Implementing Selective Greywater Treatment

Greywater treatment

As discussed in the previous chapter, the most rational solution to treat and recycle greywater is to use it untreated for watering the garden. The present chapter addresses other alternatives to traditional wastewater purification. Remember that under EAUTARCIE’s ECOSAN (SAINECO), conventional wastewater purification is to be avoided.

It is curious to note how a wastewater treatment system’s environmental efficiency is inversely proportional to its cost. This is also true of selective greywater treatment.

The greywater batch reactor (or greywater tank)

The reactor will normally consist in a standard prefabricated two-compartment septic tank, having a capacity that is measured with respect to a household’s daily greywater output. The reactor’s overflow (second compartment) will be designed to prevent escape of the floating grease-laden bacterial scum layer, in the form of an inverted trapped overflow, or an adequate baffle or filter screen. The flow of fluids between the two compartments will ideally be situated at the bottom. Considering that grey wastewater treatment is simpler than that of combined wastewater, choose the simplest and least expensive tank. Even a one-compartment tank can do.

Greywater must be held in the reactor for 3 to 4 weeks. The mean daily output of greywater produced by the household must be estimated in order to properly size the tank. The estimated value must be multiplied by 18 to 30 to determine an ideal tank size [1]. A multiplication factor of 30 applies mainly to households with children, when laundry is done almost every day. A factor of 18 is sufficient for those who are particularly conscientious about not polluting water.

To obtain the required volume, you can also install a series of interconnected tanks.

To attain the desired tank volume, it’s also possible to interconnect several smaller single-compartment septic tanks.

How the greywater tank works

Greywater produced by the household is usually warm or hot, rarely cold. This characteristic insures spontaneous and rapid growth of bacteria that decompose grease, detergents and soap. Laboratory readings taken after 18 to 21 days have shown that about 80% of the pollutant load expressed in COD 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[2].

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. During a laboratory batch reactor test, it was shown that the sludge ferments in anaerobia, producing a bit of methane, carbon dioxide and atmospheric nitrogen. Anaerobic denitrification in fact explains the weak nitrogen content of treated water: even the nitrates contained in mains water supply [3] used by households disappear during their stay in a batch reactor pit [4].

About grease traps

Laws sometimes oblige (as in Belgium) the installation of a grease trap (also called a grease interceptor) at a home’s wastewater outlet. This is fine for restaurants and hotels, but in homes choosing selective greywater treatment, a grease trap is not only unnecessary, but is actually detrimental. The grease trap will unavoidably cool down the wastewater’s temperature before its exposure to the greywater tank’s bacteria. In addition, grease and fats form a scum layer ensuring an anaerobic environment in which a profusion of bacteria thrive, decomposing soaps and detergents. At the end of their life cycle, the bacteria settle at the bottom to form a layer of sludge. This sediment is in turn decomposed by mesophilic bacteria, producing a bit of methane. Thus, the quantity of sludge remains stationary, precluding the need for any maintenance or emptying of the tank.

In a greywater batch reactor, the pollutant load is more quickly decomposed than in a conventional septic tank. In fact, a household’s greywater output is almost always warm or hot. Thanks to these extra degrees, biological reactions within the reactor take place more quickly.

The bacterial scum layer closes up the water, enclosing a strict anaerobic milieu that decreases the rH₂ to below 14, making it chemically reductive.

For these two reasons, a grease trap located upstream from a batch reactor is unnecessary, even detrimental, because it cools down the greywater 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 greywater 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, unaffected by any possible mismanagement or misuse.

The absorption pit

To disperse the greywater reactor’s outflow, the most cost-effective solution is the absorption pit (or « soakaway »). In its simplest and least expensive version, this will consist in digging a 1- to 2-m3 cavity in the ground right next to the reactor, into which overflows the batch reactor’s « digested » greywater effluent. You must first check the subsoil’s permeance. Impervious subsoil such as clay requires a larger pit, or a dispersal drain. Conversely, sandy subsoil requires a smaller cavity. The absorption pit will be filled with mineral-based materials. These include construction scraps – bricks, tiles, broken (and washed) pieces of cement blocks –washed stones or non-crumbly rocks having a diameter between 40 and 60 mm and up to 80 mm. Broken bricks can be used in varying sizes, but to ensure creating larger voids, their set-up should be disordered. It is best to avoid crumbly materials such as cement mortar, which can erode into dust and thus clog up the system. The infill material is then covered up with a heavy plastic (PVC or EPDM) tarp. A 30-cm layer of soil is added on, and the purification system is ready for use.

