Sixteen full-scale tracer studies were completed at two water treatment pla
nts to assess disinfection performance under the concentration-time (CT) co
ncept. The step residence time distribution (F RTD) was developed for each
case. The value of the effective contact time, t(10), in the CT concept was
then obtained. For reservoirs without baffles, the t(10) values were found
to be much smaller than the expected values, indicating poor performance u
nder the CT concept. Several models were used to interpret the F RTD charac
teristics, but the results were unsatisfactory. The standard jet model was
then applied and was able to match the field data F RTD curve up to the rel
ative concentration c/c(o) approximate to 0.2. This showed that the momentu
m causing jet was responsible for the rapid movement of water through the s
ystem causing small t(10) values. The work shows the importance of the mome
ntum causing jet in reservoirs, and that in addition to traditional criteri
a it should be considered in the evaluation of water treatment component de
sign. Other models that are commonly used to predict the t(10) value should
be applied carefully as a result of this jet effect.