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

 

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.

Composition

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.

Acidity

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.

Bacteria

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

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

 

Onsite Stormwater Dention – Why

If you want to redevelop or subdivide an existing urban house block, you might find that a planning condition is that you will need to provide On-site Stormwater Detention (OSD).

You may also find it is a condition on individual blocks on smaller subdivisions.

Why Is Onsite Stormwater Detention Needed?

Before development of towns and cities a large proportion of the rain that fell in an area soaked into the ground or flowed slowly across the land to a creek or river. When areas started to be developed two things happened:

  • More and more of the land was built on, or paved, which meant rain was unable to soak into this ground.
  • Stormwater drains were built to carry the rainwater quickly away from the houses to be discharged into streams and creeks.

Initially while Australia had a small population this didn’t cause too many problems.

Since the mid 1950’s and the rapid growth in population more and more land has been built on.

The result has been more and more water has been discharged surface water drainage systems causing overloading of the piped systems and flooding of the rivers.

In order to try and reduce flooding Planning Authorities are attempting to reduce rainwater flows from developments to a flows similar to an undeveloped site.

OSD On Large Subdivisions

If you buy a block on a large subdivision it is unlikely that you will be asked to provide OSD on your Block.  This is because large developers  as a condition of the overall development have to provide Stormwater Detention Storage for the whole development.

The way they usually do this is by making much of the open space they also have to provide as Ponds, Lakes or Wetlands, which can fill up during periods of rain and then slowly empty. (Now you know why so many developments have a reference to Water in their name)

Other posts will explain more about how the Storage Volumes are Calculated and will look at various storage options.

Guide to Buying a Block has lots of info like this on what to look for before you buy land.

 

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!

Problems

  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).

Solutions

  • 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.

 

ADVANTAGES

  • 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.

 

DISADVANTAGES

  • 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

 

Submersible Pumps

As well as the more common pressure pumps you will frequently see submersible pumps at your local supplier . . . . . So what’s the difference?

 

Characteristics of Submersible Pumps:

Most domestic submersible pumps come with an attached float switch which switches the pump on when the water level rises, and off before it runs dry.

  • They’re Quiet  The surrounding water absorbs any noise.
  • Don’t Need Priming  As the pump sits under the water it doesn’t need to be primed, which also improves efficiency.
  • Rarely Block These are centrifugal pumps, which can pass reasonably large solid particles.
  • Pump From Bottom of Tank/Well Useful  for emptying things, but bad if you need the best water quality (The worst water is typically at the bottom of a tank see: Understanding Tank Water Quality)

Uses

Best for transferring water in constant flow rate applications such as:

  • Emptying tanks
  • In a sump to keep basements dry
  • Pumping Storm Water
  • Pumping Grey Water
  • Irrigation systems (make sure there is a filter smaller than any nozzles)

 

Make sure you get the correct size pump, see: Pump Size

 

Rainwater – Pump Size

One of the reasons why most rainwater systems use too much power is because because the pump is too big. For other reasons see Rainwater – Pump Issues

Most pump suppliers and design charts will specify a larger pump than you really need (They sell a more expensive pump, It’s less likely that the pump will be returned because it isn’t big enough, and they aren’t paying the power bill)

If you want to pick the most economic pump here is how to go about it.

Instantaneous Flow

What is the most flow you need at any one time. Here are some figures for typical house fittings.

Fitting Flow Litres/min
Tap 10 to 15
Tap with Flow Restrictor 4 to 6
Low Flow Shower 7 to 9
Washing Machine* 4 to 10
Dishwasher 4 to 6
Toilet 3 to 5
Garden Sprinkler** 10 to 15

*To get to the lower figure you will need to close the supply valve this will add a few minutes to the wash but will help with issues like Water Hammer.

**It’s really better to irrigate the garden with a separate pump.

Add together the highest flow rate fittings that you think you will want to run together, which will give you a Total Flow Rate ‘Q’

Pressure

You need to aim for a pressure at the fitting of around 150kPa (15m of Head)

To get this pressure you need to:

  1. Measure the height of the furthest fitting above the lowest level in  the tank ‘Hs)
  2. Calculate the Pressure  Loss ‘Hf‘ due to Friction in the pipes See table below
Max Flow Rate
Litres/min
Hf  m head per 100m of pipe
20 25 32 40
12 10.9 3.7 1.2 0.4
24 13.4 3.9 1.3
36 8.3 2.8

The Required Pump Pressure is then calculated from:

Required Pump HeadP‘ m = 15 +Hs + Hf
Or
Required Pump Head ‘P‘ KPa = 150 +(Hs +Hf)) x 10

Ordering

To order a pump you just need to quote the Total Flow and Required Pump Head

 

When you are ordering a pump think about getting a Pressure Tank

Rainwater – Pressure Tank

A pressure tank,  which I think is the most important upgrade to a standard rainwater supply system,  looks like the photo on the right.

