CEE 370 Environmental Engineering Principles. Disinfection of PWS

Transcription

1 Updated: 19 November 2015 Print version CEE 370 Environmental Engineering Principles Lecture #30 Water Treatment IV & Wastewater Treatment I: Disinfection, Adsorption, WW characteristics, 1 & 2 treatment Reading M&Z, Chapter 9 Reading: Davis & Cornwall, Chapt 6-1 to 6-8 Reading: Davis & Masten, Chapter 11-1 to 11-7 David Reckhow CEE 370 L#30 1 Disinfection of PWS One of the greatest achievements in public health during the 20 th century CDC One of the greatest engineering feats of the 20 th century National Academy of Engineering David Reckhow CEE 370 L#30 2 Lecture #30 Dave Reckhow 1

2 Disinfection Kill or inactivate pathogens Bacteria, viruses protozoa Disinfectants Chlorine (Cl 2, HOCl or OCl - ) Chloramines (NH 2 Cl or NHCl 2 ) Ozone (O 3 ) Chlorine Dioxide (ClO 2 ) Others: Bromine, UV light Primary purpose for drinking water treatment David Reckhow CEE 370 L#30 3 Chick s Law In the early 1900's Dr. Harriet Chick postulated that the death of the microorganisms was a first order process. So, for a given disinfectant and concentration: dn dt kn This can be separated and integrated (with N = N o at t = 0) to yield: kt N N N 0 e or: ln kt N 0 David Reckhow CEE 370 L#30 4 Lecture #30 Dave Reckhow 2

3 Chick-Watson Law The fraction inactivated is a function of the specific lethality (λ) of the disinfectant-organism couple and the disinfectant concentration (C ) n k C ln C Many studies have found that n is in the range of 0.8 to 1.2 for most microorganisms. In engineering practice, it is usually assumed that n is unity, thus the equation becomes: so N N 0 n t David Reckhow ln N N 0 Ct CEE 370 L#30 and k C Ct 2. 3x x log 5 Chick-Watson II Use of Ct values for various log removals is general practice Here is how Ct corresponds to specific lethality of Chick s Law (for n=1) % Removal x Log Removal N, if N 0 = 10,000/L /λ /λ /λ /λ Model is not always accurate, but it is usually a good first approximation David Reckhow CEE 370 L#30 6 Ct Lecture #30 Dave Reckhow 3

4 Specific Lethality (λ) at 20 o C General hierarchy Disinfectants: O 3 >ClO 2 >HOCl>OCl - >NHCl 2 >NH 2 Cl Organisms: bacteria>viruses>protozoa Units: L/mg-min Some may change with ph, dose; all are affected by temperature Disinfectant E. coli Poliovirus I Entamoeba histolytica Cysts O HOCl ClO OCl NHCl NH 2 Cl David Reckhow CEE 370 L#30 7 Chick-Watson Law: HOCl & Giardia Direct plot Specific Lethality = mg/l HOCl 2 mg/l HOCl 4 mg/l HOCl 1 log removal 2 log removal 0.6 N/N logs David Reckhow CEE 370 L#30Time (min) 8 1 log Lecture #30 Dave Reckhow 4

5 Chick-Watson Law: HOCl & Giardia Log plot 10 1 Specific Lethality = mg/l HOCl 2 mg/l HOCl 4 mg/l HOCl 1 log removal 2 log removal 3 log removal N/N log logs logs David Reckhow CEE 370 L#30Time (min) 9 Ct values for Giardia lamblia cysts H&H, Table 7-4, pg.245 David Reckhow CEE 370 L#30 10 Lecture #30 Dave Reckhow 5

6 Ct values for Viruses For Viruses at various temperatures ph 6-9 H&H Table 7-5, pg 245 David Reckhow CEE 370 L#30 11 t 10 concept US EPA regulatory approach Use the t 10 value 90% of water has a residence time greater than t 10 10% of water has a residence time less than t 10 A conservative or safe approach Protection of public health Value ranges from: 100% of t R for PRF 10.5% of t R for CSTR (-ln(0.9)) In between for all real reactors David Reckhow CEE 370 L#30 12 Lecture #30 Dave Reckhow 6

