GB2191769A - Method of odour control - Google Patents

Abstract

A method of odour control comprises passing an odorous gas through a column of inert packing material on which is deposited iron oxide. An iron salt solution is recirculated through the column to cause deposition of iron oxide. The process may be used to remove hydrogen sulphide from sewage off-gas.

Description

SPECIFICATION Method of odour control The present invention relates to a method of controlling odour. The method is particularly, but not exclusively, applicable to controlling odorous emissions from sewage works, although many other applications can be envisaged.

Odorous compounds present in gaseous sewage works effluent include hydrogen sulphide and other sulphur containing compounds such asthiols. Many attempts have been made to improve the processing of sewage to reduce the problem of odour emission and have been successful to a point. However the problem of odorous gaseous emission remains and the present invention is concerned with the purification of such collected gaseous effluent. The process has been developed primarily to this end but is, of course, equally applicable to a number of other industries where odorous emissions may cause nuisance to those living nearby.

In the past, the removal of hydrogen sulphide was of particular concern in the conversion of coal to towns gas. Here the problem was reduced by passing the gases through beds of bog iron ore in a relativelyfinely divided state. Although partial removal of hydrogen sulphide could be accomplished in this way, the method cannot achieve a sufficient reduction in the concentration of hydrogen sulphide to allow release of odorous airto the environment. The necessary costs of compressing the gas and of wasting some ore could be borne when the product, town gas, was a saleable commodity. Clearly when, as in the present case, the gas to be purified is a waste product, such costs cannot be contemplated.

A more recent effort at odour removal has been to pass the waste gas through a column of iron swarf. This has been successful to a degree but several problems have been associated with the process. It is almost impossibleto pack the column satisfactorily with iron swarf. The reaction relies on the swarf rusting and thus the swarfgraduallyturns to rust and disintegrates into a sludge. At this point, the column must be dismantled and refitted. It is an object of this invention to provide a method of and apparatus for odour control which can be carried out cheaply and over long periods without constant attention.

According to the present invention, there is provided a method of odour control comprising the steps of providing a column of inert packing material, passing therethrough a solution of an iron salt, whereby to deposit iron oxide on the packing material, and passing an odorous gas through the column. Preferably, the iron salt solution is recirculated to maintain the iron oxide deposit.

Advantageously additional ferrous salt may be added to the solution at intervals.

Alternatively, the recirculating solution may be run off and a fresh ferrous salt solution be introduced.

The method may include the additional steps of measuring the pH of the solution and in response thereto, adding alkali or acid to maintain a desired pHvalue.

The packing material may be of inert plastics material which has a high surface area but which allows free passage ofcountercurrent streams of gas and recirculating liquid.

The column may also contain microorganisms, possibly growing on the same packing material, which also act to remove or reduce odorous gases.

An embodiment of the present invention will now be more particularly described byway of example and with reference to the accompanying drawings, in which: Figure lisa schematic view of an apparatus for use in a method embodying the invention; and Figure2 shows graphically the hydrogen sulphide removal rate atvarious loadings.

The apparatus shown comprises a container 1, having an upper column portion 2 and a lower reservoir portion 3. The reservoir portion may alternatively be formed as a separate but interconnected container.

The column portion 2 comprises a bed of conventional packing material such as pall rings, shorttubes of inert plastics material having apertured walls and internal ledges. Above the column is a spray head 4which is fed via a pump 5 from the reservoir 3. In the recirculation system is an inlet 6, which may also be used for introduction of make-up water if required, through which is introduced a solution of ferrous sulphate or other ferrous salt.

Before use, the solution is allowed to pass through the column a number of times until iron is oxidised to ferric oxide by atmospheric air, which ferric oxide is deposited on the elements of the column. If necessary, furtherferroussulphate or other salt can be added, atthistimeoranysubsequenttime,via inlet7. The pH of the solution is checked by pH meter 8 and if necessary, acid or alkali can be added via inlet7.

The column is then ready for use. Odorous gas is introduced via inletS and fan 10. It passesthroughthe column 2to outlet 11,whence it may be discharged to the atmosphere. Within the column any hydrogen sulphide in the gas reacts with the deposited ferric oxide, which acts substantially catalytically in the presence of air, to form elemental sulphur or other sulphur compounds. Any ferric oxide washed from the column into the reservoir is, of course, returned by the recirculating solution. Occasionally the reservoir maybe drained and a fresh solution added, although the column may be used continuously for long periods.

It has been found that the preferred solution is slightly alkaline, since this helps the hydrogen sulphide into solution, where it is reacted more effectively with the ferric oxide.

In a preferred embodiment of the invention, the column also supports a growth of microorganismsinclud- ing both heterotrophs and autotrophs, such as thiobacillus ferrooxidans. This is useful in deodorising the gas of other odorous compounds, for example certain carboxylic acids and organicsulphurcontaining com pounds, e.g. dimethyl sulphide, or in converting such compounds to compounds which will then reactwith the iron oxide on the column.

