AU2004240145A1 - Water treatment process - Google Patents

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RELATED ART: NAME OF APPLICANT: ACTUAL INVENTOR(S): ADDRESS FOR SERVICE: INVENTION TITLE: GOLDWAY HOLDINGS PTY LTD RAE CAMPBELL DAVISON, KYLE RAYMOND BLIGHT, DOUGLAS ALFRED CLARKE AND DAVID EDWIN RALPH LORD AND COMPANY, Patent Trade Mark Attorneys, of 4 Douro Place, West Perth, Western Australia, 6005, AUSTRALIA.

WATER TREATMENT PROCESS DETAILS OF ASSOCIATED PROVISIONAL APPLICATION NO'S: AUSTRALIAN PROVISIONAL PATENT APPLICATION NUMBER 2003906922 FILED ON 16 DECEMBER 2003 The following Statement is a full description of this invention including the best method of performing it known to me/us:

TITLE

"WATER TREATMENT PROCESS" The present invention relates to a water treatment process, in particular to a water treatment process for the treatment of water samples contaminated with entrained organic chemicals, such as herbicides and pesticides, having long residence times in the environment.

(Ni Contamination of ground water with entrained organic chemicals, such as herbicides S 10 and pesticides, is becoming of increased concern, particularly because these chemicals typically have long residence times in the environment. Although many of these chemicals can be decomposed by ozonolysis or wet oxidation, such treatment processes are capital intensive or relatively energy inefficient and are not viable treatment options for large volumes of water which contain low levels of entrained organic pollutants.

The present invention attempts to overcome at least in part some of the aforementioned disadvantages.

In accordance with a first aspect of the present invention there is a process for the treatment of contaminated water containing entrained organic chemicals, which comprises electrolysing the contaminated water in the presence of an alkali metal halide so as to generate hypohalous acid, thereby reducing the concentration of contaminants in the contaminated water.

In accordance with a second aspect of the present invention there is provided an apparatus for the treatment of contaminated water containing entrained organic chemicals, which comprises a reactor arranged to receive the contaminated water and means for supplying alkali metal halide to the reactor, and electrolytic cell means, and 0 means for circulating the contaminated water between the reactor and the electrolytic O cell means. Preferably, the apparatus also comprises means for removing reaction byproducts.

The present invention will now be described, by way of example, with reference to the accompanying drawing, in which: Figure 1 is a schematic diagram of an apparatus for treating water contaminated with ri entrained organic chemicals in accordance with the present invention.

0The term "entrained organic chemicals" will be understood to include water-soluble

(N

organic chemicals, emulsions and micro-emulsions of organic chemicals, and fine suspensions of particulate organic chemicals. Typically, the entrained organic chemicals are herbicides, pesticides, speciated phenols, methylene blue active substances (MBAS) including, but not limited to, atrazine, simazine, MCPA, 2,4-D, 2,4,5-T, 2,4,6-T, dicamba, dimethoate, diuron, molinate, fluometuron, prometryne, propazine, trifluralin,terbutryn, chlorpyrifos, endosulfan sulphate, 2-chlorophenol, 2,3,5,6-tetrachlorophenol, 2,3,4,6-tetrachlorophenol, 2,3,4,5-tetrachlorophenol, 2,4dichlorophenol, 2,6-dichlorophenol, 2,4,5-trichlorophenol, 2,4,6-trichlorophenol, 2,3dichlorophenol, 4-chloro-3-methylphenol, 4-chlorphenol, 2,4-dimethylphenol, 2nitrophenol, 4-nitrophenol, 2-methyl-4,6-dinitrophenol, pentachlorophenol, dinoseb, phenol, chloromethylphenol, paraquat and glyphosate.

The treatment process of the present invention can be effectively employed to reduce concentrations of water-soluble organic chemicals to at least ppb levels in contaminated water having pre-treatment concentrations of entrained organic chemicals of up to 10-100g L'.