7 Determining t 10 Conduct tracer study Add a conservative substance to tank inlet at a particular time Fluoride is good; doesn t change, just moves with the water, non toxic Can be either a pulse (slug), step-up, or step-down Monitor concentration of conservative substance in tank outlet Data Analysis Prepare graph of concentration vs time Identify when concentration reaches 10% of breakthrough value David Reckhow CEE 370 L#30 13 Case Study I: Amherst Ozone Contactor Four chambers Under/over baffled Fluoride Tracer test Step t=0 2.4 mg/l Added to inlet Measure F - at outlet vs time David Reckhow CEE 370 L#30 14 Lecture #30 Dave Reckhow 7

8 Amherst O 3 Contactor II C/C Atkins WTP, Ozone Contactor Fluoride tracer study Q=1000 gpm V=22,980 gal C 0 =2.4 mg/l 0.4 Fluoride Data Ideal PFR Ideal CSTR 0.2 C C 01 e David Reckhow CEE 370 L#30 Time (min) 15 t t R Data from :Teefy, 1996 [AWWARF Report] Amherst O 3 Contactor III 0.5 Calculation of t min or 65% of t R 0.3 Atkins WTP, Ozone Contactor C/C Fluoride Data Ideal PFR Ideal CSTR 0.1 C/C 0 =10% Data from :Teefy, 1996 [AWWARF Report] 2.5 min 14 min 23 min David Reckhow CEE 370 L#30 16 Time (min) Lecture #30 Dave Reckhow 8

9 Amherst O 3 Contactor IV Use of t 10 for disinfection compliance Conventional treatment requires 2 log virus inactivation by disinfection For ozone 0.9 mg/l min is worst case (0.5 o C, in H&H table 7-5) With a t 10 = 14 min, then we need to have mg/l ozone residual at outlet of tank C min Ct t required 10 mg 0.9 L min mg 14 min L David Reckhow CEE 370 L#30 17 Sorption and Ion Exchange Adsorption The physical and/or chemical process in which a substance accumulates at a solid-liquid interface Natural solids (soil, sediments, aquifer) Anthropogenic (activated carbon) Sorption The combined process of adsorption of a solute at a surface and partitioning of the solute into the organic carbon that has coated the surface of a particle David Reckhow CEE 370 L#30 18 Lecture #30 Dave Reckhow 9

10 Sorption Naphthalene: Aqueous System with Sediment Reactive Surface Sites Adsorption Partitioning Coating of organic matter Solid Sediments David Reckhow CEE 370 L#30 19 Isotherms Freundlich Multi-layer adsorption q KC 1 n q (mg/g) Amount Adsorbed 1/n < 1.0 1/n = 1.0 1/n > 1.0 Amount Dissolved In Water C (mg/l) David Reckhow CEE 370 L#30 20 Lecture #30 Dave Reckhow 10

11 Isotherms (cont.) Simple partitioning When 1/n = 1.0 q = KC Incorporating organic carbon layer K oc = K/f oc Octanol/water partition coefficients [ A] K ow [ A] octanol water Good correlation with K oc Relatively easy to measure David Reckhow CEE 370 L#30 21 Adsorption Removal of Dissolved compounds industrial solvents, pesticides taste & odor compounds chlorination byproducts biodegradable substances (biological filtration) doesn t require regeneration Several Applications for activated carbon granular (GAC) in a fixed bed powdered (PAC) in a rapid mix Can be expensive when used strictly as an adsorbent David Reckhow CEE 370 L#30 22 Lecture #30 Dave Reckhow 11

12 Fixed Bed Adsorber Saturated Bed Mass Transfer Zone Clean Bed Adsorbed Conc. David Reckhow CEE 370 L#30 23 Other Sorbents Activated Alumina David Reckhow CEE 370 L#30 24 Lecture #30 Dave Reckhow 12

13 Membrane Processes Reverse Osmosis (RO) Demineralization, desalination Nanofiltration (NF) softening, NOM removal Ultrafiltration (UF) particle & pathogen removal Microfiltration (MF) particle removal David Reckhow CEE 370 L#30 25 Membranes cont. Membranes are carefully configured into: hollow fibers spiral wound tubular plates & frames Recent advances in membrane manufacture have made this technology more practical. David Reckhow CEE 370 L#30 26 Lecture #30 Dave Reckhow 13

