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| Underground concrete tank (A) before and (B) after chemical cleaning. |
By Ulrich Reimann-Philipp, FLORAN Technologies
Producing and distributing high-quality drinking water is becoming increasingly challenging. While rapid technological progress has greatly improved and facilitated water treatment, water providers have to adjust to tightening water quality standards and increasing concerns about drinking water safety. Recent examples of those issues are arsenic, disinfection by-products (DBPs) and heterotrophic plate count (HPC) bacteria.
These problems have been approached primarily from the treatment and source water aspects. Methods to reduce arsenic and alternatives to free chlorine disinfection are being implemented on a large scale. However, not all water quality problems can be solved through improved treatment. Water quality declines if the storage and distribution system is not properly maintained, somewhat like drinking clean water from an unclean glass. An integrated approach to water quality management, including water supply, treatment, storage and distribution can result in improved water quality at the customer's tap and also reduce maintenance and treatment expenses. Chemical surface cleaning is part of the Clean Glass Concept.
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| Underground concrete tank (A) before and (B) after chemical cleaning. |
Problems caused by surface deposits
All surfaces that are in contact with water accumulate deposits over time. These include pipelines, reactors, clarifiers, filters, and storage facilities. Depending on source water, temperature and light conditions, and flow characteristics, this contamination can be primarily biological or inorganic. The most visible are calcium carbonate scale, iron and manganese deposits, permanganate stains and algae.
Biofilm is a component of many types of deposit, but might by itself go undetected. The visible contamination of filter and clarifier surfaces might, by some, be considered primarily an aesthetic issue, but problems associated with surface deposits go much further.
Chlorine reacts with both organic and inorganic surface contaminants. This leads to a depletion of the chlorine residual in the bulk water, which is referred to as chlorine demand of tanks and distribution lines. In order to maintain an effective disinfectant residual throughout the system, chlorination at the treatment plant has to be increased or the water has to be booster-chlorinated downstream.
However, high levels of chlorine may promote corrosion and lead to increased production of disinfection by-products such as trihalomethanes (THMs). Switching to alternative disinfection procedures such as UV, ozone, chlorine dioxide and chloramine is helpful to reduce DBP formation but cannot prevent disinfectant residual decline caused by surface deposits. Particles or cells that slough off of mature biofilms and heavy scale end up in the water and cause high HPC readings or affect taste and clarity.
Current cleaning technology
At this time, the American Water Works Association has not established a cleaning standard for drinking water facilities. Existing standards specify methods for chlorine-based disinfection but not deposit removal. Tanks are usually cleaned on a 3-5 year schedule. The most commonly applied cleaning practices are on-line sediment removal and off-line high-pressure washing. While both methods have their specific benefits, often only loose debris is removed, with biofilm and scale remaining in place.
Corrosion cannot be properly assessed once heavy deposits cover the paint. Available chemical surface cleaning technology has either been too expensive, not compatible with drinking water facilities or too corrosive to be widely applied. As a result, many tanks have to be sandblasted and repainted long before the projected lifetime of the surface coating is reached.
The necessity to reduce chlorination to comply with lower chlorine and THM limits has led to the development of novel surface cleaning formulations in Germany. Inclusion of regular tank and line cleaning into water quality management has enabled German water providers to reduce chlorine demand and comply with limits of 0.3 mg/l total chlorine and THM limits of either 10 µg/l at the plant or 50 µg/l at the tap. These formulations have recently been introduced to the US and Canadian markets under the Floran brand name.
Chemical cleaners
Floran cleaners are ANSI/NSF standard 60 certified for surface cleaning of water treatment and storage facilities. While being widely applied in Germany for more than a decade, the products have been proven highly effective in trials in the US and are gaining acceptance rapidly. The liquid cleaners are primarily used in treating interior and exterior tank surfaces as well as all accessible treatment plant surfaces. Granular formulations for filter media maintenance and rehabilitation are also available. The main benefits are efficient deposit removal, surface restoration, reduction or elimination of chlorine demand and short facility downtime for treatment.
This article was abridged from Environmental Science & Engineering magazine, which also contains many more articles not posted on our Web Site. See our home page on how to order your subscription. We regret we can only accept orders from Canada and the United States.