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preamble
Attempts to make ideal, enclosed artificial swimming environments have for many years been hampered by the "double edged swords" of systems intended to keep the water clean. On this page, I outline the natural and inoffensive means that I have used to sanitise the water in a private outdoor swimming pool in the south of Spain.
The motivations for this work are my loves of swimming and natural form, and strong dislikes of rectilinear architecture and chlorine. My work has been facilitated by encountering the work of the German biologist Kaethe Seidel, who first applied reedbeds to water cleaning in the mid 20th century, in the face of less than indifference from a chauvinistic establishment. Kaethe Seidel's work is difficult to find but is described at that link on the web site of geobiologist Elisabet Sahtouris, herself revealed in an interview at http://www.scottlondon.com/insight/scripts/sahtouris.html
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living machine?
There are those who would scoff at the need to provide sanitisation for any body of water they are to swim in, but not everyone has a robust immune system that can resist the invasion of hoards of almost any micro-organism shed by previous "users" of the water. For all of us without the ultimately robust constitution, unwilling to endure either chemical or concerted microbiological assault, here is the human-friendly living swimming place.
The El Valero pool combines elements of both a biological system and a technical one. The steel and concrete "hard" engineering is dealt with on the The El Valero pool technology page. This section deals with the living bits, though there is some overlap.
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the contamination process
The El Valero pool is in the open air and surrounded by overhanging trees and shrubs; bird droppings fall in it and it is used by pets and wild animals for bathing and drinking; large quantities of soil dust and organic debris blow into it; children and pets in particular repeatedly try to bring large amounts of contaminants into it on their feet and bodies. Each of these conditions is a potential source of pathogenic organisms as well as the body fluids of humans swimming in it.
The infectiousness of organisms ingested by humans in particles is far greater than that of any isolated micro-organism. The particle environment protects the organisms, is often nutritious to them, and allows them to breed in close proximity. And as most of the natural pathogenic agents in the water will be in visible particles, getting such particles out of the swimming environment quickly is an important part of the El Valero pool water hygiene system. In static, unbleached bodies of water, particle removal can happen by precipitation and consumption by other organisms. Without help, neither of these methods is sufficient for a small, intensively used pool. However, the water most likely to be contaminated can be selectively removed and replaced with treated water. The faster the circulation and isolation, the lower the concentrations of organisms and the safer the pool water.
In the particular case of the El Valero pool, the source of original or refill water and loss-replacement water is in itself a potential source of contaminants. There is no drinking water available at the site, and the potential sources include a spring or river. The spring is frequently stood in by goats and other animals, above where the supply can be drawn off for use. The river has untreated sewage dumped in it from small villages and ham-producing factories, further up in the mountains. The pool is therefore only filled from the river when the snow-melt causes a flood which is low in contaminents and, usefully, minerals.
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algae
Another major cause of contamination to pool water are algae, which, whilst mostly not themselves toxic to humans, can cause the water to develop an uninviting appearance, taste and odour, as well as causing unhygienic, anaerobic conditions as they decay. Algae are a bit like weeds in a garden. Unless you remove them fanatically, perhaps using herbicides, you will have some of them around, but hopefully not sufficient to bother you. Much of the water treatment system at El Valero also controls algae, and the result for most of the time is that one can see clearly to the bottom of 2.4 metres of water, though it may have a green tinge to it in spring when algae growth is most rapid. Because sufficient concentrations of chlorine kill plants as well as animals, algae are not a problem in highly dosed pools. In private pools where lower concentrations of chlorine are used, algae can become a problem when nutrient levels in the water are high, incurring the use of other controls.
Barley straw was in the news during 2003 as a useful, natural means of controlling algae in natural lakes and garden ponds. We are also trying it out at El Valero, with a wrapped bale in the pond. Its efficacy is dependant upon its decay (ie. fermentation) in highly aerobic conditions. The substances which are produced are also known to be harmful to animals and humans in higher concentrations, so it is important not to exceed appropriate usage. On a short term basis, it does not seem to make much improvement in a system already using nutrient stripping, rapid turnover and biologically active filtration.
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zoning
Whilst it is desirable to restrict the contamination of the main pool water body caused by material carried in on human and animal feet, many people have an innate dislike of artificial barriers which can also mar the aesthetics of the bathing environment. One of the ways I have addressed this problem in the El Valero pool is to make the skimmer-overflow water flow in a wide, shallow, slightly puddling stream around the edge of the half of the pool bordering onto the garden environment. Thus, animals and children get their feet washed by spent water when processing between the garden and pool, and do not carry so much over, into the pool. This feature is far from an encumbrance and pleases most users. The famous parrot actually bathes regularly in the shallow stream.
