Calidad del agua de lluvia

La acidez del agua de lluvia

Hay que distinguir entre el agua que cae del cielo y el agua que se almacena en una cisterna adecuada. Si el primero es casi siempre ácido y prácticamente no contiene minerales, este último es neutro y ligeramente mineralizado.

Hay varios conceptos erróneos acerca de la calidad del agua de lluvia que no se resulta satisfactoria ante las pruebas de laboratorio. Algunos dicen que llamar el agua de lluvia «pura» es excesivo, teniendo en cuenta la contaminación atmosférica que se puede encontrar en el cielo. No decimos que el agua de lluvia esté libre de contaminación. Sin embargo, sin duda es mucho menos contaminado que la gran mayoría de las fuentes de agua que se encuentran en la naturaleza. Es un hecho analítico. No olvidemos que la lluvia y la nieve son las fuentes primarias de agua dulce en la Tierra. Su contaminación influirá en la calidad de todas las demás fuentes naturales de agua dulce.

Así, antes de entrar en la cisterna, el agua de lluvia inevitablemente absorbe la contaminación atmosférica. El efecto más destacado de la contaminación es la acidez.

Incluso sin contaminación, la lluvia es naturalmente ácida debido al dióxido de carbono (CO₂)siempre presente. Esta acidez natural es amplificada por la presencia de óxidos de nitrógeno (NO_x) y dióxido de azufre (SO₂). Tales óxidos son producidos por la combustión de combustibles fósiles (petróleo, carbón), especialmente a altas temperaturas. Cuando se disuelven en agua, estos óxidos se convierten en ácidos: el óxido de nitrógeno se convierte en ácido nitroso (HNO₂) o ácido nítrico (HNO₃) mientras que el dióxido de azufre produce ácido sulfuroso (H₂SO₃) que se oxida en el aire para convertirse en ácido sulfúrico (H₂SO₄).

La lluvia ácida es una molestia para los bosques de coníferas. También ataca monumentos de piedra caliza. Sin embargo, para la reutilización doméstica del agua de lluvia, la acidez constituye una ventaja..

El contenido mineral en el agua de la cisterna

Los componentes ácidos del agua de lluvia reaccionan con los componentes alcalinos del mortero de hormigón o cemento de la cisterna, disolviendo sales minerales. Durante este proceso, la acidez desaparece: el agua se vuelve neutra. La gran mayoría de las sales disueltas se componen de bicarbonato de calcio [Ca(HCO₃)₂]. Los óxidos de nitrógeno se convierten en iones de nitrato. El dióxido de azufre se convierte en sulfatos. Estos iones tienen concentraciones débiles. Nunca he medido más de 9 miligramos por litro de nitratos en el agua de la cisterna, siendo el promedio está entre 3 y 5 mg/l. Comparativamente, en agua potable «legalmente», puede haber hasta 50 mg/l. Los carbonatos y sulfatos de hidrógeno son sales (es decir, iones) que son inofensivas en el agua potable.

Volviendo con lo anterior, el agua que cae del cielo es ácida y contiene muy pocas sales minerales. El agua de lluvia cosechada en una cisterna de hormigón o piedra caliza es neutra o ligeramente alcalina. (PH entre 7,7 y 8,5) y está débilmente mineralizada. Su contenido en electrolitos oscila entre 50 y 80 mg / litro. Debajo de 10 mg / l, el agua es demasiado pobre en electrolitos, y usted necesitaría agregar algo. Por encima de 200 mg / l, puede sobrecargar los riñones de alguien que sufre de insuficiencia renal. Una persona sana puede tolerar el consumo de agua que contiene 600 mg / l de minerales. Sin embargo, uno no sabe a priori si su función renal es perfecta o está genéticamente predispuesta a fallar con el tiempo.

Comparativamente, las mejores aguas minerales embotelladas disponibles en el mercado belga, por ejemplo, pueden contener tan poco como 16 mg/l, y hasta 35 mg/l.

La siguiente tabla muestra las lecturas tomadas en 18 ocasiones distintas sobre muestras de agua de lluvia almacenadas en las cisternas de siete instalaciones de PLUVALOR. Las características fisicoquímicas del agua de lluvia cosechada en una cisterna de hormigón están cerca de lo ideal.

Parámetros físico-químicos del agua de cisterna (en instalaciones de tipo PLUVALOR)
Parámetros	Unidades	Mín. valor	Máx. valor	Lecturas promedio	Normas de agua potable
Acidez alcalinidad: pH	-	6,31	8,01	7,23	6,5 - 9,5
Conductividad	µS/cm	36	190	90	< 2100
Nitratos NO₃²	mgN/l	0,2	4,7	1,5	< 11,3
Amonio NH₄⁺	mgN/l	0,010	0,059	0,022	< 0,5
Cloruros Cl^-	mg/l	1,0	16,7	6,5	< 350
Sulfatos SO₄^2-	mg/l	< 8	< 8	< 8	< 250
Calcio Ca²	mg/l	4,3	15,3	10,1	< 270
Magnesio 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
Dirigir 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 has a pH close to 10! Don't worry, as this phenomenon is temporary. Nevertheless, such alkalinity, basically harmless in itself, makes the water quite unpleasant for personal hygiene. 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.

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 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 may 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 any 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, a simple microfiltration or reverse osmosis system will eliminate the problem.

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].

[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 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 roof stormwater runoff, a bit below the legal potable water limit. The average 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. 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.

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 new zinc surface treatment technologies can help 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. electrolytical 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), than it is preferable to resort to reverse osmosis instead of microfiltration, to totally eliminate this metal.

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. P.S. One must, however, avoid drinking too much of such water when taking a bath or a shower.

In light of the above, if your roof or gutters have copper, lead or aluminium components, it is best to have the cistern water analyzed 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 (a little bit more expensive) 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 «safe» 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 a specific application, 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 the presence of these mineral salts and of 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 and its bubble diffuser, these algae end up fermenting in anaerobia (absence of air) giving the water an odour of rotten eggs. 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. Under light, bacteria and soap/detergent molecules tend to cake together to form particles (micelle) that settle at the bottom. In the resulting sediment, other bacteria take charge, decomposing and transforming it into water and carbon dioxide. In deeper ponds, there can even be a bit of methane produced. In planted wetland systems, daylight is absolutely indispensable. 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. 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, insuring constant water quality. 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. Nevertheless, it remains useable for irrigation.

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