The present invention relates to the treatment of industrial wastewaters. In particular, though not exclusively, the invention relates to an improved treatment of produced water from offshore hydrocarbon production and drilling operations using a combined advanced oxidation process (AOP).

Industrial wastewaters are generated as a by-product from industrial operations which include chemical manufacturing or processing, food and beverage production and the oil and gas industry amongst others. For instance, in the oil and gas industry, produced water is the wastewater by-product from hydrocarbon extraction. Before this water can be discharged, it must be separated from the oil and gas fractions. However, the separated water will contain low but measurable amounts of organic contaminants that have the potential to harm the environment and so the water must be treated.

One method commonly used to treat industrial wastewaters is a combination of ozone and ultraviolet (UV) light and this is termed an advanced oxidation process. Typically, the ozone is injected into the wastewater stream before exposure to the UV light within a reaction vessel. The ozone and UV light work together to produce hydroxyl radicals which are a powerful oxidizer.

It has been found that the efficiency of an advanced oxidation process is greatly dependent on the particular set up used. There is a great demand to improve upon existing systems, such as in efficiency, possible flow rates through the system (and the ability to vary the flow rate) and processing time.

According to a first aspect of the present invention there is provided a fluid treatment apparatus for treating a fluid using an advanced oxidation process, the apparatus comprising:

a primary flow line connecting an inlet and an outlet of the apparatus; an ozone injecting device adapted to introduce ozone to the fluid;

a reactor vessel including a UV light source for treating the ozonated fluid, the vessel being provided along the primary flow line and downstream of the ozone injecting device; and

a secondary flow line arranged in parallel to the primary flow line to provide variation of a fluid flow rate within the apparatus.

The secondary flow line may be a bypass line having a first node provided at or near the inlet and a second node provided at or near the inlet of the reactor vessel.

Alternatively or in addition, the secondary flow line may be a recirculation line having a first node provided at or near the inlet and a second node provided downstream of an outlet of the reactor vessel.

The apparatus may include an overflow line such that the apparatus is operable in both a bypass mode and a recirculation mode. A non-return valve may be provided along the overflow line. The ozone injecting device may be provided along the secondary flow line. Alternatively, the ozone injecting device may be provided along the primary flow line.

The apparatus may include pumping means for driving fluid from the inlet to the outlet. The pumping means may be provided upstream of the ozone injecting device. The pumping means may comprise a centrifugal pump.

The apparatus may include a flow adjusting device to provide control of a fluid flow rate within the apparatus. The flow adjusting device may be provided along the primary flow line at or near the inlet. The flow adjusting device may comprise one or both of a first control valve and a flow meter. The flow adjusting device may include a second control valve provided along the secondary flow line. The second control valve may be arranged to control a turn-down limit of the apparatus. The apparatus may include one or more non-return valves. A first non-return valve may be provided along the primary flow line at or near the inlet. The first non-return valve may be arranged to prevent ozonated fluid flowing to the inlet.

The inlet may be arranged to be at a lower height than one or both of the primary and secondary flow lines. The outlet may be arranged to be at a greater height than the inlet and the primary flow line.

The ozone injecting device may receive ozone from an ozone generating device. The ozone generating device may receive substantially pure oxygen for generating the ozone. Oxygen may be supplied to the ozone generating device in the form of liquid oxygen. Alternatively or in addition, oxygen may be supplied from an oxygen concentrator.

The apparatus may comprise means for recycling oxygen. The apparatus may include a degasser vessel. The degasser vessel may include a gas outlet where oxygen may be collected.

The degasser vessel may be provided downstream of the reactor vessel. Alternatively, the degasser vessel may be provided upstream of the reactor vessel for passing degassed liquid to the reactor vessel.

The degasser vessel may include a liquid outlet for degassed liquid. The apparatus may be adapted to recirculate the degassed liquid via the secondary flow line.

The degasser vessel may comprise a separator having a liquid and gas outlet. The pumping means and ozone injecting device may be provided along the secondary flow line with the pumping means operating in a reverse flow mode to drive ozonated fluid towards the primary flow line upstream of the reactor vessel.

