METHOD FOR LIBERATING, IN AN AQUEOUS PHASE, SUBSTANCE(S) WHICH IS/ARE IMPOSSIBLE, OR DIFFICULT, TO BIODEGRADE, FROM A COMPOUNDED MATERIAL WHICH IS NOT SOLUBLE IN WATER

Abstract: The invention relates to a method for liberating, in an aqueous phase, substance(s) which is/are impossible, or difficult, to biodegrade from compounded material which is not soluble in water and comprises this/these substance(s) fixed to, or enclosed in, an organic substance, by bringing the compounded material in an aqueous phase into contact with matter which is suitable for liberating the substance(s) in question, in which method the said matter consumes the organic substance while liberating, in the aqueous phase, the substance(s) which is/are impossible, or difficult, to biodegrade, and if desired separating off the substance(s) which has/have been liberated in the aqueous phase in this way. Preferably, the matter which is suitable for liberating the substance(s) in question is selected from the group consisting of microorganisms, bacteria, predators, protozoa and enzymes, while the compounded material which contains the substance(s) which is/are impossible, or difficult, to biodegrade is advantageously brought into contact, in a membrane bioreactor, with matter which is suitable for liberating this/these substance(s).

Description: Short title: Method for liberating, in an aqueous phase, substance (s) which is/are impossible, or difficult, to biodegrade, from a compounded material which is not soluble in water. The invention relates to a method for liberating, in an aqueous phase, substance (s) which is/are impossible, or difficult, to biodegrade from compounded material which is not soluble in water and comprises this/these substance (s) fixed to, or enclosed in, an organic substance, by bringing the compounded material in an aqueous phase into contact with matter which is suitable for liberating the substance (s) in question. A method of this nature is relatively conventional and consists, more particularly, in the bond between the substance (s) which is/are impossible, or difficult to biodegrade and the organic material being broken. The matter which is suitable for liberating the substance (s) is in practice usually a compound which is able to react with the substance (s) in question, forming a material which can be separated off from the reaction medium more easily. A method has now been found in which the first step is not to break the bond between the substance (s) which is/are impossible, or difficult, to biodegrade and the organic material, but rather to be able to convert the substance (s) which is/are impossible, or difficult, to biodegrade and is/are fixed to, or enclosed in, an organic substance, into the aqueous phase, and then separate it/them off in this way. More particularly, the invention relates to the method mentioned in the preamble which is characterized in that the said matter consumes the organic substance while liberating, in the aqueous phase, the substance (s) which is/are impossible, or difficult, to biodegrade, and if desired separating off the substance (s) which has/have been liberated in the aqueous phase in this way. The materials which are to be treated using the method according to the invention may, for example, be derived from the recovery of minerals, from water sanitation and/or soil sanitation, or biological surplus sludge from a wastewater purification installation (referred to as "WWPI") . Although the invention is explained below with reference to a specific embodiment, namely the treatment of biological surplus sludge from a WWPI, it will be clear to the person skilled in the art that at any rate the applications mentioned above are suitable for the present treatment process. It should be noted that in a WWPI, the microorganisms present therein are used to remove contaminants which are present in the wastewater from households and/or industry by the abovementioned microorganisms breaking them down so as to form new microorganisms with the formation of carbon dioxide, water, gaseous nitrogen or by enclosing these contaminants in or fixing them to the microorganisms, depending on the nature of the contamination. Examples of the latter category are, for example, heavy metals. An important condition which WWPIs impose on the microorganisms is that they can be separated from the water using separation techniques which are based on a natural or forced difference in density between microorganisms and water, such as settling and floatation systems. For this purpose, they must (be able to) accumulate in flocculent agglomerates and/or grow in flocculent colonies under the conditions used in the system. Microorganisms which do not satisfy this condition cannot be separated off by means of sedimentation and/or floatation systems and leave the system together with the water which is separated off. After the water has been separated from the flocculent microorganisms, it is drained. In order for these installations to operate correctly, it is necessary for the total volume of microorganisms to remain below a defined level . The excess of microorganisms is withdrawn from the purification process as an aqueous solution and is referred to as the "discharge sludge" or "surplus sludge" . Preferably, the surplus sludge is used in agriculture as an organic fertilizer. However, this relatively inexpensive disposal method may be undesirable or even prohibited if concentrations of heavy metals in the surplus sludge are too high. In the Netherlands, surplus sludge is a "remainder organic fertilizer" , as referred to in the Dutch Decree on Remainder Organic Fertilizers, known in Dutch as the BOOM. The BOOM decree prohibits the use of surplus sludge from a large number of WWPIs as an organic fertilizer in agriculture in the Netherlands, because the concentrations of one or more heavy metals in the dry matter exceeds the standard. Since separation of the heavy metals from the dry matter fraction is impossible or is economically unattractive using existing techniques, the surplus sludge has to be disposed of using an alternative method. Alternative methods of disposing of surplus sludge, such as export, fuel for power stations after initial drying, using