Method of Purifying Polluted Water

Abstract

In a first treatment step part of at least one difficulty biodegradable or non-biodegradable organic compound is oxidized in polluted water by bringing said polluted water into contact with an ozone containing gas. The effluent is subsequently diluted with an aqueous liquid before a second treatment with ozone is carried out. If the concentration of the organic compound is still not sufficiently low, the treated effluent is diluted again and subjected to a further treatment with ozone. It was found that a more effective ozone treatment was obtained by the dilution or dilutions and that in particular no pH control was needed, so that no formation of additional salts occurred.

Description

IV. COMPARATIVE TESTS

1. Test Set-Up

In order to carry out the test, use was made of a test set-up which corresponds to the first part 1, 2, 3, 4 of the purification plant shown in FIG. 1. In order to dilute the water purified with ozone after a particular treatment step, some of the water was removed and replaced with diluent water. The diluted effluent was subsequently treated with ozone again in the same purification set-up.

• Ozone injector: 150 grams per hour (ozone concentration 11.9% based on pure oxygen); gas/water ratio: approx. 10 normal m³ gas/m³ water.
• The gas contact system: formed by a water hose 8 mg and with a diameter of 25 mm.
• Water volume pipe system: 4 l
• Retention time: 6 s
• Working pressure: 60 to 70 kPa
• Phase separator: Water volume vessel of 90 l Retention time: 135 s Working pressure: 60 to 70 kPa
• Circulation pump: Output of 35 to 40 l/min

2. Carrying out the Tests

Test Sequence:

1. The vessel (3) of the test set-up is filled with a representative sample of 90 litres of the effluent to be treated.

2. Ozone generator is started up and circulating pump (4) is started up.

3. After 1 hour, the ozone generator is stopped, the circulating pump (4) is stopped and 60 litres are drained off in such a way that 30 litres remain in the vessel (3).

4. A quantity of 60 litres of the desired water (tap water or other lightly polluted effluent) is added to the vessel (3) of the test set-up, in order to dilute the water treated in the first step.

5. The ozone generator is started up and the circulating pump (4) is started up.

6. After 1 hour, ozone generator is stopped, circulating pump (4) is stopped and 60 litres are again drained off from the vessel (3).

7. A quantity of 60 litres of the desired diluent water (tap water or other lightly polluted effluent) is added to the vessel (3) of the test set-up.

8. The ozone generator is started up and the circulating pump (4) is started up.

9. After 2 hours, the experiment is stopped.

3. Test Results

A number of characteristics of the original effluent to be treated:

• COD: 1,100 to 1,800 mg/l
• HCN: 75 to 85 mg/l
• Benzene: approx. 2,000 μg/l

The effluent corresponded to water, wherein the combustion gases, produced during the reticulation of polyurethane foams by means of the oxidation of hydrogen gas, were trapped. The effluent contained a large range of difficultly degradable organic compounds, such as benzene (2,650 μg/l), toluene (338 μg/l), ethyl benzene (8 μg/l), para-xylene and meta-xylene, (4.7 μg/l), styrene (143 μg/l) and o-xylene (4 μg/l) (MAH). It emerged from tests that with the method according to the invention, more particularly by use of the test sequence described above, the content of all these difficultly degradable organic compounds could be reduced to below the detection limit of 0.5 μg/l. Further tests with ground water containing vinyl chloride, dichloromethane, tr1,2-dichloroethylene, 1,1-dichloroethane, cis1,2-dichloroethylene, trichloromethane, 1,1,1-trichloroethane, tetrachloromethane, trichloroethylene, 1,1,2-trichloroethane, tetrachloroethylene, chlorobenzene, 1,3-dichlorobenzene, 1,4-dichlorobenzene and 1,2-dichlorobenzene (VOCl) showed that the content of these compounds could also be reduced to below the detection limit of 0.5 μg/l. Hereinafter there will be more particularly a discussion of the results obtained from measurement of the COD values and the HCN values after the treatment of effluent coming from the reticulation of polyurethane foam.

A total of 6 tests with the following specifications was carried out:

Test 1: no pH check, poor circulating pump (diaphragm pump), treatment of the effluent lasts 4 hours and comprises the continuous addition of ozone with existing ozone generator.

Test 2: no pH check, good circulating pump, treatment of the effluent lasts 8 hours and comprises the continuous addition of ozone with existing ozone generator.

Test 3: pH check by addition of NaOH (guide point pH 7 to 8), treatment of the effluent lasts 7 and 8 hours respectively and comprises the continuous addition of ozone with existing ozone generator.

Test 4: test was carried out according to the above test sequence, with the addition in step 4 of 60 litres of effluent with COD of 42 mg/l (light nitrogen and phosphorus pollution) and in step 7 of 60 litres of tap water.

Test 5: test was carried out according to the above test sequence, with the addition in step 4 of 60 litres of effluent with COD of 127 mg/l (chlorinated products such as vinyl chloride) and in step 7 of 60 litres of tap water.

Test 6: test was carried out according to the above test sequence, with the addition in step 4 of 60 litres of tap water and in step 7 of 60 litres of tap water.

The test results for COD and HCN are given below for the various tests. The percentages indicate the reduction relative to the original value, with the dilution effect being taken out by way of a back calculation.

Example: the COD value of the purified water after step 9 of the test sequence is 11 mg/l, but is “back calculated” to 99 mg/l (two dilutions 3 times, so in all diluted 9 times. The original COD was, for example, 1,183 mg/l, so that the COD drops from 100% to 9.29%.

