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Greywater Management
Selective Greywater Treatment

Implementing Greywater Treatment

The Problem with Phytopurification

The TRAISELECT System on the Market
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The following pages will only make sense to those who collect their greywater separately from black water (containing toilet waste). When one is commited to managing wastewater in a sustainable manner, one must come to accept that domestic wastewater purification is a mistake. For the environment’s sake, we must prioritize wastewater reuse/recycling, not wastewater treatment/purification. The types of wastewater management we so choose are conditioned by their end-use. Selective greywater purification, as described on this website, takes a holistic approach to sustainable water management. It seeks to ensure properly functioning ecosystems, sustainable agriculture and the continued replenishment of water tables, within an ecological sanitation framework.

In 1992, Joseph Országh publicly launched the idea of collecting and treating greywater and black water separately; [Tribune de l'eau (CEBEDEAU-Belgique), vol.45, pp. 89-94, (1992)]. Two years later, the same idea was taken up at the 11th annual symposium on water management (called the « Journées Information Eau » or Water Information Days) in Poitiers (France), September 28-30, (1994). In 2000, the concept was further developed and presented at the 14th edition of the Poitiers Symposium, September 13-15, (2000), the proceedings of which were published under the title of Assainissement Intégré : une nouvelle vision de la gestion des eaux usées domestiques, (or Integrated sanitation : a new vision for domestic wastewater management). The same year, an article was published on the subject in China: Urban Water Management Technology in Respect to Protecting the Agriculture Ecosystems. Water Resources and Hydro-power Engineering, Beijing, China, Vol. 31, N°7, p.58-60 (2000)].

To see examples of EAUTARCIE homes that are self-sufficient and self-contained in terms of water consumption, click here.

It is interesting to read a testimonial from Andalusia (Spain) on EAUTARCIE’s benefits in dry regions. Yet the solutions that have been adopted in the testimonial are inspired by the will to purify wastewater. As we shall show, there are simpler, less expensive and more efficient solutions to wastewater purification.

To visualize the general schematic of a TRAISELECT system,
click here.

The text within this page was first published in French on in 2003

The original text has been adapted and translated in English by André Leguerrier and was first published on this page at

Last update: 2017-02-06

Selective Greywater Treatment

Sanitation: sustainable or not sustainable ?

The simplest solutions

Considering that reading through the entirety of this chapter may be burdensome for some readers, here is a summary of the simplest solution, showing how to implement the selective treatment of greywater, containing no toilet waste (i.e. black water).

In rural and peri-urban areas (where you most often find homes with gardens), the selective treatment of greywater remains possible, in a very efficient and environmentally-friendly way, for those who wish to continue using conventional flush toilets. To start off, a separate greywater outlet needs to be installed for the home. The black water would continue to be (provisionally) discharged into existing sewers. In areas not serviced by mains sewerage, a septic holding tank (to be periodically emptied) would be reserved exclusively for the storage of black water.

Of course, those who adopt a proper dry toilet such as a BioLitter Toilet would have minimal transformations to do within the home. In the absence of black water, only greywater would go through the home’s drainage system, making it possible to treat greywater in the home’s garden.

In summer, greywater will simply be brought to the foot of plants for irrigation purposes, using one or more flexible pipes. These would be covered with mulch to limit evaporation. Alternatively, one can consider holding greywater in an open basin, which water will be clarified by the process of photo-purification even if it involves greater evaporation.

During winter and on rainy days, greywater would be discharged into a simple septic tank, to be set up to overflow into a soakaway, a dispersal drain or better, an absorption pit. This solution is fit for those who don’t want to manage their wastewater.

These systems require no maintenance. Once installed, one can « forget about them ».

NB. Those who are mainly interested in the practical aspects of selective greywater treatment can skip the philosophical sections and go straight to the technical ones.

What is sustainable sanitation ?

During a meeting held in Verviers (Belgium) in March 2012 under the auspices of the newly formed Réseau d'Assainissement Durable (or « Sustainable Sanitation Network »), some people protested that « sustainable sanitation » was an inappropriate denomination for the network, insisting that « sanitation cannot be anything short of sustainable ». Most participants viewed the idea of « unsustainable » sanitation as senseless because according to them, sanitation can only be beneficial for the environment.

First, they consider that sanitizing our wastewater is a means of restoring better quality materials (e.g. treated wastewater and/or sewage sludge) from our waste. « How can this not be sustainable? » will they assert. Thus, « sanitation » and « wastewater purification » are considered synonymous. Ultimately, this means their main objective is depollution.

Second, they seemingly consider that pollution is inevitable, a fatality against which we are powerless, except by remediating the waste we produce.

These are the reflections of what I qualify as the classic vision, the dominant view shared by regulating authorities and many die-hard environmentalists.

Sanitation - wastewater purification ?

There has usually been a linear relationship between how the science of sanitary engineering has evolved and whatever concerns of the moment needed to be addressed: a problem crops up, find a solution; another problem comes along, find another solution; and so forth and so on. When you look at the history of sanitation, you discover with amazement that all solutions have systematically resulted in newer problems, in turn giving rise to other solutions, again resulting in other problems, endlessly escalating.

