By Ulrich Reimann-Philipp, Ph.D.

Summary: The quality of drinking water is often tied to what cleaning technology is implemented by small system operators. Additional benefits include reduction of chlorine and maintenance costs of rather large and complex facilities. Utilizing improved technology on hard to clean surfaces, some products have found promising results.

Providing ever-growing communities with high-quality drinking water is becoming both technically challenging and increasingly costly. New technologies and procedures are being developed at a fast pace for water acquisition, treatment and distribution. Those that have the potential to both improve water quality and lower operating expenses are especially attractive.

European drinking water suppliers have applied an integrated approach to comply with stringent water quality standards. A combination of source water protection, innovative treatment methods and rigorous storage and distribution system maintenance was necessary to achieve maximum contaminant levels of ≤ 0.3 milligrams per liter (mg/L) chlorine, ≤ 0.01 mg/L total trihalomethanes (TTHMs) and ≤ 0.01 mg/L parts per million (ppm) arsenic. Regular cleaning and disinfection of water tanks, treatment facilities and distribution lines has proven to be one effective and economical tool in water quality management. The development of novel chemical surface cleaners has been instrumental in facility maintenance. Several such cleaners have recently become available in the United States. The cleaners are ANSI/NSF Standard 60 certified for drinking water facility cleaning and disinfecting.

Why cleaning?
Every surface in contact with water will accumulate biological and mineral deposits. Deposit formation usually begins with attachment of bacteria and development of a biofilm. Deposition of extracellular polysaccharides, trapping of debris and scaling contribute to the formation of rigid or slimy layers.1 Depending on water supply, environmental conditions and surface structure, these mixed organic or inorganic deposits can be transparent or colored.

Surface deposits can affect water quality, cause disinfectant residual decline and promote corrosion. Particles and aggregated bacteria, which dissipate from mature biofilm (sloughing), can contribute to turbidity, discoloration and downstream contamination of the distribution system. Reaction of free chlorine and chloramines with organic matter present in biofilm leads to increased chlorine demand of reservoirs, resulting in the need for increased chlorination. The corrosive properties of chlorine and formation of corrosion cells under surface deposits lead to accelerated deterioration of metal and concrete surfaces.2

One logical answer to problems associated with surface deposits is regular and thorough cleaning. Re-growth of biofilm is delayed on clean surfaces. Cleaning can frequently reduce chlorination needs, resulting in reduced accumulation of disinfection by-products, reduced corrosion and savings in treatment costs. Furthermore, the surface under the deposits can be inspected and corrosion sites can be spot-repaired. Without cleaning, sandblasting and repainting is the only remaining option.

Regular deposit removal results in operations and maintenance cost savings as well as improved water quality; however, no chemical cleaning products with combined efficiency, safety and convenience were available until recently. Water quality problems are addressed almost exclusively by improving treatment methods. This is reflected in the American Water Works Association (AWWA) 2001 conference proceedings—out of 280 articles, only two addressed reservoir cleaning.

Limits of methods
Many water storage tanks aren’t cleaned on a regular basis. While the AWWA standards C652-92 and C653-97 describe procedures for chlorine-based disinfection of water treatment and storage facilities, no cleaning standard exists. The most commonly applied procedure is high-pressure washing followed by surface chlorination. Diving and robotics technology is available for sediment removal. Sandblasting and repainting are the last resort to solve deposit-related problems. Table 1 summarizes surface cleaning procedures.

Of the procedures listed in Table 1, only resurfacing and chemical cleaning are suitable to remove heavy deposits. High-pressure washing and chlorination leave most deposits and biofilm in place. This is obvious when deposits are colored. Several studies have shown that bacteria become very resistant to high chlorine concentrations once a biofilm is established. In addition, rapid re-growth occurs on insufficiently cleaned surfaces. To limit bacterial growth, pressure washing and chlorination have to be carried out frequently. Underwater tank inspection and sediment removal have become valuable tools in tank maintenance; however, scale and biofilm cannot be removed using this technology. Corrosion can only be assessed when the original paint surface remains visible.

