CA2921749A1

CA2921749A1 - Method and system for removing hydrogen sulfide from wastewater

Info

Publication number: CA2921749A1
Application number: CA2921749A
Authority: CA
Inventors: James R. Ward; James R. Warren
Original assignee: 0984767 Bc Ltd
Current assignee: 2266725 Alberta Ltd
Priority date: 2016-02-23
Filing date: 2016-02-23
Publication date: 2017-08-23

Abstract

Method and system for removing hydrogen sulfide from wastewater. Nanobubble-containing fluid is first produced and mixed with wastewater to oxidize sulfide to sulfur, sulfite, and sulfate. An electrocoagulation unit is then used to remove the sulfur, sulfite, and sulfate from the system. The nanobubble-containing fluid is produced by introducing water into pressurized vessel through an atomizer nozzle, where oxygen dissolves in water droplets at a pressure higher than the atmospheric pressure, and then ejecting through a confined channel which is less than 50 mm in at least one dimension without causing a turbulent flow.

Description

METHOD AND SYSTEM FOR REMOVING HYDROGEN SULFIDE FROM
WASTEWATER
FIELD OF THE DISCLOSURE
[0001] The present disclosure is related generally to a method and a system for removing hydrogen sulfide (H2S) from wastewater.
BACKGROUND INFORMATION

[0002] Hydrogen sulfide can be produced through anaerobic decay of organic matter in water. In typical wastewater produced either during municipal wastewater treatment or during industrial processes such as exploration and processing of oil and gas, microbial reduction of sulfate ion is the primary mechanism for formation of hydrogen sulfide. In the absence of dissolved oxygen, sulfate-reducing bacteria such as Desulfovibrio desulfuricans convert the sulfate ion into sulfide, which may exist in three forms: hydrogen sulfide gas (H2S), non-volatile HS-, and non-volatile S2-. The ratio of these three species is dependent on the pH. For example, at pH 6, 90% of the sulfide will be present as H2S; while at pH 10, almost 100% of the sulfide will be present as S2-. In some industries, such as the oil and gas industry, H2S-containing water has been referred to as "sour water".

[0003] Hydrogen sulfide is a dense, colorless, and acutely toxic gas, recognized by its characteristic rotten egg odor. Presence of sulfide in wastewater creates challenges for wastewater treatment operations, including concerns on worker safety and corrosion problems for metal and concrete. It is a leading cause of death among works in sanitary sewer systems. Even at low concentrations in air, exposure to hydrogen sulfide has been linked to fatigue, headaches, eye irritation, sore throats, and other health problems.

[0004] Sulfide control methods can be categorized into two major approaches:
(i) prevention of sulfide formation and (ii) removal of sulfide after it is formed.
Sulfide formation may be prevented by inhibiting bacterial action, which typically involves addition of chemicals, such as chlorine dioxide and nitrate, to the system. Methods of prior art for removal of sulfide after it has been formed typically employ chemicals, sometimes known as H2S scavengers, to oxidize the hydrogen sulfide to sulfur, sulfite, and/or sulfate. These chemicals include sodium chlorite (NaC102), hydrogen peroxide (H202), sodium nitrate (NaNO3) or calcium nitrate (Ca(NO3)2), and iron salts (including ferrous salts and ferric salts). Drawbacks of adding chemicals to the system, however, include high operational cost of the wastewater treatment process and potential introduction of secondary contamination to the environment.

[0005] A major challenge of treating H2S-containing wastewater by the methods of prior art, particularly in the oil field, is that the level of H2S
in the wastewater comes back within several days after the treatment. This significantly impairs the performance of wastewater treatment processes, increases the operational cost, and poses serious safety concerns. Methods and systems that are able to permanently remove H2S from wastewater at low cost are highly demanded.

[0006] The present disclosure provides a method and a system that is able to permanently remove hydrogen sulfide from wastewater at a low cost. The system is able to reduce H2S concentration in water from hundreds of ppm down to non-detectable within seconds, and the H2S does not come back over time. The system only employs oxygen or air, and no H2S scavengers or chemicals are needed. The present disclosure is particularly advantageous for removal of H2S in large-scale applications such as in municipal wastewater treatment and in oil and gas industries.
SUMMARY OF THE DISCLOSURE

[0007] The present disclosure provides a method and a system for removing hydrogen sulfide from wastewater.

[0008] The method for removing hydrogen sulfide from wastewater can comprise 2 steps, which each can include a plurality of sub-steps, or be combined with additional steps (before or after). In the first step, a nanobubble-containing water stream is injected into the wastewater where hydrogen sulfide is oxidized into sulfur, sulfite, and/or sulfate. In the second step, the wastewater treated by the first step process is introduced into an electrochemical cell where the sulfur, sulfite, and/or sulfite are coagulated and removed from the water by applying direct-current (DC) electric field across the electrochemical cells.

