GB2471092A

GB2471092A - A process for the treatment of water

Info

Publication number: GB2471092A
Application number: GB0910354A
Authority: GB
Inventors: David Owen
Original assignee: YNYS SERVICES
Current assignee: YNYS SERVICES
Priority date: 2009-06-16
Filing date: 2009-06-16
Publication date: 2010-12-22
Also published as: GB0910354D0

Abstract

A process for the removal of hydrogen sulphide from waste water comprises: treating a mixture comprising hydrogen sulphide containing waste water and a ferric salt chelant by dissolving a source of oxygen into the mixture under pressure within a closed vessel. The mixture may be pressurized by a compressed source of oxygen. The source may be either pure oxygen or air. The process may also include the step of drawing the waste water from a sewer system by at least one pump. The chelated ferric salt may be present in the mixture at a dose rate of 0.01-110 time the sulphide on a stoichiometric basis. The chelate may be ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), inino-disuccinic acid (IDS), methylglycince di-acetic acid (MGDA), monoaminocarboxylic acids, polyatninocarboxylic acids, polyaminoalkylcarboxylic acids and polyhydroxychelating agents. An apparatus for carrying out the process is also disclosed.

Description

A Process and Apparatus for the Treatment of Water The present invention relates to a process and apparatus for the treatment of water. In particular, the present invention relates to a process and apparatus for the removal of hydrogen sulphide from a waste water system such as a sewer system.

The presence of hydrogen sulphide in sewer systems, both industrial and domestic is highly problematic. As will be appreciated, there is a need to control the levels of hydrogen sulphide within a sewer system, not least because it has a very characteristic malodour of rotten eggs, which is extremely offensive to the human nose, it is toxic at very low concentrations and moreover, it can be readily converted to sulphuric acid which can result in corrosion and subsequent failure of sewage piping and related equipment.

Chelating agents are a group of chemicals that react with metals such as iron and which, in use, provide a "protective cage" around the metal in solution such that precipitation of that metal can be avoided even when the pH of the solution approaches the normal pH of precipitation for that metal. In the case of ferric iron salts, the iron will normally precipitate as a ferric hydroxide at a pH in excess of 5.

Suitable chelating agents for use with ferric iron, in particular ferric salts, include ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), imino-disuccinic acid (IDS) and methylglycine di-acetic acid (MGDA).

*:*::* Other suitable chelants which may be considered are iron chelated with a chelating agent selected from the group consisting of monoaminopolycarboxylic acids, polyaminopolycarboxylic acids polyaminoalkylpolycarboxylic acids and their alkali metal salts, polyhydroxy-type chelating agents such as sorbitol, or polyhydroxy type such as alkaline glucoheptonate or gluconate. These chelating agents enable the ferric salts to remain in solution, even when the pH of the solution is raised above 5. As will be appreciated, the chelating agents also assist with resisting precipitation with an ionic counter-iron such as sulphide which would normally reduce the iron from the ferric state to the ferrous state such that the iron is precipitated as a ferrous suiphide.

Ferric salt chelates have been used for the scrubbing of gasses containing hydrogen sulphide. The general reaction scheme involving the use of a ferric salt chelate as a catalyst to remove the sulphide or bisulphide ions from solution can be represented as follows:-Step No. H2Svapour + H20 liquid > H2S Liquid 1 H2S liquid. > Hi-+ HS-2 HS-+ 2 Fe3+ > S + 2Fe2+ + H+ 3 /2 02 (air) +H20 liquid.> 1/202 liquid 4 2Fe2+ + 1/202 Liquid + H20 > 2FE3+ + 2 OH-5 The overall reaction being: * *S 01.

* H2S vapour + 1/202 vapour > S + H20 * 0S * * * * Il As will be appreciated, steps 1 and 2 represent the absorption of the hydrogen sulphide gas into the solution containing the ferric salt chelant and its subsequent ionisation.

Step 3 represents the oxidation of the sulphide ions to elemental sulphur and the simultaneous reduction of the ferric iron to ferrous iron; and steps 4 and 5 represent the absorption of oxygen into the aqueous solution followed by oxidation of the ferrous iron back to its ferric state such that the catalytic chelated ferric salt can be reused.

As will be appreciated, steps 3 and 5 are very rapid; however, in order to achieve rapid reaction, it is necessary to ensure good contact of the reagents within the aqueous system.