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covering
A well used pool cover will greatly reduce the cleaning required and contamination of a pool, but once again, there are those that strongly object to this option on the grounds of aesthetics or interference with their spontaneity of use of the pool. Some, needless to say, are likely to retract the cover and then not redeploy it till long after use.
Nature only covers water with ice in winter or vegetation, so a cover may be a controversial subject. As my patron at El Valero strongly objected to the use of a cover, it was necessary to develop other features to maintain the quality of water desired, for example the automatic vacuum cleaner.
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skimming
A major part of isolation of undesired micro-organisms in the El Valero pool water is their expulsion without disturbance as near source as possible, so that their often loosely particulate homes have less time to disperse. To this end, the entire perimeter of the pool forms a skimmer that precludes skimmed material being reflected back into the pool. In windy conditions, when most debris enters, it is thrown clean over the rim by the wind not long after it enters, and often before wetting occurs, by wave action as well as flow on the tense surface skin of the water, whose only exit path is over the rim. In no wind, no swim, skimming conditions, the half life of floating debris on the surface is about 15 minutes. Any water disturbance causing waves greatly reduces this time. Since much animal contamination tends to be introduced near the rim and surface, the skimmer can also provide a short-cut out for this.
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displacement rather than mixing
The El Valero pool water is fed in via a bottom inlet designed to progressively reduce its velocity to zero so that the minimum possible mingling occurs between replacement water and that already in the pool. Less mixing of fresh and stale water means that when otherwise undisturbed, the water of the pool is more effectively turned around by bulk replacement with less actual pumping. This runs contradictory to the practice of injecting water in jets, a process requiring expenditure of additional energy. By injecting processed water at low velocity into the lowest part of the pool, stagnation in the lower part is reduced. In winter and spring, a stagnant thermocline can develop when the inlet water is warmer than that in the bottom of the pool. Siphoning a small proportion of the water for recirculation from the pool floor in competition with the flow over the skimmer weir reduces this. The bottom vacuum cleaner robot also counteracts thermocline development.
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rapid circulation
The refined El Valero waterwheel - see this section on the pool technology page - and gravity filter permit water replacement rates of a similar order to conventional practice in a privately used swimming pool, but using very little energy. The entire El Valero pool volume of 125,000 litres may be circulated twice per day using less than 5 units (kW.hours) of electricity. The efficient design of the waterwheel and filter permit this, and hence the use of only solar electricity. The water turn-over is reduced to once per day if the vacuum cleaner is used about half of the time. The efficiency of the waterwheel and the treatment systems is offset by their added capital expense, size and uniqueness, for the small swimming pool investor, content with artificial water sanitisation. The natural system described here is advantageous where a larger pool with long-term economics and environmental responsibility is considered.
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automatic vacuuming design
An automatic vacuum cleaner was added to the El Valero pool, when it became clear that too much time was required to keep the pool clean by routine vacuuming using a handheld tool connected to a vacuum port, which simply siphoned to the pond. The need arose largely as a result of the owner refusing to accept the pool cover. This meant considerable modifications to the project, including the addition of major additional components - see this section of the pool technology page for details. As well as greatly improving the appearance of the pool, the vacuum cleaner removes sediment before it can be stirred up again by the movement of pool users. This sediment is composed of dead algae, amphibian and bird faeces, and other material settled from the surface or introduced on users, so it is also a good idea to remove it for hygiene reasons. The clean floor and lack of food and camouflage thus created, plus the continual movement of the robot cleaner also tend to persuade all but the most determined frog that the pond is by far the best place to live.
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treatment area separation
A swimming pool with a separate treatment pond was chosen for this scheme as the El Valero household seemed agreed that they wanted a swimming pool that looked more like a swimming pool than a pond with a swimming area. In the latter, it is less practical to maintain a high standard of water clarity with plant and animal matter migrating more easily into a swimming area due to lack of physical separation.
The treatment pond is described mainly in the sections below, being the central living part of the system, and links from the "hard technology" section concerning it point to here.
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sewage versus pool treatment
Though biological treatment of swimming pool water, municipal effluent and mains water can all use aquatic plants and ponds, the treatment systems need to be tuned for very different conditions. Sewage treatment is a linear process, with what comes in being fairly uniformly concentrated nutrient material with a very low oxygen concentration; its flow rate is small compared to the amount of work that needs to be done on it to return it to a condition compatible with return to the wider environment where it is presumably hoped more sensitive and appropriate organisms will flourish. Pool water treatment and mains water treatment are more similar, though with pool water, the same water keeps coming back to the source. A reasonable size private pool is recirculated at a rate equivalent to the total water supply for a settlement of many hundreds of homes.