As may be seen from the foregoing, the method offers easy removal of odorous gas from sewage works or other installations: without any need to pressurisethe gas, exceptforthe use of low pressure fan 10; without any need to dispose of large volumes of spent reagent, only occasionally will the reservoir need to be drained via outlet 12; without great material expense other than occasional requirements to introduceferroussulphate or otherferrous salt and a pH control; and without need of constant attention, refreshing the solution being required perhaps only everyfew months.

Although the invention has been described with reference to iron compounds, which are preferred because oftheircheapness and availability, many othertransition metals (for example manganese) may also be used. In such a case the word "ferrous" must be taken to include any stable lower oxidation state of such a metal.

Example The scrubbing column used for the Example comprised glass sections of 300 mm internal diameter, packed with 16 mm pall rings to a height of 1.4 m. Glass pipework of nominal internal diameter 25 mm was so arranged asto permit operation eitherwith once-through liquid flow, or with partial ortotal recycle.

Mixtures of hydrogen sulphide and air were produced. A container of capacity 501 was partially filled with hydrochloric acid and fitted with astonedifluserformixing and aeration. A solution of sodium sulphidewas added to this container by a dosing pump, and air was blown through the space above the liquid by a fan.A flexible hose was used to introduce the gas mixture into the scrubber.

Because of the highlytoxic properties of hydrogen sulphide, extensive safety precautions were necessary.

The precautions included extensive testing for leaks, using a portable electron-capture detector, and the continuous monitoring ofthe atmosphere in the room.

Electronic monitors forthe detection of hydrogen sulphide are widely available, and have advanced considerably insensitivity and reliability since the early designs. They are however least accurate at low con- centrations and may give high percentage errors below concentrations of 1 part per million.

Atwo-channel instrument (Neotronics Ltd., Bishops Stortford, Herts) has been used in this evaluation. One ofthe available channels has been used continuously to monitor the environment near the plant. An alarm was set to give warning of atmospheric concentrations greater than 10 parts per million, generally acknowledged to be the safety limit for prolonged exposure (8 hourTLV). The second channel was used to measure the concentration of hydrogen sulphide in the gas stream entering the plant. This channel was connected to a recorder, to give a permanentand continuous record.

Because it was intended to purify the air stream below the limit of reliability ofthe monitor, it was for many ofthe tests necessaryto use a more sensitive method for measuring hydrogen sulphide. The technique chosen was gas chromatography at ambienttemperatures, using a packed column and a photoionisation detector. The instrument used (Photovac, Ontario, Canada) is capable of detecting concentrations similarto the limits of detection with the nose. In addition, it can be left to sample automatically at a predetermined time interval and to print the results of analysis. This device has proved more than sufficiently sensitive for these tests.

Apart from concentrations of the contaminant in the gas phase, other conditions of operation are clearly important. Flows were measured by variable-area flow-meters (rotameters), and recorded manually. Temperatureswere measured using mercury-in-glass thermometers. Where appropriate, concentrations of hydrogen sulphide in the liquid phase were determined. The anlytical technique was that standard forthe water industry, but because storage ofthe samples for several days was sometimes required, the samples were "fixed" immediately after sampling by the addition of zinc acetate and sodium carbonate.

The control of pH value in the liquid was also provided for: a combined pH meter and dosing pumpwas available to add sodium hydroxide to the make-up stream. In practice control was not used, and the unit was employed only as a measuring device.

Nine runs were performed under conditions of operation shown in Table 1 and the results are shown in Table 2. The experimental runs represent a single period of continuous operation during which the conditions of operation were changed from time to time after steady state had been reached.

The calculated values of removal rate are plotted against applied loading in Figure 2. It can be seen thatthe column removed hydrogen sulphide completely up to a loading of about 1 g/h, but that above th is figure there was a rapid fall off of removal. Whereas it was expected that the plant would reach "saturation loading" above which the removal rate would remain constant, it is clear from Figure 2 that the weight removed actuallyfell above a certain loading.

