Reduce use without spoiling function, and keep reused water separate from the wholesome supply.
Water efficiency means using less water for the same useful job. A fitting that forces a second flush or a much longer running time can defeat the saving.
Follow the water's origin
Greywater is used water from sources such as baths, showers and washbasins; it excludes WC waste. Rainwater harvesting collects rainfall, commonly from roofs. Neither source becomes drinking water simply because it looks clear. Water softening changes mineral content; it does not turn mains water into greywater.
A reuse system must treat and manage its collected water for the intended use. WC flushing can use suitable-quality alternative water, but that permission does not extend to the kitchen drinking tap. Identify the separate pipes and outlets, prevent cross-connections and allow for maintenance and treatment failure.
A mains top-up still needs separation
A rainwater tank may need a mains top-up in dry weather. Connecting both supplies together through a pair of ordinary valves is not acceptable separation. Use an appropriate break-tank/air-gap arrangement for the assessed risk and agree the installation with the water undertaker. The protected mains inlet must not become submerged or contacted by the alternative water.
Keep the fitting's function
Choose a WC pan and flushing arrangement that are compatible at the intended flush volume. A reduced-flush button is part of a designed system. Throttling the cistern's inlet valve changes the refill rate; it does not reliably change the volume discharged by a full cistern. Loose objects in the cistern can obstruct mechanisms.
For taps and showers, match flow-control fittings to the appliance and supply. Repair leaks as well as reducing unnecessary running time.
Worked example: a tap runs for three minutes at 12 litres/minute: 12 × 3 = 36 litres. At 8 litres/minute for the same time, it uses 24 litres: a saving of 12 litres. If the slower tap then runs for 4½ minutes, it uses 36 litres again. Compare the completed task, not just the advertised flow rate.
Recall check: bath water collected for treatment is greywater; roof runoff is rainwater; WC discharge is neither of those reusable source streams. Their collection, treatment and distribution arrangements must remain appropriate to their different risks.
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