Evaluating Membrane Processes For Drinking Water Treatment .

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ASSOCIATION OF ENVIRONMENTAL ENGINEERING & SCIENCE PROFESSORSEvaluating Membrane Processes for Drinking WaterTreatment DesignM.E. Walsh and G.A. GagnonDept. of Civil and Resources EngineeringDalhousie UniversityHalifax, Nova Scotia, CanadaSummaryThe objective of this case study is to help undergraduate students applyengineering theory to the principles and scope of designing membranesystems for drinking water treatment. The case study outlines the activities ofan engineering consulting firm that has been contracted to submit apreliminary design for a new water treatment plant for a rural community inNova Scotia, Canada. The design is required to replace existing 20-year oldfacility and concur with new treatment standards for municipal surfacesource water treatment facilities established by the regulator. Preliminaryanalysis of water samples shows a source water that has low alkalinity,elevated color from naturally occurring matter (NOM) and seasonal spikes ofturbidity and manganese. Additional challenges associated with this projectinclude adapting the final design into the current infrastructure in terms ofoperational, maintenance and footprint constraints. This case presents thetreatment capabilities of the existing treatment train as compared to aproposed design based on membrane filtration. Specifically, a review of theresults of pilot-scale membrane trials leads the reader to critically examinethe viability of membrane technology for small-scale water treatmentapplications within the context of meeting new drinking water standards.Keywords: Water treatment plant design, membrane filtration, naturalorganic matter (NOM)Context and LogisticsLearning ObjectivesThrough this case study, students will: gain an understanding of the basic principles of membraneprocesses for drinking water treatment; andAEESP CASE STUDIES COMPILATION 200670

ASSOCIATION OF ENVIRONMENTAL ENGINEERING & SCIENCE PROFESSORS learn to analyze water quality data with respect to potential impactto treatment performance.Accommodating Course(s) and Level junior or senior level courses in water and wastewater treatmentgraduate level courses in water treatment plant designPrerequisite Course(s) Water Quality/Environmental ChemistryFluid MechanicsIntroduction to Environmental EngineeringType of Activity The case study can be discussed by the instructor as an in-classexample, however may be best suited for in-class group exercise oradministered as a group or individual student assignment.Level of Effort by Instructor Review of the case study as an in-class example would use maximumone 60-minute lecture period. If administered as a group or individualassignment, grading time would be minimized by attached solutions.Level of Effort by Individual Student Review of the case study would require maximum 60-minute, withsolutions to questions accompanying the case another one hour.Suggested Assessment Methods Question set which accompanies the case study will provide suitableassessment of student’s understanding of the proposed engineeringproblems.AEESP CASE STUDIES COMPILATION 200671

ASSOCIATION OF ENVIRONMENTAL ENGINEERING & SCIENCE PROFESSORSIntroductionCanadian Drinking Water RegulationsIn Canada, drinking water quality objectives are based on theGuidelines for Canadian Drinking Water Quality (GCDWQ) as developed bythe Federal-Provincial-Territorial Subcommittee on Drinking Water.Theguidelines can be used by provinces to establish provincial drinking waterregulations. The Province of Nova Scotia has adopted the GCDWQ asregulations for public drinking water supplies. In January, 2002 the NovaScotia Department of Environment and Labour released a drinking waterstrategy which established new treatment standards for municipal surfacesource water treatment facilities. The major objective of these standards wasto ensure public health through the development of potable watertreatment systems which produce drinking water that is free of microbialpathogens. Multi-barrier treatment strategies that incorporate both physicalremoval (ie., coagulation/flocculation, sedimentation and filtration) andchemical inactivation (ie., disinfection) have been recognized as beingcritical to ensuring that systems in Nova Scotia meet current environmentalstandards. In particular, process selection should take into consideration theability to satisfy all Maximum Acceptable Concentrations (MACs), InterimMaximum Acceptable Concentrations (IMACs) and Aesthetic Objectives(AOs) recommended in the Guidelines for Canadian Drinking Water Quality(GCDWQ).Membrane Technology for Drinking Water TreatmentThe advancement of membrane technology over the past ten yearshas resulted in an economically viable drinking water treatment solution forboth large- and small-scale applications. From a fundamental perspective,membrane technology is based on the principle that these systems act as aphysical, size-exclusion barrier to contaminants present in raw waterfeedstreams.Low-pressure membranes, microfiltration (MF) andultrafiltration (UF), effectively remove suspended or colloidal particles via asieving mechanism based on the size of the membrane pores relative to thatof the particulate matter (USEPA, 2003). Dissolved substances that aresmaller in dimension than the pores in a MF or UF membrane surface will passthrough the surface of these membranes. As presented in Figure 1, UFmembrane filtration with a rated pore size of 0.01 to 0.1 µm can effectivelyremove particulate matter and microorganisms via a size exclusionmechanism. However, based on the principle of pore size exclusion,dissolved material present in the feed water or wastewater streams (i.e.,viruses, DOC and soluble inorganics) may not be effectively removed.AEESP CASE STUDIES COMPILATION 200672

