In November 2000 the Washington Aqueduct (WA), the water treatment
provider to the District of Columbia, switched its disinfectant from chlorine to
chloramines in order to reduce disinfection byproducts (DBP). This disinfection change
proved successful as total trihalomethanes and halo acetic acids decreased considerably
in the distribution system, which is operated by the District of Columbia Water & Sewer
Authority (DCWASA). However, accompanying this change was an increase in 90th
percentile lead compliance levels from 12 parts per billion (ppb) to 70 ppb.
DCWASA conducted lead profiles that showed average peak concentrations up to
150 ppb in homes with lead service lines. (Giani, Edwards, et al. - 2004). Review of the
profiles indicated that the majority of the lead was dissolved coming from the lead
service lines. In Spring 2004, a desktop study and several pipeloop research projects evaluated
different corrosion control options and identified orthophosphate as the most optimal
corrosion control treatment. In June 2004, orthophosphate was added to an isolated
section of DC's distribution system and studied. Several lead profiles were conducted
along with routine water quality field monitoring during that time.
In August 2004, the Washington Aqueduct began adding orthophosphate in the
form of phosphoric acid at its treatment plants. Entry point residuals were slowly
increased over a two-week time frame to 3.5 mg/L as PO<sub>4</sub>. Over the next 12 months,
WASA studied the formation of lead phosphate and the stability of the phosphate scale in
the distribution system through the data collected from lead profiles.
This paper discusses the results of the research conducted prior to and after the
start of orthophosphate corrosion control. Specifically this paper describes procedures
and results from laboratory circulation loops and lead profiles conducted at residential
homes with lead service lines in relation to the lead concentrations being released, the
type of lead being released (i.e. dissolved Lead-II or particulate Lead-IV), and the amount
of time for the orthophosphate to properly passivate the lead in the service lines. Includes 3 references, figures.
| Edition : | Vol. - No. |
| File Size : | 1
file
, 400 KB |
| Note : | This product is unavailable in Ukraine, Russia, Belarus |
| Number of Pages : | 16 |
| Published : | 11/01/2005 |