O Importantly, in a preferred embodiment, the treatment process of the present invention N involves the electrolytic generation of a strongly oxidising species, hypochlorous acid, which is capable of oxidative degradation and decomposition of the watersoluble organic chemicals. Accordingly, the operating conditions of the electrolytic S 5 cell are typically arranged to favour the generation of hypochlorous acid rather than its conjugate base, the hyperchlorite anion. However, in circumstances where the ri target material for decomposition is an organic species having greater solubility at higher pH, the operating conditions will be adopted to favour formation of (Ni hypochlorite anions.

The electrolyte employed in the electrolytic cell is preferably sodium chloride. Other alkali halides, however, such as sodium bromide, potassium chloride, and potassium bromide may also serve as the electrolyte or part thereof.

Hypochlorous acid is typically electrolytically generated from an aqueous sodium chloride solution ranging from 3g per litre in NaC1 to the point of the solution saturation concentrations (depending on solution temperature), more typically from an aqueous solution of about 100 g/litre NaC1. When the entrained organic chemicals are present in an aqueous sodium chloride solution of the above concentration range, there is no further requirement to use additional electrolytes in the electrolytic cell.

The following, based on Pletcher, D and Walsh, F.C. (1990), Industrial Electrochemistry, 2 nd ed. Chapman and Hall, Great Britain, pp 269-274, describes typical conditions required for production of the sodium chlorate NaC10 3 and the reactive oxidizing intermediate compound hypochlorous acid HCIO.

Electrode reactions: overall 6NaCl 6H 2 0 3C1 2 6NaOH+ 3H2 (1) 0 anode C 6C1 3C1 2 6e- (2) O cathode 6H 2 0 6e 3H 2 60H (3)

ID

In this way, chlorine is first electrolytically generated, then hydrolysed in aqueous t' solution to yield hypochlorous acid HOC1 and hyperchlorite anion OC1- 3C1 2

H

2 0 3HC1 3HOC1 (4) C1 2 OH- Cf OC1 HOC1 H OCI- (6) Hypochlorous acid and the hypochlorite ion can react in the bulk electrolyte to produce chlorate C103- 2HOC1 OCI- C10 3 2HC1 (7) Chlorate can also be formed by direct anodic oxidation of hypochlorite 60C- 3H 2 0 2C103- 4C1 +6H 3/202 6e- (8) To avoid losses in efficiency through reaction it is best to operate cells under conditions of a high Reynolds number to prevent reaction of OC1- at the electrode surface.

The operating pH of the electrolytic cell for maximum efficiency is governed by the speciation of the chloride species, as outlined above, and is preferably maintained at pH 5-6.5 with a suitable commercially available pH control or chemical buffering system. However, in some cases the operating pH should be allowed to rise above the optimum range for hypochlorous acid activity to improve the efficacy of the system for the decomposition of organic species that contain acid groups.

In concentrated solutions at low pH a substantial proportion of the total chlorine may be available as molecular chlorine (see Table Above pH 5 the hypochlorite ion concentration increases and the concentration of molecular chlorine is negligible. The fraction of chlorine available as hypochlorous acid is independent of the total concentration of chlorine above pH 5 (see Table 2).

Table 1. Total chlorine available as HOCl% 25 °C pH 10 ppm 100 ppm 1000 ppm 5000 ppm 96.5 73.7 21.8 99.0 89.9 46.9 18.2 99.6 96.5 73.7 41.2 99.9 99.0 89.9 69.0 99.96 99.6 96.5 87.5 99.99 99.9 99.0 95.7 99.99 99,96 99.6 98.5 99.99 99.99 99.96 99.5 Table 2: Total chlorine present in a hypochlorite solution present as hypochlorous acid 25 °C pH HOCI% pH HOC1% 99.7 9.0 3.1 99.1 9.5 0.99 96.9 10.0 0.31 91.0 10.5 0.10 76.0 11.0 0.03 50.0 11.5 0.01 24.0 12.0 0.003 9.1 The treatment process can be performed at temperatures ranging from ambient to 95°C, although performing the process at higher temperatures promotes the formation of chlorate in solution which is undesirable. Since the operating conditions are chosen to maximise generation of hypochlorous acid, it is preferable that the treatment process be conducted at lower temperatures. Typically, the treatment process is performed on solutions from at 400 to 50 0

C.

