Balancing DBP Formation with Corrosion Control Concerns for a Blended Water Supply on Maui
County of Maui Department of Water Supply
PNWS-AWWA Annual Conference Kennewick, WA May 5, 2017
Bill Persich, P.E. Joanie Stultz, E.I.T.
• Background and Project Objectives • Description of Existing System and Operations • Water Quality Assessment • Recommended Chemical Feed Systems
Presentation Summary
Brown and Caldwell 2
• Customer Notification Required When Upper Kula Customers Cycle to Free Chlorine Disinfection
• Increases Disinfection Byproduct Formation of Upper Kula Water Due to Excess Free Chlorine
• Biofilm Sloughing • Possible Taste and Odor Problems
Problems of Mismatched Disinfectants
Brown and Caldwell 5 https://www.kwrwater.nl/en/onderzoek/health-en/biological-activity/
• Pre-design for a New Chloramination Station at the Omaopio Tank
• Understand Additional Chemical Feed Systems Based on Water Chemistry
Project Objectives
Brown and Caldwell 6
Existing Water Treatment
Description Water system
Lower Kula/Makawao Upper Kula
Water System Kamole Weir WTP Pookela well Piiholo WTP Olinda WTP
Treatment process Membrane Filtration Disinfection Only Direct filtration Membrane Filtration
Primary disinfection Chlorine Chlorine Chlorine Chloramines
Residual disinfection Chlorine Chlorine Chlorine Chloramines
Corrosion control Soda ash Not used Soda ash Food-grade lime
Brown and Caldwell 8
Piiholo WTP Dual Media Filter Wailoa Ditch Outlet to Kamole WTP Olinda WTP Pookela Well Discharge Head
• Pump Station • 3 Submersible pumps, one as standby • 2,400 GPM Firm Capacity
Upper Kimo Tank Booster Pumps
New Upper Kimo Pump Station
Brown and Caldwell 9
• Study Performed by University of Central Florida • Water Quality Blending Concerns After Chloramination
• Total Chlorine Residual Stability • DBP Formation (TTHM/HAA5)
• Tested Various Composite Blends in June 2016 • UKT: Blends of Lower Kula Water Only (Upper Kimo Tank) • UKS: Blends of Upper and Lower Kula Water (Upper Kula System)
Blending and Disinfectant By-products (DBP) Study
Brown and Caldwell 11
Results of Total Chlorine Residual Stability
Brown and Caldwell 12
• Lower(UKT) and Upper Kula(UKS) Blends were comparable
• Less decay and longer retention time for UKS
0
0.5
1
1.5
2
2.5
3
0 50 100 150 200
Tota
l Cl 2,
or F
ree
Cl2
,mg/
L
Time, hours
Disinfectant Residual Decay Curves Summary
UKS1 (total)
UKS2 (total)
UKS3 (total)
UKT (free)
96 hours (0.7 mg/L)
168 hours (0.4 mg/L)
Results of TTHM/HAA5 Testing
Brown and Caldwell 13
0102030405060708090
0 48 96
TTH
M, µ
g/L
Holding Time, hours
UKT1
UKT2
UKT3
MCL
MCLG
010203040506070
96
HAA
5, µ
g/L
Holding Time, hours
UKT1
UKT2
UKT3
MCL
MCLG
0102030405060708090
0 6 48 96 168
TTH
M, µ
g/L
Holding Time, hours
UKS1
UKS2
UKS3
MCL
MCLG
010203040506070
168
HAA
5, µ
g/L
Holding Time, hours
UKS1
UKS2
UKS3
MCL
MCLG
MCL
MCLG
MCL
MCLG
MCL
MCLG
MCL
MCLG
• Results Can be Impacted by Excessive Water Age in UKT Blends • Pre-formed DBP’s from Lower Kula Not Considered
Chloramination Recommendations
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• New Chloramination Station at the Omaopio Tank • Total Chlorine Residual Target: 2.5 mg/L • Chemicals
• Sodium Hypochlorite Solution (12.5% by wt.) • Anhydrous Ammonia
Corrosivity Indices of Water Sources
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Water quality constituent
Finished water
Olinda WTP
Finished water
Kamole WTP
Finished water
Piiholo WTP Finished water Pookela Well
Desired corrosivity values
TDS, mg/L 26.2 46.0 45.0 86.0
Temperature, °C 21.2 23.6 22.9 20.0
pH 9.00 9.20 9.30 8.64
Alkalinity, mg/L as CaCO3 5.6 14.6 25.8 40.0
