planktonic (free-floating) microorganisms begin to adhere on surfaces, such as pipe walls, heat exchangers, and cooling tower fill. Traditional approaches to monitoring microbial activity include measuring halogen residuals, heterotrophic plate counts, and adenosine triphosphate (ATP) levels in the bulk water. Unfortunately, no correlation exists between any of the results of these monitoring techniques and the attached, sessile microorganism levels that cause biofilm. Since traditional approaches to monitoring neither pre- dict nor indicate biofilm, mechanical approaches have been employed to monitor the efficiency of heat exchangers to determine if biofilm fouling is present. However, detecting biofilm by measuring heat exchanger approach tempera - tures, unfortunately, only indicates the presence of biofilm after the fact. Similarly, flow studies only show restrictions and loss of velocity after biofilm has formed. New approach Solenis’ proprietary ClearPoint biofilm detection and control programme provides a new approach to the costly prob- lem of biofilm fouling. This programme comprises three components: a novel biofilm analyser, proprietary chlorine stabiliser chemistry, and expert service. Employing the bio- film analyser, the programme provides early detection and accurate measurement of biofilm growth in real-time. The chlorine stabiliser chemistry is used to produce a patented, in situ stabilised active chlorine solution. The solution signif- icantly reduces microbiological activity without the adverse side effects associated with strong oxidising biocides. Field service personnel provide the expertise required to main - tain clean and efficient heat exchangers. The proprietary OnGuard 3B analyser uses a patented ultrasonic sensor, shown in Figure 3 , to accurately measure the thickness of biofilm that accumulates on a heated target assembly, shown in Figure 4 . The sensor detects biofilm with a measurement accuracy of approximately 10 μm and at a resolution of ±5 μm. The analyser mimics critical heat exchanger conditions in real-time by duplicating the shear stress on a surface while also simulating the local surface temperature to provide continuous fouling factor measure- ments that inform the adjustment of chemical feed when
0% 30% 20% 10% 40% 50% 60% 70% 80% 90%
Bio lm
CaSO
CaCO
Al O
2000
20
200
Fouling thickness (µm)
Figure 2 Thermal effect of biofilm and typical mineral scales
exceeded at typical dosages. Dosages that can exceed the demand have a negative impact on the corrosion rates of metal surfaces themselves and degrade dispersants that are used to provide protection from inorganic deposition. Non-oxidising biocides similarly have difficulty penetrating the EPS’s protective slime matrix without reacting with the EPS. Economics do not favour traditional approaches to biofilm control. Underappreciated and underestimated aspects of indus- trial cooling water treatment include the effect of biofilm on heat transfer and the resultant heat exchanger failure from microbiologically induced corrosion (MIC). As shown in Figure 2 , thinner biofilms, as compared with mineral scale, exhibit a more severe resistance to heat transfer. Microbiological fouling inhibits heat transfer up to four times that of calcium carbonate fouling. Additionally, once the biofilm exceeds 50 microns, approximately the thick - ness of adhesive tape, the resulting anaerobic conditions support the growth of acid-producing bacteria. The acidic waste products from anaerobic bacteria often aggressively pit heat exchanger tubes and eventually cause leaks, requir - ing repair or replacement. Traditional techniques for monitoring microbial growth and biofilm formation cannot measure biofilm. Biofilm forms when
Ultrasonic pulse (p)
Ultrasonic sensor
Heated target assembly
Re ection (r)
Time (p + r) Bio lm growth
Figure 3 Working principle of the ultrasonic sensor
Figure 4 Heated target assembly showing presence of biofilm
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PTQ Q1 2024
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