Wet Air Oxidation. Schematic diagram of a batch wet air oxidation reactor. for sampling

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1 CENG 576 Advanced Physico-Chemical Treatment Processes Professor Xijun Hu Wet Air Oxidation Wet air oxidation (WAO) is a well-established technique for wastewater treatment particularly toxic and high concentration organic wastewater. WAO involves the liquid phase oxidation of organics or oxidizable inorganic components at elevated temperatures (5- o C) and pressures (.5 MPa) using a gaseous source of oxygen (usually air). Enhanced solubility of oxygen in aqueous solutions at elevated temperature and pressure provides a strong driving force for oxidation. The elevated pressures are required to eep water in the liquid state. Water also acts as a moderant by providing a medium for heat transfer and removing excess heat by evaporation. In WAO Carbon CO H H O N NH, NO or N Halogen and sulfur inorganic halides and sulfates The degree of oxidation depends on Temperature Oxygen partial pressure Residence time Oxidizability of the pollutants CENG 576 Advanced Physico-Chemical Treatment Processes Professor Xijun Hu for sampling Schematic diagram of a batch wet air oxidation reactor The operating costs are almost entirely for power to compress air and high pressure liquid pumping. WAO becomes self-sustaining with no auxiliary fuel requirement when the COD (chemical oxygen demand) is above, mg/l. Incineration (combustion) becomes self-sustaining when the COD is in the range of, 4, mg/l. Adding a catalyst can achieve the same or better oxidation efficiency at lower reaction temperatures and pressures so reducing the operation cost. When a catalyst is used, the process is called catalytic wet air oxidation (CWAO)

2 CENG 576 Advanced Physico-Chemical Treatment Processes Professor Xijun Hu CENG 576 Advanced Physico-Chemical Treatment Processes Professor Xijun Hu Measurement of Organic Content determines the approximate quantity of oxygen required to biologically stabilize the organic matter determines the size of waste treatment facilities measures the efficiency of some treatment processes determines compliance with wastewater discharge limits Schematic diagram of a continuous WAO reactor. In most applications, WAO is not used as a complete treatment method, but only as a pretreatment step where the wastewater is rendered nontoxic and the COD is reduced sufficiently, so that biological treatment becomes applicable for the final treatment. For industrial wastewater treatment, COD or (total organic carbon) is often used to characterize the wastewater and to test the efficiency of the WAO process. Chemical Oxygen Demand (COD) The oxygen equivalent of the organic matter that can be oxidized is measured by using a strong chemical oxidizing agent in an acidic medium. Potassium dichromate is excellent for this purpose. Test is performed at elevated temperature Catalyst (silver sulfate) is required in some cases The principle reaction is Organic matter (C a HbOc ) Cr O 7 H catalyst Cr CO HO heat Some inorganic compounds may interfere with the test 9 9

3 CENG 576 Advanced Physico-Chemical Treatment Processes Professor Xijun Hu Total Organic Carbon () A nown quantity of sample is injected into a high temperature furnace or chemicallyoxidizing environment The organic carbon is oxidized to carbon dioxide in the presence of a catalyst The carbon dioxide produced is measured by an infrared analyser The test is very quic so it becomes popular Theoretical Oxygen Demand (ThOD) Organic compounds in wastewater is a combination of carbon, hydrogen, oxygen, and nitrogen The ThOD can be computed if the chemical formula of the organic matter is nown Its usage is limited because the organic matter in wastewater is usually a mixture of many unnown substances CENG 576 Advanced Physico-Chemical Treatment Processes Professor Xijun Hu Example: Determine the ThOD for glycine (CH( NH) COOH) using the following assumptions:. In the first step, the organic carbon and nitrogen are converted to carbon dioxide and ammonia.. In the second and third steps, the ammonia is oxidized sequentially to nitrite and nitrate.. The ThOD is the sum of the oxygen required for all three steps. Solution. Write the balanced reaction for the carbonaceous oxygen demand: CH( NH) COOH O NH CO HO. Write the balanced reaction for the nitrogenous oxygen demand: (a) NH O HNO HO (b) HNO O HNO NH O HNO HO. Determine the ThOD: ThOD = (/+) mol O /mol glycine =.5 mol O /mol glycine g/mol O = g O /mol glycine 9 9