There are a number of known systems and apparatus used to remove hydrogen sulphide. In some embodiments, these systems usually include two vessels, namely, an absorber and a regenerator in which the air is used to regenerate the ferric chelate. These systems are rather large and use a lot of energy.

As will be appreciated, the main problem associated with the removal of hydrogen sulphide from a sewer system is that the air in contact with the liquid is confined to the sewer and can escape containing high levels of hydrogen sulphide through venting points within the system. To this end, it is necessary to trap the gas at or prior to, the point of release. a...

It is an object of the present invention to provide a process and *..*** * a apparatus for the removal of hydrogen sulphide from a waste water system *:*::* such as a sewer system, which is quick, efficient, requires minimum supervision and has a low carbon footprint.

In a first aspect of the present invention, there is provided a process for *..: the removal of hydrogen sulphide from waste water, preferably and optionally originating from a sewer system, comprising the step of: dissolving a source of oxygen into a mixture comprising hydrogen sulphide containing waste water and an oxidizing chelated multivalent transition metal/metal salt, under pressure within a closed vessel. Preferably, the oxidizing multivalent transition metal salt chelate or chelated oxidizing metal salt is a ferric salt chelate or chelated ferric salt.

The applicants have surprisingly found that by dissolving the source of oxygen under pressure results in faster reaction times not least because more oxygen is present in the mixture to oxidise the catalytic chelate back to its oxidising, or in the case of the chelate being an iron salt chelate, ferric state.

This also means that less chelate need be used to achieve the desired result such that the treated mixture can be discharged back into the waste water system as the levels of chelant would be very low and would not pose a threat to the environment. Additionally, selection of the most environmental friendly types of chelant i.e. highly biodegradeable products, means that sensitive water systems may be treated without fear of further pollution. Also any excess oxygen present in the mixture on discharge would have a beneficial effect vis-à-vis the aerobic organisms common to waste water systems, such as sewer systems Preferred features of the process of the present invention include: Preferably, the mixture is pressurised by a compressed source of oxygen.

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Advantageously, the source of oxygen is air or is pure oxygen, pure bottled oxygen.

In a preferred embodiment, the process of the invention further *:*. comprises the step of drawing the hydrogen sulphide containing waste water from a sewer system into the closed vessel. To this end, preferably the hydrogen sulphide containing waste water is drawn into the closed vessel by the use of at least one pump. Moreover, it is further preferable that the hydrogen suiphide containing waste water is drawn directly from the sewer system into the closed vessel.

In addition, to prevent escape of the hydrogen suiphide into the air, it is preferable that the hydrogen suiphide containing waste water is drawn from the sewer system before reaching a venting point within the sewer system.

Advantageously, the chelated ferric salt is present in the mixture at a dose rate of 0.01 to 10 times the suiphide on a stoichiometric basis.

Further preferably, the ferric salt is chelated with a chelating agent selected from the group consisting of ethylenediaminetetraacetic acid (EDTA), nitriotriacetic acid (NTA), imino-disuccinic acid (IDS) and methyiglycine di-acetic acid (MGDA) polyaminocarboxylic acids, monoaminaocarboxylic acids, and any other chelation system as previously recited.

Preferably, the source of oxygen is dissolved into the mixture above atmospheric pressure. Further preferably, the oxygen is dissolved into the mixture at a pressure of at least 5 bar, preferably to 10 bar or 20 bar.

Advantageously, the mixture and source of oxygen are pressurised for at least isec or up to 3 minutes. * * ****

There is also provided a method of removing hydrogen sulphide from * ** waste water, including waste water from a sewer system, in accordance with the method of the present invention substantially as hereinbefore described. ***

In a further aspect of the present invention there is provided a system *..: for removing hydrogen sulphide from waste water, the system comprising: a vessel which, in use, can receive a mixture of hydrogen sulphide containing waste water and an oxidizing chelated multivalent transition metal/metal salt, including but not limited to a ferric salt chelate, and which can be closed such that a source of oxygen can be dissolved into the mixture under pressure, the vessel including at least one inlet through which the waste water can be introduced and at least one outlet through which the treated waste water can be discharged; a source of a ferric salt chelate or other oxidising multivalent transition metal/metal salt chelate in communication with the vessel; delivery means, which, in use, introduce or deliver the ferric salt chelate or other oxidising metal/metal salt chelate into the vessel; a source of oxygen in communication with the vessel; and a compressor for introducing the source of oxygen into the mixture under pressure. Preferably, the system also includes at least one pump for pumping the waste water into the vessel through the at least one inlet.

