Ineffective Cleaning of Polysulfone Ultrafiltration Membrane Systems and Corrosion by Bisulfite Used as a Sanitizer

Bisulfite solution was used to sanitize polysulphone ultrafiltration membranes. The ultrafiltration system consisted of a 380-L jacketed vat with agitator, prefilter, centrifugal pump, and two polysulphone ultrafiltration membranes in parallel with valves and pressure gauges before and after the membranes. The system was soiled by recycling 380 L of sweet whey (40°C) for 2.0 h followed by concentrating (by diverting permeate steam to drain) whey for .5 h. The cleaning cycle consisted of acid cleaner (.5 h, 40°C) followed by enzyme cleaner (10.0 h, 40°C) and rinsing (2.0 h, 40°C). A .2% sodium metabisulfite solution was circulated for 5 min and the ultrafiltration unit left idle overnight. Swabs and rinse water samples were taken immediately after soiling, after cleaning, and the next morning to check sanitizing. Samples were plated on plate count agar and potato glucose agar (acidified with tartaric acid to pH 3.5) to determine numbers of bacteria, yeasts, and molds. Effectiveness of bisulfite was pH dependent. At pH 4.7, bisulfite solution was ineffective in controlling microorganisms and at pH 3.5, bisulfite was microstatic. Compared with trials with Antibac B (30 ppm Cl), bisulfite (pH 3.5) was similar for controlling bacteria, yeasts, and molds. Extensive corrosion of stainless steel housings was apparent following long-term contact (2 wk) with bisulfite solution. Laboratory trials using stainless steel fittings, plates, and rubber gaskets found that extensive corrosion and pitting of stainless steel and leaching of gaskets by bisulfite solution required several conditions: low pH, exposure to bisulfite vapors, use of 304 type stainless steel, and oxygen. Intermittent or continuous exposure to acidic bisulfite vapors resulted in corrosion. Antibac B (30 ppm Cl) did not show similar corrosion or leaching effects.