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Water softening using forward osmosis : economic and technical feasibility for residential systems / by Kristen S. Brastad.

by Brastad, Kristen S.

Water -- Softening

Water-supply engineering

Water -- Purification -- Membrane filtration.

Water -- Purification -- Ion exchange process

Water -- Purification -- Organic compounds removal

100 leaves : ill. ; 29 cm.

Thesis advisor: Dr. Frank Mahuta

Committee members: Dr. Deborah Jackman, Tara Kowalski

Introduction and background -- Literature review -- Materials and methods -- Experimental results -- Analysis and discussion -- Conclusions and recommendations -- A: Forward osmosis laboratory experiment -- B: Lab data forms -- C: Self-regenerating water softener cut sheet -- D: Reverse osmosis small system cut sheet -- E: Reverse osmosis whole home system cut sheet -- F: Mass balance spreadsheet -- G: City of Madison water characteristics.

The emerging legislative action to prohibit the use of ion exchange water softeners presents a unique opportunity for new technologies that may help improve water quality throughout the United States. Forward osmosis offers a viable alternative to those systems with advantages in reduced energy consumption and high rejection of a wide range of contaminants. This process requires the use of a second step for creating potable water. The purpose of this report is to analyze the technical and economical feasibility of using forward osmosis as an alternative to conventional ion exchange water softening.

Bench-scale testing was conducted to evaluate the effectiveness of using forward osmosis to remove hardness from water. Four different influent hardness concentrations were evaluated for their percent hardness rejected and the flux across the membrane: 0 mg/l as CaCO3, 10 mg/l as CaCO3, 100 mg/l as CaCO3, and 500 mg/l as CaCO3. The results of these tests were used as input to a mass balance model to estimate the overall hardness removal of the forward osmosis process in conjunction with a second processing step using reverse osmosis.

This report found that forward osmosis is able to remove 70% of the hardness ions from a water stream. Using the results from the laboratory data, the performance of the forward osmosis system was compared against a conventional softener. This report found that while the forward osmosis system has a fairly competitive hardness rejection rate, it adds more than 75 times the amount of salt as ion exchange into the environment every year. Acquisition and maintenance costs are double those of a self-regenerating system while operation costs are fifty times higher. There are many areas within forward osmosis for further research, such as creating draw solution recycling, generating new draw solutions, membrane commercialization, and exploring other second step options that will allow forward osmosis to become a more viable water treatment option in the future.

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