Common Grey Water Errors and Preferred Practices

Wave of grey water misinformation threatens to swamp web surfers.

An ongoing effort to counter the tidal wave of grey water misinformation on the web

Excerpted from Create an Oasis with Grey Water (book) By  

Popular grey water system mistakes and designs that don't yield the best results are described. Grey water best management practices are recommended.


Click on the title at right to jump down to the section which interests you, or just read the whole thing…(the section needs to load before it will jump).

Page number references are from Create an Oasis with Grey Water (book), from which this list is excerpted…the page numbers don't go anywhere on the web, but the hot links do.



Common errors on this page:

Why you can trust this information

The quality and credibility of our information is the key to our livelihood; 99% of our income is from information. We have developed numerous greywater reuse innovations. All have been published unpatented into the public domain. We don't have as much stake in any particular system as we have in being a trustworthy information source for all ecological design. When no greywater system is appropriate, we gain credibility by saying so. Likewise if a manufactured system is more suited than a DIY system we developed. When a better way of reusing greywater is found, we just update our information--we don't have a warehouse full of obsolete systems to be stuck with. (Our main greywater book is currently in its 19th revision!) <More about Why you can trust this info>

You'll find older versions of this content replicated all over the web; this is the source. We appreciate your links, likes, and direct book purchases; they support our public service research, development, and free content.



Why Greywater Mistakes Don't Matter

Bad news: Greywater reuse offers much more potential benefit than are realized in most systems.

Good news: Even the worst shortfalls in greywater design rarely cause actual harm, and for the few that do, it's generally insignificant.

For every hundred greywater users in the US, probably 15 are achieving most of the benefit they should, eighty-some could do better, and a few systems have overall negative net benefit.

Of these few, most are overbuilt systems, and the problem is that the ecological cost of the pumps and pipe are greater than the saved water.

Health risk is extremely low. There are over eight million greywater systems in the US, and in the 60 years that the Center for Disease control has been keeping records, There has not been one documented case of greywater transmitted illness in the US (references).


In our area, we have curbside recycling of mixed recyclables as well as trash pick up.

I've observed that well-meaning citizens put plenty of non-recyclable stuff which looks vaguely recyclable into their recycling bins (e.g., polystyrene packing), as well as totally recyclable materials in in a form which is impractical to recycle, like thousands of bits of loose paper, broken glass, and specs of plastic. At the sorting facility, they send this sort of stuff to the landfill.

For some households, the percentage of their recyclables which are actually recycled is as low as 20%, though it could be 95% with good information. It would have been better if the homeowners had sorted correctly, but this does not mean recycling is dangerous or should be illegal.

This is a totally different kind of "failure" than, say, burning PVC in the backyard (which forms a plume of carcinogenic dioxin). These folks just need to know what they're doing wrong, and how to do it better.

The aim of this web page is to share with greywater users and regulators what they're doing wrong and point people in the right direction. The aim of our greywater books and instructional video is to provide all the detailed how-to info.

Please bear in mind as you read the exhaustive litany of common mistakes that even the most pathetically misguided attempts at greywater reuse still often wind up showing some net benefit relative to the alternatives.

Here's an overview of the failure of greywater reuse to achieve more of the benefit which it easily could:

We are public interest ecological engineers dedicated to getting the nuts and bolts of sustainability to really work. Pondering these failings has led our path further and further from the mainstream of grey water thought.

Our conclusion can be summed up in two phrases:

  1. Do what makes sense in the context.
  2. Aim lower, hit the target.

There's plenty of room for innovation and improvement still, to be sure. However, the free, DIY designs we've created to fill the design void we found are holding up well. In almost twenty book revisions we've had to take back almost nothing we've said.

The first ten years of our attempts to popularize good design didn't make as much difference as we hoped. The appeal of common mistakes is just too strong and universal. So we started this direct attack on mistakes. While the public remains vulnerable to the siren song of grey water misinformation, the presentation of which is often authoritative and polished, this effort is making a noticeable difference now.

This compilation is a free public service we are happy to provide. We love feedback and new mistakes--please Email us .

However, please don't request free private consultations on how to undo your mistakes or do it right in the first place. That's the paid work that supports the free error warning service. Please buy a book, or commission an inexpensive review of your project, or do the best you can the with info on this site, which is quite a lot. You'll find a bunch on this page, and more in the book excerpts, and the consulting examples.


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Error-Assuming it's simple

Most of the errors below stem from this fundamental error, which in turn stems from:

1) The instinctive recognition that in most cases the resource potential of the grey water (water, nutrients, embodied heat) is not worth very much in the scheme of things, and the costs of poor management (health threat, smells, etc.) are not very high either, and—

2) Failure to realize that a grey water system which achieves the common design goals (e.g., saving water and resources, and/or achieving disposal in a sanitary, low maintenance way) is a more site and user specific design problem than almost any other home-scale appropriate technology—more than solar heat, composting toilets, rainwater harvesting, ecological building materials, etc...

In other words, grey water seems like it should be simple, and people don't allocate grey water system design the effort required to achieve the performance they expect.

Preferred practice

Either lower your goals or put more thought/ energy/ money into the design. Reading this page, the grey water Q& A page, and our grey water books is a good start. If your site is difficult, your goals high, or your time short, consider a design consultation as well.


If the performance goals are low enough for the context, grey water systems ARE simple. The "drain out back" is a very common example of a "system" which doesn't ask for or give much.

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Error-Out of context design

Wetlands in the desert. Irrigation of a swamp. Sand filtration, ozonation, and reuse for flushing toilets in a residence. Each of these are valid designs but applied in the wrong context. In a culture where standardized solutions are the norm from coast to coast, it is hard to remember the necessity of paying primary attention to the context, let alone to know how to do it.

Almost every time I say that something is so, the next breath is spent explaining why it is not so in slightly different conditions. People often seize on a principle I've mentioned in one context, then vigorously misapply it to the next. As soon as I've explained the principle which overrides it in case two, they apply THAT to case three…this can go on for hours.

