GB2096983A - Process for the purification of physiologically-harmful and/or foul- smelling emissions - Google Patents

Abstract

A process for the purification of physiologically-harmful and/or foul- smelling emission in a biological effluent purification plant operating by the activated sludge process which comprises the emission, optionally in combination with air, being blown in under pressure towards the bottom of an aeration tank, without the oxygen content falling below the minimum of approximately 2 mg/l required in the aeration tank for the continuity of the vital processes.

Description

SPECIFICATION Process for the purification of physiologicallyharmful and/or foul-smelling emissions This invention relates to a process for the purification of physiologically-harmful and/or foul smelling emissions; more particularly it relates to a process for the purification of physiologicallyharmful and/or foul-smelling emissions in a biological effluent purification plant operating by the activated sludge method.

It is known that, as a result of industrial processes, physiologically harmful emission which is almost always characterised by an unpleasant odour accumulates in large quantities.

The foui-smelling organic or inorganic volatile substances contained in the emission are usually sulphur-containing compounds, nitrogencontaining compounds, such as trimethylamine and ammonia, and oxygen-containing compounds, such as formaldehyde and acetaldehyde. In the viscose industry, considerable quantities of emission containing carbon disulphide and hydrogen sulphide, for example, accumulate in the production of synthetic wool, rayon and cellophane, for example. In order to reduce the adverse effects on health which the odours released have on local populations and also the damage inflicted on vegetation, tall and expensive chimneys are erected for the purpose of discharging the foulsmelling and/or toxic gases.However, the odour problem in the neighbourhood of such industrial operations may only be reduced to a limited extent in this way, in addition to which the more or less partial success of such measures also depends upon the particular weather conditions prevailing.

Accordingly, an object of the present invention is to purify waste gases of the type in question in a more effective and less expensive manner.

According to German Offenlegungsschrift (DT OS) No. 2,839,173, the foul-smelling, physiologically-harmful gases in question are removed by means of a combination of biological solids and active carbon. In this specification, it is pointed out that biological solids and active carbon are too ineffectual on their own effectively to eliminate the harmful constituents of gas streams of the type in question.

It has now surprisingly been found that physiologically-harmful and/or foul-smelling emission may be effectively purified in a biological effluent purification plant operating by the activated sludge method providing the emission, optionally in combination with air, is blown in under pressure towards the bottom of the aeration tank without the oxygen content falling below the minimum of approximately 2 mg/l required in the aeration tank for the continuity of the vital processes.

In the majority of cases, industrial emission contains sufficient oxygen, in addition to the toxic substances to be eliminated, to be able to be directly and exclusively used as a medium for the artificial aeration of the aeration tank. Thus, in the production of viscose filaments, for example, the emission from degassing of the A-bath contains 24.4 g/m3 of H2S, 21.3 g/m3 of CS2 and 270 g/m3 (=20.5%, by weight) of oxygen. If such emission is blown in as the sole aeration medium towards the bottom of the aeration tank, a free oxygen content of 2 mg/l and higher may in principle be maintained therein. Accordingly, combination of this emission with air is not necessary for the continuity of the vital processes in the aeration tank providing the emission is available in sufficient quantities.However, if the quantities of emission to be eliminated are smaller than the quantities of air otherwise introduced into the aeration tank to maintain the minimum content of free oxygen, the emission should be combined with the corresponding balance of air which may be effected either by mixing or by separate injection. Combination of the emission with air will always be necessary when the emission has such a low oxygen content that the minimum oxygen content of approximately 2 mg/l required in the aeration tank cannot be maintained.

It has proved to be advantageous to blow the emission into the aeration tank in the form of fine bubbles by means of fine-pored aerators having pore widths of from 0.05 to 0.25 mm. The aerators normally used for aeration in the form of fine bubbles may be used for carrying out this measure. In other words, suitable aerators are the tubes, filter boxes and filter plates of porous filter material normally used and also porous steel tubes which are coated with porous plastics or wrapped in plastics cord and of which the pore width amounts to from 0.05 to 0.25 mm. In objective terms, the pore width of 0.05 mm is not an actual limit, but rather a necessity which practical experience has revealed.If the pore width falls below this limit, the danger of the aeration openings becoming blocked will of course increase, so that, in the absence of additional measures, it generally appears advisable for the pore width not to fall below the limit in question.

in addition, it is possible in principle for the emission, optionally in combination with air, to be blown in even closer to the bottom of the aeration tank in the form of bubbles of medium size. In that case, perforated tubes or plates having a perforation diameter of from 2 to 5 mm are used as the aerators and the emission is blown in approximately 30 cm above the bottom of the aeration tank. However, in view of the resulting lower utilisation of oxygen, it is generally preferred in accordance with the present invention for the emission to be blown into the aeration tank in the form of fine bubbles.

In this connection, it has proved to be advantageous for the emission introduced by means of fine-pored aerators, optionally in combination with air, to be blown into the aeration tank at a depth of from 3 to 4 metres. In this way, oxygen utilisation and elimination of the impurities present in the emission is optimally effected, irrespective of the type of substances to be eliminated. Even emission containing the physiologically-harmful and foul-smelling constituents, hydrogen sulphide and carbon disulphide, in concentrations of far more than 1000 ppm (in each case) is purified to the point where it is completely odourless.

