Designing a Rainwater Tank and a First Flush System

• Rainwater Tanks in Australia
• Selecting a Tank
• Calculating the Tank Size
• System Design
• Health Issues
• Maintenance

Rainwater Tanks In Australia

Australians love water but unfortunately we just don’t have enough to go around anymore.

With a drier climate and a growing population it is clear that our current water supply in Australia is not sufficient so we are left with 2 options:

Use less water

OR

find more water

We can all try to save water but the fact is that we need to water the garden, fill the swimming pool and flush the toilet so it seems that if we want to keep the standard of living we are accustomed to then we need to find more water.

Good news. Its all around us!

We don’t need to spend billions of dollars on desalination plants or recycling sewerage. The easiest way to do that is by simply collecting the water that lands on your roof and storing it in an Australian water tank.

Most places in Australia get enough rainfall each year for each home to collect enough water to keep them going most of the year. Obviously this depends on how big your roof is, how big your water tank is and how much water you use, but even if you could collect just 20% of the water you use each year into a water tank it would make a big difference to the amount of water we use in Australia

With a Supertank System of modular water tanks you can easily store any volume of water from 300L to 300,000L or more! and for around 75c per litre to buy it is one of the most economical slimline water tanks in Australia. (A 395L tank costs around $300). With rising water prices a tank can save you money in just a few years

The other great thing about the Superwall Water Tank is that its patented folding rectangular shape means that it looks just like a regular wall or fence so you don’t have to try and hide it away, or lose valuable space in your backyard.

Supertanks are designed, owned and made in Australia for Australian conditions.

You can make a difference!

Installing a rainwater tank is one of the easiest and most effective ways to save water. Rainwater (provided you take a few simple precautions) is cleaner, more pure and usually tastes better than your mains tap water. Best of all its free!

Selecting a Rainwater Tanks for Australia

Before you buy a water tank you need to decide how much water you need. Ask yourself a few questions and use the tables to determine your storage requirements

How much water do I use?

What do I use the water for? (gardening, washing, drinking?)

How much do I rely on it (Do I have mains backup)

How much rain do I get?

How much water can I collect (how big is my roof?)

How often does it rain?

Water Consumption Table for Australian Homes

Average person	50,000- 100,000 litres/ year (including outdoor use)
Average Home	230,000- 280,000 litres/ year (including outdoor use)
INDOOR
Garden	35- 50%
Bathroom	10%
Hot water	16%
Toilet Flushing	10- 20%
Laundry	10- 20%
Kitchen	5%
Shower	10- 30 litres per minute (depending on showerhead)
Bath	120 litres approx
Washing Machine	150 litres/ load (medium)
Dishwasher	50 litres/ cycle
Toilet	11 litres full/ 6 litre half flush
OUTDOOR
	Small	Average	Large
Lawn	Up to 70,000	90,000	320,000
Garden	Up to 50,000	55,000	60,000
Pool (uncovered)	Spa 20,000	40,000	80,000
Hose (20mm)	3460 litres/ hour
Hose (15mm)	1090 litres/ hour
Sprinkler	1000 litres/ hour (average pop up)

Pool Evaporation Rates (litres per square metre)

	Oct	Nov	Dec	Jan	Feb	Mar	6 mth Average
Sydney	5.8	6.5	7.4	7.2	6.4	5.3	6.4
Melbourne	2.6	5.7	7.4	7.5	7.3	5.8	6.4
Brisbane	6.4	6.7	7.6	7.9	6.7	5.8	6.9
Adelaide	5.3	7.2	8.1	8.9	8.4	6.4	7.4
Perth	5.3	7.4	9.1	10.2	9.8	7.9	8.3

Factors influencing evaporation rates

• Pool Cover reduces evaporation between 20- 90% depending on type
• Exposure to wind will increase evaporation. Protection from wind will decrease it. A gentle breeze can DOUBLE the evaporation rate by constantly removing the layer of humid air directly above the surface of the pool.
• Air temperature:
• Humidity. Dry air (low humidity) will increase evaporation
• Useage. Using the pool will increase the evaporation. Splashing and getting out will remove water. Broken/ wavy water (either by use, filtration or wind) can increase the evaporation rate.
• Pool temperature. Heated pools will evaporate more. The hotter it is, the more it will evaporate.
• Rainfall. Rain will top up the pool but doesn’t affect the evaporation rate.

