SNEDDON ANDREW DOUGLAS (GB)
| WO2004033376A1 | 2004-04-22 |
| NL1004856C2 | 1998-06-23 | |||
| US5126111A | 1992-06-30 |
| CLAIMS
1. A single pass continuous process for the treatment of wastewater, the process comprising the steps of: introducing a wastewater stream at a first flow rate; introducing ozone gas into at least a portion of the wastewater stream; illuminating the wastewater stream with UV light; passing the wastewater stream over a photocatalyst; and discharging the wastewater stream at a second flow rate into the environment.
2. A process as claimed in Claim 1 , wherein the first and second flow rates are substantially equal.
3. A process as claimed in Claim 1 or 2, wherein the photocatalyst is fixed upon a surface in a reaction vessel containing the UV light.
4. A process as claimed in any preceding claim, wherein the ozone gas is produced in situ from an ozone generator.
5. A process as claimed in any preceding claim, wherein the ozone is injected into the wastewater stream by an injection mechanism.
6. A process as claimed in Claim 5, wherein a venturi member is used to create a vacuum from the wastewater flow to draw the ozone gas into the wastewater stream.
7. A process as claimed in Claim 5 or 6, wherein the injector is arranged in series with the wastewater process piping.
8. A process as claimed in Claim 5 or 6, wherein the injector is arranged on a by-pass to the main wastewater process pipe.
9. A process as claimed in any preceding claim, wherein the reaction vessel contains at least one UV lamp and the photocatalyst.
10. A process as claimed in any preceding claim, wherein the photocatalyst is coated on a plurality of rods located within the reaction vessel.
11. A process as claimed in Claim 11 , wherein the rods are made from titanium dioxide and/or covered in the powder anatase form of titanium dioxide.
12. A process as claimed in any preceding claim, wherein the reaction vessel is arranged to provide a convoluted path through the vessel such that the wastewater steam has a higher residence time within the vessel.
13. A process as claimed in Claim 10 or 11 , wherein the rods are attached to a removable rack that slides into the reaction vessel.
14. A wastewater treatment apparatus comprising: a first inlet for the introduction of the wastewater into a pipe; an injector mechanism for introducing ozone gas to the wastewater in the pipe; and a reaction vessel, wherein the reaction vessel includes one or more UV light sources and a photocatalyst fixed upon a surface in the vessel; and an outlet for discharging the processed wastewater stream into the environment.
15. A wastewater treatment apparatus as claimed in Claim 14, including an ozone generator.
16. A wastewater treatment apparatus as claimed in Claim 14 or 15, including a venturi member adapted to create a vacuum from the wastewater flow to draw the ozone gas into the wastewater stream.
17. A wastewater treatment apparatus as claimed in any of Claims 14 to 16, wherein the reaction vessel is arranged to provide a convoluted path through the vessel.
18. A single pass continuous process for the treatment of wastewater, the process comprising the steps of: introducing a wastewater stream at a first flow rate; introducing ozone gas into at least a portion of the wastewater stream; inputting the ozone enriched wastewater to a first reaction tank at substantially the first flow rate; maintaining pressure in the first reaction tank to maintain the dissolution and reaction of ozone in the wastewater stream; outputting the wastewater stream at substantially the first flow rate directly into a second reaction tank; exposing the wastewater stream to UV light radiation in the second tank while dissolving or dispersing substantially all of the ozone in the wastewater stream; outputting the wastewater stream at substantially the first flow rate from the second tank; and discharging the wastewater stream into the environment.
19. A process as claimed in Claim 18, wherein the first tank is used for ozone contacting.
20. A process as claimed in Claim 18 or 19, wherein the second tank is used for UV illumination and degassing of the wastewater stream before being discharged to the environment.
21. A process as claimed in any of Claims 18 to 20, wherein the ozone gas is produced in situ from an ozone generator.
22. A process as claimed in Claim 21 , wherein the ozone generator provides a positive pressure of ozone which at least assts with rapid mixing of the ozone with the formation of micro-bubbles.