Swimming pool water conditioning means ideally, keeping near drinking-quality water in drinking condition. The treatment system must be able to survive on water with very low nutrient levels as well as creating these conditions from newly added water with much higher nutrient levels. In hot climates, the large rates of evaporation, especially from the plants, can cause mineral build-up problems.
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enter the pond
Having isolated the oldest and most likely to be unhygienic fractions of the pool water, they are passed to the conditioning pond. The skimmed water, with floating matter passes via a canal between the pool and the garden. The vacuum cleaner effluent enters via a vacuum settlement chamber followed by a pump. When the gravity sand filter is washed, the wash water, which is taken from the downstream end of the pond, may be returned with sediment to the input end of the pond to settle out and for the fish to pick over. Filter washing often repatriates small tadpoles and frogs hatched in the filter from dislodged eggs, and germinating seeds from reedbed flora.
The first vegetation and filter bed also acts as a surface barrier, trapping any floating objects and oils potentially noxious to fish, such as in sunscreens and cosmetics. The fish have elective access to this area via a tunnel and often come to check out what is in the water entering the pond, which arrives via small cascades.
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the plant treatment beds
The biological conditioning pond has an area of the order of half of that of the pool, and a depth of approx. 70 cm except in a sump region crossing its centre. The pool users wanted a treatment pond that was significantly smaller than the pool, so a system was constructed such that the plants in the El Valero pond could have more extensive root access to the flow of water through the pond, and the planted area did not need to be as extensive as if the plants were mainly in closed peripheral beds or floor beds.
All but the first bed acting as a surface barrage with a fish and water passage, are arranged as a labyrinth rather than a barrier. Bulk flow of water may circumnavigate a bed as well as passing through it. The idea behind this is that if the beds themselves become choked by root growth, the filtration process will continue externally in the labyrinths which are of the order of 40 cm in width and exposed to roots from beds on both sides.
The mechanical structure of the beds was suggested by their purpose which is normally to pass large volumes of relatively clean water. The beds are a horizontal flow type, with a width of only 30 - 40 cm. They are constructed on the solid floor of the pond using perforated brick laid with the perforations in the horizontal plane. They resemble a cavity wall with the cavity filled with 20mm rock chippings. The vegetation is planted in a thin 'nurse' layer of alluvial silt at the top of the bed which is covered with more chippings. The beds thus offer very little resistance to the passage of water and there is very little soil medium which is rapidly depleted of major nutrients by the growing plants which are then obliged to strip the water of its nutrients in order to survive.
The effectiveness of filtration by the roots is shown by the accumulation of flocculated matter on them in the labyrinths. The roots generate electrical charges to attract suspended material onto their hairs - the electrophoresis process.
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sedimentation
When floculated particles on the plant roots are disturbed or become too large to be supported mechanically or by electric charge, they fall to the bottom of the pond as sediment. The pond was drained and cleaned at the end of the first year, and more than 200 litres of sediment emptied via a 110 mm soil pipe and drain valve to the farm terrace below, growing sheep fodder beneath orange trees.
Sedimentation in the pond became far greater than initially expected when large amounts of sludge accumulated over winter were vacuumed from the pool, before the vacuum cleaner had its own separate easily drained sedimenter fitted. Later, when it became clear that filter wash water reclamation in the pond was possible, an ongoing extra sedimentation load was imposed upon the pond. Left without intervention, the pond floor becomes anaerobic and disturbance by fish displaces sulphurous smelling matter. Sedimentation in the intended sump region of the pond needed no further attention, as the steep floor of that region allowed the sludge to be drained off without emptying the pond completely. One of the options for dealing with the remainder of the pond is to annually drain and sweep it, with the associated problems of water loss, fish catching, and damaging the roots filling the labyrinth walkways. Another possibility under trial at the moment is to remove excess self-sown Juncus and other seedlings from other specie beds, and toss them weighted by the gravel they are growing in onto heavily silted floor areas. This is intended to have the effect of re-oxygenating and assisting with deflocculation and decomposition in the floor environment without extensive work.
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the pond plantings
Within the first year, the more vigorous of the species planted had sent fibrous roots out of the beds themselves and into the surrounding water up to 30 cm as expected. The roots in the labyrinths were a well developed mesh within the first year of planting.
Sewage treatment reed bed management schemes recommend that beds be cleaned and replanted to maintain a vigorous growth on a 4 yearly cycle. Whether the El Valero pond floor and beds will be maintained in this way remains to be established.
