Rainwater Quality

There are several misconceptions about rainwater quality that do not stand up to lab tests.

Rainwater Acidity

Before entering the cistern, rainwater obviously 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 (NO_x) 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 reuse, acidity constitutes an advantage. 

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 harmless in drinking water.

Commissioning a New Cistern

When putting a new cistern into operation, the tank’s concrete and mortar are initially too alkaline, which makes the reclaimed rainwater too basic. It so often happens that the first harvested rainwater has a pH close to 10! Don’t worry. Nevertheless, such alkalinity, basically harmless in itself, makes the water quite unpleasant for personal hygiene. It is preferable to drain off the first reclaimed waters 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.

Odour Problems in the Cistern’s Water

Sometimes, cistern water used in the home takes on a yellowish colour along with an unpleasant odour. Fortunately, this is rare, but it’s still better to get to the source of the problem.

Most odour-related problems are tied to a discontinuous use of the cistern. The rainwater’s replenishment within the cistern is a factor that prevents the advent of odours. As to the source of these odours, two areas are critical: the accumulated sediment at the bottom of the tank, and the hot-water heater.

To prevent or delay the accumulation of sediment, insure proper gutter design and maintenance. Sediment-related odours are caused by anaerobic fermentation within the sediment. Fermentation-causing bacteria die in the presence of air. To kill them off – and eliminate the odours and yellowish colour also - use an aquarium aerator with its bubble diffuser placed at a depth at which the air bubbles can still escape (through the « sludge » ). Turn it on 24 hours a day for one to two weeks. If odours persist, cistern maintenance becomes necessary. In larger cisterns (greater than 30 m³), an aquarium aerator may be insufficient. In this case, use a tray aeration system

Odour problems caused by the hot-water heater have to do with the equipment’s temperature setting. If the water temperature is not constantly above 50 degrees Celsius, anaerobic bacterial colonies can develop on the heat exchanger, giving the hot water an unpleasant odour and a yellowish colour. These bacteria are harmless, but the odour can become disturbing. Therefore make sure the water heater’s setting is above 50^oC. It may become necessary to increase the setting to 70^oC once a week for 24 hours to prevent bacterial growth within the hot water tank, in the event of  water temperature drops due to abundant hot water use.

Another problem occurs in cottages or secondary residences that are not occupied all year. The same goes for households disposing of a too small cistern and/or a too small roof for continuous water catchment. When the PLUVALOR system has not been used for many weeks or months, odours can appear on start-up. To check this out, take a sample of cistern water in a glass (straight from the cistern) and smell it. In most cases, there will be no odour. This means that the odour detected at a faucet simply comes from the anaerobic bacteria that developed within the plumbing circuit or in the unutilized pump’s pressure tank. The problem disappears by letting a few hundred litres of water run by opening all the house’s faucets. If yet the odour persists, turn on the aquarium aerator. As soon as the odour disappears, turn off the aerator so as to not increase the rH2 value of the water unnecessarily.

Some people connect their aquarium aerator to a timer switch to aerate their cistern every day for ½ to 1 hour. It is better to program this for early in the night. The bubble diffusion tends to disturb a bit of the tank bottom sediment, which rises into suspension, potentially leading to a premature blockage of the filters. The sediment will settle back a few hours after the aerator has been used.

Other Odour Origins

Inappropriate Roofs

Other odour-causing problems include asphalt or tar-based roof coverings (Derbygum, shingles) or plastic-based roofing that can give off tar, rubber or plastic odours. Asphalt shingles tend to stabilize after time, except when rain falls on sun-exposed hot areas of the roof. Roof material odours cannot be overcome by cistern aeration.

Dead Animals in the Cistern

Odours caused by dead animals are altogether different form those caused by anaerobic fermentation. Such odours indicate the presence of a drowned animal carcass in the cistern. A dead bird may be found in a gutter. However, rodents (mice, rats), but also frogs or toads have the unfortunate tendency to come and drown into the cistern. It’s important to keep them out, by placing appropriate screens at the gutters and downpipes, but also at the cistern’s overflow. The cistern hatch must also be designed to keep out animals.

When a dead animal is found in the cistern, the water must be thrown out and the cistern completely cleaned up. In light of chlorine bleach’s effect on a cistern’s natural bacteria, it is preferable to limit the use of such a disinfectant to cases of extreme contamination. To eliminate the chlorine after disinfection, an abundant clean water rinse is required on the cistern walls and floor. The rinse water must also be thrown out.