[0009] In some embodiments of the present disclosure, the nanobubble-containing water stream used in the first step contains oxygen and/or air. The dissolved oxygen level in the nanobubble-containing water stream is between ppm and 2000 ppm, preferably between 100 and 1000 ppm, arid more preferably between 200 and 600 ppm.

[0010] In some embodiments of the present disclosure, the nanobubble-containing water stream used in the first step is produced by the following sub-steps. First, gas (oxygen and/or air) and water is injected into a pressure vessel at a pressure higher than atmospheric pressure, where water is atomized and gas dissolves in water. Second, the gas-containing water is sent from the pressure vessel to an array of capillary tubes. Third, the gas-containing water is ejected from the capillary tube to an opening environment with a lower pressure than that in the capillary tube without causing a turbulent flow.

[0011] In some embodiments of the present disclosure, the absolute pressure under which the gas and water are mixed in the first sub-step to make gas-containing water ranges from about 0.1 MPa to about 100 MPa, more preferably from about 0.2 MPa to about 20 MPa, and still more preferably from about 1 MPa to about 3 MPa.

[0012] In some embodiments of the present disclosure, the diameter of the capillary tube used in the second sub-step to make gas-containing water ranges from about 1 micrometer to about 50 mm, more preferably from about 100 micrometer to about 10 mm, and still more preferably from about 1 mm to about 5 mm.

[0013] In some embodiments of the present disclosure, the gas-containing water is ejected from the capillary tube to an opening environment with lower pressure than that in the capillary tube without causing a turbulent flow. The Reynolds number of the fluid flowing inside the capillary tube is less than 2x105, preferably less than 5x104, and still more preferably less than 5x103.

[0014] In some embodiments of the present disclosure, the DC current density in the electrochemical cell used in the 2nd-step treatment process ranges from about 1 to about 1,000 A/m2, more preferably from about 10 to about 200 A/m2, and still more preferably from about 50 to about 150 A/m2.

[0015] In some embodiments, as water passes through the electrodes, sulfate and sulfite ions dissolved in water may change from a dissolved state to a suspended state, and charged ions are introduced into water from the electrodes, which neutralize the charge on the surfaces of the suspended solids including the sulfur, sulfite, and sulfate, as well as the bacteria in the water.
The charge neutralization causes these contaminants to coagulate.

[0016] In some embodiments, oxygen and hydrogen gas may form during electrocoagulation, causing the coagulated contaminants to rise to the surface of water. As the DC electrical current passes through water in the electrocoagulation system, reactive oxygen species may be produced, which may kill the bacteria, particularly the sulfate-reducing bacteria in water that are responsible for converting the sulfate ion into sulfide.

[0017] The system for removal of hydrogen sulfide from wastewater can comprise 2 major units and multiple mixing, retention, and separation tanks.
The 2 major units are a nanobubble-generating unit and an electrocoagulation unit.

[0018] The nanobubble-generating unit can comprise 2 major components.
The first component can comprise a pressurized vessel where liquid and gas are mixed and gas is dissolved into liquid under a pressure higher than atmospheric pressure. The second component can comprise a hose with a delivery nozzle which comprises one or more capillaries or plates that form channels with at least one dimension less than 1 mm.

[0019] In some embodiments of the present disclosure, the liquid is introduced into the pressurized vessel through an atomizer nozzle in the first component of the system. The liquid passing through the nozzle forms droplets within the pressurized vessel, and the gas diffuses into the droplets.

[0020] In some embodiments of the present disclosure, the liquid introduced into the pressurized vessel may be water from a clean water supply such as a tank, pond, lake, stream, or river. In another embodiment of the present disclosure, the liquid introduced into the pressurized vessel may also be treated wastewater recycled after the treatment by the electrochemical cells.

[0021] In some embodiments of the present disclosure, the gas introduced into the pressurized vessel may be oxygen from an oxygen gas supply assembly such as an oxygen cylinder, an oxygen separator that separates oxygen from air, or an oxygen generator. In another embodiment of the present disclosure, the gas introduced into the pressurized vessel may be air.

[0022] In some embodiments of the present disclosure, the capillaries used in the delivery nozzle in the second component of the nanobubble-generating unit can be tubes with the internal diameter of the capillaries ranging from about micrometer to about 50 mm, more preferably from about 100 micrometer to about 10 mm, and still more preferably from about 1 mm to about 5 mm.

[0023] In some embodiments of the present disclosure, the delivery nozzle used in the second component of the nanobubble-generating unit can be plate-based nozzles that include one or more plates having a plurality of channels formed therein. The cross-sectional profile of the channels may be a variety of shapes including circular, square, rectangular, oval, and triangular, with at least one dimension less than 50 mm, ranging from about 1 micrometer to about 50 mm, more preferably from about 100 micrometer to about 10 mm, and still more preferably from about 1 mm to about 5 mm.

