La puesta en práctica del sistema PLUVALOR

Hay que evitar los tejados y canalones de cobre, de plomo y de aluminio. Estos metales (excepto el aluminio) son solubles en el medio ácido (la lluvia que cae sobre el tejado es siempre ácida), y son tóxicos. En caso de duda sobre una instalación existente va a ser mejor hacer un análisis del agua de la cisterna. Si el contenido excede los estándares de agua potable se puede considerar un uso seguro del agua de lluvia para fines no alimentarios, con un filtro primario ( de 35 a 10 micras). Sin embargo, para la producción de agua potable habrá que recurrir a la osmósis inversa (en lugar del sistema de microfiltración).

A pesar de su aspecto rústico las tablillas de madera (alerce, cedro u otros) no constituyen una buena solución para la recuperación del agua de lluvia. En sus rugosidades la madera retiene una cantidad de agua significativa. Tras una llovizna, el agua fluirá más rápido hacia la cisterna sobre un tejado de tejas que sobre un tejado de madera. El agua retenida en las asperezas de la madera terminará evaporándose y no llegará a la cisterna. Este tipo de pérdida puede estimarse alrededor del 5% de las precipitaciones totales. Si el tejado del que disponemos no es muy grande en comparación con las necesidades del hogar, esta cantidad está lejos de ser despreciable.

Pero en los tejados de madera el inconveniente principal se debe al deterioro de la calidad del agua recuperada. El color del agua se vuelve marrón o amarillento debido a la presencia de aceites esenciales aplicados sobre la madera. En esta agua habrá además, sobre todo al principio, una suspensión de materias orgánicas. Estas constituyen un verdadero medio de cultivo para las bacterias. Afortunadamente estas bacterias no son patológicas, pero pueden ser molestas. Habría que esperar entre 5 y 10 años para que el agua se volviese incolora y relativamente limpia.

A flat roof consisting of exposed concrete is suitable as long as it is not used as a lounging area. Flat roofs that are covered with other materials are more challenging, as the choice of materials usually installed is predominated with tar- or asphalt-based membranes (see prior comments).

It is best to forget green roofs, very inefficient for domestic water reuse (and that's not forgetting the long term waterproofing problems, and the extra loads requiring an appropriate, more expensive structure). On the one hand, a green roof reclaims little water for the cistern, only during major rainfalls. In addition, the water will be loaded with impurities: humic particles giving the water a brownish colour, and lots of bacteria. The filtering system(s) will quickly be saturated, requiring frequent replacement of filtering components, mainly the 10-micron filter for non-food water production and the 5-micron primary filter on the microfiltration system.

We often forget that flat roofs, be they green or not, preclude any possibility of attic space in a house design, therefore eliminating any long term hope of refitting such a space for added bedroom space, as the family grows.

Artificial slate roofs sometimes contain asbestos, but this does not constitute a major inconvenience for rainwater reutilization. Asbestos is only toxic by inhalation. As far as I know, there is no proof of toxicity through ingestion. But it is still best to avoid this type of roof for new housing. Those who fear even the ingestion of asbestos fibres (which will not be filtered by ceramic based microfiltration) will no doubt resort to reverse osmosis for their drinking water production.

Apparently, there exist many roofing products on the market where the primary material (or "substrate") is coated with a laminated synthetic compound during fabrication, e.g. pre-painted aluminium or pre-painted galvanized steel, imitation roof tiles with finish coats that aim to imitate traditional products, etc. The coatings can be rubberized spray-applied coatings, or synthetic enamels that are baked either chemically or with heat, etc. Some even integrate fine aggregates to give a texture to the product. There are many such products, each coming under their specific technology and manufacturing process. Unfortunately, we have no experience with the impact of these products on quality of rainwater running off their surfaces. There are generally no studies available on this. The question of aging of these coatings is also an issue. In doubt, for drinking water production, it is best to place an activated carbon filter upstream from the 5-micron filter. Moreover, when the primary roofing substrate is aluminium, it's best to periodically monitor rainwater quality in a cistern to check out the aluminium content with respect to standards.

Water running down the roof must be filtered or decanted. To protect the home's filtration equipment, a sand filter is the least expensive, and very efficient. Manufactured concrete filters are also sold on the market.

