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The TRAISELECT system:

Selective grey water treatment

Note: This page is subdivided into many paragraphs which each constitute a hypertext link, intended to those who seek the answer to a precise question. That is the reason why, when reading the paragraphs in sequence, one will come upon redundancies.

About the TRAISELECT system

The TRAISELECT system is a selective biological grey water purification system. 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).

Selective grey water treatment only makes sense when no sewage is produced (e.g. black water from flushable 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 not a purification system using plants. Selective grey water treatment is a technique for those who wish to go to the root of the problem, for those who really take the notion of sustainable water management very seriously.

The TRAISELECT system is not a commercially manufactured off-the-shelf system. Consequently, no corporation has exclusive rights to its installation, thus reducing its cost. 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.

Implementing the TRAISELECT System

An Ecological Sanitation System

Soapy water produced by bathing, laundry, dishwashing and general cleaning contains very little nitrogen compared to black water. Grey water treatment and discharge into the environment therefore answers to other criteria than those for conventional wastewater. Neither laws or sanitation engineers know about or recognize the characteristics of grey water. To apply the same sanitation guidelines and legal restrictions to grey water treatment leads to regrettable conflicts and errors as well as unnecessary expenditure.

In the present legal context, especially in areas serviced by centralized treatment plants, the use of dry toilets and the desire to treat one’s own grey water leads to conflicts with sanitation authorities. In fact, in sectors serviced by centralized sanitation, ecological sanitation is forbidden. That is a true shame, because those who compost their dry toilet effluent in their own yard and treat their grey water as per the TRAISELECT system effectively cease to pollute water. This system protects the environment and its water like no conventional sanitation system can or probably ever will (and that includes water purification with plants).

Grey Water Characteristics

Compared to water effluent from a home with WC’s, grey water contains almost no nitrogen and thousands to tens of thousands less fecal contaminating bacteria. Grey water’s pollutant load mainly comes from soaps, detergents (from household cleaning products, laundry, dishwashing and personal hygiene, etc.), grease 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

Nitrogen comes only from our excreta. The use of flushable WC’s is far from the concept of ecological sanitation or sustainable water management. In a world of sustainable development, WC’s and black water treatment should not exist.

Considering its composition, grey water could even be infiltrated in the soil, untreated, by means of an adequate dispersion system such as a drain or an absorption pit. In practice however, a prior treatment is necessary to prevent the dispersion system’s blockage.

This assertion is based on laboratory experiments and on field observations. Grease and detergents contain large organic molecules that are composed of carbon, oxygen and hydrogen. These electrically polarized molecules adsorb (stick) to soil particles and, with the help of bacteria, spontaneously decompose into water and carbon dioxide.

Sulphur (as sulphates and detergent sulphonates) and phosphorus (from phosphate-containing laundry detergent) 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. Actually, treatment of grey water before discharge is mostly necessary because of its grease content.

Comment:

When infiltrating treated water in the soil, the only real hazard for the water table is nitrogen ions, which can get through all geological formations in their mineral or inorganic states: nitrates, nitrites and ammonium. These ions are not formed during selective grey water treatment. However, conventional electromechanical black water treatment for individual installations (as imposed by law outside sewered neighbourhoods in Europe) [6] transforms excreta’s organic nitrogen into nitrate ions. One person’s annual dejecta produces an average 25 kg of nitrates in this manner. These nitrates divide up between treated water and sludge. In fine, both of these end up in the environment and pollute water. In this sense, the better the individual treatment system works, the more organic nitrogen is transformed in nitrates, the greater the environmental pollution. It’s important to know that untreated black water (before nitrogen is transformed into nitrates) that is correctly infiltrated in the soil– near plants’ root systems – would do less environmental harm than the conventionally imposed (and expensive) treatment systems. It’s obvious that black water infiltration is cheaper, and also inadequate. But it is ultimately less harmful than individual conventional electromechanical treatment systems as found in Europe. [6]

Nitrogen comes only from excreta. The use of flushable WC’s is far from the concept of ecological sanitation or sustainable water management. In a world of sustainable development, WC’s and black water treatment should not exist.