Preferred practice

The most general principle of grey water system design is that there are no general principles. All appropriate technologies are context specific (that is part of what makes them appropriate) however grey water reuse is EXTREMELY context specific. The final section on each of the common mistakes describes the inevitable exceptions when it isn't a mistake. Reuse or treatment, new construction or retrofit, soil and climate conditions, legal considerations; each of these variables in particular have the potential to change the design completely. Carefully check the list of grey water system design variables (page 8) and what to do about them in the balance of the grey water books. General background on context specific design can be found in Living with Nature (see inside back cover) from which this summary is excerpted:

Context specific design: While there are no solutions which apply universally, there are a variety of approaches and patterns which can be applied to generate the optimum solution for any need in any context.
This is a vitally important principle of ecological design. In all cases the greatest efficiency—and quality as well—is achieved when the power of the tool is well matched to the task at hand. Frequent technological overkill is one of the saddest sources of waste in our society. Elimination of overkill does not involve real sacrifice. The resources saved by using simple tools for easy tasks can be applied towards performing more difficult tasks. Using transportation as an example, walking would be used for the tasks for which it is adequate, bicycles for distances too long to walk, busses and trains for distances too long to bike or in bad weather, planes for speed or great distances, and cars for special applications like ambulances, mobile homes, or workshops. Getting superfluous cars off the road would enable the necessary ones to get around without being choked by traffic.

Cleverly matching the power of the tool to the task at hand is cheaper, healthier, has lower environmental impact and is more enjoyable—but ultimately more powerful than any single solution. Though uniform solutions appeal to centralized bureaucracies, attempting to implement a single solution across the board, without regard to context, will generate a host of new problems. Bare sufficiency produces optimal growth; deficiency is stunting, excess unbalancing.

Those readers who are used to single solutions will be quick to point out situations where, for example, a composting toilet is unsuitable. None of the solutions proposed here are applicable across the board, nor is it suggested that any of the technologies criticized here should be eliminated completely. What is suggested is that a range of solutions be matched to the range of contexts using common sense.


There are no exceptions to the "no general principles" rule.

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Error-Overly complex, delicate and/or expensive systems with negative net benefit

As a class, these systems fail to refer to the big picture. A typical residential grey water system will save $5-$20 worth of freshwater a month, at best. This means that if the system costs several thousand dollars, the owner would have been better off just paying for the extra water, and the Earth would have been less impacted by the wasted water than the wasted pumps, valves, piping, filters, and electricity used by the overbuilt system. It is this constraint more than any other which makes good grey water system design a real challenge. Complying with the "actual net benefit" requirement is so difficult in practice that it is very common for systems to just be built anyway, often with vague allusions to it being a "demonstration project"…I feel this justification should be used sparingly, otherwise we will end up with lots of demonstrations of how yet more resources can be wasted.

Expensive, complicated, ill-conceived greywater system at trade show.

This system spotted at the Los Angeles Eco Expo demonstrates a number of mistakes.
First, there's overkill. For several thousand dollars it does what a $400 "drum with pump" does. Second, the pictures on the poster board show filtered but unsterilized grey water coming out at high pressure through sprinklers and inundating a concrete walkway from both sides. That violates three health principles (at least it didn't show kids dashing through the sprinklers). The tank is also too big, so that violates a fourth…probably a record for one system.

In a residential context, any system which uses a pump, filter or costs more than you spend on water in a year is suspect. Disinfecting is extremely suspect. Systems which entail massive, permanent disturbance to the planted area, such as the state of California's old mini-leachfield system, which involves burying truckloads of gravel in your garden, are also missing the point.

Unfortunately, strict interpretation of the requirements of grey water laws which are still works in progress tends to drastically reduce or completely destroy any possible economic benefits, in some cases without any benefit (except that reducing the proliferation of grey water systems was a perceived benefit to some of the major architects of the laws).

Preferred practice

Choose the SIMPLEST possible design, and BUILD IT AS WELL as you possibly can. Keep one eye on your original goals and the big picture throughout the process (see page 8). Keep asking yourself what the system is likely to look like in flood, drought, or in twenty years after three owners. What is most likely to have failed or been abandoned? How is the system likely to have been patched? Often the way systems get patched or modified later to comply with reality is the way they should be designed to be used in the first place. If no patch seems acceptable, there is a good chance the system should not be built that way at all.


When and wherever grey water is more valuable, the ground rules shift. Grey water skyrockets in value during drought emergency, and anywhere that other water sources are not available.

If grey water is the only way to save a $20,000 landscape during a drought, an expensive system that falls apart in a year may be justifiable. On the other hand, a very well-built, simple system which lasts for decades could possibly be made for the same price, if the design resources are available.

I designed a series of simple, very well-made grey water systems for a house where the only water supply is rainwater, stored for the eight month dry season in giant cisterns. The grey water system cost a few thousand dollars, but can provide the same amount of water as a cistern which would cost about ten thousand dollars. It also uses a fraction of the material and also solves the problem of disposing of the grey water.

Acute disposal problems can change the picture. I got a call from one Alaskan oil camp where the grey water from several hundred people was boiled down to ash, and the drums of ash shipped to the continental US for disposal (don't you love oil company engineers?) Compared to this, almost any system would be cheaper, simpler and more ecological.

A hotel in Big Sur was sending out their laundry at a cost of a few thousand dollars a month because they could not make a conventional on-site disposal system which could handle the water. A large, complicated, but well-built grey water system enabled them to do the laundry themselves, and they were able to pay off the investment in less than a year.

Sheer volume shifts the economics drastically. Almost any institution with several thousand gallons a day of grey water production and a like amount of irrigation demand would find that a complex system capable of treating the water so it could be distributed efficiently through irrigation hardware could be paid off in a few months to a few years.

US regulators are more comfortable with complex, expensive engineered systems than mysterious biological systems. It may be the case that an overly complex grey water system is the best alternative to an even more overbuilt conventional system, if that's all they'll let you do.

