Field

The invention relates to an apparatus and method tor treating wastewater, in particular, wastewater that contains high oil contents.

Background

Oily wastewater comes from various sources such as ship, ship repairs, food processing plant, food court, etc. The liquid from the waste generated daily has higher oil contents. The wastewater contains higher level of Chemical Oxygen Demand (COD), nitrogen, nutrients and other contaminants. It can cause several environmental problems, such as water pollution and eutrophication. As such, it must be treated before it is released to infrastructures such as watercourse, sewers, etc. Governments and international organizations have set regulations on the oil levels in discharged water.

The typical treatment technology is an activated sludge process, by which oily substances are oxidized to carbon dioxide and simple organic compounds. In order to achieve such a goal, higher amount of energy is applied to provide oxygen and stir wastewater. Another approach for the treatment is a physiochemical technology that is composed of coagulation, sedimentation and filtration and optional activated carbon adsorption. Although this approach can achieve the level of removal that meets the requirements from governments or regulatory authorities, the cost of the chemicals used is rather high and the handling of chemicals requires skilled personnel and equipment.

Summary

According to one aspect of an example of the present disclosure, there is provided an apparatus for treating wastewater, the apparatus comprising: at least one reaction zone for producing chemicals to destabilize contaminants in wastewater to be treated and to form contaminant aggregates in the wastewater, the at least one reaction zone comprising a plurality of electrodes for contacting the wastewater, wherein the plurality of electrodes comprises one or more anode and one or more corresponding cathode and the plurality of electrodes is configured such that polarities of the one or more anode and the one or more corresponding cathode are switchable every 10 to 60 seconds to cause the plurality of electrodes to react with the wastewater to produce the chemicals.

Each of the plurality of electrodes may include a casing made of an electrically conductive and chemically stable material.

The casing may comprise a frame for housing a plurality of plates made of a reactive material for reacting with the wastewater to produce the chemicals when the polarities of the one or more anode and one or more corresponding cathode are switched periodically every 10 to 60 seconds.

The casing may contain a material including stainless steel, titanium or titanium coated with rare metal oxide.

The plurality of electrodes may be spaced apart from one another by about 10 to 20 mm.

The plurality of electrodes may comprise one or more sets of electrodes, each set of electrodes having 8 or less electrodes.

More than one of the one or more sets of electrodes may be electrically connected in parallel to one another.

One or more of the plurality of electrodes may contain a reactive material including Aluminium.

One or more of the plurality of electrodes may contain a reactive material including Iron. One or more of the plurality of electrodes may contain a reactive material including Aluminium and Iron.

One of the one or more anodes of the plurality of electrodes may contain a reactive material including titanium or titanium coated with rare metal oxide for generating total residual oxidants for killing organisms and microorganisms and preventing regrowth of organisms and microorganisms. The apparatus may comprise at least one settling tank for separating or settling solids in the wastewater, wherein the at least one settling tank comprises inclined plates spaced apart from one another, each inclined plate is disposed 60 degrees from an axis parallel to direction of wastewater flow.

The inclined plates may be in parallel arrangement with one another and the inclined plates are spaced apart by a gap distance of 20 to 40 mm from one another.

The rate of flow of wastewater to be treated may be controlled such that hydraulic retention time of the wastewater to be treated is 2 hours or more.

The apparatus may comprise at least one flotation zone for generating gas bubbles for attaching to the contaminant aggregates and for causing the attached contaminant aggregates to float upwards in the wastewater.

The at least one flotation zone may include a plurality of electrodes for contacting the wastewater to generate the gas bubbles.

One or more of the plurality of electrodes in the at least one flotation zone may contain titanium, stainless steel, titanium coated with rare metal oxide or rare metal oxide based titanium.

An anode of one or more of the plurality of electrodes in the flotation zone may contain a reactive material including titanium or titanium coated with rare metal oxide for generating total residual oxidants for killing and preventing organism regrowth.

An upper part of the at least one flotation zone may be attached to a structure having downward inclined slope to allow the floated contaminant aggregates to exit the flotation zone through the downward inclined slope.

The structure may be attached to a plate for blocking the floated contaminant aggregates so that the floated contaminant aggregates only exit through the downward inclined slope.