The period in which the contaminated water is treated with hypochlorous acid electrolytically generated in a sodium chloride cell in accordance with the present invention may vary from minutes to hours, depending on the volume and concentration of the contaminants.

The treatment process of the present invention may be performed with one of two possible cell configurations, either a monopolar system operating with relatively high currents and relatively low voltages, or a bipolar system which operates with relatively low currents and relatively high voltages. Typical working voltages range Sfrom about 2.9 10 V. Typical current densities range from 12.5 to 17.5 mAcm 2 -2 although higher current densities of up to 100 mAcm 2 can be employed.

Monopolar systems are easier to control for operational purposes and where some variation in feed composition is expected. Bipolar systems provide greater efficiency but suffer from cross current leakage and may prove difficult to control where the feed composition varies.

Operating the electrolytic cell at higher voltages decreases the overall economic efficiency of the process. It is also preferable to operate the electrolytic cell at lower voltages and current densities to maintain electrode longevity by reducing side reactions.

Typically, the electrolytic cell is an undivided cell with no partition to separate the cathode and the anode. Although it is envisaged that, alternatively, a divided cell could be utilised in the process of the present invention, care would have to be taken to choose a substantially porous diaphragm or membrane to divide the electrolytic cell as fouling of the membrane by the organic chemicals in solution and their corresponding intermediate degradation products will readily occur and could result in detrimental current density in part of the cell.

O The inventors have also identified that fouling of the electrodes has proven to be a Scritical problem. Fouling arises from two sources, the first being agglomeration of organic chemicals on the electrode surface. The second source of fouling in the electrolytic cell arises from the deposition of carbonate through the entrainment of S 5 carbonate onto the electrode surface. Fouled electrodes result in a change in the current density through the sections of electrode exposed to the solution electrolyte, c leading to increased electrode degradation.

It will be understood that the electrodes may be constructed from material known to be suitable for electrolysis. The electrodes include, but are not limited to, titanium ruthenium oxide electrodes with a ceramic oxide coating of ruthenium iridium platinum, magnetite coated titanium, bulk magnetite electrode, diamond coated electrode, platinum, graphite, and any other suitable type of non-sacrificial electrode.

It is generally regarded as inappropriate to use a sacrificial electrode, such as Cu or Ag because of the formation of metallic species in solution, and the resulting requirement to then separate them from solution.

The electrodes are periodically provided with an acid wash, typically with 5% v/v HC1, to remove fouling agents adsorbed on the electrode surface, and thus promote electrode longevity. It is envisaged that other dilute strong acid solutions could be employed as acid wash solutions. The electrodes are also fully submerged in the aqueous electrolyte solution and the fluid velocity in the cell is accelerated by means of pumps to promote turbulent flow across the electrode surfaces.

The action of species such as hypochlorous acid, hypochlorite, chlorine dioxide and chlorate on organic waste is known to produce a small proportion of halogenated by products, commonly known as "disinfection by products". The principal halogenated O products formed in the treatment process are chloramine, trichloroacetic acid (TCA), Sand chloroform. Heating the solution during or preferably after electrolysis treatment to between 650 C and 100' C, preferably 95' C, was found to effectively reduce the concentration of TCA and eliminated the chloroform and chloramines from the S 5 electrolysis solution to below detection limits. In the present invention chloramines are decomposed in the off-gas scrubbing system and are collected as ammonia and N, chloride salts. Chloroform is recovered as a liquid from the condenser system.