The confounding factor between sanitation and wastewater treatment is the result of two centuries of historical evolution. Sanitation, as in to « sanitize » or « make clean », was concerned with getting rid of our dirty water, to keep our cities clean. That is why sanitation was initially associated with sewers (although not yet confounded with wastewater purification). Even when dirty water is removed, it remains dirty, wherever you remove it to: thus, our rivers are being polluted. Instead of going to the root of the problem with profound questioning [2], technicians have chosen to address the symptoms, giving rise to wastewater purification (also known as wastewater treatment).

Looking more closely, late 19th and early 20th century engineers still had a more pragmatic view on what has become commonly known as pollution. Sewers were not made watertight: the goal was not to drive the wastewater of that time into rivers (which were already reported as contaminated), but rather to disperse wastewater in the soil. It was the most reasonable solution, especially considering that flush toilets were not yet the rule in cities. Grey wastewater contained practically no nitrogen. So grey wastewater’s most environmentally-friendly destination was – and still is – infiltration into the soil. With the advent and extension of flush toilets, wastewater’s composition has changed. To prevent nitrate pollution of groundwaters, it then became necessary to make sewers watertight.

At this point another bad option emerged: for convenience, treated water had to be evacuated to the nearest river. In the minds of technicians, the river was the perfect outlet because its water came from a renewable resource; so it was considered « inexhaustible ». Similarly, seas and oceans in which rivers poured were considered infinite, reservoirs into which we could dump anything and everything, indefinitely. The dominant view was to evacuate everything that bothers us, out of sight [3]. We did not think about the consequences.

That is the only reason the flush toilet was created. The ecological life cycle analysis of the flush toilet is only catastrophic because of wastewater treatment. When flush toilets are connected to a sewerage network that discharges wastewater into life-sustaining water bodies, water is wasted, that could have been used to irrigate farmland, while human dejecta are utterly wasted away, when they could otherwise have been composted with plant-based material to help fertilise the land that produces our food. A flush toilet user destroys his or her own food base each year. Human dejecta only become waste, pollution and a threat to the environment after treatment in a sanitation plant.

In the late 20th and early 21st century, despite the misgivings I expressed, astronomical amounts have been spent for the collection and treatment of wastewater. The result is a failure that sanitation technicians still refuse to recognize: despite the generalized implementation of sanitation (in Europe), the quality of our groundwater reserves has continued to deteriorate. Similarly, the average quality of our rivers has continued to fall, even if there have been improvements locally. What is even more grievous is that water experts have still not come to acknowledge the fact that the depletion of our groundwater reserves is the indirect result of the « all-to-the-sewer » system, also known as all-mains sewerage.

The first paradigm of present-day sanitary engineering, which postulates that « to protect the environment, you need to purify wastewater as efficiently as possible », has clearly shown its limits. It may be shocking to some, but it can easily be shown that the opposite assertion is the true one: the more you purify domestic wastewater, the greater you pollute and the more you damage the environment. In no way can current sanitation be qualified as « sustainable ». Therefore, « unsustainable » sanitation does indeed exist!

Unsustainable sanitation

Thus we come to the concept of unsustainable sanitation, which includes a set of techniques currently imposed by laws, with important negative impacts. In a village, for example, the real pollution and environmental damage occur when a sewerage network or an « approved » septic treatment is implemented [4].

In France, « approved » septic systems for individual homes include the new generation of electromechanical systems called « compact sewage treatment plants »: in fact, the better their performance, the more environmentally-harmful they are. They are also expensive and require substantial electrical consumption and maintenance. In their treatment efficiency, they better transform the organic nitrogen component of human dejecta into nitrate pollution, when in fact it should be harnessed as an agricultural resource. In the best case, nitrogen is denitrified – thus permanently lost to the biosphere.

But nitrate pollution and the eutrophication of our waterways is only a minor aspect of wastewater treatment’s harmfulness. This environmental mess really takes root in our disregard for (or ignorance of) the intimate relationship between global food production and domestic wastewater treatment. This is well summarized in our video entitled « The end of all-mains sewerage ».

Hygienics: an ideology of unsustainable development

Human dejecta has somehow become « an evil that must be destroyed », that must be removed from our sight. Yet this anthropocentrism is an outdated ideology. Whether we admit it or not, humanity is part of the biosphere. Now in the biosphere, nothing happens without a purpose, not even the fact that we defecate and urinate – just like animals. Human and animal dejecta do not constitute waste to be eliminated; they partake in a series of necessary transformations that ensure the functioning of the whole. When we were only one or two billion people on this earth, we could still ignore this fact. But the dejecta of 7 billion humans constitute a biomass comparable to that produced by animals. Together with plant biomass, they are the foundations of sustainable food production in an increasingly populous world. In this context, destroying the organic matter of our dejecta under pretence of sanitation – and transforming it into pollution – is a suicidal activity. The image suggested by Friedensreich Hundertwasser is relevant: sewers are the open veins through which the blood of the Earth is lost.