[0024] In some embodiments of the present disclosure, the capillaries or the plates used in the delivery nozzle in the second component of the nanobubble-generating unit can be made of silica or silicate glass. Alternatively, the capillaries or the plates used in the delivery nozzle in the second component of the system may be constructed by using a variety of metals, metal alloys, glasses, plastics, polymers, ceramics, and other suitable materials.

[0025] In some embodiments of the present disclosure, the electrochemical cell used in the electrocoagulation unit of the treatment system consists of a cylindrical cathode made of copper sheet or aluminum sheet and an anode made of aluminum rod placed inside the cylindrical cathode. Both the anode and the cathode are placed vertically and water flows from the bottom to the top of the cells.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] FIG. 1 is a schematic illustration of the two major water treatment steps for removal of hydrogen sulfide for some embodiments of the present disclosure.

[0027] FIG. 2 is a schematic illustration of the two major water treatment steps for removal of hydrogen sulfide, where the feed water to the nanobubble-generating unit is recycled after the 2nd treatment step.

[0028] FIG. 3 is a schematic illustration of a confined space that has channels formed between plates.

[0029] FIG. 4 is a schematic illustration of the contact angle of a liquid on a solid.

[0030] FIG. 5 is a schematic illustration of a system for producing nanobubbles and nanobubble-containing liquids.
DETAILED DESCRIPTION

[0031] As used herein in the specification and claims, including as used in the examples and unless otherwise expressly specified, all numbers may be read as if prefaced by the word "about", even if the term does not expressly appear, unless otherwise expressly stated. Also, any numerical range recited herein is intended to include all sub-ranges subsumed therein.

[0032] In the present description, where used or otherwise designated to apply as described above, the terms "about" and "consisting essentially of' mean 20% of the indicated range, value, or structure, unless otherwise indicated.
It should be understood that the terms "a" and "an" as used herein refer to "one or more" of the enumerated components. The use of the alternative (e.g., "or") should be understood to mean either one, both, or any combination thereof of the alternatives, unless otherwise expressly indicated. As used herein, the terms "include" and "comprise" are used synonymously, and those terms, and variants thereof, are intended to be construed as non-limiting unless otherwise expressly stated.

[0033] In the following description, certain specific details are set forth in order to provide a thorough understanding of various embodiments of the disclosure. However, upon reviewing this disclosure one skilled in the art will understand that the disclosure may be practiced without many of these details. In other instances, well-known structures, systems and methods in the relevant fields have not been described in detail to avoid unnecessarily obscuring the descriptions of the embodiments of the disclosure.

[0034] Whenever the terms, for example," "such as," or variants thereof are used herein, the provided examples are assumed to be without limitation or restriction, unless otherwise expressly indicated.

[0035] The present disclosure provides a method and a system that is able to permanently remove hydrogen sulfide from wastewater at a low cost. The system is able to reduce H2S concentration in water from hundreds of ppm down to non-detectable within seconds, and the H2S does not come back over time.

[0036] The method for removing hydrogen sulfide from wastewater comprises 2 major steps, as shown in Figure I. In the first step 100, a nan.obubble-containing water stream is first produced by a nanobubble-generating unit 110 that dissolves oxygen or air into water droplets, and then injected into the wastewater where hydrogen sulfide is oxidized into sulfur, sulfite, and/or sulfate. In the second step 120, the wastewater treated by the first step process 100 is introduced into an electrochemical cell where the sulfur, sulfite, and/or sulfite are coagulated and removed from the water by applying DC
electric field across the electrochemical cells.

[0037] In some embodiments, the water that feeds into the nanobubble-generating unit 110 is recycled after the 2nd-step treatment 120 by the electrochemical cells, as shown in Figure 2.

[0038] A key component of the disclosed method and system is the nanobubble-generating unit which dissolves oxygen or air into water droplets and creates a nanobubble-containing water stream. The nanobubbles play an important role in oxidizing the hydrogen sulfide into sulfur, sulfite, and/or sulfate. The advantage of the nanobubbles and the mechanism that underlines this process is described below.

[0039] Bubbles are gas-filled cavities in liquid. The size of bubbles plays an important role on processes that involve multiphase flow in many industrial applications, including aeration, wastewater treatment, oil and gas exploration, and petrochemical production. Bubbles can be produced by vigorous mixing of gas and liquid and the size of bubbles can vary in a wide range. Based on the length scale, bubbles of different sizes have been known as millimeter-sized bubbles, micrometer-sized bubbles (also known as "microbubbles"), and nanometer-sized bubbles (also known as "nanobubbles"). The size of these bubbles has significant effect on heat and mass transfer, chemical reaction kinetics, and thermodynamic equilibrium at the gas-liquid interface. Reducing the size of bubbles may facilitate heat and mass transfer, significantly change the thermodynamic equilibrium, and accelerate chemical reactions taking place at the gas-liquid interface.

