Assessing Post-Treatment Aeration Variables to Reduce Disinfection Byproducts(THMs) for Small Systems

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1 Assessing Post-Treatment Aeration Variables to Reduce Disinfection Byproducts(THMs) for Small Systems Ethan S. Brooke & M.R. Collins Water Treatment Technology Assistance Center Civil Engineering University of New Hampshire 1

2 What Are Disinfection Byproducts? + = Free Chlorine (CL) Natural Organic Matter (NOM) Disinfection Byproducts (DBP)

3 Stand Pipe Aeration

4 Chlorine Contact Basin Aeration 4

5 A Potential Solution for Small Systems Particularly useful for concurrent systems Do not have control over water quality Alternative to precursor removal / alternative disinfectant 5

6 Diffused Aeration Surface Aeration Spray Aeration 6

7 Aeration Kinetic Basics Equilibrium is the driving force for all forms of Aeration! 7

8 Equilibrium is Expressed by Henry s Constant 8

9 Factors Affecting Henry s Constant ph Complex Mixtures: Co-solvents and Co-solutes Ionic Strength: Dissolved Salts Suspended Solids Dissolved Organic Matter Surfactants Temperature 9

10 Henrys Constant Temperature Correction Equation 10

11 Henrys Constants and Temperature Correction Factor for TTHMs at 20⁰ C and 1⁰ C THM Species H cc 20⁰ C H cc 1⁰ C B Chloroform (CF) Bromodichloromethane (BDCM) Chlorodibromomethane (CDBM) Bromoform (BF)

12 Henrys Constants for Haloacetic Acids at 20⁰ C Haloacetic Acid Species H cc 20⁰ C Monochloroacetic Acid Dichloroacetic Acid Trichloroacetic acid Monobromoacetic Acid Dibromoacetic Acid

13 The tendency of a system to seek equilibrium is the driving force for all forms of aeration. Equilibrium is expressed by Henry s constant. Temperature has a large effect on Henry s constant. Haloacetic acids are not volatile enough to strip effectively; however, THMs are amenable. 13

14 Methodology I. BENCH-SCALE STUDY- Diffused Aeration Assess selected design variables: Air flow rate, contact time (air/water ratio), diffuser arrangement (bubble size) Assess selected operating conditions: Air temperature, Water temperature, THM concentration 14

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16 Diffused Aeration Apparatus 16

17 Diffused Aeration Apparatus 17

18 Bench Scale Variables Air Temperature Water Temperature Air Flow Rate Number of Diffusers THM Concentration Contact Time 4º C and 20º C 4º C and 20º C 1L/Min and 3L/Min 1 and ug/l and 400 Ug/L 40 Min and 80 Min 18

19 Experimental Design and Analysis 19

20 ANOVA Table and % Contribution of Data Variation for Selected Design Variables and Operating Conditions for Bench Scale Study Source DF Sum of Squares F Ratio Prob > F % Contribution Water Temperature < % Air Flow Rate < % Aeration Time % Air Flow Rate*Aeration Time % Error 26 17% Total Contribution 100% 20

21 % Removal % Removal Vs. Air to Water Ratio for THMs at 20 C CF DBCM CDBM BF TTHM /1 30/1 45/1 60/1 Air to Water Ratio 21

22 Bench Scale Experimental Conclusions Water temperature and air to water ratio have a significant effect on removals Air Temperature and initial concentration did not have a significant effect on removals Bubble distribution did not appear to have a significant effect on overall removals 22

23 Diffused Aeration Minimum Air to Water Ratio 23

24 5% 10% 15% 20% 25% 30% 35% 40% 45% 50% 55% 60% 65% 70% 75% 80% 85% 90% 95% Air to water ratio Percent removal vs. air to water ratio for THMs at 25º C 60 Chloroform Bromodichloromethane Chlorodibromomethane Bromoform Percent removed 24

25 A simple air diffuser can be placed in a storage tank or chlorine contact basin for THM removal Effective for small system THM compliance No removal of HAAs Most effective for Chloroform Dominant species in most chlorinated water Most effective during warm weather months 25

26 Methodology I. BENCH-SCALE STUDY- Diffused Aeration Assess selected design variables: Air flow rate, contact time (air/water ratio), diffuser arrangement (bubble size) Assess selected operating conditions: Air temperature, Water temperature, THM concentration II. PILOT SCALE STUDY- Spray Aeration Evaluate the role of temperature on spray aeration removal rates Develop a relationship between percent removals and physical characteristics of aeration equipment with respect to Henry s constants of THM species Assess the role of operating parameters (e.g. operating pressure / head loss and flow rate) in determining mass transfer coefficients and interfacial surface area (K L a) 26

27 Assessing Mass Transfer Coefficients and Interfacial Surface Area With Respect to Spray Aeration Equipment 27

28 Mass Transfer Coefficient (K L a) and Two Film Theory 28

29 Droplet Size and Interfacial Surface Area (a) a = M 2 / M 3 = (M -1 ) a = 6 / d p d p = Sauter Mean Diameter (SMD) (average. droplet diameter determined by photographic analysis, provided by nozzle manufacturer) d p is a function of flow rate and nozzle characteristics 29

30 Spray Aeration Pilot Schematic 30

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Height (m) Removal 22 1 0.00379.74 31 22 1 1.135.74 35 22 2 0.

32 Initial Pilot Scale Study: Assessing Selected Shower Heads Temperature Flow Rate % TTHM ( C ) Shower Head (m 3 /min) Height (m) Removal

33 Pilot Scale Assessment of Selected Spray Aeration Variables Nozzle Height 0.74 m, 2.13 m, 4.27 m Water Temperature 2º C, 22º C, 36º C Water flow rate m 3 /min (a= m -1 ) m 3 /min (a = m -1 ) 33

34 ANOVA Table and % Contribution of Data Variation for Selected Design Variables and Operating Conditions for Pilot Scale Study Source DF Sum of Squares F Ratio Prob > F % Contribution Height % Temp % Flow % Error % Total Contribution 100% 34

35 % Removal 100 % Removal of Chloroform as a Function of Interfacial Surface Area (a) and Water Droplet Travel Distance (d) CF Hcc 0.50 ( T 2C) CF Hcc (T 22 C) CF Hcc (T 36C) R² = R² = R² = a(.06) x d(.94) (Dimensionless) 35

36 % Removal ` 100 % Removal of Bromoform as a Function of Interfacial Surface Area (a) and Water Droplet Travel Distance (d) BF Hcc (T 2C) BF Hcc (T 22C) BF Hcc (T 36C) R² = R² = R² = a(.06) x d(.94) (Dimensionless) 36

37 Concentration (μg/l) Chlorine Residual Results Penn State-Harrisburg (Xie et al, June 2008) Chlorine residual (mg/l) Chloroform Chlorine Residual Time (hours)

38 Chlorine Chemistry 38

39 % of Total Chlorine in a Nonionic Form (HOCl) as a Function of ph 100% 90% 80% % of Total Chlorine in a Nonionic form as a Function of ph 70% 60% 50% 40% 30% 20% 10% 0% ph 39

40 Conclusion Aeration provides a way to remove THMs after they have formed with a minimal capital investment Temperature has a significant effect on removal efficiencies Diffused aeration can achieve significant removals based on air to water ratio Spray aeration can achieve significant removals of THMs dependent primarily on shower head type and droplet travel distance 40

Meeting the new maximum contaminant levels (MCLs) for

Meeting the new maximum contaminant levels (MCLs) for disinfection Posttreatment aeration inside water tanks or in chlorine contact basins to strip trihalomethanes (THMs) after formation is an underused and cost-effective treatment option to reduce disinfection