Natural Drinking Water System for Schools

We are looking for an energy-efficient and natural drinking water treatment system for several schools with about 1000 pupils and 100 teachers. The available water is groundwater with various loads like Iron. The capacity should be about 4,000 liters per day. Reverse osmosis, nano or membrane filters are not desired. Sustainability and easy maintenance are a must. Which natural water treatment system is the best to use?  Who has experience with such conditions and systems and knows appropriate providers?