrity. The plastic leachate extraction pipe also needed to be replaced because it also showed signs of being crushed.
Having also discovered several wells that had air lines plugged with debris and loose connections, the team cleaned out the lines, tightened connections and performed other repairs.
Then, another problem was uncovered: moisture entering the lines had frozen in several places, causing more blockages. The consultants determined that the desiccant in the air compressor building could not handle the constant heavy moisture loading that continually entered the system from the leaks. Moisture and ice were found mostly in the air system's lower portion near the compressor building.
Using the compressor, the team blew water and ice from the system to force the material out from an opening located at EW#7. Once the lines were clear, air was properly delivered to the extraction wells, and the leachate extraction system worked.
Gravity Underdrain System Problems
However, all the problems still were not solved. The leachate collection system in the expansion area also was not producing as much leachate as anticipated. The east lateral line was not working, although the area above the lateral was soft and saturated. And leachate from the refuse piled on the site's west end also was not entering the collection system because it was running over the refuse cover and off the expansion cell.
The team did not extensively investigate the gravity leachate collection system, but determined there were several possible explanations for the malfunction:
Collector piping may have been damaged or clogged with biological growths or sediments;
Filter fabric may not have been installed properly around the collector piping or around the drainage material surrounding the pipe;
The drainage layer of sand may have been clogged with biological material or fines;
The cover material may have prevented infiltration of the leachate into the initial refuse lift and the collector system because of its impermeability and grade;
The expansion area main collector line was approximately 2,000 feet long with no cleaning provision;
Filter fabrics may have been clogged with oils or filamentous growth; and
Abnormally high rainfall may have caused more runoff across the expansion cell surface.
The team decided to concentrate its efforts on the pipe bedding material. The sand layer was sampled for field and laboratory testing, since the slow permeability of the sand layer surrounding the perforated leachate collection pipe may have been responsible for the poor leachate production.
The permeability tests showed a rate of 10-3 centimeters per second (cm/s), meeting the Iowa's Administrative Code permeability requirement — although a higher rate would have been desirable.
From the field sample, the team concluded that clogging from biological growths or matting of the drainage layer surface was not reducing permeability.
Examining the 15-foot, underground leachate collection pipe was impractical. So the team and MWA decided that well-designed and carefully placed interceptor trenches that extended up to the current surface of the expansion cell could improve leachate collection.
The team installed a new interceptor trench and vertically extended an existing cutoff trench. The new interceptor trench was constructed across the cell's width at a location that approximately bisected its length. The trench area between the top of rock and the surface of the cell was backfilled with clay-free refuse. The total trench depth was 12 to 15 feet.
The existing cutoff trench is located across the width of the refuse cell near the lower west end of the cell. The trench's vertical extension included washed rock with filter fabric placed above the existing trench rock up to near the cell's surface. A thin cap of clay was placed over the rock at the surface.
Construction work on the two interceptor trenches allowed surface runoff to be captured before it left the expansion cell and increased collected leachate.
The final step was preparing the O&M manual. This included leachate collection pipe network drawings to help future troubleshooting teams become familiar with each component of the leachate collection piping system, including the compressed air delivery system, leachate storage and handling system.
As a result of its efforts, the team learned that:
Accurate as-built drawings are critical for facilities that are buried or hidden from view. “As-builts” need to record all changes to the design.
Diligence during construction is critical to ensure construction complies with design and that connections are secure.
Post-construction activities must be monitored to prevent damage to installed system components. In the case of shallow buried pipes, designated crossing locations for heavy equipment must be provided.
MWA's leachate collection system now operates successfully, and improvements intercept leachate and contaminated runoff from the surface of the refuse cell along with subsurface migration of leachate down gradient through the refuse.
Edwin R. Slattery is a principal environmental engineer in the Des Moines office of Stanley Consultants Inc. E-mail the author at [email protected].