The Pond Edge:
Geomembrane Liners In Wastewater Treatment Ponds: Whales, And Their Prevention
by Ian D. Peggs
Geomembrane liners manufactured from polymers such as high density polyethylene (HDPE), linear low density polyethylene (LLDPE), polypropylene (PP), polyvinyl chloride (PVC), ethylene propylene diene monomer (EPDM), chlorosulfonated PE (Hypalon ®), and PVC alloy (XR-5®), and even more sophisticated polymer alloys are used very effectively in reservoirs to contain potable water, and in wastewater treatment plant (WWTP) lagoons to prevent contamination of the groundwater. But occasionally performance problems occur that are the result of inexperienced design, sometimes poor installation, but rarely inadequate liner material.
The predominant problem is an assumption by the design engineer that an installed geomembrane liner will not leak. It may not, but the chances are that it will. While this appears counter-intuitive, it is a design philosophy that experienced engineers and regulators have come to accept, to the benefit of all, since the early 90s.
When wastewater leaks through a liner and remains in the subgrade soil microbiological reactions continue, water reacts with any organic matter in the soil, and gases (mostly methane) are generated. The gases collect under the liner of the lagoon which is often designed with a flat floor. The gases then lift the liner until it becomes influenced by the water being drawn toward the aerators ultimately contacting the aerators. The liner is torn, increasing the leakage rate and damaging the aerators. Increased leakage accelerates the formation of “whales”; large blisters of liner above the surface of the water . Typically the whales contain gas above the water line and leaked water below the water line. Thus the level of water below the liner is the same as that above the liner.
Frequently, a nonwoven geotextile is placed under the liner to act as a cushion for the geomembrane, a drainage medium, and a gas venting medium. Gas vents may or may not be placed at the tops of the slopes to vent gases. However, on more than one occasion gas vents have been seen below the maximum operating water level! In some cases the geotextile is augmented, or replaced, by strips of geotextile/geonet/geotextile composite (geocomposite) across the floor and up the slope to each vent. This often does not work. The in-plane flow capacity of the compressed geotextile or the geocomposite strips is insufficient to handle the leak flow rate. When the floor of the lagoon is flat, as mistakenly required by some state regulations, the leaked water cannot be drained off to a sump and removed. Therefore it completely fills the void space within the geocomposite. Thus, the gas that is generated cannot diffuse to the gas vents. It accumulates in the unavoidable high spots on the floor, lifts the liner, and forms the whales. The resulting tensions in the geomembrane liner cause additional damage, typically at fixed points such as pipe penetrations, resulting in more leakage and accelerating whale formation.
Statistics show that the number of leaks in a geomembrane lining system is a function of the area, or complexity of the liner. Larger areas imply a lower proportion of detailed liner work and therefore a fewer defects per unit area. Typically a liner with an area of 2.5 acres will have about 12 leaks while larger area liners have 1 leak per acre. The number of leaks can be significantly reduced if knowledgeable geosynthetics construction quality assurance (CQA), in addition to geotechnical CQA, is performed during liner installation. They can be reduced even further if an electrical leak location survey is performed before, during, or after first filling of the lagoon.
Target Action Leakage Rates, above which leaks must be found and repaired, are typically 20 gallons per acre per day (gpad) for landfill primary liners with a maximum 1 ft hydraulic head, and 500 gpad for WWTP lagoon single liners with 6 ft hydraulic head. Therefore, these are the types of leak flow rates that must be safely handled by underliner drainage systems. This requires specific liner design features.
The floor of the lagoon must be sloped to a sump, both for easy dewatering/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.
Liner design in lagoons with aerators should require ballast on the liner to prevent the liner’s uplift and to provide a pad to support the aerator when the water level is lowered. Perfect cross-shaped cuts in the liner matching the aerator inlet geometry have been seen in some cases where the aerator settled on the liner or where the liner was lifted into the aerator. Other aerator damage is frequently evidenced as cuts in the liner along a specific elevation on the side slope where the aerators have been pulled to shore for maintenance. A 60 mil thick HDPE liner stands no chance against the sharp edge of a 0.25 in. thick stainless steel plate of an aerator pulled against the slope or lowered by a crane.
