Chapter 2: Conventional Wastewater Treatment (continue)

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1 ENGI 9605 Advanced Wastewater Treatment Chapter 2: Conventional Wastewater Treatment (continue) Winter 2011 Faculty of Engineering & Applied Science 1

2 2.2 Chemical treatment processes 1. Coagulation (1) Why coagulation Coagulation the process of adding chemicals to water to collect colloidal solids into clusters that can be removed through sedimentation and filtration Substances in water = chemicals in solution + colloidal solids + suspended solids Colloidal solids do not dissolve in water size range: nm (somewhere in the range between a molecule and bacteria in size) can be seen only with a high-powered microscope 2

4 Colloidal solids % of the organic matter in domestic wastewater composed of colloidal solids color, turbidity, viruses, bacteria, algae and organic matter in water treatment primarily either in the colloidal form or behave as colloids have an extremely large surface area per unit volume of the particles 1 sq yd to 1 acre/gram tend to adsorb substances (e.g., water molecules and ions) from the surrounding water because of the large surface area 1 sq yd = m 2 and 1acre = 4047 m 2 4

5 Colloidal solids tend to acquire an electrostatic charge due to the ionization of surface groups and the adsorption of ions from the surrounding solution most colloids in water have a negative charge repel each other due to the negative charges, large surface area and small size prevent the colloids from coming together to become heavy enough to gravity settle (e.g., it would take a 1 micron colloid 1 year to settle a distance of 1 foot by gravity) too small to be filtered by standard filtration devices Colloidal solids will not settle or filter until they agglomerate to a larger size through coagulation 5

(2) Coagulation processes There are two major forces acting on colloids Electrostatic repulsion simply, negative colloids repel other negatively charged colloids

6 (2) Coagulation processes There are two major forces acting on colloids Electrostatic repulsion simply, negative colloids repel other negatively charged colloids Intermolecular attraction van der Waals force The colloidal stability depends on the relative magnitude of the forces of attraction and the forces of repulsion 6

Coagulants added to the rapid mixing tank can be used to reduce the electrostatic repulsive forces (e.g., the electrostatic repulsion reduced by the addition

7 Coagulants added to the rapid mixing tank can be used to reduce the electrostatic repulsive forces (e.g., the electrostatic repulsion reduced by the addition of counter charged ions [Al 3+ ]) Continuous mixing or flocculation causes the destabilized (reduced charge) colloids to cluster removed through sedimentation 7

8 (3) Coagulants The most widely used coagulants in water treatment are aluminum sulfate and iron salts Iron salts ferrous sulfate, ferric sulfate, ferric chloride Aluminum sulfate is cheaper but iron salts are more effective over wider ph range Alum used as a popular coagulant Al 2 (SO 4 ) H 2 O Alum dosage used in water treatment 5-50 mg/l 8

9 Add alum to provide Al (OH) 2+ with positive charges for neutralization of colloidal solids (charge neutralization mechanism) Al 2 (SO 4 ) H 2 O + 2H 2 O 2Al (OH) H + + 3SO H 2 O (Viessman et al., Water Supply and Pollution Control, 2009 ) 9

10 At particular ph conditions and high enough alum concentration Al(OH) 3 precipitation formed colloids then tend to adsorb to the precipitation and become enmeshed (enmeshment mechanism) Enmeshment mechanism Sweep floc coagulation Al 2 (SO 4 ) H 2 O + 6H 2 O 2 Al(OH) 3 + 6H + + 3SO H 2 O (Viessman et al., Water Supply and Pollution Control, 2009 ) 10

Another type of primary coagulants polymers Polymers man-made organic compounds made up of a long carbon chain with smaller active molecules such as amine, nitrogen, or sulfate groups along the chain

11 Another type of primary coagulants polymers Polymers man-made organic compounds made up of a long carbon chain with smaller active molecules such as amine, nitrogen, or sulfate groups along the chain The chain is long enough to allow active groups to bond to multiple colloids Polymers can be either cationic (positively charged), anionic (negatively charged), or nonionic (neutrally charged) (Viessman et al., Water Supply and Pollution Control, 2009 ) 11