The apparatus may include a booster pump for pressuring fluid within the apparatus. The booster pump may be provided at the secondary flow line.

According to a second aspect of the present invention there is provided a fluid treatment apparatus for treating a fluid using an advanced oxidation process, the apparatus comprising:

a primary flow line connecting an inlet and an outlet of the apparatus;

an ozone injecting device adapted to introduce ozone to the fluid;

an ozone generating device for supplying ozone to the ozone injecting device; and

a reactor vessel including a UV light source for treating the ozonated fluid, the vessel being provided along the primary flow line and downstream of the ozone injecting device,

wherein the ozone generating device is adapted to receive substantially pure oxygen for generating the ozone.

The apparatus may comprise means for recycling oxygen. The apparatus may include a degasser vessel. The degasser vessel may include a gas outlet where oxygen may be collected.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a schematic layout of a wastewater treatment system according to a first embodiment of the invention; Figure 2 is a typical chart of pressure against flow rate for a pump and injector;

Figure 3 is a two phase flow map for flow within a horizontal pipe; Figure 4 is a schematic layout of a wastewater treatment system according to a second embodiment of the invention;

Figure 5 is a schematic layout of a wastewater treatment system according to a third embodiment of the invention;

Figure 6 is a schematic layout of a first type of degasser that can be used with the embodiment of Figure 6;

Figure 7 is a schematic layout of a second type of degasser that can be used with the embodiment of Figure 6;

Figure 8 is a schematic layout of a wastewater treatment system according to a fourth embodiment of the invention; and Figure 9 is a schematic layout of a wastewater treatment system according to a fifth embodiment of the invention.

Figure 1 shows a first embodiment of the invention which is termed an inline injection system 10. In this system 10, the fluid flow is linear and passes through each element in series from an inlet 20 and via a primary flow line 22 to an outlet 52 of the system 10.

Water to be treated enters the system via the inlet 20. The inlet 20 is arranged to be at a lower height than the primary flow line 22 (or, in other embodiments, at a lower height than a T-piece connection point where a bypass stream joins the primary flow line 22). The outlet 52 is located at a higher level than the inlet 20 and the primary flow line 22. Also, in the primary flow line 22, low points and high points are avoided such that the pipework is horizontal or oriented upwards. The wastewater therefore flows in an upward direction from the inlet 20 to the outlet 52 and this helps to direct gas bubbles downstream.

The pipework of the system 10 is also configured to ensure that fluid flowing from the inlet is exposed to ozone as early as possible (either directly through an injector or indirectly in other embodiments by merging the primary flow line 22 with the bypass stream downstream of the injector). This entails that the inlet 20 to be positioned either at the pump suction or at the injector discharge. The inlet 20 cannot be positioned between the pump and the injector as the fluid is pressurized at this location.

Wastewater flows through a non-return valve 24, which prevents the fluid from flowing in the opposite direction in the event that the system 10 is blocked or restricted or the pressure at the outlet 52 becomes greater than the pressure at the inlet 20.

Fluid then flows through a flow adjusting device in the form of a flow meter 26 which allows measurement and control of the overall treatment flow rate as well as the flow rate through the injector 40 (which determines the amount of gas suction).

The fluid is then pressurized using pumping means such as a centrifugal pump 30. A positive displacement pump may over-pressure the system when a valve is closed accidentally or if a line is blocked. A centrifugal pump is also a source of shearing energy and this helps in dispersing the contaminants to be treated by the ozone in the wastewater stream. The water stream then flows through a control valve 32 which can be manual or automatic. This allows for slight adjustment of the treating flow rate and can be considered to be part of the flow adjusting device. Fluid then passes through the ozone injector 40 where a mixture of ozone and air (or oxygen) is injected. The pressure at the inlet and the outlet of the injector 40 is measured and controlled using pressure gauges 42 to ensure it is operating correctly. The type of injector is selected depending on the flow rate of the gas mixture to be injected in the wastewater stream and the flow rate required through the injector 40. This last parameter highly depends on the position of the injector 40 (whether it is in the primary flow line 22 or in a bypass stream). The combination of these two parameters determines the injector model and size, as well as the pump pressure required. The pump specifications are chosen to match the injector 40 selected. The working point of the pump and injector assembly is obtained by crossing the pump curves and the injector curves on a pressure against flow rate chart, such as shown in Figure 2. Due to the injection of air and ozone into the wastewater, the process pipework is exposed to a two phase flow regime downstream of the injector 40. The desired flow regime is a "dispersed bubble flow" regime as shown in Figure 3.

