NZ287397A - Waste water treatment; method of purifying waste water from dairies by anaerobic conversion and then separating off reaction products - Google Patents

Abstract

Disclosed ia s method of purifying dairy waste water by anaerobic conversion and subsequently separating off the reaction products thus produced, the method comprising the following steps: (a) pretreatment of the waste water with a base; (b) introduction of the pretreated waste water into a fermenter, anaerobic fermentation of the lactose present in the waste water to give lactic acid, and subsequent purification of the fermentation bath; (c) reduction of the lactate concentation in the waste water and concentration of the lactic acid and base by bipolar electrodialysis. Also disclosed is a device for carrying out the invention.

Description

New Zealand No. International No. 287397 TO BE ENTERED AFTER ACCEPTANCE AND PUBLICATION Priority dates: 08.06.1994; Complete Specification Filed: 02.06.1995 Classification:^) C02F3/28; C12P7/56 Publication date: 24 June 1997 Journal No.: 1417 NEW ZEALAND PATENTS ACT 1953 COMPLETE SPECIFICATION Title of Invention: Method of purifying waste water from dairies Name, address and nationality of applicant(s) as in international application form: FRAUNHOFER-GESELLSCHAFT ZUR FORDERUNG DER ANGEWANDTEN FORSCHUNG E.V., Leonrodstrasse 54, D-80636 Munchen, Federal Republic of Germany New Zealand No. International No. 287397 NEW ZEALAND PATENTS ACT 1 953 COMPLETE SPECIFICATION Title of Invention: Method of purifying waste water from dairies Name, address and nationality of applicant(s) as in international application form: FRAUNHOFER-GESELLSCHAFT ZUR FORDERUNG DER ANGEWANDTEN FORSCHUNG E.V., of Leonrodstrasse 54, D-80636 Munchen, Federal Republic of Germany /\ Fraunhofer-Gesellschaft... eV. 9721a: Description - 1 - Process for purifying dairy wastewater The present invention relates to a process for purifying dairy wastewater by anaerobic metabolism and separation of the reaction products formed and an apparatus for carrying out this process.

In conventional wastewater purification processes, the degradable constituents are degraded either anaerobically to form methane and C03 or aerobi-cally to form C02, water and biomass. In these processes, a considerable part of the energy contained in the wastewater constituents, or in the case of aerobic degradation, all of the energy, is lost. In the aerobic process, in addition to the high biomass formation, considerable amounts of sludge occur. The principal disadvsuitage of the known anaerobic wastewater treatment process is the slow reaction rates which are required for mass conversion (max. 1 g/1 h COD degradation) . The anaerobic treatment of dairy wastewaters is surveyed by Temper et al. in "Berichte aus Wassergutewirtschaft und Gesundheitsingenieurwesen", [Water Industry and Health Engineering Reports] , Technical University, Munich, 1986, No. 67.

The object of the present invention is to provide a process and an apparatus for purifying wastewaters from milk-processing enterprises and, in particular, whey-containing wastewaters, in which the principal carbon source is not converted to methane, C0a and water, but to 287 3 97 287 3 9 7 organic acid, so that a major part of the energy contained in the starting substances is retained. Using the novel solution of this object, a wastewater having only a low COD load is obtained, together with lactic acid which can in turn be used in the food industry, in agriculture or in the chemical industry.

The production of lactic acid from whey is known. Processes therefor are described in EP0265409 A1 and FR2555200. However, the purpose of these processes is the inexpensive production of the product from whey-based substrates. They therefore differ in essential features from the invention described in more detail below.

The accompanying figures are briefly described below.

Figure 1 is a process diagram which shows in outline an exemplary embodiment of the process according to the invention together with some variants.

Figure 2 shows the concentration of dissolved calcium ions and phosphate ions (in mg/1 in each case) as a function of the pH established during the pretreatment.

Figure 3 shows the transmembrane flux during the filtration of a whey permeate fermented by casei as a function of time, with or without pretreatment with base (microfiltration membrane Carbosep M20, (manufacturer; Rhone-Poulenc) 0.2 mm nominal pore diameter, 1 bar, 1 m/s) .

Figure 4 shows the formation of lactic acid from whey permeate (P) with unhydrolyzed whey protein (M) and whey protein (M) hydrolyzed in the reactor by Alcalase. 287 3 97 Figure 5 shows the productivities of the various reactor systems at various lactose concentrations.

Figure 6 shows a comparison between ultrafiltration and microfiltration in the filtration of L^. casei from whey fermentation solution under the same conditions (1 bar, 1 m/s) Carbosep M 20 (0.2 mm) and Carbosep M9 (300,000 Daltons) (manufacturer: Rhone-Poulenc).

Figure 7 shows a possible configuration of the bipolar membranes in the third stage of the process according to the invention.

