Demonstrating the integrity of a large scale microfiltration plant using a Bacillus spore challenge test

Abstract

For protection against Cryptosporidium and Giardia New Zealand drinking water regulations established a goal of removal of all particles of 1μm and larger. The City of Tauranga, New Zealand, commissioned a 36 ML/d (9.5 mgd) microfiltration (MF) plant, the Joyce Road Water Processing Plant (JRWPP), to meet this stringent reduction goal. The plant consists of ten USF Memcor 90M10C units. This paper details a bacillus spore challenge test conducted on the JRWPP. The objectives were to (1) confirm that the plant could achieve the required 5 log reduction of particles, and (2) evaluate direct and indirect integrity testing methods as means to confirm log-5 particle reduction performance over the life of the plant. The test used Bacillus megaterium spores dosed into the feed water with the plant operated to maximise the challenge on the MF hollow fibre membranes. The results showed (1) the ability of an integral MF plant to reject Bacillus megaterium spores and achieve at least log 5.9 reduction in organisms; (2) Bacillus spores are a good medium for testing as they are robust, easy to handle, cultured in large quantities, and easily identified; (3) the spore challenge test cannot be economically used to monitor full-scale plant performance; (4) particle counting results correlated poorly with bacillus counts in feed or filtrate; and (5) the USF Memcor integrity test methods, which included a pressure hold test and a diffusive airflow test, underestimated spore rejection.