4 CENG 576 Advanced Physico-Chemical Treatment Processes Professor Xijun Hu Biochemical oxygen demand (BOD) measurement of the dissolved oxygen used by microorganisms in the biochemical oxidation of organic matter 5-day or -day BOD (BOD 5 or BOD ) is most used CENG 576 Advanced Physico-Chemical Treatment Processes Professor Xijun Hu B = dissolved oxygen of seed control after incubation, mg/l f = ratio of seed in sample to seed in control = (% seed in D )/ (% seed in B ) Determination of BOD dilution to cover different ranges of BOD seeding with a bacterial culture (saprophytic and other micro-organisms and some autotrophic bacteria) that has acclimated to the organic matter or other materials in the wastewater incubation period of five days at the constant temperature of o C measure dissolved oxygen before and after incubation Non-seeded BOD (mg / l) = D D P D D -B B f Seeded BOD (mg / l) = P D = dissolved oxygen of diluted sample immediately after preparation, mg/l D = dissolved oxygen of diluted sample after 5 days incubation at o C, mg/l P = decimal volumetric fraction of sample used B = dissolved oxygen of seed control before incubation, mg/l 94 95

5 CENG 576 Advanced Physico-Chemical Treatment Processes Professor Xijun Hu CENG 576 Advanced Physico-Chemical Treatment Processes Professor Xijun Hu Example: A wastewater sample is diluted by a factor of using seeded dilution water. If the following results are obtained, determine the 5-day and 6-day BOD. The ratio of seed in sample to seed in control, f =. Dissolved oxygen, mg/l Time (day) Diluted sample Seeded sample P., 5-day BOD: f ( D ) ( ) ( / ) D B B f BOD mg L P (8.55.4) ( )() BOD5 59.( mg / L). 6-day BOD: ( D ) ( ) ( / ) D B B f BOD mg L P (8.55.) ( )() BOD6 6.9( mg / L)

6 CENG 576 Advanced Physico-Chemical Treatment Processes Professor Xijun Hu The inetics of the BOD reaction can be formulated with first-order reaction inetics, for practical purposes: dlt L t dt where L t is the amount of the first-stage BOD remaining in the water at time t and is the reaction rate constant. This equation can be integrated as ln Lt t L t or t t e L where L or BOD L is the BOD remaining at time t= (i.e., the total or ultimate first-stage BOD initially present). The amount of BOD remaining at time t is t Lt L( e ) and y, the amount of BOD that has been exerted at any time t, is yt LLt L( e t ) Note that the 5-day BOD equals 5 y5 LL5 L( e ) For polluted water and wastewater, (base e) is around. (.5 -.) day -. CENG 576 Advanced Physico-Chemical Treatment Processes Professor Xijun Hu BOD is mg/l. The reaction constant (base e) =. day -. Solution:. Determine ultimate BOD t Lt L( e ) 5 y5 L L5 L( e ) 5. L e L=9 mg/l. Determine -day BOD y L L L( e ) = 9(-e -. ) = 6 mg/l Example: Determine the -day BOD and ultimate first-stage BOD for a wastewater whose 5-day, o C 98 99

7 CENG 576 Advanced Physico-Chemical Treatment Processes Professor Xijun Hu. Nitrogenous Biochemical Oxygen Demand (NBOD) - the oxygen demand associated with the oxidation of ammonia to nitrate. Carbonaceous Biochemical Oxygen Demand (CBOD) - suppressed BOD. Elimination of the interference of nitrifying bacteria by pretreatment or by the use of inhibitory agents. Limitation in the BOD Test A high concentration of active, acclimated seed bacteria is required Pretreatment is needed when dealing with toxic waste, and the effects of nitrifying organisms must be reduced Only biodegradable organics are measured An arbitrary, long period of time is required to obtain results Inhibition and Toxicity An organic substance that is biodegradable at one concentration can become persistent at higher concentrations by inhibiting the growth of the microbial culture. At even higher concentrations, the substance can become toxic to the culture. CENG 576 Advanced Physico-Chemical Treatment Processes Professor Xijun Hu WAO of cotton desizing wastewater at 9 o C. COD Reduction (% Removal (%) Reaction Time (min) without O with O without O with O The organics in wastewater are stable to heating but oxidizable by oxygen

8 CENG 576 Advanced Physico-Chemical Treatment Processes Professor Xijun Hu Effect of reaction temperature on the WAO of cotton desizing wastewater at.5 MPa partial oxygen pressure. COD Reduction (%) Removal (%) MPa MPa.5 MPa MPa MPa Reaction time (min) P 5 o C o O C.5 MPa4 o C MPa 7 o C.5 MPa9 o C MPa MPa P O 5 o C o C 4 o C 7 o C 9 o C 5 CENG 576 Advanced Physico-Chemical Treatment Processes Professor Xijun Hu Effect of reaction pressure on the WAO of cotton desizing wastewater at 4 o C. COD Reduction (% Removal (%) Reaction time (min) P O.75 MPa.75 MPa.5 MPa.5 MPa.5 MPa P O.75 MPa.75 MPa.5 MPa.5 MPa.5 MPa WAO is better at a higher temperature Near 8% COD and removals at 9 C WAO is better at a higher oxygen partial pressure