Note: there's a web page posted by a manufacturer that claims that the info here about pumps and complex systems does not take into account recent technological improvements. It's important to us that our info be accurate; we update it all the time. We hoped we were wrong...based on over a year testing their system, we were right. We applaud their efforts; it's certainly getting closer, and we look forward to the day when we can confidently, broadly recommend this type of system for residential scale flows.

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Error-Mansion with a grey water system

This is a specific case of the general problem described above. A grey water system for a 2500-5000 square foot house with a half to five acres of irrigated area and four or six inhabitants is virtually assured to be a grey water system with negative net benefit. In this situation, the resource value of the grey water is literally a drop in the bucket compared to all the other waste going on, and attempting to capture it just adds more waste. A one-bathroom house might require thirty feet of extra pipe to be dual plumbed, a mansion could easily require hundreds of extra feet of pipe for dual plumbing. When you add the level of construction perfection and use convenience required to fit in with the rest of the house, the resource cost of the system compared to its use value can become ludicrously out of balance.

Preferred practice

Build a smaller house. A house half as big built with completely conventional building materials and systems is almost certainly more ecological than a "green" mansion. In the US in the fifties the average house was nearly half as big as now. Focus all that money and energy on making a good, space efficient design, and you'll have lots left over to do state of the art green materials and systems (See Living with Nature)

If you must have a huge house, don't try to dual plumb it. Go after the combined grey water and blackwater, treat it with (for example) a septic tank and sand filter, and distribute it through efficient drip irrigation. You drastically reduce the plumbing cost, you get significantly more water and nutrient volume, and the drip irrigation can reuse it with many times more efficiency.


The argument is frequently advanced that huge "green" custom homes nourish providers of green technologies with a huge infusion of money. As one of the affected professionals, I am not immune to this line of logic. My personal standard for taking on a job of this sort is that if the owner is willing to commit themselves and other users to significant lifestyle accommodation for the earth, and take the risk of pioneering new designs, then I'll do it. Redefining the comfort and convenience standard (lifestyle accommodation) can easily cut the environmental impact of the project in half, and furthermore, promotes the idea of earth awareness and accommodation to an influential class of people. The benefits of supporting pioneering are obvious. Still, these goals can be accomplished even better in fewer square feet. One of the most powerful lifestyle statements I've personally witnessed is a very successful attorney and his wife living happily for several years in a tasteful 120 square foot cottage on a multi-million dollar lot in Santa Barbara (that's the size of the master bedroom closet of the house you'd expect these people to own!)

For a letter on this topic, click here.

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Error: Pump enthusiasm

For some reason many people reflexively reach for a pump when they want to move grey water around. There is no denying that it is very cool the first day to see grey water squirting out where you want it thanks to a pump. What you don't realize is that the pump is the first step on a slippery slope- this error is the most common subset of the one above. It won't work very long without filtration, and filtration is a big hassle (see Error: Grey water to drip irrigation for a typical filtration story). Grey water handling is a long term issue—your house is still going to be generating grey water in thirty years, but pumps are not a long term solution. Odds are about 10 to 1 that your grey water system isn't going to be still be in use in even five years if it uses a pump. If the pump hasn't been killed by two foot long hairs which worked their way through the 75 micron filter and wrapped around the rotor, or by you when the float switch hung up at three in the morning and stayed on noisily for the third night in a row, that's about the time it will die of old age. Unless you're really fanatically enthusiastic about cleaning fetid filters after doing it 100 times in five years (often during parties when the use peaks ) you probably won't buy a replacement.

Even if everything works perfectly, the pump sucks a lot of electricity; it will typically be the number 2 to 5 energy using appliance in the home, so for all your effort you've just substituted electricity waste for water waste.

Preferred practice

Use one of the systems which doesn't need a pump. There is information about pumps and filtration options in the appendices of Create an Oasis with Grey Water.


If 100% of your potential irrigation area is higher elevation than your grey water sources, you'll have to either forget about reusing your grey water or get a pump. For institutional contexts where there is more than 300 gallons a day of grey water generation and a like amount of irrigation need, the water can easily be worth more than the fancy filtration and pumping hardware needed to manage the large volume, even if it all has to be replaced every five years. At present the market is very weakly developed for volume grey water reuse hardware and support, but at least theoretically it is a legitimate application for a pump.

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Error: Storage of grey water

Storage rapidly turns grey water into blackwater (see photo, page 4). The word "storage" should immediately sound an alarm, as should anything that includes a tank bigger than 55 gallons (for residential systems). If you doubt this, just fill a bucket with grey water and observe it as it progressively darkens and becomes more fetid. Bacteria multiply to blackwater levels as well, at least the indicator bacteria. In Mexico the trampa de grasa (grease trap) often included in grey water systems is a very popular way to commit this mistake-omitting or bypassing the trampa de grasa would be much better.

A beautifully made, poorly designed festering gray water storage tank; sulfuric acid from anaerobic decomposition has rotted the lid.
Beautifully made but ill-conceived grey water storage tank. This four foot wide, four foot deep tank is the final of a four-chamber system which converts grey water to foul-smelling blackwater over the course of the week it takes to get through the system. The water is dark black, smells wicked, and generates hydrogen sulfide which has eaten through the steel lid.

Preferred practice

24 hours is generally considered the prudent maximum time for storage. Since this is not enough time to, for example, store grey water from a time when irrigation is not needed to one in which it is, I find myself tuning designs to eliminate pooled grey water anywhere it occurs; just send it all straight to the soil. The fewer little anaerobic corners and pockets the better. My latest designs drain COMPLETELY…all the collection plumbing, distribution plumbing, and surge tanks (if any) slope at least 2% across their bottom surfaces.

Manually distributed grey water can be stored for the day to allow for manual distribution all in one session. Tanks for this purpose should be designed to drain COMPLETELY (not leave a bit of fetid grey water at the bottom to inoculate the next batch) and NOT BE TOO BIG as this invites misuse in the form of letting the water sit too long.


Highly treated grey water (for example, after a septic tank and constructed wetland) can supposedly be stored for up to a month before it goes septic, depending on the BOD and temperature.

Surge tanks, which absorb peak flows (say from a bathtub and washing machine discharging simultaneously) then let them out immediately at a reasonable rate are OK.

