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A good rainwater cistern in none other that the artificial reconstitution of a subterranean rock cavity, in which water conserves well. Contrary to popular belief, water stored in a concrete or masonry cistern contains enough minerals to make it biocompatible.

The text within this page was first published on in 2003

The original text has been adapted and translated in English by André Leguerrier and was first published on this page 2009-09-22

Last update: 2017-04-23

Quality of Rainwater in a Cistern

Rainwater acidity

One must distinguish between water that falls from the sky and water that is stored in a proper cistern. If the former is almost always acidic and practically contains no minerals, the latter is neutral and lightly mineralised.

There are several misconceptions about rainwater quality that do not stand up to lab tests. Some say that calling rainwater « pure » is excessive, considering the atmospheric pollution that can be « washed » from the sky. We do not contend that rainwater is pollution-free. However, it is certainly much less polluted that the vast majority of water sources found in nature. This is an analytical fact. Let us not forget that rain and snow are the primary sources of freshwater on Earth. Its pollution will influence the quality of all other natural freshwater sources.

Thus, before entering the cistern, rainwater inevitably absorbs atmospheric pollution. The most spectacular effect of pollution is acidity.

Even without pollution, rain is naturally acidic due to always-present carbon dioxide (CO2). This natural acidity is amplified by the presence of nitrogen oxides (NOx) and sulphur dioxide (SO2). Such oxides are produced by the burning of fossil fuels (oil, coal), especially at high temperatures. When dissolved in water, these oxides become acids: nitrogen oxide becomes nitrous acid (HNO2) or nitric acid (HNO3) while sulphur dioxide produces sulphurous acid (H2SO3) that oxidizes in the air to become sulphuric acid (H2SO4).

Acid rain is a nuisance to coniferous forests. It also attacks limestone monuments. However, for domestic rainwater reuse, acidity constitutes an advantage.

Mineral content in cistern water

The acidic components of rainwater react to the alkaline components of the cistern's concrete or cement mortar, dissolving mineral salts. During this process, acidity disappears: water becomes neutral. The great majority of dissolved salts are composed of calcium bicarbonate [Ca(HCO3)2]. Nitrogen oxides become nitrate ions. Sulphur dioxide turns into sulphates. These ions have weak concentrations. I have never measured more than 9 milligrams per litre of nitrates in cistern water, the average being between 3 and 5 mg/l. Comparatively, in « legally » potable water, there may be as much as 50 mg/l. Hydrogen carbonates and sulphates are salts (i.e. ions) that are harmless in drinking water.

To resume, water that falls from the sky is acidic and contains very little mineral salts. Rainwater harvested in a concrete or limestone masonry cistern is neutral or slightly alkaline. (pH between 7.7 and 8.5) and is weakly mineralised. Its electrolyte content ranges between 50 and 80 mg/litre. Below 10 mg/l, water is too poor in electrolytes, and you would need to add some. Above 200 mg/l, it can overload the kidneys of someone suffering from renal failure. A healthy person can tolerate the consumption of water containing 600 mg/l of minerals. However, one does not know a priori if one’s kidney function is perfect or genetically predisposed to fail over time.

Comparatively, the best bottled mineral waters available on the Belgian marketplace for example can contain as little as 16 mg/l, and as much as 35 mg/l.

The following table shows the readings taken on 18 separate occasions on samples of the rainwater stored in the cisterns of seven PLUVALOR set-ups. The physicochemical characteristics of harvested rainwater in a concrete cistern are close to ideal.

Physicochemical Parameters of Cistern Water
(in PLUVALOR-type set-ups)
Parameters Units Min Max Average
Acidity alkalinity: pH - 6,31 8,01 7,23 6,5 - 9,5
Conductivity µS/cm 36 190 90 < 2100
Nitrates NO32 mgN/l 0,2 4,7 1,5 < 11,3
Ammonium NH4+ mgN/l 0,010 0,059 0,022 < 0,5
Chlorides Cl- mg/l 1,0 16,7 6,5 < 350
Sulphates SO42- mg/l < 8 < 8 < 8 < 250
Calcium Ca² mg/l 4,3 15,3 10,1 < 270
Magnesium Mg² mg/l 0,14 0,52 0,21 < 50
Zinc Zn² µg/l 50 1731 466 < 5000
Iron Fe² µg/l < 50 < 50 < 50 < 200
Cadmium Cd² µg/l < 10 < 10 < 10 < 50
Lead Pb² µg/l < 50 < 50 < 50 < 50

When commissioning a new cistern

When putting a new cistern into operation, the tank's concrete and mortar are initially too loaded with soluble alkalis. The water's pH, its hardness and its mineral salt content can be abnormally high which makes the reclaimed rainwater initially too alkaline. It so often happens that the first harvested rainwater in a brand new cistern has a pH close to 10! Don't worry, as this phenomenon is temporary. Nevertheless, such alkalinity, basically harmless in itself (except for drinking), makes the water quite unpleasant for personal hygiene. It dries and irritates the skin. Even so, the first reclaimed water must not be used for drinking or cooking. It is therefore preferable to evacuate such water with a submersible pump and wait for the cistern to refill. Many homebuilders use the first water harvest to finish up on some of the fit-up work in the home.