Advantageously, the vessel is located above ground and the at least one inlet is in communication with a sewer system below ground via a piping.

Preferably, the system includes at least one additional pump for pumping the treated waste water out through the at least one outlet back into the waste water system. *... * I ****

Advantageously, the system further includes automated control means * ** *** * such that the system can carry out the process of the present invention. * ** * S * * S.

There is also provided the use of the system of the present invention to carry out the process of the present invention.

The present invention will now be exemplified by way of reference to the following non-limiting examples.

EXAMPLE I

As it is known that ferric chelates in the presence of a source of oxygen can scrub gasses containing hydrogen suiphide, sodium sulphide was dissolved in water to a concentration of approximately 4-5 ppm as measured by a commercial test kit.

The resulting solution smelt of free hydrogen suiphide and had a pH of 6.

The solution was admitted to a closed vessel, which was capable of being pressurised to 7 bars with compressed air.

The solution was subsequently pressurised to 5 bars with the compressed air; the compressed providing a source of oxygen, which would have become dissolved within the solution.

After three minutes, the solution was depressurised to atmospheric pressure and was extracted from the vessel.

The extracted solution was immediately sampled and tested for * suiphide. It was established that there had been no reduction in the amount of sulphide present in the liquid. This confirms that the dissolving of oxygen * S....

* into the solution to be treated under pressure in the absence of a suitable *:*::* oxidising metal chelant had no effect on the levels of hydrogen sulphide in the :. time frame. S. * . S * S

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EXAMPLE 2

In this study, a solution containing 40 ppm of suiphide in water was added to the closed vessel capable of being pressurised. A sufficient amount of ferric chloride chelated (approx 10% so'ution of chelate) with NTA was added. The NTA chelated ferric chloride was present in the solution in an amount of 50 ppm.

The solution within the closed vessel was pressurised to 5 bars with compressed air as the source of oxygen for three minutes.

The reactants where then depressurised and the pH and resultant suiphide level was measured. The pH was shown to be 9, whereas the resultant sulphide concentration was measured to be 4 ppm thereby showing a 90% reduction in sulphide concentration in one pass.

EXAMPLE 3

This time, instead of using NTA as the chelant, MGDA was used.

Apart from the use of MGDA as the chelating agent, the reactants and reaction conditions were the same as those in Example 2. That is, the solution being treated had 4oppm of sulphide and 5oppm of the chelated ferric * ***** * chloride, and the solution was once again pressurised to 5 bars for three *:::* minutes with compressed air as the source of the oxygen, which would have dissolved in the solution It was established post reaction that the level of sulphide had been **.: reduced from 40 ppm to 5 ppm.

EXPERIMENT 4 This time the solution to be treated comprised 4 ppm of suiphide at a pH of 6 and 50 ppm of ferric chloride chelated within MGDA.

As with Example 3, the resulting solution within a closed vessel was pressurised by the use of compressed air as the source of oxygen to 5 bars for three minutes.

Our results indicated that after pressurising the reactant with compressed air for three minutes resulted in there being less than 1 ppm of sulphide remaining in the treated water.

EXPERIMENT 5 This time a solution containing 4 ppm of suiphide of a pH of 5 was treated with ferric chloride chelated in NTA present in the solution at 50 ppm.

Once again, after pressuring the liquid with compressed air including a source of oxygen to 5 bars and three minutes, the amount of sulphide was reduced to less than 1 ppm within the solution.

EXPERIMENT 6 *..* * S S...

*5**s* * In this experiment, we took a solution containing 4 ppm of suiphide at a *:*::* pH of 6. We then added a ferric chloride chelated in NTA such that the mixture contained 10 ppm of the NTA chelant.

After treating the solution under the same conditions as Experiment 6, it was observed that the resulting mixture included sulphide at a level of less than I ppm.