The cistern dimensions are calculated with respect to the roof area servicing the cistern. Do not size the cistern on the basis of the number of inhabitants, nor the expected household consumption. These criteria only come into play when the house disposes of an extra large roof.

Within measure of the site's potential, when designing a home, it is preferable to choose a one-story instead of a two-story house to increase the roof area available for rainwater harvesting. Obviously, all roof slopes should be connected to the cistern.

For each 100 m² (e.g. 10m x 10m) of roof area measured horizontally, calculate a cistern capacity of at least 15 m³. A 5x10m house will thus have a 7 500-litre (7 m2) cistern; a 10x20m home will have a 30 000-litre (30 m³) cistern. You can obviously round out the figures, preferably upwards.

When building the cistern, it will have two compartments, the first for sedimentation (or decanting) will represent 20% of the total volume, and the second for storage, representing 80%. Note: the cistern volume must be calculated up to the level of the overflow.

All roof water arrives in the cistern's sedimentation compartment. Its overflow spills into the water storage compartment. To prevent the passage of floating impurities, the sedimentation basin's overflow must be a trapped overflow, i.e. equipped with a protective skirt or an inverted elbow pipe.

Try to avoid plastic and metal cisterns. To properly neutralize rainwater's natural acidity, prefer concrete, standard masonry or limestone.

Nevertheless, considering the fact that plastic cisterns (PVC, polyethylene, polypropylene) are less expensive and more easily transportable than prefabricated concrete cisterns, I recently conducted an experiment in order to see if rainwater could be stored in a plastic cistern, after it has passed through a small concrete cistern to neutralize it. The experiment was conclusive. Rainwater harvested from an 80-m² roof was passed through a 1500-liter concrete sedimentation tank. This was sufficient to neutralize rainwater's natural acidity. From this tank's overflow, water can be conveyed to and stored in one or more plastic cisterns, placed underground.

You could even consider placing limestones or bags of limestone granules within plastic cisterns to neutralize the water. This last solution however makes maintenance a bit more difficult, i.e. to properly remove the sludge resulting from sedimentation when a cleaning is required.

To prevent unwanted underground water infiltration within the cistern, it is preferable to waterproof the outside of the tank with an appropriate membrane. The cistern's inside should be covered with a cement plaster or a polymer-modified waterproofing system containing cement. This is important to neutralize the water's acidity. Avoid synthetic coatings that do not. Watertight coatings composed of polymer-modified cement can neutralize rainwater. However, their long-term performance is unknown. The plastered finish must be smooth to prevent bacterial growth and make cistern maintenance easier. Generally, a cement plaster coating is quite satisfactory and adequately water tight. Some cisterns still exist in old fortresses, built of limestone masonry and lined with a limestone mortar and yet still function perfectly after many centuries.

Many potential users want to know if any and all concrete is appropriate for rainwater storage planned for human consumption. What comes to mind concerns toxic residues that may come from the use of «substitute fuels» (various wastes) that can be burned in cement factory furnaces. This is not a problem. Consider that Portland cement used for concrete is made at a temperature of 1700°C which no organic molecule, toxic or not, can resist. Any heavy metal components will be incorporated in the end product, but in silicate and oxide states that are practically insoluble in water.

When monitoring the water quality in cisterns, we have never come upon a heavy metal content greater than that which is prescribed in potable water standards. This is unfortunately not the case for some mains water supply in which a temporary exceedance is possible and even accepted by law.

For prefabricated cisterns, a cylindrical or oval shape is normally imposed by the manufacturer. Concrete cisterns of this type are quite adequate, but must sometimes be adapted to the rainwater system. This may involve correcting the inside surface roughness by the addition of a smooth thin cement mortar, or introducing some sort of pit or sump at the bottom, or even enlarging the manhole access, and eventually installing a ladder to the bottom. Also consider replacing the concrete cover plate by a lighter reinforced metal hatch.

