This chapter applies to homeowners as well as professionals. With the advice herein contained and the assistance of tradesmen (materials suppliers, plumbers, masons), anyone can implement a rainwater harvesting and recycling system compatible with their needs and capabilities.
After careful reading of the present documentation, a discussion is highly recommended with your architect, your contractor and your materials supplier.
To visualize the general schematic of a PLUVALOR system, click here.
The text within this page was first published on www.eautarcie.com
: in 2003
The original text has been adapted and translated in English by André Leguerrier and was first published on this page at www.eautarcie.org : 2009-10-01
Last update : 2015-10-17
The PLUVALOR rainwater recycling system cannot be improvised. The technical aspects developed below obviously apply to the PLUVALOR system. Those that resort to other systems must refer to their specific recommendations. Concerning PLUVALOR, more than 30 years of field experience has highlighted various problems that can easily be prevented, but that should be addressed when designing the home. Unfortunately, few companies are experienced with the PLUVALOR system. Consequently, design errors are numerous, which complicates the proper management and reuse of rainwater.
Remember that a properly conceived domestic rainwater cistern is none other than the artificial reproduction of a natural underground rock cavity in which water conserves well. For good water conservation, the water's temperature must remain constant (which is the case of an underground cistern) and the water must be neutralised within the cistern. This can be done by means of a concrete or cement-lined masonry cistern (without adding lime in the cement mixture). Neutralization instils a small quantity (about 50 mg/l) of dissolved mineral salts. Failing these, rainwater may quickly become putrid and smell bad. That is the case of plastic or metal tanks. (This also applies to tanks that are not buried underground.)
To visualize the general schematic of a PLUVALOR system, click here.
For a roof, the best materials are those that are less likely to alter the quality of rainwater runoff. This includes clay tiles (preferably enamelled, to avoid growth of moss), concrete tiles, natural slate, stainless steel, zinc  and glass. Synthetic slates are tolerable, as are corrugated galvanized steel or corrugated plastic or fibreglass.
For gutters and downspouts, proper materials include zinc, stainless steel, galvanized steel, PVC and other plastics, and glazed clay.
When you want to install a rainwater harvesting system with an existing roof and gutters that are inappropriate, water filtration must be adapted to the situation. This doesn't mean that you need to replace the faulty materials, but you should certainly consider it when it's time to replace an aged roof or failing gutters. In the meantime, depending on the type of undesirable substance your rainwater is exposed to, you need to choose appropriate filtration systems within the home's plumbing network. When there are significant quantities of solids and bacterial impurities, filters can quickly become clogged. This means more frequent replacement of filtering components, meaning that water thus obtained can become more expensive than mains water supply.
Copper and lead are quite inappropriate. Except for aluminium, these are very soluble in an acidic environment (rainwater that falls on the roof is always acidic) and are poisonous. In doubt as to an existing roof, it is better to have the cistern water analysed for toxic metal content. If you already have copper or aluminium gutters, have the water analysed in accordance. If the metal content exceeds potable water standards, you can still safely consider using rainwater for non-food purposes, with primary filtration (35-micron, and then 10-micron). However, for the production of one's drinking water, it is afterwards necessary to resort to reverse osmosis (instead of microfiltration).
In spite of their rustic appearance, wood shingles (hemlock, cedar or other) do not represent a good solution for rainwater harvesting. Comparatively, on a tile roof, water will runoff much quicker than on a shingle roof. By its inherent roughness, wood retains a certain quantity of water that is then lost to evaporation, never arriving into the cistern. This type of water loss is estimated at 5% of total precipitation. If the roof is already a bit small with regards to the household's water needs, this is not a negligible quantity.
Wood's inconvenience is mainly due to its negative impact on the harvested water's quality. The water colour becomes lightly brown or yellow due to the presence of essential oils diffused by the wood. This water will at the outset be laced with organic matter in suspension, a genuine breadbasket for bacteria. Fortunately, such bacteria are not pathogenic, but it can be annoying. One must wait 5 to 10 years before seeing the reclaimed rainwater clear and clean.