Really cold grey water takes longer than 24 hours to stink, though I'd have to have a really persuasive reason to want to rely on this for the design of a system.

Grey water can be filtered effectively by settling in a septic tank, but then it will stink to high heaven and must be handled like clarified blackwater from a septic tank. In this case, the longer it sits in the septic tank the better (lower suspended solids and BOD).

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Error: Cavalier disregard for legitimate public health concerns, and/or excessive paranoia about negligible health concerns

Some voices on the Web advocate growing lettuce and carrots with untreated grey water. Others fret about distributing grey water under nine inches of soil without disinfecting. Some people worry about eating fruit which contains molecules from biodegraded dish soap, forgetting that they imbibe larger traces of dish soap directly with every glass of water and plate of food.

Preferred practice

Each user has to find their own comfort point on the paranoia vs recklessness continuum, and each community has to determine the outer limits of recklessness it will tolerate. Grey water reuse poses a very mild health threat in overdeveloped countries. Despite all sorts of grievous misuse (brought on in part by lack of useful regulatory guidance), there has not been a single documented case of grey water transmitted illness in the US. At the same time, it's definitely poor form to construct pathways for infecting people into your design, and totally unnecessary. Proper handling (using the same principles on page 4) can eliminate the health threat from grey water in third world countries.

Here's an overview of health concerns and what to do about them, from Create an Oasis with Grey Water:

Health Considerations Concerning Grey water Use

Gray water runs down a street in Tijuana, Mexico.
Grey water "main" running down a Tijuana street —a bona fide health threat. In theory, this water could produce fruit and green relief in a sanitary way. Unfortunately, extremely high salt concentration from hand washing with small amounts of hand-carried water and generous amounts of "Fab one-Shot" from little day-glo packets renders this resource unusable; even though it is year-round water in a desert, not even weeds grow from it. A enlightened soap factory with a line of biocompatible cleaners and a suitable marketing plan could dramatically transform the colonia environment, exchanging fetid, mucky streets for thriving, shade, and fruit-providing large trees.

All grey water safety guidelines stem from these two principles:

1) Grey water must pass slowly through healthy topsoil for
natural purification to occur.
2) Design your grey water system so no contact takes place
before purification.

Here are examples of applying these principles to correct possible problems:

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Error: Treatment before irrigation

Plants and soil, especially the upper, most biologically active layer of soil, are fantastically effective for wastewater treatment. Pretreatment is often presented as an essential element in a grey water system, when in fact it may be more pointless than treating your wastewater before sending it down the sewer. Plants and soil are fine with funky, chunky water; it is pipes and people who may have a hard time with it. Pretreatment is only necessary to overcome limitations of the distribution plumbing to handle funky water. With a properly designed system, even straight kitchen sink water (very high suspended solids) can be reliably and safely distributed with no filtration whatsoever (photo, page 30).

Preferred practice

For simple residential systems, the preferred alternative to pretreatment is to 1) design the distribution system so it can handle funky water, in particular, high levels of suspended solids, and 2) design the distribution system so human (or animal) contact is unlikely to occur before the water has passed slowly through healthy topsoil (i.e., before it's purified).
"The Branched Drain to Mini-Leachfields" system I developed is an example of a system that fulfills these requirements in their most fanatically stringent interpretation; the smallest orifice in the system is an inch and a half, and there are no filters, pumps, valves or surge tanks to foul with solids. Distribution can be fully subsurface (see page 13 and pages 11-14 in the Builder's Grey Water Guide).
These requirements can be fulfilled adequately by several other systems where grey water daylights for two inches before disappearing into mulch filled basins.


The systems described above are restricted to the use of large diameter, drain-type distribution plumbing, which is inherently less efficient than, say, drip irrigation plumbing. For small-scale systems, it is generally best to eat this efficiency loss, or go to a more labor-intensive, less sanitary system like bucketing.
For larger flows, say several hundred gallons a day or more, and a like amount of irrigation need, it can be very economical to pre-treat grey water to the point that it can be distributed by more or less standard irrigation hardware.

^Back to contents ^ Books and DVDs on Greywater

Gray water contaminates a pristine creek near a hot springs resort. Gray water from a washer runs down a pipe into the street in a wealthy neighborhood in Los Angeles.
Soapy creek from a legacy system at a hot springs resort constructed in pristine wilderness around the turn of the century. A laundry pipe runs out the formal drive of an estate in the foothills above Los Angeles, and discharges into the gutter, in which it runs for a hundred yards before running into a gulch.

Error: Discharge of grey water directly into natural waters or hardscapes

You'd think this would be more rare, because it is so obviously wrong. They come in two categories; legacy systems, from places built 50 or 100 years ago before they knew any better and/or before it mattered, and new systems, which are generally more furtive and criminal-feeling, or just so unconscious it is beyond belief (photo below, page. 4).

Preferred practice

Almost anything would be preferred. Just dumping the water on the surface of the soil is a big improvement over dumping it in natural waters or impermeable surfaces. Dumping it into a mulch-filled basin would be a vast improvement.


When the receiving body of water has sufficient capacity to purify the water, and there is no other reasonable alternative. Sailboats on the open ocean are the only example which comes to mind. Even if you are in the middle of a vast wilderness next to a large river, you can just take a few steps back and rinse the shampoo onto soil, which will purify it before it goes into the river.

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Error: Use of grey water for irrigating lawns

The only proven safe and reliable way of irrigating lawns with grey water is through underground drip tubing supplied by a backwashing sand filter type system; far beyond what most residences are likely to install. Unfortunately, turf accounts for the bulk of the irrigation need in the typical landscape, and lawn grey watering is by far the most prevalent violation of common sense grey water safety rules.

This is awkward to write. Do I criminalize thousands of grey water users who see no harm in what they are doing, or do I condone a marginal activity?

