GB2320020A

GB2320020A - Removing halogenated compounds from water

Info

Publication number: GB2320020A
Application number: GB9625478A
Authority: GB
Inventors: Frank Schwerdtner
Original assignee: Dow Deutschland Inc
Current assignee: Dow Deutschland Inc
Priority date: 1996-12-06
Filing date: 1996-12-06
Publication date: 1998-06-10
Also published as: GB9625478D0

Abstract

A method of reducing the AOX (ie the concentration of halogenated organic compounds) in waste water comprises feeding water having an AOX of at least 1mg/l into a biological treatment unit and contacting the water with activated sludge and either powdered lignite or activated carbon. Preferably the contact is for 3-24 hours and reduces the AOX by up to 95%. The coke/carbon may be added to the activated sludge prior to introduction of the waste water. The water may contain chlorinated organic compounds and originate from production of propylene oxide, epihalohydrin, cellulose ether, epoxy resin or a halogenated hydrocarbon. The water may be pretreated by stripping, absorption, pH adjustment or by adding nutrients for bacteria used in the activated sludge.

Description

METHOD OF REDUCING THE CONCENTRATION OF HALOGENATED ORGANIC COMPOUNDS IN WASTE WATER The present invention relates to a method of reducing the concentration of halogenated organic compounds in waste water.

Much effort has been made to purify waste waters using activated carbon or powdered lignite coke. In Chemie Ingenieur Technik 9 1088 (1995) "Weitergehende Reinigung von kommunalem Abwasser durch Pulverkohleeinsatz mit dem Ziel einer Wiederverwendung zur Grundwasseranreicherung" (Further purification of municipal waste water using powdered coke with the aim of recycling the water to ground water), C. Bornhardt et al. discuss the use of several types of activated carbon and powdered lignite coke in a pilot clarification plant with biological phosphorous and nitrogen elimination. The potential to reduce the concentration of AOX in waste water of an initial AOX of 50 microgram/l is discussed (AOX = organic halogens which are adsorbable on activated carbon). When 100 g of activated carbon per m3 of waste water was added to the aeration basin, the AOX concentration in the clarifier overflow could be reduced by 20 to 30 percent. When 100 mg of powdered lignite coke per litre of waste water was added to the aeration basin or to the clarifier overflow, an AOX reduction of only 12 percent was achieved.

In Korrespondenz Abwasser (Waste Water Correspondence) 36, (1989), Nr. 3, pages 282-286, "Lignite Coke to Enhance Cleaning Efficiency - A Contribution to Advanced Waste Water Treatment", G.W. Felgener et al. discuss the efficiency of pulverised lignite coke in the biological section of waste water treatment. The lignite coke was added at a concentration of 30g/m3. Continuous measurements over 24 hours were made. In one trial the average AOX concentration was 60 microgram/l without addition of lignite coke and 13 microgram/l with addition of lignite coke, i.e. the AOX reduction was over 70 percent. In another trial the AOX concentration without addition of lignite coke was higher, i.e. in the range between 100 and 200 microgram/l, in exceptional cases over 250 microgram/l. An AOX reduction of 50 to 65 percent was achieved.

While the above-mentioned references relate to the treatment of municipal waste waters, European Patent Application EP-A-0 406 733 relates to a process for removing halogenated organic compounds from industrial waste waters.

The European patent application discusses the known processes and their disadvantages. Adsorption on activated carbon or on an adsorber resin is considered to be too expensive and recovery of the halogenated organic compounds by extraction and distillation is considered to be too complicated. Therefore, the European patent application discloses a process wherein the waste water is contacted with sewage sludge that has been mixed with a lipophilic compound, such as vegetable or animal fats or oils or mineral oil. After this contact treatment the purified waste water is separated from the sewage sludge. Lignite coke, animal charcoal, powdered hard coal, sawdust and/or bentonites may be added to further increase the absorption properties of the sewage sludge. However, the lipophilic compounds mentioned dissolve partially in the waste water.

Even if the dissolved amount is small, it will increase the COD/BOD of the waste water in the aeration step of the waste water treatment. The European patent application is silent about this COD/BOD increase. Moreover, traces of the fats or oils may remain in the clarifier overflow after the settling of the sludge, which is clearly undesirable.

Furthermore, the European patent application is silent about the initial concentration of the halogenated organic compounds in the waste water prior to and after the contact with the sewage sludge.

One object of the present invention is to find a method by which the concentration of halogenated organic compounds in waste water can be reduced efficiently, even if this concentration is high. Another object of the present invention is to find such a method in which the COD/BOD loading of the waste water is not increased.

The present invention provides a method of reducing the concentration of halogenated organic compounds in waste water, wherein waste water having an AOX of at least 1 mg/l is fed to a biological treatment unit and is contacted with activated sludge to which powdered lignite coke or activated 3 carbon in a concentration of from 0.5 to 30 g per m of waste water has been added.