Below a pH of 10, harvested rainwater is suitable for all purposes except drinking. One must sometimes wait 3 to 6 months for the water's pH to go below 9, or better 8.5. From that point on, the microfiltration or reverse osmosis system can be put in operation for potable water production.

Potential contamination of cistern water

Besides the minerals that come from the cistern itself, various types of impurities can be washed into the cistern during rainfall. You can blame rain itself, which absorbs atmospheric particles of all sorts (salt, smoke, gas fumes, etc.). Rainwater runoff on the roof, in gutters and downspouts, and anywhere in the water conveyance circuit can alter the water's quality. This includes organic and mineral impurities of all sorts: bird droppings, dead leaves, dust, dead birds in gutters, dead rodents or frogs in the cistern, etc.

The choice of materials for the roof, gutters, chases and the cistern itself is important. The chemical reaction speed of roof materials with water is a fundamental factor. You must also take into account the duration of actual contact of materials with water runoff, and temperature. Actual contact is short as water only transits on the roof. Conversely, the duration of contact is very high within the cistern.

Organic compounds

The situation concerning trace organic compounds in water is somewhat complex. You need very sophisticated and expensive equipment to measure traces of such compounds (less than 1 µg/l). And again, you must know what you're looking for. Otherwise, for each analysis, you're looking at months if not years of research. For potable water, standards impose severe limits concerning pesticide residues. In Europe, the overall pesticide content must not exceed 1 µg/l. Yet, this limit is exceeded in many wells and rivers, and also in the water table. In such cases, water supply and distribution companies are supposed to use activated carbon filtration to reduce the pesticide content to approved limits. In reality, very few water distribution companies resort to activated carbon for filtration of water intended for the population. When one uses mains water, the health risks are higher than with rainwater treated as per the PLUVALOR system. And don't forget the harmful effects of chlorine.

Organic compounds in rainwater essentially come from atmospheric pollution and roofing materials. Wood shingles, for example, convey a light brown or yellow colour to the water, 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. Some roofing materials (plastic, tar, asphalt and other bituminous roofing membranes) convey an odour an unpleasant taste to rainwater. Bituminous membranes can obviously release toxic and aromatic hydrocarbons. Fortunately, our nose is sensitive to these very odoriferous compounds. We can detect their presence before their concentration becomes harmful – at least in external use, for personal hygiene. Nevertheless, taking a bath in water that smells like rubber is not very pleasant.

Activated carbon filtration removes all colour from water and holds back any smell or taste conveyed by roofing materials. Consequently, water filtered in this way for drinking is quite safe. However, filters will quickly become saturated with higher impurity levels.

Noxious substances can also come from smoke and gas emissions. Such compounds are sometimes toxic and can be carried by rainfall. Used externally, such water is practically harmless. For drinking and cooking, water must comply with potable drinking water standards: a simple microfiltration or reverse osmosis system will eliminate the problem as they totally remove such compounds.

On the issue of zinc

The (European) press occasionally points out a too high zinc content in rainwater. What immediately comes to mind are zinc gutters and roofs (at least in Europe). Yet, some of the studies invoked have been based on stormwater runoff collected from sidewalks, streets and even garden alleys. Zinc contamination of rainwater can be substantial on such surfaces. From a rainwater harvesting point of view, it is actually more important to measure stormwater runoff from zinc roofs, as was done in a 2002 Paris study [2].

Reference: M.C.Gromaire, G.Chebbo, A.Constant, « Incidence of zinc roofing on urban runoff pollutant loads. The case of Paris ». Water Science and Technology, Vol. 45, n°7, 2002, p.113-122. The authors assert that zinc and cadmium concentrations are so high that « they even threaten the Seine river ». In Table 2 of this study, we find that the highest measured zinc concentration in the roofwater was double (exactly 9855 µg/l) that of legally imposed potable water (5000 µg/l). As for cadmium, the highest measured value was less than 5 µg/l, exactly 10% of potable water limits (50 µg/l). Table 1 of the same study showed an average measured zinc concentration of 4874 µg/l from zinc roof stormwater runoff, a bit below the legal potable water limit. The average measured value for cadmium was 2,5 µg/l, about 5% of potable water limits.