23. A process as claimed in any of Claims 18 to 22, including an injection device to produce a vacuum, and wherein the outlet pressure from the injection device going into the first reactor tank is held above atmospheric pressure.
24. A process as claimed in any of Claims 18 to 22, wherein the flow path through the first tank is convoluted to increase the residence time for the ozone to react.
25. A process as claimed in any of Claims 18 to 24, wherein the second tank is substantially held at atmospheric pressure to remove any residual ozone gas that may be in the system.
26. A process as claimed in any of Claims 18 to 25, wherein the treatment of a wastewater stream including produced water offshore is a continuous or single pass process in which the wastewater only passes once through the twin tank reactor system.
27. A wastewater treatment apparatus comprising: a first inlet for the introduction of the wastewater into a pipe; an injector mechanism for introducing ozone gas into the wastewater in the pipe; a first tank including a flow path greater than the widest dimension of the tank; a second tank including at least one UV light source; and an outlet for discharging the processed wastewater stream into the environment.
28. A wastewater treatment apparatus as claimed in Claim 27, wherein the first tank contains a series of baffles in which the water flows down to the bottom of the tank to increase the residence time for the ozone to react. |
WASTEWATER TREATMENT APPARATUS AND METHOD
This invention relates to the treatment of industrial wastewaters and in particular, though not exclusively, to the treatment of produced water from offshore hydrocarbon production and drilling operations. The treatment process may remove organic components from the wastewater by means of a combined advanced oxidation process (AOP) with or without a photocatalyst.
Industrial wastewaters are generated as a by-product from industrial operations. Examples of industries that produce wastewater effluents include: chemical manufacture/processing, food & beverage and the oil & gas industry amongst others. Produced water is the wastewater byproduct from hydrocarbon extraction. This water is mixed with the oil & gas as it is retrieved from the production reservoir. The quantity of produced water is increasing due to the aging of the oil & gas platforms throughout the world; also seawater is being increasingly injected into the reservoirs to maintain pressure to drive the hydrocarbons from the well.
Before this water can be discharged, it is separated from the oil & gas fractions by a series of separators. Once the water has been separated, it can be discharged to the surrounding environment or re-injected into an accommodating well if available.
As with produced water and all industrial processes that produce a wastewater stream, the effluents produced contain low level but measurable amounts of organic contaminants that have the potential to harm the environment. In the majority of countries worldwide, regulations are applied to control the level of organic material in the wastewater discharges to reduce the potential harmful effect on the environment.
However with increasing levels of wastewater being produced and stronger regulatory limits being imposed, it is becoming more difficult to reach the discharge limits with current technology.
A method which has been used extensively to treat industrial wastewaters is a combination of ozone and UV light providing an advanced oxidation process (AOP). Typically, the ozone is injected into the wastewater stream before exposure to the UV light. The injection point is normally immediately upstream of the reaction vessel containing the UV light source. The process acts as a powerful oxidising mechanism to reduce the level of organic material in the wastewater stream. In order to remove acceptable levels of organic material, the stream is recycled past one or both of the ozonation and UV light sources. The disadvantage of this required recycling is that, to process industrial quantities of water, the water needs to be stored as the UV reaction vessel can only process small quantities at a time. In treating produced water on an offshore installation, space for such storage is not available. Additionally, this recycling produces a time inefficient process of treatment.
A continuous single pass AOP has been proposed for the treatment of produced water from oil and gas installations in WO 03/091167. This document describes a method for treating industrial wastewaters in a single pass configuration as required in the treatment of produced water on an offshore installation. The wastewater stream is initially separated from large volumes of solids, oils and gases before entering the treatment process. After separation, the wastewater passes through the treatment process once before being discharged overboard. The single pass system is used due to space and weight constraints that exist for equipment being utilised on offshore platforms.
