PARTICLE-ASSOCIATED COLIFORM IN SECONDARY EFFLUENTS - SHIELDING FROM ULTRAVIOLET-LIGHT DISINFECTION

In this research, the effectiveness of various blending and sonication techniques were investigated for extracting particle-associated colif orms from secondary effluent samples. The amount of shielding from ult raviolet disinfection afforded these coliforms by the particulates was then assessed. It was found that blending samples for 1.5 minutes at 19000 rpm at 4 degrees C in a mixture of chemicals (resulting in conce ntrations of 10(-6) M Zwittergent 3-12, 10(-3) M EGTA, 0.01 M Tris buf fer, 0.1% peptone, and pH 7) resulted in the greatest recovery of part icle-associated coliform, as detected by the multiple tube fermentatio n (MTF) test. Fragmentation of large flocs during homogenization, as e videnced by the change in particle size distribution, was most likely the responsible mechanism. Sonication proved ineffective. It was found that particle association and shielding of coliforms significantly af fect accurate measurements of coliform density in secondary effluent u sing the standard MTF test and that significantly more coliforms may b e present in disinfected secondary effluents than is indicated by the standard enumeration procedure.