12 Coagulant aids Coagulant aids add density to slow-settling flocs and add toughness to the flocs they will not break up during the mixing and settling processes Primary coagulants are always used in the coagulation/ flocculation process but coagulant aids are not always required and are generally used to reduce flocculation time Lime a coagulant aid used to increase the alkalinity in water where natural alkalinity is insufficient to produce a good floc Bentonite a type of clay used to john with the small floc, making the floc heavier and thus settle more quickly 12

13 Lime used together with alum to provide the necessary alkalinity and control the ph of the coagulation process Al 2 (SO 4 ) H 2 O + 3Ca (OH) 2 2 Al(OH) 3 + 3CaSO H 2 O Example 2-5: A surface water is coagulated with a dosage of 30 mg/l of alum and equivalent dosage of lime to neutralize the hydrogen ions produced in the hydrolysis of aluminum. (a) How many pound of alum are needed per mil gal of water treated? (b) How many pounds of quicklime are required, assuming a purity of 70% CaO? 13

14 (4) Jar testing The selection of a coagulant requires the use of laboratory or pilot plant coagulation studies Jar testing usually used to determine the proper coagulant and coagulant aid, if needed, and the chemical dosages required for the coagulation of a particular water 14

15 Jar testing procedure Samples of the water are poured into a series of containers Various dosages of the coagulant and coagulant aid are added The contents are rapidly stirred to simulate rapid mixing Then the contents are continuous stirred to simulate flocculation After a given time, the stirring is ceased and the floc formed is allowed to settle The most important aspects to note during jar testing the time for floc formation, the floc size its settling characteristics the percent turbidity and color removed the final ph of the coagulated and settled water the chemical dosage determined from the procedure gives an estimate of the dosage required for the treatment plant 15

16 2. Lime-soda ash softening (1) Why lime-soda ash softening to remove hardness from water Hardness caused mainly by metallic cations (e.g., Ca 2+, Mg 2+, Sr 2+, Fe 2+, Mn 2+ ) expressed in equivalents of CaCO 3 Molecular weight of CaCO 3 = 100 Since Ca 2+ and CO 3 2- have valence of 2 equivalent weight CaCO 3 of =100/2 = 50 Total hardness = Ca hardness + Mg hardness (in most cases) Calculate total hardness if Ca 2+ = 70 mg/l and Mg 2+ = 9.7 mg/l? 16

17 17

18 Relationships between Hardness and Alkalinity Total hardness = Carbonate hardness + Non-carbonate hardness Carbonate hardness (CH) due to HCO 3- and CO 3 2- Ca(HCO 3 ) 2, Mg(HCO 3 ) 2, CaCO 3, MgCO 3 Non-carbonate Hardness (NCH) due to non-carbonate anions CaSO 4, CaCl 2, MgSO 4, MgCl 2 Alkalinity (ALK) the ability of natural water to neutralize acid due to HCO 3-, OH -, CO 3 2-, and H + in a carbonate dominate system Both the hardness and alkalinity expressed in equivalents of CaCO 3 we can compare them 18

19 If the total hardness (TH) > alkalinity (ALK) Carbonate Hardness (CH) = alkalinity (ALK) Non-carbonate Hardness (NCH) = total hardness carbonate hardness (CH) If the total hardness (TH) is less than alkalinity (ALK) Carbonate Hardness (CH) = total hardness (TH) Non-carbonate Hardness (NCH) = 0 Example 2-6: A sample of water has 210 mg/l alkalinity, 330 mg/l total hardness and 290 mg/l calcium hardness. Find out the following? a) Magnesium hardness; b) Carbonate hardness; c) Non-carbonate hardness. 19

20 Solution: TH = 330 mg/l, Alk = 210 mg/l, Ca-H= 290 mg/l a) TH = Mg-H + Ca-H Mg-H= TH Ca-H = = 40 mg/l Since TH is greater than Alk, Carbonate Hardness (CH) = alkalinity (ALK) Non-carbonate Hardness (NCH) = total hardness (TH) carbonate hardness (CH) b) CH = ALK = 210 mg/l c) NCH= TH CH = = 120 mg/l 20

21 (2) Lime-soda ash softening processes Lime (CaO) and soda (Na 2 CO 3 ) ash softening reactions First lime reacts with CO 2 : Then lime reacts with carbonate hardness: More lime reacts with soluble MgCO 3 produced in reaction (3): 21

