Residence time distribution analysis as related to the effective contact time

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.