Initially ozone is injected into at least a portion of the produced water stream, with the stream at substantially three times atmospheric pressure. The ozone enriched stream is exposed to UV radiation in a UV reaction vessel before being discharged overboard. Due to the space constraints, the ozone injection point is on the inlet pipe close to the UV vessel.
An inherent issue with this method of treatment is the short residence time that exists to efficiently treat the wastewater stream. As a result, the quantity of ozone that can be injected may be limited and then, once mixed in with the wastewater stream, there may be insufficient time and therefore insufficient oxidation occurring to reduce the contaminant concentration to the required level. The highest level of reduction is seen in the UV reaction vessel. Ideally, the wastewater would spend an extended time passing through the UV reaction vessel in order for all of the ozone to react with the contaminants within the wastewater. Using the single pass method, there may not be enough residence time in the UV reaction vessel for all of the ozone to be consumed. Therefore the residual ozone continues to react down the outlet piping before reaching the discharge point. Thus insufficient oxidation of the organic material may occur and this process is thus limited in the level of organic material which can be removed.
A further known technique for the treatment of wastewater by AOP is the combination of a photocatalyst and UV radiation. The addition of a photocatalyst, activated by UV radiation, enhances the oxidation.
However, a disadvantage of these systems is that the photocatalyst is used in a powder form, in a slurry type reactor. The powder is mixed with the wastewater to be treated and illuminated with UV light until the necessary level of degradation has been achieved. As a result these reactors require an extended period of reaction time, UV illumination and
continual mixing to achieve the required results. This type of system can only operate as a batch process method due to the requirement for removing and reusing the photocatalyst in the reactor after completion of the treatment process. The powder form of the photocatalyst means that the treated wastewater requires filtering before the water can be discharged to the environment. Thus a single pass continuous process is not feasible. There are few full-scale photocatalyst and UV treatment plants. The majority of photocatalyst and UV treatment systems are laboratory or pilot scale models. The opportunity for transition into full- scale industrial processes has been limited due to difficulties with removing and filtering the photocatalyst from the wastewater after treatment and due to the large residence times required in both slurry and fixed bed reactor systems.
It is an object of the present invention to provide a process for the treatment of wastewater which obviates or mitigates at least some of the disadvantages of the prior art.
According to a first aspect of the present invention there is provided a single pass continuous process for the treatment of wastewater, the process comprising the steps of: introducing a wastewater stream at a first flow rate; introducing ozone gas into at least a portion of the wastewater stream; illuminating the wastewater stream with UV light; passing the wastewater stream over a photocatalyst; and discharging the wastewater stream at a second flow rate into the environment.
The combination of the three technologies above constitutes an advanced oxidation process (AOP). The combination of the oxidising powers of the
photocatalyst and ozone while being activated by the UV light results in a faster oxidation of the organic material in the wastewater stream. This provides in a single pass a significant reduction in the level of organic material in the wastewater stream.
Preferably the first and second flow rates are substantially equal so that the process is continuous.
Preferably the photocatalyst is fixed upon a surface in a reaction vessel containing the UV light.
Preferably, the ozone gas is produced in situ from an ozone generator. Preferably the ozone is injected into the wastewater stream by an injection mechanism. Preferably, a venturi type mechanism is used to create a vacuum from the wastewater flow to draw the ozone gas into the wastewater stream. The injector may either be in series with the wastewater process piping or set on a by-pass to the main wastewater process pipe. The ozone generator provides a positive pressure of ozone, which in conjunction with the vacuum produced by the venturi injection device produces rapid mixing of the ozone with the formation of micro-bubbles. The micro-bubbles aid the dissolution/dispersion of the ozone into the wastewater stream. The wastewater stream is now enriched with ozone.
Preferably the ozone enriched wastewater stream flows into the reaction vessel containing at least one UV lamp and the photocatalyst. More preferably, the reaction vessel contains one or more medium pressure ultraviolet lamps that perform two process functions. Advantageously, the UV lamp/s illuminate the ozone enriched wastewater stream to activate the generation of hydroxyl radicals as well as activating the photocatalyst
within the reaction vessel to initiate redox reactions and molecular transformations.
