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20 Jun 2022

Reuse every drop! Why tank sizing is key to any rainwater harvesting system

Are you working on a project which involves rainwater harvesting, or interested in how we size your system to maximise water and carbon savings?

Out of sight, out of mind. Incorrect! Storage tanks are an important part of any rainwater harvesting system. Tanks help to prevent stormwater going directly to drainage infrastructure, ensure there is a constant water supply to the facility and hold some of the filtration devices which improve water quality. So, you have probably gathered by now that any rainwater harvesting system requires a correctly sized storage tank to meet the bespoke site requirements.

In most instances, a water recycling specialist like Stormsaver will help customers to size a rainwater storage tank to get the most efficient system, but on some occasions tank sizes may already be specified as part of the mechanical and electrical package. This is where getting a tank sized correctly matters for the long term performance of the system.

Site specific sizing

No two sites are the same. Location, scale, ground conditions and demand will all impact on the specific size of tank required to achieve maximum water, monetary and carbon savings. So, what are the factors we consider when sizing a system?

Weather – it would be fair to say that the British climate is quite unpredictable at times! However, using Met Office rainfall data from the past 20 years, annual rainfall within a given location can be calculated and used to predict rainwater runoff. This explains why giving us the site location is crucial to giving an accurate proposal!

Demand – nobody wants to be left without water. In fact, many facilities would be required to close without water! Ensuring the rainwater harvesting system can keep up with the demand is key to building safety. It also ensures pumps are not constantly running; prolonging their lifespan. We will always ask for the estimated occupancy of the building and give a unique rainwater harvesting solution based on this.

Think of it like this; you’ve got a rainwater harvesting system which is supplying 20 WCs. There are around 500 workers and each of them will start their breaks at a similar time. Flushing demand is higher in peak times, like breaks or lunch times because of the number of workers using the WCs. To conserve the most amount of water, we want to make sure that the majority of these flushes use recycled rainwater instead of carbon intensive mains water.

Collection area – In many cases, large roof areas require larger rainwater storage tanks. If a building has a large collection area, but a small tank, then most storm water runoff will go to straight drain. This increases pressure on local drainage systems and limits the effectiveness of the rainwater harvesting system.

Not too small, or not to big

Yes – getting it just right matters to achieve optimum efficiency! Storage tanks which are oversized risk keeping water for long periods of time, risking stagnation and having an effect on water quality. Too small a tank will see precious rainwater going to drain instead of being recycled as the tank will already have reached capacity.

However a large roof area doesn’t necessarily follow that you will need a large storage tank. If you have a huge warehouse, but only 6 toilets it is a waste of money to pay for a huge tank. In reverse, a tall block of flats will have a high demand but a small roof area, so a large tank would never fill. We will always take into account both your collection area and demand to ensure you are getting the most value for your money.

Leave it with us

Rest assured, you’re in safe hands. By placing your trust in us, our team of rainwater harvesting experts will always propose the most efficient system based on your bespoke system requirements. So, no matter how big your project is, why not take advantage of our free rainwater consultancy service? Our friendly team can be contacted here.

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