Branched Drain Greywater Systems
Reliable, Economical Sanitary, Low Maintenance Distribution of Household Greywater
to Downhill Plants without Filtration or Pumping
The Branched Drain Greywater System was originally developed by Oasis Design. You can find information on it all over the internet and several books; Oasis Design is is the original source. If you find unattributed information on Branched Drains please Email us and we'll ask for a link.
Buying Create an Oasis with Greywater 
, our book on this topic, directly from us most effectively supports our development unpatented, public domain designs for this and many other systems for living better with less use of resources.
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"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
Complete information on Branched Drain Greywater Systems can now be found in Oasis greywater-book
(Branched Drains used to be it's own stand alone book, and this page was it's description.) A large amount of free information on Branched Drains can be found here and on the Create an Oasis with Greywater web page.
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...
Topics
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 Oasis greywater-book. The Builder's GW Guide-book is recommended for new construction & remodeling.
Is a branched drain system right for your project?
- Is the area to be irrigated downhill from where the greywater is generated?
- Does the house generate less than 300 gallons a day of greywater? (15-35 gal a day per person is typical)
- Are you either a) indifferent to irrigation efficiency, or b) willing and able to lay pipes with a great deal of detail orientation?
If you answered "yes" to all three questions, a branched drain system is an excellent candidate for your project.
Introduction
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?
Advantages of the “drain out back”
- No filtration necessary
- No pumping necessary
- No surge tank necessary
- Little pipe is needed
- Little or no required maintenance
- Low economic and ecological cost
- It can be built by anyone
- Low failure rate (that is, they are little worse in year 100 than year one)
- They last forever
Drawbacks of the “drain out back”
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%.
Poor sanitation
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 soil’s 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.
Illegal
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 wouldn’t
be permitted for new construction or a remodel).
Branched drains to the rescue
The branched drain system solves most of the drawbacks above while retaining
most of the advantages. Here’s 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:
- With a double ell or other “tee” fitting, which splits the flow in two. By then splitting it again, and again, one big flow can be split into 16 little ones, in a “family tree”-like branching network (hence the name).
- By not combining the flows in the first place. Each fixture or fixture set will water its own area.
- By manually moving an outlet extension.
- By a combination of 1), 2), and/or 3). (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 world’s 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. Don’t 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.
What’s 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 I’d 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 it’s 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!
Limitations of the branched drain system
A branched drain network cannot deliver water uphill
If your irrigated area is above the greywater sources, you’re 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 example—see 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. You’ll 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 aren’t 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 you’ll 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. Don’t even think of it
if you don’t have a lot of pressure—it 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 (what’s 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 haven’t 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 that’s 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 hasn’t 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:
- Define system goals and design assumptions
- Assess greywater sources
- Assess irrigation need
- Connect greywater sources with irrigation need on a site plan
- Decide whether or not to combine all flows before splitting them
- Design the plumbing details
- Design the mulch basins and outlets
- Make fine adjustments to the design during construction (this is unavoidable!)
Now we’ll work through them in detail, one by one...
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Flow SplittersFor Branched Drain Greywater Systems $24 — Flow splitter, 2" $19 — Flow splitter, 1.5" <out of stock>
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