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Rainwater Harvesting

Rainwater Collection

Rainwater harvesting (RWH) is a simple process of collecting and storing rainwater that falls on a catchment surface for re use.  Typically this requires a capture area, some form of filtration, a storage unit and a means to convey the captured rainwater to where it is required. 

The benefits of rainwater harvesting are:

  • reduces demand on the mains supply resulting in a cost saving for metered properties
  • the reduction in local demand has the effect on the upstream network to reduce abstraction and relieve water stressed areas
  • reduces the amount of energy used for water treatment and transportation
  • If well planned, collection and diversion of surface run-off can also mitigate flood risk by cropping the peak flows into the surface water network

Rainwater can be used for non-potable purposes such as flushing toilets and urinals, supplying washing machines, irrigation systems, vehicle washing, sprinkler systems and so on.  Typically toilet flushing , clothes washing and outdoor use accounts for a whopping 50% (30%, 13% and 7% respectively . Source: Waterwise).  

Rainwater Harvesting is scaleable and though there has been an increased interest in domestic RWH a full range of solutions is available for the small business, commercial and light industrial sectors.

Guidance and Policy

The Code for Sustainable Homes has provisions to restrict surface water run-off for new build developments and encourages RHW for  new-build homes to collect rainwater. In addition, rainwater harvesting scores highly in the BREEAM and LEED rating systems.

BS EN 16941-1:2018 (which supersedes BS 8515:2009) 'Rainwater Harvesting Systems, Code of Practice', provides recommendations on the design, installation, testing and maintenance of rainwater harvesting systems for non-potable applications in the UK . It includes standards for filtration, for the manufacture and installation of storage tanks and a series of approaches for calculating the sizes of tanks. Annex C also sets out colour standards for pipework to distinguish it from the mains supply.

RW  is considered to have an important part to play in 'Future Water', the government’s water strategy, and in October 2012, Schedule 3 of the Flood and Water Management Act 2010 was enacted, requiring sustainable drainage of surface water to be included in developments that require planning approval or have drainage implications. RWH is now seen as a potential dual purpose tool to address flooding issues and provision of rainwater for non potable uses.

The following is some further useful guidance documents which cover in part water efficiency and RWH:

Water Wise White Paper:  Waterwise is a UK NGO focused on driving water efficiency. It is the leading UK authority on  water efficiency. Waterwise is developing the Evidence Base for Large Scale Water Efficiency  in Homes.

Harvesting Rainwater for domestic uses (Environment Agency).  This document provides information on rainwater harvesting systems in the UK. It  covers the supply of non-potable water for  domestic uses such as flushing the  toilet, watering the garden and washing clothes  using a washing machine. It does  not cover systems supplying water for  drinking, food preparation or personal  hygiene

Rainwater Harvesting : an on -farm guide : Water is essential for farming but often is taken for granted. Yet in many  parts of England and Wales water  resources are already scarce and are  likely to become even scarcer as a result of climate change . This document provides an overview of the techniques, and challenges of delivering on-farm rainwater harvesting

Types of System

BS8515 identifies the following 3 types of rainwater harvesting systems:

At freeflush we believe RWH should be simple, effective have minimal maintenance and an appreciable cost benefit ideally a payback within 10 years . Our systems are developed with these design principles in mind.


A number of calculations methods are available to determine tank volume and these are well summarised in Roebuck et al (Journal of Water and Environment Journal Vol 26, Sep 12, (3).  Likewise these simplicity calculations tools area available online.  However  as the rainwater harvesting industry develops these calculation tools are seen to be overly simplistic and do not result in optimum tank size. Roebuck et al suggest that simplified approaches should not be relied upon.

Here at freeflush we agree with the sentiments of Roebuck et al and have developed our own in house detailed simulation model which we believe results in optimum tank sizes with peak payback. Each system we supply is provided with a design statement based on our unique software which provides a detailed indication of how we predict the package will perform. 

Design Principles and  Cost-Benefit

We’ve been working with a wide range of clients to develop some guiding principles to design our systems for wide appeal : 

  • minimal maintenance
  • minimum footprint
  • Easily retrofitted
  • Quick pay back, ideally <10 years


Rainwater harvesting is a simple reusing rainwater at the point of capture. Rainwater harvesting packages must filter, store and convey to the point of use.

Existing rainwater harvesting systems typically cost thousands of pounds, are complicated to fit , maintain,  require pumps, electricity supply and space and don’t always have a suitable payback period.

Freeflush is different.

Our designed packages are shown to provide tangible cost benefit and address a number of the issues  raised by our clients.



Rainwater harvesting systems for stormwater management:feasibility and sizing considerations for the UK (Gerolin A, Kellagher R B, Faram M G, Novatech 2010)

Environment Agency. Harvesting rainwater for domestic uses: an information guide. s.l. : EA, 2010.

'BREEAM. BREEAM: BRE Environmental Assessment Method. [Online] 2007. [Cited: 11 25, 2012.]

DEFRA. Future Water: The Government's water strategy for England. [Online] 2008.

Rainwater harvesting: model-based design evaluation. Ward, S, Memon, F A and Butler, D. 61, 2010b, Water Science & Technology, Vol. 1, p. 85.

A whole life costing approach for rainwater harvesting systems. Roebuck, R M. 2008b.

BSI British Standards. BS 8515:2009 Rainwater harvesting systems - Code of practice. s.l. : BSI, 2009.

A review of models for low impact urban stormwater drainage. Elliott, A H and Trowsdale, S A. 3, 2007, Environmental Modelling & Software, Vol. 22, pp. 394-405.

Environment Agency: Re-using and harvesting water.

Greenspark Public Impact Profile