On this page:
"Branched drains are the best residential greywater system since indoor plumbing. It's an awesome, ingeniously simple design. I am really impressed with how easy it was to design and install, and how little attention it's required since. We made it out of off-the-shelf pipe fittings and it outperforms thousands of dollars of complex, delicate machinery. That's what I love about it."
-Rainy Fackler-Adams, organic gardener, owner of a branched drain greywater system at a state of California greywater test site
A decade of experience with dozens of residential greywater systems led ecological designer Art Ludwig to question everything about conventional greywater system design. He stripped away every possible bit of complexity until the essence of a greywater system remained: a pipe network for distributing water from the house to a number of trees around the yard. Branched drains provide economical, reliable, sanitary, low maintenance distribution of household greywater to downhill plants without filtration, pumping, or surge tanks.
Advantages, disadvantages, and limitations of branched drain systems, designing a system, assessing greywater sources and irrigation need, checking levels with a water level, selecting plants, estimating irrigation need, cost/benefit analysis, installation, laying pipe with marginal slope, branched drains for Third World materials and conditions, mulch basin design, maintenance, system examples, and references & suppliers can be found in The New Create an Oasis with Grey Water (book). The Builder's Grey Water Guide (book) is recommended for new construction & remodeling.
If you answered "yes" to all three questions, a branched drain system is an excellent candidate for your project.
Most greywater industry offerings are elaborate systems featuring filters, pumps, tanks, valves, and sometimes disinfection and electronic controllers, etc. They cost from $1000 to $30,000 (gasp!) for a single-family residence. The vast majority of these are either newly installed, or abandoned, or failing to meet their original goals. However, these are a small percentage of the systems in use: 5% or less.
What about the other 95%? 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?
Low reuse efficiency
With all the water dumping in one spot, the result tends to be an over watered patch and everything else under watered. Usually nothing is planted to utilize the water, in which case the reuse efficiency is zero. If the flow is high, plants at the outlet might be able to utilize only a fraction of the water available. In a few lucky instances there is a fruit tree which grows until its water need equals the available flow, in which case the reuse efficiency is close to 100%.
Without a basin to contain it and mulch to cover and slow it, greywater applied to the surface from a drain out back could flow into a creek, onto the street, or whatever, especially if it is raining. In older installations near natural waters, the pipe typically discharges directly into them.
Without mulch or soil covering it, greywater could be lapped up by dogs, played with by children, or prowled for food bits by vermin. This litany sounds worse than it really is in most first-world installations (where not a single case of greywater-transmitted illness has been documented). Amazingly, wastewater just flowing over the ground for some distance (say fifty feet) receives spectacularly high treatment by the same beneficial bacteria that live in soil.
In third world conditions, especially in shanty towns where people live in appalling intimacy with their greywater, it is just as bad as it sounds above.
Soil overload/poor aesthetics
With a typical drain out back, a mucky, anaerobic grey-white material on the ground indicates where the soils purification capacity is overloaded. This patch can measure from a few inches to several feet in length. If it slopes so the greywater runs off and infiltrates into the surrounding soil, there is usually no odor unless you stick your face in it. If the greywater pools at the outlet, odor is likely and mosquitoes are possible. Some plants may be harmed by root suffocation.
Though common throughout the entire world, the drain out back is illegal, primarily because the effluent is applied to the surface (this virtually never results in official censure with existing structures, but it wouldnt be permitted for new construction or a remodel).
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:
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.
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.
Carefully consider the limitations below and determine whether any might apply
to your site. Do this step diligently. If you set out to do something the system
is incapable of doing, no amount of energy is going to get you there!
A branched drain network cannot deliver water uphill
If your irrigated area is above the greywater sources, youre stuck. Look in Create an Oasis: Choosing a greywater system for other system options.
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%)
The steeper your lot and the higher your house plumbing is above grade, the less painstaking it is to lay out the pipe. If your lot slopes 1" per foot (8%) or more, you can pretty much just dig a trench, throw the pipe in, and the dirt after it. If it slopes 1/4" to 1/2" per foot (2-4%), it is a tedious multi-step process to level the pipe. If your lot slopes less than 1/4" per foot (2%) wow, its going to be rough. In this case, the further from the house you go, the deeper your lines are going to end up and you will not be able to irrigate very far from the house. Youll also have to double the design effort and redouble the installation precision. If you have a lot of water to get rid of and water percolates slowly into your soil (low perk)...it may not be feasible.
The pipes must be sloped to exact tolerances
The tricky part of branched drain systems is designing and installing them so the pipes slope constantly and correctly downhill, within close tolerances. This is also one of the trickiest parts of doing greywater collection plumbing under a house. Hiring a plumber to get the inside pipes right is a good idea, however plumbers arent that great in the yard. First, they are expensive. Second, they are easily disoriented in the garden. Third, plumbers tend to be conservative and tradition-bound. You might have a hard time coaxing them to adhere to your design. You are likely to have to do the work yourself, or at least supervise it. Unless you are naturally detail oriented you may have a hard time warming to the simple but at times unreasonably painstaking task of getting those darn pipes to slope right.
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 pressureit is sure to fail.
Branched drains cannot be used to irrigate lawns or small plants
Besides the other reasons not to irrigate lawns with greywater (described in Create an Oasis: How to use greywater/ A note on lawns), the branched drain system, with only a few dozen outlets maximum, is poorly adapted to irrigating thousands of tiny individual plants. It works best with trees, especially fruit trees (whats the point of irrigating a tree unless it gives fruit?)
Exception: if your only goal is to get rid of the water, it may be appropriate
to use plants whose only claim to fame is sucking up water.
Branched drains are poorly suited to high flows
There may be an elegant way to reuse more than a few hundred gallons a day through a branched drain network, but I havent thought of it. The amount of pipe and the area required to deal with a single large source soon starts to seem overwhelming. This is a limitation shared by all but a few of the systems described in Create an Oasis. The average, mildly conservative first-world household generates 35 gallons of greywater per person per day, which is well within the comfortable range for the branched drain system. There is no lower limit.
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
There is no easy way to supply a small amount of water to a young tree and then several times more when that tree is large, or shift from watering deciduous trees to evergreens when the deciduous trees drop their leaves. You make your best approximation when you design the system, and thats it.
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
It is certainly possible that more could be watered, but it hasnt been tried to my knowledge. With ordinary flows, 16 is a reasonable limit. With pulsing output from a dipper box 64 outlets would likely work (see Possible variations & improvements/ More than four levels of splitting).
Designing a system
These are the steps to designing a system:
Now well work through them in detail, one by one...
The New Create an Oasis with Grey Water (book)
For Branched Drain Greywater Systems
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