Optical fouling monitor

Papermakers operate their process to optimise production, quality, and cost objectives. Therefore, it may be necessary to monitor and control surface deposition to allow for an appropriate period of operation prior to maintenance and cleaning. During this period of operation, deposits accumulate throughout the whitewater loop in the process. A stream of whitewater is continuously pumped through the OFM, so deposition measurements can be made. The unit of measure is the fouling index . The fouling index is measured every hour so that trends can be established. These trends can be used for the following objectives  

Remember OFM data only show the degree of fouling and fouling rates in the test water. If you collect machine performance data such as hole counts, non-mechanic breaks, etc., and relate these data to OFM data, the OFM becomes a more powerful tool for monitoring performance of microbial control programs. 

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In parallel to lab experiences, optical fouling monitors and coupons (removed at 14 days intervals and analysed for colony forming units of aerobic bacteria and sulphate-reducing anaerobic bacteria) have been used to monitor and measure biofilm formation on machines. 

Figure 15 Optical fouling monitor general description.

Practical methods are now available for diagnostics and monitoring but more research, innovation, and commercial development presents a very great potential to improve the performance of industrial programs. Several new innovations have been recently introduced such as an effective optical fouling-deposit monitor12. 

Phattaranawik J, Fane AG, Pasquier ACS. Studies of air slug distributions and preliminary membrane fouling by optical monitoring in a side-stream membrane module. Sep. Sci. Technol. 2009 44 3793-3813. 

Projector technique (PT), one early application of optical techniques to membrane fouling visualization, utilizes a projector and screen to monitor fouling deposition. This technique was applied to measure biofilm thickness on the silicone rubber tubular membrane during wastewater filtration [63]. The thickness was then measured and photographed from direct visual observation on the projector screen. 

There is synergy between efforts to monitor shear distribution using electrochemical probes and foulant monitoring, as poor hydrodynamics eventually lead to foulant build-up. In situ monitoring of fouling in industrial MBR systems still remains elusive however, a combination of small optical and shear sensing probes may provide long-term solutions. 

Additionally, online monitoring methods have been developed to adapt off-line characterization methods into in situ (i.e., in-reactor) probes for determination of kinetics and monomer conversion with optical methods such as mass spectroscopy (MS), ESR, FTIR, near IR, and Raman spectroscopy. However, frequently, due to high turbidity and viscosity of the polymer reaction milieu, the optical surfaces are easily fouled, leading to frequent sensor failure. Furthermore, data acquired with these probes are model dependent the empirical and inferential calibration schemes used can be expensive and time consuming to develop and can drift and become unreliable as reactor conditions change and as sensors become fouled. Another limiting feature of these methods is that they usually measure only one characteristic of the reaction, such as monomer conversion and are not directly sensitive to polymer molecular mass and intrinsic viscosity. More detailed discussion of these techniques can be found in Chapters 6-10 of this book. 

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