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Although “film thickness” and “viscosity” represent two of the most important physical parameters in lubricated tribological contacts, their in situ measurement in tribosystems is still difficult. Sensors based on ultrasonic reflectance are a class of sensing devices that allow the direct measurement of these properties. For tribologists especially, the condition changes of tribosystems are of interest, as they determine the smooth operation, proper lubrication condition or the tribosystems’ change to impending failures. Here, ultrasonic reflectance sensors can provide valuable transient information, as they are capable of measuring the evolution of film thickness and viscosity directly, non-invasive, and in situ, thereby bringing a large benefit over classic commercial condition monitoring sensors such as vibration sensors. This paper reviews the methods of operation of ultrasonic reflectance devices, their historical evolution, limitations, potentials, and outline trends of application for the challenges faced by the tribological community and society. [ABSTRACT FROM AUTHOR]

The effect of pressure on fouling layers formed by diluted activated sludge was studied in real time with ultrasonic reflectometry by fouling microfiltration membranes for 300 min at constant pressure (0.15 bar (15 kPa) and 0.25 bar (25 kPa)), as well as by performing a series of pressure-step experiments in which the pressure was instantaneously increased from 0.15 to 0.25 bar. For the constant pressure experiments, the change in ultrasonic signal amplitude was inversely proportional to the degree of fouling as represented by changes in permeability and the amount of deposited material. This finding was verified by a series of replicated filtration experiments of 15, 30, and 60-min duration at 0.15 bar, which indicated a statistically significant correlation between the degree of fouling (quantified by permeability loss and post-mortem characterization metrics) and ultrasonic amplitude change. Diluted activated sludge filtrations at varying pressures revealed that there is less ultrasonic amplitude reduction at higher pressure where the resistance of the fouling layer is higher. This finding reflects the formation of a hydrated fouling deposit that serves as an impedance matching layer with the water-filled membrane that produces lower reflection at lower pressure. Thus, the fouling layer is thought to be less hydrated and denser at higher pressures, which confirms that the fouling layer is a compressible structure. Given that fouling layer mechanical behavior may well influence membrane filtration performance, it may be possible to improve membrane bioreactor filtration by engineering fouling layer compressibility. [ABSTRACT FROM PUBLISHER]

Abstract: The summative and formative fouling potential of industrial fermentation broth engineered to produce valuable industrial enzymes was investigated. A resistance-in-series approach was adapted to estimate the relative fouling potential of broth components at two different cross-flow conditions under high-loading scenarios. Bench-scale tests were executed in cross-flow cell modules fitted with polysulfone microfiltration (MF) membranes (0.2μm nominal pore size) where fermentation broth and its isolated components, i.e., bacterial cells, particle-free media, surrogate proteins, or virgin media, were separately introduced. Setting and fouling behavior were assessed in situ, using continuous ultrasonic and permeate flow observations. Fouled membranes were characterized using scanning electron microscopy (SEM) and scanning acoustic microscopy (SAM). The largest flux declines were observed when membranes were challenged with fresh industrial broth. Flux was least affected by virgin media that had been clarified by centrifugation and by a solution of washed bacterial cells. Acoustic spectra showed significant increases in reflected power immediately following broth challenges. Above a threshold of 14μgcm−2, SAM showed that acoustic reflection patterns from fouled membranes were statistically different than their otherwise clean-condition counterparts. SEM observations were consistent with acoustic spectra and biochemical autopsy revealing rapid, progressive biomass build-up from dynamic conglomeration of “patchy”, heterogeneous biomass deposits that are loosely anchored to the membrane surface. Results were interpreted via modeling approaches using resistance-in-series as well as combined pore-blockage and cake filtration. [Copyright &y& Elsevier]

Abstract: Oil fouling during crossflow microfiltration of oily wastewater using a single hollow fiber membrane filtration module as outside-in conformation was analyzed experimentally by ultrasonic reflectometry and wavelet transform in real time. Three 10MHz ultrasonic sensors mounted along the tubular test module were utilized to monitor the fouling profile of the hollow fiber membrane. Results showed that the instantaneously rapid flux decline at the onset of fouling was caused mainly by concentration polarization and the fast oil adsorption on the lower part of the hollow fiber near the inlet of the membrane module. Further, wavelet analysis of the ultrasonic spectra revealed that the amount of oil deposited on the lower part of the hollow fiber near the inlet of the membrane module was more than those on the other parts of the hollow fiber due to the inertial impaction of oil droplets and local shell-side hydrodynamic effects. Moreover, the oil diffusion behavior (the relaxation and disappearance of the fouling layer) was also visualized by ultrasonic reflectometry in real time after the microfiltration system was shut down. The flux decline data and SEM micrographs corroborated the ultrasonic measurements and wavelet analysis. Overall, this technique will provide a useful and quantitative approach to the on-line assessment of fouling remediation and cleaning strategies of hollow fiber membranes. [Copyright &y& Elsevier]

Abstract: Biofilm growth on polymeric surfaces was monitored using ultrasonic frequency-domain reflectometry (UFDR). The materials utilized for this study included nonporous polycarbonate (PC) sheets, polyamide (PA) nanofiltration composite membranes and porous polyvinylidene fluoride (PVDF) microfiltration membranes (nominal pore size: 0.65 μm). Coupons of each material were placed in a biologically active annular reactor for up to 300 days, and subjected to a constant shear field (0.12 N m−2), which induced sessile microbial growth from acetate amended municipal tap water. Acoustic monitoring was non-destructively executed by traversing coupons in a constant temperature water bath using a spherically focused 20- MHz immersion transducer. This semi-automated system was configured to obtain reflections from 50 regions (c.a. 120×103 μm2) distributed evenly near the centerline of each coupon. The resulting reflected power distributions were compared with standard biochemical and microscopic assays that described surface associated biofilms. When compared to clean (virgin) conditions, biofilms growing on coupons induced consistent attenuations in reflection amplitude, which caused statistically significant shifts in reflected power (p <0.01). Using exocellular polysaccharides as a surrogate measure of total biofilm mass, UFDR was able to detect biofilms developing on any of the materials tested at surface-averaged masses ≤150 μg cm−2. Above these threshold levels, increasing amounts of exocellular polysaccharides correlated with significant decreases in total reflected power (TRP). The distribution of biomass on the coupon surfaces determined by acoustic spectra was consistent with that observed using environmental scanning electron microscopy (ESEM). These results suggest that UFDR may be used as a non-destructive tool to monitor biofouling in a wide variety of applications. [Copyright &y& Elsevier]

This review describes the use of ultrasonic reflectometry (UR) for characterizing membranes and membrane processes. A growing body of literature documents the capabilities of UR as a versatile nondestructive, noninvasive, real-time, and low-cost methodology that can provide important information about a wide range of membrane-based separations. A compact but thorough explanation of the ultrasonic measurement concepts relevant for use of the methodology for membrane applications is first presented. This section is followed by a description of the many studies in which UR has been employed for characterization of membrane structure, formation, compaction, and inorganic and organic membrane fouling, the latter in both real-time and post-mortem modes. Examples of recent work that incorporates the innovative use of UR for scaling in nanofiltration and reverse osmosis applications as well as results that suggest the potential of the ultrasonic slow wave to monitor pore closure in early-stage fouling are then highlighted. UR is then compared with other techniques for fouling detection so that the advantages and limitations of UR can be placed in proper perspective. Finally, valuable future directions for the incorporation of UR in membrane research, development, and practice are considered. [ABSTRACT FROM AUTHOR]