Figure 8 shows the influence of lactic acid concentration in the wastewater on the electrodialysis performance.

Figure 9 shows the decrease of the COD value in the wastewater during the bipolar electrodialysis of (prepurified) fermentation broth fed batchwise.

Figure 10 shows the isolation of free lactic acid and sodium hydroxide solution by bipolar membranes.

Figure 11 shows the procedure described in more detail in the text in electrodialysis for combined wastewater purification and product isolation (batchwise feed of the fermentation broth).

Figure 12 shows salt being separated off from the lactic acid by a downstream monopolar electrodialysis (with chloride-enriched fermentation solution).

Figure 13 shows diagrammatically by way of an example an apparatus for carrying out the process according to the invention.

The process according to the invention essen- 287 3 97 tially includes three stages: in the first stage, the wastewater is pretreated with base. In the second stage, che pretreated wastewater is introduced into a fermenter, where the lactose present is fermented to lactic acid, and the fermentation broth formed is given a secondary purification. In the third stage, the lactic acid concentration in the wastewater is reduced and lactate ions and base are concentrated using bipolar electrodialysis.

The term "lactic acid" is used here in the broad sense and includes all chemical forms of lactate (that is, the acid itself, its salts or the lactate anions) as a function of the respectively chosen or prevailing conditions, in particular the pH.

All dairy wastewaters which contain milk or whey residues can be used for the process according to the invention. However, preferably, the permeate produced in the ultrafiltration of whey-containing wastewaters is used. For the ultrafiltration (for sterile filtration and for protein isolation), an exclusion limit of 15,000 to 30,000 is recommended. Sterile take-off and feed to the buffer tank is ensured by this means if the procedure is otherwise carried out correctly.

The wastewater is treated in the first stage with base, preferably with aqueous base. Bases which are suitable are alkali metal hydroxides, alkali metal carbonates and/or ammonium compounds. Very particularly preferably, the base used is that alkaline solution which is produced in the third stage in the concentration of 287 3 97 the filtrates. By this means, the addition of external chemicals (which would possibly further increase the COD load of the wastewater) can be avoided to the greatest possible extent or avoided entirely. The amount of base fed should be sufficient to precipitate out a portion, as far as possible the majority, of the Ca3* and tig2* ions and P043" ions present in the wastewater (see Figure 2) . It is preferred that the pH of the wastewater is adjusted to a 7. In one embodiment of the process, the wastewater is adjusted to a pH of 2 10 and is charged into a reservoir vessel in which the calcium phosphate precipitating out is sedimented. Obviously, the calcium phosphate can also be separated off by centrifugation or filtration, in a hydrocyclone or by other comparable measures.

In one specific embodiment, the above described base treatment is carried out in at least one buffer tank (reservoir vessel). This is equipped with a stirrer and a heater. It can be sterilized as far as possible and should be equipped with a pH controller or at least with a pH meter. It has em opening in the bottom of the vessel for take-off of the calcium hydrogen phosphate precipitated out and other possible precipitates and it has another take-off which is adjustable in height and serves for removing the permeate. Two (or more) of such buffer tanks (reservoir vessels) cam be operated alternately. One buffer tank then serves each time for charging the fermenter. The other tank is filled and the appropriate pH is established and the calcium hydrogen phosphate and the like are separated off.

Advantages of the novel wastewater pretreatment are the marked decrease of the phosphate load in the wastewater and the avoidance of fouling and scaling problems in those downstream process steps which require the use of membraneb (eg. microfiltration (see Fig. 3) or electrodialysis), and the reduction of the sterility problems in the buffer vessel. Furthermore, no undesirable precipitations occur during fermentation, since only soluble lactic acid salts are formed.

The wastewater pretreated as described above is then, in the second stage of the novel process, fed to a fermentation reactor (fermenter). It is desirable that a pH which is optimum or at least favorable for the fermentation organisms is established and maintained in the fermenter, for example a pH range from 6.0 to 6.5. This can be performed in a plurality of ways: for a start, as described in the prior art, the pH in the fermenter can be regulated by addition of base to the fermenter itself. However, preference is given to an embodiment in which the pH in the fermenter is adjusted via regulation of the volume of the wastewater made alkaline flowing into the fermenter. This can be performed using a pH stat which measures the pH in the fermenter and regulates the addition of the wastewater automatically in such a way that the lactic acid produced in the fermenter is neutralized and the pH is maintained at the previously described value. This achieves an automatic Betting to a desired material conversion rate and thus an essentially constant product concentration. The reactor should, in 287 3 97 addition, preferably have a level controller in order to ensure a constant level.

