Rainwater Tank Problems

Rainwater Tanks aren’t always trouble free, so here is some advice on a couple of problems that you can encounter.


Even with pre-filters there will eventually be a build up of dirt and cloudy water at the bottom of the tank.

There are professional tank cleaning companies that will provide the safest way of cleaning a tank.

As they can filter the water out of the sludge they minimise the water lost from your tank.

If you are going to do it yourself the best time is to do it when the tank is low.

Working from out side the tank use a pressure washer to wash the sides and agitate the sludge in the bottom, then flush it out of the drain.

Keep Out of the Tank

Whatever you do DO NOT Climb Inside The Tank.  

Working inside tanks (known as Confined Space Work) is a very risky thing and several people have died inthe last few years.


Regular chlorination of your rainwater tank should not be necessary, although there will be some bugs in the water you will develop an immunity.

However if:

  • People are getting ill, or:
  • You suspect the water in your tank is contaminated.
  • You have a new baby, or
  • Small children from the city are visiting

. . . .you can chlorinate the water as it would be sad if your visitors became ill because they weren’t used to drinking tank water.

To chlorinate add either swimming pool chlorine (calcium hypochlorite 60-70%) or sodium hypochlorite (bleach) 12.5% by volume.

The dose to treat the contamination should be 7 grams of calcium hypochlorite or 40mL of sodium hypochlorite per 1000 litres of water in the tank at the time of treatment.

Rather than just add the chlorine fill a plastic bucket with water to 2/3 full and then add the chlorine, in the open air.

DO NOT add Water to Chlorine. Empty the bucket into the tank, being careful not to spill it, and then mix the contents of the tank.

An easy way of mixing if you have a pressure pump is to put a hose into the tank and leave it running for 15 – 20 minutes.

Once the water is mixed leave it to stand for at least 24 hours to allow the chlorine taste and smell to dissipate.


Have you had rainwater tank problems?


For more about tank water quality see Rainwater Safety

For similar posts see Sustainability


What Size Rainwater Tank 3

Other postshave explained how to calculate:


From your calculations find out the month of lowest average summer rainfall which is likely to be the peak month of water demand.

Design Case 1

If each monthly average rainfall is above the estimated use for that month.

You will need a tank with around 10% more volume than required for  the peak month of use. (This allows for evaporation, and the water that’s unavailable because its below the tank outlet.)

Design Case 2

If the monthly average rainfall below the estimated use for that month.

For this case you will need to add the differences in volume between the expected use and the rain for each month leading up to the peak month to the amount in Design case 1 above.

When you have the expected storage volume you can then look at storage tanks.

Not enough room, or enough rain?………….. then you need to think about reducing your use or getting more collection area.

Bushfire Zone

If you are in an area with a bushfire risk you will need additional storage,

This needs to be in tanks that will be resistant to flame attack. (Concrete or Steel. . . NOT Plastic tanks)

See this post for more information:  Bushfire Reserve


Before choosing a tank look at Round or Slimline Tanks.


What Size Rainwater Tanks Do You Need – 2

How Do You Plan To Use The Water?

The most common uses for rainwater are for toilet flushing, garden watering and car washing.

Toilet Flushing

Basically a new duel flush Australian toilet will use 6 L for it’s main flush and 3L for it’s half flush.

Older toilets will use 9 L and 4.5L respectively.

Assuming two main flushes and 2 half flushes per person per day gives a monthly average usage between 540 – 810L per person.

For a family of four that’s between 2,160L and 3,240L of water.

If you are reading this in other country your numbers may be a bit different!

Garden Watering

This depends on the area that you want to water, whether you are watering an area or spot watering, and how often you want to water.

A reasonable summer months watering of an area would be the equivalent of 10mm a week, which would be 10L/m2 of garden watered so for each m2 of garden you need to store 45L/month. (To water say 200m2 of garden weekly would use 2000L which would add up to 9,000L per month)

You can see from this how much water can be used on the garden compared with indoors.

Approximatly 3 times the volume used in the toilet!

Car Washing

Washing a car using a pressure washer would probably only take about 30L so the monthly total per car for washing once a week should be less than 150L.

Rainwater is Sole Water Supply

Every family is different so its hard to predict your water use.

We are fairly careful, don’t water the garden,  only have 3 minute showers, use a front loading washing machine and only run the dishwasher when full.

All this means we use around 75L/person/day say 28kL/year.