One pragmatic attitude I have seen justifiably advocated in order to select a flora and fauna to achieve, maintain and regulate a certain environment, is first look for the organisms that naturally thrive in the desired conditions. For instance, beautiful yellow-flowered Iris pseudacorus thrive in the El Valero pond: they are tolerant of the mineral build-up that occurs through evaporation and the highly mineralised water used for topping-up. Their floating seeds are busy establishing them anywhere on the pond they can get a grip.
Juncus species - mainly effusus, though it is not a particularly attractive plant, is robust and virtually unkillable - regarded as a weed by local farmers - and does the business of root zone oxygenation and nutrient stripping. It is best levelled to just above the ground every winter. In contrast, french and local Reedmace (Typha) began by easily reaching the greatest height - 2.2 m - of any of the plants in the beds in the first year, but are no longer thriving, having exhausted their nurse nutrient. The initially more robust french import has all but died out. These may be re-tried in the rich floor silt!
Other species on trial in the pond vary from a couple of species of Willow (Salix) to three species of Papyrus (Cyperus)! Pontederia cordata (pickerel weed) is, so far, thriving in a new bed laced with a nurse layer of sediment; it has beautiful blue spathes of flower that glow in twilight and are loved by insects. Water mint (Mentha aquatica, etc) is an essential part of any water treatment bed. In the El Valero pond, it is having to fight very hard against bigger, bullying plants for its root space, but it is hanging on with a little help. To reiterate the Pools Centrefold page, its roots secrete substances into the water which destroy many undesirable micro-organisms including parasitic worm eggs, in a matter of a few hours, without causing apparent harm to higher animals or the environment.
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the biologically active sand filter
The structure of the filter is described here, in the technology section. Biologically, the sand filter is the polish on the water from the pond, or for short periods, an alternative to the pond's filtration.
A side effect of the tubulent flow of the water though the matrix in the compact,"fast", pressurised filter of a conventional pool is that living organisms are effectively unable to survive on the matrix, so the filter simply functions as a mechanical barrier to relatively big or flocculated particles. Since conventional pool systems are intended to be sanitised by powerful oxidising - bleaching - agents rather than living organisms, this is of consequence only to the amount of power consumed, since the bleaching also prevents the establishment of useful phagocytic organisms.
If, however, non-toxic, oxygenated water is passed slowly through a filter bed, micro-organisms will establish themselves on the matrix; they will trap, feed upon, digest and thus reduce or remove pathogens and their nutrients from the water. The kind of water pressure that produces such a slow flow through a filter bed can easily be provided by gravity - typically about 2 metres head of water, at most, above the filter bed. The El Valero waterwheel is ideally matched to efficiently providing this low head, large flow, very highly oxygenated water supply.
And so the imperatives to reduce energy consumption and the use of poisons such as chlorine becomes a major reason for the existence of this so called eco folly.
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fauna
One of my greatest joys in being at the El Valero project is to watch, at different times of day, swallows and bats drinking in flight from the uncluttered surface of the swimming pool. Other birds and flying insects stop for a drink on the rim of the pool on hot, dry, calm days, when the thin film of skimming water is just what they want to keep cool.
The bulk water of both pool and pond have other residents. Small snails or insects such as water boatmen or pond skaters don't mind the vast open space of literally sweet water or apparently clean tiles. In the numbers these occur, they do not present an intrusion for most swimmers and perform useful functions - the insects consuming mosquito larvae and the snails consuming algae. Though clouds of tiny insects are to be seen swarming above the pool on warm evenings, few larvae get a chance to develop in the water itself, unlike the many that develop in vessels with no established ecosystem.
The frogs, besides serenading the bathers, feed on the mosquitos that try to colonise the pond, that would otherwise feed on us. Their tadpoles are like tiny vacuum cleaners in the pond. Few frogs bother with the pool itself, preferring the more friendly environment of the pond. Eggs tend to get skimmed or vacuumed out of the pool, and end up back in the pond when the filter is backwashed. A small turtle was often to be seen in the pool during a winter in which the bottom was left undisturbed; it too moved over to the pond once cleaning began.
Goldfish - carp - were originally placed in the pond, partly as a novelty and in the belief that they would be needed to control insects, and partly because the patron had an interest in raising fish for human consumption. Though they have never been fed, they remain and do not appear to cause problems, despite being bottom feeders, having bred several generations at the time of writing. A heavier load of fish would be expected to cause sanitary problems with the pool environment, particularly if they were fed. Fish farms using reedbeds for fish effluent treatment were studied as part of the research for this project. Specialist fish may have applications in other pool environments, but at El Valero, the daily and seasonal variation in water temperature and flow would have implications for their survival.
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