The rate of TCA decomposition was found to be a function of pH and temperature, faster decomposition rates were achieved at higher pH values and higher temperatures, at pH 11. heating the electrolysis solution to 95' C for three hours reduced the TCA concentration to less than 10% of its initial value. The undivided cell system is of advantage for the decomposition of TCA, as the spent electrolyte is alkaline, thereby aiding the decomposition. If required an alkali hydroxide or alkaline earth hydroxide can be added to the electrolyte solution to facilitate the decomposition process.

Referring to Figure 1 of the drawings there is schematically shown an apparatus for treating water contaminated with water-soluble organic chemicals in accordance with the present invention.

Water contaminated with entrained organic chemicals enters the reactor vessel from a feed reservoir (not shown) by first feed line 12, the first feed line 12 being in fluid communication with the feed reservoir and the reactor vessel 20. Sodium chloride electrolyte solution or solid sodium chloride enters the reactor vessel 20 from an electrolyte reservoir (not shown) by second feed line 14, the second feed line 14 being in fluid communication with the electrolyte reservoir and the reactor vessel Adjustment of solution pH within the reactor vessel 20 to from about 4-6.5 is achieved by addition of sodium hydroxide to the reactor vessel 20 from alkali reservoir 22 by a third feed line 16 and corresponding pump, the third feed line 16 being in fluid communication with the alkali reservoir 22 and the reactor vessel 20, or by means of addition of hydrochloric or sulphuric acid to the reactor vessel 20 from first acid reservoir 24 by a fourth feed line 18 and corresponding pump, the fourth (NI feed line 18 being in fluid communication with the first acid reservoir 24 and the Sreactor vessel The mixture of contaminated water and sodium chloride electrolyte is circulated from the reactor vessel 20 to the electrodes 30 via first circuit line 32 and a pump 34, then returned back to the reactor vessel via second circuit line 36. Typically, for a 25000L scale system, the pump 34 recirculates 1500 L.hr 1 per electrode of solution mixture from the reactor vessel A second acid reservoir 26 containing about 5% HCI is also provided and is in fluid communication with first circuit line 32 and the pump 34 via first acid line 21. The HCI solution is directed to the electrodes periodically via the first circuit line 32 to remove fouling agents adsorbed on the electrode surface, and thus promote electrode longevity. The 5% HC1 solution is then redirected back to the second acid reservoir 26 via second acid line 23.

During the treatment process, oxygen and chlorine gases are electrolytically generated at the anode surface whilst hydrogen gas is electrolytically generated at the cathode surface. The reactor vessel 20 is provided with an air blower 28 and a condenser to vent gases evolved during the treatment process. Off-gases are vented through the condenser and out of the reactor vessel 20 via a gas exhaust line 42 in fluid o communication with the reactor vessel 20. The gases also pass through a series of gas Sscrubber assemblies 44, 46 before being vented to the atmosphere. The gas scrubber 44 is typically a glass packed cylinder of 25 cm diameter and 250 cm in height circulating a 2 M NaOH scrubber solution. The gas scrubber 46 is typically a glass S 5 packed cylinder of 25 cm diameter and 250 cm in height circulating a 1 M sulphuric acid scrubber solution. The gas scrubbers 44, 46 are provided with pumps operating at a volume throughput of 10 L.min to circulate the scrubber solution.

After the completion of the electrolysis process, if required, the electrolyte solution in reactor vessel 20 can be heated, either directly by heating elements or by a heat exchange assembly (not shown) to the optimum temperature for the removal of disinfection by products. TCA, ideally 95 if necessary an alkali or alkaline earth hydroxide can be added by feed lines 14 or 16 to adjust the pH to a suitable value to facilitate optimum TCA decomposition, generally greater than pH 10. The TCA breakdown product chloroform can be collected at condenser assembly 40 for disposal or purification. The chloramines pass into the gas scrubbers 44 and 46 where they are removed before the remaining gaseous products are vented to atmosphere. If necessary the decomposition of TCA can take place a separate vessel to that in which the electrolysis reactions occurred.