Preferably the photocatalyst is a semiconductor material of a metal oxide or sulphide that is excited by the application of UV light. Suitable photocatalytic materials include: TiO2 (titanium dioxide), ZnO (zinc oxide), CdS (cadmium sulphide), Fe 2 O 3 (iron oxide) and ZnS (zinc sulphide). It will be appreciated that this list is not exhaustive. Preferably, the photocatalyst is used under the principle of a fixed bed reactor.
Preferably, the photocatalyst is coated on a plurality of rods located within the reaction vessel. Preferably also the rods are substantially cylindrical but may have a polygonal cross-sectional area. Preferably the rods are made from titanium dioxide and covered in the powder anatase form of titanium dioxide, which is known to be the most photoactive. The use of rods allows all of the rod surface area to be illuminated by the UV light source.
The reaction vessel may be arranged to provide a convoluted path through the vessel, such that the wastewater steam has a higher residence time within the vessel. This ensures optimum degradation of organic material before discharge.
More preferably the rods are attached to a removable rack that slides into the reaction vessel. By mounting the rods on a metal rack that can be inserted and removed from the UV reaction vessel this provides for ease of use and maintenance.
Preferably, hydroxyl radicals are produced from the UV activation of the ozone and the photocatalyst. The hydroxyl radicals are thought to be
second only to fluorine in terms of oxidation power. This fact allows nonselective degradation of organic material in wastewater streams.
It is understood that, when ozone in a water, wastewater medium, is illuminated by UV, the UV splits the ozone molecule apart to form diatomic oxygen and an oxygen radical. The oxygen radical then proceeds to react with a water molecule to produce two hydroxyl radicals. This can be shown by the reaction sequence below: hυ O 3 ► O 2 + O '
O ■ + H 2 O ► 2OH ■
It is also understood that when the ozone enriched wastewater enters the reaction vessel, it can react directly with a water molecule in the presence of UV light to produce hydrogen peroxide (H2O2) and oxygen. When UV light illuminates H2O2 it splits to form 2 hydroxyl radicals. This can be shown by the reaction sequence below: hυ O 3 + H 2 O ► H 2 O 2 + O 2
hυ H 2 O 2 ^2OH-
The semiconductor photocatalyst can also produce hydroxyl radicals. The unique electronic structure of the semiconductor photocatalyst includes a filled valence band and an empty conductance band. The area between the two bands is termed the energy gap. The energy gap corresponds to the minimum amount of energy required for the semiconductor to become electrically conductive. For TiO2 the band gap energy is 3.2eV.
When the UV light source in the reaction vessel illuminates the photocatalyst above the required band gap energy, an electron is promoted from the valence band to the conductance band. This results in a hole (h + ) being produced at the valence band and a spare electron (e ~ ) at the conductance band. The photocatalyst can now perform a redox reaction by the hole acting as an oxidising agent and the electron acting as a reducing agent.
The hole (h + ) is considered to be a powerful oxidising agent. Typically, the hole (h + ) can react with water to produce a hydroxyl radical. This can be shown by the reaction sequence below:
H 2 O + h + ► OH + H +
The hole (h + ) and the hydroxyl radical are powerful oxidants that can oxidise many organic materials.
It is generally understood that oxygen accepts the spare electron to form the super-oxide ion 02" " by the reaction shown below:
O 2 + e " ► O 2 "
The super-oxide ion is also very reactive and able to oxidise the organic material in wastewater streams.
The hydroxyl radical and other reactive particles above are all very powerful oxidants. There are a number of reaction pathways that exist in which the oxidising species can break down the organic material. It is understood that the most common reaction pathway is hydrogen abstraction. This can be shown by:
OH- + RH-^ R + H 2 O
The reaction pathway leads to the splitting of the organic material with the formation of organic and peroxyl radicals that continue in a chain reaction. As the reaction continues the organic material breaks down into simpler molecular species and structures. In time the final reaction end products are CO 2 , H 2 O and inorganic salts.
