PVC Liner - Taber Sewage Lagoon
Owner: Town of Taber
Location: Taber, Alberta, Canada
Material: 20 mil PVC - 2,500,000 ft/2
The Taber Sewage Lagoon, A PVC Ten Year Case History
by Andrew Mills
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This article was first published in Canadian Environmental Protection Magazine, in its special Geosynthetics Journal section. Canadian Environmental Protection Magazine is available free to qualified readers from Baum Publications. This article is reprinted with the permission of Baum Publications. For further information on Baum Publications and their excellent magazines please visit their web site at www.baumpub.com.
We often think of the use of geomembranes as being a new technology, and in certain applications they are. But there are applications where geomembranes have been in service for many years. This case history takes a look at the town of Taber industrial sewage lagoon system that was lined with a 0.5 mm (20 mil) PVC lining system ten years ago. This article goes through the history of this effluent treatment system and investigates the performance of the geomembrane over this time.
The town of Taber Alberta is located 2 hours southeast of Calgary in the dryland prairie on the Oldman River. The Taber area receives substantial sunlight, and the area is farmed extensively for potatoes, corn and sugar beets. Water is scarce in the area and almost all crops rely on irrigation.
There are a number of food processing plants in Taber that place considerable demand on these water resources for washing and other processes. The effluent water from these industrial plants is collected in the Taber industrial sewage treatment system, treated, and then used to irrigate farm land. Municipal sewage is completely separate from industrial sewage and uses a separate mechanical batch process. By separating the municipal from the industrial sewage treatment systems the town of Taber is able to return a substantial quantity of water back to irrigation purposes.
The industrial sewage system uses a series of three aeration ponds for treatment, and one large storage cell to retain treated water. After treatment the water is applied to agricultural land with five centre-pivot sprinkler systems. In total approximately 260 hectares (640 acres) of land are irrigated with treated water from this system.
The industrial sewage treatment system accepts 2 to 3 million gallons (9 to 14 million litres) of effluent water per day. Most of the effluent water is in the form of wash water from vegetable processing plants. After treatment, the system delivers some 400 million gallons (1.8 billion litres) of treated water per year into the field irrigation system. This volume of water represents about 70 cm of irrigation per year (about 27 inches). The irrigated land is leased to local farmers.
Accumulation of heavy metals, sometimes a problem in municipal sewage treatment is not a problem in this system as the collection, treatment, and disposal of the industrial sewage system is completely separate from the municipal sewage system. Sludge in the industrial system is removed from the water by settling before it is sprayed onto the fields. The town has been spraying these fields with treated water from the industrial system since the late 1970's.
Since water cannot be sprayed onto the fields during the winter (when the ground is frozen) the main storage pond allows treated water to accumulate during the winter months. In the spring the stored water is sprayed onto the fields at a rate that will empty the storage pond in time for the following winter.
In the mid 1980's leakage in an existing clay-lined lagoon became excessive and Alberta Environment asked the town to construct a new treatment and storage system. El-San Industries of Medicine Hat Alberta started construction on a clay-lined replacement lagoon in the summer of 1986, however once construction started it was apparent that insufficient quantities of clay were available to complete the project.
El-San was familiar with geomembrane lining work in irrigation canal liners and proposed that a liner material be used for the project. The existing Alberta Environment guidelines allowed for a backfilled 0.5 mm (20 mil) PVC lining material for sewage lagoons. El-San did a cost comparison between the nearest proven clay borrow pit and the PVC geomembrane. The PVC geomembrane was substantially less expensive.
Layfield Plastics was selected as the supplier of these PVC materials. Due to delays in the project caused by the change in lining material, the actual go-ahead for the PVC was received very late in the season. It would take everything Layfield had available to meet the project deadlines.
Layfield Plastics began fabrication of the PVC material in late September and started deliveries to the field of fabricated panels on the 1st of October 1986. Additional truckloads of fabricated panels were shipped to the site as soon as they were completed. Layfield committed additional manufacturing space to fabrication and added two additional welding machines during this time to keep ahead of schedule. In the end, all fabricated material was delivered on time so that site delays were avoided.
