Method and system for treatment of organic contaminants by coupling Fenton reaction with membrane filtration

Abstract

An organic contaminants treatment system comprises a first pH adjustment tank, a Fenton reaction tank, an H.sub.2O.sub.2 purging tank, a second pH adjustment tank, a holding tank, and a membrane tank. An organic contaminants treatment method couples Fenton reaction and membrane filtration.

Claims

1. An organic contaminants wastewater treatment method, comprising: receiving from an external source wastewater that contains organic contaminants; adjusting pH value of the received wastewater by acidic reagents and dosing the received wastewater with ferrous reagents; adding H.sub.2O.sub.2 into the pH-adjusted and ferrous-dosed wastewater for Fenton reaction; purging the H.sub.2O.sub.2 from the Fenton reaction-treated wastewater; adjusting the pH value of the H.sub.2O.sub.2-purged wastewater by a caustic reagent; wherein the pH value of the caustic reagent-treated wastewater is in the range of 7-10; thereby iron (III) becomes insoluble to form iron (III) complexes in the form of iron hydroxide; filtering the caustic reagent-treated wastewater by filtration membrane to produce filtrate (i.e. water) and retain the iron (III) complexes and solids within a membrane tank housing the microfiltration or ultrafiltration membrane modules; wherein the membrane is operated at Trans-membrane Pressure (TMP) in the range of 10-30 kPa; recirculating the iron (III) complexes and solids at a recirculation rate to the step of adjusting pH value of the received wastewater by acidic reagents and dosing the received wastewater with ferrous reagents; wherein recirculation rate is in range of 50%-700% wastewater flow rate; and wherein the wastewater flow rate is the flowrate of wastewater entering the step of adjusting pH value of the received wastewater by acidic reagents and dosing the received wastewater with ferrous reagents; acidic cleaning the microfiltration or ultrafiltration membranes using a mixture of acid and water, wherein the water is from the filtering step; and caustic cleaning the microfiltration or ultrafiltration membranes using a mixture of caustic reagent and water, wherein the water is from the filtering step.

2. The organic contaminants wastewater treatment method of claim 1, wherein the acidic reagent is any acid suitable for pH adjustment.

3. The organic contaminants wastewater treatment method of claim 1, wherein the pH value of the pH adjusted influents is in the range of 2-6.

4. The organic contaminants wastewater treatment method of claim 1, wherein the pH value of the pH adjusted influents is in the range of 3-4.

5. The organic contaminants wastewater treatment method of claim 1, wherein a range of chemical dosage ratios are used: $\frac{H_2{O}_2}{{\mathrm{COD}}_{\mathrm{influent}}}=0.1.fwdarw.5\mathrm{and}\frac{{\mathrm{Fe}}^{2+}}{H_2{O}_2}=0.1.fwdarw.5.$

6. The organic contaminants wastewater treatment method of claim 1, wherein the caustic reagent for adjusting the pH value of the H.sub.2O.sub.2-purged influent is sodium hydroxide.

7. The organic contaminants wastewater treatment method of claim 1, wherein the pH value of the caustic reagent-treated influent is in the range of 6-8.

8. The organic contaminants wastewater treatment method of claim 1, wherein the steps of acidic cleaning and caustic cleaning are performed sequentially.

9. The organic contaminants wastewater treatment method of claim 8, wherein the caustic reagent for the caustic cleaning is sodium hypochlorite and/or sodium hydroxide.

10. The organic contaminants wastewater treatment method of claim 1, further comprising air-scrubbing of the surface of the filtration membrane.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Preferred embodiments according to the present invention will now be described with reference to the Figures, in which like reference numerals denote like elements.

(2) FIG. 1 shows a functional block diagram of a conventional Fenton reaction oxidation system.

(3) FIG. 2 shows a functional bloc diagram of an organic contaminants treatment system in accordance with embodiments of the present invention.

(4) FIG. 3 shows a flow chart of the organic contaminants treatment method in accordance with embodiments of the present invention.

(5) FIG. 4 is a dot graph showing the plot of absolute COD removals at different Fenton reagent dosages.