Placing a single prefabricated cistern requires large heavy machinery (truck, crane). For an existing home, limited space and access will sometimes make this option impossible. There are three alternate options:

1. Building in situ a single concrete or masonry cistern; this can can be relatively expensive.
2. Interconnecting a series of smaller concrete prefab cisterns (sized to the desired overall volume).
3. Building in situ a smaller concrete or masonry sedimentation cistern, followed by a series of plastic storage tanks

Interconnected cisterns can more safely be linked together at the top, in the event of potential terrain or soil movement. Hence, the pipes interconnecting many cisterns must be flexible enough to absorb such movement. However, when you have more than one storage cistern, you need to adapt the configuration of your home's well pump set-up, as you may need to draw your water from more than one tank, meaning multiple piping and cistern equipment.

On the other hand, some propose interconnecting cisterns with piping located near their bottom. The advantage to this obvious: the cisterns become communicating vessels whereby their respective water levels automatically equalize. Thus, the well-pump dilemma is resolved. However, you become more vulnerable to water leakage that could happen surreptitiously at faulty pipe connections to the tank, or even total water loss resulting from a piping rupture in case of unexpected subsoil movement. In such events, repairs can be quite complicated, and expensive.

Finally, don't forget that the first tank (the sedimentation cistern) must have a trapped overflow to prevent the transfer of floating impurities to the storage cistern. The same type of overflow must be used between two or more storage cisterns, when piping is at the top.

Also consider ease of access and maintenance to the cistern.

The plumbing systems inside the house require careful attention. For mains water supply, a half-inch pipe diameter is fine. When pumps and pressure tanks come into play, the pipe diameter needs to be bigger, preferably one inch in diameter. This is to avoid pressure loss when more than one faucet is turned on. City water pressure is supported by a load of hundreds if not thousands of litres of water with a compressibility that easily absorbs pressure fluctuations. That is not the case for a residential well pump system.

Let us insist on the fact that contrary to the usual recommendations in the industry, a PLUVALOR rainwater harvesting and reuse system does not require doubling the piping network within the house, i.e. one for rainwater, and the other for city supply water. The system is perfectly adapted to existing plumbing set-ups, without transformations.

When choosing rainwater harvesting systems other than the PLUVALOR system, it is generally recommended to double the pipes in the house. The systems recommended by most merchants are expensive and less efficient. If a merchant is not familiar with the PLUVALOR system, one may end up buying expensive equipment of dubious utility.

When the roof area is limited and harvested water does not cover the household needs, one can consider a mixed-use plumbing set-up in the home. In this case, an interesting solution is to connect the water-closet(s) and exterior taps to city water, and to feed the rest of the house with reclaimed rainwater. This reduces the rainwater needs by 30 to 40%. This obviously only applies to those with a PLUVALOR system.

The basic thinking behind the PLUVALOR system is not to save city water by harvesting rainwater, but it is rather to provide better quality water (naturally fresh) and preserve one's health by the use of non-chlorinated and weakly mineralized water. High quality rainwater is thus reserved for more noble uses (drinking, cooking, personal hygiene) and lesser quality city water will be used to feed the water closet (if one insists on keeping one, against all environmental logic) and for garden irrigation.

In certain countries, even European, more than one water supply source is authorized to feed the home's plumbing system. The only restriction is that there be a check-valve placed right after the water meter. This protects the city water network from potential contamination from a non-potable source. From that point on, it is clear that a house can be serviced by two separate valves, one at the city water meter, and the other at the cistern. It is a simple, practical and efficient solution. In fact, the probability of introducing rainwater in the public water distribution network is very weak. It is rare that a well pump system can produce a higher water pressure than that of the city water.

From a technical point of view, this is a rational approach, and is certainly very practical. But many water supply and distribution companies will not accept it. Some companies accept the connection of one or the other source to the house's plumbing network, as long as this is done with a flexible and detachable pipe. In any case, it is absolutely necessary to include a check valve (non-return valve) right after the water meter.

Another technique would be to install a pipe from the water meter's outlet to the cistern: this could even be a simple flexible garden hose. Make absolutely sure however that the water meter's connection to this pipe is situated above the cistern's overflow level.

Before the cistern goes dry, one will put in a bit of city water. Thanks to the cistern's content, the city water introduced into the cistern will be somewhat improved in quality due to its dilution by better quality rainwater.

Important : in all cases, make sure that the cistern water cannot get into the city water network. A proper check-valve placed after the water meter is mandatory. To avoid all risks, I recommend separating the city water fed plumbing circuit from the cistern fed circuit.