ADVANCED OXIDATION OF WATER USING CATALYTIC OZONATION

Abstract: Contaminants are removed from waste water (1) by an advanced oxidation process in which the waste water is contacted (7) with ozone (8) in the absence of a catalyst to oxidize-ozone-oxidizable contaminants and to dissolve ozone in the water, and the resultant ozone-containing water (3) is contacted with a solid ozone activating catalyst (5) to oxidize ozone refractory contaminants in the water. Effluent (6) from the catalyst treatment (5) can be contacted (7) with ozone and recycled (2) for further contact with the catalyst (5). The preferred catalyst is an undoped monolithic structure of gamma alumina having low surface area, high porosity and low pressure drop.

Description: ADVANCED OXIDATION OF WATER USING CATALYTIC OZONATTON The present invention relates to the treatment of surface water or aqueous effluent to remove organic impurities therefrom. Conventional microbiological waste water treatments are effective in removal of biodegradable organic contaminants from water. However, recalcitrant organic contaminants which are not biodegradable and/or are inhibitory to microbiological activity require chemical oxidation for removal . The extent of contamination is measured in terms of the amount of oxygen required to remove the contaminants. COD (Chemical Oxygen Demand) is the amount of oxygen required to oxidize all oxidizable contaminants in the water to be treated; BOD (Biological Oxygen Demand) is the amount of oxygen required to oxidize the biodegradable contaminants,- and hard COD is the amount of oxygen required to oxidize the non-biodegradable contaminants . Chemical alternatives to microbiological waste water treatment are known. The most usual chemical treatment is the Zimmerman (or ZIMPRO) wet air oxidation process, in which waste water is contacted with air at elevated temperature and pressure, usually 200°C to 370°C at 20 to 200 Atmospheres (2 to 20 MPa) . This method is only economical for waste water with an organics contents higher than 1% or an oxidation heat value sufficient to maintain the elevated temperature required. Various catalysts have been used in the Zimmerman process . These catalysts include noble or heavy metals such as palladium, platinum, cobalt or iron deposited on a carrier of, for example, alumina, silica-alumina, silica gel, activated carbon, titania or zirconia (see JP-A-49- 44556 (1974) ; JP-A-49-94157 (1974) ; and JP-A-58-64188 (1983) ) . The use of ozone to treat water after microbiological bulk BOD removal is well known. Ozone is the strongest molecular oxidant for water treatment and it has been used since the beginning of this century in drinking water treatment to provide disinfection, removal of colour, taste and odour, and destruction of organic compounds. However, ozone is very selective in its reactions; it predominantly reacts with compounds containing unsaturated bonds such as olefins, aromatic compounds and/or compounds containing electron rich groups such as sulfur and nitrogen. For other organic compounds such as saturated alkanes and chlorinated organics (most of pesticides and priority pollutants) , the reactivity of ozone is limited. These ozone refractory contaminants can be oxidized by hydroxyl radicals which are usually formed by activating ozone with hydrogen peroxide or ultraviolet light ("advanced oxidation") . Hydrogen peroxide has an operation cost higher than ozone and ultraviolet light requires capital and operational costs at least equal to that of ozone generation. Accordingly, the activation of ozone to hydroxyl radicals by these two methods results in a large increase (50 - 200%) in the cost of water treatment. Moreover, since hydroxyl radical reactions are much less specific than those involving ozone alone, the hydroxyl radicals generated in solution by hydrogen peroxide or ultraviolet light activation of ozone can be wasted by reaction with non-target inhibitors or scavengers, such as carbonate and bicarbonate ions, which do not require oxidation. Thus, a more cost effective method of ozone activation is needed. PL-A-56775 (1969) reported that ozone-containing gas had been used in a continuous oxidation process to purify waste water from coke ovens but that the treatment was uneconomic for industrial use. It was proposed in PL-A- 56775 that the waste water should be continuously treated with ozone-containing gas in a froth phase by blowing the gas countercurrent to the waste water in a scrubber packed with Rashig rings, slag and/or oxides of silver, copper, aluminium, zinc, magnesium, tin, lead, iron, or manganese as catalysts. US-A-4007118 (1977) discloses the ozonation of waste water using a transition metal catalyst, such as manganese trioxide, ferric oxide, copper oxide or nickel oxide, which is present as a powder contained in fabric bags, disposed on a substrate or dispersed within the waste water US-A-4040982 (1977) discloses a method of removing contaminants