(6) FIG. 5 is a graph showing the operating pressures of the organic contaminants treatment system under different membrane cleaning protocols.

(7) FIG. 6 is a graph showing the stability of operational pressures for the organic contaminants treatment system between the 9.sup.th-14.sup.th cleaning cycles.

DETAILED DESCRIPTION OF THE INVENTION

(8) The present invention may be understood more readily by reference to the following detailed description of certain embodiments of the invention.

(9) Throughout this application, where publications are referenced, the disclosures of these publications are hereby incorporated by reference, in their entireties, into this application in order to more fully describe the state of art to which this invention pertains.

(10) The present invention provides a system for treatment of organic contaminants. Briefly, the organic contaminants treatment system is capable of integrated two-stages treatment of organic contaminants, i.e. Fenton Oxidation stage and membrane-filtration stage. The filtration can be microfiltration or ultrafiltration. The organic contaminants treatment system leverages on the production of .OH hydroxyl radicals by the Fenton reaction to provide strong but non-discriminatory oxidative degradation of organic substances in the influent.

(11) Referring now to FIG. 2, there is provided a functional bloc diagram of the organic contaminants treatment system in accordance with embodiments of the present invention. As shown in FIG. 2, the organic contaminants treatment system 200 comprises a first pH adjustment tank 210, a Fenton reaction tank 220, an H.sub.2O.sub.2 purging tank 230, a second pH adjustment tank 240, a holding tank 250, a membrane tank 260, and a product water tank 270.

(12) The first pH adjustment tank 210 receives influent via an influent pump, where the received influent is from an external source (not shown) and contains organic contaminants. The exemplary influents that can be effectively treated by the present invention include wastewater streams from industrial processes, such as but not limited to petrochemical, tannery and pharmaceutical production. These wastewater streams have characteristically low biodegradability, which manifests as low biochemical oxygen demand (BOD) to chemical oxygen demand (COD) ratios (commonly known as B/C ratio) due to abundance of complex chemical structures in the influent. Then first pH adjustment tank 210 receives an acidic reagent via an acid metering pump, where the acidic reagent adjusts the pH value of the received influent. Acidic reagents that are suitable for the present invention include most commonly available acids in the industry. In certain embodiments, the pH value is in the range of 2-6, preferably in the range of 3-4. The first pH adjustment tank 210 also receives a ferrous reagent via a ferrous metering pump and undergoes thorough mixing. Ferrous sulphates are almost exclusively used as the ferrous reagent in the Fenton reaction, mostly due to its low costs and availability.

(13) The Fenton reaction tank 220 is fluidly coupled with the first pH adjustment tank 210, and receives the pH-adjusted and ferrous-dosed influent from the first pH adjustment tank 210; then the Fenton reaction tank 220 receives H.sub.2O.sub.2 via a H.sub.2O.sub.2 pump, where the received H.sub.2O.sub.2 and ferrous reagent (from the upstream first pH adjustment tank 210) undergo Fenton reaction in the Fenton reaction tank 220 to produce .OH hydroxyl radicals, providing strong but non-discriminatory oxidative degradation of organic contaminants in the pH-adjusted influent. Iron complexes, contaminants, and oxidation by-products in the influent, if in the form of colloidal or suspended solids (collectively termed as solids) are formed after Fenton reaction. In certain embodiments, the Fenton oxidation systems operate under the following range of chemical dosage ratios;

(14) $\frac{H_2{O}_2}{{\mathrm{COD}}_{\mathrm{influent}}}=0.1.fwdarw.5\mathrm{and}\frac{{\mathrm{Fe}}^{2+}}{H_2{O}_2}=0.1.fwdarw.5$

(15) The H.sub.2O.sub.2 purging tank 230 is fluidly coupled with the Fenton reaction tank 220, and receives the influent treated under the Fenton reaction from the Fenton reaction tank 220; the unreacted H.sub.2O.sub.2 within the influent received from the Fenton reaction tank 220 is purged by a blower through diffusers installed at the base of the H.sub.2O.sub.2 purging tank 230.