If the lawn receives traffic, by applying grey water to the surface you are short circuiting the all-important purification step (see health rules pages 4, 8), inviting direct contact with untreated grey water and the possibility of transmitting pathogens. The likelihood of transmitting disease is small (it would be laughed off in most developing countries) but it exists. The nightmare scenario: the day care center that "saves money and the environment" by irrigating the lawn with diaper wash water, which a dozen toddlers from other families then play in (I know you think I'm making this up, but I saw it at my daughter's very highly regarded day care; they were just trying to do the right thing and spaced out a bit about the context).

If the lawn doesn't receive traffic, then it is less risky to irrigate it with grey water but it shouldn't be a lawn in the first place; the only legitimate reason to have one of these resource hogs is that they are more fun to play on than, say, a gravel and cactus garden. A better solution would be to replace the un-trafficked lawn with something else and irrigate that with grey water, if it needs irrigation at all.

Besides the health issue, grey watering a lawn is a pain in the rear. The system almost universally used is a hose from the washing machine or house plumbing which is moved around. Since the water has to be applied within the root system to benefit the plant, you have to move this hose to numerous locations in a very small grid, as compared to say, a large fruit tree, which would benefit from water left to dump anywhere within an area of hundreds of square feet.

Perforated pipe under the lawn will have efficiency in the single digits, and leave some areas completely dry.

Preferred practice

We suggest that you replace most of your turf with something else, replace what's left with a water-conserving grass such as Tall Fescue, watered with the freshwater you save from using grey water elsewhere, or just let your lawn go dormant when there's not enough rain to sustain it.


Lawns can be irrigated well and safely through subsurface drip ($1500 on up. 300 gpd grey water generation/irrigation need is the break even point where such a system starts to make sense).

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Error: Use of grey water for irrigating vegetables

The primary reason not to use grey water on veggies is concern about transmitting disease (see page 4). Some people use grey water on veggies anyway. Why? See exceptions, below.

Preferred practice

If your goal is to just get rid of grey water responsibly and irrigation is not needed there is no reason to put it on food crops. If your goal is water reuse to lower overall water consumption, chances are that you will have more irrigation demand than you have grey water supply. In this case, use grey water first on ornamentals, then on fruit trees, and then use the fresh water you saved on veggies.


A gray water greenhouse in Massachusetts, supplied by a Clivus gray water system.
A probable exception: veggies irrigated subsurface with grey water in a greenhouse (the plumbing is shown in a photo on page 22).

In the past I have categorically recommended against using grey water for irrigating vegetable gardens. As a certain fraction of grey water users have always and will always do it anyway, I've decided to illuminate the boundary between responsible resource reuse and reckless public health threat in this area. Also, after irrigating veggies with grey water myself, I understand the attraction better. We have a vast drip irrigation system, but it does not adapt well to irrigating veggies. The hardware is not that great; sprayers have very uneven coverage, they put the water on top of mulch instead of under, and you have to build the bed to match the spray pattern, except that wind blows this pattern completely off course. Lines with emitters in line are super expensive, and have clogged rapidly from filtered potable water. Additionally, I can't just hook up to one of the other irrigation zones; the veggies need about ten minutes every day, instead of, say an hour and a half three times a week. Also, during weeding, seeding, transplanting, growth and harvest the irrigation need varies tremendously. Veggies also are much more sensitive to daily (even hourly) changes in weather than fruit trees. They are a very poor fit to an automated system; I would probably have to settle for less than 30% irrigation efficiency, whereas I'm used to 80% for my fruit trees. Even if I wanted to pay for it, our small community water system doesn't really have this water to spare during the hot, dry summer months. Hand watering is the obvious alternative, and if I'm hand watering anyway, why not use hoses or buckets of free grey water, instead of fresh water which costs me nearly a penny a gallon? When hand watering with grey water I can get 80% irrigation efficiency, and am much more tuned into the plants.

With the grey water systems I recommend there are several layers of protection, each capable of preventing the spread of infectious microorganisms on its own. When irrigating veggies with grey water the only possible protection is from 1) not happening to have anything nasty in the water, 2) not splashing grey water on the edible portions, 3) washing veggies, 4) cooking and 5), not getting sick even if you eat something nasty. Each of these offers tenuous protection. Additionally, grey watering vegetables is often manual, which inevitably results in some direct contact with grey water. If you're going to engage in this reckless practice, pay attention to what's happening with this slim margin of safety. If anyone in your household has an infectious disease, protection 1) is not operative and you should stop using grey water on veggies. For 2), exercise care in applying grey water, and give crops which are splashable and eaten raw a wide berth, even more so as harvest time approaches (e.g., carrots, salad greens). Always wash grey watered raw veggies with soap, iodine, or equivalent. Try not to splash too much, and wash your hands after grey watering (this extra wash water has to be considered in the irrigation efficiency for this system). As a last resort, hope for not getting sick even if you eat something nasty. I'm reluctant to mention this factor, but it seems to work in much of the Third World, where people catch plenty of nasty diseases but still not as much as you'd think.

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A mature oak tree toppled by application of gray water to the roots in the dry season.
This is the most dramatic case I've ever seen of this problem. These two giant coast live oak trees toppled, with dry season grey water application as the suspected culprit (dry season irrigation promotes root disease in these trees). Mysteriously, oaks with grey water applied to them across the street don't seem any worse for the wear.

Error: Irrigation of plants which can't take it or don't need it

Certain plants cannot take grey water, and certain plants can't take too much of any kind of water. Acid-loving plants tend to have a hard time with grey water, and certain plants which are native to dry areas have a hard time handling any irrigation in the dry season. Also, some cultivated plants have problems with soggy conditions.

This problem is not a commonly encountered one.

Preferred practice

Avoid irrigating plants which don't want the water, and design your system so it doesn't create soggy conditions for plants that can't handle it.


A related problem is irrigating plants which don't need it. For example, greywater is good, and native plants are good, so why not do both? Well, California natives don't need much water in the summer, so it's a waste of a resource that could, for example, irrigate fruit trees.

Preferred practice

Match the plant's water needs to the greywater supply.