Rainfall Averages

State	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	Total
													mm/year
Melbourne	48.2	47.0	50.6	58.2	56.6	49.8	47.9	50.3	58.7	67.4	59.3	59.1	653.2
Albury	52.8	34.6	40.8	47.5	62.9	71.9	86.7	88.5	71.4	72.5	53.2	53.9	736.7
Bendigo	33.0	32.2	36.0	41.4	54.8	61.0	56.4	58.6	54.2	52.4	37.4	33.1	550.5
Canberra	59.8	51.2	55.6	49.3	47.5	37.9	52.4	47.6	65.2	61.9	58.7	46.0	633.1
Sydney	103.3	117.4	131.2	127.2	123.3	128.1	98.1	81.5	68.7	76.9	83.1	78.1	1,217.0
Newcastle	91.4	105.6	121.9	115.9	118.6	117.8	97.2	76.2	73.7	74.3	69.5	82.4	1,144.6
Wollongong	106.8	108.7	118.5	131.2	115.8	109.0	91.0	61.4	66.7	68.1	71.9	86.5	1,135.6
Brisbane	159.6	158.3	140.7	92.5	73.7	67.8	56.5	45.9	45.7	75.4	97.0	133.3	1,146.4
Cairns	386.6	453.0	420.3	197.7	93.2	46.5	28.7	27.9	34.6	39.6	94.4	180.4	2,002.8
Townsville	262.8	297.5	187.8	67.6	34.6	19.6	13.0	16.5	10.1	25.6	59.3	127.0	1,121.5
Gold Coast	175.3	190.0	202.0	135.8	131.5	93.0	74.6	55.8	57.9	86.7	103.8	132.1	1,438.6
Adelaide	19.2	13.7	26.2	38.7	62.6	83.1	77.8	68.1	63.6	48.5	29.6	26.8	558.1
Port Lincoln	13.2	14.8	20.0	36.4	57.4	74.6	78.9	69.0	49.7	35.9	21.9	19.1	490.9
Perth	8.6	13.3	19.3	45.5	122.7	182.4	172.9	134.6	79.9	54.5	21.7	13.9	869.4
Albany	23.7	23.1	38.3	67.9	119.6	132.5	144.8	127.4	102.0	80.4	43.4	29.6	932.6
Broome	173.0	185.0	99.6	26.6	28.7	18.2	6.3	1.7	1.5	1.4	8.1	49.1	599.2
Katherine	271.6	256.1	188.7	37.7	6.1	0.3	1.4	1.0	7.7	38.0	87.0	203.6	1,099.2
Hobart	48.0	39.8	45.7	51.8	47.0	54.6	52.8	53.0	52.4	62.6	54.5	57.2	619.5
Devonport	44.8	37.8	48.1	62.5	77.8	82.9	99.9	90.9	76.3	67.0	57.0	52.5	797.4
Launceston	51.8	28.9	34.7	53.5	67.9	67.3	79.4	83.0	66.4	55.7	49.3	45.5	683.3
Darwin	423.3	361.1	319.3	98.9	26.5	2.0	1.4	5.7	15.4	70.7	141.8	247.9	1,713.9

To Calculate how much water you can collect each year use the following calculation:

Annual Rainfall (mm) x Catchment (roof) Size (m²) x run-off percentage* – any first flush = Maximum Harvest Volume

* ‘run- off percentage’ is the percentage of the total roof area that is used for collection.

Example:

Your house in Broome has a 100m² roof. But you will only use 70% of the harvest area because you will not collect the water under a large tree that overhangs some of your roof. You also have a Superhead first flush device that will discard 30L each time it rains. Assume it rains 50 times per year. Your annual rainfall is 599.2mm

1) Calculate the first flush that will be discarded= 30L x 50= 1,500

2) Put the figures into the calculation: 599.2mm x 100m² x 70% – 1500L= 40,144L per year

Calculating the Tank Size

To calculate the size of the tank you need and the number of Supertanks you need, you will need to have a look at how often it rains (the period between significant rain), how much water you use, and what will happen if you run out of water. If you have mains water then it is not a big problem because you can switch to mains water if you run out, but if you rely solely on rainwater then you will need to factor in periods of drought or little rain

The time between significant rain is a bit of a guess and you will need to use your best guess based on your past experience. You can also use the rainfall table to give you a better idea of the amount of rain you get and when. Don’t forget these are AVERAGES so a worst case scenario could be DOUBLE, possibly more.