In the event of a decrease of the microbial conversion rate, eg. due to operating faults or contaminations, the feed rate is automatically decreased, but th.3 product concentration remains the same. If the biological activity in the fermenter increases again, the inflowing rate is automatically increased again. If the lactose content in the feed is known, this embodiment of the invention enables the fermentation to be operated automatically at complete conversion of substrate. The pH-statting has, moreover, the advantage that, in the further work-up of the lactic acid, a constant product feed concentration results and, due to the complete conversion in the fermenter, colonization of downstream filtration and electrodialysis membranes is prevented (which leads to an improvement in the transmembrane filtrate fluxes) , and the maximum achievable COD reduction in the wastewater can be achieved.

In the fermenter, the lactose present in the wastewater and any other sugars are converted homofer-mentatively to lactic acid using lactic acid bacteria. r Lactic acid bacteria utilize only a very small spectrum of the metabolic pathways present in nature for the degradation of nutrients: they convert certain sugars such as lactose to lactic acid anaerobically and produce from this reaction energy for growth and maintenance of cell function. Degradation of the sugar and growth of the organisms are substantially coupled to one another and 287 3 97 take place simultaneously.

The choice o£ the strain or strains for the novel process is optional, but the lactic acid bacteria are preferably organisms from the genera Lactobacillus. Lactococcus and Streptococcus.

The growth of the lactic acid bacteria is to a great extent dependent on the presence of certain additives. These are, in particular, vitamins, trace elements and nitrogen in organic form, ie. in the form of (hydrolyzed) proteins, peptides and amino acids. In a preferred embodiment of the invention, in the conversion of lactose to lactic acid, the addition of such additives and, in particular the addition of an organic nitrogen source for the lactic acid bacteria, can be dispensed with entirely or for the most part. The known processes for lactic acid production from whey permeate describe the necessity of such an addition of external nitrogen sources such as yeast extracts, com steep liquor and the like (see EP 0 265 409 A1 and 0 393 818 Al) . According to the invention, the addition of nitrogen-containing compounds can be omitted entirely or for the most part if the nitrogen-containing substances present in the wastewater which cannot be naturally metabolized by lactic acid, or cam only be slowly metabolized by lactic acid bacteria, are converted during the fermentation or even in advance to compounds which can be relatively rapidly utilized by the organisms. Thus, proteins can be hydrolyzed, for example by addition of enzymes such as proteases which are optionally introduced directly into 287 3 97 the fermenter or can be fed in a previous process stage. Proteases are suitable which are active at the fermentation conditions present, eg. HT Proteolytic 200 (Solvay), Keutrase and Alcalase (Novo Nordisk) (see Figure 4) . Alternatively, the nitrogen source can also be produced by acid hydrolysis or the like. The temperature in the fermenter is set in dependence on the microorganisms selected. It can be, for example, 30-45°C.

The fermentation itself can be carried out in any fermentation reactor, for example in a fluidized-bed reactor, in which the microorganisms are immobilized on porous sintered glass or other supports which are inert as far as possible, or in a stirred-tank reactor. The reactor is preferably made of stainless steel. In order to achieve high productivity rates, it is advisable to equip the stirred-tank reactor with a cell retention system. This leads to significantly higher productivities (see Figure 5) . A COD conversion rate up to 20 times more rapid results in comparison with the known anaerobic wastewater treatment processes. For the cell recycling, a part-stream is taken from the fermenter and circulated via a cell removal stage. The cells can be removed by membrane processes or by centrifugation. If a fluidized-bed reactor is employed, a stream can be taken off at the top of the reactor and introduced below centrally into the reactor in order to fluidize the fluidized bed. A further stream cam be removed in order to keep the reactor volume constant.

Membrane processes are preferred for the cell Id 7 Z97 removal. A cell-free permeate stream is taken off from the cell removal module. In addition, a bleed stream is provided which bears cell-containing fermentation broth and can be taken off directly downstream of the filtration unit or directly from the fermenter. By varying the ratio of bleed stream to permeate stream the desired degree of concentration can be set. The socalled recycle ratio is defined in this case as R _ Permeate stream Permeate stream + bleed stream.

If the fermenter balance equation for the biomass is considered, the great effect of the recycle ratio R on the system becomes evident. The differential equation below is obtained: VrIf = v&(|1 " kd> * " d Vr(1-R)x x biomass (g/1) VR reactor volume (1) kd death rate (1/h) D throughflow rate (1/h) H growth rate (1/h) t time With increasing recycle ratio, the biomass concentration in the system increases and at the same throughflow rates, many times the amount of submerged biomass is obtained. Since the lactose conversion rate is 287 3 97 generally proportional to the biomass itself and to the biomass growth, at higher biomass concentrations, there is higher '.ubstrate consumption per unit of reactor volume, i'.i enhanced conversion rate and thus an improved product formation rate. On the other hand, it is undesirable to operate the reactor with complete cell retention, ^i^ce in such a case, the changes in the cultures due to <ne corresponding selection pressure and, as a result, shifts in the product spectrum, may be expected. Moreover, undesirable substances which pass into the reactor via the medium accumulate to a very great extent. These have unfavorable effects on the biological and technological system.

