Grayling Wastewater Treatment
The City of Grayling Michigan needed to expand it's wastewater treatment plant to accommodate the city's continued growth, but the sandy Northern Michigan soil was unacceptable for containing the effluent from the city sanitary sewer system.
Richards and Associates designed the 1.6 million square foot expansion to the existing facility. This consisted of a new 15 acre secondary pond and a 20 acre holding pond. Constructed by Devere Construction in 1991, the ponds utilized 1.6 million square feet of 40 mil PVC geomembrane fabricated and installed by EPI. Since there was not an acceptable or affordable source of clay for the secondary liner, another 1.6 million square feet of Gundseal GCL was installed by EPI under the PVC geomembrane.
The Gundseal was manufactured with 20 mil smooth HDPE and granular bentonite applied at the rate of 4.9 kg/m2. The Gundseal was installed with the HDPE side down and the bentonite side on top. The PVC geomembrane was installed directly on top of the GCL. The resulting composite geomembrane provides the most secure containment available.
PVC, fabricated in large panels, also provides the advantage of being able to install large quantities of GCL and rapidly cover the GCL by deploying the PVC panels. This can serve to protect the GCL if inclement weather unexpectedly occurs. And since PVC has a low coefficient of thermal expansion, there are no waves caused by temperature differentials during the day. Therefore, intimate contact with the underlying bentonite is much more likely.
Another advantage to this system is avoiding premature hydration of the bentonite layer. Should damage to the PVC liner ever occur, the bentonite is hydrated, swelling to more than 30 times its original volume, to seal off the opening and prevent leakage of any fluid. The side slopes of the lagoons were covered using an 8 oz/sq. yd non-woven geotextile, a uni-axial reinforcing geogrid, and 8 inches of stone cover. The bottom of the lagoons were covered with 12" of sand.
This unique covering of the side slopes has provided the protection needed for the geomembrane and the erosion control needed for the large surface lagoons. These wastewater treatment lagoons have been in operation for since 1991, with leak free, trouble free performance of the PVC geomembrane liner.
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