Tom Goddard
June 2, 2016, first pumped gray water
Here is my system for pumping shower drain water to my backyard garden in San Francisco. I wanted to give a large avocado tree more water since it gives us hundreds of avocados in good years.
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| Backyard | Avocado tree before (April 2016) and after (July 2016) gray water | |
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My backyard rises from the house to the back fence 60 feet away, so I need to pump the water up hill. I added a valve to divert the shower drain water to a pump. The water goes into small bucket containing a sump pump that automatically turns on when the bucket fills.
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The pump output goes through PVC pipe under my deck, then a garden hose up to a barrel at the back of the yard.
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The barrel water is distributed to plants using drip irrigation tubing with holes poked in it.
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Screen mesh in the shower drain to catch hair.
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We use biodegradable soap and shampoo.
1) Backyard is 25 feet wide by 60 feet deep with about 100 plants. It rises about 12 feet from the house to the back fence.
2) Valve controlling whether water goes to sewer or to pump: Pentair 263037 3-Way PVC 1-1/2 inch (2 inch slip outside) Pool And Spa Diverter Valve.
3) Pump is a Liberty Model 404 1/3 hp Compact/Low Profile Drain Pump. Specifications say the pump flow drops to zero raising water 21 feet. My pump is about 4 feet below the house floor and rises to the rim of a barrel about 3 feet off the ground, plus about 10 feet rise in ground level, for a total elevation gain of about 17 feet. Flow is plenty fast emptying the 4 gallon bucket in about 30 seconds. One shower takes about 15 - 20 gallons so the pump goes on 4 or 5 times for 30 seconds at a time separated by 2 minute intervals. Flow out of the shower head is typically 2.5 gallons per minute. The pump is very quiet, a deep hum, similar noise level to a modern refrigerator, barely noticable from a few feet away. It requires close attention to hear it while taking a shower.
4) Plumbing. The pump output goes through 15 feet of 1 1/2 inch schedule 80 PVC under my deck. Then I switch to a 50 foot garden hose up to a barrel at the back of the yard. Schedule 80 PVC is gray, has thicker walls, and is durable in direct sunlight exposure, but costs about twice as much as the more common white schedule 40 PVC. The schedule 80 was overkill for under my deck but I liked the gray color, less visible.
5) Barrel. The 50 gallon Ivy rain barrel from Rain Water Solutions is made for catching run-off from a house roof and has an outlet at the bottom.
6) Cost $500 for parts, with free installation done by me. Breakdown: pump $250, pvc pipe and fittings (four 45 degree elbows, check valve, slip couplings, end bushing, brass garden hose coupling, nohub clamp couplings, drip irrigation tubing...) $120, barrel $50 (with SF city discount), pentair valve $40, hose $0 (had extra lying around), GFI electrical outlet I installed for pump $30. Could probably do this for half the cost with cheaper pump and plumbing, but most gray water web sites discourage pumping because the system fails soon, so I went for better quality parts.
7) Water savings. The three members of my family take showers every day, total of 60 gallons per day in shower water. Total water use at my house is about 100 gallons per day when not watering the yard (wet winter months). Water cost is about $1 per 60 gallons (about half billed as water cost, San Francisco water from Hetch Hetchy resevoir in Yosemite, and half billed for sewer treatment). So the gray water pumping saves $1 per day in water charges compared to using clean water. It will take about 2 years to pay for the cost of the pumping system.
8) Water use. Our normal yard watering since 2002 uses about 50 gallons per day for about 8 months of the year, no yard water during wet months (December - March). In theory gray water could replace all of the clean water irrigation, but not in practice. The problem is that distributing the gray water to our hundred plants is impractical for two reasons. First soap would clog our clean water drip irrigation system, and second the water is not under enough pressure to get even distribution to all our plants. Air lock where the irrigation line rises and drops as little as an inch traps air in the line that the low pressure water cannot force out, slowing water flow to almost nothing.
9) Electricity cost. About 10 minutes per day running pump, 500 watts, or 0.08 kilowatt-hour per day at about $0.10 per kilowatt-hour, so about $0.01 per day in electricity.
10) Soap. My wife investigated soap safe for plants. General advise was to avoid salts. We did not get any quantitative information about this. Liquid soap was advised over bar soap. We switched to Dr Bronner's liquid, and Griffin Remedy and Alaffia shampoos (from San Francisco Rainbow Grocery). The extra cost may be more than the value of the recycled gray water.
11) Clogging. The millimeter size holes we poke in the drip irrigation line get clogged probably with soap. I've been poking them with an ice pick every few weeks to clear them. This is not a good way to distribute gray water. The recommended way is to have large (1/2 inch?) discharge into buried mulch pits (maybe a few gallons in an upside down pot) to avoid clogging. I've been too lazy to do that so far. My small holes are to allow the water to soak into the ground slowly without any run-off. Water squirts out in a very fine stream, or drips. I have about 20 holes and the clogging never blocks them all, but I clear them to get more even water distribution.
12) Check valve. At the pump outlet is a 1-way valve called a "check valve" to prevent water flowing back into the pump when it stops. It is a flap valve, there are also spring type check valves, but the flap valve does not restrict pump flow as much as a spring valve. One concern is that if the check valve leaks the water in the line going to the barrel will drain back into the pump bucket causing the pump to start and pump it back, repeating over and over all day long. This would only happen if the line to the barrel contains enough water to fill the pump bucket. 15 feet 1 1/2 inch schedule 80 pvc (1.5 inch inside diameter, 318 cubic inches) plus 50 feet of 5/8 inch garden hose (5/8 inch inside diameter, 184 cubic inches) is 502 cubic inches or about 2.2 gallons (231 cubic inches per gallon). This is probably not enough water to start the pump in its 4 gallon bucket. I have only noticed the pump run one time when a shower was not in progress. The hose appears to be filled with water right to the end at the barrel even many hours after a shower suggesting that the check valve is not leaking at all one month after installation (July 2016).
13) Avocado tree. The largest tree it my backyard is a Bacon avocado about 35 feet high and 25 feet wide. It has had hundreds of avocados each year for about 5 of the last 10 years. It is probably about 30 years old, as told by long-term neighbors. It looks very unhealthy with all leaves browning half-way from the leave tip to the stem. A report on Avocado Production in Home Gardens from San Diego reports water evapotranspiration for a 20 foot wide canopy avocado tree of around 50 gallons per day for May through August dry months. San Diego is hotter than San Francisco, but our tree is larger. I live half way up Liberty hill, about a 500 meter diameter, 50 meter high hill in San Francisco. The only source of water in dry months will be houses higher up irrigating yards. A drain in my basement that takes water from my back house foundation runs constantly at a trickle (5 gallons / day?) during dry months.