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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 the Oasis greywater-book  By  

Popular grey water mistakes and designs that don't yield the best results are described. Grey water best management practices are recommended, with links to detailed how-to info.

Click on the in-page links below to jump down to the section which interests you, or just read the whole thing.

Ths material is excerpted from our  Oasis greywater-book, the best selling greywater how-to resource since 1990, now in it's 6 edition. Most other greywater info in books and on the internet is derived from ourGreywater centralinfo, or is one of the common mistakes described below.

Why you can trust this information

The quality and credibility of our information is the key to our livelihood; most of our income is from information. We have developed numerous greywater reuse innovations, perhaps more than any other single source. All have been published unpatented into the public domain. We don't have much stake in any particular system, whereas our livlihood depends on being a trustworthy information source for all ecological design. Where 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 sell through. (Our main greywater book is currently in its 19th revision). <More about Why 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 much

Bad news: Greywater reuse offers much more potential benefit than is 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. There is not, for example, a single documented instance of grey water-transmitted illness in the United States.

For every hundred greywater users in the US, probably 10 are achieving most of the benefit they should, eighty-some could do better, and several systems have overall negative net benefit.

Of these few with negative net benefit, 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... our quantitative analysis reversed decades of inaccurate perception of risk and set the stage for the sharp turn towards rational regulation in California in 2008).

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 close to zero, though it could be nearly perfect 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; in fact, any little bit recycled helps.

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 article 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 videos 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 a rough account of the failure of greywater reuse to achieve more of the benefit it 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 often sounds 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 other errors stem from this fundamental error. This in turn stems from the fact grey water seems on the surface like it should be simple, and people don't allocate grey water system design the effort required to achieve the performance they expect. How does this illusion take root?

1) The instinctive recognition that in most cases the value of reused grey water resources (water, nutrients, embodied heat) is not worth very much compared to just paying for more of these resources, 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...

3) Failure to recognize that the main benefit of greywater is not the resources directly reused, but rather all the collateral benefits of the actions and systems necessary to accomplish this reuse. Greywater is like the keystone indicator species for household ecosystem health. Nobody really gives a hoot about ugly little tidewater goby fish in their own right, but if the gobies are happy, that means you have an entire watershed in balance, which is super important.

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, consider a design consultation as well.


The more basic the context and performance goals the more grey water systems ARE simple. The "landscape direct" and "drain out back" are examples of "systems" which can do their humble work incredibly reliably and well. (Conversely, The fancier the house looks, the bigger this error likely is and the more trouble it's likely to cause.)

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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 need to pay primary attention to context, let alone to know how to do it.

Preferred practice

The most general principle of grey water system design is: there are no general principles. All appropriate technologies are context specific. That is a key 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, regulatory environment; 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  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, buses 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

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 "Laundry to Landscape" does. Second, the pictures on the poster board show filtered but unsterilized grey water blasting 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.

As a class, these systems fail to see the forest for the trees. A typical residential grey water system will save $5-$20 worth of fresh water a month. 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 design of fancy grey water system design a real challenge. Fancy systems are often just built despite negative net benefit, often with the vague justification of being a "demonstration project" This justification should be used judiciously, otherwise we will end up with lots of demonstrations of how more resources can be wasted.

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 rules, which are still works in progress, tends to drastically reduce or completely destroy any possible economic benefits, in some cases without any benefit (alas, reducing proliferation of grey water systems was a perceived benefit to some of the architects of the regulations).

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. 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 in the first place.


When and wherever water is more valuable, or there are acute disposal issues, the cost-benefit shifts. Grey water skyrockets in value during drought emergency, anywhere that other water sources or other disposal is more costly.

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, solid 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 provides 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.

High 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 if its all they will let you do, an overly complex grey water system is the best alternative to an even more overbuilt conventional system.