14 Pressure-Driven Membrane Process Application Guide Micron Scale Approx MW Dissolved Organics Sand Typical Size Range of Selected Water Constituents Salts Viruses Colloids Bacteria Cryptosporidium Media Filtration Membrane Process* Ultrafiltration Microfiltration Nanofiltration Reverse Osmosis * Media Filtration David (not a Reckhow Membrane Process is shown for reference only) CEE 370 L#30 27 Typical Spiral-Wound Reverse Osmosis Membrane Source: AWWA and ASCE, SOURCE WATER SOURCE WATER & FLOW SPACER Processed water passes through the membranes on both sides of the porous permeate carrier. MEMBRANE (cast on fabric backing) POROUS PERMEATE CARRIER MEMBRANE (cast on fabric backing) SOURCE WATER & FLOW SPACER SOURCE WATER MEMBRANE LEAF SOURCE WATER Concentrate Permeate water Adapted from hydranautics Water Systems diagram. Concentrate Cutaway view of a spiral membrane module The permeate flows through the porous material in a spiral path until it contacts and flows through the holes in the permeate core tube. PRESSURE VESSEL ANTI-TELESCOPING SUPPORT CONCENTRATE (bnne) SEAL SNAP RING PERMEATE WATER CONCENTRATE OUTLET MEMBRANE ELEMENT MODULE MEMBRANE ELEMENT MODULE MEMBRANE ELEMENT MODULE END CAP SOURCE WATER INLET BRINE SEALS O-RING CONNECTOR David Reckhow CEE 370 L#30 28 Cross section of pressure vessel with 3 membrane modules Lecture #30 Dave Reckhow 14

15 Typical Hollow Fine-Fiber Reverse Osmosis Membrane Module "O" RING Concentrate Source Water END PLATE (FEED) FEED TUBE The permeator in this figure is adapted from E.I. dupont de Namours & Co. (Inc.) sales literature. HOLLOW FIBERS Source: AWWA and ASCE, HOLLOW FIBERS O RING POROUS SUPPORT BLOCK CONCENTRATE HEADER EPOXY DEFLECTOR BLOCK or "NUB" Concentrate FLOW SCREEN PERMEATE END PLATE (PERMEATE) FEED HEADER (source water) PERMEATE HEADER DEFLECTOR BLOCK PERMEATE EPOXY TUBE David Reckhow CEE 370 L#30 SHEET 29 Source Water RO videos Seven Seas Water (6:40) Cartoon Style Sydney Water (4:02) Shows recovery in 3 stage system David Reckhow CEE 370 L#30 30 Lecture #30 Dave Reckhow 15

16 Residuals Types Settling sludge Filter backwash water Softening sludge Reject from RO or ion exchange Other Contaminated air David Reckhow CEE 370 L#30 31 Sludge Treatment Depends on type of sludge Typical process train Thickening or dewatering Conditioning Stabilization (usually for wastewater) Disposal Nonmechanical methods Lagoons Sand-drying beds Freeze treatment Mechanical methods Centrifugation Vacuum filtration Belt filter press Plate filters David Reckhow CEE 370 L#30 32 Lecture #30 Dave Reckhow 16

17 Centrifuge David Reckhow CEE 370 L#30 33 Vacuum Filter David Reckhow CEE 370 L#30 34 Lecture #30 Dave Reckhow 17

18 Belt Filter Press David Reckhow CEE 370 L#30 35 WW Parameters Conventional BOD TSS Oil & grease ph Nutrients Nitrogen Ammonia Nitrate TKN Phosphorus Toxics Heavy metals Chromium, etc. Pesticides Parathion, etc Industrial Phenol, etc. PPCPs Pharmaceuticals Personal care products Others TOC, etc. David Reckhow CEE 370 L#30 36 Lecture #30 Dave Reckhow 18

19 Wastewater Characteristics Municipal/Domestic WW David Reckhow CEE 370 L#30 37 Municipal WW Temporal Patterns in flow and quality Seasonal Weekly Daily David Reckhow CEE 370 L#30 38 Lecture #30 Dave Reckhow 19

20 On-site disposal Septic Systems Requires minor levels of maintenance David Reckhow CEE 370 L#30 39 To next lecture David Reckhow CEE 370 L#30 40 Lecture #30 Dave Reckhow 20