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Water Storage Reservoirs: Sizing for Backup Process Water Supply Systems

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In SBS Tanks™ previous post in this blog series, Backup Process Water Supply Systems | Planning for Business & Industry, we discussed how to go about completing a water usage analysis and water audit. Now it is time to put this information to work by using it to identify how much water you will need to have on hand in water storage tanks or reservoirs. Having completed your water usage analysis you can now look at how much water you need in your storage reservoirs to keep your business running when a water emergency occurs in your area.


The method to size your water storage tanks will actually be quite simple, but before you get out your calculator there is one important decision that must be considered and that is at what capacity you want your business to operate while on the backup process water supply system. There are 3 basic levels as described below:

  • Maximum Required Water Storage Capacity 

The maximum amount of water that your business needs on a daily basis to continue operating at 100% capacity. This is business as usual with no restrictions. 

  • Reduced Required Water Storage Capacity

The amount of water your business needs on a daily basis when operating at a reduced capacity. Your production line supplies all customers while items list under non-essential usage are stopped. Water available for staff usage may also be reduced but not eliminated completely.

  • Minimum Required Water Storage Capacity

The absolute minimum your business will need to continue operating. Only key customers will be supplied with essential products. Non-essential usage items and most staff usage of water is eliminated.

 When choosing one of the above options you should first consider the cost to your company when operating without water. If you can afford to operate at minimum or reduced capacity then the cost of your backup storage tanks and system may be less in comparison to those of a business operating on a 100% backup water supply.

Next, you must decide the number of days of backup water supply that you will have available. Consider carefully the frequency of water outages you experience monthly and how many consecutive days they normally last. Pay particular attention to the age and condition of the water supply infrastructure in your area. Remember, what may not be a big problem now, could become a nightmare in the near future.

Finally, consider if rainwater harvesting is an option for your business. If you have a large roof area and space for larger water storage tanks then perhaps you should think about harvesting some of this free water. It can be used either as extra backup process water supply or to supplement your daily usage requirements. You may find that adding 50kl to you water storage tanks is not as costly as you expected and that the extra water, when used for flushing toilets, washing cars or irrigating your grounds can mean big savings on your water spend over time.  To get an idea of how much extra space to allow for in your water storage tanks simply multiply the roof area you have available to harvest from by your average monthly rainfall total (annual rainfall divided by 12 months) and that will give you a good estimate of harvesting potential.

Example: An average annual rainfall of 460mm divided by 12 months gives an average monthly rainfall of 38.3mm per month, Multiply that by a 1 000 square metre roof area and that adds up to 38 300 litres average per month of potential harvested free water.

The Spanish Weather website TuTiempo.net has historical weather data for most major cities in South Africa.

With all of these considerations taken into account you can get the required capacity of your water storage tanks. First, take out your water usage assessment and add up the daily usage for each piece of equipment and all processes that you will need to run in order to meet your chosen level of operational capacity, this is your daily requirement. Next, multiply the daily requirement by the average number of consecutive days that water outages last in your area and that’s it, you have the required size for your water storage tanks.

If you would like to add rainwater harvesting as an extra then you simply add the monthly potential number of  litres that can be harvested monthly as calculated in the example above to the required operational amount and  this is the capacity of your water storage tanks.

Example:  A business owner has determined that his operation needs 80 000 litres of water per day to operate at a reduced capacity which will allow him to supply his customers and keep his employees happy. He investigated local records and found that his business has been without water an average of 3 times per month over the last 2 years and the outages last from 1 to 3 days.  He decides that storing 3 days of water is going to be his best option so he works out that he will need water storage tanks with a capacity of 240 000 litres. He also sees that harvesting rainwater could yield approximately 65 000 litres  per month of free water to use within his operations and decides to investigate the cost of adding this to his water storage tanks and sends out a request for quotation from local water storage tank suppliers like SBS Tanks™  for a 240 000 and 305 000 litre storage tank. 


I Know What Size Water Storage Tanks Are Required, What’s Next?

Armed with this information you could now go out and request some quotations from suppliers of water storage tanks but we would suggest that you wait just a bit longer, at least until after our next blog that will be coming out later this week which will discuss pumps, piping and delivery systems.  It will be most helpful for those quoting on water storage tanks to know the sizes of piping you will require in order to accurately advise and price which inlets and outlets etc. will be required.