The feed gas to the injector 40 must be of very high quality and may require pre- treatment (not shown). Ozone is fed to the injector 40 from an ozone generator 44.

The ozone generator can be fed using air to produce ozone but can also be fed with 'pure' oxygen which allows the production of approximately 75% more ozone (at 6 to 10 %wt). Two sources of oxygen can be used. Liquid Oxygen can be used, which can have purity as high as 99.8 %wt, or the oxygen can be derived from "oxygen concentrators". These are Pressure Swing Adsorption units which can generate oxygen from pressurized air with levels of purity as high as 93 %wt. Oxygen concentrators are the preferred source of 'pure' oxygen for ozone generators for ease of operation (no need for high pressure regulators), safety (no high pressure equipment or frost generating equipment is required) and because it produces oxygen on a continuous basis.

Although the efficiency of ozone production is greatly increased, using pure oxygen as a feed gas for the ozone generator requires a large amount of compressed air and an increased amount of electrical energy. In addition, pure oxygen left with the treated water can only be discharged at a safe location. However, recycling the oxygen injected in the waste water stream downstream of the UV reactor can claim back up to 75% of the oxygen initially injected which is re-used to feed the ozone generators, therefore also lowering the power requirements and the compressed air consumption by up to 75%.

An oxygen recycler which can be used is a conventional degasser vessel where the exhaust gas (oxygen) is collected from the outlet 52, retreated and re- compressed before feeding again to the ozone generator. The degasser can also be used simply to ensure gas-free water at the outlet 52.

The ozone generation produces a lot of heat and therefore requires its own dedicated cooling system (not shown).

Fluid then flows to a UV reactor vessel 50 which is installed downstream of the injector, as close as possible to it. The UV reactor vessel 50 creates the hydroxyl radicals from water and ozone to treat the water and also destroys any remaining ozone in the water stream to prevent harm to facility personnel working around the discharge point. To maximise performance, the UV reactor vessel 50 is generally positioned in the line exposed to the greatest ozonated flow. Fluid flowing out of the reactor 50 flows to the outlet 52 of the system 10. The outlet 52 is positioned so as to ensure that all the fluid at the outlet 52 has been exposed to UV light. This is to ensure that substantially no ozone is released at the discharge point downstream and therefore minimises the risk of the host facility personnel being exposed to ozone.

A booster pump can be provided to compensate for any back-pressure at the outlet 52 or to allow for pressurized discharge. The use of pressurized discharge is described in detail below.

While this inline injection system 10 has a number of advantages over the prior art, it can be low in flexibility in terms of flow rates that can be used. Therefore, it may typically be installed in a process where a pre-determined and constant flow rate of waste water is supplied to it.

Figure 4 shows a second embodiment of the invention which is termed a bypass injection system 100. In this system 100, the primary flow line 22 is linear between the inlet 20 and the outlet 52 of the system 100 but the system 100 includes a secondary flow line in the form of a bypass line 1 10.

In this embodiment, after flowing through the inlet 20, non-return valve 24 and flow meter 26, a part of the main flow in the primary flow line 22 is taken off to be directed to the bypass line 1 10. The pump 30 and injector 40 (and associated pressure gauges 42) are located along this bypass line 1 10. Downstream of the injector 40 the bypass line 1 10 merges into the primary flow line 22 and so the bypass line 1 10 is parallel to the primary flow line 22. The UV reactor 50 is located downstream of the merging point. The distance between the injector 40 and the UV reactor 50 is configured to be as short as possible. With this configuration, the flow meter 26 allows measurement and control of the overall treatment flow rate but not of the flow rate through the injector 40. The maximum flow rate is only limited by the pipework size or the treatment efficiency (depending on the ozone or UV dose). However, the system 100 requires a minimum flow through it in order to keep the pump 30 running. This minimum flow rate, also called the 'turn-down limit', corresponds to the nominal working flow rate of the pump and injector assembly.