The average usage in Melbourne with drought restrictions over the past few years has been about double that. . . so there must have been lots of people taking 30 minute showers!

Perhaps the best thing to do is monitor your water meter where you live now.

If your only supply is rainwater its worth checking with your Council if they have a minimum requirement.

Its also quite common for councils to require a bush fire reserve storage capacity for rural properties which can range from 10,000 litres to 20,000 litres depending on your State and location.

How Long Do You Want The Water to Last

Once you know the intended amount of useage, you can then find the total volume to supply a months usage.

For example: If a family of four was only using the water for toilet flushing and car washing a storage volume 3,500L (3.5kL) will be required for each months storage.

If the same family also wanted to also water 200m2 of garden 12,000L (12 kL) would be required for a summer month. (For Winter months there would be no garden watering so the storage would easily last for  several dry winter months..

As you can see the less garden watering you do the smaller the tank needs to be. . . or the longer you can survive  without rain.


Next step See What Size Rainwater Tanks 3

For similar posts see Sustainability


What Size Rainwater Tanks Do You Need – 1

With sustainability being a majot topic you may be thinking about a rainwater tank. . . but what size of tank do you need?

The bigger tank you get the cheaper the cost of storage per kilolitre (1,000L or 1kL) but you don’t want to buy one too big!

Not only will it cost more, but it will take up more space on your block.

How Much Rainfall is There in Your Area?

The first thing you need to do is find out what is the average rainfall for your area.

Remember that average rainfalls can change over quite small distances. (For example the rainfall in some Melbourne’s Western suburbs can be 30% lower than parts of the Eastern suburbs).

If you go to this web page and enter any Australian town it will give you the nearest weather stations.


For other counties try your national weather centre.

Once you have selected a weather station click on the get data button and you will get heaps of data. What you want is the mean monthly rainfall in mm.

This should look something like the table below, which is the rainfall for Werribee Racecourse, my local Weather station.



























Remember the above figures are the mean so for example one year there may be a summer thunderstorms and January may get 80mm of rain, the next year in a drought it may get no rain at all.

How Much Rain Will You Get ?

Now you need to measure the roof plan area that will be drained to the tank/s in m2.

Multiply the area in m2 by the rainfall in mm for each month and divide by 1,000 will give you the volume of rain in kilolitres.

For example; An 180 m2  roof in Werribee Household will get a mean of 97.6kL per year. (The bottom row of the table below shows the monthly means as well as the annual mean)










































The above totals are theoretical!

In practice you will usually collect 80-90% of these amounts.

This is due to losses through evaporation, and any first flush filtering systems.

The next question is how much water do you need?….To find out see Post 2 on this subject


For similar posts see Sustainability


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


Bushfire Reserve – Volume

I have previously talked about Bushfire reserve supplies. . . . But how much?

I see various minimum water volumes put forward for bush fire reserves. For example in early 2014 the following applied.

  • Victoria, regulations for new builds require 10,000Litres.
  • South Australia the country fire service suggests 22,000Litres.

Just because there is a minimum requirement that doesn’t mean that is going to be enough water to deal with an incident for your property. Here are some thoughts on what I feel is appropriate.

  1.  10,000litres which can only be accessed via a fitting that meets the requirement of your local fire fighting service. This varies from state to state so check with your local brigade. This supply is really a last ditch supply available to the fire brigade when they are activly defending your property from direct attack.
  2. 2 – 4,000litres to assist you in dealing with ember attack with a hand held hose, in advance of the fire front reaching the property, and following the passing of the fire front.
  3. Adequate Volume to run a sprinkler system from starting the system until the fire front has passed. This volume will depend on the number of sprinklers, and the time you intend to run them. In other words Flow and Time:
    • Flow For a small house you could be using around 2,400litres per hour (Say 2  impulse sprinklers on the roof and 6 spray nozzles on the side of the house under attack) For larger Houses or those with several outbuildings the volume will be much larger.
    • Time The time will vary depending on your Fire Plan.
      • If you are planning to stay and defend your property you will want enough volume to defend your property during severe ember attack, while the fire front passes through your property, and to damp down afterwards. (Say 2 hours, 5,000litres)
      • If your Fire Plan is to set all the sprinklers running and leave early on days of high fire danger you will probably need to allow 12 hours running time per day of fire danger.(Say 28,000litres per day)

The above figures are indicative and any spray system should be properly designed.

Photo from Blazecontrol.com

Rainfall Intensity

When most people think about rainfall they think about the amount of rainfall . . . . . . . . . but it’s just as important to understand how hard it can rain, the  ‘Rainfall Intensity’.