The greywater reactor’s overflow conduit will be brought to the centre of the absorption pit, but placed right underneath the plastic tarp.

Once the flush toilet has been replaced by a dry toilet, a homeowner can in fact recycle an existing septic tank and use it as the greywater batch reactor. Once the absorption pit is added on, the cost ot the entire greywater treatment system is less than the cost of connecting the home to a sewerage network. This elementary system will have no negative impact on ground waters, while being totally reliable, efficient and inexpensive. On the other hand, greywater will have quite a negative environmental impact when it is dumped into sewers.

Unfortunately, in spite of these advantages, absorption pits are not permitted in Belgium [5]. You must therefore try to convince the regulating official responsible of approving your set-up of the soundness of your greywater treatment approach. Indeed, you must insist on the fact that in the absence of faecal water, your greywater contains less nitrogen than mains water supply. To support this assertion, you can even propose taking water samples at the batch reactor’s outflow. Whatever soap and detergent residues remain will be completely decomposed into water and carbon dioxide after dispersal in the ground. There is absolutely no pollution by nitrates.

Other dispersal systems

The dispersal drain, similar to a percolation trench (or « infiltration trench »), is widely used for wastewater infiltration in the ground. Its dimensions are usually regulated. The set-up can be less extensive than what regulations usually require, due to the lesser quantity of wastewater that needs to be dispersed (remember: no more toilet wastewater (black water)). But also consider that greywater is less likely to clog the system, especially after it’s been digested in a batch reactor.

Real-life example with the TRAISELECT System

A Belgian household of 3 produced 180 litres of greywater per day. Theoretically, they needed a batch reactor measuring 18 times 180 litres, that is 3240 litres or 3,5 m³. However, with a tank of 30 x 180 litres = 5400 litres, water purification would have been more efficient. In reality, prior to the removal of the flush toilet, this family disposed of a 2 m³ septic tank, which is less than the estimated requirement above. To avoid the additional expense to cover the difference, the family decided to stay with their existing tank only.

I therefore advised them not to overdo laundry cleaning, and to properly clean out their pots, pans and dishes prior to dishwashing. They decided to adopt the use of a kitchen scraper 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, a 2-m³ absorption pit was dug in the ground next to the batch reactor. It was filled with scrap material recovered from a construction site. T

The TRAISELECT System

Considering the conditions by which the TRAISELECT System is recommended (detailed in the preceding chapter), the idea behind the system is to purify « as best can be done ». In such cases [6], treated greywater is not dispersed in the ground, but is subjected to further treatment.

To summarize, after going through a primary treatment in the greywater batch reactor, the greywater effluent is then discharged into an aeration tank before going through a planted trench filter, from which it overflows into an artificial pond for what we call the finishing treatment pond.

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

The aeration tank

Water outflowing from a greywater batch reactor is discharged into an aeration tank. Experience has shown that even though an aeration tank improves the TRAISELECT system’s purification efficiency, it isn’t indispensable for the system to properly work. Thus, wastewater can be discharged directly from the greywater batch reactor into the absorption pit. On the other hand, the aeration tank’s usefulness lies in its contribution in the control of odours at the TRAISELECT system’s other components, namely the planted trench filter and the constructed wetland.

Water outflow from the greywater tank has the stink of rotten eggs, due to the presence of sulphur ions S₂^- that come from laundry detergents, whereby small quantities of sulphur (sulphates and sulphonates) are reduced. In contact with soil, these ions quickly precipitate in the form of metallic sulphides, insoluble in water, and thus the odour disappears. Treated greywater contains very little faecal contaminating bacteria, if any (in any case, nothing that can be a health risk if the water is used for irrigation). In most cases, the smell quickly dissipates in contact with air, although some people will be annoyed by it [7].

Thus, to reduce this smell, all the while improving treatment efficiency, you must place an aeration tank downstream from the greywater batch reactor (and upstream from the planted trench filter). This underground tank (often made of plastic) will have a capacity of about 30 to 50 litres per person. In it, you will place a bubble diffuser and an aquarium aerator, or even a pond aerator. In some instances, you may also need a sump-pump equipped with a float switch to discharge the water towards an elevated planted trench filter. In this case, the aeration tank doubles up as a pumping station.

The pumping station

In implementing the various TRAISELECT system components, it is best to try and take advantage of natural gravity. Yet, a pumping station is sometimes necessary when site topography is not conducive to a natural gravitational flow between the anaerobic reactor and the planted trench filter. It will also become necessary when one chooses to build an elevated soilbox filter instead of a trench to maintain a natural gravitational flow towards the constructed wetland. In such circumstances, a small 100-litre pumping station is placed downstream from the anaerobic batch reactor.

