Managing Raw Water Storage with Geomembranes in Water Treatment Plants
Post 2: How are Geomembranes Used to Store Raw Water and How Do I Select the Right One?
In the first post of this series, the rationale for off-line raw water storage was presented, with storage in open impoundments being the most feasible method. The raw water should be separated from the earthen subgrade to maximize clarity, and geomembranes are considered the barriers of choice due to their durability and reliability.
Here are the four principal geomembrane properties needed to successfully line raw water storage impoundments.
- Hydraulic barrier. The basic function is to contain stored water and prevent loss into the ground. A geomembrane should exhibit low permeation properties and be able to lay flat against the subgrade, creating intimate contact and minimizing any liquid transmission. Installation should occur with minimal field seams as most breaches occur in the field seaming process.
- Isolation from overburden. Covering the geomembrane with anything that can add impurities to the raw water will result in increased treatment requirements and should be avoided. The exposed geomembrane should have high UV resistance and low thermal expansion-contraction.
- NSF tested and certified to Standard 61. Raw water is not officially considered part of the water treatment process and is not subject to NSF 61 surface contact regulations. However, considering all precautions to provide the purest water reasonably possible to the treatment process are desirable, utilizing a geomembrane tested by and certified by NSF per standard 61 is good practice.
- Survivability. Much attention in the geomembrane industry in recent years has been directed at survivability. The geomembrane must have physical properties (think tensile, puncture, dead load and shear seam strength) that will survive the rigors of installation and long-term operation.
Like any type of industrial equipment, geomembrane characteristics and performance vary. Make sure the membrane meets the criteria listed above.
Raw water impoundment lining case history example
Franklin, Tennessee’s raw water storage pond had largely been abandoned due to losses of water through the subgrade. Back-to-back drought years kept the water system with little reserve in water supply due to low flows in the Harpeth River, their water source. The 1.3 million sq ft (12 hectares) pond site was upgraded, and the subgrade reworked and compacted. NSF 61 tested and approved geomembranes were installed and left exposed throughout the installation. The XR PW series of geomembranes provided sufficient strength that additional puncture protection was not required. Utilizing large panels, created in a second manufacturing step, the installation was quick and field seams were minimized.
Existing Pond Site
The project engineer, AECOM, anticipated wave action in the southeast corner of the large open impoundment. Accordingly, geocells were installed on slopes in that area to aid in wave energy dissipation and protection of slopes/subgrades. The XR-5 PW geomembrane provided the hydraulic barrier to ensure slope protection. Further, the exposed geomembrane keeps the raw water clean, and diversion baffles made of the same XR PW geomembrane help in suspended solids settling pretreatment and prevention of short circuiting.
XR PW Prefabricated Panel Installation
Anchor Trench Lining
Finished Lined Reservoir
XR PW Lining of Franklin, Tennessee Raw Water Impoundment
Geomembranes help ensure resilience in water treatment operations by containing source water. Select, design and install the geomembrane system that matches the project requirements.
The full Franklin Raw Water Impoundment case history can be found here.
All photos are sourced from Seaman Corporation and AECOM, unless indicated otherwise.