Branched Drain Greywater Systems
Here is a very cost
effective alternative to the more expensive systems.
Adobe Loos & Worms offers this as information only and
provides no support or warrantees that this method will
work in your situation.
Laundry to Landscape system, a
new DIY system that
is even simpler and more economical than the branched
What about the other 95 percent? Many have been working for decades without the users even thinking about them. The majority consist of nothing more than a drainpipe pointing down the nearest hill. The classic drain out back has some serious shortcomings, but its durability and spectacular simplicity give one serious pause for thought.
From a holistic, ecological design perspective, a really complicated, expensive system is doomed from the start. At best, all it can hope to do is shift the impact from the waste of water to the waste of resources used to make pumps, valves, tanks, piping and electricity.
How about scrapping all this delicate, expensive technology which is nearly impossible to make ecological, affordable, or durable, and instead concentrate on improving the humble drain out back? What does it have to teach us?
Drawbacks of the drain out back
Branched drains to the rescue
The branched drain system solves most of the drawbacks above while retaining most of the advantages. Heres how:
Split the flow
The most intractable problems of the drain out back stem from unmanageably high flow to one place. The branched drain system addresses this by splitting the flow. These are the ways the flow can be split:
(For detail and pictures see Designing a system/ Connect greywater sources/ Ways to split the flow.)
Contain and cover the flow
If I had just two words to contribute to improve the worlds handling of greywater they would be mulch basin (Figure 2). Mulch covers the greywater and provides many other benefits (see Designing a system/ Mulch basin and outlet options).
The basin contains the water where it is needed and prevents it from escaping where it is wasted or a nuisance. The island in the middle of the mulch basin protects the delicate root crown from wet conditions and possible disease (more on basins under Designing a system/ Mulch basin design).
Mulch basins are a common feature of existing horticultural practice and could hardly be simpler to make and maintain. Dont let this fool you. Though nature takes care of their inner workings, these are fantastically complex biologically, far more complex than a municipal sewage treatment plant.
Whats more, the treatment level mulch basins provide is far higher than that of a municipal treatment plant,3 and instead of consuming copious electricity and chemicals to create polluted natural waters and piles of toxic sludge,4 mulch basins run on sunlight and yield drinkable groundwater and fresh fruit (are you convinced yet?)
The outlets can be arranged so the water falls through the air for a few inches before disappearing under the mulch (simplest), or they can be fully enclosed in chambers under mulch (legal and most sanitary). These options are fully described under Designing a system/ Outlet design.
Advantages and disadvantages of branched drain systems
By splitting and covering the flow, branched drain systems overcome all the disadvantages of the drain out back while retaining most of the advantages. Overall Id characterize them as surprisingly involved to design and install optimally, with surprisingly little to do after. The up-front investment (primarily in labor) is substantial but then its taken care of, possibly for the life of the house. In contrast, most other systems cost more, lack the opportunity to save much by doing the labor yourself, and require significant ongoing inputs in the form of electricity, maintenance, and system replacement.
Limitations of the branched drain system
A branched drain network cannot deliver water uphill
Partial exceptions: a washing machine can send water through a drumless laundry system pressurized by its own pump, to a height which is roughly even with the top of the machine, and quite some distance away (a hundred feet, for examplesee Create an Oasis: System examples/ Drumless laundry). From there you could drop it into a branched drain network. The drumless pressurized plumbing is also partially exempt from the next two rules. It will still work if: 1) not sloped exactly, and: 2) U shaped sections work, although they should be designed so the amount of pooled water in the U-section is minimized.
Conceivably, an effluent pump could also deliver greywater from a surge tank to an uphill branched drain network. It is hard to see the advantage relative to pressurizing the pipe directly by the effluent pump, or better yet, going to a sand filter to drip irrigation system (see Create an Oasis: System examples/ Automated systems).
The ground on your site needs to slope at least 1/4" per foot (2 percent)
The pipes must be sloped to exact tolerances
Exception: If you have lots of well-situated fall, getting slopes right is a breeze and youll be exempt from many of the plumbing steps.
The pipes must slope downhill continuously
The pipes cannot drop down and then back up again; for example, plunge to exit under the foundation then pop to the surface, or dip to cross a gully.
Possible exception: If there is a large amount of unvented elevation drop (ten or twenty feet) on the line, the pressure of greywater has been successfully used to blast through any clog which might occur.5 This keeps the line running, but it negates an advantage branched drain networks have when they slope continuously: no anaerobic pockets or pools of any size. The water which pools in the bottom of the U will go septic when it sits for a while, and when it gets pushed through by fresh greywater it will come out stinking. Set a system up this way only if there truly is no alternative. Dont even think of it if you dont have a lot of pressure--it is sure to fail.
Branched drains cannot be used to irrigate lawns or small plants
Branched drains are poorly suited to high flows
If you are faced with a huge water flow and a like amount of irrigation need, the economics of other more elaborate systems start to be more attractive; see Create an Oasis: System examples/ Automated systems
The way the water is distributed is difficult to alter
Partial exceptions: It is easy to add on to the ends of a system or truncate outlet extensions without changing the existing branching pattern. For example, you can design a system to accommodate tree growth by putting the flow splitter some distance from the trunk. An outlet extension from the flow splitter to a young tree can be removed when the roots have grown out farther, or an additional split can be added to send the water to two places in the (now) extensive root zone.
If one side of a flow splitter is the final outlet (as in photo 12), you can plug it and the water will continue on unsplit. This could be used to bypass a deciduous fruit tree and send the water on to an evergreen one. When the deciduous tree leafs out, remove the plug and clear the opening.
Branched drains have not been used to water more than sixteen outlets from one source
Designing a system
All Oikos pages copyright 1996 - 2006, Iris Communications, Inc.