(16) The second pH adjustment tank 240 is fluidly coupled with the H.sub.2O.sub.2 purging tank 230, and receives the H.sub.2O.sub.2-purged influent from the H.sub.2O.sub.2 purging tank 230; then the second pH adjustment tank 240 receives a caustic reagent via a caustic metering pump Sodium hydroxide, either in the form of caustic soda pearls or liquid form is preferred for this purpose of pH adjustment of the received H.sub.2O.sub.2-purged influent, where the caustic reagent adjusts the pH value of the received H.sub.2O.sub.2-purged influent. In certain embodiments, the pH value is in the range of 7-10, preferably in the range of 6-8.

(17) The holding tank 250 is fluidly coupled with the second pH adjustment tank 240, and receives the pH-adjusted influent from the second pH adjustment tank 240; where the holding tank 250 holds the received pH-adjusted influent and houses a lifting pump that lifts the pH-adjusted influent into the membrane tank, overcoming the lack of gravity flow and providing a buffer volume to smoothen its hydraulic coupling with the membrane tank 260. The operation of the lifting pump is controlled by a level sensor located within the downstream membrane tank 260, which switches off the lifting pump when the water level within the membrane tank 260 reaches an operator-specified level.

(18) The membrane tank 260 is fluidly coupled with the holding tank 250 via a lifting pump, where the membrane tank 260 houses microfiltration or ultrafiltration membranes that are chemically resistant to extreme acidic and caustic conditions (examples are PVDF membranes manufactured using the Thermally-Induced Phase Separation technique). Air scrubbing of the membrane surfaces within the membrane tank 260 is provided by the blower and is crucial to control the extent of foulant accumulation on the membrane surfacea phenomenon known as membrane fouling in the industry. It is beneficial to keep the membrane fouling phenomenon under control so as to confer operational stability to the membrane system and minimize chemical consumption associated with membrane cleaning (performed when the membrane fouling phenomenon becomes severe). The membrane tank 260 retains the solids, and produces filtrate, i.e. water. The water from the membrane tank 260 is pumped into the product water tank 270 via permeate pump, where the water in the product water tank 270 will be partially used to backwash the membranes via the backwash pump; in certain embodiments, the water can be mixed with either acid or sodium hypochlorite via an acid metering pump and sodium hypochlorite metering pump respectively to perform either acidic washing or caustic washing. Then the solids can be either discharged as excess solids via the wasting pump or returned to the first pH adjustment tank 210 via a recirculation pump; in certain embodiments, the recirculation rate is in the range of 50%-700% influent flow rate, which is the flowrate of wastewater entering the first pH adjustment tank, and preferably in the range of 300%-500%; as a result, overall solids content in the Fenton reaction tank 220 is elevated. Higher solids content in the Fenton reaction tank 220 enhances contaminant removals, reduces chemical consumption and lowers overall sludge production.

(19) The present invention also provides a method for organic contaminants treatment. In brief, the organic contaminants treatment method integrates Fenton reaction and membrane filtration.

(20) Referring now to FIG. 3, there is provided a flow chart of the organic contaminants treatment method in accordance with embodiments of the present invention. As shown in FIG. 3, the organic contaminants treatment method comprises:

(21) receiving from an external source an influent that contains organic contaminants 310; the exemplary influents include wastewater streams from industrial processes, such as but not limited to petrochemical, tannery and pharmaceutical production; these wastewater streams have characteristically low biodegradability, which manifests as low biochemical oxygen demand (BOD) to chemical oxygen demand (COD) ratios (commonly known as B/C ratio) due to abundance of complex chemical structures in the influent;

(22) adjusting the pH value of the received influent by acidic reagents and dosing the received influent with ferrous reagents 320; where the acidic reagents include most commonly available acids in the industry; in certain embodiments, the pH value of the pH adjusted influents is in the range of 2-6, preferably in the range of 3-4; where the ferrous reagents are preferably ferrous sulphates;

