CEE 371 Water and Wastewater Systems

Size: px

Start display at page:

Download "CEE 371 Water and Wastewater Systems"

Jessie Watson
5 years ago
Views:

1 Updated: 21 November 2009 CEE 371 Water and Wastewater Systems Print version Lecture #14 Drinking Water Treatment: Chlorination Reading: Chapter 7, pp , David Reckhow CEE 371 L#14 1 Forms of Chlorine applied to water Chlorine gas Cl 2 Sodium Hypochlorite liquid (Hypo) NaOCl Calcium Hypochlorite solid Ca(OCl) 2 Other forms Traditional method Becoming more common Organic-N based compounds and resins David Reckhow CEE 371 L#14 2 Lecture #14 Dave Reckhow 1

2 Chlorine Cont. HOCl + - H + OCl The hypochlorous acid ionizes to hypochlorite Although both hypochlorous acid and hypochlorite are disinfectants, hypochlorous acid is much more powerful. The equilibrium reaction is: K a = 3.16x10 8 = Fractio on of total (α) + [ H ][ OCl ] = [ HOCl] α 0 =HOCl/C T ph α 1 =OCl - /C T David Reckhow CEE 371 L#20 3 Question At ph 8.5, the percent of the total free chlorine that is in the most effective form is: A. 0% B. 9% C. 27% D. 50% E. 73% F. 91% G. 100% David Reckhow CEE 371 L#13 4 Lecture #14 Dave Reckhow 2

3 Chlorine demand I Chlorine reacts quickly with substances in water so that the effective residual is always less than the dose Chlorine residual = chlorine dose chlorine demand The effective concentration; this is the C in Ct This is what you add to the water Chlorine demand is usually measured for a particular water and it may depend on the contact time and dose It may be estimated from known water quality David Reckhow CEE 371 L#14 5 Chlorine Demand II Summation of at least 3 types Chlorine demand = inorganic demand + fast organic demand + slow organic demand Inorganic demand is based on known stoichiometry see next slide Both types of organic demand are highly variable As a first approximate you can use: Fast + slow organic demand = 0.25*TOC Where all concentrations are in mg/l David Reckhow CEE 371 L#14 6 Lecture #14 Dave Reckhow 3

4 Inorganic Chlorine Demand I Major inorganic ions responsible & reactions Manganese: 1M/M HOCl + Mn + H2O MnO2 + Cl + 3H Iron: 0.5M/M HOCl + Fe Fe Cl + 2 OH Sulfide: 4M/M when sulfate is product 2 + 4HOCl + HS SO4 + 4Cl + 5H Ammonia: 1.5 M/M, when nitrogen gas is produced HOCl + NH 1 N + 1 Cl H2O H David Reckhow CEE 371 L#14 7 Inorganic Chlorine Demand II Molar Calculation [ Inorganic Demand ] = [ Mn ] + 0.5[ Fe ] + 4[ HS ] + 1.5[ NH4 ] Where all concentrations are in moles/liter Mass-based Calculation Inorganic Demand = 1.3( Mn ) ( Fe ) + 8.8( 8( HS ) + 7.6( NH4 ) Where all concentrations are in mg/l David Reckhow CEE 371 L#14 8 Lecture #14 Dave Reckhow 4

5.5 mg/l David Reckhow CEE 371 L#14 9 Cl 2 gas: larger installations 1 ton cylinders With small (150 lb) vertical

5 Question You have a raw water with the following characteristics TOC = 2 mg/l, Fe +2 =0.5 mg/l, Mn +2 =0.2 mg/l, HS - =0.1 mg/l, NH 4+ =0.3 mg/l What is the chlorine demand? A. 03mg/L 0.3 B. 0.9 mg/l C. 3.7 mg/l D. 4.2 mg/l E. 5.5 mg/l David Reckhow CEE 371 L#14 9 Cl 2 gas: larger installations 1 ton cylinders With small (150 lb) vertical tanks in background Requires separate sealed room or bldg. Avon, CO David Reckhow CEE 371 L#14 10 Lecture #14 Dave Reckhow 5

6 Commercial Chlorinator feed Fig 7-18; pg. 236 in H&H David Reckhow CEE 371 L#14 11 Large chlorinator Vacuum created at aspirator injector Figure courtesy of Wallace & Tiernan Co. David Reckhow CEE 371 L#14 12 Lecture #14 Dave Reckhow 6

7 Dose control Flow pacing Small home system Full-scale l municipal system Fig 7-19; pg. 237 in H&H David Reckhow CEE 371 L#14 13 Dose control Feed back system Adjusts for varying chlorine demand Fig 7-19; pg. 237 in H&H David Reckhow CEE 371 L#14 14 Lecture #14 Dave Reckhow 7

8 Hypochlorite Dosing dafd David Reckhow CEE 371 L#14 15 Chlorine tanks Left side is currently feeding Right side is on reserve Lecture #14 Dave Reckhow 8