22 Finally lime reacts with magnesium non-carbonate hardness, such as MgSO 4 : (1) - (5) stochiometric addition of lime excess lime treatment 35mg/l of CaO (1.25 meq/l) Lime addition removes only Mg hardness and Ca carbonate hardness reaction (5) simplify swap Mg non-carbonate hardness for Ca non-carbonate hardness Soda ash is added to remove Ca non-carbonate hardness : Water treatment through adding lime single-stage softening or single-stage lime treatment Water treatment through adding lime and soda ash two-stage softening 22

23 (3) Recarbonation After softening water has high ph and contains the excess lime and the magnesium hydroxide and the calcium carbonate that did not precipitate Recarbonation (adding carbon dioxide) used to stabilize the water and to reduce ph to CO 2 + MgCO 3 + H 2 O Mg(HCO 3 ) 2 23

24 (Viessman et al., Water Supply and Pollution Control, 2009 ) A two-stage excess-lime softening plant 24

25 Milliequivalents-per-liter bar graph graphical presentation of the chemical composition expressed in milliequivalents per liter the top row of the bar graph consists of major cations arranged in the order of Ca, Mg, Na and K the bottom row of the bar graph aligns anions in the sequence of carbonate (if present), bicarbonate, sulfate and chloride to maintain the electro-neutrality of water the sum of the positive milliequivalents per liter must equal to the negative values in equilibrium 25

26 Example 2-7: Water defined by the following analysis is to be softened by excess-lime treatment in a two-stage system: CO 2 = 8.8 mg/l as CO 2 ; Ca 2+ = 70 mg/l; Mg 2+ = 9.7 mg/l; Na + = 6.9 mg/l; Alk (HCO 3- ) = 115 mg/l as CaCO 3 ; SO 4 2- = 96 mg/l; Cl - = 10.6 mg/l The practice limits of removal can be assumed to be 30 mg/l of CaCO 3 and 10 mg/l of Mg(OH) 2, expressed as CaCO 3. (a) Sketch a meq/l bar graph and list the hypothetical combinations of chemical compounds in the raw water. (b) Calculate the quantity of softening chemicals required in pounds per million gallons of water treated and the theoretical quantity of CO 2 needed to provide the finished water with ½ of the alkalinity converted to biocarbonate ion. (c) Draw a bar graph for the softened water after recarbonation and filtration. 26

27 Solution: (a) components mg/l Equivalent weight Meq/l CO Ca Mg Na ALK SO Cl Bar graphs and hypothetical chemical combinations in the raw water 27

28 Related chemical reactions within this case: Lime Soda Recarbonation (7) (8) CO 2 + MgCO 3 + H 2 O Mg(HCO 3 ) 2 (9) (10) 28

29 (b) List the components which may react with lime and soda ash based on the bar graph in raw water: Components meq/l Lime Soda ash # of reactions involved CO (1) Ca(HCO 3 ) (2) CaSO (6) MgSO (5) + (6) total Lime required = stochiometric quantity = excess lime = 3.5*28+35 (excess lime) = 133mg/l of CaO *8.34 = 1109 b/mil-gal b / mil gal mg / l Soda ash required = 2.0*53 = 106 mg/l of Na 2 CO 3 *8.34 = 884 b/mil-gal b / mil gal mg / l 29

30 After line and soda-ash softening treatment: Components meq/l Lime Soda ash # of reactions involved products CO (1) CaCO 3 Ca(HCO 3 ) (2) CaCO 3 CaSO (6) Na 2 SO 4 and CaCO 3 MgSO (5) + (6) Mg (OH) 2, Na 2 SO 4 and CaCO 3 total Therefore, the components in the system are: components Meq/l Excess lime 1.25 (35 mg/l = 1.25 meq/l) CaCO 3 (dissolved) 30 mg/l (as CaCO 3 )/50 = 0.6 meq/l Mg (OH) 2 (dissolved) 10 mg/l (as CaCO 3 )/50 = 0.2 meq/l Na 2 SO = 2.0 NaCl 0.3 (previously exists in the system) 30

31 components Meq/l Excess lime 1.25 Ca Mg Na =2.3 components Meq/l OH CO SO Cl Thus, the bar graph of the water after lime and soda ash additions and setting but before recarbonation is shown as follows: 31