When the principles of SAINECO (or EAUTARCIE’s version of ECOSAN) are submitted to sanitary engineering technicians for their consideration, said technicians tend to retain only what suits them. They do not realize that all these principles work as a whole, as a coherent system. Set aside but one of these principles, and the system breaks down. All-mains sewerage is the root of our water problems worldwide, and indirectly of climate change. Advanced treatment systems such as tertiary sewage treatment using plants or involving ultrafiltration, the treatment or composting of sewage sludge, phytopurification, etc. are simple « plasters » that don’t go to the root of the problem. As soon as greywater is combined with black water and that our excreta’s organic matter is destroyed, the wastage is irreversible. There is no turning back: the synthesis of humus can only work using the molecular structures contained in phosphorous- and nitrogen-rich dejecta combined with carbon-rich cellulosic plant biomass. Together, these go through a series of transformations to form humus, a process that occurs in intimate contact with the soil. Polymeric humic acids (amino) are formed through the graft of animal- and human-sourced nitrogenous materials (proteinaceous) on the carbohydrate matrix of polymeric molecules of plant cellulose and lignin. There is no other way. Wastewater treatment, even using plants, breaks this series of processes, destroying the molecular structures previously mentioned.

Humus is the « brown gold of the earth », a vital substance. Its importance is such that we can say the history of civilization is also a history of humus. Historical migrations of human populations originated in the depletion of the land’s humus. Nowadays, there are no more virgin lands to conquer. Our planet is a kind of spaceship in which we must learn to live while managing our resources efficiently. Human and animal faeces are part of these. If we do not make an abrupt shift towards sustainable management of biomass, our farmlands will eventually disappear through erosion and runoff to the seas, with humus being destroyed. Our lands’ humic reserves are currently so depleted that crops require more and more chemical fertilizers, which in turn accelerates the destruction of humus.

To save what can still be saved of our lands while ensuring sustainable food production, we must harness all plant-based and animal-based biomass (including human) to form humus by using different composting techniques. An extensive mobilization of biomass will phase out synthetic fertilizer needs and eventually even pesticides and herbicides. Thereafter, Earth, as well as humanity, can regain health. The first step in this direction is to abolish all-mains sewerage. There is no other alternative.

An undertaking at an individual scale

Since EAUTARCIE transpires – at first – as a personal undertaking by individuals, its impact is first measurable at a family garden scale. Its expansion would nevertheless have an impact far exceeding the most optimistic forecasts in the fields of water and environmental management. The pursuit of an approach where the selective treatment of greywater is done separately from that of black water opens the way to a world where dwellings will no longer pollute natural water bodies.

To understand the relevancy of selectively treating greywater, you must first acknowledge three analytical realities:

Today’s domestic wastewater is a mixture of greywater and black water, as per the conventional view on sanitation, by which both types of wastewater must be combined and treated together. Under EAUTARCIE’s version of ECOSAN (SAINECO), this « all-to-the-sewer » logic is unacceptable, as much so as consumer society’s « all-to-the-trash » logic. The composition of each wastewater being quite different, their selective treatment is a necessity. By using a proper dry toilet in households, black water need not be produced. In an urban context where dry toilets would be difficult to implement, the solution is to selectively collect black water (i.e. separately from greywater) and convey it to an integrated biomass treatment centre.

Ideally, greywater should not be treated. However, to satisfy the dominant view that demands purification of any wastewater at all cost, we developed a basic greywater purification system as well as the more extensive TRAISELECT System, two domestically-sized selective greywater treatment systems adapted for households. Despite an exceptional efficiency, the selective treatment of greywater is generally prohibited by law. Current laws (and regulating authorities) don’t recognise greywater’s distinctive characteristics. To be blunt, it is more precise to state that they don’t want to acknowledge said characteristics, based on an unscientific reasoning. To apply the same principles and legal prescriptions to greywater treatment leads to regrettable errors and unnecessary costs. Read more on this at The Traiselect System and the Law.

Important notice. In spite of its excellent efficiency, we currently do not promote the implementation of the TRAISELECT System. I developed the system to challenge the objections from my sanitary engineering colleagues. When I explained the necessity of collecting and treating greywater separately from black water, many eminent colleagues asserted that this would « never be accepted, since the nitrogen and phosphorus contained in black water are indispensable to feed the bacteria that thrive on soaps and detergents contained in greywater ». None of said colleagues found it necessary to at least visit one of our TRAISELECT set-ups, to witness how the quality of the discharged wastewater was close to potable.

When centralized sewerage is at stake, those who wish to use dry toilets and concomitantly treat their own household’s greywater inevitably come into conflict with sanitation regulating authorities. Indeed, truly sustainable sanitation is strictly prohibited in those areas serviced by mains sewerage. Laws tend to protect the sanitation industry instead of the environment, as exemplified by the obligation to hook up one’s home to city sewers, even if it can be proven that one’s private greywater treatment system is more environmentally friendly than centralized sewerage. This goes against the EU principle of « applying the best available and economically viable technology ». This is quite unfortunate, because those who compost their dry toilet effluent and treat their own greywater as recommended in these pages truly don’t pollute water. Selective treatment of greywater protects water bodies and the environment well beyond what a conventional sanitation system (including phytopurification systems) can or will probably ever do.

