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Material build-up on membrane surfaces is one of the vital challenges faced by Reverse Osmosis operations, leading to many operational and maintenance issues. To date, several modeling studies are dealt with flow behavior and concentration patterns for cross-flow membrane operations. However, the relative fouling propensities of top and bottom membrane surfaces are never addressed in any study for narrow channels filled with ladder-type spacers. In the present work, fluid flow patterns through different spacer configurations are visualized using ANSYS FLUENT by varying the dimensionless filament spacing, L (ratio of top or bottom filament spacing and channel height). Results clearly indicate that the average shear stress values at the top membrane surface are always higher (3-8 times) than that at the bottom membrane surface, but the mass transfer coefficient for the two walls yielded approximately similar average values having low to moderate filament spacings of L 3 (SP22 and SP33), indicating similar fouling propensities of membrane surfaces. Further increase in filament spacing with L ! 4 (SP44 and SP66), the average mass transfer coefficient for the top membrane indicated a sharp decline, suggesting high fouling propensity compared to bottom membrane which is not a desirable feature. Among the four spacer arrangements studied, SP44 (with L ¼ 4) was found to be the optimal arrangement, yielding moderate pressure drop with nearly equal=higher area weighted values of mass transfer coefficient for the two walls and would lead to lower and equal fouling tendencies for top and bottom membrane surfaces, respectively.

Xeroderma pigmentosum (XP) is an autosomal recessive condition characterized by an extreme sensitivity to UV rays from sunlight. It presents clinically with progressive pigmentary abnormalities and an increased incidence of skin and mucous membrane cancers at sun-exposed sites. Parental consanguinity is a significant risk factor.  Previously, cases of XP have been reported from various regions of Pakistan including Larkana, Sibbi, Karachi, Lahore, and District Dir in 1993, 2009, and 2010. Genetic studies have been conducted on seven consanguineous families with XP belonging to the Khosa tribe of Baloch ethnicity. In May 2018, XP was reported in four siblings in a family from a small village in Sindh Province, Pakistan.  Current surveillance has been carried out in the tribal village of Lundi Khosa, District Kachhi at Baluchistan, Pakistan. The disease has been endemic in the tribe since 1986, although it was brought under control in the last few years. This case report describes five patients (aged 3–12 years) with XP who have developed nonmelanoma skin malignancies.