The Board of Water Supply used a device to scrub water lines Tuesday and Wednesday, and a high concentration chlorination also was used to disinfect the system.

The bacteria count went down after the operation.

The Board of Water Supply continues to take test samples. If the low bacteria count continues to be low next week, the board may lift the request for residents to boil water, in effect since July 17.

But if the bacteria count rises again, the Board of Water Supply plans to lay temporary lines through back yards to provide residents clean water from a nearby, high-elevation reservoir, while it continues work to find the bacteria source in existing lines, maybe digging some up.

Brooks Yuen, Board of Water Supply deputy manager and chief engineer, explained the process to about 30 residents at a meeting last night. "We're still not sure what the cause is," Yuen said. "At this point in time, we still do not know where the source is."

Bill Wong, program manager, safe drinking water branch, state Department of Health, assured residents that bacteria involved are not life-threatening.

One theory is that bacteria-laden silt may have been in the lines.

Board of Water Supply officials hope the scrubbing eliminated it and the heavy dose of disinfectant wiped out most lingering bacteria.