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Error: Distribution of grey water through perforated pipe or other system where you can't tell where the water is going

There are two problems with distribution of grey water through perforated pipe:

One, it will clog

Why doesn't perforated pipe clog in septic tank leachfields? Because a septic tank is highly effective for removing suspended solids. Suspended solids are plentiful in grey water, even after the crude types of filtration sometimes attempted in home-brew systems, and these will eventually clog the pipe. If that doesn't clog it first, root infiltration will. If the pipe is big enough, and the grey water clean enough, it may take so long to clog that the durability is acceptable, but this is rare. A rarely used bathroom sink going into ten feet of four-inch pipe in gravely soil might last several years before failing. Laundry water will quickly clog almost any sized system.

Two, it is terribly unmanageable for irrigation

Any system where you don't know where the water is going is all but unmanageable for irrigation. This is only a problem if you are trying to reuse the water, not if you are just trying to get rid of it. The problem with any system where there are emitters lined up in series (like holes in perforated pipe) is that the majority of the water will dump out the first few holes, or the lowest few holes, depending if the flow is low or high. Then these holes will clog, and the water will all go out the holes next door. In order to actualize irrigation water savings, you have to somehow coordinate with fresh water irrigation so that all the plants are getting watered enough but not too much. This is all but impossible to do with perforated pipe, which will invariably water one small area way too much the rest hardly at all, in a pattern which is constantly changing and invisible (if the pipe is buried) except in the form of distressed plants.

You may be able to tune the pipe slope, hole spacing and size such that each hole spits out the same amount of water along the entire length of the system. This exercise has been performed by numerous grey water experimenters before you. However, this perfection is very fragile. If you alter the slope 1%, the flow a few percent, or if lint comes down and blocks some holes, the distribution evenness will all go to hell!

A series of garden beds with water flowing in gravel underneath is another example. Will the first bed get 90% of the water because the plants suck it up before it can move on, or the last bed, because it is low and the water rushes down to it? Who knows?

Three, gravel is nasty in the garden

Most perforated pipe systems have gravel around them. Most greywater users are keen gardeners. Gravel is the last thing you want in your garden; it degrades soil and makes digging really difficult. If you wrap the gravel with filter fabric to contain it, that will clog, as well as turn to lots of bits of plastic trash over time.

Preferred practice

Use gravelless infiltration galleys or subsurface drip for subsoil distribution. If you must have perforated pipe, add it as an extension to your septic tank leachfield and filter the grey water through the septic system so the pipe doesn't clog. The preferred practice for separate disposal of residential grey water are mulch filled basins supplied by drain out back, or a branched drain network, with pipes a few inches above the mulch if allowed or in good sized underground chambers if subsurface discharge is required (page 13, pages 11-14  Builder's Grey Water Guide (book)). The preferred practice for reuse is to plumb in such a way that you know with some certainty where the water is going, so you can adjust your supplemental irrigation accordingly. This typically means a parallel system, or one with only one outlet. Examples are drain out back, movable drain out back, branched drain networks,Laundry to Landscape , well-made and regularly serviced distribution boxes, bucketing, etc. Crude but effective parallel splitting of grey water flows can be achieved by not combining the flows in the first place; each fixture has its own separate outlet. This is difficult to manage if the fixture use is highly variable and/or unknown, but works adequately in some applications.

For reuse of large flows, high level treatment and underground drip irrigation is preferred (pages 24, 8).


Effluent with suspended solids removed, for example, by a septic tank or sand-filtration, can be discharged through perforated pipe. For even irrigation, 1" pressurized perforated pipe lines with 1/16" holes on 1" centers up to 50' long have been used (photo, page 22).

I've heard reliable accounts of a few gravel pit or trench systems which have been working for ten years or more with shower water only. Perhaps there is something magic about these installations, or shower grey water in general?

Laundry water is normally the worst, however, I have seen an old fashioned clay drain tile line which handled laundry grey water for thirty years in fairly heavy soil without clogging. Even though it was not failing, it was replaced with a modern perforated pipe as part of a remodel. The modern replacement clogged completely in less than a year and was abandoned.

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Error-Garden hose direct from the back of the washer

This method works until or unless any of these problems occur: 1) the hose kinks and the washing machine pump burns out; 2) the pump, after some months of bearing this unaccustomed load, burns out; 3) scalding hot water, with insufficient time to cool, boils your prized plant; 4) the machine tries to refill itself and the hose (with its end lower than the washer) continues to siphon fresh water out until someone shuts off the washer hours later; or 5) the pump doesn't get all of the water out and clothes stay soggy, or 6), you lift up the hose to move it to a new mulch basin before doing a load of laundry and weeks-old fetid grey water rushes backwards into the machine and onto your clothes.

Preferred practice

TheLaundry to Landscape system (page 14) solves all of these problems except the hot water one. The gravity drum, (page 15) solves all these problems.


As with many of the errors listed here, a certain percentage of installations which are the perfect embodiment of the error are used for years without any of these problems manifesting. The percentage of people who get away with this "error" because of a favorable confluence of characteristics of their washing machine internal plumbing, site geometry, and use habits is fairly high, as the incidence of each individual error is fairly low. If you have an installation like this which has been working for years, why rock the boat? On the other hand, if you're installing a new system or replacing your old one, why not choose a design which protects against these unhappy possibilities?

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Error-Fresh water designs & hardware used for grey water

Most people have more experience with fresh water than grey water. While it may seem natural to expect grey water to follow the same laws of physics, it doesn't. Here are some examples of the common pitfalls:

U-shaped pipe

Grey water should pass through a rigid "U" shaped pipe, seeking its level just like fresh water. Wrong! Crud in grey water will settle at the bottom of the U, clogging it. The same is true for the oft-reinvented alternative to a three way valve, using one ball valve and an overflow, or two ball valves; the dead end created by the closed valve will plug with crud.

Preferred practice

2% continuous slope in all rigid lines. This prevents airlocks as well, which plague inverted U pipes with low pressure fresh and grey water alike.