Table 1 Hydrogen Liquid Flowrate Gas sulphide pH Liquid Run makeup recirculation flowrate concentra- value temp no. (1/h) (1/h) (1/h) tion Inlet of ( C) Inlet liquid liquid gas (mg S/1) (ppm) 1 3.78 600 8820 40 0.18 8.2 18.2 2 3.50 600 8820 32 0.16 8.1 17.0 3 3.65 600 8820 31 0.21 8.2 16.8 4 3.65 600 12800 56 0.18 6.5 16.8 5 3.65 1300 12800 53 0.10 5.4 15.8 6 7.96 1300 16800 40 0.7 4.0 13.9 7 7.96 1300 15200 42 0.06 4.7 16.0 8 7.96 350 14400 45 0.05 6.2 16.8 9 0 0 14400 44 - - Table 2 Hydrogen sulphide Gas concentration Loading Removal Run flowrate Inlet Outlet (g/h) rate no. (1/h) gas gas (g/h) removal (ppm) (ppm) 1 8820 40 0 0.535 0.535 100 2 8820 32 0 0.428 0.428 100 3 8820 31 0 0.415 0.415 100 4 12800 56 14.0 1.088 0.816 75 5 12800 53 9.0 1.030 0.855 83 6 16800 40 1.0 1.020 0.994 98 7 15200 42 0 0.969 0.969 100 8 14400 45 0 0.984 0.984 100 9 14400 44 0 0.962 0.962 100 As can be seen from these results, levels of hydrogen sulphide as high as 45 ppm, and possibly higher, can be removed totally, provided the optimum loading is not exceeded. Bearing in mind that many people can detect hydrogen sulphide at concentrations as low as 0.01 ppm, the figure which can be removed represents a great nuisance if it were to be allowed to escape into the atmosphere.

The exact mechanism of the chemical reaction is not yet entirely certain. The first reaction is known with some confidence: 3H2S + Fe203 < 2FeS + S + 3H2O It has been assumed that the overall reaction could be represented: 2H2S + 029 2H2O f 2S that is, thatsu Iphurwas the oxidation product. However several competing reactions are possible, to form products other than sulphur, such as pyrite and sulphate, both of which are stable.

It is also known that ferrous sulphide may be oxidised biologically as well as chemically. Bacteria such as thiobaclllus ferrooxidans thrive under acid conditions and can also oxidise sulphurto sulphate as an energy source. However, hydrogen sulphide in air does not provide bacteria with nutrients for growth and replication, and a source of trace nutrients would be required to establish a colony.

The pH value shown in Table 1 indicate that some production of sulphuric acid was taking place. In the first three runs, although thetitration curves using aerated samples indicate that the drop in pH value is sufficient to accountforthe full amount of hydrogen sulphide oxidised. Visual inspection of the column however indicated the presence of significant amounts of yellow substances: probably both sulphur and pyrite. The question of reaction mechanism presently remains therefore unresolved.

Run 9 was carried out with no liquid flow and represents a deliberate attempt to cause process failure through failure of the mechanical plant. Monitoring of the inlet and outlet gas was continued as before after the recirculating pump was switched off.

Full removal of the applied load (0.96 g/h) continued forfourweeks after the liquid flow ceased, afterwhich time deterioration in performance was rapid. Afterfive weeks virtually no hydrogen sulphide was being removed.

At this point liquid flow was restored, and the performance of the process began to improve, though itwas not until seven days later that 75% removal was achieved. Dunny this recovery period performance was also found to be greatly influenced by varying pH value.

It is clearthatthe process is not susceptible to failure from short-term disruption of power supply, or by pump failure. It is also clearthatthe recirculating liquid has an important role in the efficient removal of hydrogen sulphide. It may be concluded that no elaborate precautions are necessary to guard againstfailure from these causes, provided a periodic check is carried out.

The reasons forthe slow recovery of the process after failure are not fully understood. A possible explanation is that under the dryer conditions of run the product ofthe oxidation of ferrous sulphide changed from sulphur and sulphate to, for example, pyrite.As pyrite accumulates, an increasing proportion of the iron in the column would be unavailable for reaction. If the pyrite were oxidised only slowly without bacterial activity, recoverywould initially be slow.

However,the long period of operation before deterioration may indicate that under normal operation it is the oxidation offerrous sulphide of its immediate products that is the rate-determining step rather than the reaction of iron oxide with hydrogen sulphide.

Claims (10)

1. A method of odour control comprising the steps of providing a column of inert packing material, passing therethrough a solution of an iron salt, whereby to deposit iron oxide on the packing material, and passing an odorous gas through the column.

2. A method as claimed in claim 1 ,wherein the iron salt solution is recirculated to maintain the iron oxide deposit.

3. A method as claimed in either of claim 1 or claim 2, wherein additional iron salt may be added to the solution at intervals.

4. A method as claimed in either of claim 1 or claim 2, wherein the recirculating solution may be run off and a fresh iron salt solution be introduced.

5. A method as claimed in any one of the preceding claims, further comprising the additional steps of measuring the pH of the solution and in response thereto, adding alkali or acid to maintain a desired pH.

6. A method as claimed in any one of the preceding claims, wherein the packing material is of inert plastics material which has a high surface area but which allows free passage of countercurrent streams of gas and recirculating liquid.

7. A method as claimed in any one of the preceding claims, further comprising the step of growing within the column microorganisms which act to remove or reduce odorous gases.

8. A method as claimed in claim 7, wherein the microorganisms comprise thiobacillus ferrooxidans.

9. A method of odour control substantially as described herein with reference to the accompanying drawings.

10. A method of odour control substantially as described herein with reference to the Example.