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Home Maintenance Advice Sizing Tanks
Sizing rainwater tanks for residential needs in NSW Print E-mail

 

There are four useful points to understand in order to maximise the amount of water you use from your water tanks

  1. The most important variables determining yield are, in order,
  • the number of appliances the tank is connected to,
  • the size of the roof area connected and
  • the size of the water tank.
  1. Rain harvesting systems in urban areas are being used most efficiently when rainwater tanks are empty.
  2. Estimate how much water you will need for toilets, washing machines and gardens. A rule of thumb for sizing tanks is to provide about 4 weeks supply.
  3. Use the ATA Tankulator


Maximising yield

The research is quite clear, rainwater use is demand driven, the more you need the more you will use. This is important because intuitively you would think rainwater use is supply driven, ie you can only use as much as is in your tank.

Coombes (2011, p5) found a 24% (48KL/annum) decrease in mains water use for tanks connected to outdoor uses and a 56% (90KL/annum) decrease for tanks connected to both indoor and outdoor uses in South East Queensland. The CSIRO (2014, p115) found an 11KL/annum saving from outdoor uses and a 31KL/annum saving from indoor and outdoor uses.

Coombes (2011) research showed that the number of connected appliances was a greater determinant of yield than the amount of roof area connected and both were greater determinants than the size of the tank. Co-efficients of determination for appliances, roof area and tank size relative to yield are 0.8, 0.6 and 0.3 (the higher the number the stronger the determination).

Why is this so? Because the more water you use, the more space there is in your tank to fill up the next time it rains. This is explained more fully below.

 

Water tanks in urban areas are being used most efficiently when they are empty

In the country it is important to preserve the rainwater in your tank because it is the only supply. In urban areas connected to mains water the reverse applies, you should use as much of your rainwater as possible because you have the security of the mains system to rely on.

If your rainwater tank is empty you will maximize the amount of water you can capture and store from the next rain event. Most people underestimate how often it rains and therefore how much opportunity there is to refill your tank. Here are the average number of days in Sydney where more than 1mm of rain fell each month. 1mm of rain represents 100 litres from a 100sqm roof. Of course the actual rainfall events would have ranged from 1mm to over 100mm, the point is the relative frequency of rain.

Figure 1: Number of days each month in Sydney with rainfall over 1mm.


The average and recent rainfall for any site in New South Wales can be found on the BOM website.


How much water will you need?

The following table, based on the RHAA standard, Rainwater Tank and Installation Handbook HB230, can assist in estimating rainwater requirements and savings for different uses. A rule of thumb for sizing tanks is to provide about 4 weeks supply.

 

Table 1 Water requirements for washing machines

Rainwater Use

Is the use Seasonal?

F/loader small l/annum

F/loader medium

l/annum

F/loader large l/annum

T/loader small l/annum

T/loader medium l/annum

T/loader large l/annum

Washing machine 2 washes/week

No

5,356

6,916

8,528

10,920

14,196

17,472

Washing machine 3 washes/week

No

8,008

10,400

12,792

16,380

21,320

26,208

Washing machine 4 washes/week

No

10,660

13,884

17,056

21,840

28,392

34,944

Washing machine 6 washes/week

No

16,016

20,800

25,584

32,760

42,588

52,416

Washing machine 7 washes/week

No

18,668

24,232

29,848

38,220

49,712

61,152

Washing machine 8 washes/week

No

21,320

27,716

34,112

43,680

56,784

69,888

Washing machine 9 washes/week

No

23972

31,200

38,376

49,140

63,908

78,624

Note (Source: NSW Guidelines for Greywater Reuse in Sewered, Single Household Residential Premises.)

Small – up to 5.5 kg/load, Medium 6-7 kg/load, Large Over 7.5 kg/load

Assumptions: Top Loading: average machine is larger (over 7.5 kg) with a 2-star WELS rating.

Front loading: average machine is medium (67 kg) with a 4-star WELS rating.

For one unit increase in stars there is approximately a 30% decrease in water requirements per load.

NOTE: Many washing detergents can be detrimental to the environment, if unsure it is recommended that the first wash be directed to sewer not greywater reuse. If this is the case, the volume of greywater produced from a washing machine will be less than stated in this table.

 

Table 2 Water requirements for toilets

Rainwater Use

Is the use seasonal?

1 person, l/annum

2 persons l/annum

3 persons l/annum

Each extra person l/annum

Toilet Single flush, 11l/flush WELS 0

No

20,100

40,200

60,200

20,100

Toilet dual flush, 11/5l/flush WELS 0

No

11,300

22,600

33,900

11,300

Toilet dual flush, 9/4.5l/flush WELS 1

No

9,900

19,700

29,600

9,900

Toilet dual flush, 6/3l/flush WELS 3

No

6,600

13,100

19,700

6,600

Toilet dual flush, 4.5/3l/flush WELS 4

No

6,000

12,000

18,100

6,000

Note: All figures are in litres per year (local housing type and occupants’ age may vary the above consumption volumes). Toilet flushing figures based on AS/NZS 1172 & AS/NZS 6400. Old plumbing systems designed for larger volume flushing toilets may not perform to expectation when new 4.5/3 L pans have been installed. If in doubt, evaluate the system before installing new pans.

 

Table 3 Water requirements for gardens

Rainwater Use

Seasonal

Annual usage (litres/year, Sydney)

Annual usage (litres/year, Newcastle)

Water requirements for 100sqm warm season grasses (Buffalo, Couch)

Yes

17,000

14,500

Water requirements for 100sqm cool season grasses (Rye, Fescue)

Yes

48,800

44,200

Water requirements for a drought tolerant garden based on 100sqm of garden.

Yes

1200

700

Water requirements for a garden with high water requirements

Yes

35,000

33,000


ATA Tankulator

For NSW new homes and additions the minimum tank size is specified by the BASIX tool. You may wish to use the Tankulator as a more customised guide tailored to your needs but you must still meet the BASIX requirement.

This excellent online resource allows you to select your local postcode and, like BASIX, uses local rainfall data. The Tankulator allows you to specify different uses for your rainwater and calculates the reliability of your supply based on roof area and tank size.

Arguably the most important feature of the software is that it allows you to model different combinations of uses and supply. For example you might find that an internally plumbed 5,000 rainwater tank provides you with rainwater for 79% of days throughout the year. Increasing the size of the tank to 10,000 litres might only increase reliability of supply by 6% to 85%. This increase may not justify the additional costs involved.

Our acknowledgement to the Alternative Technology Association for providing the free software.

http://tankulator.ata.org.au/interactive.php


References

Coombes (2011) Insights into Household Water Use Behaviours Throughout South East Queensland During Drought

ARID (2008), Rainwater Tank Design and Installation Handbook HB 230

Moglia M, Tjandraatmadja G, Delbridge N, Gulizia E, Sharma AK, Butler R, Gan K (2014) Household Survey of savings and conditions of rainwater tanks. Melbourne, Smart Water Fund and CSIRO, Australia.


This material has been prepared as a result of discussions with the NSW Department of Planning although RHAA is solely responsible for the material. The RHAA provides these as general principles with no implied liability for the advice. In all circumstances you should rely on your own sources for advice that will meet your specific needs. 

Last Updated on Thursday, 29 October 2015 14:10
 
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