The « zinc-contaminated stormwater » discharge into the Seine river is in fact diluted by runoff originating from roadways and other material surfaces. This appears clearly in the analysis of readings presented in Table 5 of the study. The authors calculate an estimated total quantity of 33 to 60 tons (average +/- 46 tons) of zinc, which is ultimately diluted by a volume of 4 to 8 million m3 (average 6 millions m3) of rainwater drained annually from zinc roofs. This represents an average concentration of 7750 µg/l, clearly greater than the 4874 µg/l figure pointed out in Table 1. On the other hand, 37 million m3 of water being discharged into sewers annually drain 60 to 84 tons (average 72) of zinc towards rivers. However, the authors have omitted to point out that by dividing 72 tons by 37 million cubic meters, you obtain a mean value of 1946 µg/l of zinc being discharged into the Seine river, i.e. about 40% of the value accepted for potable water. Considering this, is it right to assert that « the Seine river is threatened by zinc pollution »? Even less so concerning cadmium, whereby readings in Table 5 show an average concentration of about 1 µg/l, 2% of legal potable water limits.

A person without any scientific background, who reads the figures presented on roof runoff will inevitably come to the logical conclusion that « it is dangerous to consume rainwater, due to zinc contamination ». This was the case from one of my correspondents who, after reading the article, considered abandoning zinc as a roofing material for his prospective new home.

When confronted with assertions of « harvested rainwater’s pollution », it is useful to be reminded that the study’s authors are directly linked to public/private urban water interests that perceive rainwater as a kind of « unfair competition ».

The Paris study presented readings exceeding potable water standards. Measurements in this study were taken from old zinc roofs, sometimes over a century old. Older zinc roofs are more prone to corrosion from rainwater. Over 30 years ago, the zinc used on roofs wasn’t as pure as today. It contained notable amounts of impurities (lead, iron, cadmium). Nowadays, zinc alloys used for roofing are manufactured almost pure, with a 99.995% zinc content, the rest being taken up by minute amounts of copper and titanium. With such a level of zinc purity, corrosion is strongly diminished, unlike older zincs. Nevertheless, the presence of atmospheric sulphur dioxide SO2 can accelerate corrosion, especially in large cities where sulphur-containing gas emissions are greater. Yet current zinc surface treatment technologies mitigate this situation.

Another consideration that should not be overlooked for zinc roofs is the use of copper roofing nails and hooks by older roofing craftsmen. When copper and zinc come into contact, in presence of water (i.e. electrolytic contact), zinc corrosion is inevitable. Preference should be given to zinc or stainless steel fasteners and accessories.

Outside of big cities and industrial zones, the atmosphere’s SO2 content is fortunately weak, even negligible. In the absence of SO2, zinc is not very soluble under normal atmospheric conditions, including the slightly acidic conditions of rainwater. Remember, rainwater on a roof is always acidic, even in the absence of pollution. Numerous analyses were carried out on water drawn from rainwater cisterns where zinc roofs were concerned, in collaboration with Professor Paul Vander Borght of the Université de Liège. Our readings indicated an average zinc content of 500 µg/l (micrograms per litre), and a maximum zinc concentration of 1750 µg/l. The admissible zinc concentration for potable water is 5000 µg/l. Some people can be sensitive to zinc and even develop an allergy. Fortunately, like all allergies, this affliction has a greater chance of arising with prolonged use of city water than it does with rainwater.

In conclusion, harvesting rainwater from a zinc roof does not generally present a problem. In large cities and industrial zones, when rainwater is harvested from zinc roofs, especially from older roofs, it is best to have the cistern water analyzed for zinc (and other heavy metals). If zinc concentrations exceed 2000 µg/l (something we have never observed), then it is preferable to resort to reverse osmosis instead of microfiltration, to totally eliminate this metal (…and remember, drinking water standards accept a zinc content of up to 5000 µg/l).

Heavy metals: copper, lead and aluminium

It so often happens that certain roof elements (chimneys, skylights, solar panels, etc.) are waterproofed with lead flashing. The water that trickles down the roof comes into contact with such flashing for a very short time. One must obviously avoid lead gutters because water can eventually stagnate and dissolve some of the metal. In certain analyses that we performed on different set-ups, we never measured a lead content exceeding the potable water standards. The measured content was systematically ten times less than the accepted standards.