from waste water by treatment with ozone- containing gas in the presence of a catalyst comprising ferric oxide supported on catalytically active alumina and having a surface area of 150 to 450 m 2 /g and a pore volume of at least 0.3 cm 3 . The exemplified alumina is gamma alumina but reference is made to eta alumina, amorphous alumina and activated alumina. JP-A-58-37039 (1983) discloses a method of removal of an aromatic organic compound from waste water by mixing first with a surfactant, then with a transition metal or alkaline earth metal compound, and contacting the resultant mixture with ozone-containing gas to oxidatively decompose the organic compound. NL-A-9001721 (1991) discloses the treatment of iron- containing waste water by forming and removing precipitated Fe(III) by mixing with hydrogen peroxide,- then forming and removing precipitated carbonate by adding, for example, calcium hydroxide, calcium chloride and/or alkali metal hydroxide; and subsequently oxidation with, for example, ozone-containing gas alone or with a solid catalyst, to remove residual organic compounds. Specified catalysts are activated carbon, alumina or silica. It is stated that the catalyst must have a surface area of at least 50 m 2 /g and a pore volume cf greater than C.l cm 3 /g and that its activity can be improved by addition of a transition metal such as copper, iron, molybdenum or cobalt. In the exemplified process, ozone-containing gas is passed through a bubble reactor countercurrently to liquid effluent from a catalyst bed and the gas exiting the bubble reactor is passed through that catalyst bed cocurrently with the partially treated waste water. If required, additional ozone- containing gas can be added to supplement the ozone content of the gas exiting the bubble reactor. US-A-5, 145,587 (1992) discloses the treatment of waste water by wet oxidation with a molecular oxygen-containing gas in the presence of a solid catalyst comprising (i) titanium dioxide; (ii) an oxide of a lanthanide element; and (iii) at least one metal selected from manganese, iron, cobalt, nickel, tungsten, copper, silver, gold, platinum, palladium, rhodium, ruthenium and iridium or a water insoluble or sparingly soluble compound thereof. The catalyst is formed by adding component (iii) to a calcined mixture of (i) and (ii) and preferably is in the form of an integral or monolithical structure, such as an extruded honeycomb (having straight through channels) , of (i) and (ii) impregnated with (iii) . The preferred oxidants are oxygen, ozone, hydrogen peroxide or mixtures of oxygen with ozone or hydrogen peroxide. US-A-5352369 (1994) discloses a method of treating water to kill bacteria therein by contact with a silver catalyst in the presence of oxygen to form an active oxidizer in the water. The silver catalyst is formed by depositing elemental silver on an alumina matrix and heating to a temperature of at least 300°C. In the preferred embodiment ozone-containing gas is used as the source of oxygen but it is required that the water be exposed to the silver catalyst as soon as the ozone- containing gas has been added to the water. To the best of our knowledge and belief, the prior art processes of treating water with ozone-containing gas in the presence of a catalyst has involved the passage of ozone-containing gas through a catalyst-containing reactor containing the water or through which the water also is passed in cocurrent or countercurrent flow to the gas. It has now been found that the catalytic ozonation of water is improved if the ozone is dissolved in the water prior to contact with the catalyst. This simple modification reduces costs by removal of those contaminants oxidizable by ozone prior to the formation of hydroxyl radicals to remove ozone refractory contaminants. Thus, according to the present invention, there is provided a process for the removal of contaminants from water which comprises contacting the water with ozone in the absence of a catalyst to oxidize ozone-oxidizable contaminants and to dissolve ozone in the water, and contacting the resultant ozone-containing water with a solid ozone activation catalyst to oxidize ozone refractory contaminants in the water. The process of the invention permits the use of some relatively inexpensive solid catalysts which are widely used in the chemical industry for chemical synthesis. In this invention, the waste water first reacts with ozone in a gas-liquid contactor and the easily oxidisable contaminants are removed. The thus treated waste water (free from gas bubbles but with residual dissolved ozone) passes through the ozone activation catalyst where the residual ozone is activated to secondary oxidants more reactive than ozone and which decompose contaminants remaining after treatment in the gas-liquid contactor. The effluent from the catalyst treatment can be re-injected to the gas-liquid contactor to absorb more ozone for reaction if the concentration of contaminants