To calculate how much water you consume, use the table above. You can either be general and simply use the ‘average household’ figure divide by 12 (12 months in the year) or if you want to be more specific adjust the figures to what you think is appropriate. Keep in mind you will use more water on your garden in Summer, and if you have a pool or ponds then that can change your useage too. The number of people in your home will also change your useage

To calculate the tank size you need

Period of no significant rain (days) x Daily Consumption (L)= Tank Size

Example

You live in Adelaide and during Summer you can remember times it hasn’t rained for 4 weeks (30 days). You have a family of 4 and use the rainwater for the lawn and garden only.

Daily Consumption: Lawn and Garden= (90,000 + 55,000) / 365 = 402L per day

Storage Volume Required: 30 days x 402L = 12,060 L

Supertanks Needed: 12,060/ 395= 31 tanks

Rainwater System Design

Now you know how many tanks you need you can plan your rain harvesting system. Things to consider are:

• space requirements
• distance from garden and/ or plumbing
• location of trees etc that may affect water quality
• visual impact
• distance from gutters/ downpipes
• connection to existing plumbing
• the use of filtration devices- gutter screens, first flush rainwater filters, rain- heads, outlet filtration.
• Pumps if required (you will probably need a pump if connecting to household plumbing)

Rainwater Health Issues

Installing and maintaining a Supertank is very simple, however you need to be aware of the potential hazards of storing water, especially if you are using it as drinking water. Some councils don’t recommend drinking rainwater so check to see if there are any restrictions first before you buy a tank.

Most problems occur because of dirt and pollutants from the roof building up and being washed into the water. By keeping your roofs and gutters clean and/ or using suitable filters like the Superhead rain head and first flush rainwater filters can prevent most problems.

Below is a list of the common problems and ways to prevent them

• Bird and Animal Droppings

Prune overhanging branches and keep tank covers closed and seal gaps to prevent access. Use a first- flush filter to discard the first few litres of rain collected.

• Mosquitoes and insects

Protect inlets and overflows with mosquito proof mesh. Prevent water from pooling and stagnating such as near the overflow or in gutters

• Lead Contamination

Check for and remove lead paints and primers on roof and gutters (pre 1970’s)

• Rust and corrosion

(rotten egg or sewerage odours)

Rusty gutters and downpipes can taint the water. This is from low pH caused by contact between leaves and metals for long periods so clean gutters regularly

• Bacterial Growth

(Musty, fishy or ‘rotten egg’ smells, or small white flakes)

Some bacteria (biofilms) are OK, but some others are bad. Anaerobic growth in the sediment at the bottom of the tank is usually not good so try to minimise sediment entering the tank and keep the water oxygenated by using it regularly. Also make sure no light can enter the tank.

Chemicals can also be added to treat bacterial growth.

• Coloured water

Caused by leaves rotting on roofs and gutters, or from sediment coming from the roof tiles or paint. Keep the gutters clean and use a first- flush filter

• Slime

A light coating of slime (bio-film) is common and good for maintaining the quality of water. Heavy slime may be a concern can be treated with chemicals.

You might also like to download these articles on ‘Making Use of Rainwater’, and ‘Water Tanks and gardening

Maintenance

Keeping rainwater tanks and first flush rainwater filters is easy! The main problems are usually caused when things like leaves collect on your roof and gutter. They will start to rot and breakdown making them easier to get washed into the tank. The rotting process can also encourage bacteria and insects to breed. So by simply keeping the gutters clean you can make sure your tank is clean and safe. You can even gut a range of devices like the Superhead to help keep your gutters clean.

It also work on a cycle. Most of us will get rain in the winter months, and preceding that is Summer and Autumn when there is little rain, and the leaves are falling off the trees. So a bit of common sense says that Autumn is a good time to check that the roof and gutters are clean, and also let the first few litres of rain first flush all the dust and other dirt off the roof if it hasn’t rained for a while.

You should try and use the water stored in your tank regularly to prevent it stagnating and growing unfriendly algae and bacteria. This won’t be a problem if it is plumbed into your toilet and laundry, but if you are just using it for the garden then try to use a little every week in winter.

Tagged as: first flush, Rainwater filters, Rainwater Tanks, water filters, water tanks