LOW LEVEL EFFLUENT LIMITATIONS: IS IT MBR, MEMBRANE ... level effluent limitations: is it mbr,...
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LOW LEVEL EFFLUENT LIMITATIONS:LOW LEVEL EFFLUENT LIMITATIONS:IS IT MBR, MEMBRANE FILTRATION, IS IT MBR, MEMBRANE FILTRATION,
TWO STAGE FILTRATION OR TWO STAGE FILTRATION OR CONVENTIONAL TERTIARY TREATMENT?CONVENTIONAL TERTIARY TREATMENT?
Presented byPresented by
Onder Caliskaner, Ph.D., P.E.Onder Caliskaner, Ph.D., P.E.Project Manager, Kennedy/Jenks ConsultantsProject Manager, Kennedy/Jenks Consultants
Brian Davis, P.E.Brian Davis, P.E.Project Manager, Kennedy/Jenks ConsultantsProject Manager, Kennedy/Jenks Consultants
Presented at thePresented at theCWEA Sacramento Section Meeting CWEA Sacramento Section Meeting
December 13, 2006December 13, 2006
LINDA COUNTY WATER DISTRICTLINDA COUNTY WATER DISTRICT
Located in Yuba County, south of Marysville Provides water and sewer service to over 12,000 customersServes primarily residential and light commercialRural community experiencing rapid growth due to new development
LINDA COUNTY WATER DISTRICT LINDA COUNTY WATER DISTRICT WWTPWWTP
Plant constructed in 1962 with land disposal and permitted river discharge.Secondary level plant with trickling filter, primary and secondary clarification andchlorination/de-chlorination.Capacity to treat average dry weather flow of 1.8 MGD.
DISTRICTDISTRICT’’S STRATEGY FOR S STRATEGY FOR EFFLUENT DISPOSALEFFLUENT DISPOSAL
In 2004, the RWQCB requested that the District abandon their disposal ponds. The District weighed its alternatives:
Land disposal at new siteUse District’s permitted NPDESdischarge to riverRetain usage of existing ponds
After extensive engineering and cost analyses, the best option was found to be the continued use of the NPDES discharge.
LINDA COUNTY WATER DISTRICT LINDA COUNTY WATER DISTRICT NPDESNPDESPERMIT RENEWAL PROCESSPERMIT RENEWAL PROCESS
Permit to Discharge to Feather River, administrated by the Central Valley Regional Water Quality Control Board (RWQCB)Existing NPDES permit was to expire in June 2005Report of Waste Discharge submitted November 2004
Reasonable Potential Analysis for CTR and SIP contaminantsMixing Zone Study for discharge to Feather River
Administrative Draft from RWQCB December 2005Continued discussion with RWQCB staff regarding:
Compliance timelinesAssimilative capacityVarious other requirements
NEW NEW NPDESNPDES PERMITPERMIT
Adopted by RWQCB on 22 September 2006Allowed for expansion from 1.8 MGD to 5.0 MGDRequired tertiary level treatment with plant expansionAllowed for a mixing zone to relax effluent requirements for contaminants based on human health criteriaAdded re-opener clauses to allow future consideration of aquaticlife-based criteriaRequired full compliance by 22 September 2011
HOW TO MEET REQUIREMENTS OF HOW TO MEET REQUIREMENTS OF NEW PERMITNEW PERMIT
Significant plant improvements requiredExpansion of capacity to 5.0 MGD average dry weather flowNew secondary and tertiary treatment processesPossible advanced treatment
