GB2029227A - Deodorising waste gases - Google Patents

Abstract

A method of deodorisation of factory gases comprises passing the gases through layers of peat which include bacteria. To ensure bacterial development various substances may be added as alternatives or in combination including glucose, phosphates and lacto-serum. The peat may also include marl sand to permit free passage of the gases therethrough. The peat preferably is arranged in layers so that the gases pass through the layers in turn. The bacteria may be obtained by utilisation of sludge. The method is particularly applicable to the treatment of waste gases formed in the processing of animal carcasses, and containing e.g. amines and mercaptans.

Description

SPECIFICATION Process of deodorising of factory gases The present invention relates to a method of deodorisation of the gases produced in factories in which animal carcasses are cut up. It also relates to an apparatus for carrying out the method.

Factories in which carcasses are cut up are sources of bad smells. In these factories the scraps of animals comprising muscles, viscera, bones feathers etc., are cooked by vapour in stoves. The solid residues are recovered. The vapours are condensed in condensation towers at the outlet of the cookers.

However, some gases do not condense and are generally discharged directly into the atmosphere.

These gases are mal-odorous and there results therefrom in factories in which animal carcasses are cut up disagreeable conditions of work and pollution of the environment. It has therefore proved to be necessary to deodorise the gases before they are discharged into the atmosphere.

According to studies already made regarding the composition of these non-condensable gases, the bad smells are probably due to the presence of nitrogenous compounds and sulphurated carbonyls produced at the time of fermentation of the materials of animal origin; the compositions in these products may vary to a wide extent according to the animal materials treated. As regards the nitrogenous products from chromatographic analysis show in particularthe presence of ammoniac, methyl amine and trimethyl amine. As regards the sulphurated products they show the presence besides sulphurated hydrogen, of methyl mercaptan, ethyl mercaptan, propyl mercaptan and butyl mercaptan.

The methods known at present for the deodorisation are simple combustion, catalytic combustion, ozonization and the chemical washing of gases. These methods are onorous, difficu It to maintain and only cause the polutionto be modified or transferred.

One object of the present invention consists in providing a method of deodorisation by absorption of gases in layers of natural material which is not very onorous, the coefficient of absorption of which is distinctly greater than that of the earth.

Another object of the invention consists in providing a method for deodorisation permitting of obviating the saturation of the said absorbent layers by degradation in situ of at least a large part of the gases absorbed.

According to one feature of the invention there is provided a method of deodorisation of gases produced in factories in which animal carcasses are cut up in which the gases pass through layers of peat sown with bacteria, for the absorption of undesirable constituents from the gases.

According to another feature glucose and phosphates in suitable quantities are added to the layers of peat to ensure the bacterial development.

According to another feature lacto-serum and phosphates in suitable quantities are added to the layers of peat to ensure the bacterial development.

According to another feature marl sand is added to the peat.

According to another feature the bacteria is su pplied to the peat in the form of muds from sludge purification stations.

The features of the present invention mentioned above as well as others will appear more clearly on reading the following description of embodiments of the method, the said description being made in respect of the attached drawings among which:~ Fig. 1 is a diagrammatic view of a deodorisation installation according to the invention.

Figs. 2a, 2b and 2c are diagrams representing spectra obtained by chromatography of gas samples taken at different levels of a column of untreated peat; Figs. 3a to 3c are diagrams representing spectra obtained by chromatography of gas samples taken at different levels of a column of peat to which sludge has been added; Figs. 4a to 4c are diagrams similar to those of Figs.

3to3c; Fig. 5 is a diagram representing the curves of saturation of the untreated peat at different levels of the column; Fig. 6 is a diagram showing curves of efficacity of the peat to which sludge, and periodically glucose and phosphate have been added; and Fig. 7 is a diagram representing curves of efficacity of the peat to which sludge, and periodically lactoserum and phosphates have been added.

In Fig. 1, a series of cut up animals carcass cookers are shown at 1, the vapours from which are, after passage into a compressor 2, condensed in a tower 3 in which the cold source is water cooled by the ambient air.

The condensate can be withdrawn through a drain 4. The non-condensed gases leave at the top of the tower and, through a pipe 5, are introduced at the bottom of a deodorisation column 6 which forms the subject of the invention.

In practice, the column 6 has a height of a few metres its cross-sectional size depending on the supply of gas to be treated. It is constituted by the peat or a mixture of peat, and marl sand or peat and earth. The pipe 5 emerges under the column in a compartment 7 limited at the top by a grill 8 on which the peat rests. With the compartment 7 is associated a purifier 9 to avoid condensate wetting the peat by accident. Gas collectors 10 are provided to supply gas samples to analysis apparatus by means of pipes 11, for observing the behaviour of the column 6 at different levels.

