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Rainwater Harvesting Calculator

In the UK we refer to the British Standard 16941-1:2018 (previously BS 8515:2009+A1:2013) for guidance on rainwater harvesting. 

The British Standard gives recommendations on the design, installation, testing and maintenance of rainwater harvesting systems supplying non-potable water in the UK. 

In terms of calculation of the size of a rainwater harvesting tank the a three tier approach is introduced. 

1. Simplified approach for residential properties  where there is consistent daily demand 

2. An intermediate approach which uses simple formulae to calculate a more accurate estimation of of storage capacity

3. A detailed approach for non-standard systems where there is variable demand over the year  

The Simplified Approach

The simplified approach is, well, simple! All you need to know is the roof area in plan (not along the slope),  the annual rainfall in your area and the demand (number of residents). The BS provides a look up graph from which the optimal tank size is determined. The example below is completed for a house with 4 occupants, annual rainfall of  1000mm and repeated for 2 roof areas 45m2 and 90m2. Since the graph is  straight line the storage required goes up proportionately 1.5m3 and 3m3 respectively. The graph also shows an upper limit of storage of 3.6m3 (solid black line) for all scenarios. 


Rainwater harvesting Tank calculator


The Intermediate Approach

To calculate the storage requirements to meet non-potable demand the lesser of 5% of the annual yield (supply) or 5% of the annual non-potable demand should be calculated. 

Annual yield is given as: 

Yr = A x e xAAR x h x0.05 

Yr is 5% of Annual rainwater yield (litres) 

A is the collection area (m2) 

e is the yield coefficient (assume 0.9 in most cases)

AAR  is the site specific annual average rainfall (mm) 

h is the hydraulic filter efficiency (varies) 

Non-Potable water demand  is given as: 

Dn = Pd x n x 365 x 0.05 

Dn is 5% of annual non potable water demand (litres) 

Pd is the daily non potable requirement per person (litres)

n is the number of persons

So in our example above, assuming AAR= 1000mm, A is 90m2, e =0.9, h=0.9 results in an annual yield of 3,645 litres. 

Calculating non-potable water demand assuming n=4, Pd = 33 l/d results in 2,409 litres. 

In this example the lesser of the two values is 2,409 litres and so this is the preferred option. It can be seen that varying Pd to a maximum of 49 l/d would have resulted in a similar value to the the annual yield of 3613 litres. 

Use the rainwater harvesting calculator (intermediate approach) below to calculate the optimum rainwater harvesting system tank size/ Simply change the grey cells to represent your set up. 

The Detailed Approach

For larger schemes where there are significant economic benefits might be realised or where there is a temporal variation in demand it is suggested that continuous rainfall time series (or simulation modelling) should be used to address this variability. In terms of rainfall data 3 years daily rainfall should be used as a minimum and 5 is preferred. The method should provide information on the frequency of overflow operation and should provide statistics of water saved annually and days when no rainwater is available.

At freeflush our in house software fulfills all the requirements of the British standard and then some! We tend to use the detailed approach for all applications as it allows us to interrogate cost-benefit a little more closely. 

Contact us today and we'll assess your site using our unique Rainwater Harvesting software offering.

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