the surplus sludge as landfill, optionally after (mechanical) dewatering and/or drying or burning the surplus sludge, optionally after (mechanical) dewatering and/or drying and using the residual ash as landfill, are expensive. The Dutch government is looking for a disposal method which in the long term will provide least pollution of the environment, and the preferred solution is mechanical dewatering followed by burning and disposing of the residual ash as landfill. This method is known for short as surplus sludge combustion. The associated investment costs are enormously high, owing to the investment required in infrastructure work (e.g. sludge combustion installations) , process equipment, auxiliary substances, logistics, labour, etc. It has now been found that these drawbacks can be counteracted by bringing the material which contains the contaminants (i.e. the compounds which are impossible, or difficult, to biodegrade) (i.e. the surplus sludge) into contact, in a membrane bioreactor, with matter which is suitable for liberating the said contaminants. It should be pointed out that membrane bioreactors are devices which are known per se. In this context, reference may be made, for example, to EP-B-0 463 062. In reactors of this nature, the contaminated wastewater is preferably treated in such a manner that the harmful organic compounds present are converted into harmless compounds such as carbon dioxide and nitrogen using the bacteria population which is present in the reactor. Hitherto, the use of a membrane bioreactor related to the treatment of all the wastewater, involving the treatment of a very large amount of water and contamination load. As has been explained above, it is attempted, in a process of this nature, to keep the compounds which are impossible, or difficult, to biodegrade in the reactor as far as possible, with the result that the treated water which leaves the reactor contains the minimum possible amount of these substances . By contrast, according to the invention, the substances which are impossible, or difficult, to biodegrade are liberated from the material to which they are fixed and these substances can leave the membrane bioreactor with the aqueous phase. It should be noted that the method according to the invention can be carried out both continuously and in a batchwise manner and relates to both anaerobic and aerobic systems . The present method is preferably carried out as an aerobic process. In this case, it is advantageous to work with a sludge load of at most 0.1 kg COD/ (kg dry matter. day) . With this level of sludge load, the organic substances are degraded to such an extent that the "substances to be liberated" which are present therein or fixed thereto can remain in the aqueous phase in such small dimensions that they can be separated from the reactor contents together with the aqueous phase. In this case, it should also be noted that with this level of sludge load there is also no increase, or only an extremely limited increase, in the quantity of living microorganisms . it should be noted that sludge load is to be described as a measure of the daily volume of feed given to a microorganism. The term "dry matter" in terms of the sludge load is a term customarily used in the Netherlands to quantify the abovementioned microorganisms and also includes inactive substances. COD refers to the chemical oxygen demand which represents a measure of the quantity of substances present which can be oxidized by dichromate. Owing to the fact that the substances which are impossible, or difficult to biodegrade are separated off with the aqueous phase in the method according to the invention, microorganisms which do not (are unable to) accumulate to form flocculent agglomerates and/or do not grow into flocculent colonies under the conditions employed in the system can also be used. The invention also relates to the use of a membrane bioreactor for liberating phosphorus compounds and/or halogenated hydrocarbons and/or heavy metals and/or heavy metal compounds from the bacteria population which is contaminated with heavy metals and/or metal compounds, by feeding this contaminated bacteria population as a feed source to another bacteria population in a membrane bioreactor so as to form an aqueous solution which contains phosphorus compounds and/or halogenated hydrocarbons and/or heavy metals and/or metal compounds, as the permeate. Such an application has the following advantages : accumulation and/or growth of bacteria in flocculent colonies is not necessary, while furthermore it is possible to avoid flushing out certain selected bacteria. higher bacteria concentrations are possible than with conventional separation techniques . Conventional separation techniques are based on natural or forced differences in density between bacteria and water, such as for example during settlement or floatation. The membrane bioreactor for carrying out the method according to the invention is preferably provided with a bacteria concentration of 30-50 g/1. This is considerably greater than the concentrations which are customary in floatation and settlement, namely 6-12 g/1 and 3-4 g/1, respectively. The advantages will be clear to the person skilled in the art . The invention will be explained in more detail with reference to an example and appended figure which diagrammatically illustrates a membrane bioreactor. The appended figure diagrammatically illustrates the principle of a membrane bioreactor during use in treating surplus sludge from a WWPI . The surplus sludge formed in a WWPI is fed, via line 2, to a membrane bioreactor 1, in this case an aerobic membrane bioreactor. A (second) microorganism population or biomass is present in this membrane bioreactor and can be maintained due to the fact that the (first) microorganism population which is present in the surplus sludge is able to serve as a feed source. For the sake of simplicity, this description refers to microorganisms; however, it will be clear to the person skilled in the art that this term can also be understood to mean bacteria, single-cell organisms and also higher organisms . Examples of higher organisms of this nature are various groups of predators, as well as many