ASSOCIATION OF ENVIRONMENTAL ENGINEERING & SCIENCE PROFESSORSBacteria (1 – 10 µm) UF Membrane SurfaceProtozoa (2 – 20 µm)FeedStreamMembrane Pore (0.01 – 0.1 µm)UF PermeateDOC / Humic Acids (0.01 – 0.1 µm)Soluble Inorganics ( 0.2 µm)Viruses (0.005 – 0.1 µm)Figure 1. Conceptual drawing of UF membrane pore sieving mechanism(Walsh, 2005).Design Parameters1. Flow configuration. As presented in Figure 2, membrane systems can beoperated in various process configurations. In a cross-flow configuration,a percentage of the concentrate (water that does not permeate throughthe surface of the membrane) is recirculated and blended with thefeedwater. In a direct filtration configuration (dead-end filtration) there isno recirculation of the concentrate and system operation is based on 100% recovery of the feedwater.2. Transmembrane pressure (TMP) is the pressure that is used to drive waterthrough the membrane.Positive pressure systems involve thepressurization of the feedwater that is then fed to the membranes. Forimmersed systems, the membrane modules are submerged in tankscontaining the feed water and a negative pressure (i.e., vacuum) isapplied to pull the treated water (permeate) through the fiber lumens.TMP can be calculated according to Tutujian (1985):Ptm (Pi Po) - Pp2AEESP CASE STUDIES COMPILATION 2006(1)73

ASSOCIATION OF ENVIRONMENTAL ENGINEERING & SCIENCE PROFESSORSwhere Ptm transmembrane pressure, [psi], Pi pressure at the inlet of themembrane module, [psi], Po pressure at the outlet of the membranemodule, [psi], and Pp permeate pressure, [psi].Concentrate RecirculationFeedWater FlowConcentrateMembrane SurfacePermeate FlowCross-FlowFeed Water FlowMembrane SurfacePermeate FlowDead-EndFigure 2 Membrane flow configurations.3. Permeate flux, or ratio of permeate (membrane filtrate) flow rate tomembrane surface area expressed as (L/ m2/ hr or gal/ ft2/ hr), is a majordesign factor used to determine the number of membrane units requiredfor a specific plant capacity. To correct for temperature effects on waterviscosity in flux calculations, Jacangelo et al., (1994) proposed thefollowing correction equation:J20 Qp e-0.0239 (T-20)SAEESP CASE STUDIES COMPILATION 2006(2)74

