Percent Backwash Water

Every gallon of backwash is a gallon the plant treated and did not sell, so this ratio is a direct efficiency number. Sixty thousand gallons of wash against three million gallons filtered in the run is 2.0%, which is the target most operators are held to; 1–3% is normal, and above 4% something is wrong. Note the two very different ways to reach a bad number — either the wash is too long or too fast, or the run is too short.

Short runs are almost never a filter problem. They are a coagulation problem: too little coagulant lets fine particles migrate deep and break through, too much builds a mud layer on the surface that blinds the bed and drives head loss to the limit in hours. Plot filter run length against coagulant dose for a month and the optimum usually announces itself. The other half of the equation is what happens to the spent washwater — most plants recycle it to the head of the works, which is efficient but concentrates Giardia and Crypto oocysts, so the US Filter Backwash Recycling Rule requires that the return be metered and introduced ahead of all primary treatment rather than sneaked in downstream.