The column 6 may be limited by the walls of a hole dug in the ground, these walls being covered by a layer of impervious material. The top of the column is open to allow the stripped gases to escape into the atmosphere.

In the course of experiments carried out to ensure the efficacity of the method of the invention, a column of the type of column 6 was used which had a diameter of 1 metre and a height of 3 metres. The peat was placed in baskets of 50 cm heith and the distance between two superimposed baskets was 10 cm thus leaving a free space permitting the easy placing of the gas collectors 10. In practice, these collectors were simple tubes leading into the three spaces and connected to small output pumps con veying the gases to the measuring apparatus.

The use of baskets of limited height permits effec tive contact of the gas with materials mixed with the peat such as earth or marl sand.

In the course of experimentation it has been found that there is very little pressure drop as the gas flows through the peat which is a very light material. In the experimental column for a height of peat of 2.50 m, the pressure drop is in the order of 30 mm water for a speed of gas to be treated corresponding to outputs from 20 to 25 m3/m21h. It has likewise been found that mixtures of peat and earth can be used comprising up to about 30% earth without problem due to pressure drop. For higher proportions of earth cloggings are produced which render the column practically unusable.

The gases used for the experiments were noncondensable gases from an animal carcass cutting up factory. The composition in total nitrogen of these gases was very variable from 100 to 1200 ppm with an average of the order of 400 ppm. The amounts of sulphurated hydrogen and methyl mercaptan of these gases were likewise very variable, most generally of the order of a few ppm but with peaks capable of going up to 600 ppm for H2S and 200 ppm for CH3 SH during certain periods (decompression of the hydrolyser pens) in the animal carcass cutting up factory.

First series of experiments The series of experiments permitted of evaluating the performances of the peat itself, of peat mixed with earth and of peat mixed with marl sand.

Table 1 hereinafter summarises the tests carried out with the experimental column defined above, containing five baskets of peat alone, the gases being admitted continuously in the column from the factory with an output of the order of 25 mVmSh. The first column of the table indicates the period, after the commencement of the trials, when the associated readings were made. The second column gives the concentration in sulphurated compounds in the gases admitted on the day indicated, this concentration being expressed in ppm of H2S. The third column gives the concentration in nitrogenous compounds in the gases admitted on the day indicated this concentration being expressed in ppm of NH3.

The five last columns give the concentrations in nitrogenous compounds expressed in ppm of NH, at five levels between the baskets.

In the table 1 the concentrations in sulphurated compounds taken at 5 levels of the column have not been shown because there has been detected at these levels throughout the experiment only traces of sulphurated compounds. It appears therefore that the peat absorbs perfectly the sulphurated compounds. The result is remarkable because as the particulars of the second column show the concentrations at the beginning vary very much.

The figures of the third column indicate that the concentration of nitrogenous compounds in the crude gas has varied from one day to another but that practically on one day out of two the concentration is practically close to the average of 450 ppm of NH3 and the number of days in which the concentration departs very much from this average is very small. One may therefore consider as a first approximation that the column of peat is fed in a rather consistant manner.

The figures referring to the Level 1 show that the peat commences by absorbing the nitrogenous compounds then is saturated at the end of 25 to 30 days. Consequently, the figures reveal an instability by not remaining constant at the level of saturation.

The figures referring to the level 2 show that the same phenomenon are produced but with a certain delay. This is likewise for levels 3 to 5.

TABLE NO. 1 time ppmH2Sin ppmofNH3 Level 1 Level2 Level3 Level4 Level 5 days crude gas crude gas 50 cm 100 cm 150 cm 200 cm 250 cm considered not measured 423 1.74 0.74 1.00 0.74 0.50 4.5 51.8 399 0.98 0.69 0.23 0 0 6 - 512 2 1.2 0.65 0 0 8 - 516 1.16 1.55 0.62 0 0 10 49 770 0.38 0.40 0.24 0 0 12.5 20 850 1.60 0 0 0 0 14.5 - 500 59.6 0 0 0 0 16 42.3 93 93 0.3 0 0 0 18 6.6 264 256 1.4 0.9 0.7 0.5 20 167 195 234 1.9 1.1 0.6 0.4 22 66 406 364 0.50 0.50 0.37 0.25 25 traces 131 172 104 3.73 2.30 1.08 30 traces 425 786 770 4.04 1.87 0.25 32 traces 429 451 460 104 1.0 0.57 34 traces 909 744 542 1.6 0.6 36 traces 242 398 348 360 0.2 0.2 38 traces 372 675 472 533 116 0.76 40 traces 533 345 373.5 273.5 158.5 2.1 42 traces - 447 382 397 279 44.6 44 - 514 478 469 445 46 traces 360 - 535 490 443 12 48 traces 334 570 512 489 461 20 Furthermore, the first spectrum of Fig. 2a is an example of the spectrum obtained by chromatography of the gases supplied to the column of peat. In the spectrum at I one has the peak of the solvent, at II the peak of the ammoniac, at III the peak of the methyl amine, at IV the peak of the dimethyl amine and at V the peak of the trimethyl amine.