The cell retention system is fed by the circulating stream from the fermenter. It can be operated by ultrafiltration membranes. However, microfiltration membranes are preferably used, since, because of the controlled-coating process, complete removal of all components can also be achieved by these membranes, but, at the same time, at the same energy input, higher transmembrane fluxes are achieved, which can decrease the membrane area required (see Figure 6) . Moreover, the use of microfiltration membranes enables lower pressures to be employed. Ceramic multichannel tube modules having a porn diameter of about 0.2 fim which are arranged in stainless steel housings are preferably used. However, other modules, membrane materials and pore diameters are also possible. The tubing should preferably be made of stainless steel. The filtration unit should be equipped 287 i with an apparatus for external back-flushing of the modules. The back-flushing can be performed by a permeate-side pressure pulse, which can be produced by sterile-filtered compressed air on an intermediate vessel. Upstream and downstream of the filtration modules, pressure sensors and volumetric flow meters are preferably mounted.

The filtration unit can be planned to have multiple lines and, very particularly preferably, two lines, in order to clean one module as required and nevertheless not to interrupt the filtration. The system preferably has the capacity for CIP. The bleed stream can be set by a ratio flow controller at a defined ratio to the permeate stream from the filtration unit. The permeate is preferably continuously fed to the electrodialysis (see below), more precisely preferably directly via a nanofiltration unit or via a selective ion exchanger.

The ion exchanger is used to free the cell-free fermentation broth from any calcium residues still present. The weakly acidic macroreticular cation exchanger resins Duolite XE 702 or Duolite C 467 from Rohm & Haas, Paris, for example, are highly suitable here. Owing to their specific reactive groups, these ion exchangers preferentially bind divalent ions, such as calcium and magnesium, while they allow to the greatest extent possible the passage of the monovalent ions which do not interfere during the third process stage of wastewater purification and lactic acid isolation. When £8 7 3 9 the ion exchanger is used, the calcium concentration of the fermentation broth may be very greatly decreased, for example to below 5 ppm. Since electrodialysis membranes, as are subsequently used during the third stage, are generally sensitive to the precipitation of Ca(OH)2 in the basic concentrate, this gives the advantage that the nembranes used have a substantially longer service life. The ion-exchange resin is situated in one, but preferably in two (or more), ion-exchange columns, through which the permeate stream from the filtration flows continuously in alternation. Downstream of the exchange column should be situated a measurement instrument for measuring the exchanger loading. If one exchanger is laden, flow can automatically be switched to the second or another column, so that a continuous stream is fed to the electrodialysis. The ion exchangers are regenerated by acid and alkali solution which, after the ion-exchange material becomes laden, are passed through the respective exchanger column(s) not in operation.

Alternatively or additionally to the use of an ion exchanger, a nanofiltration unit can be used. The nanofiltration unit comprises a high-pressure pump and filtration modules which are equipped with nanofiltration membranes. Nanofiltration membranes are suitable which retain calcium and lactose but essentially allow the passage of lactic acid. As an example, the Filmtec membrane XP 45 (Dow Chemical) may be mentioned here. This shows a retention rate of up to 99% for lactose and calcium, whereas lactate is only retained at 3 0%. If the 287 3 87 wastewater, for example after the pretreatment in the reactor, still has a concentration of 1.15 g/1 of calcium, the calcium concentration can be decreased to 40 ppm. Preferably, pressures between 5 and 20 bar are established. The nanofiltration retentate is preferably recycled to the fermenter. A further advantage of this process procedure is that any lactose present, which cannot pass through the nanofiltration membrane, is recycled back to the fermenter together with the retentate stream and thus, even with incomplete conversion in the fermenter, cannot pollute the wastewater. Alternatively, the retentate is concentrated and correspondingly continuously removed. The permeate is fed to the electrodialysis unit.

In a special embodiment of the present invention, the nanofiltration process step directly follows the lactose fermentation process step, ie. the liquid leaving the reactor is not conducted through a microfiltration unit or the like. In this embodiment, passing the liquid through em ion exchanger cam also be dispensed with. Cells, lactose and calcium are retained by the nanofiltration unit. The liquid leaving the reactor is subjected to only a single process step, that is to nanofiltration, a permeate being produced which in its composition is an ideal product for further processing. This embodiment is particularly advisable if pressure-insensitive orgemisms are used.

Both the first and the second stage of the novel process, the pretreatment and the fermentation with 287 3 97 secondary purification of the fermentation broth, can be carried out discontinuously. However, preferably, they are operated continuously.

