Selective Grey Water Purification

An Ecological Sanitation Technique

Since EAUTARCIE transpires as an individual undertaking, 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 grey water 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 grey water, you must first acknowledge three analytical realities:

• Nitrogen is the key element of water pollution, for once treated, it is converted into polluting nitrate;
• 98 % of the nitrogen contained in domestic wastewater comes from flush toilet use;
• There are ten- to one-hundred-thousand less pathogenic bacteria in grey water than in black water.

Currently generated domestic wastewater is a mixture of grey water and black water, in compliance with the conventional view on sanitation, according to which both types of wastewater must be combined and treated together. According to ecological sanitation precepts, this «all to the sewer» logic is inadmissible, as much so as our consumer society «all to the bin» logic. The composition of each wastewater being quite different, their selective treatment presents numerous advantages. Hence, the selective treatment of grey water only makes sense when black water from flush toilets is not produced. When black water is removed from the equation, domestically produced grey water no longer contains environmentally harmful elements such are nitrates and pathogenic bacteria.

Unfortunately, in spite of its exceptional efficiency, selective grey water treatment is generally prohibited by law in those areas serviced by a centralized sewerage network. Current laws don’t recognize grey water’s specific characteristics. Sanitation technicians are also quite unfamiliar with these. To apply the same principles and legal prescriptions to grey water treatment leads to regrettable conflicts, errors and unnecessary costs.

In the current legal context, especially where centralized sewerage is concerned, the wish to use dry toilets and concomitantly treat one’s grey water leads to conflict with sanitation regulating authorities. Indeed, ecological sanitation is prohibited where centralized sewerage prevails. This is quite unfortunate, because those who compost their dry toilet effluent and treat their own grey water as per the TRAISELECT system truly don’t pollute water. Such a system protects water bodies and the environment well beyond what a conventional sanitation system (including wetland purification systems) can or will probably ever do.

Grey Wastewater Discharge

Grey Water Characteristics

When compared to wastewater effluent from a conventional home using flush toilets, grey water contains almost no nitrogen, and thousands to tens of thousands less faecal contaminating bacteria. Grey water’s pollutant load mainly comes from soaps, detergents (from household cleaning products, laundry, dishwashing and personal hygiene, etc.), grease and fat, and phosphates from certain laundry products.

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

Untreated Grey Water

To a certain extent, when considering its composition, grey water could even be infiltrated into the soil, untreated, by means of an adequate dispersion system such as an underground drainage network or an absorption pit. Many have taken up this appealing idea.

Soaps and detergents contained in grey water 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. Organic 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 little chance of reaching the water table. Thus, even without treatment, sole grey water that is simply infiltrated in the soil will have practically no impact, whatever the quality of detergents used by the household (soaps, laundry and dishwashing detergents, etc.)

In practice however, a prior treatment is necessary to prevent the dispersion system’s blockage, due to grey water’s grease content. This is the probable reason that direct dispersion into the ground has apparently been abandoned.

Treated Grey Water

Thus, to prevent a system’s blockage, grey water must first be treated in an appropriate bioreactor, herein called an anaerobic batch reactor. Based on laboratory experiments and field observations, 60 to 80% of grey water’s pollutant load (expressed as COD) is already decomposed after an average stay of 18 days in the reactor. The wastewater that has been so treated no longer clogs a dispersion system. Even the simple infiltration of such grey water directly into the soil constitutes a simple solution.

Infiltration of grey water into soil has been laboratory-tested. This has shown that seepage of grey water through a few centimetres of earth is enough to make it clear and odourless. Thanks to the soil’s and soil organisms’ remarkable purifying capacity, the weak residual pollutant load at a batch reactor’s outlet decomposes all the quicker, into water and carbon dioxide. In anaerobic conditions, a small part of the sulphates and sulphonates 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 grey water batch reactor contains less nitrogen than what can be measured in a home’s city water supply. Hence, if such grey water were to reach the water table, it would in fact improve ground water quality in the vast majority of cases. Thus, the environmental impact on ground waters is nil.

In France, direct dispersion of pre-treated grey water into the ground is authorized. 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, but only grey water. 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, in flood plains or when the soil is comprised of fractured rock, infiltration of grey water in the ground is not recommended, even after prior treatment. In such cases, the grey water’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.

The TRAISELECT System

The TRAISELECT system is a selective biological grey water purification system that was developed at the Université de Mons in Belgium, financed by the Walloon Region. It is adapted specifically for sole grey water treatment. It is the first component of an approach that aims to treat grey wastewater distinctly from black wastewater, in a selective manner. 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. It differs from other grey wastewater disposal techniques in its holistic approach to water management. It seeks to ensure properly functioning ecosystems, sustainable agriculture and the continued replenishment of water tables.

It comes in two versions. The simplest solution is called the Basic TRAISELECT System, and involves simple infiltration of soapy grey water directly in the soil after it’s prior treatment in a bioreactor called a grey water batch reactor. The Complete TRAISELECT System is required where soil conditions have inadequate filtering capacities, whereby infiltration in soil is replaced by two more stages: filtering in a planted trench filter and finishing in an artificial wetland. An optional aeration pit can also be placed at the batch reactor’s outlet.

Selective grey water treatment only makes sense when no sewage is produced (e.g. black water from WC’s). Thus, the TRAISELECT system is normally recommended alongside the use of dry toilets and the subsequent composting of the toilet effluent. As a result, only grey water actually needs to be treated.

The TRAISELECT system is 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 sustainable ecological sanitation.

To continue reading, go to chapter on implementing the TRAISELECT system.

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