Strategic Filter Backwashing Techniques and Resulting Particle Passage

A new filter backwashing strategy, extended terminal subfluidization wash (ETSW), aimed at reducing particle passage into the filtered water during the initial filter ripening sequence immediately following backwashing has been evaluated on pilotscale filters at a conventional water treatment plant using alum as the sole coagulant. Turbidities and particle counts were measured following backwashes extending across a range of ETSW flow rates. The ETSW procedure was effective in significantly reducing the passage of particles through filters that originated from the backwash remnant water flowing out of the filter during the first 20  min of the filter run. A secondary turbidity peak associated with the filter influent water leaving the filter was observed following all fluidization backwashes with or without the addition of air-scour or ETSW. The secondary peak was termed the "additional collector" peak and was attributed to cold-water impaired (CWI) alum coagulation at the plant. The secondary peak was not observed with the particle counter as it was with the turbidimeter, which indicates the particles passing were predominantly particles <2  µm in diameter. A backwash procedure consisting of only subfluidization water flow was evaluated because it was expected to leave significantly higher number of particles attached to the media grains to serve as "additional collectors." The subfluidization wash procedure virtually eliminated all filter ripening peaks in spite of the CWI coagulation conditions.