El tratamiento selectivo de las aguas grises

Saneamiento: sostenible o no sostenible?

Las soluciones más simples

Teniendo en cuenta que la lectura de la totalidad de este capítulo puede resultar pesada para algunos lectores, aquí hay un resumen de la solución más simple, que muestra cómo implementar el tratamiento selectivo de aguas grises, que no contienen residuos del inodoro (es decir, aguas negras).

En las zonas rurales y periurbanas (donde suele encontrar casas con jardines), el tratamiento selectivo del agua gris sigue siendo posible, de una manera muy eficiente y respetuosa con el medio ambiente, para aquellos que deseen seguir utilizando los inodoros convencionales (aquellos en los cuales se puede tirar de la cadena). Para empezar, es necesario instalar una toma separada para las aguas grises. El agua negra seguiría descargándose (provisionalmente) en las alcantarillas existentes. En áreas no atendidas por alcantarillado, un tanque de contención séptica (para ser vaciado periódicamente) se reservaría exclusivamente para el almacenamiento de aguas negras.

Por supuesto, aquellos que adoptan un inodoro seco adecuado, como un sanitario seco biocontrolado (o toilette à litière biomaitrisée en francés / biolitter toilet en inglés), tendrían transformaciones mínimas que hacer en el hogar. En ausencia de agua negra, sólo el agua gris pasaría por el sistema de drenaje de la casa, haciendo posible tratar el agua gris en el jardín de la casa.

En verano, las aguas grises se trasladarán a la zona de las plantas con fines de riego, utilizando una o más tuberías flexibles. Éstas se cubrirían con un mantillo de tierra para limitar la evaporación. Alternativamente, se puede considerar depositar las aguas grises en una cuenca abierta, agua que será aclarado por el proceso de fotodepuración, incluso si implica una mayor evaporación.

Durante el invierno o días lluviosos, las aguas grises seran descargadas en un simple tanque séptico, que se preparará para desbordar en un pozo de infiltración, mejor dicho drenaje de inflitración, un pozo de absorción. Esta solución es apta para aquellilos no quieren administrar sus aguas residuales.

Estos sistemas no requieren mantenimiento. Una vez instalados, uno puede « olvidarse de ellos ».

NB. Aquellos que están principalmente interesados en los aspectos prácticos del tratamiento selectivo de aguas grises pueden omitir las secciones filosóficas y pasar directamente a las técnicas.

Que es un saneamiento sostenible?

Durante una reunión celebrada en Verviers (Bélgica), en marzo de 2012, bajo los auspicios de la recién creada Rede d'Assainissement Durable, algunas personas protestaron que el « saneamiento sostenible » era una denominación inapropiada para la red, insistiendo que « el saneamiento no puede ser nada menos que sostenible ». La mayoría de los participantes consideraron la idea de un saneamiento « insostenible » como insensata porque, según ellos, el saneamiento sólo puede ser beneficioso para el medio ambiente.

En primer lugar, consideran que la desinfección de nuestras aguas residuales es un medio para restaurar materiales de mejor calidad (por ejemplo, aguas residuales tratadas y/o lodos de aguas residuales) de nuestros desechos. « ¿Cómo puede esto no ser sostenible? », afirmarán. Así, el « saneamiento » y la « depuración » de las aguas residuales se consideran sinónimos. En última instancia, esto significa que su objetivo principal es la despolución.

En segundo lugar, parecen considerar que la contaminación es inevitable, una fatalidad contra la cual somos impotentes, excepto si pudieramos remediar los desechos que producimos.

Estas son las reflexiones de lo que califico como la visión clásica, la opinión dominante compartida por las autoridades reguladoras y muchos ambientalistas duros.

Saneamiento= depuración?

Generalmente ha habido una relación lineal entre cómo la ciencia de la ingeniería sanitaria ha evolucionado y como las cualesquiera preocupaciones del momento necesitaban ser tratadas: un problema surge, encuentra una solución; otro problema viene, encontrar otra solución; y así sucesivamente y así sucesivamente. Cuando nos fijamos en la historia del saneamiento, descubrimos con asombro que todas las soluciones han dado lugar sistemáticamente a nuevos problemas, a su vez dando lugar a otras soluciones, dando lugar nuevamente a otros problemas.

El factor de confusión entre el saneamiento y la depuración es el resultado de dos siglos de evolución histórica. El saneamiento, como para « esterilizar » o « hacer limpio », se preocupaba por deshacernos de nuestro agua sucia, para mantener nuestras ciudades limpias. Eso es por lo que el saneamiento se asoció inicialmente con alcantarillas (aunque aún no se confunde con la purificación de aguas residuales). Incluso cuando se elimina el agua sucia, ésta permanece sucia, donde quiera que la retire: así, nuestros ríos están siendo contaminados. En lugar de ir a la raíz del problema con preguntas profundas [2], los técnicos han optado por abordar los síntomas, dando lugar a la depuración.

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.

In the late 20^th and early 21^st 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!

Saneamiento insostenible

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].

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:

• Nitrogen is the key element of water pollution, for once treated, it is transformed into polluting nitrate or is converted into atmospheric nitrogen N₂;
• 98 % of the nitrogen, 90% of the phosphorus and 99% of the bacteria contained in domestic wastewater come from flush toilets;
• Human dejecta only represent 1% of the total domestic wastewater volume (in developed countries). Yet significant environmental pollution is concentrated in this small 1%.

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 finish treatment, 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 4^th 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:

• The most harmful products (disinfectants, biocides) are used in flush toilets. Greywater does not contain such products, except where households make an excessive use of disinfectants.
• Other household cleaning products (for laundry, dishwashing, cleaning) were laboratory-tested on the basis that they would be combined with black water and discharged in a mains sewer, not used directly for irrigating plants, All tests have involved the discharge of these products in natural aquatic bodies, never in the ground. The purifying capacity of soil is remarkable, even for highly toxic substances.

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:

• The laboratory experiments by which such assertions are made have not been done in real-life field-tested conditions, such as a test culture irrigated only partially with greywater, followed with analyses of the plants themselves. Those assertions are extrapolated from tests that were done on hydroponic systems fed with water containing significantly more boron salts then what can be found in greywater.
• Such warnings are based on the third paradigm of conventional sanitary engineering, by which the sanitation plant technician has no control over the pollution generated. In a sustainable world, we should act before problems arise. In a sustainable world, we should act before problems arise. If it can be shown that specific products are harmful to plants or to human health, such products should be prohibited from being marketed and even manufactured. In the meantime, users should refrain from purchasing products containing boron salts.

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].

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

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

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.

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:

• A person annually produces an average 5.2 kg of N nitrogen with their dejecta (urine and faeces);
• A person consumes an average XXX litres of water per day, depending on the consumption patterns in their region (for example, 119 l/day in Belgium, 137 l/day in France, 150 l/day in the UK, 229 l/day in Canada, 262 l/day in the US)

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]?

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

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

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

Conclusion

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.

Para seguir leyendo, ir a la página Mejora del tratamiento selectivo de aguas grises.

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