Integrity management for low pressure membrane plants: Impact of log removal value (LRV) parameter selection

Water quality and monitoring is of the utmost importance in drinking water systems. In terms of pathogen removal quantification, conventional systems are only able to rely on turbidity for this monitoring. Membrane facilities, on the other hand, can and are therefore also asked to also monitor and estimate the Log Removal Values (LRV) of the filtration units through the use of pressure decay tests. Correlating the results of the pressure decay test to obtain an estimate of the membranes’ pathogen removal performance is accomplished through a set of calculations. In North America, the USEPA has led both the development and implementation of the LRV concept for membranes with the introduction of the Membrane Filtration Guidance Manual (MFGM) back in November 2005. The use of the LRV to demonstrate pathogen removal has become widespread as utilities upgrade their facilities to comply with more stringent regulatory requirements worldwide. Managing LRV has now become a crucial part in the design, permitting and operation of membrane facilities.

The multiple equations involved to compute the LRV on membrane systems require multiple raw and derived parameter inputs. Proper selection of the operating data is crucial to obtaining representative LRV values that can be used by the operations staff to properly assess the plant integrity. Several theoretical models and options are currently available for the end user to choose from. With the increase in the number of facilities utilizing LRV calculation for integrity management, the calculations and assumptions used have come under frequent scrutiny by the regulatory agencies.

This paper will examine key LRV parameters and present evidence comparing empirical data with theoretical calculations and relate the differences to the recommendations of the Membrane Filtration Guidance Manual. An overview of the different options available for parameters such as flow rate, transmembrane pressure, flow regime, and temperature will be provided. The timing of the integrity test and its relationship with the plant operation will also be reviewed since its impact can be significant on the outcome of the test. Lastly, due to its significant influence on the LRV calculations, a comparison between the theoretical model and experimental data will be presented on the Volumetric Concentration Factor.