Copper gutters service some older houses. When restored, if they are not replaced, these must be properly repaired to eliminate any water stagnation, primary source of verdigris or copper corrosion. For aged copper gutters and roofs, rainwater cistern water must be analyzed for its copper content. If this exceeds 1 mg/l, a microfiltration system is unsuitable for potable water production and one must resort to reverse osmosis, which eliminates copper ions in water. For non-food purposes, a weak copper content (up to a few milligrams per litre) is not inconvenient. In fact, it is a bactericide.

In light of the above, if your roof or gutters have copper, lead or aluminium components, it is best to have the cistern water analysed for heavy metal content. Except for lead, if the metal content exceeds potable water standards, this doesn't mean that this water is unfit for non-food purposes. However, it is better to choose a reverse osmosis system instead of microfiltration when considering potable water production. Reverse osmosis will eliminate any heavy metals to be found in the cistern's water. As for lead, it's the only metal which, when it exceeds water content standards, precludes such water from even being used as « innocuous » domestic water for non-food purposes, due to its cumulative effect in a living body, and considering that it can be absorbed through the skin.

I must insist on the fact that a heavy metal content that exceeds recognized standards is only dangerous if ingested in great quantities. Whatever quantity one accidentally swallows in a bath, shower, or while brushing one's teeth is much too weak to constitute a health risk. Let me also insist on the fact that most supply water, measured not at the production plant, but at the homeowner's faucet, contains already more heavy metals than can be found in the reclaimed water of a rainwater cistern.

Sea salt in rainwater

Some users living near the Atlantic coast or the North Sea have noted the presence of sea salt within their cistern. Fortunately, the concentration is weak. But it could still hinder the reuse of this rainwater as potable water.

The sometimes-violent winds from offshore can in fact draw in a kind of sea salt aerosol that can find its way into the cistern. However, such winds are seasonal, and overall, they bring little salt.

Therefore, such reclaimed water can safely be used for all non-food domestic purposes. As for potable water production, one will resort to reverse osmosis instead of microfiltration. Go to the Making rainwater potable page for further reading. With this precaution, the PLUVALOR system remains suitable for coastal regions also.

Algae in the cistern

Some literature in the industry suggests that algae in the rainwater cistern are to be avoided. Other specialists assert the opposite saying that the presence of algae is a factor of biological stability tending to purify water. It is our opinion that both assertions are founded. However, for different applications, it is best to consider the objectives pursued. Light is inappropriate in an underground cistern, yet it is an equilibrium factor in exposed outside water reserves.

A rainwater cistern for domestic use is none other than an artificial reconstitution of a natural underground rock cavity in which water conserves well. For good water conservation, temperature must remain constant (this is the case of an underground cistern). The water must be neutral to lightly alkaline (due to concrete/cement/limestone as a cistern material), and contain small quantities of dissolved mineral salts (about 50 mg/l, which comes from the neutralisation of acidic rainwater by the said materials at cistern walls). If these pre-conditions are not respected, rainwater tends to become putrid and cause odours. That is the case of plastic and metal cisterns in which rainwater acidity is not neutralised..

In the presence of these minerals as well as organic matter (from roof catchment), daylight penetration into the cistern will promote the growth of algae on the cistern walls and in the water. Without forced aeration of the water with an aquarium aerator (or pond aerator) and its bubble diffuser, these algae end up fermenting in anaerobiosis (absence of air) giving the water an odour of rotten eggs, although perfectly harmless. In fully lighted cisterns, algae-containing water tends to become yellowish or greenish. This leads to a premature blockage of the system's filters. In other words, it is preferable to prevent daylight from entering a cistern.

The situation is completely different when it comes to a decorative garden pond such as a constructed wetland that is part of a selective greywater treatment system. Here, daylight plays a capital role in the greywater's clarification. That is not the case for a rainwater cistern.

Rainwater for gardening

When building a rainwater storage tank only for irrigation purposes, the situation is altogether different. Such a tank can be made of plastic, concrete, metal or other material. The tank will be relatively shallow (max. 1.2 m or 4 feet) and completely open. Ultimately, one can even put in aquatic plants. However, during a hot summer, there will be greater evaporative water loss. For watering the garden, the water temperature will be a bit higher than in underground tanks. The sun and summer heat will bring the water to a temperature that plants like. Water stored in such tanks is inappropriate for domestic consumption.

A thick biofilm containing algae will grow on the tank's walls, ensuring constant water quality and biological stability. Note however that the introduction of mains water supply, river water or well water in the irrigation water tank will generate a splurge of algae growth, including filamentous algae. In such conditions, the rainwater tends to become putrid, although it remains useable for irrigation

To continue reading, go to page on Household Water for Non-food Uses


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