The Basic TRAISELECT System

It is curious to note how a water treatment system’s environmental efficiency is inversely proportional to its cost. This is also true of selective grey water treatment. The basic TRAISELECT system is the most efficient, simple and inexpensive solution.

As mentioned above, grey water can even be infiltrated in the soil, without prior treatment. Those that have attempted this solution have noted that their dispersion system was quickly clogged up. To avoid this, the grey water must go through a grey water batch reactor.

The Grey Water Batch Reactor

The reactor will normally consist in a standard prefabricated two-compartment septic tank. The flow of fluids between the two compartments is normally situated at about mid-height in the tank to prevent escape of the scum layer that will develop within the reactor from grease and bacteria. It’s also important to make sure that the tank’s outlet (second compartment) be equipped with an inverted trapped overflow (or an adequate baffle or filter screen), to prevent the escape of whatever floating impurities could have escaped from the tank’s first compartment.

After removal of a WC in the home and adoption of the dry toilet, the homeowner can in fact reuse an existing septic tank as a grey water batch reactor.

Batch Reactor Volume

Grey water must be held in the reactor for 2 to 3 weeks. The mean daily output of grey water produced by the household must be estimated in order to properly size the tank. The estimated value must be multiplied by 15 to 20 to determine an ideal tank size. To attain the desired tank volume, it’s also possible to interconnect several smaller single-compartment septic tanks.

A multiplication factor of 20 applies mainly to households with children, when laundry is done almost every day. A factor of 15 is sufficient for those who are particularly conscientious about their water pollution.

Batch Reactor Mode of Operation

Grey water produced by the household is usually warm or hot, rarely cold. This characteristic insures spontaneous and rapid growth of bacterial flora that decomposes grease, detergents and soap. Laboratory readings taken after 18 to 21 days have shown that about 80% of the pollutant load expressed in COD [7] is degraded. The reactor outflow is still cloudy, but its infiltration in the soil presents no danger of clogging. The COD readings taken at various installations showed between 120 and 250 mgO₂/l. The legal limit is 180 mgO₂/l.

Treated grey water infiltration has been simulated in lab tests. This has shown that seepage of water through a few centimetres of earth is sufficient to make the water clear and odourless. Its nitrogen content is much weaker than that of mains water supply. As a result, if this water were to reach the underground water table, in the vast majority of cases, instead of polluting, it would actually improve underground water quality. Soap residues and detergents are retained in the soil and progressively decomposed.

Grey water that comes out of the batch reactor smells like rotten eggs [8]. This comes from the reduction of sulphur (sulphates, sulphonates) contained in laundry detergent. Fortunately, this odour disappears in contact of air. 

To reduce odours, and to make grey water purification more efficient, you can consider installing an additional aeration tank downstream from the anaerobic batch reactor. This is usually an underground plastic tank having a 50 to 100 litre per person capacity, in which is placed an aquarium aerator with bubble diffuser. Such a tank remains optional, but it does improve the purification efficiency of the system.

As for the batch reactor, bacteria that degrade soap, detergents and grease eventually die out and settle in the form of sludge at the bottom of the reactor tank. Scientific monitoring of a real-life set-up has shown that after reaching a thickness of 10 cm, the layer of sludge no longer increases as a stationary equilibrium is reached. In a lab batch reactor test, it was shown that the sludge ferments in anaerobia, producing a bit of methane, carbon dioxide and gaseous nitrogen N₂. Anaerobic denitrification also explains the weak nitrogen content of treated water: even the nitrates contained in mains water supply [9] used by households disappear during their stay in a batch reactor pit [10].

Thus, after its passage through the anaerobic batch reactor, the effluent can simply be infiltrated back into the soil with an adequate dispersion system such as a dispersal drain or an absorption pit (also called a drainage well or «soakaway»).