9 CENG 576 Advanced Physico-Chemical Treatment Processes Professor Xijun Hu WAO of chemical fibre desizing wastewater at MPa partial oxygen pressure at various temperature. COD Reduction (%) Removal (%) Biodegradability (%) o C o C 4 o C 7 o C Reaction time (min) 5 o C o C 4 o C 7 o C 5 o C o C 4 o C 7 o C WAO is better at a higher temperature 9% COD & 8% removals at 7 Biodegradability = BOD/COD BOD = biochemical oxygen demand CENG 576 Advanced Physico-Chemical Treatment Processes Professor Xijun Hu Possible Reaction Kinetics COD as reactant (C) Reaction mechanisms: fast Wastewater CO & H O (COD) fast slow Intermediate organic products (COD) Rate data modeled by first order inetics dc C dt where t is reaction time, and is the specific reaction rate constant which has the following temperature dependency: exp E / RT where is a pre-exponential factor, E is the activation energy, R is the universal gas constant and T is the temperature in Kelvin. Integration gives C ln C t where C is the initial COD value. By plotting ln(c /C) versus time, the slope is the specific reaction rate constant. A typical plot of the WAO treatment of cotton desizing wastewater at a fixed partial oxygen pressure of.5 MPa and four 4 5

10 CENG 576 Advanced Physico-Chemical Treatment Processes Professor Xijun Hu different reaction temperatures is shown in the following figure. The data fit well into two straight lines for a given temperature, indicating that oxidation proceeds in two distinct steps: a fast initial reaction of large molecules decomposed into intermediate products, followed by a slow reaction of further oxidizing the intermediate products into end products of low molecular weight organic acids, carbon dioxide, and water. CENG 576 Advanced Physico-Chemical Treatment Processes Professor Xijun Hu The specific rate constant is a function of temperature: E exp RT or ln ln E RT ln(k) K fast LnC /C Reaction time (min) o C 4 o C 7 o C 9 o C K slow.8.. /T (K - ) Effect of temperature on rate constants of cotton desizing wastewater at.5 MPa partial oxygen pressure. The activation energies are: E fast = J/mol; E slow = 9 J/mol WAO of cotton desizing wastewater at.5 MPa partial oxygen pressure (theoretical oxygen requirement). 6 7

11 CENG 576 Advanced Physico-Chemical Treatment Processes Professor Xijun Hu If oxygen is not in excess, then '. P O n CENG 576 Advanced Physico-Chemical Treatment Processes Professor Xijun Hu.5 K fast..8 K. InC /C Reaction time (min) P O.75 MPa.75 MPa.5 MPa.5 MPa.5 MPa.5. K slow Oxygen partial pressure (MPa) WAO of cotton desizing wastewater at 4 o C. Effect of oxygen concentration on rate constants of cotton desizing wastewater at 4 o C. The slow reaction is independent of oxygen partial pressure. The fast reaction strongly depends on the oxygen supply when it is less than the theoretical oxygen requirement (.5 MPa), with excess oxygen, even the fast reaction becomes independent of oxygen partial pressure. 8 9

12 CENG 576 Advanced Physico-Chemical Treatment Processes Professor Xijun Hu For most WAO operations, the reaction is assumed to consist of two steps: the decomposition of large molecules into intermediate products and the further oxidation of the intermediates into the end products of carbon dioxide and water. If starch is assumed the major content of the wastewater, which can be hydrolyzed into glucose at first, and glucose is oxidized into carbon dioxide and water thereafter. Furthermore, it is assumed that a portion of the organic compound is very difficult to be oxidized. Therefore, the following reaction routes were assumed as: S K G CO + H O N Where S is the substrate organic (starch) of wastewater, G is glucose, and N is a non-oxidizable organic. Reactions and do not change the COD or value of the solution. CENG 576 Advanced Physico-Chemical Treatment Processes Professor Xijun Hu To simplify the analysis, it is further assumed that the reactions are inetics controlled and the dissolved oxygen concentration is a constant since enough oxygen gas is supplied. The conversion between the substrate organic and glucose is a fast reversible reaction and reaches equilibrium very quicly, represented by the equilibrium constant K. Reactions and are assumed to follow first order inetics. Let the total organic in the solution during reaction be X, its removal rate is then dx = G () dt where [G] stands for the concentration of glucose. There exists equilibrium between the substrate and glucose concentrations: [G]=K [S] ()

