Chlorine disinfection is vitally important as it is typically the final barrier to pathogens and algal toxins at a water treatment plant (WTP). Fluoride tracer testing studies were undertaken at Rylstone WTP as a method to determine the mixing conditions and residence time in the clear water tank (CWT) in order to validate and verify Chlorine Contact Time (C.t) calculations.
The existing fluoride dosing system was used to collect data and enable determination of the baffling factor under various plant conditions. The tracer testing revealed that the baffling factor was much lower than expected and changed “unexpectedly” under CWT levels. The baffling factor was also in fact found to vary and was not a constant. Following the tracer testing study, immediate modifications were made in an attempt to reduce short-circuiting within the CWT with the aim to improve C.t. Additional tracer testing was then undertaken to assess improvements to disinfection following the modifications.
With the recent introduction of Microbial Health Based Targets in the Australian Drinking Water Guidelines (ADWG) and an increasing algae toxin risk profile across many parts of the country, there has never been a more important time to undertake tracer testing to confirm baffling factors and your C.t, because applying a constant baffling factor assumption could overestimate disinfection effectiveness.
Author
- Michael Carter, Principal Process Engineer, Beca Hunter H2O
- Peter Dunn, Water and Sewer Operator, Mid-Western Regional Council