32 (c) The purpose of recarbonation is to (1) remove excess lime and OH - in the system and (2) convert part of the CO 3 2- into HCO 3 - components Meq/l Excess lime 1.25 removed Ca Mg Na =2.3 components Meq/l OH (changed to CO 2-3 based on reaction #8) CO = 0.8 SO Cl Half of the CO 3 2- becomes HCO 3- (reactions # 9-10) both CO 3 2- and HCO 3- are 0.4 meq/l in the system the bar graph of the water after two-stage recarbonation and final filtration shows: Total CO 2 required = ( /2 * 0.8) meq/l* 22 * 8.34 = 340 b/mil-gal 32

33 3. Adsorption (1) Why adsorption? Adsorption is mass transfer of chemicals in liquid phase onto solid phase Adsorbent adsorbing phase Adsorbate chemicals being adsorbed Adsorption is used in water treatment to remove organic contaminants taste and odor-causing chemicals synthetic organic chemicals color forming organics some disinfection by-products 33

34 Adsorption chemicals adhere to surface of solid Absorption chemicals penetrate into solid Sorption includes both 34

35 (2) Adsorption isotherm If the adsorptive process is rapid compared with the flow velocity contaminant chemicals will reach an equilibrium condition adsorbed phase and the process can be described by an equilibrium adsorption isotherm Adsorption depends on properties of activated carbon, chemistry of adsorbate, ph and temperature of water each application requires development of adsorption isotherm An adsorption isotherm relates S (solid phase concentration = mass of absorbate / mass of adsorbent) to C (liquid phase concentration of absorbate) Adsorption process is quantified via an adsorption isotherm which can take multiple forms 35

36 The linear adsorption isotherm can be described by the equation: S = K d C Where S = mass of chemicals adsorbed per dry unit weight of the solid adsorbent (mg/kg) C = concentration of chemicals in solution in equilibrium with the mass of chemicals adsorbed onto the solid adsorbent (mg/l) K d = distribution coefficient (L/kg) Samp le # Initial C (mg/l) Equilibrium C (mg/l) Mass adsorbed (mg/kg)

37 The Freundlich adsorption isotherm mathematically expressed as or S S x m x m KC KP 1 n 1 n Where x = mass of adsorbate m = mass of adsorbent p = Equilibrium pressure of adsorbate c = Equilibrium concentration of adsorbate in solution K and 1/n constants for a given adsorbate and adsorbent at a particular temperature S C 37

38 The Langmuir isotherm the equation is stated as Г max 1 KC KC Where Г(or S) amount adorbed (equilibrium absorbent-phase concentration of adsorbate) Г max maximum amount adorbed as C increases K Langmuir adsorption constant C Equilibrium concentration of adsorbate in solution C 38

39 Linear isotherm Freundlich isotherm Langmuir isotherm Г max 39

(3) Adsorbent Most popular adorbent activated carbon Activated carbon

, wood, coal, coconut shells) is heated in oxygen-starved environment

Activation carbonized material is exposed to steam or hot CO 2 to cause

40 (3) Adsorbent Most popular adorbent activated carbon Activated carbon is made in two-step processes Carbonization carbonaceous material (e.g., wood, coal, coconut shells) is heated in oxygen-starved environment to liberate carbon creates carbon to which organic chemicals will sorb Activation carbonized material is exposed to steam or hot CO 2 to cause pores and fissures to form increases surface area available for sorption 40

41 Activated carbon comes into two main forms Powdered activated carbon (PAC) ~ 24 µm, suspended in water/wastewater to be treated, then settled/filtered out and may be recycled Granular activated carbon (GAC) mm, placed in packed beds usually in pressure tanks GAC a granular media, approximately the size of medium fine sand has high interstitial surface area, 800 to 1100 m 2 /g the surface activation allows organic molecules to adsorb to the interstitial surface remove dissolved organic compounds from water 41

42 (Shanahan, Water and Wastewater Treatment Engineering, 2006 ) 42

43 GAC adsorption in water and wastewater combination of physical and chemical adsorption Physical adsorption Van der Waals attraction between adsorbate and adsorbent The attraction is not fixed to a specific site and the adsorbate is relatively free to move on the surface This is relatively weak, reversible, adsorption capable of multilayer adsorption Chemical adsorption chemical bonding between adsorbate and adsorbent characterized by strong attractiveness Adsorbed molecules are not free to move on the surface There is a high degree of specificity and typically a monolayer is formed The process is seldom reversible 43

