An Integrity Sensor for assessing the performance of low pressure membrane modules in the water industry William B. Krantz ⁎, Corliss S. Lin, Priscilla Ching Yee Sin, Adrian Yeo, Anthony G. Fane Singapore Membrane Technology Center, Nanyang Technological University, Singapore abstract article info Article history: Received 10 December 2010 Received in revised form 4 May 2011 Accepted 9 May 2011 Available online 1 June 2011 Keywords: Low pressure membranes Membrane integrity monitoring Microfiltration Ultrafiltration Pre-filtration There is a need for a low cost, sensitive, on-line sensor to assess the integrity of membrane modules used in the water industry for water treatment, membrane bioreactors and for pre-treatment prior to using nanofiltration or reverse osmosis. A sensor addressing this need is described whose operating principle is based on measuring the differential pressure across a membrane that intercepts a portion of the permeate from the pre-treatment module(s) relative to the differential pressure across a valve. The latter can be adjusted to maximize the sensitivity of the instrument. A dimensionless metric based on the differential pressures is defined and corroborated with the Silt Density Index. The sensitivity of this Integrity Sensor is demonstrated by its response to cutting one fiber in a UF pre-treatment module that contains 1500 hollow fibers. © 2011 Elsevier B.V. All rights reserved. 1. Introduction Low pressure membranes are used for water treatment, in membrane bioreactors and as pre-treatment in water reclamation plants and desalination. For example, wastewater reclamation via nanofiltration (NF) or reverse osmosis (RO) requires pre-treatment of the feed water using microfiltration (MF) or ultrafiltration (UF) membrane modules. The performance of these pre-filtration modules is critical to maintaining efficient operation of the NF or RO membrane modules. Any compromising of the pre-filtration modules can cause fouling of the NF or RO membrane modules via the deposition of particulate materials that would otherwise have been removed by the upstream pre-filtration membrane modules. This fouling of the NF or RO membrane modules causes a decrease in the permeation rate that can necessitate shut down of the module(s) for cleaning or replacement. The economic operation of both large-scale centralized membrane plants as well as the rapidly expanding use of smaller scale decentralized treatment facilities requires sensitive, reliable, efficient monitoring of the integrity of the low pressure membrane modules. The techniques used to assess the integrity of membrane modules overlap those used to detect membrane fouling. There have been several recent reviews of the broad spectrum of techniques used to assess membrane integrity [1,2] and membrane fouling [3–6]. Indeed, optical methods [7], ultrasound [8], and impedance spectroscopy [4] have been used or are being explored for both purposes. A distinguishing feature of the methods used to assess membrane integrity is their relative simplicity; that is, these methods focus on addressing whether a membrane module has been compromised without necessarily asses- sing any of details regarding the nature of the particulates in the permeate. Techniques for assessing membrane integrity are broadly classified via two characteristics: whether they are direct or indirect methods; and whether they are off-line or on-line techniques. Direct methods are applied to the membrane module and include sonic or acoustic sensing [9], porosimetry [10] and techniques based on determining whether the module can hold pressure or sustain vacuum such as the pressure decay, bubble point and vacuum-hold methods. Indirect methods measure some property of the permeate from the module and include microbial monitoring, turbidity monitoring, particle counting and particle monitoring. Off- or on-line distinguishes integrity sensing techniques based on whether they are applied when the module is shut down or when it is in operation. All direct methods as well as microbial monitoring are necessarily off-line. Off- line techniques are not optimal for operation of water treatment facilities since the delay in determining the integrity of a pre- treatment module could result in severe fouling of the downstream NF or RO modules that could necessitate shut-down for cleaning or replacement. Nonetheless, the pressure decay test is a frequently used direct technique for assessing membrane module integrity [11]. Microbial monitoring of the permeate whereby the bacteriological count in samples that are taken on some regular basis is a highly sensitive technique but is labor intensive and has a slow response time. Any technique based on holding pressure or sustaining vacuum is not applicable to flat sheet membrane modules that are finding increasing use in membrane bioreactors and other applications. Measuring some property of the permeate from the pre-treatment Desalination 283 (2011) 117–122 ⁎ Corresponding author. E-mail address: krantz@colorado.edu (W.B. Krantz). 0011-9164/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.desal.2011.05.029 Contents lists available at ScienceDirect Desalination journal homepage: www.elsevier.com/locate/desal