9 Chlorine feeding system Lecture #14 Dave Reckhow 9

10 Dose adjustment knob Lecture #14 Dave Reckhow 10

11 Chloramines I Inorganic chloramines are formed by the reaction of free chlorine with ammonia. The reaction is stepwise, giving monochloramine (equation 1) followed by dichloramine (equation 2). The dichloramine is quite unstable, forming nitrogen gas (equation 3) and some nitrate. This decomposition is responsible for the classic breakpoint chlorination phenomenon. NH 3 + HOCl > NH 2Cl + H 2 O (1) NH 2 Cl + HOCl > NHCl 2 + H 2 O (2) NH 2 Cl + NHCl > 3H + + 3Cl - + N 2 (3) David Reckhow CEE 371 L#20 21 Chloramines II Advantages Less reactive so that it persists longer in distribution systems May be better at penetrating biofilms on pipe walls Forms smaller amounts of disinfection byproducts (DBPs) Disadvantages Weak disinfectant; appropriate as a secondary disinfectant only May lead to tastes & odors May stimulate nitrification David Reckhow CEE 371 L#14 22 Lecture #14 Dave Reckhow 11

12 Chloramines: Breakpoint Curve 4 mg Cl 2 mg N Combined The Breakpoint 7.6 mg Cl 2 mg N MCA = monochloramine DCA = dichloramine FRC = free residual chlorine Free MCA DCA FRC Compare with Fig 7-17 in H&H, pg. 234 Chlorine Dose David Reckhow CEE 371 L#20 23 Northampton s Ground Storage 4.0 MG Two Concentric cells Can be isolated to service one while keeping the other in service NaOCl added just prior to entry David Reckhow CEE 371 L#9 24 Lecture #14 Dave Reckhow 12

13 Raw Water Reservoirs & Transmission Mains Clearwell 9 Sept 06 Lecture #14 Dave Reckhow 13

14 Clearwell 9 Sept 06 Clearwell 9 Sept 06 Lecture #14 Dave Reckhow 14

15 Clearwell 9 Sept 06 Clearwell 29 Sept 06 Lecture #14 Dave Reckhow 15

16 Clearwell From the plant site 29 Sept 06 Clearwell Dropping a panel into position 29 Sept 06 Lecture #14 Dave Reckhow 16

17 Clearwell 29 Sept 06 Northampton Ground Storage Finished water storage at plant Know as a Clearwell View from Outer ring Under construction David Reckhow CEE 371 L# Sept 06 Lecture #14 Dave Reckhow 17

18 Clearwell Exit portal 29 Sept 06 Clearwell In outer ring, looking SW? 29 Sept 06 Lecture #14 Dave Reckhow 18

19 Clearwell or Ground Storage Multi-purpose p Chlorine contact tank for achieving Ct Giardia controls 3 log Giardia is more restrictive than 4 log virus when using chlorine 2.5 log credit given for Giardia (clarification + filtration), leaving 0.5 log for Ct Northampton has decided to see 1.0 log for Ct Buffering system flows Fire Flow Backwash Storage David Reckhow CEE 371 L#14 37 Evaluation of Clearwell (cont.) Ct requirements Design is for maintenance of Ct even when one of the two concentric tanks is taken out of service t is normally evaluated for peak hourly flow Outer cell is considered less efficient based on length to width ratio 22:1 for outer cell 38:1 for inner cell Design conditions Q = 6.5 MGD max plant flow C = 0.5 mg/l Temp = 0.5 o C ph = log inactivation Ct = 79 mg/l - min t min David Reckhow CEE 371 L#14 38 Lecture #14 Dave Reckhow 19

Disinfection Profiling & Benchmarking David Reckhow CEE 371 L#14

20 CT for Giardia & Free Chlorine Portions of H&H Table 7-4 are extracted from this table Source: EPA, 1999, Guidance Manual for Disinfection Profiling & Benchmarking David Reckhow CEE 371 L#14 39 asdfasdf David Reckhow CEE 371 L#14 40 Lecture #14 Dave Reckhow 20

21 Storage profile Also below 542 ft surface elevation, the washwater pumps can no longer draw suction David Reckhow CEE 371 L#14 41 Tracer Test I Tracer Test Conditions Depth = 26.5 ft Volume = 2,073,200 gal Q = 3000 gpm = 4.32 MGD Fluoride concentration Background = mg/l Target = 1.33 mg/l Temperature = 10.1 C ph = 8.14 t R = hr David Reckhow CEE 371 L#14 42 Lecture #14 Dave Reckhow 21

22 Outer Cell Data Step Dose December 13, 2007 Lag time = 6.6 min = days David Reckhow CEE 371 L#14 43 Testing for Ct criteria Step dose tracer test t R = hr = min t 10 = 318 min t 10 /t R = 0.46 At 6.5 MGD MG = = 0.319d = 459 min 6.5MGD David Reckhow CEE 371 L#14 44 t R t 10 t10 = tr = 459(0.46) = 211min tr tracer Greater than 157 min Lecture #14 Dave Reckhow 22

23 To next lecture David Reckhow CEE 371 L#14 45 Lecture #14 Dave Reckhow 23

CHLORAMINATION AND CHLORAMINE ANALYSIS SW AWWA 2014

CHLORAMINATION AND CHLORAMINE ANALYSIS SW AWWA 2014 COMBINED CHLORINE - CHLORAMINATION Chlorine (HOCl and OCl - ) reacts with ammonia to form chloramines, commonly referred to as combined chlorine The