The floated contaminant aggregates may be collected and utilised as an additive to fuel.

An upper part of the at least one reaction zone may be attached to a structure having downward inclined slope to allow the floated contaminant aggregates to exit the flotation zone through the downward inclined slope.

Concentration of each of Cr, Cd, Co, Cu, Mn, Ni, and Pb may be less than 1 ppb level in the treated wastewater originating from a shipyard and concentration of Fe and Zn may be 1 .5 and 1 ppm respectively in the treated wastewater originating from the shipyard.

According to another aspect of an example of the present disclosure, there is provided a method comprising: producing chemicals to destabilize contaminants in wastewater to be treated and to form contaminant aggregates in the wastewater by switching every 10 to 60 seconds polarities of one or more anode and one or more corresponding cathode in a plurality of electrodes placed in contact with the wastewater to cause the plurality of electrodes to react with the wastewater to produce the chemicals.

The method may comprise generating gas bubbles for attaching to the contaminant aggregates and for causing the attached contaminant aggregates to float upwards in the wastewater.

The method may comprise controlling rate of flow of wastewater to be treated so that hydraulic retention time of the wastewater to be treated is 2 hours or more.

Brief Description of Drawings

It will be convenient to further describe the examples of the present invention with respect to the accompanying drawings that illustrate possible arrangements/configurations of the examples. Other arrangements/configurations are possible and consequently, the particularity of the accompanying drawings is not to be understood as superseding the generality of the preceding detailed description.

Figure 1 is a schematic front view of an example system or apparatus.

Figure 2 is a schematic back view of the system or apparatus.

Figure 3 is a schematic cross view of the system or apparatus.

Figure 4 is a schematic right and left view of the system or apparatus.

Figure 5 is a schematic top view of the system or apparatus. Figure 6 illustrates the construction of electrodes.

Figure 7 illustrates arrangement of electrodes.

Detailed description

The technology in the invention described herein is directed to address environmental issues surrounding industries using, handling and treating oily substances, organic waste and heavy metal waste due to, for instance, ship repair and cleaning, food processing, soft drink production, metal mining and processing, metal coating, manufacturing of electronic components, oil extraction and refining, pharmaceutical manufacturing waste, and waste from fine chemical operation/production.

It has been observed that wastewater from ship repair and cleaning, food processing, oil extraction and refining, etc, generally contains high oil contents. It would be beneficial to find solutions for recovery of the oil from the wastewater, which can then be served as a raw material for generation of new energy.

In this disclosure, an example herein known as electrochemically assisted flotation technology (EAFT) is devised for the treatment and recovery of oily substances from the wastewater that is from industries, specifically from their operation and locations. Oils and greases (O&Gs) and organic matters obtained from the wastewater of said industrial operations are observed to be quite stable due to their "charge" characteristics. EAFT can also treat contaminants such as heavy metal ions, fluoride, arsenic, phosphate, dye, and organic matters.

A method is provided to minimize energy consumption, improve ease in operation and reduce physical, chemical and biological fouling of electrodes via rapid polarity switching within seconds (for example, 10-60 seconds) in the EAFT for treating wastewater containing O&Gs, and other said contaminants. EAFT comprises at least two zones known as a reaction zone and a flotation zone with each having electrodes. Wastewater first enters reaction zone and then flotation zone. Among the electrodes in these two zones, half of them are cathodes and the other half are anodes. The reaction zone is for producing chemicals to destabilize said contaminants and to form contaminant aggregates (also known as sludge in the present disclosure), which float up to upper level of water and are removed; the separated sludge has higher contaminant content and can be reused by industry. The flotation zone is for generating fine gas bubbles to float the remaining sludge from reaction zone, so that pollutants (in particular O&Gs) can be concentrated and separated, and clean water can be obtained after separation from the pollutants. The word "flotation" refers to such process of floating the aggregates. In the reaction zone, the electrodes are aluminum or iron plated and mounted on a casing or framework having chemically stable but electronically conductive materials such as stainless steel (For example, SS316) or titanium (Ti). The electrodes in the flotation zone are chemically stable metals such as stainless steel (For example, SS316), titanium, and/or titanium coated with rare metal (e.g., Ru, Pb, Ir and the like) oxides.