ASSOCIATION OF ENVIRONMENTAL ENGINEERING & SCIENCE PROFESSORSwhere J20 permeate flux corrected for 20oC, [L/ m2hr], Qp permeateflow [L/ hr], T permeate test temperature [oC], and S membranesurface area [m2].Often, design engineers will try to reduce overall capital cost of themembrane system through implementation of higher membrane fluxes.However, operating at higher design fluxes will inevitably increaseoperating costs due to higher operating pressures, more frequentcleaning and potential membrane replacement costs.Specific flux (e.g., permeability) is defined as the ratio of permeate flux toTMP according to the following equation:Jsp J20Ptm(3)where Jsp specific flux [L/ m2hr kPa], and Ptm transmembrane pressure(TMP) [kPa].4. Recovery is defined as the ratio of permeate flow to feedwater flowrateaccording to the following equation:% R QPQF(4)Low-pressure membranes (MF and UF) typically operate within the rangeof 85 – 97 % recovery. The rate of fouling and TMP will tend to increasewhen systems are operated at higher recovery rates.Case StudyBackgroundAn engineering consulting firm was contracted to evaluate theintegration of low-pressure membranes into the current drinking watertreatment facilities for a small community (i.e., 1,000 residents) in rural NovaScotia, Canada.The raw water source for the plant is the French Riverwhich flows through surrounding agricultural and natural land-use areas. Theraw water quality is influenced by seasonal spikes in the Spring and Fall dueto periods of high precipitation. A preliminary analysis of the raw waterquality is presented in Table 1.AEESP CASE STUDIES COMPILATION 200675

ASSOCIATION OF ENVIRONMENTAL ENGINEERING & SCIENCE PROFESSORSTable 1. Raw water quality analysis.AnalyteAlkalinity, mg/L asCaCO3Turbidity, NTUAverage Value 10Seasonal Peak172.0 – 4.04.5TOC, mg/L4.0 – 6.017.2Color, TCU40 – 50197Conductivity, µS0.5 – 5068The existing water treatment plant for the community has a capacityof approximately 75,000 gal/day (0.3 ML/day) and consists of direct filtration(anthracite-sand) without coagulation or pre-oxidation unit operations.Liquid chlorine (NaOCl) is added to the filtered water to maintain a 0.2 mg/ Lfree chlorine residual in the distribution system. The key treatment concernswith the current plant treatment train are: Achieving a filtered turbidity less than 0.2 NTUMaintaining a free chlorine residual of 0.2 mg/ L during spikes inturbidity/organic matterAchieving a 0.5-log inactivation of Giardia lambliaMinimizing the potential to form disinfectant by-products (DPBs)Pilot StudiesA membrane pilot study was conduced by the engineering consultingfirm to investigate the treatment capabilities of low-pressure membranesystems (MF and UF) without chemical pre-treatment (i.e., coagulants) for thissource water. This particular membrane application was somewhat uniquein that the raw water quality is generally very good for the majority of theyear. However, it requires robust treatment to respond to source qualityvariations during seasonal run-off periods.Specifications for the twomembrane systems evaluated in the pilot studies are presented in Table 2.Water Quality ResultsThe initial phase of the pilot trials involved operating the membrane moduleswithout chemical pre-treatment. Samples were routinely taken from the rawwater and permeate sample locations and analyzed for the water qualityparameters outlined in Table 3.AEESP CASE STUDIES COMPILATION 200676

