AWWA MTC53921

As freshwater supplies continue to become fewer and harder to come by, municipalwater utilities are required to turn to supplies of lesser quality. Thesesupplies may contain high levels of dissolved salts and/or taste and odor causingcompounds such as hydrogen sulfide (H2S). Membrane desalting technologies such asreverse osmosis (RO) and nanofiltration (NF), have been used to treat thesequality-impaired supplies. Traditional pretreatment for membrane desaltingtechnologies included the addition of an acid such as sulfuric acid orhydrochloric acid to lower feed stream pH and reduce the potential for calciumcarbonate precipitation, and scale inhibitor to sequester salt precipitation notcontrolled by pH. Acid addition has benefit not only for inhibiting calciumcarbonate scale, but also for the removal of hydrogen sulfide. Hydrogen sulfideexists predominantly as a dissolved gas at low pH and is well removed by suchdegasification processes as packed tower aeration. The addition of mineral acidto membrane desalting processes has been the source of many operations andmaintenance problems. Improper acid delivery and storage and feed system design haveresulted in direct operator contact with acid in addition to various maintenanceproblems. Poor acid specification has contributed to the addition of undesirabletrace contaminants (i.e., iron, arsenic, sulfate, etc.) that may complicatemembrane fouling and/or byproduct disposal. Recent developments in the field ofmembrane desalting pretreatment include the development of new scale inhibitorsthat can sequester the formation of calcium carbonate at Langelier SaturationIndex (LSI) values as high as 1.8 to 3.0. At these high LSI values, acid is notrequired. Additionally, for systems that experience naturally high levels ofsulfate, recovery may actually be increased since sulfuric acid is no longerrequired. United Water Florida (UWF) is currently evaluating membrane desaltingfor the treatment of their Floridan Aquifer supply. This supply contains sulfateat concentrations of approximately 340 mg/L and hydrogen sulfide in excess of 2mg/L. No-acid RO/NF offers the benefit of saving capital and operational expensesassociated with using acid. However, no-acid RO/NF carries with it thecomplication of removing hydrogen sulfide from both the product and byproductstreams at higher pHs. Work performed includes the planning level decision makingprocess for evaluating a no-acid treatment approach in a new treatment facility.Decision making involved evaluating process alternatives, cost savings, andoperational challenges associated with hydrogen sulfide removal at high pH. Suchissues as product water post treatment, formation of colloidal sulfur, andbyproduct disposal to a wastewater treatment plant, reuse system, and surfacewater were all incorporated into the decision making process. Final processrecommendation consisted of a no-acid RO process, use of packed tower aeration ata pH of approximately 6.0 (i.e., using carbonic acid for a pre-aerationtreatment), and in-line chlorine oxidation followed by aeration for byproductstream treatment. Includes 14 references, tables, figures.

Product Details

Edition:

Vol. - No.

Published:

05/01/2001

Number of Pages:

20

File Size:

1 file , 310 KB