An example treatment system or apparatus is provided in EAFT. The treatment system or apparatus comprises at least one reaction compartment, followed by at least one flotation compartment, and at least one optional settling tank (known as primary settling tank) for separating solids in incoming water and at least one settling tank (known as final settling tank) for separating solids in treated water coming out from the flotation compartment(s). The water in the final settling tank may be further processed. The operation sequence of the components of the treatment system is in the order of the optional primary settling tank, the reaction compartment, the flotation compartment, and followed by the final settling tank. Most of the contaminants are removed by the treatment system after processing by the treatment system. The treated effluent obtained after processing by the treatment system may be optionally further treated by treatment systems such as activated carbon adsorption and/or membrane filtration, by which the effluent can be discharged or recycled.

The terms "treatment system" and "treatment apparatus" will be used in the present disclosure interchangeably to refer to the same thing.

The treatment system leads to

1 ) generation of concentrated oily substances when treating oily wastewater, which are suitable for use in industry,

2) clear water for reuse or

3) less contaminated water for discharge dependent upon arrangement of treatment system in terms of hydraulic retention time, energy input, number of reaction and flotation compartments, primary settling tanks and final settling tanks.

The concentrated oily substances from the treatment system once mixed with fuels such as the heavy fuels used in ships can greatly improve the combustion efficiency of the fuel. Such use of the concentrated oily substances can lead to cost savings due to lesser fuel usage in ships.

Detailed Process Description

The terms "compartment" and "zone" are used in the present disclosure interchangeably to refer to the same thing.

With reference to Figures 1 to 5, the treatment system (100) comprises at least one reaction zone (23) and at least one flotation zone (24). The treatment system (100) further comprises at least one primary settling tank (25) for separating solids in incoming water (1 ) to the treatment system (100) and at least one final settling tank (22) for separating solids from treated water coming out from the at least one flotation zone (24). In the said figures, there is shown one rectifier (98) connected to the electrodes (7) in reaction zone (23) and another rectifier (198) connected the electrodes (10) in flotation zone (24) in the treatment system (100). The rectifiers (98 and 198) provide direct current (DC) to the electrodes (7 and 10) shown in Figures 3 and 5. The polarities of power (namely DC from rectifier (98)) can be switched. Sludge or solids can be released from the bottom of primary settling tank (25) through a valve (18), from that of reaction zone (23) through a valve (20), from that of flotation zone (24) through a valve (21 ) and from that of final settling tank (22) through a valve (17). In Figure 2, there is shown a structure (32) with a downward inclined slope attached to an upper part of the reaction zone (23) and flotation zone (24) to allow floating sludge in both zones (23, 24) to exit the zones (23,24) through the downward inclined slope by gravity.

With reference to Figures 3 and 5, the wastewater labelled as (1 ) enters the treatment system (100) through an inlet valve (2) and go downward through a small chamber (3) till the wastewater (1 ) reaches a bottom (26) of the primary settling tank (25). The wastewater moves upwards to a top area (4) of the primary settling tank (25) through inclined plates (41 ) with a gap of 20-40 mm between the inclined plates (41 ) so that particles in the wastewater can be removed, flows downwards to a small chamber (5) and enters a reaction zone (23). The inclined plates in the primary settling tank (25) are inclined 60 degrees from a horizontal axis (99) of the treatment system ( 100) in Figure 3 or in other words, 60 degrees from direction of water flow, which in this case is parallel to the horizontal axis (99).

The water then moves upwards through spaces (6) of the reaction zone (23) surrounding a plurality of electrodes (7) made of metal plates residing in the reaction zone (23). A rectifier (98) connected to the electrodes (7) provides DC to the electrodes (7) during operation. The electrodes (7) may be made of a reactive material such as aluminium, iron or other metal plates that can be a combination of Al or Fe or other metals. At least one set of the electrodes (7) is arranged parallel to water flow direction in the reaction zone (23), which is also shown in Figure 5 with the connection to rectifier shown in Figure 7. One pair of electrode is made of one piece of electrode used as an anode and another piece of electrode as cathode. Typically, more than one pairs of electrodes are used. The fabrication and connection of the electrodes as well as control of power supply through DC will be addressed in later parts of the present disclosure.

