Application of HDPE at a sewage treatment plant

Abstract

A pair of aeration lagoons at a wastewater treatment facility were lined with 0.75-mm HDPE geomembrane in 1993. The ponds were placed in an area that previously contained an unlined sewage stabilization pond. This paper discusses activity at the east lagoon only. Prior to liner installation, a gas vent system was added in the design in the event that gases formed under the liner. That system employed approximately 30-cm-wide strips of geotextile/geonet geocomposite placed on 30-m centers in a grid pattern. Gas vents were cut in the liner around the top perimeter of the pond at 15-m centers. Thorough construction quality assurance practices were employed to insure that the geomembrane and seams were of a high quality. Seven months after installation, gas bubbles began to form under the liner, and large bubbles resulted. The bubbles were pierced to remove the gas. At that time, the lagoon was taken out of service and drained for inspection. Crescent moon-shaped cuts (about 300 mm in size) were then found at the location of some of the baffle curtain anchors. This damage was repaired in conjunction with the holes created by venting the gas bubbles. The lagoon was then partially filled, and more bubbles were formed under the liner. Analysis of the gas resulted in the opinion that it was the result of anaerobic digestion of organic matter under the liner. Also found were holes approximately 60 mm in size that were grouped in random locations throughout the lagoon. These holes were in pairs and spaced 3 to > 30 mm apart. The source of those holes remains unknown. In retrospect, a gas vent system capable of removing gases at a greater rate than it was generated from beneath the liner was required. It is suggested that when a previously existing pond is to be lined, extensive testing for organic matter be performed on subgrade material. Such testing is not currently well defined and should occur prior to design. Other investigations may include hydrological surveys that include several years' data on underlying water level activity. Also, a thicker geomembrane may have had enough resistance to mechanical damage to prevent the formation of the crescent moon and smaller holes.