A 'Whale' of a Problem
Geomembrane liners are vital containment materials in wastewater treatment plants. Leaks and 'whales', large blisters of liners that show above the water surface, can create major headaches. The keys to preventing these problems are proper design and installation.
Whales occur when a plant's geomembrane leaks. Wastewater leaks through a liner and remains in the subgrade soil;microbiological reactions continue; water reacts with organic matter in the soil; and gases, mostly methane, are generated.
The gases collect under the liner of the lagoon, which is often mistakenly designed with a flat floor. The gases lift the liner until it is drawn toward the aerators. The liner is torn, increasing the leakage rate and damaging the aerators. It's important to keep in mind that increased leakage accelerates the formation of whales.
CQA is the Right Way
The number of leaks occurring in a geomembrane lining system depends on the liner's area and complexity. Large areas imply a lower proportion of detailed liner work and, therefore, fewer defects per unit area. It's not uncommon for a liner with an area of 2.5 acres to have about 12 leaks, while larger liners have one leak/acre.
The number of leaks can be significantly reduced if knowledgeable geotechnical construction quality assurance (CQA) is performed during liner installation. Leaks can be reduced further if an electrical leak location survey is performed before, during, or after the lagoons first fill
Design and Installation Help
Underlying drainage systems must be designed to handle certain leak flow rates. For wastewater lagoon single liners, the rate at which leaks must be repaired typically is 500 gallons/acre/day (gpad) with 6-foot hydraulic head.
Managing leak flow rates requires specific liner design features. The lagoon floor must be sloped to a sump, both for easy dewatering and desludging of the lagoon and for drainage of the leaked liquid. There must be a pipe leak drainage system, or preferably a full geocomposite leak drainage layer under the liner, also draining down gradient to a sump system where the leak flow rate can be monitored and the leaked water removed. With the water removed, the sloping drainage system will also function as an effective upgradient gas-venting system to the gas vents at the top of the side slopes.
In lagoons with aerators, ballast on the liner prevents the liner's uplift and provides a pad to support the aerator when the water level is lowered. Ballast on the liner is usually achieved with a concrete slab.
When lowering precast slabs into place, regardless of the connection method used between the liner and slab, be careful not to tear the liner. Geomembranes are not designed to be load bearing. A heavy geotextile cushion is advised. And when casting in place a slab, be careful with heavy equipment and rebar around and on the liner.
Proceed cautiously when placing and removing aerators in and out of the water. Even a 60-mil-thick high-density polyethylene (HDPE) liner stands no chance against the sharp edge of a 0.25-inch-thick stainless steel plate of an aerator pulled against the slope or lowered by a crane.
To address these construction concerns:
- Install a thick concrete embedment liner on top of the concrete slab poured on the subgrade.
- Take extra care in compacting the subgrade at the edges of the slab (with subgrade surface level and embedment liner surface).
- Weld the geomembrane liner to the concrete embedment liner.
This method allows a continuous geomembrane liner that can be tested electrically for leaks.
Extensive whaling leads to costly repairs involving liner repair/replacement, saturated subgrade removal and recompaction, aerator damage, operational delays, and more. Proper design and CQA will greatly reduce these risks.
Ian D. Peggs, Ph.D., P.E., is owner of I-CORP International, Ocean Ridge, Fla. Chris Kelsey, editorial consultant for geosyn thetica.net, is based in St. Paul, Minn.