Examples of chemicals such as metal ions and hydroxides generated according to the following reactions (1 a) to (1 c) at anodes of the electrodes (7) in the reaction zone (23) are as follows.

Al = Al ³⁺ + 3e - [if aluminium is present as anode electrodes (7)]

Fe = Fe ²⁺ + 2e - [if iron is present as anode electrodes (7)]

Fe ²⁺ = Fe ³⁺ + e - [if iron is present as anode electrodes (7)]

40H =2H ₂ 0+0 ₂ +4e- [if Al or Fe is not used and Ti or raw metal/oxide coated Ti is used]

2Ci ^" = Cl ₂ + 2e- [if Al or Fe is not used and Ti or raw metal/oxide coated Ti is used]

The said metal ions can cause destabilization of the particles and contaminants in the wastewater in the reaction zone (23), as these metal ions are positively charged and thus can neutralize negatively charged contaminants. As a result, the destabilized contamainants can be formed larger particulate matters, which can be removed by settling and/or through flotation.

The said metal ions can even easily form metal hydroxides in the reaction zone (23) as M ^m+ (OH) _n ^;T1 " (m=2 or 3, n=1 , 2, 3..) (namely floe). The contaminants in the waste rwater such as heavy metals, small particles and organic matters attach to these metal hydroxides, which can be removed by settling and/or flotation.

Side reactions such as Reactions (2a)-(2b) may occur. However, the chance is lesser if Al and/or Fe typed electrodes are used; thus these side reactions may be ignored.

The above description is based on use of Al and/or Fe plates as the electrodes (7). In the event that such plates are not used, Reactions (2a) and (2b) may occur. At the anode electrode(s) (7) located in the reaction zone (23), production of chlorine may occur when the anodes made of titanium or titanium coated with rare metal oxides (Ir, Ru, etc) are used according to Equation (2b) if there is sufficient chloride ion in the wastewater, which is always the case in food waste or other oil-containing waste. The chlorine being a gaseous oxidant is typically converted to an oxidant known as OCI in the wastewater. In the event that the choride content is low, some oxygen bubbles are expected to be generated according to Equation (2a). In addtion, some chemically powerful oxidants such as OH free radicals and ozone may be generated. These oxidants termed as total residual oxidants (TRO) can oxidize organic matters in wastewater with high efficiency. If the contaminated water or wastewater contains mico-organisms, these oxidants can kill the micro-organisms and/or prevent their growth. After the wastewater is treated, it may contain un-reacted oxidants (e.g. OCI , and 0 ₃ ). A neutralization process by reductants such as Na ₂ S0 ₃ may be used to neutralize these oxidants. If one wishes to keep away microbial presence and growth in the treated water, he/she may consider having several or many of the aforementioned of electrodes (titanium or titanium coated with rare metal oxides (Ir, Ru, etc)) (7) in the reaction zone (23).

Hydrogen gas in the form of gas bubbles can be generated at cathodes of the electrodes (7) according to the following reaction (3), which can facilitate the separation of the oily substance from the wastewater. The gas bubbles and the sludge formed in the reaction zone (22) are attached to each other and float to the upper layer of water in reaction zone (22) and flow through the inclined structure (32) due to the gravity and are collected for disposal, or reuse as an oil additive. The remaining solids are to be removed by the flotation zone (24).

2H~ + 2e = H ₂ (3) The wastewater after the treatment in the reaction zone (23) enters a flotation zone (24) through a small chamber (8). In the flotation zone (24), at least one set of chemically stable/inert electrode (10) residing in the flotation zone (24) is used. Such electrodes (10) may comprise an anode and a cathode placed in parallel arrangement with each other and the water as shown in Figures 3 and 5. DC is applied to the electrodes, which can be in the form of stainless steel (For example, SS316) plates, titanium plates, titanium coated with rare metal oxide and/or rare metal oxide based titanium plates. Numerous fine gas bubbles are formed at the cathode(s) as a result of Reaction (3). The particles, sludge and contaminants in the wastewater coming from the reaction zone (23) are attach to the surface of these fine bubbles. Due to their lower density, the bubbles attached with contaminants float up to the surface of the wastewater. The resulting floating sludge comprising of the bubbles, oily matters, organic matters and other contaminants flow through the structure (32) located at the upper part of the reaction and flotation zones (23, 24) illustrated in the right view in Figure 4. The structure (32) has a downward inclined slope to facilitate the floating sludge to exit the reaction and flotation zones (23,24) by gravity to a collection point, which may be a tank (not shown in the figures), for further disposal or reuse. The sludge advantageously contains a high content of oily matters. Production of TRO in the flotation zone (24) may be possible when the anodes made of titanium or titanium coated with rare metal oxides (Ir, Ru, etc) are used in the flotation zone (24) according to Equations 2a and 2b. Organisms and micro-organisms may be killed as a result.