Careful ! Grey water that has been treated in a batch reactor cannot be dispersed or infiltrated in a flood zone (i.e. water table close to ground surface), nor in a soil composed of fractured rock. In these cases, water purification must be completed with the help of a planted trench filter and the wetland filtering method.

Finally, treated grey water contains very little fecal contaminated bacteria: anyway too little to be a health risk when used for watering a yard or garden.

About Grease Traps

Law obliges the installation of a grease trap (also called a grease interceptor) at a home’s wastewater outlet. This involves added expense. For selective grey water treatment, a grease trap is not only unnecessary, but is actually harmful.

Grease that comes from dishes forms a bacteria scum layer that is quite useful for the water’s purification. This layer closes up the water and thus provides the strict anaerobic conditions required. An inverted trapped or baffled overflow at the batch reactor’s outlet is useful to prevent the passage of this layer.

For these two reasons, a grease trap located upstream form a batch reactor is unnecessary, even harmful, because it cools down the grey water temperature that is to be treated and eliminates the grease that is useful for the reactor’s operation. Thanks to the stationary equilibrium of the sludge content in a grey water batch reactor, no maintenance is needed. Once placed underground this treatment system can simply be put out of one’s mind.

It works without electrical power and without risk of improper use. Contrary to what happens in a conventional aerobic purification system, the introduction of biocides like chlorine bleach has very little effect on the bacterial flora. In effect, the batch reactor is a chemically reducing milieu. As a result, chlorine bleach spilled into the reactor does not even have time enough to disperse in the tank water that it is already reduced into harmless chloride ions (kitchen salt is a chloride for example). On the other hand, the discharge of such a biocide into a small-sized commercial sanitation plant destroys all bacterial flora and halts water’s purification for many days or weeks.

Real-life Example

A Belgian household of 3 produces 180 litres of grey water per day. Theoretically, they need a batch reactor measuring 15 times 180 litres, that is 2.700 litres or 2,7 m³ which can be rounded off at 3 m³. However, with a tank of 20 x 180 litres = 3.600 litres, water purification will be more efficient. In reality, prior to the removal of the WC, this family disposed of a 2.5 m³ septic tank, which is less than the estimated requirement above. To avoid the additional expense to cover this difference, the family decided to stay with their existing tank only.

I therefore advised them not to abuse laundry cleaning, and to properly clean out their pots, pans and dishes prior to dishwashing. They decided to adopt the proper tool for the job.

The water analyses that were subsequently done on the outflow of their reactor were conclusive: about 65 % of the pollutant load (expressed in COD = chemical oxygen demand) disappeared. This is largely sufficient to avoid clogging up the dispersion system downstream from the reactor.

To reduce costs, instead of placing a dispersal drain (i.e. underground French drains used to distribute wastewater effluent in soil), a 2 m³ hole was dug in the ground next to the batch reactor. It was filled with broken bricks, scrap material recovered from a construction site. They covered the bricks with water-pervious interlocking concrete pavers, and covered these with a layer of 30 to 40 cm of earth. That was all.

Fortunately, the soil in their yard was a sandy soil that obviously did not hinder water infiltration into the soil. However, with an impervious clay-type soil, a dispersal drain would have been necessary.

The Complete TRAISELECT System

In some cases, infiltration of treated grey water into the soil is impossible. At the same time, for the environment’s sake, this water can in no way be discharged directly in a receiving body of water (e.g. river), even if it complies – as is often the case – to discharge standards.

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. Many households choose this solution in order to dispose of a decorative wetland in their yard. I must insist on the fact that when grey water can technically be infiltrated in the ground, a complete system is unnecessary, and even somewhat inappropriate, because it takes up a lot of water by evaporation.

Here is another essential point. The TRAISELECT system is not a lagooning system. Nor is it a system that is designed to use plants for water purification. Plants are present in a trench filter system only for a light filtering of the water, and they are part of a TRAISELECT constructed wetland only for decorative purposes. In a lagooning system, plants are called upon to eliminate nitrogen from WC effluent.

