Inactivation of particle-associated coliforms by chlorine and monochloramine

Abstract

Sieves and nylon screens were used to separate primary sewage effluent solids into particle fractions of <7- or >7-μm size. The efficiency of separation was determined by using a particle counter. Indigenous coliforms associated with the particle fractions were tested for their resistance to chlorine and monochloramine. Coliforms associated with the <7-μm fraction were inactivated more rapidly by 0.5 mg of chlorine per liter at 5°C and pH 7 than coliforms associated with the >7-μm fraction. Homogenization of the >7-μm fraction not only resulted in an increase in the number of <7-μm particles, but also increased the rate of inactivation to a size had no appreciable effect on the rate of inactivation. At pH 8, however, the <7-μm fraction was inactivated more rapidly than the >7-μm fraction. The time required for 99% inactivation of the particle fractions with monochloramine at pH 7 or 8 was 20- to 50-fold greater than the time required for the same amount of inactivation with chlorine at pH 7. The results indicate that coliforms associated with sewage effluent particles are inactivated more rapidly with 0.5 mg of chlorine per liter than with 1.0 mg of monochloramine per liter. However, >7-μm particles can have a protective effect against the disinfecting action of chlorine.