The Virus-Removal Ceiling of Gravity Ceramic and "Nano" Point-of-Use Filters: A Sieving and Adsorption Accounting

Gravity ceramic and 'nano' point-of-use filters sold at nominal pore ratings of 0.2, 0.5 and 0.9 micrometres carry virus log-removal claims of 4 to 6 log10. This paper prices those claims against the stated ratings. With a lognormal pore distribution (rating as median, geometric standard deviation 1.5), over 99.99% of pores exceed the 20-30 nm and 60-90 nm virus classes, and a flow-weighted sieving model of the candle wall gives a maximum virus removal of 0.66 log10 (largest viruses, finest rating) and 0.000 log10 for the 20-30 nm class at every rating - short of a 4-log claim by 3.34 log10, a factor of 2,163 in viral passage. Adsorption is exhausted by the competing organic load of ordinary source water after a central estimate of 117 litres, 3.9% of a 3,000-litre rated life. The same model confirms protozoan cyst removal at the 12-log cap at all ratings and strong bacterial removal at finer ratings: the devices are bacteria and cyst filters, not virus purifiers.