The third stage of the novel process, reducing the concentration of lactic acid in the wastewater and thus producing a wastewater having only a very low COD load, is performed by electrodialysis using bipolar membranes. Using this technique, it is possible at the same time to isolate free acid in high concentration and purity directly from the fermentation solution. As a third product in this case, alkali solution is formed which preferably serves for pH elevation during pretreatment of the wastewater, as described above, or can be used for the pH regulation of the fermenter.

Membranes for the bipolar electrodialysis are known. In the simplest case, they are composed of laminates of two ion-selective drop membranes of opposite polarity. The overall potential drop across the membrane is made up of the individual resistances of the membranes and the resistance of the solution between the membranes. From this it follows that the thickness of the intermediate space between the membranes is desirably as small as possible. Therefore, in the present process, those membranes are particularly preferably used as are disclosed, eg., in the German Offenlegungsschrift 4211267 Al. These are multiple-layer membranes having an anion-selective and a cation-selective layer which were produced by generating a first ion-selective layer from a first polymer solution in the presence of an amount of 2 8 7 3 9 7 steam such that, on the one hand, condensation of the water is avoided and, on the other hand, the miscibility gap in the polymer/solvent/water phase diagram is achieved, whereupon, after removing the solvent on the side of this first ion-selective layer which had been exposed to the steam, a second ion-selective layer with opposite charge is generated. Using this process, bipolar membranes can be produced in which the thickness of tho charge-neutral intermediate layer between the anion- and the cation-selective layers is very small. The preferred membranes, despite the very low resistance, have chemical properties and a perm selectivity which are comparable to those of other membranes.

The use of bipolar electrodialysis for purifying lactic acid is prior art (see EP 0393818 A1 and 0346983 A2) . A possible configuration of the bipolar membranes in the novel process is depicted in Fig. 7. The fermentation solution filtrate circulates in the diluate circuit. In accordance with their charge, the lactate anions migrate into the acidic circuit and the sodium ions migrate into the basic circuit. The pHs in the two circuits are set by the two bipolar membranes. The free lactic acid can then be taken off from the acidic circuit and the alkali solution formed can be taken off from the basic circuit, which alkali solution, as already described previously, can be recycled and added to freshly supplied wastewater for pH elevation.

Isolating lactic acid as pure as possible, as highly concentrated as possible, or lactate of such 287 3 97 quality, as product is not the principal aim of the present invention. Rather, this is in particular producing wastewater with COD values as low as possible. Obviously it is desirable here, at the same time, to isolate lactic acid in high concentration and good purity. Since, with decreasing concentrations in the diluate - that is low COD values in the wastewater - the current yields and the maximally achievable concentration of the acidic and basic products markedly decrease, both aims cannot be achieved together by simple bipolar electrodialysis with good results in each case (see Fig. 8) . Therefore, it is particularly preferably proposed to carry out the bipolar electrodialysis as follows: the fermentation broth is reduced in concentration batchwise, ie. discontinuously, to the desired wastewater concentration. The COD decrease is shown in Fig. 9. The acidic and the basic concentrates are concentrated here up to the maximum product concentration (Fig. 10) . During this process step, water is continuously transported through the membranes. Therefore, the product streams produced (alkali solution and lactic acid) can be taken off continuously and at constant concentration (Fig. 11) . The products of this process step are therefore produced continuously; accordingly, the alkali solution produced can be continuously returned to the wastewater during the wastewater pretreatment stage, if this stage is likewise carried out continuously. The lactic acid produced can also be additionally purified in further continuous process steps. If a product concentration lower than the 287 3 maximum is desired, the product can be diluted by water in the concentrate cycle or the concentrate can equally be produced in the batch process.

The electrodialysis unit includes one or more electrodialysis stacks, in which the monopolar ion-exchange membrane and the bipolar membranes are appropriately arranged. The above described, preferred electrodialysis with discontinuous sequence with continuous production of the worked-up fermentation broth can be implemented as follows: 4 vessels are arranged around the electrodialysis stacks. These include two diluate vessels, one always being filled by the continuously arising feed stream and the second always being circulated by pumping via the electrodialysis module. The diluate circuit is preferably equipped with a conductivity meter. When a defined value is undershot, the circuit is switched over to the second vessel filled in the meantime. The desalted vessel is automatically emptied and then refilled with fresh feed. The procedure effects a transfer from the continuous operation to cyclic batch operation. In the other two vessels are situated the products lactic acid and the alkali solution. The solutions are also circulated from the vessels by pumping via the electrodialysis stacks. A pH meter and a conductivity meter are built into the lactic acid circuit. Only a conductivity meter is built into the alkali solution circuit. In all three circuits, the pressure is measured before entry into the module. A continuous stream can be taken off from the two vessels, 287 3 9 7 or else concentration may also be carried out up to a desired concentration and then the vessels are emptied in a cyclic batch operation as far as a defined residual amount. The electrodialysis unit is generally constructed in plastic or stainless steel. Possibility for CXP is preferably provided. Obviously, the number of the vessels can be higher, as those skilled in the art can readily recognize, if the plant size or the throughput requires further parallel streams.