It is preferable to avoid lightweight synthetic materials like tar and bituminous membranes, which can convey an unpleasant taste and colour to water, and even release potentially carcinogenic aromatic hydrocarbons in a cistern. With such materials, the microfiltration system needed to produce drinking water must be supplemented with an activated carbon filtering system. And even for the production of safe water (not for drinking), a large capacity activated carbon filter should be included. On the other hand, many testimonials received suggest that asphalt roof shingles are appropriate for rooftop rainharvesting. This is supported by an American study , which suggests that the composite shingle configuration substantially reduces runoff of aromatic hydrocarbons, and even of heavy metals.
Over the years, flat roof bituminous and tar-based roofing will age, and odours will eventually diminish in the reclaimed water. Unfortunately for rainwater, this can take many years. On the other hand, this is usually a sure sign that the roofing material is approaching the end of its useful lifecycle and will soon need to be replaced.
I am led to understand that certain types of synthetic waterproofing are commonly used in commercial, institutional and industrial buildings, and appear to give good results in terms of rainwater quality. These include Polyvinyl Chloride (PVC), Ethylene Propylene Diene Monomer (EPDM – an artificial rubber), and thermoplastic polyolefin (TPO). These can be successfully adapted for housing : one of my correspondents is quite satisfied with his EPDM membrane. Yet, although water quality may be good on the short term, one may rightly wonder what happens during aging of such membranes, especially when they are exposed to the sun's UV. To our knowledge, this has not been effectively researched. Consequently, it is best to closely monitor the emergence of odours. Fortunately, microfiltration and reverse osmosis systems that produce drinking water usually include an activated carbon filter that eliminates odours if ever they crop up. For non-food uses, the odour may be unpleasant but remains harmless.
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 sedimentation filter on the microfiltration or reverse osmosis 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.
However, with reverse osmosis (RO) filtration for drinking water production, such precautions are unnecessary. In fact, reverse osmosis is currently cheaper than ceramic microfiltration. With RO, one can afford to be less attentive to the roofing materials and material coatings, even for gutters and downspouts as RO provides a « clean slate » to water for drinking and cooking. As for non-food purposes of water filtered by 10-micron filters, water remains safe for use, notwithstanding the nature of roof and gutter materials.
When falling leaves are expected on the roof, a removable protective screenshould be placed over the exposed gutters. To avoid intrusion of dead birds in the cistern, also place a screen in the gutter above each downpipe. Please note that these screens require frequent inspections, every two months, and even more frequently in the fall. They are easily blocked, which can provoke gutter overflow. Leaf interceptors are also useful on downspouts, but also need regular inspection as they tend to block frequently, provoking substantial water loss.
In addition to regular gutter and downpipe inspections, the gutters need to be cleaned twice a year, before and after winter. When designing a home and garden, you should consider easy access for ladders.
Water running down the roof must be filtered or decanted. To protect the home's filtration equipment, a sand filter is an inexpensive yet quite efficient solution. Manufactured concrete filters are also sold on the market.
You can also find sediment filters having a porosity of 100 microns, which are to be placed under every gutter downspout. They usually combine sediment filtration and leaf interception. Such filters usually come with a screening basket for easy maintenance, are relatively inexpensive and easy to use. These are very practical, but need to be inspected and cleaned out regularly.
In warmer countries with extended dry rainless periods and concurrent dust-carrying winds, an additional sedimentation pit could also be considered upstream from the main cistern and downstream from the main feed from the gutter downspouts. Such a pit will have a volume of about 200 to 300 litres at the bottom of which the coarser particles will settle. The pit will be covered to prevent intrusion of rodents and frogs, and a trapped overflow with inverted elbow will again be provided to prevent floating impurities in the pit from entering the main cistern's sedimentation compartment.
The first rainfall following an extended dry spell is liable to carry a certain quantity of dust into the cistern. This dust will settle as sediment. To delay this process, some recommend the introduction of a tilt type first flush diverter system at the downspout. It consists of a 1-meter segment of gutter connected to a float placed in a 200-litre container. When the container is empty, the float would be in its low position. The gutter segment is thus sloped towards the container, and at first rain, the water is diverted into the container. As the container fills, the float lifts, and so does the gutter which eventually tilts in the other direction, towards the cistern inlet.