The spectrum of Fig. 2b is an example of the spectrum of the gases coming from the first level and that of Fig. 2c is an example of the spectrum of the gases from the second level, the peaks being marked as in Fig. 2a.

It will be noticed that the peak II no longer exists in the Figs. 2b and 2c but that on the contrary the other peaks subsist. One may conclude that the ammoniac is effectively absorbed in the peat but that it is not the same for the amines. The absorption of the ammoniac in the peat is confirmed by the pH of the peat which passes from 4 to 9 or 9.5.

In the course of the experiments, it has been found likewise that slight variations of ambient temperature did not have any or any appreciable effect on the absorption of the peat.

Other experiments on the columns of peat mixed with earth have shown that the earth absorbed not the nitrogenous and sulphurated compounds but caused a rapid clogging of the column entailing a prohibitive loss of charge. To reduce the pressure drop there has been likewise been mixed peat and marl sand which has not brough about modifications as regards the absorption of the polluting gases.

From details collected regarding the absorption of the ammoniac in the peat alone it has been possible based on the Adams Boharttheory to calculate the characteristics of absorption of the peat. The curves of Fig. 5 show the various levels absorb the gases with time and are in fact curves of saturation of untreated peat at different levels of the column. Thus it has been possible to determine that a factory having to treat 5,000 m3lh of noncondensable gases containing 400 ppm of NH3 on the average must have a filter of peat of 200 m2 in section by 2m high to work at the rate of 25 m31m2/h. The peat would be found to be saturated at the end of 6-7 weeks of operation.In order that such a method of deodorisation may be usable in the industrial field it is necessary to arrange to regenerate the peat on the spot because its replacement at the frequence envisaged would be of a prohibitive cost.

Second Series of Experiments These series of experiments have permitted of evaluating the performances of the peat sown with sludge to which has been added periodically phosphates for promoting bacterial development.

In the course of preliminary experiments, noncondensable gases were caused to bubble in two tanks containing muds coming from the purification station. It is known that such muds may contain two classes of bacteria: heterotrophic on the one hand and autotrophic on the other hand.

In each tank, the elements for promoting the development of the bacteria were supplied: that is phosphates, oligoelements, then the pH was adjusted to 8 or 8.5 to favour this development.

In the first tank the development of the heterotrophic bacteria were favoured by the supply of glucose serving as carbonated substratum.

In a second tank, the development of the auto trophic bacteria was favoured by providing there in milieu exempt of D.C.O. (demand consumption of oxygen) the effluent studied being itself free of D.C.O. and applying there carbonate periodically.

It has been able to be established that the muds of the first tank had become black and are very rapidly developed whilst those of the second tank have become light maroon and very little concentrated.

It has been deduced therefrom that the ammoniac is very rapidly eliminated by the heterotrophics.

Furthermore, in the first tank neither nitrates nor nitrites have appeared. Therefore there has only been an assimilation there. In the second tank, a group of autotrophics eliminates much more slowly the products with the formation of nitrates and nitrites.

In the experiments on the column of peat sown with muds, first of all carbonated substrata has not been applied, that is to say glucose has not been supplied. The spectrum of Fig. 3a concerns the crude gases admitted at the bottom of the column. It causes a peak I corresponding to the solven to appear, a peak II corresponding to the ammoniac, a peak Ill corresponding to the methyl amine, a peak IV corresponding to the dimethyl amine and a peak V corresponding to the trimethyl amine. The spectrum of Fig. 3b concerns the gas recovered at the first level after having passed through a layer of sown peat.

The peaks Ill to V have disappeared, the peak II of the ammoniac subsists. The spectrum of Fig. 3c concerns the gas recovered at the second level. It is similay to that of Fig. 3b.

It can be concluded therefrom that the amines are completely eliminated in a layer not very thick of sown peat therefore that the bacteria assimilate particularly well the amines. On the contrary, the action on the ammoniac is less clear.

Experiments have likewise been carried out mixing marl sand with the peat, the marl sand having to serve a source of CO2 may likewise serve as a bacteria support. The results have shown that no substantial improvement was recorded in respect of the sown peat alone.