1) flexible grey water lines won't develop this problem if moved once in a while
2) pressurized grey water lines blast the crud through
3) traps are short U’s at the bottom of vertical drops that (usually) blast crud through

Cheap electric valves

Grey water distribution can be controlled elegantly and automatically using $12 drip irrigation electric valves. Wrong! Crud in grey water will prevent the valves from closing, grey water will corrode the valves in short order, and grey water rarely has enough pressure to make this type of valve work right even on the first day, as they are mostly powered by water pressure.

Preferred practice

1) forget about automated valves
2) buy super-expensive electric sewage valves


This nifty sand/gravel/carbon/reverse osmosis/fill in the blank filter will filter my grey water just like it does my fresh water so I can…Wrong! Crud in grey water will clog that filter in the blink of an eye.

Preferred practice/Exceptions

Check page 29 for grey water filtration options.

Soaker hose

"I'll just run my grey water through these here soaker hoses…"Wrong! Soaker hoses are such a poor technology that they typically have 30% variation in flow when brand new and used with fresh water…and they soon clog with all but the cleanest FRESH WATER, let alone grey water.

Corrugated flexible French drain tubing

Corrugated flexible drainpipe has small, rapidly clogging outlets and will collect festering crud in all it's ups and downs, as well as the corrugations, which is likely to elevate bacteria and smell levels.

Drip irrigation

See grey water to drip irrigation, below.

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Effect of changing gray water system pipe size.
Comparison of how the same size solid passes through the same flow in 1-1/2", 2", 3" or 4" pipe. Note that while it floats in the smaller pipe sizes, it is scraping bottom in the bigger ones. In this case, the smallest pipe shown provides the best flow.

Error-Combined wastewater designs used for grey water

Most plumbers have more experience with combined wastewater (grey water + blackwater) than dedicated grey water plumbing, and sometimes misapply principals of one to the other.

Here are some examples of the common pitfalls:

Too big of pipe

Combined wastewater generally requires 3 to 4 inch pipe in residences. This is too big for grey water. If you just ask your plumber to run grey water and blackwater separately until outside the house, you've got a good chance that you'll end up with a three or four inch grey water stub out. How can a pipe be too big? First, it is more awkward, expensive, and burns up valuable elevation faster. Second, crud in small flows doesn't even flow down it as well:

Preferred practice

Use 1-1/2 or 2" pipe. This prevents airlocks as well, which plague inverted U pipes with low pressure fresh and grey water alike.


Huge multifamily or institutional flows.

Distribution in perforated pipe

See Error-Perforated pipe or other indeterminate system for grey water distribution

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Error: Gravel

Gravel has been used for years for subsoil leachfields and french drains, and it is used in constructed wetlands, so it is natural enough to reach for it...but don't: it is wholly unsuited for greywater.

Issues with gravel:

Preferred practice

Use wood chips. If you used gravel, dig it up and replace it with wood chips. Advantages of wood chips:

The disadvantage of wood chips is that they need to be replenished from time to time, usually each year or two. This can sometimes be gotten around by using plants that generate their own mulch layer. Redwood or other rot-resistant wood will last much longer, though it will contribute less fertility. Also, the useful life of a layer of wood chips can be extended by placing them on top of a layer of pruned branches cut to lay flat-ish. The branch layer allows such rapid water movement that it will keep the wood chips (or other mulch) draining longer.

Note: these same factors make stormwater infiltration systems that are based on open mulch basins outperform buried gravel systems, especially over time.



If it is a constructed wetland, gravel is required to resist the continuous submersion; the maintenance issues with gravel noted above still apply but you are stuck with gravel.

If the system is buried subsoil, you won't have access to replenish wood chips. In this case, use plastic infiltrators masonry gravelless infiltration galleys—still no gravel.


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Error: Grey water to drip irrigation

Backyard tinkerers naturally tend to converge independently on the idea of running grey water through drip irrigation hardware to distribute it. There is no other way to achieve 80% irrigation efficiency, short of manually bucketing the water out plant by plant. The only problem is that it doesn't work. Of the hundreds of systems built during the 1990's California drought, every one that I know of was quickly abandoned. I think we're getting closer, but every system going in during the 2011- California drought seems likely to meet the same fate within say, five years (see exceptions, below).

The most common configuration was a surge tank with an inlet filter and a float-actuated sump pump to pressurize the grey water lines. These systems work great for the first few weeks, then the filter clogs. At first, the drains are just slow. Then there is no denying it; it's time to clean that filter; the pipes are not draining at all. When you remove the filter for cleaning (with rubber gloves, of course; a wise precaution), you realize the lines are full to the brim; fifty or a hundred feet of two-inch pipe's worth of chunky, days-old backed-up grey water comes out in a pressurized deluge into the surge tank. Fortunately, you are quick on your feet and you are only lightly spattered. Not more than a third of the solids caught in the filter have spilled into the surge tank. As you struggle to get your rubber gloves off so you can wipe the flecks off from around your lips and eyes, the sump pump cycles on. Horrified, you stand there paralyzed for a moment as the sump pump charges $300 worth of drip line with last week's split pea soup; by the time you wiggle under the crawl space and get the plug pulled on the pump, the drip line is history and the pump is stopped by hair anyway.

Preferred practice

Do something simpler and more robust with less efficiency, likeLaundry to Landscape orBranched Drain Grey Water Systems, or go to a more labor-intensive, less sanitary system like bucketing.


Hi volume systems, such as schools or hotels, can make sense. These, however, cost tens of thousands of dollars, hundreds of pounds of plastic, lots of power,  and require a maintenance contract for frequent service and replacement of parts. This gives an idea of what it takes to make this kind of system really work. (Orenco is the originator of this class of system and the leading vendor).

AGWA systems, now out of business, made residential grey water to drip systems which were so well designed and built, out of such expensive components, that they actually worked. They cost a couple to several thousand dollars. This is not a homebrew-type system. Even these eventually failed after AGWA wasn't around to maintain them. Someone really ought to figure this out.


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Error: Automated reuse systems for flushing low usage (e.g., residential) toilets

Flushing with untreated grey water will result in fouling of the tank and fetid anaerobic smells (see "Error: storage of grey water").