The ‘Rainfall Intensity’ is important when thinking about things like; Roof Gutters, Down Pipes, Stormwater Pipes , Tank Overflows, and even how high your house is above ground level.

Photo courtesy of www.mirror.co.uk

Recurrence Interval

When talking about heavy rainfall you will frequently hear talk on the news about things like a ‘1 in a 100 year storm’. What that means is that a statistical calculation indicates that a storm of that strength is only expected to occur once in any 100 year period, the ‘Recurrence Interval’.

There are two rainfall intensities that the building codes normally consider. They are based on the peak 5 minutes during a storm for vaious ‘Recurrence Intervals’.

Intervals can be from: 1 in 20 years to: 1 in 100 years (See this link for more information: Roof Choices)

Intensity Calculation

Statistical intensities have been calculated for all locations in Australia and are available at the Bureau of Meteorology (BOM) Website.

Search for ‘Intensity Frequency Duration (IFD)’

To Use the IFD

  1. Enter “your town name ”  “coordinates”  into Google and write down the coordinates
  2. Click on the Create An IFD button
  3. Enter the coordinates into the Decimal Coordinate box
  4. Enter the town name in the Location Name box
  5. Tick that you accept the Conditions of Use
  6. Press the Submit button
  7. Click on the Table tab
  8. The following is the top line of the table for Werribee in Victoria.


1 year


2 years


5 years


10 years


20 years


50 years


100 years

5 mins








The units are mm of rain in 5 mins (Multiply by 12 to get an hourly flow rate)

Low Rainfall Intensity

Parts of the Plumbing and Building Codes refer to areas of “Low Rainfall Intensity”.

A location  of “Low Rainfall Intensity” means the 5 minute rainfall intensity for an average recurrence interval of 20 years is not more than 125 mm/hour.


For more on rainwater plumbing see Drainage

Rainwater includes more on rainwater collection





Rainwater Collection – Quality Issues

This is a fairly typical rainwater collection installation.

Gutters discharging to a pipe which discharges onto a screen fitted to the tank access point, cheap and cheerful!

I must have seen it hundreds of times . . . . . . but it has some negative implications on the water quality you will get from the tank!


  1. Although the screen keeps leaves, mozzies, and other debris out of the tank it lets sunlight in. This will encourage algae growth inside the tank.
  2. At the top of the tank the screen is out of view and consequently hardly ever gets cleaned. As a consequence the leaves rot and organic materials gets washed into the tank providing nutrients for that algae.
  3. Discharging water into the top of the tank isn’t the best location (see this link:  Inlet Improvements).


  • Install a cover over the access point (Something like a round dustbin lid with a hole for the inlet pipe) This will keep the light out, but the issue of the rotting leaves will remain.
  • Screen the leaves out at the gutter discharge using a rainwater diverter with a self cleaning screen (for an example see this link: Supadiverta), and pipe the inlet  flow to the bottom of the tank.


For more posts see Rainwater


Charged Rainwater Collection

Thanks to SaveH2O, of Supadiverta

This diagram indicates a charged rainwater collection system. These are sometimes called either a “wet” or a “pressure” system.


NB. This diagram has been simplified for clarity.  A leaf diverter, and an adequate overflow, must also be fitted.

With this type of system a section of the pipework always remains full.

As the pipes are under pressure it is essential all the joints in above ground and underground pipework are fully watertight.



  • Allows you to discharge water at a height above the ground level at the down pipe (But not at the top of the tank – see the post: Inlet Improvements)
  • Neater than above ground pipework
  • Additional down pipes can be connected without diverting pipes along walls.



  • It is more likely to block as the flow through the pipes can be fairly slow and the low points can collect silt. (It is also more difficult to unblock.)
  • Higher cost than above ground systems due to trench excavation, and additional inspection fittings.
  • Potential for mosquito  breeding  in water unless appropriate screes are installed.
  • Harvesting additional downpipes  without adequate overflow capacity can result in the tank overtopping during heavy rain.


Also see Underground Pipes


Rainwater Collection Fail

How Not to collect rainwater!

I suppose its cheap…….but it’s not the most convenient for filling your watering can from.

You can also see that algae is growing in all those open containers.

It’s not something most of us would want on our front gardens!

On the plus side it probably looks very spectacular in heavy rain as the water overflows into the various containers.


For more fails and unusual houses go to What the………………….?