The overall flow through the system 100 is controlled by a primary control valve 132 which is located along the primary flow line 22. A secondary control valve 134 can be installed in the bypass line 1 10 between the pump 30 and the injector 40 to control the flow rate of water going through the injector 40 and consequently to control the turn-down limit of the system 100.

Due to its turn-down limit, the bypass injection system 100 is particularly suited for large flows of water containing low amounts of pollutants or easy to treat water. Because the pump 30 and the injector 40 are installed on the bypass line 1 10, the pressure between the inlet of the pump 30 and the outlet of the injector 40 is equalized when the primary control valve 132 is fully open.

As with the first embodiment, pure oxygen can be used as the feed gas for enhanced ozone production, and a degasser vessel can be installed downstream of the UV reactor 50 to provide gas-free water at the outlet 52 and to recycle oxygen. A booster pump can be provided to compensate for any back-pressure at the outlet 52 or to allow for pressurized discharge. Figure 5 shows a third embodiment of the invention which is termed a recirculation system 200. This system 200 includes a secondary flow line in the form of a recirculation line 210. The system 200 is configured to recirculate a portion of the ozonated water through the pump and injector assembly. One advantage of this system 200 is that water can be treated at flow rates less than the nominal working flow rate of the pump and injector assembly (in other words, lower than the turn-down limit). In the system 200, the entire inlet flow is directed along the primary flow line 22 which includes the pump and injector assembly. The UV reactor 50 is positioned downstream of the injector 40 as close as possible to it. Downstream of the UV reactor 50, the flow line splits into two flow lines. One is directed towards the outlet 52 and the other is a return (recirculation) line to the pump 30. Since air and ozone have been injected into the water stream, it must be degassed before entering the recirculation line 210 and the pump 30. A degasser 60 can be used to achieve this. The use of the degasser 60 has other advantages. When using pure oxygen as the feed gas, for safety reasons and explosion risk reduction, the oxygen must be taken out of the fluid being discharged in the host facility processes and vented off in a safe area. Also, recycling the feed gas provides a substantial saving in energy/gas consumption. Two different types of degasser 60 can be used.

If a conventional degasser is used, the fluid, such as shown in Figure 6, a mixture of water and gas flowing in the primary flow line 22 enters the holding tank from one side, at the top. The volume and length of the tank are sized in such a way that the gas mixture can separate by gravity in accordance with Stokes law. The exhaust gas only is collected at the top of the tank and passed to a vent 62 provided in a safe area. The degasser has only one outlet 64 at the bottom of the vessel 60 for degassed fluid which is passed to the outlet 52 for subsequent discharge.

The degassed fluid can be used to feed the pump and injector assembly for recirculation systems. The level of water in the degasser 60 is controlled as any excess water in the degasser would lead to water through the exhaust line and then, when the level is low, the pump suction would be exposed to gas which can damage the pump 30. Control of the level of water in the degasser is achieved by the adjustment of two metering valves 66, one in the gas exhaust line and one at the outlet of the degasser. Alternatively or in addition, level switches may be used.

Alternatively, as shown in Figure 7, a separator 160 can be used which is a type of degasser that does not vent any gas off into the atmosphere. The fluid enters the tank from one side at the top from the primary flow line 22. On the opposite side of the inlet are two outlet lines. The first outlet line 162 is positioned in the middle of the top half of the tank and this line is connected to the outlet of the system 200. The second outlet line 210, of smaller diameter, is at the bottom of the tank and directly connected to the suction side of the pump 30. Due to gravity, gas bubbles cannot reach the suction of the pump. They flow away, with some of the treated water through the top outlet line 162.

