Sieves and nylon screens were used to separate primary sewage effluent solids into particle fractions of less than 7- or greater than 7-micron 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 less than 7-microns fraction were inactivated more rapidly by 0.5 mg of chlorine per liter at 5 degrees C and pH 7 than coliforms associated with the greater than 7-microns fraction. Homogenization of the greater than 7-microns fraction not only resulted in an increase in the number of less than 7-microns particles, but also increased the rate of inactivation to a rate similar to that of the less than 7-microns fraction. With 1 mg of monochloramine per liter at 5 degrees C and pH 7, particle size had no appreciable effect on the rate of inactivation. At pH 8, however, the less than 7-micron fraction was inactivated more rapidly than the greater than 7-micron 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, greater than 7-micron particles can have a protective effect against the disinfecting action of chlorine.
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