Here's How to Drain and Backwash Pools Legally*:
When you need to backwash your pool filter or drain the pool, try to use the water to irrigate landscaping. Do not drain water into the street, alleyway or other city of Phoenix right of way.*
Take care when using pool water on landscaping since it contains more salt and chlorine than tap water. Bermuda grass and Oleanders can be watered without much problem, but avoid using this water on citrus, hibiscus, or other salt-sensitive plants. Also, avoid spraying the water directly onto leaves or watering the same area repeatedly.
If you have to drain a large amount of water - such as emptying the whole pool - put it down the home's sewer clean-out. The clean-out usually is located next to the house at the point closest to the city sewer line and usually outside a bathroom or the kitchen. On some older homes, the clean-out is located in the wall. On other homes, it is at ground level, but may be hidden by landscaping. Older homes usually have only one opening, while new ones have two clean-out pipes leading to the sewer line. A threaded cap, usually black and 3 to 4 inches in diameter, covers the opening. If your neighborhood has alleys, your clean-out probably is in your back yard. If there is no alley, it may be in the front or side yard. If you cannot locate it, or a sidewalk or a patio has covered it, consult a plumber about installing one.
Caution: Using a clean-out in the wall is risky and the potential for water backing up into the home is great.
The maximum recommended discharge rate is 12 gallons per minute (720 gal/hr). However, the safe flow rate may be less, depending on the size of the drain line, distance to the sewer main, and the condition of the pipe. Most pool filter pumps will discharge too much water too fast and may cause water to backup into the yard or the house. The safest approach is to rent a submersible pump, connect it to a garden hose and slowly empty the pool. Refer to the "Hours Required to Drain a Pool" chart to learn more. A pump that operates at 700 gallons per hour is about the right size.
Use the graphics below to understand the clean out at your home, then follow the outlined procedure
Procedure for using the sewer clean-out
Locate the clean-out. If there are two, use the one closest to the home.
Remove the cap and insert the drain hose a few inches into the pipe.
Secure the drain hose so it won't pop out.
Turn on the pump.
Immediately check to make sure no water is backing up into the house. Drains for showers and tubs are the first places to check.
If the water backs up, turn off the pump immediately. You may have a blockage, or the flow rate is too high.
Note: Do not install the pool drain line as a permanent fixture. This may violate the city plumbing code or county health regulations and could contaminate the water when you refill the pool.
* City Code, Section 31-8 and Section 23-33 prohibits discharging or allowing water to escape into a city street, other right of way, or other city property. A violation of either of these sections constitutes a class 1 misdemeanor and may result in a fine of up to $2,500, imprisonment of up to six months, probation not to exceed three years, or a combination of all three. Each incident or day of violation constitutes a separate offense.