It is quite rare to find biodegradation in rapid sand filtration for drinking water treatment. This might be
due to frequent backwashes and low substrate levels. High chlorine concentrations may inhibit biofilm
development, especially for plants with pre-chlorination. However, in tropical or subtropical regions, bioactivity
on the sand surface may be quite significant due to high biofilm development—a result of yearround
high temperature. The objective of this study is to explore the correlation between biodegradation
and chlorine concentration in rapid sand filters, especially for the water treatment plants that practise
pre-chlorination. In this study, haloacetic acid (HAA) biodegradation was found in conventional rapid
sand filters practising pre-chlorination. Laboratory column studies and field investigations were conducted
to explore the association between the biodegradation of HAAs and chlorine concentrations.
The results showed that chlorine residual was an important factor that alters bioactivity development.
A model based on filter influent and effluent chlorine was developed for determining threshold chlorine
for biodegradation. From the model, a temperature independent chlorine concentration threshold
(Clthreshold) for biodegradation was estimated at 0.46–0.5 mg L1. The results imply that conventional filters
with adequate control could be conducive to bioactivity, resulting in lower HAA concentrations. Optimizing
biodegradable disinfection by-product removal in conventional rapid sand filter could be
achieved with minor variation and a lower-than-Clthreshold influent chlorine concentration. Bacteria isolation
was also carried out, successfully identifying several HAA degraders. These degraders are very commonly
seen in drinking water systems and can be speculated as the main contributor of HAA loss.