Take it one step at a timeAddress issues we can confidently control, such as Ammonia, BOD, Turbidity, etc…After construction of the new plant, then we can see where we stand; further steps may require additional study and a permit re-opener rather than employing advanced/high level treatment options
CHANGE IN EFFLUENT LIMITATIONSCHANGE IN EFFLUENT LIMITATIONS
2.2240Total Coliform (MPN/100 ml)1---Nitrite (mg/l)
10---Nitrate + Nitrite (mg/l)1.22 / 1.80 (a)---Ammonia (mg/l)
< 2---Turbidity (NTU)1045TSS (mg/l)1045BOD (mg/l)
New PermitOld PermitMonthly Averages
(a) Summer Limit / Winter Limit
LINDA COUNTY WATER DISTRICT WWTP LINDA COUNTY WATER DISTRICT WWTP EXISTING TREATMENT PLANT SITE PLANEXISTING TREATMENT PLANT SITE PLAN
SECONDARY SECONDARY CLARIFIERCLARIFIER
PRIMARY PRIMARY CLARIFIERSCLARIFIERS
TRICKLING TRICKLING FILTERFILTER
INFLUENT INFLUENT PUMP PUMP
STATIONSTATION
GRIT GRIT REMOVALREMOVAL
ANAEROBIC ANAEROBIC DIGESTERSDIGESTERS
CHLORINE CHLORINE CONTACT CONTACT
BASINBASIN
SLUDGE SLUDGE DRYING BEDSDRYING BEDS
LINDA COUNTY WATER DISTRICT LINDA COUNTY WATER DISTRICT WWTPWWTPEXISTING SITE PLANEXISTING SITE PLAN
HEADWORKSHEADWORKS••Coarse screeningCoarse screening••Influent pump stationInfluent pump station••Medium screeningMedium screening••Grit chamberGrit chamber••20 % 20 % TSSTSS removalremoval••6 % 6 % BODBOD removalremoval
LINDA COUNTY WATER DISTRICT WWTP LINDA COUNTY WATER DISTRICT WWTP EXISTING SITE PLANEXISTING SITE PLAN
PRIMARY CLARIFIERPRIMARY CLARIFIER••6060--feet diameterfeet diameter••77--feet side water depthfeet side water depth••44 % TSS removal44 % TSS removal••16% BOD removal16% BOD removal
LINDA COUNTY WATER DISTRICT WWTP LINDA COUNTY WATER DISTRICT WWTP EXISTING SITE PLANEXISTING SITE PLAN
TRICKLING FILTERTRICKLING FILTER••110110--feet diameterfeet diameter••44--feet side depthfeet side depth••54% BOD removal (Trickling 54% BOD removal (Trickling Filter/Secondary Clarifier)Filter/Secondary Clarifier)••No significant Ammonia/ No significant Ammonia/ Nitrogen removalNitrogen removal
LINDA COUNTY WATER DISTRICT LINDA COUNTY WATER DISTRICT WWTPWWTPEXISTING SITE PLANEXISTING SITE PLAN
SECONDARY CLARIFIERSECONDARY CLARIFIER••7575--feet diameterfeet diameter••1414--feet side water depthfeet side water depth••59 % 59 % TSSTSS removalremoval
LINDA COUNTY WATER DISTRICT WWTP LINDA COUNTY WATER DISTRICT WWTP EXISTING SITE PLANEXISTING SITE PLAN
CHLORINATION/DECHLORINATIONCHLORINATION/DECHLORINATION••30 feet wide30 feet wide••100 feet long100 feet long••Average effluent 14 MPN/ 100 mlAverage effluent 14 MPN/ 100 ml
EVALUATED TERTIARY EVALUATED TERTIARY TREATMENT ALTERNATIVESTREATMENT ALTERNATIVES
Conventional secondary effluent filtration
Secondary effluent membrane filtration
Secondary effluent two-stage filtration
Membrane Bio-reactor
EFFLUENT WATER QUALITY OBJECTIVESEFFLUENT WATER QUALITY OBJECTIVES
0.50.50.55-10 (<5%)Daily Maximum<0.2 (95%)<0.2 (95%)<0.2 (95%)2Daily AverageTurbidity
(NTU)
11120Daily Maximum0.40.40.44Monthly AverageTSS (mg/l)20202020Daily Maximum10101010Monthly Average