(23) adding H.sub.2O.sub.2 into the pH-adjusted and ferrous-dosed influent for Fenton reaction 330; the Fenton reaction produces .OH hydroxyl radicals, providing strong but non-discriminatory oxidative degradation of organic contaminants in the pH-adjusted and ferrous-dosed influent; iron complexes, contaminants, and oxidation by-products in the influent, if in the form of colloidal or suspended solids (collectively termed as solids) are formed after Fenton reaction; in certain embodiments, the following ranges of chemical dosage ratios are used:

(24) $\frac{H_2{O}_2}{{\mathrm{COD}}_{\mathrm{influent}}}=0.1.fwdarw.5\mathrm{and}\frac{{\mathrm{Fe}}^{2+}}{H_2{O}_2}=0.1.fwdarw.5$

(25) purging the H.sub.2O.sub.2 from the Fenton reaction-treated influent 340; in certain embodiments, the unreacted H.sub.2O.sub.2 is purged by a blower through diffusers;

(26) adjusting the pH value of the H.sub.2O.sub.2-purged influent by a caustic reagent 350; the caustic reagent includes sodium hydroxide, either in the form of caustic soda pearls or liquid form; in certain embodiments, the pH value of the caustic reagent-treated influent is in the range of 7-10, preferably in the range of 6-8;

(27) filtering the caustic reagent-treated influent by filtration (i.e. microfiltration or ultrafiltration) membrane to produce filtrate (i.e. water) and solids 360; where the solids are retained by a membrane tank housing the filtration membrane;

(28) recirculating the solids from the membrane tank to step 320 370; in certain embodiments, the recirculation rate is in the range of 50%-700% influent flow rate, which is the flowrate of wastewater entering the first pH adjustment tank, and preferably in the range of 300%-500%; as a result, overall solids content for the Fenton reaction is elevated; higher solids content in the Fenton reaction enhances contaminant removals, reduces chemical consumption and lowers overall sludge production;

(29) air-scrubbing of the surface of the filtration membrane 380; in certain embodiments, the air-scrubbing is performed by the blower; this is crucial to control the extent of foulant accumulation on the membrane surface; and

(30) backwashing the filtration membrane 390; in certain embodiments, the filtrate (i.e. water) from step 360 can be partially used to backwash the membrane; in certain embodiments, the water is mixed with either acids or sodium hypochlorite before being used for acidic or caustic backwashing.

(31) The following examples are provided for the sole purpose of illustrating the principles of the present invention; it is by no means intended to limit the scope of the present invention.

EXAMPLES

Example 1Effect of Solids Recirculation

(32) A conventional Fenton Oxidation pilot system as shown in FIG. 1 and an organic contaminants treatment system as shown in FIG. 2 have been operated to treat the same wastewater influent stream. Their operating conditions are summarized in Table 1.

(33) TABLE-US-00001 TABLE 1 The operating conditions of the conventional Fenton Oxidation pilot system and the organic contaminants treatment system Conventional Fenton Organic contaminants Parameters Oxidation pilot system treatment system Tank volume First pH adjustment tank 180 180 (Liters) Fenton reaction tank 180 180 Air purging tank 180 180 Second pH adjustment tank 180 180 Coagulant dosage tank 180 N.A Sedimentation tank Surface loading rate = N.A 1.0 m.sup.3/m.sup.2 .Math. h Holding tank N.A 1000 Membrane tank N.A 400 Influent flow (L/h) 380 150 Acidic reagents Any commonly available Any commonly available (for First pH adjustment tank) acids acids Fenton HRT (mins) 28 24 reaction H.sub.2O.sub.2:COD.sub.influent 1.9 1.5 Fe.sup.2+:H.sub.2O.sub.2 1.2 0.3 Air purging conditions 400% of influent flow rate 400% of influent flow rate Caustic reagents Caustic soda pearls, 30% Caustic soda pearls, 30% (for Second pH adjustment tank) caustic soda caustic soda Coagulant dosage tank Bypassed during N.A experiment Holding tank N.A Houses the lifting pump Continuous and complete mixing Membrane system Membrane flux (LMH) N.A 10-30 Membrane area (m.sup.2) N.A 80 Permeation cycle N.A 9 min suction 1 min relaxation Maintenance cleaning N.A Ranges from frequency 1x/3 days .fwdarw. 1x/5 days Recirculation conditions N.A 50-700% of influent flowrate

(34) FIG. 4 is a dot graph showing the plot of absolute COD removals at different Fenton reagent dosages. As illustrated in FIG. 4, the experimental data demonstrated that the organic contaminants treatment system outperformed (in terms of COD removal) the conventional Fenton process for the same chemical dosage and even maintains the performance upon further dosage optimization.

