Nowadays, Ultraviolet disinfection is observed as alternative disinfectant that overcome problems of existing chlorine and is used broadly in water and wastewater treatment or wastewater reuse system. Merits of Ultraviolet disinfection are that its contact time is so short that sterilizing power against Heterotrophic bacteria as well as virus is very excellent, by- products such as AOCs(assimilable organic carbon) and taste and odor are very few, instruments are simple, installment and operation control are very easy. In this paper, disinfection efficiency of Total coliforms and Heterotrophic bacteria, as bioindicator, was evaluated and compared with standard of water quality by adding turbidity and color agent to treated wastewater(A2O process in laboratory scale) as source Cylinder type batch reactor with single low- pressure lamp was adopted as experimental instrument and average UV intensity(Iavg) in reactor was calculated according to the methods of PSS(point source summation). The concentration of Total coliforms and Heterotrophic bacteria was measured by MPN method and HPC(heterotrophic plate count) method. 

The results are as follows;

1) Inactivation curve of Total coliforms and Heterotrophic bacteria was showed as typical lagging region, log phase and tailing region. And requirements of UV dose to inactivate Total coliforms and Heterotrophic bacteria under 99.9% was respectively 24.9 s/ , 24.3 s/ . 

2) Inactivation rate constant(k) of Total coliforms and Heterotrophic bacteria was shown similarly as 0.28 / s in consequence of applying first- order kinetics to log phase.

3) The maximum inactivation ratio of Total coliforms was more than 99.99%(- 4 log unit) which is 1 log-unit higher than Heterotrophic bacteria until typical tailing region is shown. That of Heterotrophic bacteria fell short of 99.9%(- 3 log unit).

4) While bentonite added as turbidity agent gave little influence on disinfection efficiency because absorbance is low, humic acid put as color agent lowered disinfection efficiency because UV absorbance increased hugely according to its concentration.

5) The concentration of TSS(total suspended solids) including bentonite and humic acid changed disinfection efficiency according to absorbance characteristics of main constituents of TSS.