Environmental impact of NaOH and lime milk in drinking water treatment

All kinds of chemicals are used to treat drinking water. Caustic soda and lime milk are used on a large scale. On the basis of information from the literature and manufacturers, calculations have been made to determine the current environmental impact of these chemicals and the extent to which it can be reduced.

Current environmental impact of caustic soda and lime milk

In view of the large-scale use of caustic soda (NaOH) and lime milk (Ca(OH)2) in drinking water treatment, water companies try to include the environmental impact of these chemicals in their tenders. However, it is difficult to obtain up-to-date information in this regard from manufacturers, who do often claim that the current impact is less than stated in the databases that are consulted for this purpose. This project was established with the aim of using LCA calculations to determine the current environmental impact of caustic soda and lime milk on the basis of the literature and data from manufacturers.

Available impact data

The impact per kg NaOH or Ca(OH)2 was estimated on the basis of literature data, supplemented by interviews with suppliers of these chemicals for drinking water treatment. The results were compared with other relevant information in databases such as AgriFootprint and EcoInvent.

What level of impact reduction is realistic?

The databases consulted use an impact of approximately 0.86 kg CO2/kg NaOH for NaOH calculations. That level results from the energy used in the production process and it is based on a combination of three different production processes. Currently, only the most energy-efficient membrane process is used in Europe and so average energy consumption has fallen. Carbon emissions have also fallen as a result. A 30-50% reduction in carbon emissions can be achieved by using green energy.

In the case of Ca(OH)2, 70% of carbon emissions are generated during the burning of lime. The amount of CO2 released here is inherent in the reaction from CaCO3 to CaO. However, the overall impact can be reduced by using optimised lime kilns, green energy and less environmentally burdensome modes of transport. That brings a reduction of 10-15% within reach. In addition, it is possible to capture the CO2 and use it, for example, to grow certain plants. However, whether this can be taken into account in the LCA calculation depends on the selected system limits.