Rainwater – Safety

Millions of people have lived long healthy lives drinking rainwater . . . . but that’s doesn’t mean that rainwater is entirely pure, or completely safe for everyone.


The following Chemicals are commonly found in rain: Hydrogen, Ammonium, Calcium, Magnesium, Potassium, Sodium, Suphur Tetroxide, Nitrus Oxide, Chlorine, Fluorine, Lead, Iron, Bromine, Manganese, Vanadium, Aluminium.

These chemicals are generally in safe, extremely low concentrations.


Rain pH is typically acidic, with a pH in the range around 5.0 – 5.6 9Neutral water is 7).

With local thunderstorms the pH can drop as low as 2.0.

The acidity is primarily due to the presence of the two strong acids in dilute form, sulfuric acid (H2SO4) and nitric acid (HNO3).

Although some of these contaminants are from pollution many of them are as a result of natural processes.

If nothing is done to reduce the acidity you can corrode the copper in your pipes. (See this link: ABC)

If you have concrete tanks the water should be neutral as the concrete leaches lime, which raises the pH.

However if you have plastic tanks you might like to consider adding a bag of limestone chippings to the tank, which also help to raise the pH.


Rainwater tanks are also likely to contain low levels of bacteria as a result of contaminants on the roof.

People who regularly drink this water will rapidly build up a resistance to the bacteria.

When people, who have no resistance to the bacteria, drink the water however there is a real chance of developing a gastro problem.

This is a particular concern if that person in very young, old, or already unwell.


Precautions you should take include

  • Rinse a new roof
  • Keep the roof clean and clear of leaves including removal of overhanging branches.
  • Screen the inlet and overflow of the tank with fine mesh to prevent birds, animals and insects getting in.
  • A well maintained leaf trap will reduce the amount of organic matter that enters the rainwater tank through the inlet.
  • The tank should be covered to prevent light from reaching the water as it will encourage the growth of algae and bacteria. The cover to allow access to the tank for cleaning and inspection should have a tightly sealed manhole.
  • Use only “food grade” plastic pipe and fittings.
  • Clean the gutters and tank inlet every three or four months.
  • Remove the sludge in the tank bottom every two to three years.

For cleaning and disinfection of tanks see Rainwater Problems

For Similar Posts see Sustainability


Rainwater Tank – Outlet Location

In Understanding Tank Water Quality I explained that the typical tank outlets is located close to the bottom of the tank. This takes the dirtiest water from the bottom of the tank,rather than the cleanest water near  the top.

So what can you do?

Well the simplest solution is to put a second  outlet further up the tank, as shown below.

The top outlet is then used for the supply to the house, while the lower outlet is used to supply a garden tap.

The following schematic shows a typical installation.

Normal operation is with Valve 1 open supplying the pump, and Valve 2 open supplying a garden tap. Valve 3 is left shut.

The garden top would only have the pressure of the tank so it could only be used for low pressure jobs like filling a watering can. At extra cost you could add a separate pump.

When the tank level drops below the top outlet Valve 3 can be opened to supply the pump. Although this water is taken from the bottom of the tank most of the substandard water should have been drained off to the garden.

The outlet should also be on the opposite side of the tank to the Inlet.

Outlet Modification shows a low cost modification to further improve the quality

To help you pick a tank see Round or Slimline Tanks


Understanding Tank Water Quality

The drawing on the right shows a fairly typical rainwater tank layout.

I have seen lots of tanks set up like this and have also seen this layout in tank supplier’s brochures.

The set up is probably OK for garden watering and toilet flushing but not much else………………………..”So what are the Issues?”

Variable Water Quality From Top to Bottom

Even with ‘leaf screens’ and ‘first flush divertors’ there is going to be some particles in the water coming of your roof, These particles will either be lighter than water and float to the top, or heavier than water and sink to the bottom.

The smaller the particle the longer it will take to sink to the bottom.

The 2nd diagram shows how the water quality varies through the tank a few days after it has rained.

  • There are some particles floating on the surface.
  • There is some material close to the bottom which can include rotting organic matter. Sometimes called the Anaerobic zone.
  • The water between the bottom and the top gradually improves as the height increases with the best water being about 1 cm below the surface.


Because of the variable water quality problems are:

  • The outlet is close to the zone of worst water quality.
  • When it rains the turbulence from the inlet mixes the tank which then takes time to settle.
  • The overflow takes some of the better quality water.

Over the next few weeks I will provide  information about ways of improving the water quality in your tank.


For more about tank water quality see Rainwater Safety


Round or Slimline Tanks?

In addition to the standard round tanks there are a whole range of alternative rainwater storage tanks available.

One popular shape is the slimline tank which will fit in a narrow space. I have even seen a row of them used as a boundary fence.

Before you make a choice it could be worth thinking about finding out if you can find room for a round tank.

Round tanks cost less because they are easier to make and use less material. Typically the cost increase for a slimline tank over a round tank of the same volume is at least 50%.

A comparison from my local Masters store:

  • $800 for a Slimline 3,000 Litre tank
  • $500 for a Round 3,000 Litre tank

If you haven’t the width to fit a 3000L tank you could perhaps look at getting two round 2,000Litre tanks for the same price as the 3,000Litre Slimline tank.

Other advantages of the round tanks are they are a lot stronger, and easier to clean out.


See this post to find out How Much Rainwater Storage You Need