Ca, mg/L as CaCO3 9.70 4.25 2.75 17.00
Cl-, mg/L 5.2 9.1 5.1 4.0
SO4-2, mg/L 0.6 1.1 0.5 2.1
Calculated Langelier index -0.92 -0.64 -0.49 -0.24 => 0 (minimize
corrosion risk)
Calculated alkalinity/(Cl- + SO4
-2) 1.0 1.4 4.6 6.6 >5
Calculated Ryznar index 10.84 10.49 10.29 9.12 <6–7
Calculated aggressiveness index 10.73 10.99 11.15 11.47
>12 (not corrosive) 10–12 (mildly corrosive) <10 (corrosive)
• Chloramination Will Slightly Reduce Corrosivity of Blended Water
• Sodium Hypochlorite Adds Non-Carbonate Alkalinity • Raises pH > 9.0
• Evaluation Conditions: Most Conservative • Assume Potentially Most Corrosive Lower Kula Water
100% Kamole WTP • Assume All Lower Kula Free Chlorine Residual Gone • Assume Total Chlorine Residual 2.5 mg/L required
Evaluate Supplemental Corrosion Control Measures for New Facilities
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1. Make No Further Corrosion Control or pH Adjustments • Valid if No Current Lead/Copper Rule Violations • Valid if Current State of Piping Corrosion is Acceptable • pH Will Increase Slightly
2. Reduce pH to Target 9.0 • Add CO2 at the New Chloramination Station
3. Provide Additional Corrosion Control and Adjust pH • Select Target LI = 0.0 (Theoretical “Ideal” Water) • Add Soda Ash • Control pH to 9.0 with CO2
Three Possible Corrosion Control Options
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Feature Existing Upper Kula Water
Option 1a Option 2a Option 3a
Description Existing Only Chloraminate Only Adjust pH Adjust LI and pH
Total Chlorine Residual, mg/L
2.5 2.5 2.5 2.5
Soda Ash Dose, mg/Lb
0 0 0 92
CO2 Dose, mg/L 0.0 0.0 1.3 36
pH 9.0 9.4 9.0 9.0
LI -0.92 -0.43 -0.82 0.0
Three Options for Supplemental Corrosion Control Measures
Brown and Caldwell 19
a. Calculation based on 100% Kamole WTP Water for Blending b. To be dosed at existing WTPs
Results
Ideal Water
3. Add Soda Ash and CO2 if “Ideal” Water is Desired
1. Corrosion Potential Reduced for All Options 2. Add CO2 if pH Adjustment is Desired
Recommended Disinfection Feed System for New Chloramination Station
Brown and Caldwell 21
• 12.5% Liquid Sodium Hypochlorite • Design Dose: 0.5 to 5.0 mg/L Cl2 /L • Design Flow: 1,100 – 2,400 gpm • Four(4) 275 gallon totes for 30-day supply • 3 diaphragm metering pumps (60 gpd)
Recommended Ammonia Feed System for New Chloramination Station
Brown and Caldwell 22
• Anhydrous Ammonia (Liquefied gas) • 5:1 Mass ratio to Cl2 (Mass Cl2/Mass N) • Design Dose: 0.1 to 0.5 mg/L NH3-N/L • Design Flow: 1,100 – 2,400 gpm • Four(4) 150 lb Cylinders for 30-day supply • 2 Ammoniators (18 ppd NH3)
Optional CO2 Feed System for New Chloramination Station
Brown and Caldwell 23
• Carbon Dioxide (liquefied gas) • Design Dose: 0 to 36 mg/L NH3-N/L • Design Flow: 1,100 – 2,400 gpm • Seven (7) tons CO2 for 30-day supply • 1 Carbonic Acid Generator (1,033
ppd CO2)
Construction Cost Estimate
Brown and Caldwell 28
Cost Item Total Cost
Base Construction Cost $799,000
Supplemental Constructiona $440,000
Allied Construction Costsb $985,000
Subtotal $2,220,000
Sales tax (4.166%) $92,700
Total Construction Cost $2,310,000
a. Supplemental: Demolition/Abandonment, Civil site work, Electrical, I&C b. Allied Construction Costs: Escalation, Startup, Contingency (30%), Earthquake insurance, Liability, Bonds, change orders
• Bulk Hypochlorite Delivery • No CO2 System
Chloramination
Brown and Caldwell 30
HOCl H+ + OCl-
Cl2 + NH3 NH2Cl + H2O
Free Chlorine Disinfection Monochloramine Disinfection
Cl2 + H20 HOCl + HCl Cl2 + H20 HOCl + HCl
HOCl + NH3 NH2Cl + H2O
5:1 Mass ratio to Cl2 (Mass Cl2/Mass N) https://commons.wikimedia.org/wiki/File:Chloramination_breakpoint_curve_(theoretical).jpg