Removal Efficiency and Integrity Monitoring Techniques for Virus Removal by Membrane Processes

Abstract

Membranes are a mechanical form of disinfection that works by physical separation of the target pathogen. In theory, an intact membrane is a barrier to pathogens that are larger in size than the pore size of the membrane. However, in practice the pore size distribution or molecular weight cutoff will provide an indication of the separation efficiency and in many cases complete rejection of the virus is not demonstrated by various membrane processes. The general mechanism of pathogen removal by membrane processes is predominantly achieved by size exclusion, influenced by the physicochemical properties of the membrane, surface properties of the pathogens, and the solution environment. However, in order to evaluate the intrinsic performance of the membranes for a specific objective, a standard protocol with defined set of operating conditions is required.

Any anomaly with the membrane surface (e.g., abnormally big pores, compromised glue line, holes) and the filtration system (e.g., compromised O-rings, broken mechanical seals) will result in microbial contamination risk of the product water. Therefore, monitoring membrane treatment system integrity is essential for the protection of public health from microbial risk. The demand for real-time monitoring is increasing as the usage of membrane-processed water is on the rise. Monitoring membrane integrity can be achieved by regular noncontinuous direct integrity testing performed on the membrane itself, or continuous indirect tests carried out on the filtrate. These monitoring techniques provide ongoing assurance that the water quality objectives are continually met. Existing integrity monitoring techniques have been shown in various instances to be reliable and sensitive for detecting contaminants of 1.0 μm and larger, while enteric virus particles are in the size range 0.01–0.04 μm. Some of the indirect monitoring techniques are capable of detecting a membrane breach less than 1 μm, but the practical challenge resolution is typically around 1–2-log. Similarly, the process control monitoring parameters lack sensitivity and therefore virus breakthrough occurs even before a loss of integrity is detected.

At present it is difficult to demonstrate virus removal without conducting challenge tests due to a lack of sufficiently sensitive process monitoring. On the other hand, regulators are under increasing pressure to grant log removal credits for enteric viruses to membrane treatment processes due to the inability of present integrity tests to verify ongoing removal of virus-sized particles. Therefore, finding reliable and sensitive integrity tests for viruses is a high-priority issue in the water treatment sector. The authors aim to review the present state of knowledge on the process efficiencies of virus removal by membrane processes and the associated membrane integrity monitoring practices for virus-sized particles. Some of the knowledge gaps and research needs associated with the existing guidelines and standing protocols for integrity monitoring are also identified.

KEYWORDS:

• membrane integrity
• virus rejection
• removal efficiency
• challenge testing
• MS2 phage