Third series of experiments This series of experiments permitted of evaluating the performances of the peat sown with muds to which has been added periodically not only phosphat but also glucose.

The spectra of Figs. 4a to 4c, which are similar to the spectra of Figs. 3a to 3c, commented on above, show that the peaks of the amine have disappeared in the passage through a layer of peat and that the peak of the ammoniac has disappeared after the passage through two layers.

In practice, as the curves of percentages of elimi nation per level of Fig. 6 show, when one ceases to supply glucose after a certain time there is a saturation of the peat which had already been ascertained in the course of the second experiments. From the new supply of glucose, elimination of the ammoniac resumes and the percentage of elimination increases.

These experiments confirm the preliminary exper iments mentioned above, namely that the assimilation of the ammoniac is mainly due to the heterot rophic bacteria.

In conclusion, the experiments above show that the peat is an excellent absorbent of noncondensable gases and that the saturation of the peat may be avoided by sowing it with heterotrophic bacteria existing in the activated muds of the purification station on condition of supplying to these latter phosphates and a carbonated substratum. It is obvious that the glucose utilised in the third series of experiments may represent a non-negligible cost and other experiments have been carried out to discover a product which is not very expensive and capable of serving as a carbonated substratum.

Fourth series of experiments This series of experiments permitted of evaluating the performace of the peat sown with muds to which has been added phosphates and lacto-serum serving as a carbonated substratum.

The lacto-serum is a dairy effluent not costing very much, being found practically on the spot because the animal carcass cutting up factories are normally established in a rearing zone and are therefore in the proximity of dairies. The lacto-serum has a relatively high consumption of oxygen demand (about 80 g of oxygen per litre) and is thus poor in nitrogenous products. Its average balance composition is the following: Rennet Lactic Water 93.5 93.5 Lactose 4.5 4.0 Nitrogenous materials 0.9 1.0 Fats 0.3 0.1 Mineral materials 0.6 0.8 Lactic acid 0.2 from 0.6 to 1.0 Fig. 7 shows according to the time the percentage of purification of the gases, indicating the percentage of nitrogenous products in the residues extracted at the five levels of the column. The lactoserum is added to the first layer of peat at the rate of about 80 once every five days.The crude gases at the beginning of the column contains from 200 to 300 ppm of NH3. The output of gas was 15 m3/h, which corresponds to a speed of 20 m3lm2lh at the commencement of the experiment, then it was 40 m3/h after the 75th day of the experiment. Phosphate at the rate of 200 g was supplied about every 10 days.

It follows from Fig. 7 that the gases leaving at the 5th level no longer contain nitrogenous compounds and that between the 30th day of treatment and about the 80th day the concentrations remain constant at the other levels. Afterthe 90th day, the rising of the curves is due to a decrease in the ambient temperature combined with an increase in the output which entails a supply of condensate at the bottom of the column to the point that the two first layers were saturated with humidity. In order to mitigate this drawback the drain 9, Fig. 1, must be opened to drain off residues.

It must also be noted that the sulphurated compounds of gases admitted in the column are as in the preceding experiments absorbed in the first layer of peat.

It follows from the preceding results that the lacto-serum acts like the glucose. The spectra of chromatography likewise confirmed these results.

Of course, the method of the invention is applied likewise to gases originating from other factories than animal carcass cutting up factories when these gases comprise similar components. Other carbonated substrata charged with D.C.O. may be utilised instead of glucose or lacto-serum.

Claims (9)

1. A method of deodorisation by absorption of factory gases, characterised in that the gases pass through layers of peat sown with bacteria.

2. A method according to claim 1 characterised in that glucose and phosphate in suitable quanties are added to the layers of peat to ensure bacterial development.

3. A method according to claim 1 characterised in that lacto-serum and phosphates in suitable quantities are added to the layers of peat to ensure the bacterial development.

4. A method according to one of the claims 1 or 3 characterised in that marl sand is added to the peat.

5. A method according to one of the claims 1 to 4 characterised in that the bacteria are applied to the peat in the form of muds issued from biological purification of stations.

6. A method according to claim 1 characterised in that a carbonated substratum charged with D.C.O.

and phosphates in suitable quantities are added to the layers of peat to ensure the bacterial development.

7. A method of deodorisation by absorption of factory gases substantially as hereinbefore described with reference to the accompanying drawings.

8. An apparatusforthedeodorisation of factory gases comprising a treatment column containing peat sown with bacteria, means for passing the cases to be deodorized in the column and outlet means for discharge of deodorized gases which have passed through the peat.

9. An apparatus for deodorisation by absorption of factory gases substantially as hereinbefore described with reference to the accompanying drawings.