The invention will now be further described with reference to the following Example.

EXAMPLE

Two water samples containing various herbicides, pesticides, phenols, methylene blue active substances (MBAS), in the amounts as listed in Table 3 were treated in an litre pilot plant with hypochlorous acid electrolytically generated from a 140g/L sodium chloride solution under the following operating conditions: Applied voltage was 2.99 V with 10 amp maximum current. Surface area of the anode was 756 cm 2 Electrolysis was conducted under ambient conditions for about 72 hours with the operating temperature rising to about 46'C over the duration of the treatment process as a result of energy input from the cell pump.

After 72 hours, samples of the treated water were analysed. The amounts of various herbicides, pesticides, phenols, and methylene blue active substances (MBAS), detected after treatment in accordance with the present invention are listed in Table 4.

Table 3 Concentration of herbicides, pesticides, phenols, and methylene blue active substances prior to electrolysis treatment pH 8.6 8.6 Comment Sample 1 Sample 2 Measurement units Mg/1 Mg/1 MBAS (mg MBAS/1 calc as LAS 342) 62 Total Dissolved Solids 140000 140000 Total Suspended Solids 150 350 Measurement units pg/1 pg/1 Atrazine 2300 3100 Simazine 810 840 MCPA <5 2,4-D 10 11 2,4,5-T <5 2,4,6-T <5 Dicamba <5 Dimethoate <50 Diuron 2200 2300 Molinate <50 Fluometuron 83 68 Prometryne 94 78 Propazine <50 Trifluralin 80 3200 Terbutryn <50 Chlorpyrifos <50 Endolsulfan Sulphate <50 2-chlorophenol <50 2,3,5,6-Tetrachlorophenol <50 2,3,4,6-Tetrachlorophenol <50 2,3,4,5-Tetrachlorophenol <50 2,4-Dichlorophenol 390 290 2,6-Dichiorophenol <50 2,4,5-Trichlorophenol <50 2,4,6-Trichlorophenol <50 2,3 Dichiorophenol <50 4-Chloro-3-methytylphenol <50 4-Chiorophenol <50 2,4-Dimethylphenol <50 2Ntohnl<50 4-irphnl<50 2,-Dntrpenl<50 2-Methyl-4,6-dinitrophenol <50 Pentachiorophenol <50 Dinoseb <50 Phenol <50 Chioromethyiphenol-other than <50 4-chloro-3 methyiphenol Paraquat 290 280 Glyphosate <10 Table 4: Concentration of herbicides, pesticides, substances after electrolysis treatment phenols, and methylene blue active Comment SamplelI Sample 2 pH 5.5 Measurement units mg/l mg/l MBAS (mg MBAS/l calc as LAS 342) 2.2 Total Dissolved Solids 140000 140000 Total Suspended Solids 400 300 Total Organic Carbon 470 470 Measurement units Pg/I Pg/i Atrazine I11 Simazine 4 4 MCPA <1 <1 2,4-D <1 <1 2,4,5-T <1 <1 2,4,6-T <1 <1 Dicamba <1 <1 Dimethoate <1 <1 Diuron <1 <1 Molinate <1 <1 Fluomneturon <1 <1 Prometryne <1 <1 Propazine <1 <1 Trifluralin <1 <1 Terbutryn <1 <1 ChIorpyrifos <1 <1 Endosulfan Sulphate <1 <1 2-chlorophenol <1 <1 2,3,5,6-Tetrachlorophenol 3 1 2,3,4,6-Tetrachlorophenol <1 1 2,3,4,5-Tetrachlorophenol 2 1 2,4-Dichlorophenol <1 <1 2,6-Dichlorophenol <1 <1 2,4,5-Trichlorophenol <1 <1 2,4,6-Trichlorophenol <1 <1 2,3 Dichlorophenol <1 <1 4-Chloro-3-methytylphenol <1 <1 4-Chlorophenol <1 <1 2,4-Dimethylphenol <1 <1 2-Nitrophenol <1 <1 4-Nitrophenol <1 <1 2,4-Dinitrophenol <1 <1 2-Methyl-4,6-dinitrophenol <1 <1 Pentachlorophenol 3 3 Dinoseb 1 1 Phenol <1 <1 Chloromethylphenol-other than <1 <1 4-chloro-3 methylphenol Paraquat <20 Glyphosate <10 In this example, the spent electrolyte liquor from the electrolysis process was heated to 95° C. Prior to heating Ca(OH) 2 was added to achieve a pH of 11.56. As the decomposition reaction proceeded, the pH was observed to decrease. The chloroform formed by the TCA decomposition was collected in a receiver vessel connected to a condenser assembly. Table 5 shows the concentration of TCA detected in the spent electrolysis liquor heated to 95° C as a function of time.