(35) The reason for enhanced COD removals can be understood as such: The recirculation of iron (III) complexes and solids back to the upstream Fenton reactors allow for their dissolution to form ferric ions, which catalyzes the Fenton-like oxidation process. Non-patent prior arts have examined Fenton-like reactions to demonstrate a 10% lower COD removal [10], but in the case of the present invention (a membrane-coupled Fenton process), the cyclic retention-recirculation-dissolution of the ferric species directly translates into reduction of ferrous catalyst dosages with a purported 10% performance compromise. The concept of cylic retention-recirculation-dissolution can be further illustrated with reference to FIG. 3; where step 370 utilizes a recirculation pump to recirculate ferric solids that has been retained in step 360 back to step 320 so that the retained ferric species can be made available via acid dissolution in step 320. Thus, the pH-adjusted and ferrous-dosed influent in step 320 is supplemented with ferric ions that were dosed into the organic contaminants treatment system at an earlier time. As both ferrous and ferric ions can participate in Fenton and Fenton-like oxidation within step 330, respectively, the reuse of ferric ions causes a lowered need for fresh ferrous dosage. The cycle is repeated when the caustic reagent-treated influent from step 350 is filtered by Microfiltration or Ultrafiltration membranes in step 360 and the ferric solids gets recirculated upstream again by step 370.

(36) However, long term operations have proven that the 10% compromise does not exist and long-term performances remain elevated over the conventional counterpart even at lowered dosages. The discrepancy can be accounted for by the enhanced adsorptive removals by Solids of the iron species under high concentrations. The difference in Solids content is pronounced between the two systems, where the once-through conventional Fenton process averages 500 mg/L of MLSS readings while the organic contaminants treatment system operates at levels between 2000-10,000 ppm.

Example 2Effect of Appropriate Membrane Cleaning Protocol

(37) In the initial stages of the operation of the organic contaminants treatment system, it was assumed that acid cleaning alone was sufficient to remove membrane fouling, which was logically assumed to be dominated by iron complex attachments (i.e. inorganic fouling). However, in the short-term fouling control study, it was found that acid cleaning alone is inadequate and a caustic cleaning (in succession to an acid cleaning) has been found to be critical to eliminate organic fouling. In particular, the respective cleaning reagents are added into a clean water stream via two chemical metering pump (one for acid addition and the other for Sodium hypochlorite addition). This stream is generated by the backwash pump and flows in a reverse direction into the membrane system, providing a driving force that allows attached contaminants (on the membrane surface) to be flushed outa process known as chemical backwashing in the industry. During the acidic cleaning, only the acid metering pump and backwash pump is in operation, creating a final concentration of 0.5%-2% before entering the membrane system. Caustic cleaning commences immediately after the completion of the acidic cleaning, where the Sodium hypochlorite metering pump operates alongside the backwash pump to create a final concentration of 250 ppm. At the end of the caustic cleaning, the membrane system resumes normal water production operation. Results of the fouling study are disclosed on FIG. 5, where a combination of acid, hypochlorite and caustic cleaning is capable of stabilizing operational pressures (where fouling rates have transformed from an initial rate of 12-1.5 kPa/day to 0.09 kPa/day, an approximate 90% decrement) and also reduce the frequencies of membrane cleaning (from 1/day to 1/3 days).