Fabrication of the PVC lining materials was done with heat welding techniques. The specifications in 1986 were for a seam peel strength on fabricated 0.5 mm (20 mil) PVC of 2.0 N/mm (10 ppi). Layfield consistently exceeded a 4.0 N/mm peel (23 ppi) strength on all fabricated panels. This was a substantial improvement over previous shop welding methods. To confirm the consistency of welding random samples were taken from all shop and field welds and tested at an independent laboratory.
Installation of the PVC lining material took place from the beginning of October until early December. Field seams were prepared with solvent welding techniques, augmented by applied heat on cold days. A 600 mm (2 ft) sand backfill was placed on the installed liner to provide long term protection. The main problem during installation was wet ground and snow accumulation. This required additional care during backfill. Wind was also a significant problem and one panel was damaged during the passage of a particularly energetic storm front.
The Site in 1996
In October of 1996 Layfield Plastics returned to the Taber Industrial Sewage Lagoon to note the progress of the system at the ten year mark. In an interview with Roger Miles, R.E.T., Director of Field Operations, we discussed the performance of the lagoon to date. "We haven't had any problems with the plastic liner." In fact, the only serious problem that they have had was with erosion of the eastern slopes of the berm in the storage pond.
Although some protection gravel had been included in the initial backfill, it was not of a large enough size to prevent erosion. Taber is subjected to frequent strong winds and wave action quickly eroded the slopes on the east side of the storage pond. The large size of the storage pond contributed to this erosion as the wind was able to whip up quite surf. In the years that followed the pond slopes were repaired by placing a heavy rip-rap erosion control layer on all pond slopes. This erosion control repair solved the problem and there has been no difficulty since.
The storage pond regularly freezes to a depth of 1 meter in the winter. This could have led to ice damage, however no ice damage has been observed in ten years of operations. The ice forms a large "pan" that continually rises through the winter as treated water is added to the pond. This pan stays fairly uniform throughout the pond and has not been observed either breaking up, or riding onto the slope of the down-wind side of the pond. The water level in the storage pond is always rising in the winter and this appears to eliminate the potential for ice damage.
Another area that is of concern to lagoon operators is animal damage. We asked Roger if there had been any problems with animals. The animals seen most often in the ponds are ducks; however muskrats, beavers, and gophers (prairie dogs) have been seen near the ponds. The biggest concern is with the burrowing animals such as the gophers. Although there is a lot of wildlife activity on the outside slope of the pond berms, there has not been a problem with burrowing animals on the inside of the pond. Beavers and muskrats are seen in the immediate area but do not enter the storage pond and tend to concentrate in areas near vegetation (trees) that are outside the ponds. The town has not had any problems with larger wildlife (such as deer) getting trapped in the ponds, likely because of the shallow, backfilled, slope construction.
An operational area of concern was in the accumulation of solids (silts etc) near the inlet of the first aeration pond. In the ten years of operations this accumulation had overwhelmed one of the aerators, however in the fall of 1996 these solids were removed by dredging.
The town of Taber carries out a weekly inspection of the lagoon system by driving around the perimeter of the berms and looking for any unusual disturbances in the backfill. On our visit to the site there were no apparent areas of difficulty.
When asked to characterize the performance of the industrial lagoons, Roger Miles offered that, "I would say that they have been very trouble free." When asked if he would consider another PVC lined pond in the future, he said he would have no difficulty considering it.
The Taber industrial sewage lagoon is an example of a functioning 0.5 mm (20 mil) PVC lining system that has been in service for over 10 years. Although there were intially some problems with erosion of the backfill, the lining system has proven trouble-free for the first ten years of service. Part of the success of this system is undoubtedly due to the care with which the town maintains its system. By regularly inspecting these ponds they are keeping ahead of any problems that might develop. At the ten year mark there is every reason to believe that the Taber industrial sewage lagoon will continue to function successfully for many years to come.