In the application of treating produced water, the long chain hydrocarbon molecules and aromatic ring compounds are broken down into smaller chain hydrocarbons, alcohols and weak organic acids before eventually decomposing to CO 2 , H 2 O and inorganic salts. The inorganic salts are formed from the metal ions in the produced water reacting with the oxidising agents to form insoluble metal oxide species.
According to a second aspect of the present invention there is provided apparatus for the treatment of wastewater, the apparatus comprising: a first inlet for the introduction of the wastewater into a pipe; an injector mechanism for introducing ozone gas to the wastewater in the pipe; and a reaction vessel, wherein the reaction vessel includes one or more UV light sources and a photocatalyst fixed upon a surface in the vessel; and an outlet for discharging the processed wastewater stream into the environment.
Preferably the injection point is immediately upstream of the reaction vessel containing the UV light source.
Preferably, the apparatus includes an ozone generator. Preferably, a venturi type mechanism is used to create a vacuum from the wastewater
flow to draw the ozone gas into the wastewater stream. The injector may either be in series with the wastewater process piping or set on a by-pass to the main wastewater process pipe.
Preferably, the reaction vessel contains one or more ultraviolet lamps that perform two process functions.
Preferably the photocatalyst is a semiconductor material of a metal oxide or sulphide that is excited by the application of UV light. Preferably, the photocatalyst is coated on a plurality of rods located within the reactions vessel. Preferably also the rods are substantially cylindrical but may have a polygonal cross-sectional area. Preferably the rods are be made from titanium dioxide and covered in the powder anatase form of titanium dioxide, which is known to be the most photoactive.
The reaction vessel may be arranged to provide a convoluted path through the vessel.
The rods may be attached to a removable rack that slides into the reaction vessel.
According to a third aspect of the present invention there is provided a method of treating produced water from an oil or gas installation, the method comprising the steps of: locating a wastewater treatment system according to the second aspect on an offshore installation; passing wastewater from a well through a separation system; and treating the wastewater stream according to the first aspect of the invention.
Thus the process as described for can be configured to treat produced water on offshore platforms. Preferably the system is of suitable size to fulfil the strict space and weight requirements required by process equipment offshore. Preferably, the system handles produced water volumes in the range of 1000 to 10,000 barrels per day. Preferably the method reduces the hydrocarbon content of the produced water by 50% typically, from 40ppm to 20ppm.
In the application of treating produced water, the long chain hydrocarbon molecules and aromatic ring compounds are broken down into smaller chain hydrocarbons, alcohols and weak organic acids before eventually decomposing to CO2, H 2 O and inorganic salts. The inorganic salts are formed from the metal ions in the produced water reacting with the oxidising agents to form insoluble metal oxide species.
The treatment of produced water offshore may be a continuous or single pass process where the wastewater only passes once through a reaction vessel. The process as described for this invention may be configured to allow multiple passes of wastewater through a reaction vessel.
According to a fourth aspect of the present invention there is provided a single pass continuous process for the treatment of wastewater, the process comprising the steps of: introducing a wastewater stream at a first flow rate; introducing ozone gas into at least a portion of the wastewater stream; inputting the ozone enriched wastewater to a first reaction tank at substantially the first flow rate; maintaining pressure in the first reaction tank to maintain the dissolution and reaction of ozone in the wastewater stream;
outputting the wastewater stream at substantially the first flow rate directly into a second reaction tank; exposing the wastewater stream to UV light radiation in the second tank while dissolving or dispersing substantially all of the ozone in the wastewater stream; outputting the wastewater stream at substantially the first flow rate from the second tank; and discharging the wastewater stream into the environment.
By processing the wastewater stream through a first tank, a sufficient residency time is provided without compromising the flow rate to ensure substantially all of the ozone is dissolved or dispersed so that complete degradation of the organic components in the initial wastewater stream is achieved before the wastewater stream leaves the second tank following UV radiation.