is too high to reduce in a single pass through the catalyst (i.e. the oxidant demand of the waste water is higher than the maximum ozone solubility in water under the operational conditions) . Having regard to the capacity and efficiency of conventional waste water treatments, the waste water treated by the process of the invention usually will have a COD of at most 5000 mg/l. The extent of the advanced oxidation by the process will be determined mainly by environmental requirements, which presently require the COD of waste water to be reduce to at most 125 mg/l before discharge. In addition to improving the efficiency of ozone usage in advanced oxidation, the use of a two phase catalysis (liquid/solid) instead of the conventional three phases (gas/liquid/solid) used in prior art advanced oxidation catalysis improves the reaction rate and reduces catalyst erosion. Usually, the ozone-containing gas will be an ozone/oxygen or ozone/air mixture but pure ozone or ozone in admixture with any inert carrier gas can be used. The catalyst can be in any solid form but usually will be in the form of granules, pellets or an integral or monolithic structure especially having a three dimensional continous pore phase. Preferably, the catalyst will be monolithic having a low surface area (≤ 20 m 2 /g) and/or high porosity (≥ 5 pores per linear inch,- ≥ 2 pores per cm) . It is especially preferred that the catalyst is in the form of foamed monolithic structure with high porosity and low pressure drop (≤ 0.1 bar g; ≤ 10 kPa in a cylindrical reactor of 1000 mm high and 24 mm ID at a water flow rate of 7 litre/min) . The catalyst can be any of those conventionally employed in advanced oxidation catalysis of water. For example, it can be a transition metal oxide, such as cobalt oxide, copper oxide, ferric oxide, manganese trioxide or nickel oxide, optionally on a carrier of, for example, alumina, silica-alumina, silica gel, activated carbon, titania or zirconia. However, the catalyst preferably is a gamma-alumina catalyst, especially undoped gamma alumina optionally on a carrier, especially alpha alumina. The following is a description by way of example only and with reference to the accompanying drawings of presently preferred embodiments of the invention. In the drawings : Figure 1 is a graph of percentage COD removal (ordinate) against ozone consumed (abscissa) for a process of the invention using a gamma alumina catalyst (Catalyst C3 ,- see Example 1 infra) and for a conventional (0 3 /UV) advanced oxidation process,- Figure 2 is a graph of percentage COD removal (ordinate) against ozone consumed (abscissa) for a process of the invention using a monolithic catalyst of gamma alumina on an alpha alumina carrier (see Example 2 infra) and for a conventional (0 3 /UV) advanced oxidation process,- Figure 3 is a diagrammatic representation of apparatus for the removal of organic contaminants from high strength waste water using a process of the present invention,- and Figure 4 is a diagrammatic representation of apparatus for the removal of organic contaminants from low strength waste water using a process of the present invention. Referring to Figure 3, a stream (1) of waste water which has been treated by conventional microbiological or chemical processes to reduce the COD to 5000 mg/l or less is mixed with a recycle stream (2) of water containing dissolved ozone. The resultant mixed stream (3) is pumped (4) upwardly through a fixed catalyst bed (5) and the bed effluent (6) is passed to a gas-liquid contactor (7) in which it is thoroughly mixed with an ozone-containing gas (8) from an ozone generator (not shown) . The bulk of the ozonated water is removed in recycle stream (2) but a smaller portion is removed (9) for discharge or further treatment. Undissolved gas is removed in a gaseous discharge stream (10) for reuse and/or return to the ozone generator. The process of Figure 4 differs from that of Figure 3 in that the waste water stream (1) is pumped (14) to the gas-liquid contactor (17) where it is mixed with ozone- containing gas (8) . All of the ozonated waste water from the gas-liquid contactor (17) is passed upwardly through the catalyst bed (5) . In this process, the spent ozone containing gas is removed (10) from the gas-liquid contactor (17) but the treated waste water discharge (19) is from the catalvst bed (5) . Example 1 Six water stable industrial catalysts in granular or pellet form were evaluated in a process of the present invention. The identity the catalysts is shown in Table 1 and their chemical compositions and geometric characteristics are shown in Table 2. Table 1 Identity of industrial catalysts evaluated Catalyst Ref Manufacturer Trade Designation Cl Air Products 159Cp Si0, Aln0 7 C2 Air Products CO-ZSM5 C3 Harshaw Al-4126 C4 Harshaw Cθ-0502 C5 Engelhard MgO C6 Harshaw Aα-0105 Table 2 Characteristics of industrial catalysts evaluated amorphous (i.e. mixture of Al-O-Si bonds rather than Al 2 0 3 /Si0 2 mixture) gamma 3 alpha When treating a synthetic waste water (prepared by dissolving lOOOmg/l glucose in tap water; COD 1000 mg/l) containing ozone by the process of the invention (0 3 dissolved prior to contact of water with catalyst) , these catalysts demonstrated different effectiveness in destruction of ozone refractory contaminants as shown in the following Table 3. Table 3 COD destruction bv catalytic ozonation process As can be seen from Table 3, Catalyst C3, an undoped gamma alumina, had the highest activity for COD destruction by the catalytic ozonation treatment. Surprisingly, the catalytic activity was in the order C3 > C5 > C4 > C2. Catalysts Cl and C6 did not present any significant catalytic effect in the catalytic ozonation treatment. The combination of ozone with Catalyst C3 offers advantages over the current 0 3 /UV advanced oxidation process in the destruction of COD as indicated by the comparative results in Figure 1. The removal of COD was much higher with the Catalyst C3 catalytic ozonation than the 0 3 /UV process at the same ozone dosages. The ozone required for the same degree of COD removal by the Catalyst C3 catalytic ozonation is less than 50% of that by the 0 3 /UV process, representing a significant reduction in treatment cost. Catalyst C3 maintained its catalytic activity after 100 hours operation. Municipal secondary effluent, landfill leachate, and waste water from a hospital sewer were all satisfactorily treated using this catalyst. Example 2 The procedure of Example 1 was repeated using, as catalyst, a foamed monolithic material of 92% alpha alumina (low surface area) and coated with 5% gamma alumina (RETICEL ™ HPA washcoat reticulated ceramic) sold as a filter media. The base material has a pore density of 10 pores per linear inch (4 pores per cm) and a calculated surface area of 2290 m 2 /m 3 (< 5xl0 "3 πr/g) . The 5% washcoat increases the surface area to 15 m 2 /g. This material achieved similar COD removal rate as the 0 3 /UV process with little back-pressure (see Figure 2 ) . For example, when a laboratory reactor (1000 mm high and 24 mm ID) is packed with the monolithic material, a pressure drop of 0.02 bar g (2 kPa) was recorded at a water flow rate of 7 litre/min compared with 0.8 bar g (80 kPa) for Catalyst 3 (in granular form) under the same conditions. It will be appreciated that the invention is not restricted to the exemplification above but that numerous modifications and variations can be made without departing from the scope of the following claims.

Claims: CLAIMS : 1. A process for the removal of contaminants from water which comprises contacting the water with ozone in the absence of a catalyst to oxidize ozone-oxidizable contaminants and to dissolve ozone in the water, and contacting the resultant ozone-containing water with a solid ozone activating catalyst to oxidize ozone refractory contaminants in the water. 2. A process as claimed in Claim 1, wherein the water to be treated has a COD of at most 5000 mg/l. 3. A process as claimed in any one of the preceding claims, wherein the ozonation treatments reduce the COD of the water to at most 125 mg/l. 4. A process as claimed in any one of the preceding claims, wherein effluent from the catalyst treatment is contacted with ozone and recycled for further contact with the catalyst. 5. A process as claimed in Claim 4, wherein the recycled effluent is mixed with fresh waste water prior to recycle through the catalyst. 6. A process as claimed in any one of the preceding claims, wherein the catalyst is a foamed monolithic structure with a pressure drop of at most 10 kPa (0.1 bar g) in a cylindrical reactor of 1000 mm high and 24 ram ID at a water flow rate of 7 litre/min. 7. A process as claimed in Claim 6, wherein the catalyst has a surface area of at most 20 m 2 /g. 8. A process as claimed in Claim 6 or Claim 7, wherein the catalyst has a porosity of at least 2 pores per linear cm (5 pores per inch) . 9. A process as claimed in any one of the preceding claims, wherein the catalyst is a gamma alumina catalyst . 10. A process as claimed in Claim 9, wherein the catalyst is undoped gamma alumina optionally on a carrier. 11. A process as claimed m Claim 10, wherein the carrier is alpha alumina. 12. A process as claimed in Claim 11, wherein the catalyst is a foamed monolithic alpha alumina substrate coated with undoped gamma alumina 13. A process as claimed in any one of the preceding claims, wherein contact with the catalyst is conducted in the absence of added hydrogen peroxide.

Inventors: ARMOR JOHN NELSON, BARRATT PETER, MAGNOTTA VINCENT LOUIS, XIONG FENG, ARMOR, JOHN, NELSON, BARRATT, PETER, MAGNOTTA, VINCENT, LOUIS, XIONG, FENG
Applicants: AIR PROD & CHEM, ARMOR JOHN NELSON, BARRATT PETER, MAGNOTTA VINCENT LOUIS, XIONG FENG, AIR PRODUCTS AND CHEMICALS, INC., ARMOR, JOHN, NELSON, BARRATT, PETER, MAGNOTTA, VINCENT, LOUIS, XIONG, FENG
Application year: 1996
Main classification: C02F1
IPC codes: C02F1