Evaluating the Impact of Reverse Osmosis Treatment on Finished Water Carbon Dioxide Concentration and pH WATERCON2012 March 21, 2012 Presented By: Jerry Phipps, P.E. Outline • Purpose • Background • Reverse Osmosis (RO) Fundamentals • Post-Treatment Processes • Pertinent Aquatic Chemistry • Case Studies • Conclusions and Recommendations • Questions Purpose • Review RO and carbonate chemistry fundamentals • Review data from full scale projects to determine change in carbon dioxide and pH from feed to permeate stream Background • The Reverse Osmosis (RO) process uses a semipermeable membrane to separate an influent stream into two streams, a purified permeate stream and a concentrated reject stream • Many references indicate that dissolved gases such as carbon dioxide (CO2) may pass through the membrane with no rejection • The combination of low alkalinity and high carbon dioxide results in an aggressive permeate stream with a lower pH than the influent stream Reverse Osmosis Membranes The most common RO membrane material today is aromatic polyamide, typically in the form of thin-film composites. They consist of a thin film of membrane, bonded to layers of other porous materials that are tightly wound to support and strengthen the membrane. Flow Recovery • Percent recovery is a key design parameter • Defined as permeate flow / influent flow • Recovery is limited by solubility products, Ksp • Typical value for groundwater is ~75%-85%, but varies with application • Example: Influent stream = 100 gpm, 75% recovery. Permeate stream = 75 gpm, concentrate stream = 25 gpm Flow and Mass Balances • Flow Balance ▫ Qf = Qp + Qc • Mass Balance on Conserved Solute ▫ QfCf = QpCp + QcCc • Mass Balance on Conserved Solute in Terms of Flow Recovery (R) ▫ QfCf = QfRCp + Qf(1-R)Cc Post-Treatment Processes • RO permeate blended with RO feed to create desired finished water quality, but still aggressive ▫ Langelier saturation index (LSI) ▫ Calcium carbonate precipitation potential (CCPP) • Need to raise pH to stabilize • CO2 stripping reduces chemical cost Carbonic Acid Equilibria -1.5 KH = 10 CO2(g) = CO2(aq) mole/L-atm at 250C CO2(aq) + H2O ↔ H2CO3 (aq) -2.8 Km = 10 + - H2CO3 ↔ H + HCO3 pK1 = 6.42 - + 2- HCO3 ↔ H + CO3 pK2 = 10.45 - 2- CT = [H2CO3*] + [HCO3 ] + [CO3 ] Carbonic Acid Equilibria • Ionization Fractions - 2- α0 = [H2CO3*]/CT α1 = [HCO3 ]/CT α2 = [CO3 ]/CT Case Study – Ft. Madison, IA • 180 ft deep Mississippi River alluvial aquifer wells • 75% recovery (two stage) Case Study – Ft. Madison, IA • All alkalinity values mg/L as CaCO3 assumed to match pilot • pH measurements taken both lab and in-line sensor • Lab ▫ Raw: pH 7.82, CO2 6 mg/L, alkalinity 184 mg/L ▫ Permeate: pH 6.09, CO2 9 mg/L, alkalinity 5 mg/L • In-line instrument ▫ Raw: pH 7.4, CO2 17 mg/L, alkalinity 184 mg/L ▫ Permeate: pH 5.9 mg/L, CO2 15 mg/L, alkalinity 5 mg/L Case Study – Wellman, IA • 264 ft deep buried sand aquifer well • 75% recovery (two stage) • Internal recycle • Raw Stream: 1,914 mg/L TDS ▫ Consider ionic strength and activity coefficients for feed stream Case Study – Wellman, IA • Data Set 1 ▫ Raw: pH 7.59, CO2 13 mg/L, alkalinity 246 mg/L ▫ Permeate: pH 5.75, CO2 48 mg/L, alkalinity 10.8 mg/L • Data Set 2 ▫ Raw: pH 7.69, CO2 10 mg/L, alkalinity 241 mg/L ▫ Permeate: pH 5.66 mg/L, CO2 78 mg/L, alkalinity 14.4 mg/L Case Study – Hartley, IA • 600 ft deep Dakota aquifer well • 75% recovery (two stage) • pH and alkalinity from feed, permeate, and reject streams to complete mass balance on total carbonate (CT) • Raw Stream: 2,091 mg/L TDS ▫ Consider ionic strength and activity coefficients for feed and reject streams Case Study – Hartley, IA • Data Set 1 ▫ Raw: pH 7.1, CO2 61 mg/L, alkalinity 380 mg/L ▫ Permeate: pH 5.4, CO2 38 mg/L, alkalinity 3.6 mg/L • Data Set 2 ▫ Raw: pH 7.1, CO2 62 mg/L, alkalinity 385 mg/L ▫ Permeate: pH 4.8 mg/L, CO2 172 mg/L, alkalinity 3.5 mg/L Case Study – Hartley, IA - Raw • The raw water pH and alkalinity were 7.1 and 380 mg/L as CaCO3 • At 520 F, total raw water carbon dioxide concentration was calculated to be 61 mg/L as CO2 (includes H2CO3) and total carbonate, CT, was calculated to be 8.99 x 10-3 moles/L -1 • Alpha 0, α0 = 1.54 x 10 (15.4% H2CO3*) -1 - • Alpha 1, α1 = 8.45 x 10 (84.5% HCO3 ) -4 2- • Alpha 2, α2 = 6.38 x 10 (0.1% CO3 ) Case Study – Hartley, IA - Permeate • Permeate pH was 5.4 and permeate alkalinity was 3.6 mg/L as CaCO3, resulting in a permeate carbon dioxide concentration of 38 mg/L as CO2 (includes H2CO3) and an average CT of 9.39 x 10-4 moles/L at 520 F • 38/53% removal of H2CO3* (concentration/mass) • 90.0/92.0% removal of CT (concentration/mass) • 1.7 unit reduction in pH • 99.1% removal of alkalinity (concentration based) Mass Balance on Total Carbonate • QfCf = QfRCp + Qf(1-R)Cc -3 • Feed CT was calculated to be 8.99 x 10 moles/L from raw pH and alkalinity -4 • Permeate CT was calculated to be 9.39 x 10 moles/L from raw pH and alkalinity • R (fractional recovery) = 0.75 • Calculate Cc based on mass balance using raw and permeate CT ▫ C = 10-3(8.99 – 0.75*0.939)/0.25 = 3.314 x 10-2 moles/Lc ▫ Reject C = 3.137 x 10-2 moles/L calculated from reject pH and alkalinityT ▫ % error = 100*(3.314-3.137)/3.314 = 5.3% error Conclusions • Rejection of bicarbonate alkalinity results in an aggressive permeate stream with a lower pH than the influent stream • Permeate pH range 5.4 – 6.09 with raw pH range 7.1 – 7.82 • Permeate CO2 relative to feed varies • CO2 may potentially concentrate with recycle – demonstrate during pilot testing Recommendations For Future Studies • All pH measurements should be taken from a properly calibrated in-line instrument (not lab pH). This will prevent equilibration of the sample with the atmosphere prior to measurement • The reject stream must be sampled to allow for a complete CT balance since CT is conserved. Minimum sampling to complete this mass balance includes pH, temperature, total alkalinity, and total dissolved solids • Feed samples should be taken after pre-treatment chemicals are injected Questions? .