Table 5: Concentration of TCA present in the spent electrolysis liquor, initial pH 11.56, and 95° C.

Time (min.) TCA (mg 0 270 206 184 108 120 54 2004240145 16 Dec 2004 [3-

Claims (13)

1. A process for the treatment of contaminated water containing entrained organic chemicals, which comprises electrolysing the contaminated water in the presence of an alkali metal halide so as to generate hypohalous acid thereby reducing the concentration of contaminants in the contaminated water.

2. A process according to Claim 1, in which the entrained organic chemicals N include one or more of water soluble organic chemicals, emulsions and micro- Semulsions of organic chemicals, and fine suspensions of particulate organic chemicals.

3. A process according to Claim 2, in which the entrained organic chemicals are one or more of herbicides, pesticides, phenols and methylene blue active substances.

4. A process according to any one of the preceding claims, in which the alkali metal halide is a chloride so that the electrolysis generates hypochlorous acid. A process according to Claim 4, in which the alkali metal halide is sodium chloride.

6. A process according to any one of the preceding claims, in which the electrolyte is maintained during electrolysis at a pH of 5 to

7. A process according to any one of the preceding claims, in which the electrolysis is carried out at a temperature in the range from to 400 to 500 C.

8. A process according to any one of the preceding claims, in which the electrolysis is carried out at a working voltage in the range from 2.9V to

9. A process according to any one of the preceding claims, in which current density during the electrolysis is in the range from 12.5 to 100 mA cm 2 density during the electrolysis is in the range from 12.5 to 100 mA cm- A process according to any one of the preceding claims, in which the N electrolysis is carried out in an undivided cell. d

11. A process according to any one of the preceding claims, in which the INO electrolysis is carried out by electrodes which are non-sacrificial electrodes. S12. An apparatus for the treatment of contaminated water containing entrained organic chemicals, which comprises a reactor arranged to receive the contaminated water and means for supplying alkali metal halide to the reactor, and electrolytic cell means, and means for circulating the contaminated water between the reactor and the electrolytic cell means.

13. A process by which contaminated water treated with a hypohalous acid or a hypohalite ion, is heated to within the range of 65' C to 100- C to decompose disinfection by products.

14. A process by which an alkali or alkaline earth hydroxide or oxide is added to the contaminated water in claim 13 to adjust the pH to a favourable range for the decomposition oftrichloroacetic acid by heating. A process as in claim 13 by which the hypohalous acid, and or the hypohalite ion, is generated in-situ by electrolysis, by addition of a halogen gas, or by addition of a solution of the hypohalous acid or hypohalite ion

16. A process for the treatment of contaminated water substantially as hereinbefore described in the Example.

17. An apparatus for the treatment of contaminated water substantially as hereinbefore described with reference to Figure 1. DATED THIS 16 TH DAY OF DECEMBER 2004. O DATED THIS 16 TH DAY OF DECEMBER 2004. O Goldway Holdings Pty Ltd U By their Patent Attorneys LORD AND COMPANY D\ PERTH, WESTERN AUSTRALIA.