The East Bay Municipal Utility District (EBMUD) operates six water treatment plants
throughout the East Bay in California. The primary source water is snowmelt from the
Sierra Nevada collected at the Pardee Reservoir. The water travels from the reservoir
through 90 miles of aqueduct before reaching either immediate treatment or subsequent
storage at one of several terminal reservoirs. EBMUD is interested in lowering total
trihalomethane (THM) levels and haloacetic acid (HAA) levels in its treated water. An
important mechanism leading to the formation of these disinfection byproducts (DBPs)
is the application of chlorine in aqueduct water to suppress biofilm. Biofilm control is
critical to mitigate headloss and biocorrosion in the aqueduct. A potential method of
controlling biofouling while minimizing THM and HAA formation would be to replace
free chlorine with ultraviolet (UV) light as primary disinfection. There is potential that UV treatment
would reduce chemical dose, thereby decreasing THM and HAA formation.
The focus of this project was to determine the effectiveness of UV treatment on its own
and in combination with free chlorine (Cl2) and chlorine dioxide (ClO2) in suppressing
biofilm growth in aqueduct water. The study also assessed DBP concentrations and
assessed the potential for UV to reduce chemical disinfectants. The technical approach involved eight annular reactors (ARs) set up at the Pardee Reservoir to simulate the aqueduct. Two trains of water, one treated with UV light and one un-treated, entered
separate clearwells and were dosed with a sodium tetraborate solution to adjust pH to a
level similar to the full-scale aqueduct. The adjusted water was then pumped into each
AR, which represented the first section of the aqueduct where residual disinfectant is still
measurable. One of the ARs was used as a control for the UV train and one AR was a
control for raw water. Two ARs were dosed with ClO<sub>2</sub>, one that was pre-treated with UV
and one that was not. The final four ARs were dosed with Cl<sub>2</sub>, two having been pretreated
with UV and two without. The ARs were set at a rotational speed that created the same shear stress at the outer wall
of the AR's cylinders as that which would be seen at the outer wall within the aqueduct.
Chlorine dioxide was generated according to Method 4500-ClO<sub>2</sub> of Standard Methods for
the Examination of Water and Wastewater, 19th Edition. Free chlorine was measured using the DBP colorimetric method. Heterotrophic plate counts (HPC) were performed using a spread plate technique on R2A
agar (BD, Franklin Lakes, NJ). Includes 5 references, table, figures.
| Edition : | Vol. - No. |
| Number of Pages : | 7 |
| Published : | 06/17/2005 |
