A field study evaluation for mitigating biofouling with chlorine dioxide or chlorine integrated with UV disinfection

The drinking water industry is continually seeking innovative disinfection strategies to control biofouling in transmission systems. This research, conducted in collaboration with the East Bay Municipal Utility District (EBMUD) in California, compared the efficacy of chlorine dioxide (ClO 2) to free chlorine (Cl 2) with and without pre-treatment with low-pressure ultraviolet (UV) light for biofilm control. An additional goal was to determine disinfection by-product (DBP) formation with each disinfection strategy. Annular reactors (ARs) containing polycarbonate coupons were used to simulate EBMUD's 90-mile aqueduct that transports surface water from a source reservoir to treatment facilities. ARs were dosed with chemical disinfectants to achieve a residual of 0.2 mg/L, which is a typical value mid-way in the aqueduct. The experiment matrix included four strategies of disinfection including UV/ClO 2, ClO 2, UV/Cl 2 and Cl 2. Two ARs acted as controls and received raw water (RW) or UV-treated water. The data presented show that the UV/ClO 2 combination was most effective against suspended and attached heterotrophic (heterotrophic plate count, HPC) bacteria with 3.93 log and 2.05 log reductions, respectively. ClO 2 was more effective than Cl 2 at removing suspended HPC bacteria and similarly effective in biofilm bacterial removal. UV light alone was not effective in controlling suspended or biofilm bacteria compared to treatment with ClO 2 or Cl 2. Pre-treatment with UV was more effective overall for removal of HPC bacteria than treating with corresponding chemical disinfectants only; however, it did not lower required chemical dosages. Therefore, no significant differences were observed in DBP concentrations between ARs pre-treated with UV light and ARs not pre-treated. Disinfection with ClO 2 produced fewer total trihalomethanes (TTHMs) and haloacetic acids (HAAs) than chlorination but did produce low levels of chlorite. These data indicate that replacing Cl 2 with ClO 2 would further control microbiological re-growth and minimize TTHM and HAA formation, but may introduce other DBPs.