protozoa. Oxygen and/or oxygen-containing gas mixtures, such as air, and possibly other nutrients, is/are fed to the membrane bioreactor 1 via line 3. If desired, denitrification may also take place in the membrane bioreactor. Denitrification is the conversion of nitrate into nitrogen gas. The gaseous products (such as carbon dioxide) which are formed in the membrane bioreactor can be released into the atmosphere via line 4. Some or all of the contents of the membrane bioreactor are fed to a membrane separation system 5, for example via line 6. In this membrane separation system 5, the water, containing the liberated substances, is separated from the rest of the mixture which has been fed into the membrane separation system 5. This residual fraction is fed back to the membrane bioreactor, for example via line 8, while the water containing the liberated substances is obtained as the permeate via line 7. This permeate can be treated further using the method known in the prior art for recovering the liberated substances (for example phosphates and/or the heavy- metals) , such as for example ion exchange. If desired, the phosphates can be precipitated using lime so as to form, for example, calcium phosphate. If desired, line 7 includes a device (not shown) in which any inert (inorganic) material entrained, such as silicon dioxide and the like is separated off via line 9. Example A WWPI treats wastewater with a contamination load of 100,000 P.E. (population equivalent) per day. This corresponds to a flow rate of 15,800 m 3 of wastewater per day with a contamination load of approximately 5500 kg BOD 5 20 per day (biological oxygen demand after 5 days at 20°C) . The volume required for a conventional WWPI can be calculated, on the basis of this information, as follows: A sludge load which is often used in the Netherlands is 0.05 kg BOD 5 20 per kg dry matter per day, the amount of dry matter often being approx. 4 kg per m 3 . It should be noted that in this context "dry matter" is understood to mean the solid substance suspended in the liquid, mainly the active sludge. The reactor volume V is then calculated as: V = 5500/(0.05 x 4) = 27,500 m 3 . Such a WWPI for treating domestic wastewater could perhaps form approximately 0.8 kg of dry matter surplus sludge per kg BOD 5 20 , i.e. approximately 4400 kg of dry matter surplus sludge per day. The dry matter surplus sludge can be concentrated, using static dewatering systems, to approx. 20 kg per m 3 (approx. 2% of dry matter surplus sludge) . The surplus sludge volume is then V (sludge) = 4400/20 = 220 m 3 /day. This corresponds to approximately 1.4% of the original volume of wastewater. As a result, a much smaller membrane filtration unit system can be used than that which is required to purify the original volume of wastewater. The surplus sludge is treated further using the method which has been described above . For many heavy metals, such as arsenic, cadmium and mercury, the Dutch government is striving for a 0 discharge, meaning that they must be returned to the environment in the minimum possible quantities . By processing the surplus sludge according to the invention, it is possible for the substances which are in the surplus sludge or in some other way fixed and are impossible, or difficult, to biodegrade, such as heavy metals or heavy metal compounds, to be converted into the aqueous phase and, in this way, to be separated off as a permeate. Then, such substances can be separated off in a usable form and no longer have to be disposed of as residual substance for landfill, and may, to a large extent, satisfy the 0-discharge policy of the Dutch government.

Claims: CLAIMS 1. Method for liberating, in an aqueous phase, substance (s) which is/are impossible, or difficult, to biodegrade from compounded material which is not soluble in water and comprises this/these substance (s) fixed to, or enclosed in, an organic substance, by bringing the compounded material in an aqueous phase into contact with matter which is suitable for liberating the substance (s) in question, characterized in that the said matter consumes the organic substance while liberating, in the aqueous phase, the substance (s) which is/are impossible, or difficult, to biodegrade, and if desired separating off the substance (s) which has/have been liberated in the aqueous phase in this way. 2. Method according to claim 1, characterized in that the matter which is suitable for liberating the substance (s) in question is selected from the group consisting of microorganisms, bacteria, predators, protozoa and enzymes. 3. Method according to claim 1, characterized in that the compounded material which contains the substance (s) which is/are impossible, or difficult, to biodegrade is brought into contact, in a membrane bioreactor, with matter which is suitable for liberating this/these substance (s) . 4. Method according to one or more of claims 1 to 3 , characterized in that the compounded material used comprises the biological surplus sludge which is formed in a purification installation for purifying wastewater. 5. Method according to claim 4, characterized in that the membrane bioreactor deals with a sludge load of at most 0.1 kg COD/ (kg dry matter. day) . 6. Method according to claim 4 or 5, characterized in that the method is carried out as an aerobic process. 7. Method according to one or more of claims 1 to 6, characterized in that the substances which are impossible, or difficult, to biodegrade comprise phosphorus compounds, halogenated hydrocarbons, (heavy) metal compounds or complexes thereof . 8. Use of a membrane bioreactor for liberating heavy metals and/or heavy metal compounds from a bacteria population which is contaminated with heavy metals and/or metal compounds, by feeding this contaminated bacteria population as a feed source to another bacteria population in a membrane bioreactor so as to form an aqueous solution which contains the heavy metals and/or metal compounds, as the permeate .

Inventors: LUPPENS JAN EMANUEL GERARDUS, LUPPENS, JAN, EMANUEL, GERARDUS
Applicants: LUPPENS JAN EMANUEL GERARDUS, STORK MPS BV, LUPPENS, JAN, EMANUEL, GERARDUS, STORK MPS B.V.
Application year: 1998
Main classification: C02F3
IPC codes: C02F1, C02F3