BOD (mg/l)
MBRTwo StageFiltration
SecondaryEffluent
MembraneFiltrationConventionalParameter
EFFLUENT WATER QUALITY OBJECTIVESEFFLUENT WATER QUALITY OBJECTIVES
23232323Daily Maximum2.22.22.22.27-day MedianTC (MPN/
100 ml)
15151515Daily Maximum10101010Monthly AverageTN
(mg/l)
2222Daily Maximum1-1.8 (a)1-1.8 (a)1-1.8 (a)1-1.8 (a)Monthly AverageAmmonia
(mg/l)
MBRTwo StageFiltration
SecondaryEffluent
MembraneFiltrationConventionalParameter
(a) winter limit / summer limit
Membrane Bio-reactor
HeadworksHeadworks
Primary Primary SedimentationSedimentation
Activated Activated Sludge ProcessSludge Process
Secondary Secondary SedimentationSedimentation
Secondary Secondary Effluent Effluent FiltrationFiltration
DisinfectionDisinfection
TERTIARY TREATMENT ALTERNATIVESTERTIARY TREATMENT ALTERNATIVESFLOW DIAGRAMSFLOW DIAGRAMS
HeadworksHeadworks
Primary Primary SedimentationSedimentation
Activated Activated Sludge ProcessSludge Process
Secondary Secondary SedimentationSedimentation
Membrane Membrane FiltrationFiltration
DisinfectionDisinfection
HeadworksHeadworks
Primary Primary SedimentationSedimentation
Activated Activated Sludge ProcessSludge Process
Secondary Secondary SedimentationSedimentation
Secondary Secondary Effluent Effluent FiltrationFiltration
DisinfectionDisinfection
HeadworksHeadworks
Primary Primary SedimentationSedimentation
(Optional)(Optional)
Membrane BioMembrane Bio--ReactorReactor
DisinfectionDisinfection
Secondary Effluent Membrane Filtration
Two-stage FiltrationConventional
Membrane Membrane FiltrationFiltration
EVALUATED ACTIVATED SLUDGE EVALUATED ACTIVATED SLUDGE PROCESSES PROCESSES
Plug Flow Activated Plug Flow Activated Sludge SystemSludge System
EVALUATED ACTIVATED SLUDGE EVALUATED ACTIVATED SLUDGE PROCESSES PROCESSES
Plug Flow Activated Plug Flow Activated Sludge SystemSludge SystemOxidation DitchOxidation Ditch
EVALUATED ACTIVATED SLUDGE EVALUATED ACTIVATED SLUDGE PROCESSES PROCESSES
Plug Flow Activated Plug Flow Activated Sludge SystemSludge SystemOxidation DitchOxidation DitchSequencing Batch Sequencing Batch ReactorReactor
EVALUATED SECONDARY EFFLUENT EVALUATED SECONDARY EFFLUENT FILTRATION SYSTEMS FILTRATION SYSTEMS
Granular Bed FiltrationContinuous Backwash (Dynasand, Technasand)
EVALUATED SECONDARY EFFLUENT EVALUATED SECONDARY EFFLUENT FILTRATION SYSTEMS FILTRATION SYSTEMS
Granular Bed FiltrationContinuous Backwash (Dynasand, Technasand)Pulsed Bed (Hydroclear)
EVALUATED SECONDARY EFFLUENT EVALUATED SECONDARY EFFLUENT FILTRATION SYSTEMS FILTRATION SYSTEMS
Granular Bed FiltrationContinuous Backwash (Dynasand, Technasand)Pulsed Bed (Hydroclear)
Compressible Medium Filtration (Fuzzy Filter)
EVALUATED SECONDARY EFFLUENT EVALUATED SECONDARY EFFLUENT FILTRATION SYSTEMS FILTRATION SYSTEMS
Granular Bed FiltrationContinuous Backwash (Dynasand, Technasand)Pulsed Bed (Hydroclear)
Compressible Medium Filtration (Fuzzy Filter)
EVALUATED SECONDARY EFFLUENT EVALUATED SECONDARY EFFLUENT FILTRATION SYSTEMS FILTRATION SYSTEMS