13 CENG 576 Advanced Physico-Chemical Treatment Processes Professor Xijun Hu where [S] is the concentration of starch. By substituting Equation () into Equation (), one obtains dx = K S () dt The total organic in the wastewater comprises S, G and N: X = [G] + [S] + [N] (4) where [N] represents the concentration of nonoxidizable product. Elimination of [G] by substituting Equation () into Equation (4), we can get [S] = + K Thus, Equation () becomes Meanwhile X [N] (5) dx K = X -[N] (6) dt + K d[n] dt S = (7) CENG 576 Advanced Physico-Chemical Treatment Processes Professor Xijun Hu Combining Equation (5) with Equation (7) we obtain d[n] = X -[N] (8) dt + K Dividing Equation (8) by Equation (6) yields d[n] = (9) dx K with the initial condition t= [N]= () The solution for Equations (9) and () is [N] = (X K - X) () Now we substitute Equation () into Equation (6) to give dx K = X - (X - X) dt + K K with the initial condition () t= X = X ()

14 CENG 576 Advanced Physico-Chemical Treatment Processes Professor Xijun Hu where X is the total organic concentration in the wastewater at time zero. The solution for Equations () and () is K + X - t K + K = + e X K + K + (4) If we assume the value in the solution is proportional to the total organic concentration in the wastewater, X, i.e. X (5) X then the removal of,, would become =- i K i + + i K K + e K + - t + K (6) CENG 576 Advanced Physico-Chemical Treatment Processes Professor Xijun Hu where i is the initial value of the fresh wastewater, which is different from the value at the reaction time t=,, by a factor due to the thermal decomposition. Equation (6) is applied to simulate the WAO treatment of natural fiber desizing wastewater at different temperatures. Removal (%) MPa MPa.5 MPa MPa MPa Reaction time (min) 9 o C 7 o C 4 o C o C P O 5 o C 5 5 The model (lines) is in good agreement with the experimental data. The inetic parameters are 4 5

15 CENG 576 Advanced Physico-Chemical Treatment Processes Professor Xijun Hu optimized from the experimental data by a least square method, and listed in the following table. Kinetic parameters of WAO of cotton desizing wastewater at different temperatures T ( o C) K (min - ) x (min - ) From the small value of hydrolization equilibrium constant, K, at 5 o C, it can be seen that starch does not hydrolyze easily at low temperature. Once the temperature is above o C, however, the effect of reaction temperature on the rate of hydrolysis becomes less significant. The equilibrium constant is within the range of.67 to.8 for temperatures of CENG 576 Advanced Physico-Chemical Treatment Processes Professor Xijun Hu to 7 o C. The temperature dependence of and are assumed to follow the arrhenius form: - E a = exp( ) (7) RT where is the pre-exponential factor, E a is the activation energy, and R is the gas constant. ln() - ln( ) ln( ) /T (K - ) and are with the following equations 6 7

16 CENG 576 Advanced Physico-Chemical Treatment Processes Professor Xijun Hu 46 ln( ) =.87 - (8) T 778 ln( ) =.5 - (9) T The activation energy for the oxidation of glucose, 4. J/mol, is much larger than 5 J/mol, a value where mass transfer resistance can be ignored. Therefore, the reactions here are indeed inetics controlled. On the other hand, the value of activation energy obtained here is smaller than the value reported in the literature for the oxidation of glucose. This means that the fast-formed intermediates of this ind of wastewater are easier to oxidize than the pure glucose. The activation energy for the conversion of the original organic to the nonoxidizable product is 64. J/mol, much larger than that for the oxidation of glucose. This implies that oxidation is the major reaction. CENG 576 Advanced Physico-Chemical Treatment Processes Professor Xijun Hu Other possible WAO reaction mechanisms During WAO, the long molecules are oxidized to various intermediates products. Most of the initial intermediates formed (except the low molecular weight carboxylic acids) are unstable and further oxidized to end products (CO, etc.) or to low molecular carboxylic acids (mainly acetic acid). The low molecular carboxylic acids are resistant to further oxidation. Thus, the organics in the effluent from a WAO system can be divided into three groups: A: all initial & relatively unstable intermediates B: refractory intermediates lie acetic acid C: oxidation end products A + O C (CO + H O) B + O Assume oxygen is in excess, we may have dc A C A C A dt dcb CA CB dt E / RT n e C, O 8 9

