Micro-organism rejection by membrane systems

The removal of micro-organisms by membrane systems was investigated using single-element membranes and five species of micro-organisms in a plant setting at East St. Louis, MO. Single-element membranes included a cellulose acetate ultrafilter (UF), a polysulfone microfilter (MF), a cellulose acetate (CA) nanofilter (NF), and two composite thin-film (CTF) nanofilters. Micro-organism challenge studies were conducted using raw, alum coagulated-settled, and finished plant water. Model micro-organisms consisted of Clostridium perfringens (strain 26) spores (∼1-5 μm) for bacteria simulation, MS-2 (∼0.025 μm), and PRD-1 (∼0.1 μm) phage for virus rejection and Cryptosporidium parvum oocysts (∼4-6 μm) and Giardia lamblia cysts (∼8-14 μm) for cyst rejection. Sixty-eight observations of micro-organism rejection were gathered over 1 year of operation in eight separate challenge events where micro-organisms were spiked separately and as a mixture. The composite thin-film nanofilters provided significantly better disinfection than the cellulose acetate nanofilter. However, a cellulose acetate ultrafilter rejected more micro-organisms than any membrane tested, indicating disinfection by cellulose acetate membranes is a function of construction and module configuration rather than membrane film, as both the CA and CTF membranes were constructed in a spiral wound configuration. Micro-organism log rejection was independent of organism size except for the MF, which passed viruses, and was independent of membrane material but varied among membranes.