[0040] Nanometer-sized bubbles (nanobubbles) have significant advantages compared to micrometer-sized, millimeter-sized, and larger size bubbles. The specific surface area of bubbles per unit volume is in inverse proportion to the size of the bubbles (L. Albright, Albright's Chemical Engineering Handbook, CRC Press, 2008). Therefore, the specific surface area of nanometer-sized bubbles can be 1,000 times larger than micrometer-sized bubbles, and can be 1 million times larger than millimeter-sized bubbles. Larger specific surface area means larger interface area between gas and liquid, leading to higher heat and mass transfer rate.

[0041] The rising speed of bubbles is proportional to the square of the size of the bubbles (D. G. Karamanev, AIChE 1 40(8), 1418, (1994)). Therefore, the rising speed of nanometer-sized bubbles can be 1 millionth of the micrometer-sized bubbles and can be a trillionth of the millimeter-sized bubbles. Because of the low buoyance, bubbles of less than 1 micrometer in diameter are subject to random Brownian motion. As a result, gas stays in liquid for a very long time without off-gas, resulting in complete usage of gas and high mass transfer efficiency.

[0042] The gas pressure that the bubble is able to sustain without breaking the bubble due to the surface tension at the gas-liquid interface is inverse proportional to the size of the bubble (J. Holocher, F. Peeters, W. Aeschback-Hertig, W. Kinzelback, R. Kipfer, Enuiron. Sci. Technol. 37, 1337, (2003)).
Therefore, the pressure that nanobubble can keep can be 1,000 times higher than microbubles and can be 1 million times higher than millimeter bubbles.
Higher gas pressure inside the bubble leads to higher mass transfer rate.

[0043] The nanometer-sized bubbles may produce reactive oxygen species (T.
L. Hwang, C. L. Fang, S. A. Al-Suwayeh, L. J. Yang, J. Y. Yang, Toxicol Lett.
203(2), 172, (2011)) that may degrade organic contaminates in water. The highly reactive free radicals also break the emulsions. Nanometer-sized bubbles can coalesce emulsified oil droplets as small as the nanobubbles (< 1 m), which would be very difficult for microbubbles or millimeter bubbles to coalesce.

[0044] Nanobubbles have been made in research laboratories by electrolysis (K. Kikuchi, Y. Tanaka, Y. Saihara, M. Maeda, M. Kawamura and Z. Ogumi, J.
Colloid Interface Sci. 298, 914-919 (2006); K. Kikuchi, S. Nagata , Y. Tanaka, Y. Saihara, Z. Ogumi, J. Electroanal. Chem. 600, 303-310 (2007); K. Kikuchi, A. Ioka, T. Okua, Y. Tanaka, Y. Saihara and Z. Ogumi, J. Colloid Interface Sci.
329, 306-309 (2009)), and by introduction of gas into liquid at a high shear rate (K. Ohgaki, N. Q. Khanh, Y. Joden, A. Tsuji and T. Nakagawa, Chem. Eng.
Sci. 65, 1296-1300 (2010)). Nanobubbles have also been made for research purposes by using surface active agents such as surfactants and by using ultrasonication (Z. Xing, J. Wang, H. Ke, B. Zhao, X. Yue, Z. Dai, and J. Liu, Nanotechnology 21, 14 (2010)). They have been used as ultrasound contrast agents and for targeted drug delivery (S. Sirsi and M. Borden, Bubble Sci.
Eng.
Technol. 1, 3 (2009)).

[0045] A number of patents and patent applications are related to generation of micro/nano-bubbles based on a swirl-type generator. PCT Application W02009136336 disclosed a method for generating bubbles for biomedical applications, e.g., for acoustic imaging, which comprises driving the bubble fluid through vessels, repeating the driving fluid step in order to produce the bubbles together with undesirable components, e.g., foam, and removing the components. PCT Application W02010055701 disclosed a micro/nano-bubble generating mechanism for a hot/cold water circulation type bathtub unit, which restricts water flow by a restriction gap so that water flows through a restriction gap at a flow rate that is lower than that of the water flowing through the rerouting flow path. US Patent US20100080759 disclosed a method for forming nano-bubbles, useful in pharmaceutical industry, e.g. as drug carriers, which comprises performing a polymer coating process with inorganic particles as nuclei, contacting composite particles with solvent, and performing a freeze-drying process and a dissolution process.US Patent Application No. 12/574,949 disclosed a method to generate nanobubble-containing liquid by first generating microbubbles and then introducing microbubble-containing liquid into a nanobubble-generating tank.US Patent Application No. 10/591,977 disclosed a method for forming nanobubbles by applying physical irritation to microbubbles contained in a liquid so that the microbubbles are abruptly contracted to form nanobubbles. PCT Application No. PCT/CA2014/050957 disclosed a method for generating nanobubble-containing liquid by using a series of at least two sequential cavitation zones, without using external air or gas.