In a particular embodiment of the present invention, during the bipolar electrodialysis, cation-exchange membranes having low transport numbers for calcium are arranged abutting the diluate. The advantage of this embodiment is the increase in service life of the electrodialysis membrane. In this case also, the calcium-selective ion exchanger described further above, through which the fermentation broth is passed for secondary purification, can be of a smaller size or even omitted. A cation-exchange membrane which is suitable for use in bipolar electrodialysis is, for example, the membrane Neosepta CMS from Tokuyama Soda, which has for calcium and magnesium a transport number of only 0.1 in comparison to 0.9 for the sodium and potassium ions present. This markedly retards and reduces the transport of calcium ions into the concentrate chamber.

In smother embodiment of the present invention, the electrodialysis can be carried out in two stages, the fermentation broth being subjected in a first stage to bipolar electrodialysis in which it is continuously 287 3 97 reduced in concentration, but to only about 10 to 15 g/1 diluate concentration. In a second stage, a monopolar electrodialysis is carried out in which the decreasing concentration is carried out to the desired wastewater concentration, preferably in a batch procedure. The sodium lactate produced in the second stage can be returned to the feed stream of the first electrodialysis stage in order to increase the starting concentration of lactate there. Alternatively, the sodium lactate can be isolated as product. In another embodiment of the present invention, the second electrodialysis is also operated as bipolar electrodialysis, the product then being at least in part added back to the product streams of the first unit.

Using the present novel process, the wastewater is substantially purified. Thus, for example, the COD value may be decreased by 85 to 95% and free lactic acid produced at a concentration of approximately 200 g/1. The alkali solution concentration can achieve about 2 mol/l in this case.

The lactic acid produced as described above can be further purified if desired. Thus, for example, the other ions in addition to lactate concentrated in the acidic concentrate from the wastewater can be separated off by a downstream monopolar electrodialysis. According to the invention, a pH of about 2-3 is maintained in this purification step, because the lactic acid, having a pKa of 3.9, is present in virtually completely undissociated form under these conditions. In previous processes for 287 3 97 the fermentation of whey. It was proposed to desalt the whey by electrodialysis. However, losses of the desired product lactic acid must be accepted in the course of this. If, as is presently proposed, desalting by monopolar electrodialysis is carried out only after bipolar electrodialysis, the losses of lactic acid are substantially lower than if the salts are separated off prior to the fermentation (Figure 12) . A further advantage is that the salts are available to the organisms for their metabolism during fermentation, if the salts have not already been separated off prior to the fermentation stage.

The alkali solution produced can also be subjected to purification by monopolar electrodialysis.

The novel process is to be described by way of example below on the basis of a diagrammatic representation of a specific embodiment with some variants (see Figure 1} . One embodiment of the apparatus is also described in more detail (see Figure 13).

The process diagram in Figure 1 shows the feed of a whey-containing wastewater into an ultrafiltration unit (1) . Whereas the retentate is removed, the permeate is admixed with base to pH > 10 in the reservoir vessel (2) and the calcium phosphate precipitated out is sedimented.

Liquid taken off from this vessel - with adjustment of the inflow volume by a pH stat in the reactor -is fermented at a pH of 6.5 in a fermentation reactor (3) equipped with a stirrer, the addition of protein being possible. Alternatively, the fermentation can be per- 2 8 7 3 9 7 formed in a fluidized-bed reactor (4). The fermentation broth is fed to a microfiltration unit (5), and the cell-containing bleed stream is in part returned to the fermenter (3) and in part withdrawn from the circuit. The cell-free permeate passes through either a Ca selective ion exchanger (6) or a nanofiltration unit (7) , the (lactose-containing) retentate of which is again recycled to the feirmenter or else directly fed to further work-up. The fermentation broth thus purified is then subjected to a bipolar electrodialysis in an electrodialysis unit (8) . In this electrodialysis, the desired purified wastewater is formed as diluate and in addition an acidic concentrate is formed which contains the lactic acid and can be subjected to monopolar electrodialysis for further purification or can be passed through an ion exchanger, and a basic concentrate is formed which again can be fed to the reservoir vessel (2).