With reference to Figure 4, which illustrates left and right views of the treatment system (100), the structure includes a plate (31 ) used to block the floating sludge so that it can appropriately drain to a collection point through the inclined structure (32). As such, the sludge can be collected a storage tank for reuse or disposal.

The wastewater then flows towards the final settling tank (22), through an area (1 1 ), upwards through chamber (12) and downwards through chamber (13). Thereafter, the water moves upwards to the top of the wastewater through inclined plates (141 ) with a gap of 20- 40 mm between the inclined plates (141 ) so that heavy and large sludge can be further removed by the inclined plates (141 ). The treated water with lesser solids then flows to a left direction of an upper level (14) of the final settling tank (22) and reaches an effluent weir (15). Thereafter, the treated water finally flows through an outlet valve (16).

System Control

Hydraulic retention time (HRT), electrode construction and connection, and control in

DC supply are the most important for the success of the operation of the EAFT. Typically, one can achieve 80 to 95% removal for oil and COD for oily wastewater containing 1000 to 20,000 mg/L COD. The treated effluent may have oil content as low as 1 to 5 mg/L.

HRT is calculated as: HRT = volume of tank/flow rate of water. It should be controlled at above 2 hours for treating the oily wastewater. This can be done via controlling flow rate of incoming wastewater.

Figure 6 illustrates the construction of a metal plate of the electrodes (7) used in reaction zone (23) as follows. A casing (601 ) made of metals such as Ti, titanium coated with rare metal oxide, and/or stainless steel (for example. SS316) is chemically stable in wastewater and is a good electrical conductor. Such a casing (601 ) is used to house aluminium or iron plates (602). In the example given in Figure 6, the casing (601 ) is like a photo frame made by six chemically stable metal plates (601 ) to house the Al and/or Fe plates (602). The Al and/or Fe plate (602) is placed between the chemically stable metal plates (601 ); all these metal plates are tighten up together through screw (604) and nuts (603), similar to "sandwiches". The internal metal plate of Al and/or Fe is replaced by the chemically stable metal plates (such as Ti and stainless steel) (602) at the upper part of the electrode, which is connected to the rectifier (98) according to the arrangement given in Figure 7. This upper part of electrode is exposed to air. The Al and Fe plates in the present example have a size of less than 500 mm x 500 mm. Thickness is 10 to 20 mm. Such construction is beneficial as it can cause better distribution of electrons in the said reactions to save energy, and saving in electrode materials (602). The metal frame (601 ) can be reused.

The wirings of the electrodes (7) used in reaction zone (23) described earlier is illustrated in Figure 7. Figure 7 shows a side view of the electrodes (7) arranged parallel to one another. As shown in Figure 7, positive polar of rectifier (98) is applied to the anode electrode (1001 ). Due to inductive effect, the electrode (1002) next to the anode (1001 ) is cathode (1002); the electrode (1003) is anode (1003) while the electrode (1004) is cathode (1004) and so on. Similarly, the negative polar of rectifier (98) is connected to a cathode electrode (2001 ) and the electrode (2002) next to the cathode electrode (2001 ) is anode (2002), the electrode (2003) is cathode (2003) and the electrode (2004) is anode (2004). The same concept can be applied for all the electrodes. Each of the electrodes (7) has the construction shown in Figure 6. Typically, multiple sets of the electrodes (7) are needed for treatment of wastewater with flow rate above 20 L/hour. In the present example, each set of electrodes contains no more than eight metal plates (i.e. the construction shown in Figure 6) with a gap of 10-20 mm between the eight metal plates. Individual sets of the electrodes are electrically connected in parallel to one another. The figure shows the polarity of one arrangement. When the rectifier (98) switches the polarity, the polarity of electrodes (7) is changed accordingly. For example, the cathode (2001 ) becomes an anode and similarly the anode (1001 ) becomes a cathode. Such electrodes connection illustrated in Figure 7 can ensure high performance in the wastewater treatment while using lesser energy.