A good handyman can build such a system. However, it is not essential. The problem is that the container must be emptied at the right moment: not too early, nor too late. When it's emptied systematically, water loss is not negligible. When one forgets to empty it, the system fails to do what it should, and the first rains, loaded with dust go straight to the cistern. I consider that a good decanting system is sufficient to avoid a too rapid accumulation of sediment. In addition, the system needs no surveillance, as would a tilt type diverter system.
Alternatively, better-designed first-flush water diversion systems are already available on the market, as seen on the Australian Rainharvesting web site.
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 16 000 litres (16 m³). A 5x10m house will thus have a 8 000-litre (8 m³) cistern; a 10x20m home will have a 32 000-litre (32 m³) cistern, i.e. 160 litres of capacity per square metre of roof area. You can obviously round out the figures, preferably upwards. Note: thirty years ago, we recommended 140 litres per square metre. Due to climate change and increasingly irregular rainfall, it has become necessary to increase this figure.
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.
When using prefabricated cisterns, consider at least two tanks, the first and smallest to be used for sedimentation, and the larger one for storage. These cisterns are to be interconnected with piping at the top. Again, the sedimentation cistern's overflow will be a trapped overflow to prevent the transfer of floating impurities.
Try to avoid plastic and metal cisterns. To properly neutralise rainwater's natural acidity, prefer concrete, standard masonry or limestone.
Nevertheless, considering the fact that plastic cisterns (PVC, polyethylene, polypropylene, polycarbonate, polyester, etc.) are less expensive and more easily transportable than concrete cisterns, I have 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 neutralise it. The experiment was conclusive. Rainwater harvested from an 80-m2 roof was passed through a 1500-litre concrete sedimentation tank. This was sufficient to neutralise rainwater's natural acidity. From this tank's overflow, water can be conveyed to and stored in one or more plastic cisterns, necessarily placed underground.
You could even consider placing limestones or bags of limestone granules within plastic cisterns to neutralise 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 (but no added lime!). This is important to neutralise the water's acidity. Avoid synthetic coatings that do not. Watertight coatings composed of polymer-modified cement can neutralise 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 mains water supply in which a temporary exceedance is possible and 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.
To ensure good water conservation for whole-house reuse, a cistern must absolutely be put underground to minimize temperature fluctuations. It can be placed under a terrace. A waterproofed room at basement level can also be used as a cistern, as long as this is not under part of the house , but rather under a garage or a building annex for example. For an existing construction, it can be placed in the yard, under a pergola for example (the pergola being optional!). The cistern can be covered with decorative landscaping, plants, stone pavers, etc. Note however that grass planted above a cistern will yellow much quicker during dry spells. (Be reminded that the roofs of all annexes like the garage, greenhouse, garden canopy, firewood shelter, etc. should also be connected to the cistern to maximize water harvesting capacity.)
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:
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. The simplest solution would be to install an automatic siphoning system between each cistern.
Finally, don't forget that the all the tanks, and most especially the sedimentation cistern, must have a trapped overflow to prevent the transfer of floating impurities to the storage cistern.
When designing a system, one should also consider that a cistern also requires appropriate equipment for proper use and maintenance.
The inlet pipe to the home's pump system must have a one-inch minimum diameter. The pipe within the cistern must be flexible and equipped with a floating strainer so as to prevent transfer of sediment from the bottom and floating impurities from the top of the storage tank.
Include a tube feed to the bubble diffuser for the aquarium aerator or a pond aerator(available in garden and home renovation stores). It consists of a flexible plastic tube of less than 8 mm diameter. An aquarium or pond aerator is optional, but it may come in useful if ever odours are a problem in the water. Even if it were not included, it would be wise to plan for its eventual addition, including the electrical outlet and a control switch with indicator lamp within the house. In order to properly inspect its operation, the bubble diffuser should be located at the bottom of the cistern, preferably just below the access hatch.
When using reverse osmosis to produce potable water, also include a return pipe to the cistern's sedimentation tank for the backwash of the reverse osmosis unit's membrane. (Note that a RO requires the use of a pressure tank in the well-pump system.)
Avoid connecting the cistern overflow to a sewer. Laws in many countries, including Belgium, France and the UK in fact forbid this. A cistern's overflow will rarely serve if the cistern is in continuous use. Thus, a simple absorption pit or underground dispersion system  will suffice to receive excess water. You can find manufactured overflows on the market. It is also important to cover the overflow's outlet with an appropriate screen to prevent intrusion of rodents and frogs into the cistern.