17 CENG 576 Advanced Physico-Chemical Treatment Processes Professor Xijun Hu C C B A C C B,,, A, e e e E / RT E / RT e t C A, C C ( ) t [ e t n O n O e C B, can be assumed to be zero: CA + CB = - e C C B + - A, , e - + t ( ) t The COD or in wastewater should be C A + C B, so - t - - e + e + t t ] CENG 576 Advanced Physico-Chemical Treatment Processes Professor Xijun Hu Improved WAO The efficiency of WAO can be improved by various means, such as adding a catalyst or using a stronger oxidant. Catalytic wet air oxidation (CWAO) The catalyst used may be metal salt solution, metal oxide powders, or porous solid supported metals. By using metal ion solutions and metal oxide powders as catalysts in the treatment of wastewater, The benefits: Higher COD and removals Lower reaction temperature and total pressure The disadvantages: Cause secondary pollutants The solution: Immobilize metals onto granular porous solids Used catalysts can be recovered by filtration

18 CENG 576 Advanced Physico-Chemical Treatment Processes Professor Xijun Hu CENG 576 Advanced Physico-Chemical Treatment Processes Professor Xijun Hu COD Removal (%) Cu(NO ) CuSO 4 Mn(NO ) FeSO 4 No Catalyst COD Removal (%) PtO CuO Fe O MnO TiO PtO No catalyst Removal (%) Reaction time (min) Cu(NO ) CuSO 4 Mn(NO ) FeSO 4 No Catalyst Removal (%) 4 PtO Reaction time (min) MnO CuO TiO Fe O PtO No catalyst Effect of catalysts on the CWAO of dyeing and printing wastewater at o C, p O =.65 MPa. Use of catalysts greatly improves the oxidation The effectiveness of catalysts is Cu(NO ) > CuSO 4 > Mn(NO) > FeSO 4 Effect of metal oxide catalysts on the CWAO of dyeing and printing wastewater at o C, p O =.65 MPa. Use of catalysts greatly improves the oxidation The efficiency of catalysts is CuO > Fe O > TiO > MnO > PtO

19 CENG 576 Advanced Physico-Chemical Treatment Processes Professor Xijun Hu CENG 576 Advanced Physico-Chemical Treatment Processes Professor Xijun Hu Addition of H O as promoter COD Removal (%) Removal (%) Color Removal (%) Reaction time (min) Cu-Al O Cu(NO ) CuO No catalyst Cu-Al O Cu(NO ) CuO No catalyst Cu-Al O Cu(NO ) CuO No catalyst Effect of various copper catalysts on the CWAO of dyeing and printing wastewater at o C, p O =.65 MPa. Oxidation Removal (%) Color Oxidation Removal (%) WAO CWAO PCWAO Reaction Time (min) WAO of dyeing wastewater at o C, p O =.65 MPa. CWAO & PCWAO: Cu/AC (copper supported on activated carbon) catalyst was used. PCWAO: % H O of the theoretical oxidation requirement was added in additional to oxygen. 4 5

20 CENG 576 Advanced Physico-Chemical Treatment Processes Professor Xijun Hu Wet Peroxide Oxidation (WPO) Completely replace oxygen by H O. Oxidation Removal (%) Colour Oxidation Removal (%) Reaction Time (min) Reaction is very fast High & color removal at o C H O is expensive 7 o C o C o C 5 o C CENG 576 Advanced Physico-Chemical Treatment Processes Professor Xijun Hu Oxidation Removal (%) Colour Oxidation Removal (%) Reaction Time (min) 5%Q th %Q th %Q th Effect of hydrogen peroxide dosage on WPO of dyeing wastewater concentrate. Increasing H O dosage accelerates the reduction when it is below its theoretical amount. However, when the H O dosage is above the theoretical requirements it little affects the final 6 7

21 CENG 576 Advanced Physico-Chemical Treatment Processes Professor Xijun Hu and color reductions, although the initial reaction rate increases as the H O dosage increases. This indicates the maximum final removal efficiency can not be improved by increasing the H O dosage. The reason for this might be that the excess H O reacts with the hydroxyl radical to form water and HO radical which will further react with H O to form water and hydroxyl radical. Therefore, H O is self-consumed. H HO O OH H H O H O HO O O OH CENG 576 Advanced Physico-Chemical Treatment Processes Professor Xijun Hu Catalytic Wet Peroxide Oxidation (CWPO) Oxidation Removal (%) Color Oxidation Removal (%) No Catalyst Fe ++ mg/l Cu ++ mg/l AC-Cu g/l Reaction Time (min) Effect of catalyst on WPO of dyeing wastewater at o C. 8 9