Impact of Point-of-Use Water Softening on Sustainable Water Reclamation: Case Study of the Greater Phoenix Area

摘要

Point-of-use (POU) water treatment is becoming more widespread due to concerns over the quality of municipal drinking water, specifically the presence of pharmaceuticals and personal care products (PPCPs), organohalogen pesticides, excessive hardness and excessive concentrations of total dissolved solids (TDS) also known as salinity. This report investigated the unintended consequence of POU water softening - the issue of salt accumulation in sewer systems across the nation, which forces regulatory authorities to examine possible salinity management tactics to preserve future water supplies. In the Greater Phoenix Metropolitan Area, increasing salt concentrations are already a concern for many municipal water supplies. To assess current salinity conditions, the Central Arizona Salinity Study (CASS) was initiated to establish a salt balance within the Phoenix area and provide potential management solutions. The results of the CASS showed that the second largest contributor to the salt residual in Phoenix is society, which includes residential, commercial, and industrial sectors. A common and well known way society contributes salt loading to wastewater is through POU ion-exchange water softeners. By expanding upon the research done by the CASS and narrowing the focus to a typical residential home, this report was able to quantify the amount of salts contributed by residential POU water softeners to wastewater. Salt residual and population data taken from the CASS were extrapolated to find the time period when the buildout condition for the Greater Phoenix area would occur. According to the CASS assumption, buildout would occur when the metropolitan population reached a maximum of 12 million people, as forecasted by the Maricopa Association of Governments (MAG). Assuming that salt residuals follow population growth trends, the buildout year was calculated to occur between 2065 and 2085, depending on population growth dynamics. Based on the known amount of salt released by POU water softeners and the calculated number of households with a water softeners, the annual salt contribution was calculated. The calculations predict annual salt loadings to wastewater from water softening to be approximately 45,500 tons in the year 2000; 142,200 tons in the year 2040; and 192,300 tons at buildout of the City of Phoenix. For the year 2000, the amount of salts contributed by water softeners alone accounted for 38.5% of society's salt contribution and 3% of the total salts entering Phoenix. For the year 2040, the amount of salts from water softeners are projected to account for 49% of society's contribution and 8% of the total salts entering Phoenix. At buildout, 41% of society's contribution and 8.6% of the total salt loading will come from water softeners. These calculated amounts of salt from water softeners are due only to the population increase. In actuality, these numbers will be even larger as they do not account for additional water softeners purchased for pre-existing homes that will be used to treat the increasing concentrations of TDS.