In the TRAISELECT system, at the outset, grey water contains practically no nitrogen. The little nitrate ions contained in mains water supply is eliminated in the grey water batch reactor by the process of anaerobic bacterial denitrification. Thus, the nitrate content in the reactor’s outflow is close to zero.

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

Planted Trench Filter

Water discharged from a grey water batch reactor will go through a planted trench filter to be in turn discharged into an artificial wetland.

The trench filter is designed to allow grey water to filter through it while plants grow on top of it. It consists in an 80 cm wide and 40 cm deep trench, whose length is calculated at 50 cm per person. The trench has to be lined with a thick plastic membrane. An overflow is to be installed at a height that will maintain a minimum 15 cm of water at the bottom of the trench. This is a safety measure against drying up of the system in the event of an interruption in its use, for example when the occupants are absent on vacation.

The planted trench filter is layered with washed stones, pebbles, pea gravel, etc. in decreasing aggregate grading from the bottom up: 30-50 mm at the bottom, and smaller gravel or pebbles in the top layers [11].

Aquatic plants (reeds, water irises, phragmites, etc.) are planted in the top layer: their roots will eventually fill all voids in the pebble layer to form an efficient filter. Self sown plants (peppermint, dandelion, etc.) will also appear.

In the trench, the plant roots form a sort of filtration sieve. Before winter or in early spring, the plants are to be cut back and dumped in the compost bin. During winter, the bottom of the trench has little chance of freezing (at least in Western Europe) due to the grey water’s temperature (about 16°C) at the anaerobic reactor’s outlet. In colder regions, it is probable that a deeper trench will be necessary. Up to now, we have had no specific experience on this matter.

The grey water batch reactor being necessarily underground, a submersible pump is needed to bring the water up into the planted trench filter (in a flat terrain installation). This pump is to be installed in a 100 litre tank right next to the batch reactor’s outlet. A float switch will activate the pump when the water reaches a pre-determined level.

On a sloped terrain, it is best to try and take advantage of natural gravity to reduce energy and equipment maintenance costs. Thus the batch reactor’s overflow will discharge gravitationally into the planted trench filter, eliminating the need for the above-mentioned 100 litre tank. This also has the advantage of avoiding that the wetland system receive a sudden influx of pre-treated and smelly water: by gravity, such water will seep in slowly.

When a pump is required, it will usually function once to twice a day. At these times, a slight smell of rotten eggs (hydrogen sulphur) can be detected around the trench filter at a distance of 5 to 10 meters, for a few minutes time. It is possible to reduce this problem by placing an aquarium aerator with bubble diffuser in the submersible pump tank (when present in a flat terrain installation). However, experience has shown a relatively negligible improvement: this should therefore be considered as optional.

The water that comes out of the planted trench filter is than discharged into a decorative artificial wetland.

The Constructed Wetland

The constructed wetland is an artificial decorative pond having a minimum volume of 3 m³. Calculate an area of about 1 m² per person. Be careful not to design your wetland too big. The household’s grey water production may not be able to compensate the water loss through evaporation. In summer, the wetland’s water level could level off dangerously low. To visualize the wetland design, see the general schematic of the complete TRAISELECT system.

Another word of caution: do not compensate the wetland’s water evaporation by adding city water, spring water or natural ground water. These waters are not pure enough for a grey water wetland filtering system. A few days after having put in a few hundred litres of city water, a Belgian household’s wetland pond became a green slimy stinky mess. Initially clear and crystal clean, the pond water was invaded by filamentous algae. Therefore, remember that when you need to compensate evaporation loss, only rainwater will do. Unfortunately, dry spells tend to coincide with low water levels in the rainwater cistern. Thus, let me insist: avoid designing your wetland too big.

