Example of Reversal of Flow – Backflow Backsiphonage Case 4

The water supply to the dishwasher is not protected by a vacuum preacher and the dishwasher has a solid waste connection to the sewer. The public water system main serving the building is undersized and subjected to reduced pressures during high demand periods.

The first two floor of the building are supplied directly with water main pressure but the upper floor since there is insufficient pressure relies on the booster pump which draws water directly from the public water distribution system.  During periods of low pressure, the booster pump suction creates negative pressure in the first two floors, which produces a reversal of flow in the first and second floor piping. The dishwasher that is cross-connected to the sewer is now creating a cross-connection incident by introducing sewer water into the potable water system. Anyone drinking water on the first floor when the pump is activated will be consuming unsafe water.

The dishwasher hot and cold water should be equipped with a backflow prevention device or the supply lines should be air gapped. The dishwasher waste line shall discharge through an air gap or air break into a standpipe or waste receptor. The booster pump should be equipped with a low-pressure cutoff device.