(38) TABLE-US-00002 TABLE 2 Summary of cleaning conditions Parameters Stage 1: Acidic reagent Any commonly Acid cleaning available acids Backwashing concentration 0.5%-2% Ratio of Backwash 1:1 flowrate:Filtration rate Stage 2: Caustic reagent Sodium hypochlorite Caustic cleaning Backwashing concentration 250 ppm Ratio of Backwash 1:1 flowrate:Filtration rate

(39) This protocol was previously undiscussed in any prior arts and achieved long-term stability for the membrane-based Fenton process successfully. For a given cleaning frequency of 1/3 days, the diagram on FIG. 6 illustrates the long-term pressure stability for the 9th to 14th cleaning cycle. It is evident that the introduction of caustic cleaning after an acid cleaning provided significant stability to the fouling phenomenon, where each cleaning is able to recover the initial TMP (15 kPa).

(40) One of the key differences of the present invention than prior arts is that it is not a once-through process. The recirculation pump has made it possible for solids from the membrane tank to be recirculated to the upstream Fenton reaction stage with specified recirculation rates. Being coupled with the effective retention of solids by the membrane filtration stage, the system operates at higher but controllable solids contents (controlled via solids wasting rates, which is realized through the removal of solids from the membrane tank via the wasting pumpthis act of solids removal allows for an engineered control of the operating solids level which can be determined via material balance.

(41) The present invention has the following unprecedented advantages:

(42) 1) Chemical consumption reduction via promotion of Fenton-like oxidation: This is achieved when the recirculated solids undergo acidification in an upstream Fenton reaction stage, causing partial dissolution of the iron complexes to yield ferric ions. These ferric ions can participate in Fenton-like oxidation, reducing the need for fresh dosages of ferrous salts as catalysts.

(43) 2) Reduction of chemical consumption also directly reduces solids production, lowering costs for downstream solid wastes handling and disposal.

(44) 3) Increased contaminant removals via adsorptive processes.

(45) While the present invention has been described with reference to particular embodiments, it will be understood that the embodiments are illustrative and that the invention scope is not so limited. Alternative embodiments of the present invention will become apparent to those having ordinary skill in the art to which the present invention pertains. Such alternate embodiments are considered to be encompassed within the spirit and scope of the present invention. Accordingly, the scope of the present invention is described by the appended claims and is supported by the foregoing description.

REFERENCES

(46) 1. Barbusiski, K., Fenton reaction-controversy concerning the chemistry. Ecological Chemistry and Engineering. S, 2009. 16(3): p. 347-358. 2. Goldstein, S., D. Meyerstein, and G. Czapski, The Fenton reagents. Free Radical Biology and Medicine, 1993. 15(4): p. 435-445. 3. Bernata, X., et al., Recovery of iron (III) from aqueous streams by ultrafiltration. Desalination, 2008. 221(1): p. 413-418. 4. Bernat, X., et al., Non-enhanced ultrafiltration of iron(III) with commercial ceramic membranes. Journal of Membrane Science, 2009. 334(1-2): p. 129-137. 5. Arsene, D., et al., Combined oxidation and ultrafiltration processes for the removal of priority organic pollutants from wastewaters. Environ Eng Manag J, 2011. 10(12): p. 1967-76. 6. Cailean, D., C. Teodosiu, and A. Friedl, Integrated Sono-Fenton ultrafiltration process for 4-chlorophenol removal from aqueous effluents: assessment of operational parameters (Part 1). Clean technologies and environmental policy, 2014. 16(6): p. 1145-1160. 7. Yalili Kili, M., T. Yonar, and K. Kestio{hacek over (g)}lu, Pilot-scale treatment of olive oil mill wastewater by physicochemical and advanced oxidation processes. Environmental technology, 2013. 34(12): p. 1521-1531. 8. Cammarota, M., L. Yokoyama, and J. Campos, Ultrafiltration, chemical and biological oxidation as process combination for the treatment of municipal landfill leachate. Desalination and Water Treatment, 2009. 3(1-3): p. 50-57. 9. Primo, O., et al., An Integrated Process, Fenton ReactionUltrafiltration, for the Treatment of Landfill Leachate: Pilot Plant Operation and Analysis. Industrial & Engineering Chemistry Research, 2008. 47(3): p. 946-952. 10. Wang, S., A comparative study of Fenton and Fenton-like reaction kinetics in decolourisation of wastewater. Dyes and Pigments, 2008. 76(3): p. 714-720.