The basic issue with traditional gravity diversion / irrigation systems is the inconsistency in which water is distributed.
I recognise before professional graywater dripperline became available, gravity driven diversion / irrigation was the only practical method of using graywater on a residential scale.
A combined pumped diversion system with graywater dripperline provides more benefit, is easier to install and requires less ongoing maintenance. Graywater dripperlines can also be driven by gravity, although will run at much slower flow rates.
Design and Build It Yourself gravity tubes.
Imagine a hose laid out for 100', and water is required at 20 different locations (100 emitter locations would be far better for efficient irrigation but this would be impracticle for a gravity system).
If all the holes were the same, water would come out of the first hole much fast than the last hole (if any water).
You could drill each hole a slightly different size (smaller holes first - large holes at the end) to account for the change in water volume / pressure, and then tune the pipe by drilling extra holes to get it just right. Or you could fit valves (added cost) at each location, and keep adjusting them until the hose is balanced.
And after all of that, the realisation comes that the flow settings for each valve depend on the water pressure as it enters the hose, and this will vary depending on the graywater source - showers, washer machines, lavatories (hand basins) and baths supply graywater at different flow rates.
We are not saying it cant be done. It can and has been done. However it is initially labour intensive and requires high ongoing maintenance if you want to get the most out of your graywater for you garden.
The average household produces over 3,000 gallons of graywater per month, enough to irrigate between 2,000 and 3,000 square feet of garden beds, typically surrounding the house. This is very difficult to achieve with gravity.
Gravity Driven Graywater Dripperlines.
1' of fall provides enough pressure to drive 150' of graywater dripperline, with a flow rate of 1 gallon per minute. This compares to a flow rate of 10 gallons per minute with a pump driven system. To account for the slow flow rate, an intermdiate surcharge tank (eg a 55 gallon barrel) should be installed to temporarily store the graywater while it is slowly being irrigated. This tank would be sufficient for washing machine waste, but not necessarily shower water.
While the individual dripperline emitter flow rates will only vary a small amount (approximately 5%) with slight grade across the irrigation field, tuning valves may be required at the start of each dripperline to enable even irrigation.
Pumping systems are preferable as they provide sufficient pressure to ensure even drip rates across the entire irrigation field.
This table shows flows rates for IrriGRAY graywater dripperline, head vs total dripperline deployed.
Head pressure is effected by the orientation of the surcharge container, as illustrated below.
|| Flat ground, near floor level
Placing a barrel sideways, on a barrel stand will provide more head
than a barrel standing directly on the ground. This is only important
if a faster flow rate than 1.0 gpm is required (with 150’ of IrriGRAY).
|| Ground sloping downwards, away from the house.
Little flow rate benefit is achieved by laying a barrel on its side if
the irrigation area is down hill from the barrel location. Ensure the
barrel is placed on a firm, level surface to avoid risk of tipping.
Shower waste should be only diverted to IrriGRAY dripperlines if a 80 gallon or larger barrel is used. One shower can generate graywater at 4 gallons per minute, and possibly overflow a smaller barrel if a long shower is taken.
Top loader washing machines generate more water (typically 40 gallons) per wash. Because the water is released over several cycles, graywater build up in the barrel is unlikely.
Unlike pressurized (pumped) graywater irrigation, dripperline layout density is calculated determining the irrigation area size to suit the amount of washing machine water generated per month, and then laying between 100’ & 200’ of dripperline in that area.
Good quality loan soil gardens in a moderate climate require 2 gallons per month over summer.
A top loader washing machine is run 6 times a week, producing 960 gallons of water per month. This will irrigate 480 sq. ft.
A front loader washing machine is run 4 times per week, producing 240 gallons per month. This will irrigate 120 sq. ft.
Max Lateral Length
Recommended maximum dripperline segment length is 25’.