AWWA MTC69731

This paper describes reverse osmosis (RO) plants and operating experiences at three municipalities in Illinois that operate on well waters with 1000-4000 µS/cm conductivities and above the maximum allowed concentrations of radionuclides. In all three cases, pilot plant testing verified that RO worked as expected without any big surprises. The full-scale RO plants started up in 2002-2004, and varied in size from 0.4 million gallons per day (mgd) (72 m³/h) to four trains, each with 1 mgd (159 m³/h) permeate flow rate capacity. Antiscalants and sulfuric acid dosing to pH 6.8 prevent precipitation of inorganics in the RO elements. The three RO plants have operated trouble free, except for replacement of a PLC card and a pump motor, for 4-6 years. Low energy RO elements, which require a lower feed pressure to the expense of a higher solute passage, were used, but still the passage of hardness and radium was only 1-2 percent based on the feed concentration. The calculated passages of adjusted gross alpha and gross beta were higher, 4.5 and 33 percent respectively. This was most likely the result of progeny from radon, which is not rejected by the membrane, so air stripping the permeate to remove the radon would most likely remove most of the gross alpha and gross beta. The uranium concentrations in the feeds were too small to be measured accurately, but other studies have shown that RO membrane rejection of uranium is above 99 percent, at least at pH 6.8 and above. Two of the RO units had three stages and operated on high TDS water without an interstage booster pump, which resulted in a negative net driving pressure for the downstream elements, and thus a grossly unbalanced permeate flux. The resulting very high permeate flux for the upstream elements was probably the reason for particulate fouling that has required a feed pressure increase by up to 32 psi (2.2 bar) over 5 years to maintain the design permeate flow rate. Includes 6 references, tables, figures.