[0046] Potential uses of nanobubble-containing liquid have been identified for a variety of industries, including wastewater treatment, fishery, medicine, agriculture, and household amenities. Some of these uses have been disclosed in a number of patents and patent applications. European Patent EP2181612 disclosed a method for processing foodstuffs, e.g., beer or fruits, by contacting foodstuffs containing microorganisms or enzymes with carbon dioxide microbubbles in a pressure-resistant container and sterilizing microorganisms or deactivating enzymes in food. US Patent US7628912 disclosed a method of using micro/nano-bubble containing liquid for cleaning, e.g., building a drainage system involves controlling a blower, which supplies air to a submersible-pump type micro/nano-bubble generator based on a turbidity detection signal. US Patent Application US20090145827 disclosed a water treatment system for use in a chip fabrication factory, through generation of micro/nano-bubbles by controlling the rotational speed of the circulating pump, if the specific value of the anaerobic measuring tank exceeds a predetermined range.US Patent Application US20090051055 disclosed a method for producing drinkable water in which an integrated bubble generating portion is coupled to a tap to generate nano-bubble water only with subsistence water being physically shattered a few times the power portion. US

Patent Application US20090250396 disclosed a method for treatment of wastewater of a chip fabrication factory, which involves adding microorganisms, a micro/nano-bubble generating support agent, and nutrients to water, feeding this water to a carbon tower, and decomposing fluorine compounds in the water.US Patent Application US20090029041 disclosed a cleaning method for a magnetic-recording medium used in a hard disk drive of a computer, which involves cleaning the substrate face of a magnetic-recording medium using nanobubble-containing water.

[0047] One embodiment of the present disclosure is the method for producing nanobubble-containing liquids, which can comprise 3 steps. In the first step, gas and liquid are mixed at a pressure higher than atmospheric pressure, and gas dissolves in the liquid at a pressure higher than atmospheric pressure. In the second step, the gas-containing liquid is sent to a confined space which is less than 50 mm in at least one dimension. In the third step, the gas-containing liquid is ejected from the confined space to an opening environment with lower pressure than that in the confined space without causing a turbulent flow.

[0048] The purpose of the first step is to dissolve gas into liquid and reach a gas concentration higher than the saturation concentration of the gas in the liquid at ambient pressure. Therefore, in some embodiments of the present disclosure, the absolute pressure under which the gas and liquid are mixed in the first step ranges from about 0.1 MPa to about 100 MPa, more preferably from about 0.2 MPa to about 20 MPa, and still more preferably from about 1 MPa to about 3 MPa.

[0049] In the second step, the gas-containing liquid is sent to a confined space which is less than 50 mm in at least one dimension. The confined space serves as a capillary channel to remove existing cavitation nuclei and bubbles in the liquid as well as to prevent formation of cavitation nuclei and bubbles before and during the ejection the liquid to the environment at a lower pressure. In some embodiments of the present disclosure, the confined space used in the second step has at least one dimension less than 50 mm, ranging from about 1 micrometer to about 50 mm, more preferably from about 100 micrometer to about 10 mm, and still more preferably from about 1 mm to about 5 mm.

[0050] In some embodiments of the present disclosure, the confined space used in the second step can be capillary tubes with the internal diameter of the capillaries ranging from about 1 micrometer to about 50 mm, more preferably from about 100 micrometer to about 10 mm, and still more preferably from about 1 mm to about 5 mm. Multiple capillary tubes can be assembled together as a bundle.

[0051] In some embodiments of the present disclosure, the confined space used in the second step can have channels 310 formed between plates 320, as shown in Figure 3. The cross-sectional profile of the channels may be a variety of shapes including circular, square, rectangular, oval, and triangular, with at least one dimension less than 50 mm, ranging from about 1 micrometer to about 50 mm, more preferably from about 100 micrometer to about 10 mm, and still more preferably from about 1 mm to about 5 mm.