Winter, based on experiences in Belgium, presents no problem. If the wetland harbours fish, for their survival, it is best to place a small 20 to 40 watt pump to re-circulate the pond’s water via a fountain or other decorative element. The water pumped from the bottom will keep part of the surface clear of ice. Note that the planted trench filter keeps working even during winter.

The construction of a wetland can be entrusted to companies specialized in decorative ponds. The least expensive solution for a watertight pond basin is by installing a PVC membrane overtop a felt layer and a sand base. The membrane must be a bit larger than the pond. Its edges will be concealed a few centimetres below grade, covered with sod or peat clods that will play the role of an overflow all around the pond. There, the water will seep and disperse into the soil by capillarity.

The pond should be 80cm deep at its centre if fish are to survive winter. Around this centre, a plateau of about 30cm deep will harbour decorative aquatic plants of one’s choosing.

Here, purification is not due to the presence of plants. There is no nitrogen left to be eliminated. Water purification is mainly due to daylight, even in winter. Under light, the bacteria cluster together to form micelle. These in turn coagulate and settle at the water’s bottom where they are taken in charge by an aquatic fauna. Plants and aquatic mussels also help the filtration process. The pond becomes clear, its water without odour with a quality close to that of potable water. Its mineral composition will depend on that of the water used by the household. One can install small cascades, fountains, etc. To preserve the waters limpidity, it will be wise however to prevent undesirable visitors such as ducks and geese.

Real-life experience

Fears about the pond being a potential mosquito source are basically unfounded, except in the absence of fish and frogs. Frogs, newts and salamanders destroy mosquito larvae and in fact, there will be fewer mosquitoes than before the creation of a wetland.

The perimeter of the pond should be shallower in order to serve as a watering place for birds, which will find the place hospitable. At spring, dragonflies and colourful butterflies can be observed. Bees and small animals will also come to drink. The frogs’ typical chant will bring about a rustic atmosphere to the garden. A pond which can be seen from the home’s living room provides a pleasurable and calming backdrop.

Note that a wetland requires as much maintenance as a regular flower garden. If you don’t want your pond to disappear amidst encroaching vegetation, you must cull the wetland every fall, remove excess plants, cut back reeds and remove floating leaves. At the start of summer, in spite of the quasi-absence of nitrates, filamentous algae still tend to appear. These must be regularly removed with a skimmer,

Table of Water Quality Readings

The following table shows the mean values of readings taken on 4 separate occasions over a month-long period at six separate complete TRAISELECT installations. Samples of the effluent were taken at the wetland’s overflow.

Parameter	Units	Treated Grey Water	Discharge Standards
pH (acid-base)	-	7,9	No standard
Electric conductivity	µS/cm	463	No standard
COD (Chemical oxygen demand)	mgO2/litre	18	180
BOD5 (Biochemical oxygen demand)	mgO2/litre	2,5	70
SS (Solids in suspension)	mg/litre	4	60
Turbidity	Unit FNU	1,7	No standard
NK (Organic nitrogen compounds)	mgN/litre	1,2	No standard
NO3- (Nitrate)	mgN/litre	0,2	No standard
NH4+ (Ammonium)	mgN/litre	0,9	No standard
NO2- (Nitrite)	mgN/litre	0,01	No standard
PT (Total phosphorus)	mgP/litre	1,7	No standard
PO43- (Phosphate)	mgP/litre	1,4	No standard

Answers to FAQ's on Grey Water Reuse

Reuse of Wetland Water?

In theory, water that is held in a TRAISELECT system wetland could be used for numerous purposes, like swimming, or even reusing it by sending it back into a rainwater cistern in case of water shortage there. In reality, this is impractical. During summer, household grey water production does not even suffice to compensate water evaporation in a wetland where the water level is continually low. As this always happens concurrently to a water shortage, taking up water from the wetland pond threatens its own biological equilibrium.

On the other hand, even if there isn’t sufficient water, let’s not lose sight of the fact that even properly treated water remains treated water, with «treated» being inherent to its definition. One can obviously «modify» this information by the use of special but relatively expensive techniques, and with additional water loss to evaporation.