Surprisingly, it has been found that by the addition of powdered lignite coke or activated carbon to activated sludge in a biological treatment unit a surprisingly high AOX reduction can be achieved, even if the starting concentration is at least 1 mg/l and even if the powdered lignite coke or activated carbon has been added to the activated sludge in a concentration of only 0.5 to 30 g per m3 of waste water. The AOX of waste water treated according to the method of the present invention is generally at least 40 percent lower, typically at least 50 percent lower and in most cases even at least 60 percent lower than the AOX of waste water which has been treated in a comparable biological treatment unit but without addition of powdered lignite coke or activated carbon. Furthermore, the addition of powdered lignite coke or activated carbon to activated sludge does not lead to a BOD or COD increase in the treated waste water.

The Figure illustrates a flow sheet of a preferred embodiment of the present invention.

The AOX concentration of waste water which is treated according to the method of the present invention is at least 1 mg/l, generally from 1 to 100 mg/l, preferably from 5 to 50 mg/l, and more preferably from 10 to 30 mg/l. Waste waters with such a high AOX concentration are usually not municipal waste waters but originate from chemical production plants or from industries which use chlorinated solvents for purification or degreasing, such as metal processing or textile cleaning industries. The method of the present invention is particularly useful for the treatment of waste water which originates from a process wherein chlorine or an organic chlorine-containing compound has been present, preferably from a process for producing propylene oxide, an epihalohydrin, a cellulose ether, an epoxy resin, or a halogenated hydrocarbon.

If the concentration of halogenated organic compounds in an industrial waste water is very high, for example over 100 mg/l, the waste water is preferably first treated by known procedures, such as stripping and/or adsorption, to reduce the AOX to a concentration of 1 to 100 mg/l before the waste water is fed to the biological treatment unit. The biological treatment with activated sludge is generally preceded by one or more other steps, such as adjustment of the pH of the waste water to 7 to 9.5, and /or addition of nutrients for the bacteria of the activated sludge, such as nitrogen or phosphorous compounds, where appropriate. These previous steps are known in the art.

The waste water is then fed into a biological treatment unit and is contacted with activated sludge to which powdered lignite coke or activated carbon have been added.

The surface area of these materials is preferably more than 200 m2/g, more preferably more than 250 m2/g. Powdered lignite coke is the most preferred material. Accordingly, the subsequent description of the invention will relate to powdered lignite coke although the present invention is not limited thereto. The powdered lignite coke preferably has an average particle size from 0.03 to 2 mm, more preferably from 0.03 to 1 mm, most preferably from 0.03 to 0.3 mm. The most preferred material is commercially available from Rheinbraun AG, Germany under the designation "powdered coke".

The powdered lignite coke is added to the activated sludge in a concentration of from 0.5 to 30 g per m3 of waste water. The preferred concentration is from 1 to 20 g, more preferably from 2 to 10 g, most preferably from 3 to 5 g per m3 of waste water. It is advisable to mix the powdered lignite coke thoroughly with the activated sludge.

One way of adding the powdered lignite coke to the activated sludge is to prepare a suspension of the powdered lignite coke in water and to feed the suspension to the biological treatment unit, as described in German Patent DE 35 24 078.

However, according to a preferred embodiment of this invention the powdered lignite coke is first intensely mixed with a portion of the activated sludge to prepare a concentrate. The portion of the activated sludge to which the powdered lignite coke is preferably added generally amounts to from 0.5 to 10 percent, preferably from 0.5 to 5 percent, more preferably from 0.5 to 2 percent of the total weight of the activated sludge. This concentrate is then fed into the biological treatment unit.

Surprisingly, the powdered lignite coke is effective at a much lower amount than the amounts which are recommended in the prior art. For treatment of municipal waste waters with a much lower AOX concentration, the producer of powdered lignite coke (Rheinbraun AG, Germany) recommends addition of powdered lignite coke to activated sludge in various dosages for various purposes. 5 to 20 g per m3 of waste water are recommended if the effluent of the biological treatment unit has an undesirably high solids content. If the sludge concentration is too low because of high sludge volume index, 20 to 50 g/m3 are recommended.

30 to 100 g/m3 are recommended if the waste concentration varies and in case of toxic peaks, and up to 200 g/m3 are recommended if the waste water is too coloured.

The addition of fats or oils or other lipophilic compounds disclosed in European Patent Application EP 0406733 to the sludge is neither necessary nor desirable. Preferably, the waste water is contacted with the activated sludge without addition of such compounds.

The biological treatment of waste water with activated sludge (without previous addition of powdered lignite coke) is generally known in the art and described in the Handbook of water and wastewater treatment technology by Paul N.

Cheremisinoff, MARCEL DEKKER, INC. New York, Chapter 6, page 210 "Activated Sludhe". The use of powdered lignite coke according to the present invention does not interfere with the normal operation of biological treatment. The temperature in the biological treatment unit generally is from 200C to 400C, preferably from 25"C to 400C, more preferably from 30"C to 35"C. The residence time in the biological treatment unit generally is from 3 to 24 hours, preferably from 5 to 15 hours, more preferably from 7 to 10 hours. Then the treated waste water and the activated sludge can be fed into a clarifier where a separation of the treated waste water and the activated sludge occurs, usually by settling. Generally the activated sludge is recycled and a portion is removed as excess sludge.

