Optimizing the implementation of drinking water softening

Abstract

Drinking water utilities face increasing demands for drinking water that is not only safe and affordable, but also healthy and produced without negative environmental and societal impacts. Hard water contributes to consumer inconvenience by causing lime scaling in household installations and appliances, and by increasing soap use. Water softening is increasingly implemented to increase consumer convenience and can potentially provide socioeconomic and environmental benefits in areas with hard water. Although softening may appear as just a matter of removing hardness (i.e. Ca2+ and Mg2+), softening technologies also change the overall mineral composition of the drinking water. If a process design for softening does not consider these changes, they may lead to adverse effects on e.g. human health and corrosion. Indicators predict the effects from water quality changes and are used in decision support to evaluate the performance of technologies. However, the potential is still unexploited for using water quality indicators as decision support in implementation of softening. This PhD thesis builds on the hypothesis that by carefully selecting indicators and by considering the overall motivation for softening, the actual implementation can be optimized for local conditions, including source water quality. The hypothesis was investigated through a case study: implementation of softening in Denmark. Historically, softening has been implemented to meet regulatory guidelines for mainly copper and lead, which can be released from pipe materials. Today, softening is predominantly implemented to increase consumer convenience, and softening has thereby shifted from being a “need to have” to a “nice to have” technology without a fixed guideline concentration as treatment objective. The softening technology which is the most suitable for reducing lead release from pipe materials may not be the best for reducing lime scaling in household appliances. Consequently, the overall treatment objective for softening should be clearly defined prior to designing the softening process to ensure that it is fit-for-purpose. Decision support systems (DSSs) integrate various information and models to evaluate decision alternatives. In water treatment, DSSs often encompass technical, economic, environmental and social dimensions in a broad systems perspective. Unfortunately, the broad focus often reduces evaluation of water quality changes to removal of a single target compound (e.g. Ca2+), which is insufficient to evaluate the overall effects from softening on water quality. Decision support can be improved by expanding the traditional frameworks by evaluation of additional water quality indicators. Softening is currently being implemented for the first time in Denmark to increase consumer convenience and provide socioeconomic and environmental benefits. If softening is not implemented centrally at the drinking water treatment plant, consumers may purchase decentralized softening units in their homes. Unfortunately, decentralized softening can result in increased water usage, lack of control with the drinking water quality, and can compromise the socioeconomic and environmental benefits from centralized softening. The indicator Calcium Carbonate Precipitation Potential (CCPP) is important to include when implementing softening in Denmark. CCPP predicts the maximum potential for lime scaling in open (e.g. shower and kettle) and closed (e.g. distribution networks) systems. Other relevant indicators include corrosion indicators from the Danish Code of Practice (DS 439), investment and operating costs, building area/height requirements, by-product reuse possibilities (if pellet softening is implemented), wastewater quality and quantity, and magnesium removal for health reasons. Denmark is almost exclusively supplied by groundwater and many water utilities face recent findings of pesticides that threaten the drinking water quality. This should be considered when implementing softening by ensuring that the treatment process can be extended with pesticide removal or that hardness and pesticides are removed simultaneously by membranes. The case study of softening in Denmark revealed that it is important to consider local conditions when evaluating softening scenarios. Water quality indicators provide valuable decision support, but also increase data requirements and analysis complexity. In practice, the level of information should be balanced to provide adequate decision support. This thesis has demonstrated that water quality is not only a technical issue but affects also human health, socioeconomy, the environment etc. Optimizing one aspect can result in suboptimal conditions in other aspects. In a given case, the best softening technology meets the treatment objectives and legislative guidelines, while balancing the remaining indicators to maximize the benefits from softening, identify possible adverse effects and support environmental and socioeconomic sustainability.