TEXTILES.ORG
By Felon Wilson
How are geomembranes used in disinfection processes? Broadly stated, a geomembrane is considered in use during the disinfection process and whenever the geomembrane is in contact with residuals of the disinfection process. Here are the principal geomembrane applications in potable water (PW) and wastewater (WW) disinfection.
Contact chamber diversion baffles (PW/WW) – Either as a retrofit or new construction, floating baffles constructed from high strength geomembranes are an economical method of preventing short circuiting. Contact or detention time in a contact basin is important to maximize disinfection and to manage Disinfection Byproducts (DBP) production. Geomembrane baffles direct flow parallel to the hanging curtains, creating a soft sided wall, a proven alternative to concrete or other hard-shell materials.
Clearwell and storage tank curtains-baffles (PW) – Similar to contact chamber diversion baffles in operation, clearwells are considered in the continuing contact time after the disinfection chamber. Geomembrane curtains are typically suspended from tank roofs to maintain a first in-first out flow pattern. Included are estimated increases in the baffling factor for the tank as a result of the addition of the XR-3 PW geomembrane curtains. High seam strength, equal to or greater than the sheet material, is essential long term.
Storage tank liners (PW) – These are earthen or hard-shell tanks storing disinfected drinking water. Flexible, chemically compatible geomembranes are used to protect the containment surfaces and the integrity of the treated water. Disinfection residuals, if present, are in the stored potable water.
Floating and Fixed Covers (PW) – Sunlight stable, thermally stable geomembranes are used as engineered floating covers over finished water reservoirs or tanks. This is a very demanding application that requires high strength, both sheet and seams. Fixed covers find applications over tanks to preserve water quality, usually in the treatment or disinfection processes.
Impoundment lining (WW) – Disinfected wastewater may be stored in seasonal polishing ponds, or in the case of reuse water, in storage impoundments which may be seasonal. Overburden must not be placed over geomembranes in reuse applications to keep the water clean from debris and sediment. That requires a geomembrane with extremely low thermal expansion-contraction and high strength. An example of a successful XR-5-lined reuse impoundment can be found at this case study of Barton Creek.
The selection of a geomembrane should not cause questions with the selection of the disinfection process. Rather, the geomembrane to be used should exhibit characteristics that make it a vital part of the process, including chemical additions and reaction products. Look for these five characteristics:
Chemical Resistance – For oxidation systems, the membrane must be able to withstand contact with low and high levels of the disinfectant. High levels are mentioned in the context of shock loading for cleaning and in the case of accidental higher loading. Some geomembranes have been proven to be resistant at high levels and some have been proven to not be resistant at even low levels. Look for actual long-term immersion data.
UV and Thermal Resistance – UV, as one of the significant disinfection processes, can impact some materials. Most impoundments, all floating covers, and baffles are exposed to sunlight and resulting diurnal thermal swings. Look for high UV resistance and negligible thermal expansion-contraction.
Geomembrane sheet and seam strength – They should be high, and the latter should equal or exceed the first. If not, a weakest link has been introduced.
Wicking Prevention – Make sure the fabric reinforcing was treated during manufacturing to prevent wicking. Don’t accept covering fabric edges to prevent fabric contact with the environment. Note also that wicking of geomembrane base fabrics is a phenomenon often confused with chemical resistance.
Puncture Resistance – This is a basic property that is a definer of geomembrane endurance. Only by comparing minimum specification values can different products be compared.
Geomembranes are a valuable tool when designing or retrofitting disinfection systems in water and wastewater applications. Read about mastering geomembrane selection Part one here. Learn more about XR Geomembranes here.