There are three possible interventions to reduce excessive flow rates - adopting water saving shower heads, using restrictors or aerators, and if your system includes a pressure regulator on each feed-in pipe, adjusting this to the desired pressure.
Eco-friendly shower heads
These shower heads use modern understandings of hydrodynamics to give an excellent showering sensation with efficient use of water. There are even shower heads suitable for low pressure piping systems, which when using conventional shower heads give an unsatisfactory flow. Several models are available on the market with prices ranging from a €5.00 to €45.00 per unit.
Restrictors and aerators
Restrictors are small devices fitted to shower hoses or wash-hand basin faucets to reduce excessive flows. Models offer different levels of restriction, with water flows that typically range from 4 litres per minute to 15 litres per minute. Aerators are similar with the added advantage of agitating the water to create more air bubbles in it, both restricting and invigorating the flow. Prices range from €0.60 to €4.50 per unit.
In systems were several showers are fed from a pressure regulated pipe, it would be worth examining whether it is possible to install a pressure regulator on the water supply line at each floor that will lead to a uniform 7 litre per minute flow on all showers on the line. If this is the case then significant water savings may be possible without making plumbing interventions on each and every shower-head. If wash-hand basins are fed from the same line, restrictors or aerators can be used to further reduce their water flows to 5 litres per minute.