From the 2018 UPC Illustrated Training Manual, Chapter 6, WATER SUPPLY AND DISTRIBUTION
611.2 Air Gap Discharge. Discharge from drinking water treatment units shall enter the drainage system through an air gap in accordance with Table 603.3.1 or an air gap device that complies with Table 603.2, NSF 58, or IAPMO PS 65.
Most types of water treatment or soft water treatment systems have a discharge to the drainage system. This is normally a small diameter plastic tube that is sometimes placed in the standpipe of the automatic clothes washer – without an airgap. This of course is a cross connection; if the washer backs up there is a possibility of the waste reversing in the discharge tube and contaminating the water supply. This discharge line must always discharge through an airgap. There are many devices on the market that make this an easy install such as that shown in Figure 611.2a.
Any type of water treatment device that connects directly to the potable water supply shall be protected by the appropriate backflow protection device. If the water treatment device utilizes a discharge line to the drainage system, it shall discharge through an airgap (see Figure 611.2b).
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