Grey Water Reuse for Irrigation?

The question crops up regularly on the potential reuse of treated grey water to irrigate the yard or garden.

I have experimented with this over a period of many years, and have never come to a clear conclusion on the matter.

Water issuing from a grey water batch reactor is sufficiently treated to be used in the garden. However, it presents an annoyance: the stink of rotten eggs due to laundry containing sulphur's reduction. This odour is not dangerous but it can attract problems with your neighbours. Fortunately, the odour dissipates quickly. The addition of an underground storage tank with its own aquarium aerator is insufficient to resolve the problem.

There is also the problem of quantity. One must evaluate the actual irrigation needs. In the vast majority of cases, the grey water produced by the household covers only a fraction of garden needs. One can obviously store treated grey water issuing from the batch reactor into an underground tank. But to have sufficient irrigation water, the tank needs to be very large. This is an expensive set-up for simple irrigation water.

To avoid odour problems, some consider watering their garden with water taken up from the wetland pond. Quality-wise, this is a valid option: the water is clear and odourless. Only, as noted above, it is precisely when you need to water your garden that water evaporation is a problem for the pond too: its required water level can hardly be maintained. In reality, there isn’t enough water for irrigation, or else the wetland system will go dry.

As already noted above, reuse of wetland effluent is not very practical either for irrigation.

Grey Water Reuse for WC’s?

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

From our lab observations, bath water stored in a closed tank quickly alters into 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 filtration. From that point, some consider reusing the wetland water for the WC’s. But, as already noted above, this is an impractical use, especially during summer when there is already too little water to maintain the wetland’s water level. It is therefore wishful thinking to want to reuse this properly treated water to other purpose. Ultimately, this is purely theoretical because, as already mentioned, selective grey water treatment only makes sense when no black water is produced, which means the elimination of the WC altogether.

[6] In Europe, anaerobic treatment followed by infiltration or dispersion into the soil is considered pollutant by sanitary engineers. I simply do not agree with this vision. In Belgium for example, the corporate sanitation lobbies have succeeded in imposing complicated electromechanical systems that involve a reactor with bacterial layers on different supports, with mechanical agitation and forced aeration with the use of a compressor. The overflow is periodically pumped in a sedimentation tank. The decanted water is again aerated and discharged in the environment.

[7] COD = chemical oxygen demand. This is a measure of the overall pollutant load in chemically oxidizable matter (biologically not all degradable).

[8] This is due to the presence of sulphur ions S^2-. In contact with soil, these ions precipitate rapidly into water-insoluble sulphur salts.

[9] In Europe, mains water supply can contain up to 50 mg/l of nitrates. The value in Belgium for example varies from 25 to 40 mg/l. Sometimes, the standards are exceeded. Water standards accept a certain nitrate content in potable water. Water analyses at the outlet of a grey water batch reactor servicing a family that consumes city water showed nitrate content around 0.1 mg/l, much less than that of the water consumed by the household. Grey water’s pollutant load contains nitrates in only exceptional circumstances. In this case, the main source of nitrate in a household is city water! In the batch reactor’s anaerobic environment, the «azobacteria» reduce nitrate into atmospheric nitrogen N². That is the reason nitrates contained in city water disappear at the reactor outlet. It is an analytical fact that sanitation engineers have great difficulty in admitting and especially in understanding. In electromechanical treatment systems of mixed wastewater, purification produces more nitrates instead of reducing them.

[10] The accumulation of sludge in conventional wastewater septic tanks comes in fact from the fibrous part of fecal matter, and especially from toilet paper. In a grey water reactor, you have neither of these.

[11] Caution: the picture shown is that of a soil box filter (a variant of the planted trench filter), which is a wooden box set aboveground. This less reasonable solution was imposed due to the fact that wetland pond in which this filter was to discharge was at a slightly higher elevation. When site topography permits, a trench in the soil is the better solution.

To continue reading, go to page on Purification with Plants

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