The water supply in our apartment is hard water and is not suitable for daily use like washing clothes, drinking, bathing etc.
Are there water purifiers we can put on the main water pipe which goes into our house? How sustainable are they?
There is only one effective measure, and that's an ion-exchange water-softener.
The good news is that the major inputs are sustainable, or potentially sustainable: electricity, and salt. Similarly, the waste product, calcium chloride, has no (to my knowledge) negative ecological impact on being added to waste-water streams.
As for the catalyst itself, I don't know about its sustainability. On the positive side, it does last for years.
The casing is recyclable plastic.
The electronics (if present) should on disposal be treated as electronic waste, and can be processed to extract reusable materials.