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, which is to say, their system. It's important to us that our info be accurate and up to date. We hoped we were wrong...but based on over a year testing their system, we were right. The subsurface drip clogged, the filters clogged very rapidly, they actually gave up in the installation before we did. We applaud their efforts; it's certainly getting closer, and we look forward to the day when we can confidently recommend any complex system for residential-scale flows.

^Back to contents ^ How-to books and videos for greywater best practices

Error: Mansion with a grey water system

This is a specific case of the general problem described above. A grey water system for a huge house with a big irrigated landscape and a handful of inhabitants is very likely 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 usually just adds more waste. Why? A one-bathroom house might require a few dozen feet of extra pipe to be dual plumbed, a mansion could easily require hundreds of extra feet of pipe. (The one bathroom house may well have more people in it than the mansion, too, which means more water per fixture to reuse). When you add the level of construction perfection and convenience required to fit with the mansion, the resource cost of the system compared to the resource value of the water can become ludicrously out of balance.

Preferred practice

Redefine comfort and convenience. This is the way to cut the environmental impact at lowest cost, and furthermore promotes the idea of ecological stewardship to an influential class of people. Specifically...

Build a smaller house. A conventional house half as big is almost certainly more ecological than a "green" mansion. In the US in the 1950s the average house was a bit over half as big as now...and had twice as many people. 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 Green 4,000 ft2 home?,  Living with Nature).

One of the most powerful lifestyle statements I've personally witnessed along these lines 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 their peer's houses).

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 completely immune to this line of logic. I will take jobs like this if the owner is willing to blaze new policy or design paths that others will follow, to offset the overall environmental cost. The benefits of supporting pioneering are obvious. Still, these goals can be accomplished even better in fewer square feet.

^Back to contents ^ How-to books and videos for greywater best practices

Error: Pump enthusiasm

Many people reflexively reach for a pump when they want to move grey water. There is no denying that it is very cool to see grey water squirting out where you want it thanks to a pump...on the first day. What is not clear is that the pump is the first step on a slippery slope to system failing under its own weight. 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. Grey water is extremely hard on pumps. Odds are your grey water system isn't going to be in use in five years if it uses a pump. Pumps won't work long without filtration, and filtration is a big hassle (see Error: Grey water to drip irrigation for a typical filtration story). If the pump hasn't been killed by long hair which worked its way through the 75 micron filter and wrapped around the rotor, or if you didn't kill it when the float switch hung up at three in the morning and stayed on noisily for the third night in a row, five years is about the time a cheap pump will die of old age. Unless you're really fanatically enthusiastic about cleaning fetid filters after doing it 100 times in five years (especially 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, 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 you must have a pump, get a high quality sewage ejector pump, one of the ones that cost $200-400. These can last 20 years. If that sounds like too much money...then you probably can't afford a pumped greywater system, as this is less than buying several cheap pumps, or building a system that's quickly abandoned.


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 weakly developed for high 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. The word "storage" in connection with grey water should immediately sound an alarm, as should any tank bigger than 55 gallons (for residential systems). If you doubt this, just fill a bucket with grey water, and observe for a few days 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 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 maximum for grey water storage. This is not enough time to store grey water from a time when irrigation is not needed to one in which it is. Instead, tune 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 fine.

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).

^Back to contents ^ How-to books and videos for greywater best practices

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 ingest larger traces of residual soap from dish washing directly with every glass of water and plate of food.

Preferred practice

Grey water reuse poses a very mild health threat in developed 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 less developed 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" detergent 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.

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

^Back to contents ^ How-to books and videos for greywater best practices

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 filtration only by the strainer in the sink.

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).

I developedBranched drain greywater systemsto fulfill these requirements: the smallest opening is an inch and a half, and there are no filters, pumps, valves or surge tanks to foul with solids. If necessary, distribution can be sub mulch or sub soil (see page 13 and pages 11-14 in the Builder's GW Guide-book. 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.
Laundry to landscape is a more flexible system with smaller diameter, flexible tubing that uses the washing machine's own pump.