[0052] In some embodiments of the present disclosure, the capillaries and the channels involved in the second step need to allow the liquid to wet. This is to prevent formation of cavitation nucleus inside the capillaries and the channels.
The wettability of a liquid on a solid can be characterized by the contact angle 0. Figure 4 schematically shows the contact angle of a liquid on a solid. When the solid is flat and smooth, the contact angle is referred as the intrinsic contact angle 0, which can be correlated to the surface free energy at the solid-liquid interface (ysL), the surface free energy at the liquid-vapor interface (yLv), and the surface free energy at the solid-vapor interface (ysv) by the Young's equation: cos 9 = rsv-YsL
In the present disclosure, the capillaries and the YLv channels need to be hydrophilic with an intrinsic water contact angle of less than 60 , more preferably less than 40 , and still more preferably less than 30 . The small contact angle prevents formation of cavitation nucleus inside the capillaries and the channels.

[0053] In the third step, the gas-containing liquid is ejected from the confined space to an opening environment with lower pressure than that in the confined space without causing a turbulent flow. This is because turbulent flow may induce vortices which may initiate heterogeneous cavitation. When the bubbles are ejected from the confined space into an open environment, the flow around the bubbles may be characterized into laminar, transition, or turbulent flow. In fluid mechanics, a dimensionless quantity known as the Reynolds number (Re) is used to predict flow patterns in different flow regimes.
pVD
Reynolds number for flow around the gas bubble is calculated as: Re where p is the density of the fluid, D is the diameter of the bubble, V is the characteristic velocity (defined as the velocity of the sphere relative to the fluid some distance away from the sphere such that the motion of the sphere does not disturb that reference parcel of fluid), and p is the viscosity of the fluid. In one embodiment of the present disclosure, the Reynolds number of the fluid flowing around the ejected gas bubble is less than 2x105, preferably less than 5x104, and still more preferably less than 5x103. Small Reynolds number prevents formation of vortices that may initiate heterogeneous cavitation and growth of large bubbles.

[0054] After the hydrogen sulfide is oxidized into sulfur, sulfite, and/or sulfate in the first step 100, the wastewater is treated by the 2nd-step process 120, where sulfur, sulfite, and/or sulfite are coagulated and removed from the water by applying DC electric field across the electrochemical cells.

[0055] Depending on the pH of the effluent of the first-step treatment process 100, the pH of the water may be adjusted by an acid or a base before it is introduced into the electrocoagulation unit. An example base for adjusting pH
is sodium hydroxide. An example acid for adjusting pH is hydrogen chloride.
This pH adjustment step is optional and when the pH of the feed water is satisfactory, the water can be directed into the electrocoagulation unit without being subjected to the pH adjustment process.

[0056] In some embodiments, the electrocoagulation system can contain metal electrodes energized by a DC electrical current. As the contaminated water passes through the electrodes, charged ions are introduced into water from the electrodes, which neutralize the charge on the surfaces of the suspended solids including the sulfur, sulfite, and sulfate, as well as the bacteria in the water.
The charge neutralization causes these contaminants to coagulate, as will be appreciated by those skilled in the art after reviewing this disclosure. These suspended solids coagulate upon neutralization of the charges on their surfaces.

[0057] In some embodiments, as water passes through the electrodes, sulfate and sulfite ions dissolved in water may change from a dissolved state to a suspended state. Oxygen and hydrogen gas may form during electrocoagulation, causing the coagulated contaminants to rise to the surface of water.

[0058] In some embodiments, as the DC electrical current passes through water in the electrocoagulation system, reactive oxygen species (ROS) broadly defined as oxygen-containing reactive chemical species, including singlet oxygen, superoxide anions, and hydroxyl radicals may be produced. The reactive oxygen species may kill the bacteria, particularly the sulfate-reducing bacteria in water, which are responsible for converting the sulfate ion into sulfide.

[0059] In some embodiments of the present disclosure, the DC current density in the electrochemical cell used in the 211d-step treatment process ranges from about 1 to about 1,000 A/m2, more preferably from about 10 to about 200 A/m2, and still more preferably from about 50 to about 150 A/m2.

[0060] Based on the abovementioned method for removal of hydrogen sulfide from water, the system for implementing such a method can comprise 2 major units and multiple mixing, retention, and separation tanks. The 2 major units are a nanobubble-generating unit and an electrocoagulation unit.

[0061] The nanobubble-generating unit can comprise 2 major components, as schematically shown in Figure 5. The first component can comprise a pressurized vessel 510 where liquid and gas are mixed and gas is dissolved into liquid under a pressure higher than atmospheric pressure. The second component can comprise a hose 520 with a delivery nozzle 530 which comprises one or more capillaries or plates that form channels with at least one dimension less than 50 mm.

[0062] In some embodiments of the present disclosure, the liquid is introduced into the pressurized vessel 510 through an atomizer nozzle 540 in the first component of the system. The liquid passing through the nozzle forms droplets 550 within the pressurized vessel, and the gas diffuses into the droplets.