The apparatus depicted in Figure 13 shows an ultrafiltration unit (1) in which the dairy wastewater is pretreated. The reservoir vessel (2) here comprises two buffer tanks, into which dairy wastewater from (1) and alkali solution from a vessel (4) can be introduced, having stirrers and bottom openings for the take-off of, eg., calcium hydrogen phosphate. The additive vessel (3) which is likewise present here in duplicate can be sterilized and can preferably be heated and serves, eg., for the enzymatic hydrolysis of retentate separated off in advance or for the preparation of other additives. The appropriate additives are metered into the feed stream 287397 from the buffer tank in appropriate concentration (ratio flow controller) and conducted into the reactor (5) . The alkali solution vessel (4) is made of alkali-resistant material. It is preferably, but not compulsorily, fed with alkali solution produced in the bipolar electrodialysis and topped up as required with fresh alkali solution. It is provided with a level controller. Alkali solution streams are metered into the buffer tanks and into the reactor by metering pumps. The reactor (5) is depicted once as a stirred-tank reactor and below - as an alternative - as a fluidized-bed reactor. Xt has a pH stat, a level controller, a temperature measurement and control instrument and an inflow controller. A circulated stream is taken off from the stirred-tank reactor, conducted via a membrane unit and recycled to the reactor. A second stream, as a so-called bleed stream, is taken off directly downstream of the filtration unit or directly from the fermenter. This stream is established in a defined ratio to the permeate stream from the filtration unit by a ratio controller. A stream is removed from the fluidized-bed reactor at the top of the reactor and introduced below centrally into the reactor in order to fluidize the fluidized bed. A further stream is removed in order to keep the reactor volume constant. The filtration unit (6) , here constructed in two lines, is fed by the circulated stream from the fermenter. The filtration unit is equipped with an apparatus for external backflushing of the modules. The permeate of the unit (6) is set between bleed stream and permeate stream 287 3 97 by the ratio flow controller via the coupled fermenter level controller. The permeate then passes directly via the nanofiltration unit (7) or an ion-exchange unit (8) to the electrodialysis module (9) . The nanofiltration retentate can in turn be recycled to the reactor (5) or else removed. The ion-exchange unit (8) here comprises two columns through which filtration permeate can flow continuously in alternation. Downstream of the columns is a measurement instrument for measuring the loading of the exchanger and an arrangement which, when the column is full, automatically switches over to the other column. Regeneration is performed by passage, which is not depicted, of acid and alkali solution from corresponding storage vessels through the idle column. The electrodialysis (9) includes one or more electrodialysis stacks in which the monopolar ion-exchange membranes and the bipolar membranes are appropriately arranged. Two diluate vessels are alternately filled with the filtration unit (6) permeate - optionally given secondary purification -which is produced continuously, the respective other vessel being circulated by pumping (not depicted) via the electrodialysis module. The circuit is equipped with a conductivity meter. If a defined value is undershot, the circuit is switched over to the other vessel, while the first containing the liquid now reduced in concentration is automatically emptied and is then continuously filled with fresh permeate feed. The alkali solution and lactic acid produced in the electrodialysis unit are collected in two further vessels, alkali solution and lactic acid 287 3 97 also being either circulated until a desired concentration is reached, or else can be continuously removed from the vessels. The alkali solution vessel is connected via a line to the alkali solution vessel (4), which in turn supplies the buffer vessels (2) and the reactor (5) with alkali solution.

The apparatus shown is obviously variable and is adapted as appropriate by a person skilled in the art if other embodiments of the process described above are to be implemented. 287 3 9 7 26

Claims (23)

Claims:

1. A process for purifying dairy wastewater. including the following stages: (a) pretreating the wastewater with base; (b) introducing the pretreated wastewater into a fermenter, anaerobically fermenting the lactose present in the wastewater to form lactic acid using a lactic acid bacteria and giving the fermentation broth formod in the fermenter a secondary purification; (c) reducing the concentration of the lactate in the wastewater and concentrating lactic acid and base using bipolar electrodialysis.

2. The process as claimed in claim 1, wherein the wastewater is pretreated with a base, selected from alkali metal hydroxide, •alkali metal carbonate and/or ammonium compounds.

3. The process as claimed in claim 2, wherein the wastewater pH is adjusted to a 7 in the pretreatment and/or the amount of base added is sufficient to precipitate out the majority of the Ca2+ ions and P043" ions present in the wastewater.

4. The process as claimed in one of the preceding claims, wherein the pretreated wastewater is introduced into the fermenter in such a manner that a pH which is favorable or optimum for fermentation organisms is established and maintained in the fermenter.

5. The process as claimed in claim 4, wherein the pH is adjusted in the fermenter by adjusting the pH of the wastewater fed or by adjusting the volumetric inflow rate 28 7 3 - 27 - of the pretreated wastewater.

6. The process as claimed in one of the preceding claims, wherein the anaerobic fermentation is carried out without addition of an external nitrogen source for the fermentation organisms.