Two or four pieces of metal plates or meshes made of stainless steel (For example. SS316) plates, titanium plates, titanium coated with rare metal oxide and/or rare metal oxide based titanium plates [10] are used for generation of gas bubbles in flotation zone (24). They are in parallel to each other and water flow shown in Figures 3 and 5. The wiring arrangement is: positive, then negative and postive and finally negative. Except the last piece at the bottom, small holes of 5-10 mm are punched on the metal plates (10) shown in Figure 5 so that the bubbles can be released through the holes and float to the water surface. No polarity switching is needed.

In the reaction zone (23), periodic polarity change once every several seconds [e.g. 10- 60 second] in the rectifier (98) is performed. The fouling of electrodes in the reaction zone (23) can be avoided as a result.

Supporting information

Design of a prototype system for treatment of oily waste generated from a shipyard

A pilot-scale or prototype system having a construction similar to the treatment system described earlier with reference to Figures 1 to 7 was used to treat oily wastewater generated from a shipyard. The flow rate of the wastewater was 100 to 200 L/hour (i.e. Liter/hour). Carbon steel was used as an electrode material in the reaction zone. The COD of influent waste was 6920 to 20900 ppm. and the COD of the treated water was 450 - 900 ppm, as shown in Table 1 . The O&Gs content in the treated water was less than 10 ppm. The pH of the treated water was 7, unchanged from that of the untreated wastewater. The concentration of each of Cr, Cd, Co, Cu, Mn, Ni, and Pb was less than 1 ppb levels in the treated wastewater and the concentration of Fe and Zn were 1 .5 and 1 ppm, respectively in the treated wastewater. The applied voltage and current were: 18 V and 100 A in the reaction zone and the applied voltage and current were 5 V and 20 A in the flotation zone.

Table 1

No influent, Efflue it, COD, mg/L Removal, %

COD, mg/L 1 2 3

1 6930 900 890 850 87

2 14500 870 840 780 94

3 6920 780 820 694 89

4 16200 729 701 692 96

5 13140 520 670 512 96

6 10023 788 695 750 93

7 14200 910 920 900 96 Key Features

With reference to Figures 1 to 7, some key features of examples of the system, apparatus and method of the technology discussed herein for wastewater are as follows.

1 . Relatively rapid switching in polarities of DC from rectifier (98) supplied to the electrodes (7) periodically, for instance, in every 10 to 60 seconds can effectively ensure that the treatment system (100) can be operated for a longer period of time without fouling of the electrodes (7), which leads to less energy and material saving.

2. The design of the electrodes (7) illustrated in Figure 6 can ensure that the electrodes can be fully utilized and no waste of the electrode material is expected.

3. The distance between an anode and a cathode of the electrodes (7) is 10 to 20 mm for effective removal and recovery of oily substances from such industries of shipyard.

4. Wirings and connections of the electrodes (7) are arranged according to the configuration shown in Figure 7.

5. Inclined plates in settling tank are 60 degrees from an axis parallel to direction of wastewater flow in the reaction and flotation zones (23, 24). The gap between each inclined plates is 20-40mm to aid filtering of particles in the wastewater.

6. The technology can separate concentrated oil and water with little or no oil from wastewater. The concentrated oil is suitable for various applications such as production of biodiesel; it can also be used as an additive to fuel such as heavy fuel in ship for enhancements in combustion efficiency. The water with little or no oil can be used for washing and many other purposes.

7. The COD of in the treated wastewater can be as low as 600 ppm and the oil content in the treated wastewater can be less than 15 ppm and typically below 10 ppm.

8. The technology is suitable for the treatment of oily wastewater from different sources such as ship waste discharges, places for ship repair, places for food processing, food court, places handling machinery, places undergone cleaning, etc.

9. Living organisms can be killed by the technology when at least one pair of metal plates made of titanium or titanium coated with rare metal oxides (Ir, Ru, etc). In addition, the treated water will not create any opportunity for regrowth of organisms.