Residence time distribution analysis as related to the effective contact time

Abstract

Sixteen full-scale tracer studies were completed at two water treatment plants to assess disinfection performance under the concentration-time (CT) concept. The step residence time distribution (F RTD) was developed for each case. The value of the effective contact time, t10, in the CT concept was then obtained. For reservoirs without baffles, the t10 values were found to be much smaller than the expected values, indicating poor performance under the CT concept. Several models were used to interpret the F RTD characteristics, 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 relative concentration c/co 0.2. This showed that the momentum causing jet was responsible for the rapid movement of water through the system causing small t10 values. The work shows the importance of the momentum causing jet in reservoirs, and that in addition to traditional criteria it should be considered in the evaluation of water treatment component design. Other models that are commonly used to predict the t10 value should be applied carefully as a result of this jet effect.Key words: tracer study, F RTD, t10, CT concept, jet, water treatment component design.