Treatment is fabulously expensive and complex for the amount of water saved. The cost for one system touted on the web is $10,000. At a water rate of $0.01 a gallon, that's 5 years of 325 flushes a day to recoup your investment, not counting lost interest, electricity, or system maintenance. The $650 Homestead Utilities system (s16), which is on the cheap end, would take 23 flushes a day — provided you don't count any of the other costs.

If it actually works. It is doubtful a cheap greywater to toilet flush system would work for long or at all, given the poor track record of low budget greywater to drip systems, a less demanding application.
Extreme economic infeasibility can indicate extreme ecological infeasibility; the earth could be way better off if you just wasted the water than if you wasted all the plumbing, pumps, tanks, filters, and electricity needed to make this sort of system work.

Residential greywater to toilet systems are an area rife with hucksterism. It is a shame to see companies trying to take advantage of well-meaning homeowners who are bad at math. These companies keep cropping up, though on the plus side they don't seem to last long.

If you are evaluating claims, check the assumptions. The amounts of water saved are often based on false or misleading assumptions, such as a high number of flushes with old high flow toilets.


Preferred practice

First, put in a low flush toilet (or a waterless composting toilet—see inside back cover). Old toilets use 3.5 gpf, 1.6 gpf is currently common, and the lowest use toilets at the moment use 0.8 gpf. If you have a low flow toilet, you can reduce your toilet water usage by half just by switching to the lowest flow available toilet.

Then, "if it's yellow let it mellow, if it's brown flush it down." Average flushes per day per person are around 5, half of these could be foregone without taking things too far.

With these two measures you can save 75% of toilet water.

Third, toilets can be flushed with grey water by simply bucketing it from the bathtub/shower directly into the toilet bowl (not the tank, where it will fester). An added plus of reusing bathtub water in this way is that due to flush volume always being under direct intelligent control it is always less. Also, in cold climates you get a primitive but highly effective sort of grey water heat recovery as the bath water sits there and heats the house as it cools.

If none of these preferred options appeal to you, you and the environment are probably better off just forgetting about flushing your toilets with grey water than buying an automated system.


Every toilet should have a lid that routes clean drinking water en route to bowl-scouring flush through a basin where it can be used for washing hands. Toto handwash cascade toilet

Multifamily, institutional or any other high use installations can benefit from flushing toilets with highly treated grey water, especially when incorporated in the original design of the building. If you have highly treated water already and don’t know what to do with it, say from a constructed wetland, it may be worth supplying it to the toilet. An important advantage is that toilets need flushing every day, whereas irrigation need is usually seasonal.
"Clearwater” such as air-conditioner drip, reverse-osmosis water purifier reject water, and fixture warm up water is a natural for flushing toilets. It needs no treatment and can store indefinitely. Steven Coles of Phoenix, Arizona suggests that if you have an evaporative cooler, supply the toilet from its reservoir and you’ll keep the mineral concentration in the cooler water from rising.

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Error: Use of government agencies, trade organizations, engineering firms or salespeople for design of residential grey water systems

There is very little overlap between the set of practical grey water systems and the set of legal grey water systems. This would seriously hamper the government's ability to give out useful information, if they had it. Because grey water reuse is a new, marginal, rapidly evolving field, it is hard for ponderous bureaucracies to keep up with. The California grey water law and the pamphlet which explains it are especially misleading for hardware guidance-see error: using CA grey water law as example.

Also, there are few practical grey water systems which can be profitably installed professionally. It is likely that an established trade organization, engineer, or plumbing company would set you up with an unproven, overkill system adapted from some better known treatment technology. Therefore, you're pretty much on your own.

Appropriately designed grey water systems are not a very sales-friendly product; way too site-specific, variable, and inexpensive. The world wide web features numerous generic, expensive, prepackaged grey water systems with fantastic claims, which seem to be better systems for the salesperson than the user.

Preferred practice

So far as I know our two books are the most complete and up to date references for home grey water systems. The designs in our books promise less than in other sources…because our books stick closer to reality. If the answer for your situation doesn't jump out of our books and web site, call us for a consultation.


For new construction sometimes an over-engineered grey water system is less over-engineered than conventional treatment. If the administrative authority requires it, gritting your teeth and paying up to ten times more for the false assurance of a brand-name product (which may never have seen success in the field but still seems safer somehow than you trying something on your own) may be the best option. Also, the prepackaged grey water system may actually be good, though the odds are against it and I know of none I can confidently recommend (please  Email us  if you make or have used a good one).

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Error: CA grey water law held up as example to copy

California's new grey water law is an important step and certainly as well done as was politically possible. Too bad it's a step not quite in the right direction, as it is being emulated all over the US and the world. Some of the hardware recommendations are questionable. The mini-leachfield system, for example, is described in great detail as if it were a proven technology, but has been installed in no cases I know of and I can’t think of any application for which I would recommend it.
Unrealistic laws have poor participation rates. Santa Barbara, for example, has issued approximately 10 permits for grey water systems between 1989 and 1998. There is evidence that during this time of severe drought over 50,000 Santa Barbarans used grey water! There are so many obviously overkill requirements that the entire law, including the sensible provisions, is dismissed as a source of design guidance.

California's 2009 greywater code is a great improvement; no permit required for laundry only system, required cover down to 2 inches of mulch cover, terribly misleading drawings removed...huge improvement. Still, Arizona's greywater code is better, and this model code (pdf) better yet.

Preferred practice

If you're a homeowner in a place with a backward greywater code, don't follow it unless you can get a favorable interpretation or have no choice. For guidance turn instead to a sensible code from another jurisdiction.
If you are an inspector, sections of the code grant you nearly total discretion to approve whatever you want. Please exercise this discretion to discount the more counterproductive sections, and allow genuinely well-designed and executed systems.
If you are a regulator, don't blindly follow California's lead when writing your own grey water regulations. Check the information in our Guidance for Regulators.
A more reasonable regulatory stance would lead to greater participation and a reduction in risk from the perpetuation of unregulated systems.

Please help distribute this information as widely as possible—it's got a long way to go to catch up with this ground breaking law!


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