Reducing Sulfuric Acid Pretreatment in a Nanofiltration Process Treating Surficial Groundwater

Nanofiltration (NF) is a pressure driven membrane process employed in drinking water treatment that requires pretreatment for reliable operation. The objective of this research was to determine if NF membranes can proficiently operate with a decreased or eliminated dose of sulfuric acid pretreatment. When used as pretreatment, sulfuric acid prevents calcium carbonate scaling on NF membranes, yet is costly, hazardous, and imparts high sulfate concentrations to NF feed and concentrate streams. To conduct this research, a 0.324 million gallon per day (MGD) NF pilot plant was operated for 3,855 run-hours at a flux rate of 15 gallons per square foot-day. The NF pilot unit\u27s process performance, productivity, and water quality were monitored while the sulfuric acid dose was gradually decreased, controlled by monitoring pH that ranged from pH 6.5 (80 mg/L sulfuric acid dose) to pH 7.0 (no sulfuric acid dose). NF pilot productivity, as measured by specific flux, was found to decline when sulfuric acid was eliminated by 2.33 percent, 9.61 percent, and 4.08 percent in the first stage, second stage, and total pilot system, respectively, with no distinguishable increase in pressure drop. Noticeable water quality trends include approximately 75 percent sulfate decrease in feed and concentrate streams, and 20 percent increase of calcium hardness and alkalinity in the permeate stream. After piloting, superimposed elemental imaging analysis revealed that the second stage, tail-end membrane surface was fouled with iron disulfide, calcium carbonate, clay, and natural organic matter. However, flux recovered to normal operating conditions after a membrane cleaning was performed. Results of the pilot study indicated that sulfuric acid could be eliminated from the full-scale NF pretreatment process; however, membrane cleaning frequencies could increase. If applied to the full-scale NF process, elimination of sulfuric acid pretreatment would reduce annual chemical costs by over $70,000