To identify the best option for a new water softening system in Leiduin, five different scenarios were compared in terms of Total Costs of Ownership and life cycle analysis. Pellet softening, particularly on the basis of lime milk produced on location, seemed to be the most favourable option. However, this method did run up against practical problems in terms of the local burning and extinguishing of the lime.
New softening process
A pellet softening system is used at Waternet in Leiduin that is based on adding NaOH. The question was whether lime milk could be used instead, preferably made from calcite produced during the softening process itself. Another option was reverse osmosis (RO).
Five scenarios for softening
The Total Costs of Ownership (TCO) and a Life Cycle Analysis (LCA) were calculated for five different scenarios. They covered pellet softening with NaOH, with pH correction with HCl or CO2, pellet softening with lime milk, and RO with and without dune passage. The use of lime milk was studied to see whether calcite, which is formed in the softening process, can easily be burned itself to produce lime and be worked up to lime milk.
TCO and LCA
The main drawback of RO processes is that recovery is 15-20%. As a result, a larger flow of water is needed in the pre-treatment stage, making the total costs higher than for pellet softening. RO processes also have a larger environmental impact for the same reason. If only the carbon footprint is considered as an environmental impact when green power is used, RO processes would actually seem to be slightly more favourable than pellet processes. Processes based on lime milk are expected to have the smallest overall impact, although practical objections remain for the time being to drinking water companies burning and extinguishing lime themselves.