Maintenance will be lower in systems where grey water daylights for two inches before disappearing into mulch filled basins; hidden outlets tend to clog with roots and are hard to find.


For larger flows, say several hundred gallons a day or more, and a like amount of irrigation need, it can be economical to pre-treat grey water to the point that it can be distributed by more or less standard irrigation hardware.

^Back to contents ^ How-to books and videos for greywater best practices

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

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 a century ago. A laundry pipe runs out the drive of an estate in Los Angeles, and discharges into the street.

You'd think these systems would be 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, or just so unconscious it is beyond belief (photos).

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 gets into the river.

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Error: Use of grey water for irrigating residential lawns

Unfortunately, turf accounts for the bulk of the irrigation need in the typical US landscape, and lawn grey watering is a common violation of common sense grey water safety rules...and it doesn't work very well.

If the lawn receives traffic, by applying grey water to the surface you are short circuiting the purification step (see health rules), 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 (Sounds like I'm making this up, but I saw this at a fancy 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 then it shouldn't be a lawn in the first place. The only legitimate reason to have a resource hogging lawn is that they are more fun to play on than, say, a gravel and cactus garden.

Besides the health issue, grey watering a lawn is a pain. This is most commonly done with hose from the washing machine or a siphon from the bathtub which is moved around. Since the water has to be applied within the root system to benefit each blade of grass, 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 a large area.

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

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, for thousands of dollars to ten thousand plus. 300 gpd grey water generation/irrigation need is the minimum volume where such a system starts to make sense.

^Back to contents ^ How-to books and videos for greywater best practices

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 and 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 do my best to illuminate the boundary between responsible resource reuse and public health threat in this area.

After irrigating veggies with grey water myself (!), I understand the attraction better. We have a vast drip irrigation system, but it does not fit well to irrigating veggies. 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. 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; Unless you have a zone matched to and programmed for each cohort of veggies using drip for veggies at home will be a fraction as water efficient as for fruit trees or other perennials. Also, 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 off course. Lines with emitters in line are expensive, and clog rapidly from filtered potable water. Even if I wanted to pay for it, our small community water system doesn't really have this water to spare for inefficient drip 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 way higher irrigation efficiency, and am much more tuned into the plants.

Regarding health, 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 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, a robust, gardener's immune system well-trained from regular, low level exposure from eating bits of dirt may well protect you even if you eat something nasty.

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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 (which is alkaline), and certain plants which are native to dry areas have a hard time handling any irrigation in the dry season. Also, some plants have problems with soggy conditions.

Preferred practice

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


When drought is truly severe, even plants such as California native oak trees may be harmed less by water than by drought.


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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 problems with distribution of grey water through perforated pipe, especially with gravel:

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. Even filtered and pressurized greywater in perforated pipe will clog with roots.

Two, it is 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 next holes. 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 (if the pipe is buried) invisible 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 or the flow a tiny bit, or if lint comes down and blocks some holes, the hard-won distribution evenness will be gone.

A series of garden beds with water flowing in gravel underneath has the same issue. 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. Gravel also reduces infiltration area at the soil interface. Unlike mulch, gravel does nothing to nourish soil biota or increase soil infiltration rate.

Preferred practice

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 reliability and serviceability, useBranched drain greywater systemswith free flow outlets, which are clog-free thanks to a few inches of clear air between the outlet and the surface of mulch.

For sub-mulch or subsoil distribution, use gravelless infiltration galleys or Once installed, these are hard to even find, and will require major service or replacement at intervals of just several years, which go by surprisingly quickly.

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. A copper wire in the pipe can help a little keep roots at bay.

For reuse of large flows, high level treatment and underground drip irrigation is preferred (pages 24, 8,Oasis greywater-bookand (page 13, pages 11-14  Builder's GW Guide-book.


