Chlorine Decay Modeling and Water Age Predictions

Transcription

1 Chlorine Decay Modeling and Water Age Predictions Marvin Gnagy, P.E., President PMG Consulting, Inc. OTCO Water Workshop March 5, 2014

2 Agenda Residual Decay in Water Experimental Data Collection Data Evaluations Adjustment for Temperature Variations Decay Model Development Applications for Decay Evaluations Water Age Predictions for Water Systems Questions 2

3 Residual Decay in Water Residual decay generally follows first order reaction C t C o e kt C t = concentration at time t C o = initial concentration k = residual decay coefficient t = decay time in days Calculate decay coefficient (k) from experimental data using reaction equation Decay models developed using identified coefficients for free chlorine, total chlorine, and monochloramine 3

4 Decay Coefficients Decay coefficient (k) contains 2 components k t = k b + k w k w - decay coefficient pipe wall k b - decay coefficient bulk water k w impacted by contact at pipe wall and presence of biofilms, deposits, corrosion materials k b affected by demand-causing substances in distribution system (water quality) k t dependent on water quality and pipe conditions, site specific 4

5 Items Influencing Residual Decay Initial residual concentration Equilibrium reactions Residual half-life Water temperature Water ph Presence of reducing substances Water age or residence time S/V ratio (surface to volume ratio in pipelines, tanks) 5

6 Equilibrium reactions Changes residual concentrations based on ph and temperature variations H OCl H OCl K Free chlorine i HOCl HOCl K i Hypochlorous acid (HOCl) and hypochlorite ion (OCl - ) remain in solution, but their content may change with ph and temperature according to solubility relationships based on equilibrium chemistry Monochloramine Equilibrium chemistry can shift monochloramine levels by simple ph and temperature changes NH3 HOCl NH 2Cl H2O 6

7 Residual half-life Residual reduces according to half-life reactions for the type of residual ½ concentration reduction each half-life Results in depletion over time in distribution and storage 7

8 Chlorine Residual Half-life, t 1/2 in days Chlorine Decay Modeling and Water Age Predictions Residual half-life (free chlorine) Half-life changes about 3 times with 10 o C temperature change Water Temperature, o C R 2 =

9 Water temperature impacts Generally two fold change k each 10 o C change in temperature Affects k and residual decay reactions Temperature affects type of free chlorine residual (HOCl or OCl - ) Equilibrium impacts for free chlorine and monochloramine shown earlier Increasing temperature increases auto-decomposition of chloramine residuals 9

10 Decay Coefficient, kb Chlorine Decay Modeling and Water Age Predictions Water temperature impacts Decay coefficient (k ) changes about 2 times for each 10 o C temperature change Water Temperature, o C R 2 =

11 Water ph effects ph affects type of residual based on equilibrium Free chlorine (HOCl or OCl - ) Monochloramine balance with ammonia and free chlorine Decay of system residuals dependent on ph level and type of residual ph levels above 8 slow auto-decomposition of chloramine residuals ph levels above 8 predominantly OCl - rather than HOCl 11

12 Chloramine Residual Remaining, mg/l Chlorine Decay Modeling and Water Age Predictions Water ph effects ph 6.6 ph 7.6 ph Water Age, days 12

13 Reducing substances Water quality parameters cause depletion of residual Iron, manganese, ammonia, sulfides, bromide, organic matter, nitrite, cyanide Reactions at pipe wall cause depletion of residual Pipe Deposits Corrosion Reactions Biofilms 13

14 Free Chlorine Residual, mg/l Chlorine Decay Modeling and Water Age Predictions Reducing substances - DOC mg/l DOC 1 mg/l DOC Reaction Time, hours 14

15 Water age / residence time Most significant effects on residual decay Exponent in first order reaction Longer residence time reduces residuals More reaction time with reducing substances More decay reaction based on time More reaction with pipe wall materials Water storage affects water age and residence times Affects water temperature variations in storage tanks Water temperature impacts given earlier Stagnant water conditions erode residual concentrations 15

