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 coliforms from secondary effluent samples. The amount of shielding from ultraviolet disinfection afforded these coliforms by the particulates was then assessed. It was found that blending samples for 1.5 minutes at 19 000 rpm at 4°C in a mixture of chemicals (resulting in concentrations of 10 -6 M Zwittergent 3-12, 10 -3 M EGTA, 0.01 M Tris buffer, 0.1% peptone, and pH 7) resulted in the greatest recovery of particle-associated coliform, as detected by the multiple tube fermentation (MTF) test. Fragmentation of large flocs during homogenization, as evidenced 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 affect accurate measurements of coliform density in secondary effluent using the standard MTF test and that significantly more coliforms may be present in disinfected secondary effluents than is indicated by the standard enumeration procedure.