Effluent with suspended solids removed, for example, by filtration, can be discharged through distribution cones; seeOasis greywater-book.

There's a lot of grey water mysteries, and we don't yet know the explanation behind the few rare cases of perforated pipe and drain tiles working:

1) 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?

2) 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. What the heck accounts for the long life of this clay tile system?


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Error-Garden hose direct from the back of the washer

This method works until or unless any of these common 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) 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 (4) the pump doesn't get all of the water out and clothes stay soggy, or (5), 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

Laundry to landscape is an open source grey water system I developed which solves all of 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 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 don't seem to develop this problem perhaps because they are moved once in a while
  2. pressurized grey water lines blast the crud through
  3. p-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—or—
  2. buy super-expensive sewage valves powered by motors, not water pressure


"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

CheckOasis greywater-bookpage 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 huge 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

"or this corrugated flexible drainpipe…" Wrong! 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 trap solids, and elevate bacteria and smell levels.

Drip irrigation

"Drip irrigation is good and greywater reuse is good, so the two together would be great..." Wrong! 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

Code for combined wastewater 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 problematic 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 flow down big pipe as well (photo).

Preferred practice

Use the smallest possible pipe for gravity-flow greywater, generally 1", 1.5" or 2."


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 for greywater. Please 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 at one- to five-year intervals. 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. 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 dramatically outperform buried gravel systems, especially over time.



If it is a constructed wetland, gravel is required to resist rot with continuous submersion. The maintenance issues with gravel noted above still apply: removing, cleaning, and replacing literal tons of gravel from a constructed wetland is a nasty, heavy job... but you are stuck with gravel.

If the system is subsoil, you won't have access to replenish wood chips. In this case, use plastic infiltrators or make masonry gravelless infiltration galleys without 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 high 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 I've only heard of a handful of such systems still in use after five years (see exceptions, below).

The most common configuration in the early days—still common in new installations—is 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. What's that like? 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. Alas, when you remove the filter for cleaning (with rubber gloves, of course), you realize the lines are full to the brim; fifty or a hundred feet of two-inch pipe's full of chunky, days-old, festering grey water comes out in a deluge into the surge tank. Fortunately, you are quick on your feet and you are only lightly spattered. But now solids which should have been 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 greywater systems, or go to a more labor-intensive, less sanitary, but infinitely more reliable system like bucketing.


For sites with both a high volume of greywater and irrigation need, such as schools or hotels, grey water to drip 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. It takes a lot to make this kind of system really work over time.

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.

Orenco is the originator of this class of system for combined wastewater, and are a leading vendor that has been around for decades. Their systems cost north of $10k, and they learned from experience to require a maintenance contract to ensure the systems stay working—see Create an Oasis with Greywater-book.

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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 smell (see Error: storage of grey water.

Yet 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 high 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, 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 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 Ultra High Efficiency (UHE) toilet (or a waterless composting toilet). Old toilets use 3.5 gpf, 1.6 gpf is currently common, and UHE 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 can be 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.


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Error: Use of info from government agencies, engineers or manufacturers for design of simple residential grey water systems

There is little overlap between the set of practical grey water systems and permitted grey water systems. This seriously hampers the government's ability to give out useful information. Because grey water reuse is a 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 architect, engineer, or plumbing company would set you up with an unproven, overkill system adapted from some better known treatment technology.

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 books and videos 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…but they deliver more, because our books stick closer to reality. If the answer for your situation doesn't jump out of our books and web site, call consider ourConsulting.


For new construction sometimes an over-engineered grey water system is less over-engineered than other disposal options. 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 do  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 original grey water rules were an important step and certainly as well done as was politically possible at the time. Too bad it was a step not quite in the right direction, as was 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 rules 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 rule, 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.

Preferred practice

Follow Arizona's greywater code, or our suggested model greywater code (pdf).

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, and instead take inspiration from our model greywater code (pdf).
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 rule!


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