ASSOCIATION OF ENVIRONMENTAL ENGINEERING & SCIENCE PROFESSORSTable 2. Characteristics of membranes used in pilot FPVDFFiltration ConfigurationOperation ModeCross-flowHollow perating Pressure, psi3.0-0.3 to -2.0Operating Flux, L/ m2hr26.029.0Backwash ModeNonePermeate back-pulseNominal Pore Size, µmFiltration Area,m2PVDF: polyvinylidene fluorideTable 3. Average water quality results during phase 1 pilot trials.AnalyteRaw WaterMF PermeateUF PermeateTurbidity, NTU2.110.140.09TOC, mg/L8.224.512.34Color, TCU44.014.013.7UV254, cm-10.1890.1140.053Both the MF and UF pilot systems achieved excellent removal ofturbidity and significant reductions in color. However, the permeate waterquality in terms of TOC and UV254 showed that the formation of DBPs afterchlorination in the clearwell may pose regulatory problems for the waterutility. Therefore, the second phase of the pilot trials involved the addition ofaluminum sulphate (Al2(SO4)3·14H20), or alum as a chemical pre-treatment toevaluate the impact on permeate water quality. Due to the low alkalinity ofthe raw water and results of jar tests conducted in the lab, the alum dosageselected for the membrane pilot trials was 15 mg/L. The results of this phaseof the pilot study are presented in Table 4.Flux and PermeabilityDuring the 17-day Phase 1 pilot trials, operating data including systemtemperature, permeate flowrate and transmembrane pressure (TMP) wascollected daily (Table 5).The MF module was operated under constantpressure with variable flux, while the UF module was operated under constantflux with variable TMP to compensate for membrane fouling.AEESP CASE STUDIES COMPILATION 200677

ASSOCIATION OF ENVIRONMENTAL ENGINEERING & SCIENCE PROFESSORSTable 4. Average water quality results during phase 2 pilot trials.AnalyteTurbidity, NTUTOC, mg/LColor, TCUUV254, cm-1Raw Water2.417.9844.00.178MF Permeate0.110.867.70. 501UF Permeate0.070.433.30.012Table 5. Membrane operating data during phase 1 trials.Operating Time,daysSystemTemperature, oCPermeateFlowrate, 921615.235650020321715.03105002055The loss of permeability in the UF and MF operating systems werecalculated to be 32 % and 44 %, respectively, during the Phase 1 trials withoutthe addition of a coagulant. However, permeability losses were reduced to28 % and 26 % for the MF and UF systems during the Phase 2 trials with alumaddition.Based on the water quality and operating results of the pilot study, theconsulting firm decided that a two-stage UF treatment system with alumcoagulation pre-treatment (15 mg/ L) would offer the most efficient andAEESP CASE STUDIES COMPILATION 200678

ASSOCIATION OF ENVIRONMENTAL ENGINEERING & SCIENCE PROFESSORSeconomical treatment solution for the given raw water quality and operatingconstraints of the plant (Figure 3). With this design, the first-stage UF systembackwash water is recovered and treated by a second UF membranesystem in order to optimize recovery rates and reduce system cleaningrequirements while still maintaining desired finished water quality.Specifically, the design would allow for a lower feedwater recovery in the firststage (85 %) resulting in reduced membrane surface area required for thisstage while achieving overall feedwater recovery of 99 %.AlumRaw Water QP1QP2UF-1UF-2 QC11st Stage WasteQC22nd Stage Waste ChlorineDistribution SystemClearwellFigure 3. Process schematic of the proposed two-stage UF plant.ReferencesJacangelo, J.G., Adham, S.S. and J.M. Lainé (1995) Mechanism ofcryptosporidium, giardia and MS2 virus removal by MF and UF, Journ.AWWA, 87(9): 107-121.Jacangelo, J.G., Shankararaman, C. and R.R. Trussell (1998) The membranetreatment, Civil Engineering Magazine, 68(9).Tutunjian, R.S. (1985) Scale-up considerations for membrane processes,Biotechnol., 3:615.U.S. EPA (2003) Membrane filtration guidance manual, Proposal Draft, EPA815-D-03-008.Walsh, M.E. (2005) Microbial and chemical impacts of blending membranetreated filter backwash water, Ph.D. Dissertation, Dalhousie University,Halifax, NS, Canada.AEESP CASE STUDIES COMPILATION 200679