[0063] In some embodiments of the present disclosure, the liquid introduced into the pressurized vessel 510 may be water from a clean water supply such as a tank, pond, lake, stream, or river. In another embodiment of the present disclosure, the liquid introduced into the pressurized vessel 510 may also be recycled wastewater.

[0064] In some embodiments of the present disclosure, the gas introduced into the pressurized vessel 510 may be oxygen from an oxygen gas supply assembly such as an oxygen cylinder, an oxygen separator that separates oxygen from air, or an oxygen generator. In another embodiment of the present disclosure, the gas introduced into the pressurized vessel 510 may be air.

[0065] In some embodiments of the present disclosure, the capillaries used in the delivery nozzle 530 can be tubes with the internal diameter of the capillaries ranging from about 1 micrometer to about 50 mm, more preferably from about 100 micrometer to about 10 mm, and still more preferably from about 1 mm to about 5 mm.

[0066] In some embodiments of the present disclosure, the delivery nozzle 530 can be plate-based nozzles that include one or more plates having a plurality of channels formed therein. The cross-sectional profile of the channels may be a variety of shapes including circular, square, rectangular, oval, and triangular, with at least one dimension less than 50 mm, ranging from about 1 micrometer to about 50 mm, more preferably from about 100 micrometer to about 10 mm, and still more preferably from about 1 mm to about 5 mm.

[0067] In some embodiments of the present disclosure, the capillaries or the plates used in the delivery nozzle 530 can be made of silica or silicate glass.
Alternatively, the capillaries or the plates used in the delivery nozzle in the second component of the system may be constructed by using a variety of metals, metal alloys, glasses, plastics, polymers, ceramics, and other suitable materials.

[0068] In some embodiments of the present disclosure, the electrochemical cell used in the electrocoagulation unit of the treatment system consists of a cylindrical cathode made of copper sheet or aluminum sheet and an anode made of aluminum rod placed inside the cylindrical cathode. Both the anode and the cathode are placed vertically and water flows from the bottom to the top of the cells.
WORKING EXAMPLE

[0069] The following example is intended to be illustrative and should not be construed as limiting the disclosure in any way.

[0070] Clean water was introduced into a pressure vessel filled with oxygen at 2.1 MPa through an atomizer nozzle (3/16 stainless steel, 0.25 inch npt).
Water exiting nozzles formed a fog consisting of small water droplets, and was collected at the bottom of the vessel. Water exited the pressure vessel through a hose which was connected to a delivery nozzle comprising an array of steel capillary tubes with a diameter of 2.5 mm. The dissolved oxygen content measured at the exit of the delivery nozzle was 400-600 ppm. This dissolved oxygen content was almost 100 times higher than that in the water introduced into the pressure vessel.

[0071] This nanobubble-containing liquid was sent to mix with wastewater containing 650 ppm hydrogen sulfide at a flowrate of 15 gallon per minute.
The concentration of hydrogen sulfide in the wastewater was monitored and was observed to decrease to non-detectable within 10 seconds.

[0072] After mixing with the nanobubble-containing liquid, the wastewater was sent to an electrocoagulation unit. The electrochemical cell used copper sheet as the cathode and aluminum rod as the anode. After applying the DC
current at a current density of 100 A/m2, the liquid was sent to a gravity-separation tank, where the sulfur, sulfite, and sulfate produced in the first-step process was coagulated and floated to the top of the tank. The water at the bottom of the separation tank was collected. The concentration of hydrogen sulfide was monitored after 3, 6, 10, and 30 days, which remained non-detectable after 30 days.

[0073] Although specific embodiments and examples of the disclosure have been described supra for illustrative purposes, various equivalent modifications can be made without departing from its spirit and scope, as will be recognized by those skilled in the relevant art after reviewing the present disclosure. The various embodiments described can be combined to provide further embodiments. The described systems, devices and methods can omit some elements or acts, can add other elements or acts, or can combine the elements or execute the acts in a different order than that illustrated, to achieve various advantages of the invention. These and other changes can be made to the invention in light of the above detailed description.

[0074] In general, in the following claims, the terms used should not be construed to limit the disclosure to the specific embodiments disclosed in the specification.

Claims

What is claimed is:

1. A method for removing hydrogen sulfide from wastewater, comprising:
injecting a nanobubble-containing water stream into the wastewater; and introducing the mixture of wastewater and nanobubble-containing water stream into an electrocoagulation unit.

2. The method of claim 1, wherein the hydrogen sulfide is oxidized into sulfur, sulfite, and sulfate by mixing with nanobubble-containing water stream.

3. The method of claim 1, wherein the sulfur, sulfite, and sulfate are coagulated and removed by applying DC current across the electrochemical cells.

4. The method of claim 1, wherein the nanobubble-containing water stream contains oxygen and/or air, and the dissolved oxygen level is between 10 ppm and 2000 ppm.