The essentials of a selective greywater treatment system

Greywater Characteristics

The following information can be used to draft an application for the authorisation of a selective greywater treatment system. Experience has shown that when openminded local officials are better informed, applicants can succeed in getting some form of authorisation to use greywater in their garden.

When compared to wastewater effluent from a conventional home using flush toilets, greywater contains almost no nitrogen and faecal contaminating bacteria. Greywater’s pollutant load mainly comes from soaps and detergents (from household cleaning products, for laundry, dishwashing and personal hygiene, etc.), greases and fats, and (decreasingly) phosphates from certain laundry products.

It contains practically no nitrogenous organic matter (proteins, urea), no medicinal residues (oestrogen, antibiotics, etc.) and no organic phosphorus of metabolic origin (contained in human excreta). Greywater treatment and discharge into the receiving milieu answers to criteria that are quite different from conventional sewage.

About detergents and phosphates, these only represent a threat to the environment after wastewater has been treated and discharged in a water body. It is true for all household products. When greywater is infiltrated into the ground, none of these products can reach the water table.There are no known cases of contamination from soaps, detergents or phosphates in underground drinking water reserves. Ground waters are only polluted by chemical fertilizers, livestock manure and pesticides. When using greywater to irrigate plants – the most rational solution –phosphates from detergents provide valuable phosphorus for plants in one’s garden, instead of being detrimental to the environment via a wastewater treatment plant.

When studying the environmental impact of irrigation using untreated greywater, laboratory testing has surprisingly shown that there is practically no difference between petro-chemical based detergents and those made of natural substances. Both are remarkably bound by all types of soil and decomposed by bacteria that spontaneously appear. All detergents, be they natural or synthetic, are only environmentally harmful when dumped into sewers to be treated in a sanitation plant.

The problem with phosphorus in wastewater

The problem with phosphorus in worldwide food production is underestimated by agricultural specialists. Sanitary engineering technicians, for their part, do not like talking about the phosphorus balance (nor the nitrogen balance) of their sewage systems. They simply argue that thanks to wastewater treatment plants’ phosphorus removal units, phosphorous is « eliminated » from wastewater.

For their part, environmentalists focus on phosphates from detergents, and easily forget that at least four-fifths of the phosphorus from ou urban wastewater comes from the black water component of sewage. Even if we altogether cease the use of phosphate-containing detergents, the consequences of discharging treated wastewater via wastewater treatment plants, namely the eutrophication of rivers and the invasion of algae on ocean beaches, remains unresolved.

What everyone seems to ignore is the fact that the treatment of urban waters, even when phosphorus removal units are used, still rejects a significant amount of phosphorus in the sea. This phosphorus, which is essential for world food production, is removed from the biosphere’s land base by wastewater treatment. In the long term, this can only lead to a serious imbalance that is currently unrevealed due to the massive input of phosphates from mines. These phosphates are used to make chemical fertilizers. Yet, world phosphate mines are being depleted. Production is expected to peak in about 30 years. After that, worldwide food production - whatever the agricultural techniques used – will inevitably decrease, unless a shift is made towards the concepts presented in these pages.

Before the advent of wastewater treatment, the amount of phosphorus on the biosphere’s continents remained relatively stable, as human and animal dejecta were not channeled to the oceans. This simple fact should get policymakers thinking, especially when they proclaim that all cities in the world should have centralized sanitation.

Our correspondents (mostly environmentalists) sometimes accuse us of being too radical. They deem it is an exaggeration to say that wastewater treatment is a major environmental problem. Ultimately though, given the significant indirect impacts of all-mains sewerage on climate change, even we of the EAUTARCIE group are far below the truth. Indeed, all-mains sewerage is a suicidal activity, on a world scale.

Untreated greywater

To a certain extent, considering greywater’s characteristics, greywater could be dispersed in the soil totally untreated, with a proper dispersion system such as an absorption pit or a dispersal drain. This has been known for years, yet some are suddenly « rediscovering » that greywater can be used for watering plants, but with incoherent and unnecessary sanitary restrictions.

Soaps and detergents contained in greywater are organic macromolecules composed of carbon, oxygen and hydrogen. When infiltrated into the ground, these electrically-dipolar molecules easily adsorb (stick) to soil particles. With the help of the soil microorganisms, they spontaneously decompose into water and carbon dioxide. The sulphur from household detergents (released as sulphates) and phosphorus (from phosphate-containing laundry detergents) precipitate in the soil as not very soluble salts, due to calcium ions that are almost always present there. As a result, these molecules have no chance of reaching the water table. Thus, even without treatment, sole greywater that is simply dispersed in the soil will have no impact, whatever types of detergents are used by the household (soaps, laundry and dishwashing detergents, etc.)