The greywater batch reactor being necessarily underground, a submersible pump is needed in the aeration tank to bring the water up into the planted trench filter or the elevated soilbox filter. A float switch will activate the pump when the water reaches a pre-determined level.

Nevertheless, taking advantage of natural gravity will not only eliminate the need for such a pumping station, but it will also reduce energy and equipment maintenance costs. It also has the advantage of avoiding that the wetland system receives a sudden cyclic influx of smelly pre-treated water: by gravitational flow, such water will slowly seep through the trench filter to the wetland.

The planted trench filter

Water issuing from the aeration tank will discharge or be pumped into a planted trench filter (or a soilbox filter) to be in turn discharged into an artificial wetland. This is a sort of greywater filtering system, a simple trench in the ground with a gravel or pebble drainage substrate for plants (without soil), and an impervious bottom lining. It is designed to allow greywater to filter through it while plants grow on top of it. In the trench's substrate, the plant roots fill in the voids to form a sort of filtration sieve. It’s important to point out that this is not a purification system using plants. As greywater issuing from an anaerobic batch reactor is nitrogen-free, plants don’t absorb elements from the water. Their role is simply limited to physical filtering of suspended particles in greywater [8].

The trench filter [9] is designed to allow greywater to filter through it while plants grow on top of it. The trench should be sized at about 1 m² per person, (or about 80 cm wide x 1,25 m long (per person)) by 40 cm deep [10]. The trench has to be lined with a thick plastic (PVC or EPDM) 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 [11], and smaller gravel or pebbles in the top layers. 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.

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 greywater’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 trench filter’s overflow

The filter’s overflow is to be located at the trench’s low point. The simplest solution consists in placing it at about 15 cm above the trench’s bottom. You can also implement a more complex scheme that regulates the water level within the trench. In such a case, the trench’s overflow must go through an upturned pipe elbow. As can be seen on the schematic, by pivoting this elbow, you can adjust the trench’s water level. This option is important when the system is not in operation for extensive periods. The water contained in the trench will help the plants survive extended periods of non-use. The trench’s water outlet shaft made up of a gravel separator screen must be protected from freezing.

The constructed wetland

The water that comes out of the planted trench filter is than discharged into a decorative artificial wetland for finishing treatment. This is a watertight pond, i.e. having an impervious bottom lining, with decorative plants. In this pond, daylight is the primary bio-filtration catalyst, by provoking coagulation and sedimentation of whatever residual pollutants may still be present. It is not a phytopurification system. Plants are only for highlight [12].

The constructed wetland is an artificial decorative pond having a minimum volume of 3 m³[13]. Calculate an area of about 1.5 to 2 m² per person [10]. Be careful not to design your wetland too big. The household's greywater production may not be able to compensate the water loss through evaporation. In summer, the wetland's water level could level off dangerously low.

Another word of caution: do not compensate the wetland's water evaporation by adding mains water, spring water, well water or river water. These waters are not pure enough for a greywater 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 (in temperate regions) 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 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.

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 peat clods or bricks that will play the role of an overflow all around the pond. There, the water will seep and disperse into the soil by capillarity.

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.

Reusing wetland water

In theory, water that is held in a TRAISELECT System’s 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 greywater 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 « erase » this information by means of turbulence techniques inspired by the works of Theodore Schwenk. When water cascades, the « wastewater information » seems to get « erased ». This can be highlighted by experiments in fractal crystallization.

Real-life experience with the TRAISELECT System

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. (Best keep your cat in the house). In the 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. After about twenty years of operation, the finishing treatment pond needs to emptied and cleaned.

Water quality readings from the TRAISELECT System

The following table shows the mean values of readings taken on 4 separate occasions over a month-long period at six separate TRAISELECT installations.

The TRAISELECT System IS NOT phytopurification

The TRAISELECT system is not phytopurification! In the planted trench filter as well as the constructed wetland, plants play no part in wastewater’s purification. Actual purification fundamentally occurs by anaerobic digestion in the greywater batch reactor, where 60 to 80% of the pollutant load (expressed as COD) is decomposed. In the trench, plant roots fill up the voids between the pebbles and stones, to filter out particles in suspension. In the finishing treatment pond, plants are present mainly for decorative purposes. The actual finishing treatment occurs thanks to daylight [14].

Once your remove flush toilets from the home, wastewater purification using plants (lagooning, waste stabilization ponds) becomes totally useless, and even environmentally harmful, especially in dry and desert regions where evaporative water loss can exceed 80%.

To continue reading, go to page on the problem with phytopurification

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