16 S/V ratio Affects volume of water reacting with pipe wall or tank wall Smaller diameter increases bulk volume of water contacting pipe surface Increases residual decay (k w ) Affected by flow velocity Increased Reynolds number increases residual decay (k w ) Affects volume to surface area ratio in pipelines increasing decay (k w ) 16

17 Free Chlorine Residual, mg/l Chlorine Decay Modeling and Water Age Predictions S/V ratio in 6in 12in 48in Reaction Time, hours 17

18 Flow velocity effects Pipe Wall Decay Coefficient, k w % change in k w with 3 fold change in Reynolds Number ,000 10,000 15,000 20,000 25,000 30,000 Reynolds Number 18

19 Experimental Data Collection 19

20 Experimental Data Collection Representative sample with known residual concentration Plant tap collected for experimental evaluation Single sample for entire test period May need to spike sample with disinfectant for evaluation Glass container, brown or amber Rinsed with chlorine and lab water to remove demand on container wall Dried prior to decay evaluation testing Stored in dark conditions Sample capped to simulate pipe conditions No evaporation or off gassing Maintained at room temperature or incubator controlled 20

21 Daily observations Time, temperature, residual concentration Record observations at nearly same time each day Conduct decay testing until maximum residence time achieved Estimate of water system residence time (8 days to 21 days common) - OR - Until residual falls below 0.2 mg/l free chlorine or 1.0 mg/l total chlorine Record observations for evaluation and k determination 21

22 Data Evaluations 22

23 Day Day Day Day Day Day Day Day Water System A Free chlorine decay for 8 days (max residence time) Graph data in Excel Select scatter graph Add trend line to data points Select exponential graph type Display equation and R 2 options Determine decay coefficient (k) from equation C t =C o e (-kt) Day

24 Free Chlorine Remaining, mg/l Chlorine Decay Modeling and Water Age Predictions Water System A 2.0 Water System A Raw data in graphical form Decay Period, days Temp = 21 o C 24

25 Free Chlorine Remaining, mg/l Chlorine Decay Modeling and Water Age Predictions Water System A 2.0 Water System A Exponential trend line selection and overlay Decay Period, days Temp = 21 o C 25

26 Free Chlorine Remaining, mg/l Chlorine Decay Modeling and Water Age Predictions Water System A 2.0 Water System A y = 1.800e x R 2 = Trend line equation and R 2 correlation Decay Period, days Temp = 21 o C 26

27 Free Chlorine Remaining, mg/l Chlorine Decay Modeling and Water Age Predictions Water System A Water System A y = 1.800e x R 2 = Identified decay coefficient (k b ) is Decay Period, days Temp = 21 o C 27

28 Day Day Day Day Day Day Day Day Day Day Day Day Water System B Total chlorine decay for 21 days (depletion minimized) Graph data in Excel Select scatter graph Add trend line to data points Select exponential graph type Display equation and R 2 options Determine decay coefficient (k) from equation C t =C o e (-kt) 28

29 Total Chlorine Remaining, mg/l Chlorine Decay Modeling and Water Age Predictions Water System B Water System B y = e x R 2 = Identified decay coefficient (k b ) is Decay Period, days Temp = 20 o C 29

30 Day Day Day Day Day Day Day Day Day Day Water System C Total chlorine decay for 19 days (depletion minimized) Graph data in Excel Select scatter graph Add trend line to data points Select exponential graph type Display equation and R 2 options Determine decay coefficient (k) from equation C t =C o e (-kt) 30

31 Total Chlorine Remaining, mg/l Chlorine Decay Modeling and Water Age Predictions Water System C Water System C y = e x R 2 = Identified decay coefficient (k b ) is Decay Period, days Temp - 21 o C 31

32 Day Day Day Day Day Day Day Day Day Day Water System D Free chlorine decay for 15 days (depletion minimized) Sample spiked with NaOCl Graph data in Excel Select scatter graph Add trend line to data points Select exponential graph type Display equation and R 2 options Determine decay coefficient (k) from equation C t =C o e (-kt) 32