Thus, greywater can be used to irrigate garden plants without prior treatment. If however greywater is infiltrated into the ground by means of a dispersion system, a prior treatment becomes necessary to prevent the dispersion system’s blockage. Said treatment consists in conveying greywater through a standard septic tank, herein called a greywater tank, as described hereafter.

Greywater treatment

To prevent a dispersion system’s blockage, greywater must first be treated in an appropriate bioreactor, herein called an anaerobic batch reactor or more simply called a greywater tank. Based on laboratory experiments and field observations, 60 to 80% of greywater’s pollutant load (expressed as COD) is already decomposed after a stay of 18 to 30 days in the reactor. But most important is the fact that wastewater that has been so treated no longer clogs the dispersion system. After such treatment, even greywater that has a high DCO or BOD5 can be harmlessly infiltrated into the ground, in a soakaway or other dispersion system. On the other hand, discharging regulatory-compliant wastewater into a natural aquatic system is extremely harmful. Thus, the simple infiltration of such greywater directly into the soil constitutes a simple solution.

This is the solution for those who do not want to manage their wastewater. For others, in summer, we recommend the use of greywater for irrigation, taking care to thoroughly disperse it through the garden. Based on the experience of a number of users, this does not seem to harm plants, regardless of the products used in the household. However, an excessive use of chlorine bleach, hydrochloric acid or ammonia will have repercussions on your cultivated plants. Fortunately, direct and immediate observation of garden plants can help the household quickly make the necessary adjustments. The « all-to-the sewer » system is lacking such feedback.

Infiltration of greywater into soil has been laboratory-tested. This has shown that seepage of greywater through a few centimetres of earth is enough to make it clear and odourless, compliant with the strictest discharge standards. Thanks to the soil’s and soil organisms’ remarkable purifying capacity, the weak residual pollutant load at a batch reactor’s outlet decomposes quickly into water and carbon dioxide. In anaerobic conditions (i.e. in a batch reactor), a small part of the sulphates and sulphonates (from laundry) are reduced into sulphur ions, which gives the water a hydrogen-sulphur smell (i.e. of rotten eggs). Sulphate and phosphate ions precipitate in the soil with calcium ions, present in the vast majority of soils. In addition, due to intense anaerobic denitrification, water issuing from a greywater batch reactor contains less nitrogen than what can be measured in a home’s mains water supply. Hence, considering the nitrogen balance of this type of treatment, if such greywater were to reach the water table, it would in fact improve groundwater quality in the vast majority of cases, by diluting groundwaters that are already contaminated with nitrates. Thus, the environmental impact on groundwaters is either nil or positive!

In France, direct dispersion of pre-treated greywater into the ground is authorised. The discharge of such water into an absorption pit or a dispersal drain has no environmental impact, as long as the anaerobic batch reactor has not received faecal-containing black water, only greywater. This is perfectly possible when using dry toilets. In such an instance, the authority in charge of inspection and control of an installation’s compliance with the law will need only make sure that no flush toilet is present in the set-up. A simple monitoring of the nitrogen content in a set-up’s wastewater discharge can easily detect any attempted cover-up of a flush toilet’s presence.

In some cases, the infiltration of greywater in the ground is not indicated, even after prior treatment in a greywater tank. In such cases, the greywater’s treatment must be completed with two extra steps: a planted trench filter and a constructed wetland finishing treatment pond, as described in the subsequent chapter.

Greywater treatment using the TRAISELECT System

If you live in regions with karst, fractured rock, insufficient topsoil or in a floodplain, the implementation of the TRAISELECT system is the best solution. It is acceptable for those who insist on having a decorative pond in their garden. However, it does not comply with the 4th principle of EAUTARCIE’s ECOSAN (SAINECO) as it comes with significant water loss through evaporation.

The TRAISELECT system is a selective biological greywater treatment system that was developed at the Université de Mons in Belgium with the assistance of the Walloon Region. It is adapted specifically for sole greywater treatment. It is the first component of an approach that aims to treat grey wastewater distinctly from black wastewater. The term TRAISELECT is a neologism that abbreviates from the French words « TRAItement SÉLECTif » (selective treatment). The TRAISELECT system is not a commercially manufactured off-the-shelf system, but is an array of solutions accessible to all.

Greywater in the garden ?

As already pointed out, it is in fact simpler to irrigate plants with greywater without prior treatment, thus taking advantage of soil’s purifying capacity. When untreated, greywater does not smell bad: at worse, the smell is that of laundry. If one aims to recycle greywater for watering plants, like what is recommended for dry regions, it is best to avoid using the TRAISELECT System, as well as any form of plant-based purification (or phytopurification) system such as a waste stabilization pond.

For those who implement the TRAISELECT system, some consider watering their garden with water taken up from the system’s constructed wetland (instead of the greywater tank), thus avoiding the odour problem. Quality-wise, this is a valid option: the water is clear and odourless. Yet, as previously noted, it is precisely when you need to water your garden on hot summer days that water evaporation is already detrimental to maintaining the pond’s required water level: drawing water for irrigation just makes things worse. In truth, there won’t be enough water in the pond for irrigation, or else it will go dry.