Granular Bed FiltrationContinuous Backwash (Dynasand, Technasand)Pulsed Bed (Hydroclear)
Compressible Medium Filtration (Fuzzy Filter)
EVALUATED SECONDARY EFFLUENT EVALUATED SECONDARY EFFLUENT FILTRATION SYSTEMS FILTRATION SYSTEMS
Granular Bed FiltrationContinuous Backwash (Dynasand, Technasand)Pulsed Bed (Hydroclear)
Compressible Medium Filtration (Fuzzy Filter)Disk Filtration
Fully submerged (Cloth Disk Filter)
EVALUATED SECONDARY EFFLUENT EVALUATED SECONDARY EFFLUENT FILTRATION SYSTEMS FILTRATION SYSTEMS
Granular Bed FiltrationContinuous Backwash (Dynasand, Technasand)Pulsed Bed (Hydroclear)
Compressible Medium Filtration (Fuzzy Filter)Disk Filtration
Fully submerged (Cloth Disk Filter)
EVALUATED SECONDARY EFFLUENT EVALUATED SECONDARY EFFLUENT FILTRATION SYSTEMS FILTRATION SYSTEMS
Granular Bed FiltrationContinuous Backwash (Dynasand, Technasand)Pulsed Bed (Hydroclear)
Compressible Medium Filtration (Fuzzy Filter)Disk Filtration
Fully submerged (Cloth Disk Filter)Partially Submerged (Hydrotech)
EVALUATED SECONDARY EFFLUENT EVALUATED SECONDARY EFFLUENT FILTRATION SYSTEMS FILTRATION SYSTEMS
Granular Bed FiltrationContinuous Backwash (Dynasand, Technasand)Pulsed Bed (Hydroclear)
Compressible Medium Filtration (Fuzzy Filter)Disk Filtration
Fully submerged (Cloth Disk Filter)Partially Submerged (Hydrotech)
EVALUATED MEMBRANE FILTRATION EVALUATED MEMBRANE FILTRATION TECHNOLOGIES TECHNOLOGIES
Secondary Effluent Membrane Filtration
Hollow-Fiber (Zenon)
EVALUATED MEMBRANE FILTRATION EVALUATED MEMBRANE FILTRATION TECHNOLOGIES TECHNOLOGIES
Secondary Effluent Membrane Filtration
Hollow-Fiber (Zenon)Hollow-Fiber (Pall)
Two-Stage FiltrationFirst Stage: Secondary effluent filterSecond Stage: Hollow – Fiber Membrane
EVALUATED MEMBRANE FILTRATION EVALUATED MEMBRANE FILTRATION TECHNOLOGIES TECHNOLOGIES
Secondary Effluent Membrane Filtration
Hollow-Fiber (Zenon)Hollow-Fiber (Pall)
Two-Stage FiltrationFirst Stage: Secondary effluent filterSecond Stage: Hollow – Fiber Membrane
EVALUATED MEMBRANE BIOEVALUATED MEMBRANE BIO--REACTOR REACTOR TECHNOLOGIES TECHNOLOGIES
Flat Plate (Kubota/Enviroquip) Hollow Fiber (Zenon)
EVALUATION CRITERIA FOR TERTIARY EVALUATION CRITERIA FOR TERTIARY
TREATMENT TRAIN ALTERNATIVESTREATMENT TRAIN ALTERNATIVES
100.0%Total12.5%Site Requirements
7.5%Environmental Considerations/Safety
25.0%Operational and Maintenance Issues
30.0%Performance
25.0%Cost
WeightingFactor (a)
(a) Specific to Linda County Water District
ECONOMIC COMPARISON FOR TERTIARY ECONOMIC COMPARISON FOR TERTIARY TREATMENT TRAIN ALTERNATIVESTREATMENT TRAIN ALTERNATIVES
38,200,000
635,000
26,700,000
Two StageFiltration
37,600,00035,900,00024,000,000Net Present Value, $(b)
935,000615,000375,000Annual Operations and Maintenance Cost (a), $
21,560,00024,770,00016,870,000Capital Cost, $ (a) (Constructed Facility)
MBR
SecondaryEffluent
MembraneFiltrationConventional
(a) Includes costs only for secondary and tertiary processes.(b) Based on 20-year operating life cycle and average annual interest rate of 1.5 percent.