Accordingly, the present invention provides a process by which it is possible additionally to use the biological effluent purification plants established for effluent purification in or in the immediate vicinity of chemical and related factories for purifying industrial emission. Since, in this case, the physiologically-harmful and/or foul-smeiling impurities are eliminated by bacterial transformation, the proposed process is basically superior to the discharge of emission into the atmosphere by means of tall chimneys. It is also more favourable in terms of cost because the aeration systems present in an effluent purification plant may be used very practically for eliminating the emission, i.e. there is, advantageously, no longer a need for emission to be blown into the aeration tank using a separate pipe system.In the majority of cases, the function of the existing aeration systems to provide the supply of oxygen, to generate turbulence in the emission tank for the purpose of preventing deposits of sludge and to mix the activated sludge as uniformly as possible with the effluent in the tank, may also be accomplished exclusively using accumulating emission. It is straightforward for those skilled in the art to determine by simple evaluation of an emission analysis whether a given quantity of emission having a given chemical composition may be blown into the aeration tank as the sole aeration medium or in combination with fresh air which is in any case necessary for the operation of the biological effluent purification plant. In either case, the optimal process variants may readily be determined by simple tests for any given emission.

In the purification of effluents from the viscose industry, an additional advantage is afforded by the process according to the present invention which, in this case, may be used for the simultaneous purification of H2S- and CS2containing emission in the same biological effluent purification plant. The precipitation of zinc by means of slaked lime required here for the elimination of zinc ions is significantly improved and the concentration of zinc in the effluent is again reduced by about half. In addition, considerable quantities of lime may be saved because, where this precedure is adopted, the pH may be reduced from above 7.2 to below 7.2.

The present invention is illustrated by the following Examples.

Example 1 The biological effluent purification plant used was one operating by the activated sludge method which simultaneously purifies effluent containing sulphuric acid and zinc from the production of viscose fibre, effluent containing hydrochloric acid and iron from the production of steel cord and effluent containing methanol, glycol and other organic products from the production of polyester fibre. The two tanks of this purification plant used for the test were each 18 metres wide, 100 metres long and 4.5 metres deep. The two tanks were aerated by means of 2350 porous aeration candles over which perforated flexible rubber tubes had been drawn.

The pore width of the porous materials was 0.18 mm. The aeration candles were situated at a depth of approximately 4 metres below the surface of the water. The two activated-sludge tanks contained 1 5,500 m3 of sludge/water suspension. After separation of the sludge, the purified effluents were respectively introduced into third and fourth tank of the same size containing approximately 50,000 carp.

By means of a 50 cm diameter polyethylene hose having a wall thickness of 0.3 mm, emission from the degassing stage of viscose fibre production was delivered to the effluent purification plant or more precisely to the intake opening of two rotary piston blowers which otherwise force 13,500 m3/h of fresh air into the activated sludge tanks. The concentration of H2S, CS2 and mercaptans resulting from admixture with fresh air taken in at the same time, was measured in the distribution pipe before the air entered the aeration candles of the biological treatment stage. The respective concentrations averaged 1700 mg/m3 of H2S and 1800 mg/m3 of CS2. The test ran for 14 days.

Samples of air taken from above the tanks showed that the emission had been completely freed from the impurities. This was underlined by the healthiness of the carp which was proof of the fact that the water coming from the activated-sludge tanks was as pure as effluent purified by the activated-sludge process using fresh air only for aeration.

By contrast, the H2S-containing emission improved the precipitation of zinc in the activated-sludge tanks. While the effluent normally contains more then 1 mg/l of zinc at pH 7.2 and, in the case of an influent having a high zinc content, the pH has to be increased to 7.4, it is now possible to reduce the pH to 6.8 without the zinc content of the effluent exceeding 0.5 mg/l. The saving of slaked lime over the process carried out at pH 7.4 amounts to approximately 10 tons per day. As for the rest, the CSB degradation and the clarity of the effluent remain unchanged. The oxygen content in the aeration tank amounted to 4 mg/l.

Example 2 The test described in Example 1 was modified to the extent that the quantities of H2S and CS2 gas were considerably increased. The emission delivered to the aeration candles now contained 5400 mg of H2S/m3 and 5100 mg of CS2/m3. The other test conditions remained unchanged. The test results obtained were the same as in Example 1. The oxygen content in the aeration tank amounted to 4 mg/l.

Claims (5)

Claims

1. A process for the purification of physiologically-harmful and/or foul-smelling emission in a bioligical effluent purification plant operating by the activated sludge process which comprises the emission, optionally in combination with air, being blown in under pressure towards the bottom of an aeration tank, without the oxygen content falling below the minimum of approximately 2 mg/l required in the aeration tank for the continuity of the vital processes.

2. A process as claimed in claim 1 in which the emission is blown into the aeration tank in the form of small bubbles by means of fine-pored aerators having pore widths of from 0.05 to 0.25 mm.

3. A process as claimed in claim 1 or claim 2 in which the emission is blown into the aeration tank at a depth of from 3 to 4 metres.

4. A process as claimed in any of claims 1 to 3 in which the emission blown in contains carbon disulphide and hydrogen sulphide.

5. A process as claimed in claim 1 substantially as herein described with particular reference to the Examples.