TABLE 1
Evolution of the COD value
	Test
	1	2	3	4	5	6
Mixing	poor	good	good	good	good	good
	(diaphragm)
pH control	none	none	NaOH
System	−	−	−	+	+	+
according to
invention
Tap water				step 7	step 7	step 4 and 7
Polluted					step 4:
ground water					COD
					127 mg/l
Effluent with				step 4
readily				COD 42 mg/l
degradable
compounds
0	100.00%	100.00%	100.00%	100.00%	100.00%	100.00%
1	95.50%	80.80%	71.50%	66.27%	65.42%	53.51%
2	85.90%	72.30%	56.20%	32.46%	37.33%	35.12%
3	81.50%	67.50%	48.40%	19.78%	17.86%	28.60%
4	83.50%	55.40%	34.90%	8.37%	7.56%	6.02%
5		43.00%	20.90%
6		29.06%	15.12%
7		21.06%	10.00%
8		19.09%	7.30%

TABLE 2
Evolution of the HCN value
	Test
	1	2	3	4	5	6
Mixing	poor	good	good	good	good	good
	(diaphragm)
pH control	none	none	NaOH
System	−	−	−	+	+	+
according to
invention
Tap water				step 7	step 7	step 4 and 7
Polluted					step 4:
ground water					COD
					127 mg/l
Effluent				step 4
				COD 42 mg/l
0	100.00%	100.00%	100.00%	100.00%	100.00%	100.00%
1	68.70%	7.20%		13.75%	15.36%	31.43%
2	37.90%	5.60%	5.30%	3.87%	4.71%	10.37%
3	40.50%	4.30%	3.70%	0.55%	0.58%	0.50%
4	34.50%	3.10%	2.80%	0.55%	0.42%	0.57%
5		3.60%	1.90%
6			1.50%
7			1.30%
8		3.60%

The results shown in Tables 1 and 2 are also shown in FIGS. 2 and 3 respectively. The results indicate that better results can be obtained with the method according to the invention and that no pH adjustment is necessary, so that no extra salt loading of the water occurs. A further advantage of the method according to the invention is that an effluent which is polluted with readily degradable compounds can also be purified by this method at the same time.

It will be clear from the invention that effluents of different origins can be purified, in particular also effluents originating from soil decontamination sites.

Claims

1. A method to purify polluted water containing a first concentration of at least one difficultly biodegradable or non-biodegradable organic compound, in which method the above mentioned organic compound is oxidized by bringing the polluted water into contact with at least one ozone containing gas, characterized in that in a first treatment step part of the above mentioned organic compound present in the polluted water is oxidized by bringing said polluted water into contact with the above mentioned ozone containing gas in such a way that the first concentration of said organic compound is reduced to a second concentration, after the first treatment step the second concentration of said organic compound is reduced by dilution with an aqueous liquid to a third concentration which is at most two thirds, preferably at most half, of the second concentration, and in a second treatment step a further part of the above mentioned organic compound present in the polluted water is oxidized by bringing the diluted polluted water into contact with the above mentioned ozone containing gas in such a way that the third concentration of said organic compound is reduced to a fourth concentration.

2. A method according to claim 1, characterized in that the second concentration of the above mentioned organic compound is reduced to a third concentration which is at least one tenth, preferably at least one fifth, of the second concentration.

3. A method according to claim 1, characterized in that in the first and in the second treatment step the polluted water and the diluted polluted water respectively are brought into contact at least 10 times, preferably at least 15 times, and more preferably at least 20 times, with the above mentioned ozone containing gas.

4. A method according to claim 1, characterized in that in the first treatment step a total of at least 10 normal cubic metres of the ozone containing gas per cubic metre of the polluted water is injected into the polluted water.

5. A method according to claim 1, characterized in that in the second treatment step a total of at least 8 normal cubic metres of the ozone containing gas per cubic metre of the diluted polluted water is injected into the diluted polluted water.

6. A method according to claim 1, characterized in that in both the first and the second treatment step the above mentioned organic compound is reacted with the ozone for a total of at least 20 minutes, preferably at least 30 minutes, and more preferably at least 40 minutes.

7. A method according to claim 1, characterized in that after the second treatment step the fourth concentration of the above mentioned organic compound is reduced by dilution with a further aqueous liquid to a fifth concentration which is at most two thirds, preferably at most half, of the fourth concentration, and in a third treatment step a yet further part of the above mentioned organic compound present in the polluted water is oxidized by bringing the further diluted polluted water into contact with the above mentioned ozone containing gas in such a way that the fifth concentration is reduced to a sixth concentration.

8. A method according to claim 7, characterized in that the fourth concentration of the above mentioned organic compound is reduced to a fifth concentration which is at least one tenth, preferably at least one fifth, of the fourth concentration.

9. A method according to claim 7, characterized in that in the third treatment step per cubic metre of the further diluted polluted water a total of at least 4 normal cubic metres of the ozone containing gas is injected into the further diluted polluted water.

10. A method according to claim 7, characterized in that in the third treatment step the above mentioned organic compound is reacted with the ozone for a total of at least 40 minutes, preferably at least 60 minutes, and more preferably at least 90 minutes.

11. A method according to claim 1, characterized in that the above mentioned ozone containing gas contains at least 3 wt.-%, preferably at least 8 wt.-%, of ozone.

12. A method according to claim 11, characterized in that the above mentioned ozone containing gas is prepared starting from an oxygen rich gas containing at least 80% of oxygen, in particular by means of an ozone generator.

13. A method according to claim 1, characterized in that the above mentioned organic compound is oxidized at a pressure of at least 30 kPa.

14. A method according to claim 7, characterized in that a water further purified with ozone in a further treatment step is used as the above mentioned aqueous liquid and/or as the above mentioned further aqueous liquid for dilution of the water after a predetermined treatment step.

15. A method according to claim 1, characterized in that the above mentioned polluted water contains combustion gases, in particular gases obtained from the reticulation of polyurethane foams by means of the oxidation of hydrogen gas.