ECONOMIC EVALUATION MATRIX FOR ECONOMIC EVALUATION MATRIX FOR TERTIARY TREATMENT TRAIN TERTIARY TREATMENT TRAIN
ALTERNATIVESALTERNATIVES
5.0%10.0%10.0%
WeightingFactor
656565100Overall Weighted Score
3334Net PresentValue
2334Annual3224Capital
MBRTwo StageFiltration
SecondaryEffluent
MembraneFiltrationConventionalCost
PERFORMANCE EVALUATION MATRIX FOR PERFORMANCE EVALUATION MATRIX FOR
TERTIARY TREATMENT TRAIN ALTERNATIVESTERTIARY TREATMENT TRAIN ALTERNATIVES
2.5%2.5%2.5%2.5%2.5%2.5%2.5%2.5%
10.0%
WeightingFactor
11311511095Overall Weighted Score4443Regulatory Requirements3433Redundancy Requirement3433Reliability/Vendor Support4443Chemical Requirements4334Backwash Requirements4443Sludge Settleability4333Shock/Toxic Load Resistance3444Aeration Efficiency4443Effluent Quality
MBRTwo StageFiltration
SecondaryEffluent
MembraneFiltrationConventionalPerformance
OPERATIONAL AND MAINTENANCE OPERATIONAL AND MAINTENANCE EVALUATION MATRIX FOR TERTIARY EVALUATION MATRIX FOR TERTIARY TREATMENT TRAIN ALTERNATIVESTREATMENT TRAIN ALTERNATIVES
5.0%
5.0%
5.0%
5.0%
5.0%
WeightingFactor
70657595Overall Weighted Score
3234MaintenanceRequirements
3433Operational Flexibility
3234Operational History/Number of Installations
2234Operational Complexity
3334Operational SkillsRequirement
MBRTwo StageFiltration
SecondaryEffluent
MembraneFiltrationConventional
Operational andMaintenance Issues
ENVIRONMENTAL/SAFETY EVALUATIONENVIRONMENTAL/SAFETY EVALUATIONMATRIX FOR TERTIARY TREATMENT TRAIN MATRIX FOR TERTIARY TREATMENT TRAIN
ALTERNATIVESALTERNATIVES
2.5%2.5%2.5%
WeightingFactor
30303030Overall Weighted Score
4444Safety Issues4444Odor4444Noise
MBR
Two Stage
Filtration
SecondaryEffluent
MembraneFiltrationConventional
EnvironmentalConsiderations/ Safety
SITE REQUIREMENTS EVALUATION MATRIX SITE REQUIREMENTS EVALUATION MATRIX FOR TERTIARY TREATMENT TRAIN FOR TERTIARY TREATMENT TRAIN
ALTERNATIVESALTERNATIVES
2.5%
5.0%
5.0%
WeightingFactor
50404338Overall Weighted Score
4233Space Requirements
4433Expansion/Modification Requirements
4343Head Requirements
MBR
Two Stage
Filtration
SecondaryEffluent
MembraneFiltrationConventionalSite Requirements
ADJUSTED EVALUATION MATRIX FOR ADJUSTED EVALUATION MATRIX FOR TERTIARY TREATMENT TRAIN ALTERNATIVESTERTIARY TREATMENT TRAIN ALTERNATIVES
328315323358100.0%Total5040433812.5%Site Requirements303030307.5%
Environmental Considerations/Safety
7065759525.0%Operational and Maintenance Issues
1131151109530.0%Performance 65656510025.0%Cost
MBRTwo StageFiltration
SecondaryEffluent
MembraneFiltrationConventional
WeightingFactor
PROJECT STATUSPROJECT STATUS
Environmental study is in the final stage, and design is 50 percent complete
Design is scheduled to be completed by April 2007 and the construction is proposed to start in August 2007
WWTP DESIGN INFLUENT CHARACTERISTICSWWTP DESIGN INFLUENT CHARACTERISTICS
mg/l395Maximum dailymg/l275Maximum monthlymg/l190Average
Biochemical Oxygen DemandMGD13.8Peak Hourly FlowMGD9.9Maximum Day FlowMGD5.0Average Dry Weather FlowUnitsFlow rates
WWTP DESIGN INFLUENT CHARACTERISTICSWWTP DESIGN INFLUENT CHARACTERISTICS
º C22.9Minimum Monthly during Warm Weather º C14.3Minimum Monthly during Cold Weather
Wastewater Temperaturesmg/l55Maximum dailymg/l45Maximum monthlymg/l35Average
Ammoniamg/l575Maximum dailymg/l435Maximum monthlymg/l305Average unitsTotal Suspended Solids
LINDA COUNTY WATER DISTRICT LINDA COUNTY WATER DISTRICT WWTPWWTPEXISTING AND PROPOSED SITE PLANEXISTING AND PROPOSED SITE PLAN
EXISTING EXISTING TREATMENT TREATMENT
PLANTPLANT
NEW LIQUID NEW LIQUID PROCESSESPROCESSES
NEW SOLIDS HANDLING NEW SOLIDS HANDLING AND EMERGENCY AND EMERGENCY
