Rainwater Harvesting

Introduction

In the UK, it is permitted to use rainwater (or recycled greywater) for:

  • Toilets
  • Washing machines
  • Garden taps

Since those can add up to a significant proportion of the total water use, both the House and the Outbuildings were built with separate pipework systems to isolate those appliances which are permitted to use rainwater from those which are not.

There is a good overview of rainwater harvesting considerations in this 2010 Guide from the UK Environment Agency. Note that rainwater – while it might look clean – will be contaminated with bacteria and must not be drunk. It is considered a “Category 5” contaminated liquid. There are dark green warning labels attached to every outside tap and near the internal stop taps in both buildings. There are no warning labels on toilets since it seems a safe assumption that nobody will attempt to drink from these.

System Overview

Rainwater is collected from five downpipes around the inside of the Courtyard, which serve about half of the roof area of the Outbuildings. (Those downpipes intentionally connect straight into the underground pipework, not via open gullies with grids, to reduce the contamination getting into the tank.) The collected water is stored in the 5,000 litre underground tank with the large, round black plastic cover near the Stable Door. The associated controls and valves are housed in the adjacent Store Room.

Annotated photo of the RainWater Harvesting controls and associated pipework

When there is a demand for rainwater (indicated by a drop in pressure in the pipework) the submerged pump in the underground tank is started by its pump controller (under the grey plastic cover). To avoid the pump starting for every single water draw-off, the pump ‘charges up’ the 24 litre white expansion vessel so often the demand is satisfied from this expansion vessel instead and the pump only starts when that is empty.1

If there is no rain for long enough that the underground tank gets close to empty, a float valve attached to the pump will open a mains water top-up valve which adds a little mains water to the tank via the clear plastic hose, ensuring the tank never runs out of water and the pump never runs dry. This does mean that the system does not provide complete exemption against hose pipe bans.

Metering and Monitoring

There are water ‘sub-meters’ which track the usage of mains water and rain water in both the House and the Outbuildings.2 These help to:

  • Show what proportion of the total water consumption is satisfied by harvested rainwater
  • Report on any mains water leaks on the 600m of underground pipework between the Severn Trent billing meter on Snelsmoor Lane and the stop taps inside the buildings
    • This isn’t automated in any way, since the Severn Trent meter readings are not easily accessible, but it is useful to compare the sub-metered mains water usage whenever Severn Trent send a water bill

All the water meters have M-Bus (Meter-Bus) reader heads and are automatically read every minute like all of the other M-Bus meters (for Heat, Electricity etc.). These readings are published as MQTT Messages and can be displayed in Home Assistant.

There is also a water level sensor immersed in the underground rainwater tank and a water pressure sensor which monitors the pressure of water in the rainwater pipework.3 These are both connected to the KNX Home Automation bus and their readings can be accessed via Home Assistant.

Finally, the electrical circuit feeding the rainwater pump and controls is fitted with an electrical sub-meter to monitor the electricity usage. The Wisy Zeta 02 Pump Controller is specifically marketed as a high efficiency unit, consuming less than 0.2W in ‘standby’ mode (though of course rather more when the pump is running) so this meter barely moves – typically incrementing by 0.1kWh every day or so.

Regular Maintenance

The main maintenance required is cleaning the filter inside the rainwater tank. This contains a fine metal mesh which tends to get blocked with a sticky, gritty residue from the gutters and once blocked it means any rainfall goes straight to the overflow rather than into the tank. It should be cleaned every 3 months.

The expansion vessel will probably need to be periodically re-charged with air. The usual trick of tapping the side of the tank will indicate how much of it is occupied by air versus water – and the pressure sensor is expected to also indicate when the pump pressure is no longer being maintained by the expansion vessel.

General Notes

  1. When making any additions or changes to the plumbing system it is important to ensure the separation between the two pipework systems is maintained. In the Outbuildings all the water pipes are exposed and there are identifying labels attached to the insulation on both sets of pipework. In the House, the mains water pipes are not marked but the rainwater pipes have labels attached to their insulation (though most of these are concealed behind plasterboard and flooring).
  2. The pump and controller, purchased from Wisy AG in Germany, were supplied pre-fitted with German-style electrical mains plugs. It seemed best to retain these and not to shorten the supplied electrical cables (hence the large coil of black cable attached to the wall) so there are two connections to German-style 230V socket outlets on the wall. Since these German plugs do not include fuses they are reliant on the overcurrent protection provided by a dedicated 16A circuit breaker which only feeds this one circuit.
  3. The rainwater pump is a single point of failure. While it is a high-quality unit with a stainless steel enclosure which should last for many years, if it does fail then all rainwater supplies will stop until the pump is repaired or replaced. Toilets can still be flushed by pouring a bucket of (mains) water into the pan (or cistern). Consideration was briefly given to allowing for some sort of ‘filling loop’ to deliver mains water via the rainwater pipework, but the rules for “Category 5” liquids make this problematic.
  4. The pump draws in water via a floating strainer which sits a little below the surface of the water in the tank, where the water is typically cleanest. With this and with a new, clean tank there has been no problem with dirt appearing in the delivered rainwater. If that does become a problem – e.g. if specs of dirt are noticed on white laundry – then it will be possible to retrofit an inline filter system – either just for the washing machine or for all of the rainwater. Perhaps one of the ‘Hydra’ models from Atlas Filtri. Note that most Web searches for “Rainwater Filter” will return results for the filter on the inlet to the tank; it’s better to search for “Water Sediment Filter” instead.
  1. Sometimes you will hear a toilet cistern start to re-fill, then gradually stop, then start again when the pump kicks in. This is perfectly normal. ↩︎
  2. There is also a further sub-meter which tracks how much of the mains water in the House is consumed as Hot Water ↩︎
  3. The main use for this is expected to be showing the effectiveness of the expansion vessel in preventing frequent pump start-ups – and indicating when that needs re-charging with air ↩︎

CC BY-SA 4.0 Rainwater Harvesting by Marsh Flatts Farm Self Build Diary is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.