The level of water in the separator 160 does not need to be controlled. Any excess water in the separator 160 will be pushed through the top outlet 162 and then, when the level is lower, no more water will be discharged.

Referring again to Figure 5, the flow meter 26 allows for measurement and control of the overall treatment flow rate but not for the flow rate through the injector 40. The flow rate through the injector 40 is deduced from the injector inlet pressure using reference tables provided by the supplier.

In this embodiment, the maximum flow rate is limited by the pump and injector assembly. Flow rates greater than the nominal working flow rate of the pump and injector assembly will cause excess water to flow through the recirculation line 210 to the degasser 60, which is undesirable as this water will not be treated. However, the system 200 does not require a minimum inlet flow. The entire flow through the system 200 is controlled by a control valve 32 installed near the inlet 20. An advantage of this system 200 is the elimination of the turn-down limit. Instead, this type of system is characterized by an Injection Ratio (IR) which is equal to the nominal working flow rate of the pump and injector assembly divided by the operational inlet flow rate. This ratio, greater than one, represents, as an average, the amount of times the waste water is directly exposed to ozone through the injector 40. Ideally, the IR of such a system should be between 2 and 10.

The recirculation system 200 offers the possibility of injecting greater amounts of ozone in the water stream. For this reason, the best application for the system 200 is the treatment of a small flow of water with medium or high levels of pollution or difficult to treat water.

The recirculation system 200 can be configured to use pure oxygen as the feed gas for enhanced ozone production. An oxygen recycler can be used in combination with a degasser 60. Use of the degasser 60 provides gas-free water at the outlet 53 as well as to the pump 30. A booster pump can be used to compensate for any back-pressure at the outlet 52 or to allow for pressurized discharge.

Figure 8 shows a fourth embodiment of the invention which is a combined bypass and recirculation system 300, a hybrid of the second and third embodiments. As the two systems are similar in terms of lay-out, they can be readily combined to provide a system which has the advantages of both of them.

The combined system 300 uses a secondary flow line 310 similar to the recirculation system 200, and additionally an overflow line 312 connects the secondary flow line 310 and the inlet of the UV reactor 50. Non-return valves 314 are installed in both the secondary flow line 310 and in the overflow line 312 to allow the automatic and adequate redirection of the flow depending on the inlet flow rate. In the event that the inlet flow rate is greater than the nominal working flow rate of the pump and injector assembly, the excess water will flow via the secondary flow line 310 and then through the overflow line 312 (the non-return valve 314 in the secondary flow line 310 will prevent the water from flowing to the separator 60). The system 300 will then work in a bypass mode.

If the inlet water flow rate drops below the nominal working flow rate of the pump and injector assembly, all the waste water will now enter the pump 30 and the injector 40. Downstream of the injector 40, the water will be prevented from flowing through the overflow line 312 by the non-return valve 314 and will be forced to enter the UV reactor 50 and the separator 60. The degassed water held in the separator 60 will flow through the secondary flow line 310 and through the dual-way flow line to compensate for the low flow rate from the inlet water at the feed of the pump 30. The system 300 will then work in a recirculation mode.

The flow meter 26 allows measurement and control of the overall treatment flow rate but not the flow rate through the injector 40; the flow rate through the injector 40 is deduced from the injector inlet pressure. The inlet flow rate of wastewater is adjusted using the control valve 32 installed near the inlet 20. The maximum treating flow rate is no longer limited by the pump 30, only by the pipework size and the treatment efficiency. Also, the minimum treating flow is no longer limited by the pump 30 and injector 40 and the turn-down limit is eliminated. To characterize this configuration, an Injection Ratio (IR) can be used, as for the recirculation system 200. It is still defined as the nominal working flow rate of the pump and injector assembly divided by the operational inlet flow rate. This ratio can however take any value from 0 to infinity. IR values greater than 1 means that the system is working in the recirculation mode whereas values lower than 1 means that the system is working in the by-pass mode. The combined system 300 offers greater flexibility than the bypass system or the recirculation system on their own. The combined system 300 is suitable for nearly all treatment conditions (high flow, low pollution; low flow, low pollution; low flow, high pollution). The system 300 is particularly suitable for host facilities that have high and unpredictable variations of water flow rates.