44 Contact time 7 to 20 minutes in typical water treatment plant A GAC tank (Shanahan, Water and Wastewater Treatment Engineering, 2006 ) 44

45 (4) GAC breakthrough and regeneration When carbon has fixed adsorption capacity chemical eventually breaks through C 3 is based on regulatory standard Typical breakthrough curve for GAC adsorption (Shanahan, Water and Wastewater Treatment Engineering, 2006 ) 45

46 Carbon Regeneration GAC is relatively expensive adsorption would not be feasible unless the carbon can be regenerated after exhaustion Spent carbon is usually regenerated at 500 ºC under low oxygen conditions in the presence of steam Activated carbon loss is about 5-15% for each regeneration Adsorbed organics are volatilized and oxidized during the regeneration process removed from GAC 46

47 Carbon regeneration 47 (Shanahan, Water and Wastewater Treatment Engineering, 2006 )

48 ADSORBING BED CONTAMINATED FLUID CLEAN FLUID REGENERATING BED REGENERATION FLUID Schematic diagram of a fixed regenerative bed carbon adsorber (Source: Suthersan, Remediation engineering: Design concepts, 1997) 48

49 4. Disinfection (1) Why disinfection? To keep microbiological contaminants out of drinking water and wastewater effluent Disinfection has two components Primary disinfection inactivation of microorganisms in the water Secondary disinfection maintaining disinfection residue in distribution system 49

50 (2) Disinfection methods Free chlorine Combined chlorine Chlorine dioxide Ozone UV (3) Free chlorine disinfection Three different methods of application Cl 2 (gas) NaOCl (liquid) Ca(OCl) 2 (solid) 50

51 Reactions for free chlorine formation Cl 2 (g) + H 2 O <=> HOCl + Cl - + H + HOCl <=> OCl - + H + Advantages Effective against (almost) all types of microbes Relatively simple maintenance and operation Inexpensive Most widely used Disadvantages Corrosive High toxicity High chemical hazard Highly sensitive to inorganic and organic loads Formation of harmful disinfection by-products (DBP s) 51

52 Application of free chlorine: flow diagram (Meschke, Environmental and Occupational Health Microbiology, 2009 ) 52

53 Chlorination and dechlorination (Shanahan, Water and Wastewater Treatment Engineering, 2006 ) 53

54 (4) Chloramines disinfection Two different methods of application chloramination with pre-formed chloramines mix hypochlorite and ammonium chloride (NH 4 Cl) solution at Cl 2 : N ratio at 4:1 by weight, 10:1 on a molar ratio at ph 7-9 dynamic chloramination reaction of free chlorine and ammonia in situ Reactions for chloramines formation HOCl + NH 3 <=> NH 2 Cl (monochloramine) + H 2 O NH 2 Cl + HOCl <=> NHCl 2 (dichloramine) + H 2 O NHCl 2 + HOCl <=> NCl 3 (trichloramine) + H 2 O ½ NHCl 2 + ½ H 2 O <=> ½ NOH + H + + Cl - ½ NHCl 2 + ½ NOH <=> ½ N 2 + ½ HOCl + ½ H + + ½ Cl - 54

55 Advantages Less corrosive Low toxicity and chemical hazards Relatively tolerable to inorganic and organic loads No known formation of DBP Relatively long-lasting residuals Disadvantages Not so effective against viruses, protozoan cysts, and bacterial spores Chloramines contributes to chlorine residue along with residual free chlorine (HOCl + OCl - ) but chloramines are longer lasting 55

56 Application of chloramines: flow diagram (Meschke, Environmental and Occupational Health Microbiology, 2009 ) 56

57 (5) Chlorine dioxide disinfection Method of generation on-site generation by reaction of chlorine (either gas or liquid) with sodium chlorite Reactions for chlorine dioxide formation 2 NaClO 2 + Cl 2 2 ClO NaCl 57

58 Advantages Highly soluble in water Strong oxidant: high oxidative potentials 2.63 times greater than free chlorine Disadvantages Unstable (must be produced on-site) High toxicity with a higher ph 2ClO 2 + 2OH - = H 2 O + ClO 3- (Chlorate) + ClO 2- (Chlorite) High chemical hazards Highly sensitive to inorganic and organic loads Formation of harmful disinfection by-products (DBP s) Expensive 58

