Is water quality worsened due to old infrastructure?

We supply drinking water to 1.8 million people. Our raw water is (historically) hard and sulfated. We are about to build a pilot plant in order to test different treatments (UF + RO, PRO + chemical precipitation + SRO, Pellet softening + UF + RO, etc.). We will try to simultaneously achieve a high quality water and a tractable reject stream (in quality and quantity). But our distribution network is, in some parts, about one century old; we expect to find scaling and biofilm. Some ideas about how can this lower the quality we want to improve? We will include a special module in the pilot plant, where we will be able to recirculate treated water along some pipes taken from the real network and check some lab indicators.