7. The process as claimed in one of the preceding claims, wherein, by addition of enzymes such as proteases into the wastewater, hydrolysis products of internal nitrogen compounds are produced which are available as nitrogen source to the fermentation lorganisms during the fermentation.

8. The process as claimed in one of the preceding claims, wherein the fermentation is carried out in a stirred-tank reactor and the cells are retained from the fermentation broth leaving the fermeuber for : secondary purification using a filtration unit and are recycled in part or entirely.

9. The process as claimed in one of the preceding claims, wherein the fermentation broth leaving the fermenter, for secondary purification, is passed through calcium-selective ion-exchange material before it is subjected to the bipolar electrodialysis.

10. The process as claimed in one of the preceding claims, wherein the fermentation broth leaving the fermenter, for secondary purification, is passed through a nanofiltration unit before it is subjected to the bipolar electrodialysis.

11. The process as claimed in one of the preceding claims, wherein the bipolar electrodialysis is carried 28 7 39 7 28 out using bipolar multiple-layer membranes, these membranes having been prepared by producing a first ion-selective layer from a first polymer solution in the presence of an amount of steam such that on the one hand condensation of the water is avoided and on. the other hand the miscibility gap in the phase diagram polymer/solvent/water is reached/ whereupon a second ion-selective layer with opposite charge being produced on the side of the ion-selective layer formed which had been exposed to the steam.

12. The process as claimed in one of the preceding claims, wherein the fermentation broth leaving the fermenter, after the secondary purification, is subjected batchwise to the bipolar electrodialysis and the acidic and b.asic product streams leaving the electrodialysis unit are continuously taken off.

13. The process as claimed in one of claims 1 to 11, wherein the fermentation broth leaving the fermenter, after the secondary purification, is continuously subjected to the bipolar electrodialysis, is there reduced in concentration to a diluate concentration of not below about 10 g/1 and is then subjected batchwise to a monopolar electrodialysis.

14. The process as claimed in claim 13, wherein the sodium lactate isolated in the monopolar electrodialysis is added back to the fermentation broth before this is subjected to the bipolar electrodialysis.

15. The process as claimed in one of claims 1 to 12, wherein the fermentation broth leaving the fermenter, 29 28 7 3 9 7 after the secondary purification, is subjected to two bipolar electrodialyses proceeding one after the other, the lactic acid solution produced in the second electrodialysis and/or the base produced in the second electrodialysis being admixed to the corresponding products produced in the first stage.

16. An apparatus for carrying out the process as claimed in one of claims 1 to 15, comprising: at least one reservoir vessel, an alkali solution vessel a fermenter/ preferably a stirred-tank reactor or fluidized-bed reactor having (a) a cell retention device and/or (b) a downstream nanofiltration unit and/or (c) at least one downstream Ca-selective ion exchanger and an electrodialysis unit having one or more electrodialysis stacks having bipolar membranes and having vessels for diluate to be passed to the unit (8) and alkali solution and lactic acid to be collected, the alkali solution vessel being connected to the alkali solution vessel for recycling into the circuit alkali solution produced in the electrodialysis .

17. The apparatus as claimed in claim 16, wherein the fermenter is equipped with a pH stat which measures the pH in the fermenter and regulates the addition of the - 30 - 28 7 3 9 7 basic liquid from the reservoir vessel in such a way that said pH has a favorable range.

18. The apparatus as claimed in claim 16 or 17, wherein the cell retention device comprises membranes, eg. microfiltration membranes and, preferably, ceramic multichannel tube modules having pore diameters of about 0.2 /xm and/or is equipped with a device for external back-flushing of the modules.

19. The apparatus as claimed in one of claims 16 to 18, wherein reservoir vessel, cell retention device,. ion exchanger and/or vessel for diluate to be passed to the electrodialysis unit are present in duplicate and are connected in parallel in such a manner that in each case one of the two units is used in the continuous process, while the other is out of processing operation in order to be regenerated, cleaned, filled or the like.

20. The apparatus as claimed in one of claims 16 to 19, wherein the electrodialysis unit includes two diluate vessels which can be filled alternately, each of these vessels being able to be connected to a circuit flowing through the electrodialysis stack(s), a conductivity meter being provided which, when a defined diluate value is undershot, switches the respective vessel into the circuit, and an additional vessel for lactic acid and an additional vessel for alkali solution, each of which are connected to a circulation likewise flowing through the electrodialysis stack(s).

21. The apparatus as claimed in one of claims 16 to 287 397 - 31 - 20, wherein the electrodialysis unit includes an electrodialysis stack having bipolar membranes and cation-exchange membranes having low transport numbers for calcium.

22. A process as claimed in any one of claims 1-15, substantially as herein described and with reference to any one of the Figures.

23. An apparatus as claimed in any one of claims 16-20, substantially as herein described and with reference to Figure 13. END OF CLAIMS