Also consider ease of access and maintenance to the cistern.
For maintenance purposes, it is useful to install cheap ceramic tile at the bottom, and only at the bottom. The floor should thus be lightly sloped towards a low point, ideally a a catch basin that will accommodate a submersible pump. Without such a set-up, the cleaning of a cistern can become a real chore.
The access hatch must be sufficiently wide for a stout person carrying a bucket. Ideally, have a ladder installed on the wall closest to it. The hatch must be light, but solid. A concrete cover is too heavy to manipulate. Prefer reinforced steel or aluminium, with a recessed handle or holes for easy opening of the cover with a hook.
For large cisterns (greater than 10 m³), it is recommended to install a sealed light fixture at the cistern's ceiling, controlled by a light switch (with an indicator lamp) that would be located within the house. This remains preferable for smaller set-ups too, to avoid lighting your way down the cistern with a portable lamp, potentially dangerous when your feet are in water. You must therefore consider providing electric power to feed the lamp fixtures, but also the aquarium aerator. The control switches for these should be within the house and should come equipped with indicator lamps.
The pump's purpose is to pressurize and inject water from the cistern in the house's plumbing system. In Belgium, this is called a hydrophor pump system. A standard home requires a pump of at least 350 Watts. Piston pumps are very good, but relatively expensive. In addition, they require a large pressure tank placed after the pump to insure a more stable operation: normally 200 litres. Less expensive centrifugal pumps also work fine: in their case, a 12- to 30-litre pressure tank suffices. However, their electric consumption is much cheaper with bigger tanks, and the cost pay-back for a 200-litre tank is within a few short years. A pressure tank, even a smaller one, is indispensable for those who resort to reverse osmosis. One can still find pump systems without pressure tanks. As soon as a faucet is opened somewhere in the house, the pump turns on. This is not the case of a system equipped with a pressure tank. With a pressure tank, the pump will function less often, use less power, and last much longer.
Note that we have had no experience with deep-well submersible pumps, but we feel that such pumps are not justified for pumping heights of less than 7 metres.
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 (too many) transformations.
If choosing rainwater harvesting systems not based on the PLUVALOR system, the vendor/manufacturer will generally recommend to double the pipes in the house. The systems recommended by most vendors 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.
Water supply and distribution companies recommend doubling the plumbing systems. The rainwater reuse recommended by these is limited to watering the garden and feeding the water closets. Some specialists will even accept the use of rainwater for the laundry, on the condition certain measures are taken. That is certainly not the objective of the PLUVALOR system, which is the result of a totally different philosophy. Users who adopt the PLUVALOR system express a way of thinking that goes beyond an institutionalized system of irresponsibility, as they become responsible managers and producers of their own water.
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 mineralised 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 flush toilet (if one insists on keeping one or has no other choice), water the garden or wash the car.
With a water level indicator in the cistern, you can easily determine when it's time to change from rainwater supply to city water supply (when there is less than 10 or 15% water left in the cistern), and vice versa. Thus, one installs a tank level gauge in the cistern connected to a water level indicator in the house. Even more efficient would be to install a water tank level sensor in the cistern (often called a pneumatic gauge or vacuum gauge / vacuum manometer) and the corresponding water level indicator displayed in the kitchen for example. Such water level sensors are usually sold by the fuel tank industry and are becoming increasingly available in the rainwater harvesting industry. Wireless models are also available. They cost between 50 and 100 €. Hi-tech lovers can even resort to ingenious systems proposed on the market, whereby the management of the city water/rainwater feed is fully automated. This solution remains dubious in light of the extra expense.
The mains water supply should be controlled by a solenoid valve. When the valve opens, the city water will flow into the cistern until a shut-off is triggered by a float switch (similar to the float in a toilet's flush tank) to limit the quantity of city water introduced into the cistern. It is unnecessary to fill the cistern with city water as is recommended in certain tank top-up systems on the market. A bit of city water is enough, until the next rainfall.
In certain countries, 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 mains 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 mains water.
From a technical point of view, this is a rational approach, and is certainly very practical. But many mains water providers will not authorize 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 mains 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 mains water-fed plumbing circuit from the cistern-fed circuit.
To continue reading, go to page on Cistern use and maintenance