OVERFLOW FACILITIESOVERFLOW FACILITIES
LINDA COUNTY WATER DISTRICT LINDA COUNTY WATER DISTRICT WWTPWWTPNEW LIQUID PROCESSES SITE PLANNEW LIQUID PROCESSES SITE PLAN
PRIMARY CLARIFIERSPRIMARY CLARIFIERS
ACTIVATED SLUDGE BASINSACTIVATED SLUDGE BASINS
CONTROL/BLOWER CONTROL/BLOWER BUILDINGBUILDING
HEADWORKSHEADWORKS
SECONDARY SECONDARY EFFLUENT EFFLUENT
FILTRATIONFILTRATION
SECONDARY SECONDARY CLARIFIERSCLARIFIERS
ACTIVATED SLUDGE BASINSACTIVATED SLUDGE BASINS
PrePre--Anoxic BasinsAnoxic BasinsBasin Side Water DepthBasin Side Water Depth 20 ft20 ftVolume per basinVolume per basin 0.23 MG0.23 MGActive Active denitrifierdenitrifier biomassbiomass 1,500 mg/l1,500 mg/lF/M (anoxic zone)F/M (anoxic zone) 0.8 d0.8 d--11
Specific Specific denitrificationdenitrification raterate 0.15 d0.15 d--11
Hydraulic detention timeHydraulic detention time 2 2 –– 3.5 hr3.5 hr
ACTIVATED SLUDGE BASINSACTIVATED SLUDGE BASINS
Aeration and Swing ZonesAeration and Swing ZonesBasin Side Water DepthBasin Side Water Depth 20 ft20 ftAeration zone volume per basinAeration zone volume per basin 0.8 MG0.8 MGSwing zone volume per basinSwing zone volume per basin 0.1 MG0.1 MGSolids retention timeSolids retention time 88--15 days15 daysMLSSMLSS 25002500--3500 3500 mg/lmg/lF/M (BOD)F/M (BOD) 0.11 d 0.11 d --11
F/M (Ammonia)F/M (Ammonia) 0.04 d 0.04 d --11BOD daily loading per 1,000 ftBOD daily loading per 1,000 ft33 1010--15 lb15 lb
ACTIVATED SLUDGE BASINSACTIVATED SLUDGE BASINS
MLSS Recirculation PumpsMLSS Recirculation PumpsNumber of pumps:Number of pumps: 1 per basin1 per basinRecirculation ratio:Recirculation ratio: 200 200 –– 450 percent450 percent
SECONDARY CLARIFIERSSECONDARY CLARIFIERS
Diameter 100 ftSide water depth 18 ftSurface Overflow Rate
ADF 360 gpd/ft²MDF 665 gpd/ft²
Solids Flux RateADF 0.7 lb/hr-ft²MDF 1.3 lb/hr-ft²
Return Activated Sludge (RAS)Return Ratio 75 – 150 %
SECONDARY EFFLUENT FILTRATIONSECONDARY EFFLUENT FILTRATION
Maximum Filtration Rate 30 gpm/ft²Filter Dimensions (L x W) 7 x 7 ftFilter Depth
Media Only 2.5 ftFilter Unit 13.9 ft
Compression Ratio Range 0 – 45 %
SECONDARY EFFLUENT FILTRATIONSECONDARY EFFLUENT FILTRATION
Filtration Rate (All in Service/ One out of Service)at ADF 13.0 / 15.6 gpm/ft²at MDF 22.9 / 27.5 gpm/ft²
Backwash Reject RatioAverage 3 %Peak 5 %
CONCLUSIONSCONCLUSIONS
Conventional tertiary treatment system with necessary design considerations for possible upgrade to two stage filtration system is recommended for Linda County Water District.
Near term more stringent requirements can be met both with conventional and advanced tertiary treatment systemsAdded benefits of advanced tertiary treatment systems with regards to near term effluent limitations was not evidentA very high level treatment (such as reverse osmosis) that requires membrane filtration as pretreatment may be required in the future for some contaminants of concernHigher capital and operational costs associated with the membrane involving alternatives is not justified for immediate and near term needs
CONCLUSIONSCONCLUSIONS
The recommended conventional tertiary system employs four plug-flow type activated sludge basins, two secondary clarifiers, and six compressible medium filters
Deferring membrane filtration provides the following benefits:Observe the outcomes from a dynamic regulatory environmentObtain performance data from the upgraded plantOverall potential capital cost savings: membrane technology cost may continue to decreaseMembrane technology is still advancing: more robust technology with less cleaning/backwash requirements in the future