Ballast on the liner is usually achieved with a concrete slab. There are three methods of liner/ slab interaction, each with its own disadvantages. The liner can be attached to the side of the slab or to the top surface, or the slab can be placed on top of the geomembrane liner. In the first case, providing proper support of the liner in the slab/subgrade corner is of prime concern. Design philosophy is that the liner serve as a barrier only and not as a load-bearing member of the system. Thus, safely bending a semi-rigid HDPE liner through 90° then attaching it to the side of the slab only a few inches up from the corner is difficult. While the liner can be easily attached to the top surface of the slab near the edge (with the soil surface at the same elevation as the slab surface) the concern is with settlement of the subgrade at the edge of the slab and deformation (stressing) of the geomembrane over a rough corner of the concrete. There are, therefore, some advantages to maintaining the continuity of the liner underneath the slab.
However, if the slab is pre-cast it will have to be carefully lowered onto the liner so that no corners or edges puncture the liner as it settles. A heavy geotextile cushion should be used between liner and slab. If the slab is cast in place extreme care must be taken with form work, handling rebar, and general traffic around the slab, to prevent mechanical damage to the liner. Again, a heavy geotextile should be used to protect the liner.
In one case leaks in an unprotected liner under an aerator had been marked for repair. Repairs had reportedly been made and a geomembrane protection layer welded to the liner for protection against a pre-cast slab that was to be placed on it. As part of the QA overview process the protective layer was pealed back and the liner examined; one marked leak had not been repaired and a new leak had been made while making the repairs. In several other locations marked leaks had not been effectively repaired. Clearly, the liner installer did not have an effective QC program, and repair QA was ineffective.
One solution to these construction concerns might be to install 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 with embedment liner surface), then to weld the geomembrane liner to the concrete embedment liner. In this way there is a continuous geomembrane liner that can be tested for leaks electrically, and there is no danger of the liner under the aerator being uplifted.
When a lagoon has experienced extensive liner whaling the liner must typically be removed so that all saturated soil underneath it can be removed. Additional damage to the liner is unavoidable. The subgrade must be re-compacted to provide a firm unyielding support for the geomembrane liner. Remember, the geomembrane acts as a barrier, not as a load-bearing member of the system. Effective drainage, leak monitoring, leakage removal, and gas venting systems must be installed. If temporary repairs are acceptable such as to provide a short term operating extension it may be sufficient to cut the liner orthogonally from slope crest onto the floor midway between each row of aerators, to insert high transmissivity drainage geosynthetic composite strips part way onto the floor and up the slopes, and to install gas vents at the top of the slopes above the strips. All evident liner leaks should be repaired at the same time – check pipe boots, repair patches, slopes at the elevation of aerator bottoms, and liner under the aerators and at the edges of concrete slabs. This temporary remedy may not stop leakage and the generation of gas but it can allow the venting of gas and the prevention of whales for some period. Do recognize though that with water on both sides of the liner and with holes in the liner, HDPE, LLDPE, and PP geomembrane liners, with specific gravities less than that for water, will still tend to float between fixed points.
Lessons still being learned? Geomembrane and other geosynthetic-based lining systems function well and are extremely cost effective when installed properly, as most are. When failures (whales) occur it is typically not a geosynthetic materials problem. To avoid such problems use a design engineer experienced in geomembrane-based lining systems, and use an experienced installer. However, it may even be difficult for an experienced installer to satisfactorily deal with inappropriate design features. Hence the need for a specified Action Leakage Rate – even CQA will not catch everything. Try to hire an experienced CQA firm reporting to the owner (engineer), not to the general contractor. Perform a final leak test. A full hydrostatic test typically takes 14 days and will tell you that there is a leak but not where. A geoelectric integrity/leak survey can cover 2 acre/day and pinpoint a leak in real time.
Liner whales should be an endangered species but I know we will continue to see many more of them.
Ian D. Peggs, president of I-CORP INTERNATIONAL, Inc., has been performing geomembrane liner failure investigations for 24 years. For more information, contact Ian at 6072 North Ocean Blvd, Ocean Ridge, FL 33435, (561)369-0795, fax (561)369-0895 or firstname.lastname@example.org.
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