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Gravity Fed Rainwater Harvesting System For Flushing Toilets

We're often asked "Can I plumb the rainwater into my toilet cistern to flush the toilet". The simple answer is yes, with some caveats!

The conventional way would be to opt for a standard undergound rainwater harvesting system which provides a large amount of storage out of sight. However, an undergound system requires a pump and controls to deliver the rainwater to where is is required and so requires power, maintenance and a large excavation for the underground tank. This isn't always possible and so an above ground rainwater harvesting water systems may be considered. 

Here we describe the principles of a simple gravity fed rainwater which allows ground floor toilet cisterns to fill and flush using rainwater. This system can also be used to irriate garden spaces assuming the garden level is below the base of the tank. This systems should not be confused with an indirect underground rainwater harvesting system which delivers rainwater to a loft tank which then drains by gravity. 

An outline diagram of a gravity fed rainwater system for flushing a downstairs toilet or for irrigation is shown below. The tank typically sited on an external wall: 

gravity fed rainwater harvesting for flushing toilets

 

System components: 

1. Primary Rainwater Downpipe Filter - this is required to screen out leaves and debris from the stored rainwater. In turn this limits decaying in the tank and potential blockage of downstream components (pipe and float valve). 

2. Rainwater Storage tank - this should be sized according to the demand , a 300l tank would provide ~50 flushes. Another consideration is how the tank is placed. If it will be fixed to an external wall then the tank size will be limited by the strength of the wall. If the tank will be located on a raised plinth the tank size limit will be determined  by the strength of the plinth. The base of the tank must be at least 0.5m above the top water level in the tank it is delivered to.

3. Mains Water Top up - this can be a mechanical or solenoid valve which simply fills the tank a small amount if the tank is nearing empty ensuring there is always some water to flush the toilet. 

Notes: 

1. Atmospheric pollution and the surfaces from which rainwater will be collected from, means rainwater is likely to contain faecal, biological and pathogenic contamination and this places it in fluid category 5, that is to say it poses a serious health hazard. To meet the Water Fittings Regulations in the UK, fluid category 5 protection can only be achieved through the installation of either a Type AA, AB or AD air gap, or through the use of a Type DC pipe interrupter (a device that incorporates an air gap), to separate the mains water supply and a water reuse system. This applies to the mains back  up device be it mechanical or electrical (solenoid). 

2. Some areas of the UK are at risk of freezing and so an above ground tank may not be suitable for these areas. 

3.Delivery pipework should be 22mm to minimise hydraulic losses. Limit the number of bends and reducers. 

4. The flow restrictor in any downstream float valve must be removed since the the operating pressure will be 0.05 bar - i.e. very low. With the flow restrictor removed from the toilet cistern float valve the fill time of a cistern under such low pressure is comparable to the fill time under mains pressure. 

Summary: 

Gravity fed rainwater harvesting systems provide a simple and inexpensive alternative to conventional underground rainwater harvesting systems. The benefits are lower capital and installation cost, reduced maintenance cost and operational cost since a pump is not required to pump the rainwater. However, gravity fed systems may be limited by tank size and exposed to freezing risk.