33 Free Chlorine Remaining, mg/l Chlorine Decay Modeling and Water Age Predictions Water System D Water System D y = e x R 2 = Decay Period, days Identified decay coefficient (k b ) is Temp - 21 o C 33

34 Adjustment for Temperature Variations 34

35 Water temperature changes k changes by factor of 2 each 10 o C change in temperature Each 1 o C changes k about 10% Increasing temperature increases k Decreasing temperature decreases k Calculate k at various temperatures to bracket range of water temperatures experienced in distribution Initial experimental data System B k = o C 10 o C o C o C

36 Water temperature changes k changes by factor of 2 each 10 o C change in temperature Each 1 o C changes k about 10% Increased temp increases k Decreased temp decreases k Calculate k at various temperatures to bracket range of water temperatures experienced in distribution system Initial experimental data System B k = o C 10 o C o C o C o C o C o C o C o C o C o C o C o C o C o C o C o C o C o C o C o C o C o C o C

37 Free Chlorine Remaining, mg/l Chlorine Decay Modeling and Water Age Predictions Seasonal Decay Variations o C, k= o C, k= o C, k= Water Age, days 37

38 Total Chlorine Remaining, mg/l Chlorine Decay Modeling and Water Age Predictions Seasonal Variations Water System C o C 29 o C 16 o C Water Age, days 38

39 Decay Model Development 39

40 Decay Model Development Using k values established decay calculations are made using first order equation k varies with temperature (vlookup function in Excel ) Bracket water age 0 days to X days Use typical initial residuals in plant effluent (t=0) Bracket residual concentration variations Insert equations for each residual and water age into table Table becomes decay model for individual water systems Known water age predicts residual at that location Known residual concentration predicts water age at that location Predict tap residuals needed to meet minimum system residuals Temperature variations often illustrate loss of residual due to decay reactions 40

41 Example Decay Model Table Chlorine Decay Model Temp, oc 21 k Residence Time, Days mg/l Tap

42 Applications for Decay Evaluations 42

43 Applications for Decay Evaluations Define residuals in areas that are not monitored Evaluate seasonal impacts to meet system residual requirements Define problem areas in storage and pipelines where persistent low residuals exist Extended water age due to lack of mixing Discover water age issues within distribution system Match decay evaluations with hydraulic modeling Relate other water quality issues in distribution 43

44 Water Age Predictions 44

45 Water System A (Hydraulic Model) 4 days 8 days 4 days 2 days Tank 2 days 8 days 2 days WTP 4 days 6 days 2 days 4 days 6 days 4 days 45

46 Water System A (Hydraulic Model vs. Decay) 4 days 8.5 days 8 days 4 days 2 days Tank 3.5 days 4.5 days 2 days 8 days 2 days 2 days 2 days 5.5 days WTP 7 days 4 days 6 days 3.5 days 2 days 4 days 6 days 4 days 46

47 Water System B 22 days Northern most tank contributes significant water age Total chlorine depleted less than 1.0 mg/l Study underway for tank mixing 4.5 days 4.5 days 1.2 days 2.3 days 1.2 days 1.2 days 0.5 days Water Plant 1.2 days System Storage 47

48 Water System C 2.4 days 1.4 days 2.2 days 0.8 days 2.3 days 7.5 days Water Plant 0.4 days 4.5 days 1.3 days 0.8 days 6.5 days 0.5 days 1.3 days 1.2 days 3.5 days 6.5 days Water Plant 8.5 days 9 days System Storage 48

49 Water System D -Hydraulic Model vs. Decay 8-10 days 6-8 days 10 days 7 days 7 days 6-8 days 3.5 days <2 days 6.5 days 6-8 days WTP 4-6 days 4.5 days 7 days 4-6 days 11.5 days 6-8 days 8-10 days 10 days 6-8 days >10 days System Storage 49 49

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