ASSOCIATION OF ENVIRONMENTAL ENGINEERING & SCIENCE PROFESSORSQuestions1. Based on the fundamental separation mechanism of pore size exclusion,explain the water quality results observed during the Phase 1 pilot trials.Based on these results, what would be the main driving factor for theconsulting firm initiating a second phase of pilot trials to include chemicalpre-treatment with alum? What other coagulants could potentially beused in this design?2. What further testing should the consulting firm have included during thepilot study to evaluate Giardia lamblia removal rates?3. The final design for the membrane plant was based on a dead-end flowconfiguration with UF membrane modules. What advantages in terms ofcapital cost would this design afford the water utility? What would be themain operational cost savings that could be achieved with a cross-flowconfiguration?4. The alum dosage selected for Phase 2 of the pilot study was 15 mg/Lbased partly on the low alkalinity of the raw water. How much alkalinity(as CaCO3) would be consumed by this alum dose? What modificationto the pre-treatment design would have to be made if the results of the jartests showed that an alum dose of 50 mg/L would be required? Would analum dose of 50 mg/L be a reasonable or realistic dosage for this design?5. Using the pilot study operating data presented in Table 5, calculate andgraph the daily permeate flux and permeability of the MF and UFmembranes during the Phase 1 pilot trials. Explain possible reasons for theperiodic spikes in permeability of the UF membrane during the 17-day trial.6. Based on the two-stage UF design (Figure 3) proposed by the engineeringconsulting firm and knowing that the first membrane can just achieve thedesign permeate goals for TOC and turbidity, what is the permeate TOCconcentration leaving the second membrane?Analysis Phase 1 water quality results showed high removal rates of turbidity(MF 93 %, UF 96%). Based on the mechanism of pore size exclusion,these results would indicate that a significant fraction of mattercontributing to turbidity was in the particulate fraction (i.e., particle size 0.04 µm). Lower removal rates for organic matter as measured by TOCAEESP CASE STUDIES COMPILATION 200680

ASSOCIATION OF ENVIRONMENTAL ENGINEERING & SCIENCE PROFESSORS(MF 45 %, UF 72%) indicate that dissolved organic carbon (DOC) mayrepresent a considerable fraction of the TOC present in the raw water.These results are supported by color and UV-254 measurements, for whichreduced removal rates were achieved with MF and UF membranefiltration indicating that the concentration of dissolved organic material inthe raw water would warrant pre-treatment (i.e., coagulation) toenhance overall removal efficiencies. Polyaluminum chloride (PACl),ferric salts such as ferric sulphate (Fe(SO4)3) or ferric chloride (FeCl3), orsynthetic coagulating agents such as polyacrylamides would also beviable coagulants that could be evaluated with this water. Particle count analysis would provide additional data on permeate waterquality in terms of removal of particles within size range of Giardia lamblia(2 – 15 µm) and other pathogens (i.e., Cryptosporidium, 2 – 5 µm). As asurrogate monitoring technique, particle counting determines particlesizes and thus warns of particles in the size range of oocysts and cysts. Dead-End Flow Configuration: Recirculation pumps and associated pipingnot required resulting in reduced capital costs and operational costsavings due to reduced energy input. Al2(SO4)3·14.2H20 3Ca(HCO3)2 2Al(OH)3 (s) 3CaSO4 6CO2 18H20Molecular Weight of Al2(SO4)3·14.2H20 597.9 g/molMolecular Weight of Ca(HCO3)2 162. 1 g/molMolecular Weight of CaCO3 100.1 g/molAlkalinity Consumed (15 mg/L) x (mol/597.9 g) x (g/1000mg) x (3mol Ca(HCO3)2/mol alum) x(2eq Ca(HCO3)2/mol) x (1eq CaCO3/1eq Ca(HCO3)2) x (molCaCO3/2eq CaCO3) x (100.1g CaCO3/mol CaCO3) x (1000 mg/g)Alkalinity Consumed 7.5 mg/ LDue to low alkalinity of raw water source (e.g., 10 mg/L as CaCO3),higher dosages of alum would require addition lime (

proposed design based on membrane filtration. Specifically, a review of the results of pilot-scale membrane trials leads the reader to critically examine the viability of membrane technology for small-scale water treatment applications within the context of meeting new drinking water standards.

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