For small gardens, the most inexpensive option is to store greywater in a watertight pond that is well exposed to sunlight and air. From there, water can be conveyed to the foot of plants using a flexible hose. Even if it is harmless, it is best not to wet plant leaves (although some of our correspondents assert that aphids have an aversion for soapy water). It is each’s responsibility to experiment how soaps and detergents affect their cultivated plants. So far, our correspondents have not reported any problems with this practice.

Is greywater harmful ?

Some of the warnings issued by scientists against using greywater in the garden are inspired by the hygienist ideology. They often confound greywater with combined sewage (containing toilet wastewater). Talking about « wastewater » at large and not specifically about « wastewater not containing faecal matter » only serves to confuse people and prompt the fear of using greywater. For them, as soon as you detect some faecal bacteria, « there is a danger ». This is a scientifically inconsistent approach. Such bacteria are present in large numbers everywhere in our environment, without causing any health problems. You're exposed to these bacteria just by putting your hand on a doorknob in public places or on grab bars in trains and trams for instance. You’ll even find some in your own plate when it hasn’t been steam-cleaned or disinfected. Instead of presenting a potential hazard, bacteria are necessary to stimulate and keep our immune system in good operating condition. Have the last rinse water for your salad analysed: faecal bacteria can be found there, yet you consume these salads regularly without catching an enteritis each time.

Nevertheless, if you fear such bacteria, simply don’t use your grey wastewater to water the garden. Simply send it in a greywater batch reactor (greywater tank) from which it flows into the soil with an appropriate dispersion system.

There are also recurrent concerns about the potentially harmful effects of household cleaning products that could end up in greywater. In this regard, let us point out two facts:

Some incriminate boron salts contained in certain detergents as harmful to plants and even animals. In face of such warnings, two things need to be taken in perspective:

Users who fear detrimental effects from particular additives in household cleaning products will normally seek out products that do not contain such additives. The role of authorities and consumer-defence organisations should be to identify potentially harmful products. Nowadays, they rather tend to instil fear of greywater utilisation. They should rather publish a list of those detergents containing products that are hazardous for irrigation. They could even create a « good for irrigation » label on the appropriate household products. In the mean time, you can follow the example of a number of our correspondents who insist on using « environmentally-friendly » products; thereafter, you no longer need to fear any harm being done to your plants.

So when you read about the environmental « harmfulness » of particular cleaning products, remember that the studies having led to such conclusions are based on the assumption that said products will end up in an aquatic milieu (which is indeed quite senstive to that type of pollution). The situation is altogether different when the same products are put into the ground. Greywater’s pollutant load is completely decomposed in the ground without ever reaching the water table. Initially bound to soil particles, the pollutant molecules prompt the growth of bacteria that deconstruct said molecules into water and carbon dioxide. It is also interesting to know that plants do not absorb such molecules.

Photo-purification of greywater

Photo-purification, or greywater purification using daylight, is an experimental technique that shows promising results (2013). Further studies are needed to narrow down the optimal conditions for water clarification by light. This simple and inexpensive method is based on observations by which soapy water exposed to daylight and air has been shown to spontaneously become clear. It eventually becomes crystal clear and can even meet the strictest discharge standards [5].

These standards are: chemical oxygen demand COD = 180mg O2/litre, biological oxygen demand after 5 days BOD5 = 70 mgO2/litre, total suspended solids TSS = 60 mg/litre.

Sanitary engineers do not seem interested in this elementary technique, which seems to present the inconvenience of costing little and being too scientifically simple. With sophisticated analyses, they can justify more complicated techniques based on nonsensical figures that are incomprehensible to most people. Photo-purification is simple enough for a child to experiment!

Greywater for flush toilets ?

I am often asked if it is possible to reuse bath water for flush toilets. This idea is obviously motivated by water conservation as the quantity of water used for personal hygiene can cover 80 to 90 % of flush toilet requirements, theoretically.

From our lab observations, bath water that is stored in a closed tank quickly alters under anaerobic conditions. As a result, the milieu becomes chemically reductive. In such conditions, sulphur from shampoos, soaps and detergents is quickly transformed into sulphur ions. These convey an odour of rotten eggs to the water that is difficult to remove without it having gone through a wetland finish treatment.

To prevent the advent of smells during storage, some German and Canadian manufacturers introduce chlorine in the storage tank. Their proposed systems are technically complex and relatively expensive, with a very long amortization period. I personally feel that this type of set-up is a sort of « make-believe » as to environmental protection. In a sustainable world where ecological sanitation precepts are applied, such as EAUTARCIE’s ECOSAN, the said « water savings » will have an altogether different significance. In individual households that adopt the use of dry toilets, greywater is not needed for flush toilets. Alternately, those who adopt ultra-low-flush toilets will not require much water. In both cases, all greywater will be recycled for irrigation or for a decorative artificial wetland.

Greywater treatment experiments

You might want to consider the following techniques or experiments prior to choosing your greywater treatment system.