As before, the combined system 300 can be configured with variations such as: the use of pure oxygen as feed gas; use of a degasser vessel rather than the separator to provide gas-free water at the outlet; use of an oxygen recycler in combination with a degasser; and the use of a booster pump.

Figure 9 shows a fifth embodiment of the invention which is termed a reverse recirculation system 400. In this system 400, the entire water stream flowing from the inlet 20 is merged with the ozonated water stream from the pump and injector assembly. The merged streams then flow through the UV reactor 50 along the primary flow line 22. Downstream of the UV reactor 50, the water enters the separator 60 and the degassed water flow from the separator 60 is used to feed the pump and injector assembly whereas the excess water and gas stream flows to the outlet 52.

With this configuration, the system 400 will always work in the same way independently of the inlet flow rate and of the nominal working flow rate of the pump and injector assembly. Therefore there is no turn-down limit for this system 400 and the maximum treating flow rate is only limited by the pipework size or the treatment efficiency. The flow meter 26 allows measurement and control of the overall treatment flow rate but not the flow rate through the injector 40, which is deduced from the injector inlet pressure using supplier's reference tables. The inlet flow rate of wastewater is adjusted using the control valve installed near the inlet 20. The reverse recirculation system 400 offers the same flexibility as the combined bypass and recirculation system 300 but with less pipework complexity. It is suitable for most treatment conditions (high flow, low pollution; low flow, low pollution; low flow, high pollution). The configuration is particularly suitable for host facilities that have high and unpredictable variations in water flow rates and pollution.

The system 400 can be designed with variations as for previous embodiments. In particular, as the system 400 requires a separator or a degasser to operate properly, it is a good candidate for using oxygen as an alternative feed-gas for the ozone generators.

All of the above embodiments of the invention allow water treatment under pressure, either due to gravity or due to the presence of a booster pump upstream of the system.

Treatment under pressure has a number of implications. When using pure oxygen as the feed gas, and when the oxygen is recycled, the pressurized treatment allows the degasser to work under pressure and release the oxygen with a small positive pressure which is useful for treating the oxygen before it is recompressed and re-used as feed gas for the ozone generators. The gas volume fraction of the gas mixture injected into the water is also reduced. This increases the mixing efficiency (or mass transfer efficiency) or allows injecting more ozone at a lower flow rate.

When a pump is installed in the primary flow line 22, either for boosting or mixing, the maximum flow rate through the system will be limited by the pump specification. However, when the pump and injector system is installed in a bypass line, the maximum flow rate through the system is only limited by the size of the process pipework. The treatment capacity in terms of flow rate is not normally limited. The minimum flow rate is normally determined by the required flow rate to keep the pump primed at all time and to ensure that enough suction can be generated at the injector 40 for mixing the gas with the water. This turn-down limit can be removed by setting up the pump and injector system in a recirculation configuration. This enables the pump 30 to re-circulate on itself in the worst case scenario where there is no flow at the inlet 20.

The flow of wastewater through the system is normally controlled upstream of the system at the existing host facility. However, as it may be required in some circumstances to control the flow of water at the system, a manual or automatic control valve can be provided.

The booster pump can be sized to meet the host facility requirements in term of flow rate and pressure. When it is required that the outlet 52 is free of gas, the booster pump can be installed at the outlet of the degasser 60. The metering valve of the degasser 60, also acting as a control valve, can be moved to the outlet of the booster pump. When the water at the outlet 52 does not need to be degassed, the booster pump can be installed upstream of the system to remove the requirement for an extra degasser. The control valve for the system can also act as the control valve for the booster pump. As well as the many advantages of the invention described above, the invention can readily be configured to be portable, compact and formed by modular units that can be installed at practically any location.

Whilst specific embodiments of the present invention have been described above, it will be appreciated that departures from the described embodiments may still fall within the scope of the present invention.