Millbrae begins unidirectional water‑main flushing in Zone 4 to reduce brown water and sediment; program uses less water than traditional flushing
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Summary
Public Works described a zone‑based, unidirectional flushing program to clear sediment from older pipes. Staff said the program cleans pipes more effectively and uses about 40% less water than conventional hydrant flushing; Zone 4 work should finish by mid‑October.
Millbrae Public Works staff reported to the City Council on Sept. 9 that the city is performing a planned unidirectional flushing (UDF) program to remove sediment and improve water clarity in older distribution mains.
Director of Engineering and Public Works Harry Burrows and Water Supervisor Leonard Harkness said UDF isolates short reaches of pipe, forces high‑velocity flows through them and tests discharge until turbidity and chlorine levels meet standards. The work clears mineral deposits that cause —brown water,— improves hydrant operation and helps identify failing valves or hydrants for repair.
Burrows said the city has divided the distribution system into four pressure zones; staff are currently flushing Zone 4, the flatter area that contains much of the older cast‑iron piping. The program identifies 192 separate reaches to be flushed in Zone 4 and staff said they were roughly one‑third complete at the Sept. 9 meeting, with an anticipated finish about mid‑October.
Staff noted UDF requires more planning and labor than conventional hydrant flushing but uses roughly 40% less water. Burrows offered a conservative accounting that Zone 4 flushing would expend about $73,000 of water that is unbilled (water flushed from the distribution system) versus an estimated $300,000 if conventional flushing were used instead.
Crews attach chlorine diffusers and neutralizing tablets at discharge points to avoid releasing chlorinated water to the storm system and test discharged water for turbidity and residual chlorine before completing each reach. The city posts an FAQ on its website and plans to update community notices as crews move through neighborhoods.
Burrows also reminded residents that Millbrae tests tap water monthly and publishes an annual Consumer Confidence Report; staff said historic sampling found no coliform contamination since 2013. The council had no objection to the phased flushing plan and staff said they will schedule zones 1–3 in subsequent years to maintain the system on a rotating basis.