59 Application of Chlorine dioxide: flow diagram (Meschke, Environmental and Occupational Health Microbiology, 2009 ) 59

60 (6) Ozone disinfection Methods of generation generated on-site generated by passing dry air (or oxygen) through high voltage electrodes (ozone generator) bubbled into the water to be treated 60

61 Advantages Highly effective against all type of microbes Disadvantages Unstable (must be produced on-site) High toxicity High chemical hazards Highly sensitive to inorganic and organic loads Formation of harmful disinfection by-products (DBP s) Highly complicated maintenance and operation Very expensive 61

62 (Shanahan, Water and Wastewater Treatment Engineering, 2006 ) Application of ozone: flow diagram 62

63 (7) UV disinfection Mechanism of disinfection Physical process Damage nucleic acid DNA pyrimidine dimers, strand breaks, other damages inhibits replication Advantages Very effective against bacteria, fungi, protozoa Independent on ph, temperature, and other materials in water No known formation of DBP Disadvantages Not so effective against viruses No lasting residuals Expensive 63

64 (Viessman et al., Water Supply and Pollution Control, 2009 ) 64

65 (Shanahan, Water and Wastewater Treatment Engineering, 2006 ) 65

66 (8) Disinfection kinetics Chick-Watson Law the most commonly used model to describe water disinfection Nt n ln KC t N0 where: N 0 = initial number of organisms N t = number of organisms remaining at time = t k = rate constant of inactivation C = disinfectant concentration n = coefficient of dilution t = (exposure) time Assumptions: Homogenous microbe population all microbes are identical single-hit inactivation one hit is enough for inactivation When k, C, n are constant: first-order kinetics 66

67 Reaction rate When T and P are constant a A + bb ==> dd reaction rate = rate of formation/disappearance of a substance = r = dc/dt = k [A] α [B] β = + k [D] γ C concentration of reactants at any time t = [A] or [B] k reaction-rate constant order of reaction = α + β when α = β = 0, r = k zero order reaction dc k C C0 kt dt C 0 initial concentration of A or B when α + β = 1, r = k [A] (or k [B]) first order reaction dc dt kc ln C C 0 kt or C C 0 e kt 67

68 when α + β = 2, r = k [A] 2 (or k [B] 2 or k [A] [B]) second order reaction dc 1 1 C0 2 kc kt C or dt C C 1 ktc 0 0 C C 0 kt ln C C 0 kt (a) zero order reaction (b) first order reaction (c) second order reaction (Viessman et al., Water Supply and Pollution Control, 2009 ) 68

69 C and t under different n If n>1 disinfection efficiency decreases with dilution concentration is more important than time If n<1 time is more important than concentraion If n=1 time and concentration equally important CT concept Disinfectant concentration C and contact time t have the same weight or contribution in the rate of inactivation when n=1 Disinfection activity can be expressed as the product of C and t the same Ct values will achieve the same amount of inactivation 69

70 (Shanahan, Water and Wastewater Treatment Engineering, 2006 ) 70

71 (Shanahan, Water and Wastewater Treatment Engineering, 2006 ) 71

72 C*t 99 Values for Some Health-related Microorganisms (5 o C, ph 6-7) Organism Disinfectant Free chlorine E. coli Chloramines Chlorine dioxide Ozone Poliovirus Rotavirus G. lamblia C. parvum (Meschke, Environmental and Occupational Health Microbiology, 2009 ) 72

73 I*t Values for Some Health-Related Microorganisms Organism UV dose (mj/cm2) Reference E.coli 8 Sommer et al, 1998 V. cholera 3 Wilson et al, 1992 Poliovirus 21 Meng and Gerba, 1996 Rotavirus-Wa 50 Snicer et al, 1998 Adenovirus Meng and Gerba, 1996 C. parvum < 3 Clancy et al, 1998 G. lamblia < 1 Shin et al, 2001 (Meschke, Environmental and Occupational Health Microbiology, 2009 ) 73

74 Overview of disinfection requirements for 99 percent inactivation (Shanahan, Water and Wastewater Treatment Engineering, 2006 ) 74

Disinfection. Disinfection is used to treat both domestic water and wastewater.

Disinfection. Disinfection is used to treat both domestic water and wastewater. Disinfection Disinfection is the selective destruction of disease causing organisms (viruses, bacteria, protozoans). It destroys most recognized pathogenic microorganisms, but not necessarily all microbial