Degree Granting Department
Daniel Yeh, Ph.D.
Reverse osmosis, Nanofiltration, Groundwater treatment, Blending, Process evaluation
At the water treatment plant in Dunedin, Florida, reverse osmosis membranes remove the hardness from groundwater sources. Reverse osmosis membranes remove salts, pathogens, and organics from the feed water but can create an aggressive permeate. The membranes strip most ions in the process and the resulting permeate, if not subjected to blending on post treatment, has a tendency to leach metals from lead and copper pipes in the distribution networks. To prevent such problems, the permeate needs to be blended with partially treated raw water or to be chemically treated to re-mineralize and add alkalinity back into the water. In the last decade nanofiltration treatment has gained an increasing foothold in the water treatment industry especially as a water softener.
Although nanofiltration membranes also have a high removal rate for organics and pathogens, the separation process is more selective towards multivalent ions (e.g., Ca²?, and Mg²?) than monovalent (e.g., Na?) ions. Most membrane softening plants blend minimally treated raw water with the membrane permeate as a means to reduce the aggressiveness of the water. However, blending can cause issues with disinfection byproducts and pathogen re-introduction. With nanofiltration membranes, fewer mono-valent ions are rejected which creates a more stable permeate and can reduce the blended water ratio. Since it is unlikely that most plants that use membrane filtration for water softening will be able to stop blending entirely, any improvement or sustainability of water quality at a reduced blend ratio should be viewed favorably within the water treatment industry.
The study evaluates three nanofiltration membranes: TFC-SR, NF-90, and ESNA1-LF in relation to the reverse osmosis TFC-S RO membrane currently in use at Dunedin. Water flux and salt rejection of the permeate water were compared using solutions of NaCl, MgSO4 and CaCl2. Since the Langelier Saturation Index (LSI) is one of the main tests of the blended finished water and is used to judge water quality prior to its release into the distribution system, this study created a 0%, 10%, 15%, 20%, 30%, and 100% blend ratio for each membrane to compare and contrast the change in the LSI. The TFC-SR membrane showed the most promise in lowering the blend ratio while improving the aggressiveness of the finished water by showing a lower rejection for divalent ions. The TFC-SR membrane also showed an improvement in the LSI relative to the other membranes over the total range of blend ratios.
Scholar Commons Citation
Keen, Michael, "Evaluation of the impact of membrane change at a membrane softening water treatment plant" (2009). Graduate Theses and Dissertations.