Preferably the first tank is used for ozone contacting. Preferably the second tank is used for UV illumination and degassing of the wastewater stream before being discharged to the environment.
Preferably, the ozone gas is produced in situ from an ozone generator. Advantageously the ozone is injected into the wastewater stream by an injection mechanism. Preferably, a venturi type or Korting injector is used to create a vacuum from the wastewater flow to draw the ozone gas into the wastewater stream. The injector outlet may be attached in series with the first reactor tank. The ozone generator provides a positive pressure of ozone, which in conjunction with the vacuum produced by the venturi injection device produces rapid mixing of the ozone with the formation of micro-bubbles. The micro-bubbles aid the dissolution/dispersion of the ozone into the wastewater stream.
Preferably, for the injection device to produce a vacuum, a drop in pressure is provided across the injector. Preferably, the outlet pressure from the injection device going into the first reactor tank is held above atmospheric pressure. This aids the dissolution of ozone into the wastewater stream.
Preferably, the flow path through the first tank is convoluted to increase the residence time for the ozone to react. From here the partially oxidised, ozone enriched wastewater enters the second reactor tank for activation from UV light.
Preferably the reactor tank contains one or more ultraviolet lamps that illuminate the ozone enriched wastewater stream to activate the generation of hydroxyl radicals. The second tank may be held at atmospheric pressure to remove any residual ozone gas that may be in the system.
Preferably, the treatment of a wastewater stream including produced water offshore is a continuous or single pass process where the wastewater only passes once through the twin tank reactor system.
According to a fifth aspect of the present invention there is provided apparatus for the treatment of wastewater, the apparatus comprising: a first inlet for the introduction of the wastewater into a pipe; an injector mechanism for introducing ozone gas into the wastewater in the pipe; a first tank including a flow path greater than the widest dimension of the tank; a second tank including at least one UV light source; and
an outlet for discharging the processed wastewater stream into the environment.
The apparatus can be considered as a twin tank reactor system for the application of ozone and ultraviolet light treatment.
Preferably the apparatus includes an ozone generator. Advantageously the ozone generator is located adjacent the injector mechanism.
Preferably the injection mechanism is a venturi type or Korting injector used to create a vacuum from the wastewater flow to draw the ozone gas into the wastewater stream.
Preferably the injector outlet is attached in series with the first reactor tank.
Preferably the first tank contains a series of baffles in which the water flows down to the bottom of the tank to increase the residence time for the ozone to react.
Preferably, the second tank contains one or more ultraviolet lamps that illuminate the ozone enriched wastewater stream to activate the generation of hydroxyl radicals produced from the UV activation of the ozone. The hydroxyl radicals provide oxidation power and allow nonselective degradation of organic material in the wastewater stream.
According to a sixth aspect of the present invention there is provided a method of treating produced water from an oil or gas installation, the method comprising the steps of: locating a wastewater treatment system according to the fifth aspect on an offshore installation;
passing wastewater from a well through a separation system; and treating the wastewater stream according to the fourth aspect through the treatment system.
Preferably the system is of suitable size to fulfil the strict space and weight requirements required by process equipment offshore. Preferably, the system handles produced water volumes in the range of 1000 to 10,000 barrels per day. Preferably the method reduces the hydrocarbon content of the produced water by 50% typically, from 40ppm to 20ppm.
Embodiment of the present invention will now be described by way of example only with reference to the accompanying drawings in which:
Figure 1 shows a schematic diagram of an apparatus according to a first embodiment of the invention to treat a wastewater stream in a single pass/continuous method; and
Figure 2 shows a cross-section diagram of a UV reaction vessel showing the layout of the UV lamp and photocatalyst within the reaction vessel; and
Figure 3 shows a schematic diagram of an apparatus according to a second embodiment of the invention configured to treat a wastewater stream using a single pass/continuous method.