Surprising AOX reductions are achieved by the method of the present invention although considerably less powdered lignite coke is added to the activated sludge than taught in the prior art. This AOX reduction in the presence of these very low amounts of powdered lignite coke is even more surprising in view of the high initial AOX concentration of the waste water, which is substantially higher than the AOX of waste waters in the known processes. The AOX of waste water treated according to the method of the present invention is generally at least 40 percent lower, typically at least 50 percent lower and in most cases even at least 60 percent lower than the AOX of waste water which has been treated in a comparable biological treatment unit but without addition of powdered lignite coke or activated carbon. For example, if the AOX concentration of the waste water can be decreased by about 90 percent in a biological treatment unit without addition of powdered lignite coke or activated carbon, the AOX concentration of the waste water can generally be decreased by about 95 percent or more in the same biological treatment unit if powdered lignite coke or activated carbon has been added to the activated sludge as described in more detail above.

The invention is illustrated by the following example and with reference to the drawing which should not be construed to limit the scope of the present invention.

Unless stated otherwise all parts and percentages are given by weight.

Example The biological treatment unit contained an aeration tank 1 and a clarifier 2. Waste water having an AOX concentration from 15 to 20 mg/l was fed via an inlet 3 into the aeration tank. Air was fed via an inlet 4 into the aeration tank.

In the comparative run, the waste water was aerated in the aeration tank at a temperature of from 28"C to 300C and a residence time of 7.5 hours during 20 days in the absence of powdered lignite coke. The mixture of treated waste water and activated sludge was fed via a conduit 5 into the clarifier 2 wherein the activated sludge was separated from the treated waste water by settling. The treated waste water was removed from the clarifier via an outlet 6. The AOX concentration of the treated waste water varied between 1.4 mg/l and 2 mg/l. About 90 percent of the settled activated sludge was recycled to the aeration tank 1 via a conduit 7. The remaining portion of the settled activated sludge was removed via an outlet 11.

For carrying out the method of the present invention, powdered lignite coke was used which is commercially available from Rheinbraun AG, Germany under the designation "powdered coke". It had the following particle distribution: 5 weight percent > 0.315 mm, 25 weight percent from 0.125 to 0.315 mm, 10 weight percent from 0.09 to 0.125 mm, 10 weight percent from 0.063 to 0.09 mm, 20 weight percent from 0.032 to 0.063 mm and 30 weight percent < 0.032 mm. Powdered lignite coke was mixed with water at a weight ratio of 1:4 until the entire material was wet. The wet powdered lignite coke was fed into a mixer 8 via an inlet 9.

About 5 percent of activated sludge, based on the total amount of activated sludge that was recycled per hour to the aeration tank, was fed into the mixer 8 via a conduit 10 and was mixed with the wet powdered lignite coke. The mixture was added to the residual amount of recycled activated sludge. The resulting initial concentration of the powdered lignite coke in the aeration tank was 0.6 g per m3 of waste water. The amount of powdered lignite coke added was gradually increased over 100 days until a concentration of 3 g per m of waste water was reached and the system was in a stationary state. When the stationary stage was reached, the AOX concentration of the waste water which was fed into the aeration tank 1 and the temperature and residence time in the aeration tank were the same as in the comparative run.

The AOX concentration of the treated waste water varied between 0.5 mg/l and 0.7 mg/l. The added powdered lignite coke did not adversely impact the handling of the excess sludge which was removed from the biological treatment unit.

Claims

1. A method of reducing the concentration of halogenated organic compounds in waste water, wherein waste water having an AOX of at least 1 mg/l is fed to a biological treatment unit and is contacted with activated sludge, and with powdered lignite coke or activated carbon in a concentration of from 0.5 to 30 g per m3 of waste water.

2. The method of Claim 1, wherein the concentration of the powdered lignite coke or activated carbon is from 1 to 20 g per m of waste water.

3. The method of Claim 2, wherein the concentration of the powdered lignite coke or activated carbon is from 2 to 10 g per m3 of waste water.

4. The method of any one of Claims 1 to 3, wherein the waste water is contacted with activated sludge to which powdered lignite coke has been added.

5. The method of any one of Claims 1 to 4, wherein the waste water is contacted with the activated sludge without addition of fats or oils

6. The method of any one of Claims 1 to 5, wherein the waste water has an AOX of from 5 to 50 mg/l.

7. The method of any one of Claims 1 to 6, wherein the waste water originates from a process wherein chlorine or an organic chlorine-containing compound is employed.

8. The method of Claim 7, wherein the waste water originates from a process for producing propylene oxide, an epihalohydrin, a cellulose ether, an epoxy resin, or a halogenated hydrocarbon.

9. The method of any one of Claims 1 to 8, wherein the waste water is contacted with the activated sludge for a period of from 3 to 24 hours.

10.The method of any one of Claims 1 to 9, wherein the AOX concentration of the waste water is reduced by 95 percent or more in the biological treatment unit.