SUMMARY OF THE RESULTS

With reference to the experimental protocols and results given above, it is clear that by exposing the resulting mixture to a compressed source of oxygen under pressure increased the amount of oxygen dissolved within the solution to oxidise the ferrous iron present in the chelate back into its ferric state thereby substantially speeding up the rate of reaction. Without carrying out the reaction under pressure, the rate of the reaction will be limited to atmospheric air saturation, which is determined by the mix ratio of the aerated water to sulphidic water. In this regard, the reported reaction times for reactions carried out at atmospheric pressure range from 15 minutes to 45 minutes, which is not only lengthy but also results in the consumption of lots of energy. As a result, and by comparison, it will also be appreciated that the amount of hydrogen sulphide is reduced substantially in a very short space of time and after carrying out the reaction only once. Such efficiencies provide a system and process which is energy efficient, has a low carbon footprint and requires minimal control and supervision.

Although it is envisaged that the process and apparatus of the invention can be used to treat hydrogen sulphide containing waste water extracted from a sewer system, it is to be appreciated that the apparatus and process of the present invention can be used to remove hydrogen sulphide from any other source of waste water or water. S...

* S. .SS * When used in this specification and claims, the terms "comprises" and "comprising" and variations thereof mean that the specified features, steps or :. integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components.

II

The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof. * *

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Claims

Claims: 1. A process for the removal of hydrogen sulphide from waste water comprising the step of: treating a mixture comprising hydrogen sulphide containing waste water and a ferric salt chelant by dissolving a source of oxygen into the mixture under pressure within a closed vessel.
2. The process of claim 1, wherein the mixture is pressurised by a compressed source of oxygen.
3. The process of any one of the preceding claims, wherein the source of oxygen is air.
4. The process of any one claims I or 2, wherein the source of oxygen is pure oxygen.
5. The process of any one of the preceding claims, further comprising the step of drawing the hydrogen sulphide containing waste water from a sewer system into the closed vessel.
6. The process of claim 5, wherein the hydrogen sulphide containing waste water is drawn into the closed vessel by the use of at least one pump.S S
7. The process of claims 5 or 6, wherein the hydrogen sulphide containing waste water is drawn directly from the sewer system into the closed vessel.
8. The process of any one of claims 5 to 8, wherein the hydrogen sulphide containing waste water is drawn from the sewer system before reaching a venting point within the sewer system.
9. The process of any one of the preceding claims, wherein the chelated ferric salt is present in the mixture at a dose rate of 0.01 to 10 times the sulphide on a stoichiometric basis.
10. The process of any one of the preceding claims, wherein the ferric salt is chelated with a chelate selected from the group, but not limited by the listing, consisting of ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), imino-disuccinic acid (IDS) and methyiglycine di-acetic acid (MGDA) monoaminocarboxylic acids, polyaminocarboxylic acids polyaminoalkylcarboxylic acids, polyhydroxychelating agents including sorbitol alkaline glucoheptonates,
11. The process of any one of the preceding claims, wherein the source of oxygen is dissolved into the mixture above atmospheric pressure.
12. The process of claim 12, wherein the oxygen is dissolved into the mixture at a pressure of at least 5 bar.
13. The process of any one of the preceding claims, wherein the mixture and source of oxygen are pressurised for at least isecond or up to 3 minutes.
14. A system for removing hydrogen sulphide from waste water, the system comprising: a vessel which, in use, can receive a mixture of hydrogen sulphide containing waste water and a ferric salt chelate and which can be closed such that a * ,* source of oxygen can be dissolved into the mixture under pressure, the vessel including at least one inlet through which the waste water can be introduced and at least one outlet through which the treated waste water can be discharged; a source of a ferric salt chelate in communication with the vessel; delivery means, which, in use, introduce or deliver the ferric salt chelate into the vessel; a a source of oxygen in communication with the vessel; and a compressor for introducing the source of oxygen into the mixture under pressure.
15. The system of claim 14, wherein the vessel is located above ground and the at least one inlet is in communication with a sewer system below ground via a piping.
16. The system of claims 14 or 15, further comprising at least one pump for pumping the waste water into the vessel through the at least one inlet.
17. The system of claims 14, 15 or 16, wherein the system includes at least one additional pump for pumping the treated waste water out through the at least one outlet back into the waste water system.
18. The system of any one of claims 14 to 17 including automated control means such that the system can carry out the process of any one of claims 1 to 13.
19. The use of the system of any one of claims 14 to 18 to carry out the process of any one of claims 1 to 13. S., a a a...S SI * . * IS * . S * SIS 4.S I* ** ** S a II *S US a