5. The method of claim 1, wherein the method for producing nanobubbles and nanobubble-containing liquid comprises:
mixing gas and liquid a pressure higher than atmospheric pressure, and dissolving gas in the liquid at a pressure higher than atmospheric pressure; and sending the gas-containing liquid to a confined space; and ejecting the gas-containing liquid from the confined space to an opening environment.

6. The method of claim 5, wherein the absolute pressure under which the gas and liquid are mixed is 0.1 MPa to 100 MPa.

7. The method of claim 5, wherein the confined space has at least one dimension less than 50 mm, ranging from 1 micrometer to 50 mm, more preferably from about 100 micrometer to about 10 mm, and still more preferably from about 1 mm to about 5 mm.

8. The method of claim 5, wherein the confined space is capillary tubes with the internal diameter of the capillaries ranging from 1 micrometer to 50 mm, more preferably from about 100 micrometer to about 10 mm, and still more preferably from about 1 mm to about 5 mm.

9. The method of claim 5, wherein the confined space has channels formed between plates, and the cross-sectional profile of the channels are circular, square, rectangular, oval, or triangular, with at least one dimension less than 50 mm, ranging from 1 micrometer to 50 mm, more preferably from about 100 micrometer to about 10 mm, and still more preferably from about 1 mm to about 5 mm.

10. The method of claim 5, wherein the capillaries and the channels are hydrophilic with an intrinsic water contact angle of less than 60°, more preferably less than 40°, and still more preferably less than 30°.

11. The method of claim 5, wherein the gas-containing liquid is ejected from the confined space to an opening environment without causing a turbulent flow, characterized by the Reynolds number of the fluid flowing around the ejected gas bubble being less than 2x10 5, preferably less than 5x10 4, and still more preferably less than 5x10 3.

12. The method of claim 5, wherein the liquid introduced into the pressurized vessel is water from a clean water supply such as a tank, pond, lake, stream, or river.

13. The method of claim 5, wherein the liquid introduced into the pressurized vessel is recycled wastewater from wastewater treatment process.

14. The method of claim 1, wherein the DC current density in the electrochemical cell used in the 2nd-step treatment process ranges from about 1 to about 1,000 A/m2, more preferably from about 10 to about 200 A/m2, and still more preferably from about 50 to about 150 A/m2.

15. A system for removing hydrogen sulfide from wastewater, comprising:
a nanobubble-generating apparatus; and an electrocoagulation apparatus.

16. The system of claim 15, wherein the nanobubble-generating apparatus comprising:
a pressurized vessel where liquid and gas are mixed and gas is dissolved into liquid; and a hose with a delivery nozzle which comprises one or more capillaries or plates that form channels.

17. The apparatus of claim 16, wherein the liquid is introduced into the pressurized vessel through an atomizer nozzle, the liquid passing through the nozzle forms droplets within the pressurized vessel, and the gas diffuses into the droplets.

18. The apparatus of claim 16, wherein the gas introduced into the pressurized vessel is oxygen from an oxygen gas supply assembly such as an oxygen cylinder, an oxygen separator that separates oxygen from air, or an oxygen generator.

19. The apparatus of claim 16, wherein the gas introduced into the pressurized vessel is air.

20. The apparatus of claim 16, wherein the capillaries used in the delivery nozzle in the second component of the system are tubes with the internal diameter of the capillaries ranging from about 1 micrometer to about 50 mm, more preferably from about 100 micrometer to about 10 mm, and still more preferably from about 1 mm to about 5 mm.

21. The apparatus of claim 16, wherein the delivery nozzle used in the second component of the system is plate-based nozzles that include one or more plates having a plurality of channels formed therein.

22. The apparatus of claim 16, wherein the capillaries or the plates used in the delivery nozzle are made of silica or silicate glass.

23. The apparatus of claim 16, wherein capillaries or the plates used in the delivery nozzle are constructed by using a variety of metals, metal alloys, glasses, plastics, polymers, ceramics, and other suitable materials.

24. The apparatus of claim 16, wherein the cross-sectional profile of the channels is a variety of shapes including circular, square, rectangular, oval, and triangular, with at least one dimension less than 50 mm, ranging from about 1 micrometer to about 50 mm, more preferably from about 100 micrometer to about 10 mm, and still more preferably from about 1 mm to about 5 mm.

25. The apparatus of claim 15, wherein the electrochemical cell used in the electrocoagulation unit consists of a cylindrical cathode made of copper sheet or aluminum sheet and an anode made of aluminum rod placed inside the cylindrical cathode.

26. The apparatus of claim 15, wherein the anode and the cathode of the electrocoagulation unit are placed vertically and water flows from the bottom to the top of the cell.