With the introduction of these cleaners, chemical surface cleaning has become a viable alternative. Even resilient deposits can be removed efficiently in a one-step cleaning/disinfection procedure. Various applications in the United States have shown that the surface coat exposed after cleaning was still in acceptable condition. Most surface coatings are designed to last at least 25 years; however, sandblasting and repainting on a 10-15 year schedule is frequently necessary because of heavy encrustation.

Cleaning isn’t an alternative to resurfacing, but can greatly extend surface coating lifetime. While resurfacing does solve deposit-associated problems instantly, it doesn’t eliminate the need for regular sediment removal and surface disinfection. The main considerations for assessing costs of the different treatments are how frequently they have to be applied, how much each application costs, how long the surface coating can be used, how the treatments affect chlorination needs and water quality, and how long the facility has to be taken off-line.

Advanced chemical cleaning
These new cleaners are being used in the United States to clean the inside and outside of water tanks, filter and clarifier surfaces, and filter media. The products are shipped as two separate components, cleaner and catalyst, which are mixed immediately before application. The liquid cleaners are applied as a fine mist using low-pressure spraying equipment on the wet surface. After a 10-30 minute reaction time, the surface is rinsed off with an excess of water. Light sponging or scrubbing is occasionally applied. A granular formulation is available for filter bed cleaning. As with the liquid cleaners, the catalyst is supplied separately. Reaction time for media cleaning is usually 24 hours.

Figure 1 shows one example of cleaning progress in a steel drinking water tank. The 0.5 million-gallon tank was heavily encrusted with black deposits, causing almost complete darkness at the beginning of the procedure. The deposits were completely resistant to high-pressure washing and scrubbing. Cleaning of the tank including pre-washing, application of chemicals, rinsing and flushing of runoff and sediment was completed by a five-man crew in half a day. The cleaning exposed the original white paint, which turned out to be in good condition. Some corroded spots were repainted immediately. The tank was refilled the same afternoon.

The combination of cleaning and disinfection has been applied when tanks developed a significant chlorine demand. The treatment resulted in stabilization of the chlorine residual. The chlorine demand of a tank is usually not known because chlorine residual isn’t monitored; however, in tanks, which are used as backup storage, the effect of deposit removal can be striking. For example, a chlorine residual of 2.8 ppm (as chloramine) was completely lost within two weeks in a concrete underground clearwell in Norman, Okla. After treatment, no decline was detected within a five-week period during which no water was exchanged.

A granular cleaning product, based on similar chemistry as the liquid cleaners, can be used to treat sand and anthracite filter media. Most scale and organic deposits can be removed in the laboratory. Figure 2 shows microscopic images of two anthracite samples that were treated. Full-scale application isn’t always feasible. In cases of heavy calcium, carbonate scale media exchange is less expensive than chemical cleaning. In contrast, iron deposits and biological contamination can be removed at lower chemical costs. Laboratory testing is necessary to develop a cleaning protocol for each filter bed.

Conclusion
Cleaning of small system water storage and treatment facilities can help improve water quality and reduce maintenance expenses. Recently introduced combination cleaners have proven effective in removing biological and inorganic deposits from a variety of surfaces. Reduction of chlorine demand and surface restoration are the most obvious benefits of chemical cleaning. With advancements in cleaning technology, storage and distribution system maintenance are likely to focus more prominently in water quality management.

References

  1. “Control of Biofilm Growth in Drinking Water Distribution Systems,” United States Environmental Protection Agency, EPA/625/R-92/001, 1992.
  2. “Corrosion manual for internal corrosion of water distribution systems,” United States Environmental Protection Agency, EPA/570/9-84/001, 1984.
  3. Herson, D.S., et al., “Attachment as a factor in the protection of Enterobacter chloacae from chlorination,” Applied Environmental Microbiology, 53: 1178-1180, 1987.
  4. LeChevallier, M.W., C.D. Cawthon and R.G. Lee, “Inactivation of biofilm bacteria,” Applied Environmental Microbiology, 54:2492-2499, 1988.

About the author
Dr. Ulrich Reimann-Philipp is director of research and development for Floran Technologies, of Norman, Okla. He is responsible for new product development and technology transfer. He can be reached at (405) 579-1121 or email: http://lab@florantechnologies.com.

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