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Brief Overview

  • Rainwater is collected from the roof area.
  • Leaves and other debris are removed via a pre-tank filter.
  • Rainwater enters the main storage tank via a calmed inlet.
  • Rainwater is stored in a central storage tank.
  • When a demand is detected, submersible pumps draw rainwater through a floating suction filter.
  • The system control panel controls an automatic mains water backup.
  • Rainwater is boosted to points of use including toilets, irrigation, vehicle wash down, and cooling.

Technical Explanation

 roof collecting surface for rainwater harvesting


Rainwater is collected from the roof area of the building. Roof spaces are ideal for rainwater collection because they are relatively free of debris and contaminants, unlike ground surfaces.

Rainwater is transported via downpipes and connected to below-ground pipework via sealed gullies. Sealed gullies are preferred over open grates as the risk of cross-contamination with the surface water is removed.  

Common pitched roofs with smooth surfaces offer optimal conditions for collecting large volumes of rainwater; however, rainwater harvesting is suitable for most surface types including green roofs.

Additional filtration will be required where rainwater is harvested from paved or hard-standing areas due to a higher pollutant load.

 a diagram of a pre-tank filter in a rainwater harvesting system

Preliminary Filtration

Rainwater collected from the roof area is sent through a pre-tank filter, located before the main storage tank. Pre-tank filters are designed to remove undissolved solids from the rainwater supply and prevent them from entering the main storage tank.

Leaves, bird debris, and other organic matter are all filtered out during this stage of filtration. Pre-tank filters work by passing rainwater through a mesh sheet of metal. Any remaining debris is collected on the surface of the mesh allowing clear rainwater into the tank.

Pre-tank filters must be easily accessible for maintenance as the filter will require regular cleaning to remove debris, even if the pre-tank filter includes a backwash spray.

Pre-tank filters should also be rated to 1000 microns (1mm).

 a diagram of an underground storage tank on a rainwater harvesting system


Filtered rainwater enters the main storage tank. Storage tanks can be located below or above ground. 

Rainwater enters the storage tank via an inlet calmer. The inlet calmer helps any remaining suspended solids to settle on the base of storage tank without disturbance from incoming rainwater.

Submersible pumps are located within the main storage tank and boost rainwater into the building. Submersible pumps work in collaboration with a floating suction filter, allowing rainwater to be collected from the cleanest part of the tank.

As part of the sizing process, rainwater harvesting storage tanks are designed to hold 18 days’ worth of rainwater to cover the total demand. This is based on the average length between rainfall events in the UK.

Main storage tanks are designed to naturally overflow several times a year to prevent stagnation.

 a diagram showing the mains water top up function on a rainwater harvesting system

Mains Water Backup

Rainwater harvesting systems include a mains water top-up function for periods of low rainfall and droughts.

Larger, more reliable systems will utilise a header tank. The mains water top-up enters the header tank via a WRAS-compliant air gap.

Smaller systems will utilise a pressure vessel and will require mains water to top up the main storage tank via an internal tundish air gap.

This function operate automatically and will be controlled by sensors and the central control panel.

Mains water top-up functions must have sufficient protection against mains water wastage in the event of a solenoid valve failure. Stormsaver utilise mechanical ball valves to protect against waste and also includes a flush-through feature to prevent stagnation.

 a diagram of additional filtration on a rainwater harvesting system

Additional Filtration

Additional filtration is recommended for optimal system performance and to protect end users.

UV disinfection is recommended for care settings where vulnerable people are present. The British Standard also recommends UV disinfection for where rainwater will be dispersed in fine vapour form, particularly dust suppression, and irrigation.

Best practice is to include a third stage of filtration to protect internal components and prevent shadowing effects in UV disinfection. Stormsaver supplies a self-cleaning, low-maintenance solution. Learn more here.

Further treatment can also be included in the rainwater harvesting system, depending on the collection surface.

 toilets being served by a rainwater harvesting system

Usage Points

Rainwater can be used to supply

  • Toilets
  • Irrigation systems
  • Vehicle and train wash
  • Manufacturing processes
  • Dust suppression
  • Cooling

Rainwater cannot be used to supply

  • Potable outlets
  • Showers

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