using a recirculating aeration system to ... - pnws-awwa 6... · using a recirculating aeration...
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Using a Recirculating Aeration System to Remove Bromoform and Other THMs
Russell Porter, P.E.May 9, 2014
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Presentation Outline
Plant background & DBP historyDescription of pilot efforts
CarbonAeration
Discussion of pilot resultsConclusions
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Eagle Cove Homeowners Association
San Juan Island – NW Washington~30 residential connectionsAll residentialWell
10 gpm capacityNaOCl added
17,000 gallon reservoirBooster station
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System Schematic17K Tank
Pressure Tanks
Service PumpDistribution
Well
NaOCL
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Eagle Cove Homeowners Well Source
Raw well water qualitypH = 7.7-7.8TOC Average = 3.4 mg/L Range 2.7-4.5 mg/LMn = 0.14 mg/L
Disinfection by-productsHigh THMsLow HAA5s
Pb/CuHigh Cu - 90% Percentile 0.73-2.15 mg/LPb well below action level
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THM History
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2005
Q1
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Q2
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Q4
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Q1
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Q2
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Q4
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Q1
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Q2
2011
Q3
Quarter
THM
, ug/
L
Quarterly Sampling
Running Annual Average (RAA)
MCL
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HAA5 History
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2005
Q1
2005
Q3
2006
Q1
2006
Q3
2007
Q1
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Q1
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Q3
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Q1
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Q1
Quarter
HA
A5,
ug/
L
Quarterly SamplesRunning Annual Average (RAA)MCL
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THMsPredominantly bromoform and bromonated THMs
Bromoform, CHBr3 – 64%Dibromochloro methane, CHBr2Cl – 25%Chlorodibromo methane, CHBrCl2 – 9%Chloroform, CHCl3 – 2%
Most systems are predominantly chloroform (Another San Juan system, for example)Bromoform, CHBr3 – <1%Dibromochloro methane, CHBr2Cl – 2%Chlorodibromo methane, CHBrCl2 – 19%Chloroform, CHCl3 – 79%
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Initial Pilot Efforts
Investigate activated carbon.
Full-scale using carbon filter in a 55-gallon drum.
UV 254 and THM testing for performance
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System Schematic17K Tank
Pressure Tanks
Service PumpDistribution
Well
NaOCL
Pilot Carbon Unit
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Carbon was used up quickly – each barrel lasted only about a month
0
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0.8
0 100,000 200,000 300,000 400,000 500,000 600,000 700,000 800,000
Gallons Treated
UV
abso
rban
ce tr
eate
d/U
V ab
sorb
ance
raw
THMs = 64 ug/L
THMs = 113 ug/L
Average monthly use ~ 120,000 gallons
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THM DataA few days after startup, total THMs in the
distribution system were sampled at 64 μg/L.
A month after startup, total THMs in the distribution system were sampled at 113 μg/L.
Carbon not the answer – cost too prohibitive to change drums frequently.
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Second Pilot - AerationAeration has been used effectively on
THMs.
Aeration also can raise the pH for Cu/Pb.Theoretical pH rise given pH and alkalinitywould be from 7.7 to 8.2.
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Second Pilot - AerationDissolved gases in water governed by
Henry’s Law generically given as:
p = Hc
Where:p = Constituent pressure in gasH = Henry’s constantc = Constituent concentration in liquid
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Henry’s Law Constants Reflect Volatility of Gasses in Water
DBP H - atmChloroform 200Dichlorobromomethane 121Dibromochloromethane 56Bromoform 35CH2ClCOOH 0.00051CHCl2COOH 0.00046CCl3COOH 0.00075CH2BrCOOH 0.00037CHBr2COOH 0.00024THMs volatile but bromoform much less volatile than
chloroform.
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Orcas Island Example – System Schematic Using Recirculating Aeration
Pressure TankTreatment Plant
Service PumpRecirculating Pump
Distribution
Aeration Pilot Spray System
and Fan
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Simple Spray Header with Recirculation 60% Removal – But Mostly Chloroform (78%)
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Aeration PilotSomething more efficient than spray
aeration would be needed.