Light exposure experiment

The following experiment can be done at home, or could be conducted at school under the supervision of a science teacher.

Mix 6.5 litres of bath or shower wastewater, 2.5 litres of laundry wastewater and 1 litre of dishwashing wastewater [6].

Take water samples from laundry or dishwashing machines at the start of each drain cycle. It is important that a representative sampling of all water cycles (soak, wash and rinse) be recovered for the experiment.

When this experiment is done in the perspective of your own household greywater treatment system, place the vessel containing your greywater sample somewhere in the garden, exposed to sunlight and air, as well as precipitation. It must be your a sample of your own greywater because greywater’s composition can vary from one household to another, depending on the products used and the water consumption patterns. When the experiment is carried out at school, you can collect the samples from several students and combine them to get an average sampling.

Over a period of days, observe what happens to the water that is exposed to light. Note the time it has taken to get clear water. (It should take between 10 days and 3 weeks.) The time it takes depends on the water’s original pollutant load, the daylight’s brightness (sunny versus overcast) and the ambient temperature during said period. The process is quicker in summer then in winter. To students: you can try to speed up the clarification process by adding half a glass (about 1 decilitre) of a solution containing one teaspoon (1-3 grams) of calcium chloride. It is a very cheap salt that is used for de-icing roads in winter.

For experiments at school, observations can obviously be refined with the science teacher’s assistance. The purpose of the experiment is to show that the separate collection of greywater from black water remarkably simplifies wastewater management. Greywater can be treated without cumbersome and expensive infrastructure, using a simple method that does not consume energy. On this subject, show students the video posted on the internet.

Before placing the vessel of greywater in the garden, draw about a decilitre in a transparent and colourless glass. To illustrate the speed of clarification using images, you should photograph the water sample glasses at regular time intervals.

For photographs, place the glass on a table in a dimly lit room, in front of a black background (a black fabric for example). Illuminate the glass laterally with intense light from a spotlight. In front of a black background, the glass will appear brighter the more the water is cloudy. This is due to the detergent-coated dirt particles that scatter incident light sideways, giving an impression of « luminescence ».

Shoot other photographs after 2, 4, 6 days, etc., using the same camera, with the same settings, with the same lighting. Compare pictures. The water contained in the glass should become growingly clearer. As a reference standard, take a picture of a glass containing perfectly clear water.

The science teacher can also solicit water testing from specialised laboratories, or even college or university labs for simple water tests. At least two analyses are needed: one on a water sample at the start of the experiment, and another after x days when the water is deemed clear. The cheapest analysis will be to measure turbidity [7], sufficient enough to illustrate the phenomenon.

An exciting experiment to do with students would be to establish the correlation (by graph) between the COD and the turbidity of soapy water. The measurement of turbidity is elementary and usually accessible to most students with relatively inexpensive measuring devices. However, measuring the COD is a bit more difficult and expensive. So, if a school has access to a turbidimeter, students can develop graphs showing the kinetics of the reduction in COD with respect to time simply by measuring turbidity. Calibration is usually done by the laboratory that provides the values of COD and turbidity for a series of soapy water samples.

There is indeed a simple relationship between the turbidity of a given aqueous medium and COD or BOD5. Once this correlation is established with a few measurements, one simple inexpensive measurement of turbidity can help estimate a sample’s COD and BOD5. Students will then make graphs showing the reduction of the pollution content (COD or BOD5, to one’s preference) of greywater over time.

With more extensive laboratory measurements, the resulting analyses can provide publishable data. The value of COD or BOD5 at the end of the experiment is to be compared to wastewater discharge standards (usually COD = 180 mgO2/l , BOD5 = 70 mgO2/l). The quantity of nitrogen in the sample will provide evidence that greywater contains very little compared to that of urban wastewater.

Students are then asked to calculate the nitrogen content of conventional domestic wastewater (containing combined black and greywater) expressed as mg of nitrate per litre of NO3-. Of interest:

Here’s a subsidiary question: from this 5.2 kg of N nitrogen released annually into sewers by one person, how many kg of NO3- can be formed after a complete purification treatment (by bio-oxidation) [8]?

It would be useful to inform students of the nitrogen balance in treated wastewater. Remember that nitrogen enters wastewater treatment plants in the form of precious organic compounds. Yet in the world’s wastewater treatment plants, most of said compounds are destroyed. In the most modern and efficient plants, about 70% of the compounds’ nitrogen is transformed into nitrates, then denitrified and thus converted into atmospheric nitrogen, whereas about 30% is not denitrified, and the corresponding nitrogen/nitrates exit the plant as treated wastewater and/or sewage sludge. The loss of nitrogenous organic matter for the biosphere is almost complete: either it has been destroyed, or the nitrogen that has been (or not) denitrified can no longer partake in the process of humus formation. A correct nitrogen balance perspective should not prioritize the quantity of nitrogen (N); it should give special weight to the place that nitrogenous organic compounds hold towards forming humus. Unfortunately, these compounds are wholly destroyed by conventional sanitation.