Ozone, ultraviolet and photocatalyst treatment of a wastewater stream can be termed an advanced oxidation process (AOP). The ozone generation equipment, UV lamps and UV reaction vessel are all tried and commercially available equipment. The ozone generation internal components and UV lamp configuration require no modification. If the process is to be used offshore for the treatment of produced water, the
external housing of the ozone generation device will require modification to comply with hazardous area certification standards. This part of the process is considered as a new problem for this type of equipment. The UV lamp configuration inside the second reactor tank will also constitute a novel engineering design for this type of application.
The photocatalyst structure inside the reaction vessel is a new concept in terms of physical layout within the reaction vessel and process when used in conjunction with the ozone and UV process. Traditionally the laboratory-based systems are small and compact and can only treat low volumes of wastewater (<10L). Typically, two types can define laboratory scale reactors: slurry and fixed bed reactors. Slurry type reactors require a constantly mixed suspension of photocatalyst powder in the wastewater medium whilst being illuminated by UV light. Fixed bed reactors consist of plates of metal, plastic etc coated in a layer of photocatalyst where the wastewater flows over the plates whilst being illuminated by UV light. Typically, the water is recycled over the plates to continue the treatment process. Full-scale photocatalyst systems are not common. For slurry type reactors, this is due to the need to remove the photocatalytic powder after treatment of the wastewater, plus the operation of the system has to be in a batch type process. Generally, fixed bed reactors require the wastewater to be flowed over the photocatalytic plates for a number of cycles before the necessary reduction in organic material is achieved. The wastewater also has to be spread in a thin layer to allow UV light transmission to the photocatalyst on the plates for the reaction to occur. The two dimensional orientation of the coated plates also only allows one surface to be illuminated at one time. This results in low volumes of wastewater being able to be treated.
The rod design as described hereinbefore for the application of the photocatalyst allows full use of the rod surface area, either directly from the UV light itself or indirectly from the reflection of the UV light from the polished stainless steel walls of the UV reactor vessel. The strong oxidising power of the photocatalyst upon illumination with UV light will immediately start to breakdown the organic material in the wastewater stream. The initial oxidation of the wastewater stream would have already begun at the point of ozone injection.
When the ozone enriched wastewater stream enters the UV reactor vessel, the production of hydroxyl radicals will accelerate the degradation of the organic material. This, in conjunction with the redox reactions from the photocatalyst, will result in an overall accelerated oxidation of the organic material in the wastewater stream.
The hydrocarbon level in produced water is in the range of 20 to I OOppm (parts per million). Typically, the average concentration is 40ppm. The invention will typically reduce the hydrocarbon concentration by at least 50% to 20ppm.
The standard AOP equipment is generally small and light. It can easily be modularised to fit into process systems that exist at many wastewater application sites. This is a specifically beneficial regarding space and weight constrictions in the application of produced water treatment offshore.
The volume of ozone to inject, UV lamp power and photocatalyst surface area required to treat an organic contaminated wastewater stream will be dependant on the characteristics of the wastewater. The equipment as
described in this invention has a number of control variables that can be changed according to application.
The ozone is generated on site by means of an ozone generator. The volume of ozone required is dependent on a number of process variables including: the wastewater flow rate and volume, the pressure drop across the injector and the quantity of organic contaminant in the wastewater stream. The ozone concentration and volume supplied can be varied on site.
The injection of the ozone gas through a venturi device into the process system will be either in series with the main process pipework or in parallel through a by-pass system. In series, the injector is sized to accommodate the full flow of the wastewater stream that is passing through the system. The by-pass configuration pumps a fixed fraction of the main process water volume into an appropriate sized injector that is capable of injecting the total amount of gas from the ozone generator. The water/ozone mix is then re-introduced back into the main wastewater process line.
