Abstract
Chloride is a key component of salt, used in many activities such as alkali production, water treatment, and de-icing. Chloride entering surface and groundwater is a concern due to its toxicity to aquatic life and potential to degrade drinking water sources. Minnesota being a hard-water state, has a high demand for water softening. Recent research has found that home-based water softeners contribute significantly to chloride loading at municipal wastewater treatment plants (WWTPs). Because of this, many WWTPs would now require water quality based effluent limits (WQBELs) to comply with the state's chloride water quality standards (WQS), unless they install chloride treatment technologies, which are limited and cost-prohibitive to most communities. A potential solution to this problem, is shifting from home-based water softening to a system where water is softened at drinking water plants, before reaching homes, i.e. centralized softening, analyzed in this paper based on its ability to address both chloride pollution and water softening needs, at reasonable cost. We estimate lifetime costs of three alternative solutions: centralized softening, home-based softening, and a Business as Usual (BAU) or baseline alternative, using annualized 20-year loan payments and Net Present Value (NPV), applied to 84 Minnesota cities with matching data on drinking water plants and WWTPs. We find that centralized softening using either Reverse Osmosis (RO) or lime-softening technologies is the more cost-effective solution, compared to the alternative of home-based softening with end-of-pipe chloride treatment, with a cost ratio in the range 1:3-1:4. Between the two centralized softening options, we find RO-softening to be the lower cost option, only slightly more costly (1.1 cost ratio) than the BAU option. Considering additional environmental and public health benefits, and cost savings associated with removal of home-based softeners, our results provide helpful information to multiple stakeholders interested in an effective solution to chloride pollution.
Highlights
Chloride is a key component of salt, used in many activities such as chlor-alkali [1] production, de-icing, fertilizer use, and water softening [2]
Considering additional environmental and public health benefits, and cost savings associated with removal of home-based softeners, our results show that centralized softening could be a critical component of effective chloride management compared to other alternatives, and provide helpful information to multiple stakeholders interested in an effective solution to chloride pollution
We considered a few options that focused on managing the landscape of home-based softening, such as: prohibiting the use of home-based softeners, upgrading to high efficiency home-based softeners in homes and industrially, switching to non-ion-exchange softeners, managing the level to which water is softened at home, and a softener column collection/exchange program, which would enable the recharging of used home-softener columns and reclamation of the brine used in the collection center avoiding its discharge to a wastewater treatment plants (WWTPs) [18]
Summary
Chloride is a key component of salt, used in many activities such as chlor-alkali [1] production, de-icing, fertilizer use, and water softening [2]. According to the United States Geological Survey, water with more than 120 mg/L as CaCO3 is hard [9], which translates to about 7.01 gpg [10]. Based on hardness prevalence [9], home-based water softening with point-of-entry ionexchange water softeners is common in many states, to treat water for domestic uses such as washing, bathing, and for use in household appliances [11, 12]. Ion-exchange is the most common type of point-of-entry softener: these work by running a sodium chloride (i.e. salt brine) solution through a resin base, a process that exchanges calcium and magnesium ions that contribute to water hardness, with soluble sodium ions, yielding soft water, and a waste brine containing the displaced and/or excess chloride ions along with the hardness ions [15]
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