When you take into account the fact that the amount of mains water used to discharge flush toilet effluent accounts for 43 litres per day (in Belgium) to 70 litres per day (in the US), one can ask how much mains water is needed to adjust the nitrate content to 50 mg/l (the standard tolerated for drinking water) considering an initial average nitrate content of 25 mg/l in mains water? Students will learn that in areas serviced by mains sewerage, each flush toilet user pollutes an impressive amount of water.

Anaerobic denitrification experiment

Here is an alternate experiment that could be carried out, based on the previous light exposure experiment. It illustrates the process of anaerobic denitrification.

Take the mixture of greywater described in the previous experiment. Instead of exposing it directly to daylight, seal it in a plastic container and store it in the dark at room temperature. After a few days, you will observe a swelling of the container. When unscrewing the container’s cap, you will notice the discharge of a noxious gas. This is a result of the accumulated pressure of gases formed by the spontaneous fermentation of greywater. You should not thenceforth reseal the container, lest it burst as the fermentation process continues. This phenomenon already illustrates the difficulty of storing greywater for eventual reuse in flush toilets.

After 3 weeks of digestion, the contents of the container is poured into a vessel that is placed in the garden, as in the previous experiment. Here, water clarification takes place more quickly. After a relatively short time, the unpleasant odour disappears and water becomes crystal clear.

Again for comparison purposes, as in the light exposure experiment, photographs are taken at regular time intervals of a sample of said water in a glass, and similar analyses can be undertaken. However in this experiment, it is important to measure the amount of nitrate in the household’s mains water (that water from which was derived the greywater) as well as the amount of nitrate in the clarified water. Compare values and draw appropriate conclusions. This experiment shows how anaerobic denitrification works in a greywater tank.

Photo-purification of greywater in watertight basins

The results of the light exposure experiment can help one properly size the intended greywater treatment system. For example, let's say that the water in the vessel was observed to be clear after 2 weeks (14 days). Considering all possible variations under real-life conditions (fluctuating daily pollution content, ambient temperature, brightness of daylight, duration of seasonal daylight, etc.), it is best to assume an additional week, say a stay of 3 weeks (21 days) in the clarification basin.

If, for example, a family of 3 produces 250 litres of greywater per day, you need to provide a total volume calculated as such: 21 days x 250 litres = 5250 litres, or just over 5 m³, that could be taken up by three basins in series, measuring about 1750 litres each. (These are not stringent numbers, but only orders of magnitude.)

Thus 3 watertight basinscould be dug out in the garden. Their depth should not exceed 50 to 80 cm in the centre. These basins can be built using the same techniques recommended for the TRAISELECT system’s constructed wetland.

Greywater will flow into the first basin, where the water will be cloudy and smell like laundry. The overflow from this basin will flow into the second, where water will be partly clarified. This one will in turn overflow into the third basin, where water will be clear enough to use for watering the garden, washing the car, etc. The overflow design of the first two basins must prevent surface impurities from overflowing downstream into the next basin. This can be done using an 80 mm diameter pipe with an inverted elbow inlet that will draw the water below surface in the upstream basin. The third basin will not have an engineered overflow. Ideally, a more « natural » overflow system would be to install peat bricks on the perimeter of the pond’s watertight membrane such that they will « pump » excess water (by capillarity) and disperse it into the neighbouring soil.

In the second and third basins, you can put decorative plants, fountains and gargoyles, to each’s fancy. Water in all basins will have a very low surface tension due to the presence of residues from soaps and other household cleaning products. Thus, female mosquitoes that try to lay eggs on the water will drown, unable to float on the water.

The third basin will tend to have low water levels in the summer, due to evaporation. Most likely there will not be sufficient water for irrigation purposes at that time of the year.

The above methodology is still experimental for now. Further experiments are needed to fully document its characteristics.

Treatment of greywater in a constructed wetland

Another option for those who do not wish to actively deal with their wastewater is to install a wetland at the lowest point of their garden, into which their household’s greywater will be discharged. This is an experimental technique that still needs to be supported by further research.

For this technique, you need to remove a layer of about 15-20 cm of soil over an area of about 30m² (for a household of 4 people). The wetland area also depends on the permeability of the soil. On compact clay soil, you should plan for a much greater area. In and around the wetland, you need to plant moisture-loving trees that draw up much water through their roots and evaporate it in the air: trees such as willows, poplars or bamboo. Also plant water irises and other aquatic plants. After planting, fill the cavity with river-washed stones and pebbles and embed the greywater inlet pipe into the layer of stones. Unlike the previous watertight basin technique, this wetland must not be made impervious with any sort of membrane. The aim of this technique is to disperse water directly into the ground.

No water is usually visible in this system, thanks to the layer of river-washed stones that will hide the water while it is being absorbed by the soil. In summer, this wetland will appear as a lush garden, even in very dry weather.


The above discussions are part of a holistic approach to water management, a reflection on the philosophy behind the entire EAUTARCIE web site. This site takes a scientific and practical approach to EAUTARCIE’s version of sustainable ecological sanitation, called SAINECO.

To continue reading, go to chapter on implementing selective greywater treatment.


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