Once the ozone has been injected into the system, the ozone enriched wastewater enters the first reaction vessel (and pass over the UV lamps and photocatalyst in the first embodiment). For the twin reactor system, the tanks will be sized in relation to the volume of water to be treated specific to the treatment location. Preferably, the reactor tanks will be large enough to provide an extended residence time for the ozone to dissolve or disperse and react within the wastewater stream but without compromising the high throughput of wastewater to be treated in a single pass.
The UV light source is a standard medium pressure light contained within a quartz tube, which in turn are positioned within the reaction vessel (the second reaction vessel for the twin reactor system). The number of lamps required or fitted will be dependent on volume of wastewater to be treated and quantity of organic material in the wastewater sample. The reaction vessels are manufactured and commercially available.
The photocatalyst could possibly be any of the metal oxides or sulphides as described hereinbefore. Tiθ2will be principally discussed as the photocatalyst to be used in this application, however it will be appreciated that other suitable materials may also be selected. Titanium dioxide exists in three crystalline structures: rutile, brookite and anatase. Rods composed of Tiθ2 are coated in the anatase form of Tiθ2, as this is understood to show the highest photoactivity. The rods are placed on a rack that can be slid into the reaction vessel to cover the area illuminated by the UV light. The TiO2 rods and anatase powder are both commercially available to order.
A diagram of the first embodiment showing the basic components of the process is shown in Figure 1. The wastewater stream enters via a pipe 1 into the process. A fixed volume of wastewater is pumped through a bypass loop 2 and is injected with ozone through a venturi type injector 3. The ozone-enriched wastewater in the by-pass flows though the loop back into the main process water pipe 4. The now mixed ozone wastewater flow is then diverted into the reaction vessel 5 where it passes over one or more UV lamps 10 and the photocatalyst rods 11 (which are shown in Figure 2). Inside the reaction vessel the wastewater stream is subjected to the treatment reactions as described hereinbefore. The treated water then leaves the process for disposal 7.
The invention process as described produces the best wastewater treatment results as a combined system with all three process components: ozone, ultraviolet light and titanium dioxide working synergistically. Ozone when injected into the system will immediately start to react with the organic material in the wastewater. Since ozone has a limited solubility, the extent of the degradation will be limited. UV light will also treat some of the organic material but to a lesser extent. The photocatalyst will only treat the organic material if illuminated by UV light.
A diagram showing the basic components of the process according to a second embodiment is shown in Figure 3. Like features are given like reference numerals.
As before, the wastewater stream enters via a pipe 1 and a fixed volume of wastewater is pumped through a by-pass loop 2 where it is injected with ozone through a venturi type injector 3. It will be appreciated that other types of injectors and more than one injection site can be used. Ozone- enriched wastewater flows though the loop back into the main process water pipe 4.
The now mixed ozone wastewater flow is then diverted into the first reactor vessel 5 where the ozone is contacted into the wastewater stream. A convoluted flow path is created through the vessel 5 by incorporating a number of baffles or plates within the vessel 5. These increase the residence time of the stream within the vessel 5.
Inside the second reactor tank 6, the wastewater stream is subjected to UV illumination by a number of UV lamps 10 where the treatment reactions as described hereinbefore are initiated. The treated water then leaves the process for disposal 7.
At all times the flow rate through the apparatus is maintained at substantially the same rate. This allows the process to operate to the required level of degradation of organic materials with flow continuously through the system in a single pass.
The invention process as described produces the best wastewater treatment results as a combined system with both process components: ozone and ultraviolet light working synergistically. Ozone when injected into the system will immediately start to react with the organic material in the wastewater. Since ozone has a limited solubility, the extended residence time gained passing through the twin reactor tank system will increase the efficiency of the process. UV light will also treat some of the organic material but to a lesser extent.
The twin tank reactor system increases the efficiency of ozone mixing plus increases the residency time for improved oxidation without compromising the fast reaction times and high wastewater volume throughput.
The embodiments of the invention described provide a process for the treatment of wastewater streams that can significantly reduce the concentration of organic material in a single pass/ continuous configuration.
Various modifications may be made to the invention herein described without departing from the scope thereof.