Small tanks are mounted on the pump, larger tanks like this one are on the ground.

Key Features

What you get is a cylindrical storage tank storage tank with an internal membrane.

The top part of the tank, above the membrane, is filled with compressed air.

The bottom part of the tank will be filled with water by the pump from a connection at the tank base.

In the top half of the tank there is an air valve where you can check the air pressure and add more air if necessary.

How It Works

 The top of the tank is pre-charged with air to a pressure slightly lower  than the ON setting of the pressure switch.

When the pump starts water is pumped into the tank compressing the air until the maximum pressure is reached.

If you turn on a tap  the compressed air will push water out of the tank until the  tank is almost empty (when the pressure will have dropped to the minimum level)

If you use less water than that stored in the tank the pressure doesn’t drop enough so the pump doesn’t start.

Once the pressure switch turns the pump back on, the pump fills the tank while at the same time supplying water to your system. Even if you turn the tap of the pump will continue to run until the tank reaches maximum pressure.

Tank Sizing

Although very small tanks are available about the smallest tank I would recommend would be an 18litre tank, which would give a flow volume of around 5litres between pump starts.

A 35litre tank with a flow volume of around 11litres between pump starts would ensure a toilet flush would not exhaust the tank.

Why Should You Get A Pressure Tank?

It can cost more than some pumps to buy the pressure tanks so why buy it is an important question.
To find out the problems with a standard set up see the post “Rainwater – Pump Issues
The pressure tank should:

    • Double, or even treble the life of the pump, by reducing the number of pump starts and pump run time.
    • Significantly reduce the power usage by ensuring the pump mainly runs at the design rate.
    • Reduces the noise nuisance by less frequent pump runs.

 

For more posts about tank water see the Rainwater Section under the Sustainability Tab

 

Charged Drainage System

Thanks to SaveH2O, of Supadiverta

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

With this type of system a section of the pipe always remains full unlike a Conventional System.

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

Advantages

  • A neater appearance than having pipes above ground.
  • Allows you to discharge water at a height above the ground level at the down pipe.

Disadvantages

  • It is more likely to block as the flow through the pipes can be fairly slow and the low points can collect silt. Silt will get deposited at flow rates below 0.6m/sec which will be the situation for most rainfall events.
  • It is also more difficult to unblock.
  • Can cost more with excavation and additional inspection fittings .
  • Potential for mosquito  breeding  in water unless appropriate screes are installed.

 

Also see Underground Pipes

 

Underground Pipes

It’s important to make sure you are getting the right type of pipes and fittings that will be underground…..you don’t want to be digging up you garden, or paths, in case of blockages.

Pipes

Some plumbers will want to use, and bury, 90 mm diameter rainwater pipes, basically plastic downpipes.

You need to make sure you are getting as a minimum 100 mm PVC pipes rated as DWV.(Stands for Drainage, Waste and Vent).

The reasons are:

      • The 90 mm pipe is thin walled and can be easily deformed once buried. This means you lose capacity without realising it. The DWV is a much thicker and thus stronger pipe.
      • Although the increase in pipe diameter is fairly small the flow capacity of the larger pipe is over 40% higher. That makes a difference in storm conditions.

Pipes are normally marked at 1m intervals with the type, manufacturer, nominal diameter, material, and the Standards reference (AS/NZS1260).

This is printed on the pipe every 1m.

Protect Underground Pipes

It’s important to protect your underground pipes.

One of the problems during a new house construction is that concrete tend to fill underground pipes, causing blockage.

A hydraulic impact cutter can remove concrete in drains and sewers.

Other problems include leaking or burst pipes caused by corrosion, tree roots, and collapsed pipes.

Roots tend to grow toward the direction of the water so a loose connecting or weak point in the underground pipes triggers tree roots to wrap around them until they burst.

That’s why the design of the pipe system is crucial to ensure a problem-free plumbing.

They should be away from trees and other structures to avoid these problems. 

You can use an experienced plumber to help protect your underground pipes.

A qualified and experienced plumber will detect common leak indicators in the underground pipes and repair them. They’ll test the repair and fill the trench.

Bends

Bend refers to a term for any change or offset of direction in the pipes, which includes elbows.

They’re fabricated as per piping specification requirement.

Elbows come in standard or pre-fabricated and are available off the shelf. 

Bends are available in 4 different angles for DFW pipes as follows: 15 degrees, 30 degrees, 45 degrees and 90 degrees.

Although 90 degree bends are available, I would NOT install them underground due to the blockage risk….. If you need a 90 degree change of direction underground:

      • For a drain or a sewer use a junction pit.
      • For a charged (pressure) rainwater system use two 45 degree bends with an inspection ‘T’ in the middle.