Selective Coagulant Recovery from Water Treatment Residuals using the Donnan Membrane Processes
The primary objectives of this research were to selectively recover coagulants such as alum from water treatment residuals using a Donnan Membrane Process (DMP) or Donnand Dialysis. This process is driven by an electrochemical gradient across a semipermeable ion exchange membrane. Laboratory results confirmed that over 70% recovery of alum is possible with essentially no particulate matter, Natural Organic Matter (NOM), or other trace metals.
During the last two decades, pressure driven membrane processes, namely reverse osmosis (RO), nanofiltration (NF) and ultrafiltration (UF) have found increased applications in water utilities and chemical industries. Unlike RO, NF and UF, Donnan membrane process or Donnan Dialysis is driven by an electrochemical potential gradient across an ion exchange membrane. Theoretically, Donnan membrane process (DMP) is not susceptible to fouling because particulate matter or large organic molecules do not concentrate on the membrane surface, as commonly observed with pressure driven membrane processes. Although information on several applications of DMP is available in the open literature (1-2), no work is reported on use of DMP to treat a sludge or slurry with high concentration of suspended solids or large organic molecules. It was conceived that a single-step Donnan membrane process could selectively recover coagulant alum (Al2(SO4)3.14H2O) (3,4) from water treatment plant sludge or water treatment plant residuals (WTR), which are the end product of coagulation. WTR contain insoluble aluminum hydroxide (50-75%) along with suspended inorganic particles, Natural Organic Matter (NOM) and trace amounts of heavy metal precipitates (5). Several efforts were made in the past to recover alum from WTR. Acid digestion process is the most commonly tried process at laboratory, pilot-scale, and plant level (6). In this process, WTR are sufficiently acidified with sulfuric acid, dissolving insoluble aluminum hydroxide in the form of Selective Coagulant Recovery from Water Treatment Residuals using the Donnan Membrane Processes