Many cistern owners have informed me of this type of accident after having only noticed it months after the animal had drowned. Fortunately, microfiltration or reverse osmosis systems deliver good healthy water, even when starting from a heavily contaminated cistern. The long-term risks come from accidental ingestion of non-potable water in the bath and shower. This is only a theoretical risk. In practice, the growth of pathogens is a progressive process that gives the body time to develop immunity to the bacteria. However, this is not true for houseguests who run a certain risk, however weak.

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.

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, light is absolutely indispensable. That is not the case for a rainwater cistern.

Rainwater to Water the Garden

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.

Mineral Salt Content in the Cistern’s Water

If rainfall is acidic and contains very little mineral salts, the rainwater harvested in a concrete/cement/limestone cistern becomes neutral or lightly alkaline (pH between 7.5 and 8.5) and is weakly mineralized. The average mineral salt content has been found to be between 50 and 80 milligrams per litre. See table below.

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.

Right after commissioning a new cistern, the water’s pH, its hardness and its mineral salt content can be abnormally high, as noted above. Fortunately, this phenomenon is temporary.

Table of Cistern Water Readings

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.

Parameters	Min.	Max.	Average  Readings	Potable Water Wtandards
Acidity alkalinity: pH	6,31	8,01	7,23	6,5 - 9,5
Conductivity	36	190	90	< 2100
Nitrates   NO32-	0,2	4,7	1,5	< 11,3
Ammonium  NH4+	0,010	0,059	0,022	< 0,5
Chlorides  Cl-	1,0	16,7	6,5	< 350
Sulphates  SO42-	<8	<8	<8	< 250
Calcium	4,3	15,3	10,1	< 270
Magnesium	0,14	0,52	0,21	< 50
Zinc	50	1731	466	< 5000
Iron	<50	<50	<50	< 200
Cadmium	<10	<10	<10	< 50
Lead	<50	<50	<50	< 50

The physicochemical characteristics of harvested rainwater in a concrete cistern are close to ideal.

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.

Mineral Salts and Drinking Water

About water quality for food purposes, there exists a preconceived notion that we need mineral salts in the drinking water we consume. On this basis, some people advise against the consumption of rainwater due to its weak mineral content, which would provoke a «demineralization» of our body.

This notion, which is far from being scientifically founded, is largely invoked by the bottled water industry. In their commercials, we read or hear things like «mineral balance» with indications of the mineral content (calcium, magnesium, potassium, sodium etc.) that the human body needs every day. By association, it is suggested that this fundamental truth is directly related to the mineral content of the bottled water being publicized. The consumer is left to apply the simple rule of three to determine how much bottled water he must drink daily to cover his mineral needs. This type of publicity constitutes a falsehood by omission.

In fact, the fixing of mineral salts within the human body is a complex process, extremely difficult to measure experimentally because of the flux interactions. Globally, one can say that during normal feeding (i.e. when not fasting) the mineral salts contained in our drinking water do not participate in our body’s tissue building process (or at best, very marginally). However, the ions resulting from electrolytic dissociation of the dissolved salts in our water can participate in our body’s biological processes, without being actually «absorbed». A dynamic equilibrium settles in between the intake of salts through feeding and the discharge of salts through urination and perspiration. Generally speaking, to be available to the body, these minerals (ions) must be interlinked (chelated) with organic molecules. These chelated ions can only be found in our food, not in drinking water. Vegetables, fruit, milk products, meat, etc. constitute this source of absorbable mineral salts.

If one could so easily absorb the calcium and magnesium found in hard water, wouldn’t that mean the end of such ailments as osteoporosis, osteoarthritis, magnesium deficiency? Similarly, the consumption of ferruginous water should mean the end of anaemia? Medical experience has not confirmed such assertions.

The human body’s fixing of mineral salts contained in drinking water only comes into effect during prolonged fasting (beyond one week). That is the reason why during fasting, one should avoid drinking rainwater or any other weakly mineralized water.

In conclusion, one can obtain appropriate drinking water from a rainwater cistern, with appropriate treatment and filtering. See the Water filtering page.

To continue reading, go to page Biocompatible Water

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