Selected technologies for centralised or decentralised drinking water softening were evaluated based on technical, economic, environmental and aesthetic indicators to identify the optimal treatment technology for a given setting. To achieve this, we demonstrated that a number of important indicators beyond hardness reduction and costs have to be included. All the evaluated centralised softening technologies could reduce water hardness to the target of 1.3 mmol/L at the Dutch drinking water treatment plant Beilen. CARIX® treatment and pellet softening with Ca(OH)2 resulted in a lower CCPP90 (0.25–0.30 mmol/L) than nanofiltration (0.30–0.35 mmol/L). Decentralised reverse osmosis had a water consumption of >100%, whereas decentralised cation exchange had a water consumption of 2.5%–4.5% which was comparable to centralised pellet softening (3.6%). Except for the electronic water conditioner that does not remove water hardness, the decentralised technologies were 7–10 times more expensive than the centralised technologies per m3 of softened water. The centralised softening technologies furthermore ensured supply of softened water to all customers in a water supply zone. Thus, in areas with hard water and limescale problems, investment in centralised softening at the local water utility is more optimal than widespread implementation of decentralised systems.

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