Options Packed towerTray aerator – chosen option
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Predictive Mass Balance Model
Recirculating Pump and Tray Aerator
Distribution
WellQwCi
QrCa QrC
C
(Qr+Q)CQC
QwCi + QrCa
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Mass Balance Model AssumptionsContinuously mixed reservoir with concentration C
C(T=n+1) = C(T=n) + inputs – outputs + dTHM/dt
Aerator efficiency (η) = (C-Ca)/C
Ci is amount formed between chlorination at well and reservoir
Qw (well) = Q (distribution system), Use average day flow in gpm
Some THMs form in distribution system after the reservoir
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Mass Balance Model AssumptionsCalibrate (as best as possible) with existing
system with no aeration system
– Ci = ~50 μg/L– dTHM/dt adjusted to have appropriate C– Final tap is C + ~10 μg/L
Example model without aeration conditionsCi=50 μg/L, C=140 μg/L, and tap
concentration is 150 μg/L
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Mass Balance Model PredictionsRecirculating aeration system can be added
after calibration.
Qr and η varied to size unit to achieve desired final THM.
25 gpm (Qr), 5-tray aerator (η = 55%) was selected.
HOA leased/purchased a used unit from a groundwater remediation company.
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Initial Pilot TestingHOA initially leased the unit for a month to test.
Initial single pass test with flow to waste to verify manufacturer’s performance claim (η = 55%) .
Contaminant
Average Influent
Concentration (μg/L)
Average Effluent
Concentration (μg/L)
RemovalEfficiency
(%)
ManufacturerPredicted
Removal (%)
Bromoform 68.2 36.7 46 31
Chlorodibromomethane 38.8 9.1 77 55
Bromodichloromethane 13.6 1.1 93 85
Chloroform 3.8 ND 100 93
TOTAL 124.4 46.8 62 -
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Pilot Test Phase 2Full scale recirculating system - Temporary piping
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Pilot Test Phase 2-Full Scale Operation
Day Finished pH Distribution THMs, μg/L
0 7.7 1324 8.0 63.35 8.1 -6 8.2 -
9 8.127.1 55.5
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Pilot Test Phase 2-Full Scale Operation
HOA decided to purchase the unit and built a permanent installation.
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Compliance Sampling – 12.3 μg/L THM
Aeration
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M, u
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Quarterly SamplingRunning Annual Average (RAA)MCL
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Regulatory Full Scale Considerations
Washington regulates VOC releases through WAC 173-460.
WAC 173-460 regulates Acceptable Source Impact Limit (ASIL) for toxic compounds.
ASIL for Chloroform = 0.0435 μg/m3
Emitters must stay below ASIL as proven by analysis.
WAC 173-460 sets Small Quantity Emission Rates (SQER) and de minimis amounts in lieu of analysis.
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Emission Rate Limits for THMs
Compound SQER, lb/yr
Chloroform 8.35
Bromoform 174
Bromodichloromethane 5.18
Chlorodibromomethane 7.1
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Eagle Cove ComplianceFor analysis assume:
2 MG Annual Water Use150 μg/L THM100% Removal
65% Bromoform 2.5 lb/yr (174 lb/yr limit)25% DiBromochloromethane 0.6 lb/yr (7.1 lb/yr limit)9% Bromodichloromethane 0.2 lb/yr (5.18 lb/yr limit)2% Chloroform 0.05 lb/yr (8.35 lb/yr limit)
All values < SQEREagle Cove is in compliance.
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Eagle Cove Compliance – Pb/Cu
Raw Water pH = 7.7Aerated Water = pH 8.1-8.2Pb/Cu scheduled for May 2014
Distribution pH used to verify aeration system operating.
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ConclusionsAeration is effective at reducing THMs at Eagle
Cove.
~ 130 μg/L reduced to <20 μg/L
Higher efficiency unit required for bromoform –tray aerator still only achieved ~50% removal single pass.
Effect of pH elevation on Pb and Cu remains to be seen.