Wastewater purification process

Abstract: Wastewater from various sources may be purified by con­ tacting it with sludge derived from an oxidative biological water purification process and subsequently subjected to a com­ plete anaerobic fermentation step followed by a dying-off step. A variety of contaminants such as metals, colorants, deter­ gents, phenols and the like may be adsorbed by the sludge in this way and removed from the wastewater by separating the sludge and wastewater after contact. The process may serve as an introductory step to a normal oxidative biological water purification process. In a special embodiment, cationic con­ taminants are removed from the wastewater, with the aid of a sludge of the aforesaid type, whilst anionic contaminents are removed with the aid of a sludge of the same type that has been pretreated with a cationic detergent.

Description: This invention relates to a process of purifying wastewater and more in particular to a method of removing metals, colorants, detergents, phenols and the like from it. Normal domestic wastewater is conventionally purified by oxidative biological methods, e.g. in oxidation beds, continuous filters, active sludge plants oxidation ditches or biodiscs. Such methods employ micro-organisms which under aerobic conditions are capable of decomposing and removing organic substances from the wastewater. In the case of industrial wastewater, such oxidative biological methods cannot always be used because the micro-organisms are sensitive to various components of the wastewater and the process might be inhibited by the presence or a high concentration of such components therein. Therefore, chemical and/or physical purification methods such as precipitation of metals in hydroxide or sulfide form, adsorption of metals to ion exchangers, adsorption of colorants to active carbon, removal of detergents by flocculation or extraction of phenols with an organic solvent, whether or not in combination with a biological purification process, are frequently used in the case of industrial wastewater. While such methods are generally satisfactory, there is always a need for other methods capable of removing such components from wastewater with high efficiency. Document EP―A―0 110 240 discloses a method of removing heavy metals from wastewater. This method employs surplus sludge from a normal aerobic biological water purification process which has been subjected for some time to anaerobic fermentation. The sludge is contacted with the wastewater under anaerobic conditions and appears to be capable of adsorbing heavy metals from the wastewater. After completion of a certain contact period, the sludge loaded with the heavy metals is separated from the wastewater and the remaining wastewater may be discarded or supplied to a normal oxidative biological water purification process. A disadvantage of the method of EP―A― 0 110 240 is that the adsorption is effected under anaerobic conditions; this means that structural provisions must be made to shield the adsorption room from the air. Moreover, it seems that the contact period must be a rather long period (up till 24 hours) if it is desired to have a sufficient adsorption of heavy metals from the wastewater. An object of the invention is to remove the disadvantages of the method disclosed in EP―A―0 110 240 and to provide a water purification method capable of adsorbing metals from wastewater in a rapid and efficient way, without need to shield the adsorption room from the air. Another object is to provide a method permitting an efficient adsorption of colorants, detergents, phenols and the like from wastewater. During extended search, it has appeared that a rapid and yet efficient adsorption of metals from wasterwater may be obtained by employing as an adsorbent sludge from an oxidative biological water purification process that has been subjected afterwards to a complete anaerobic fermentation step followed by a dying-off period for anaerobic micro-organisms (e.g. by a stay in drying beds). Such sludge is completely stabilized and substantially free of viable micro-organisms, and seems to have a high adsorption capacity for metals. Moreover, it has been found that this completely fermented and stabilized sludge is capable to adsorb other contaminants such as colorants, detergents, phenols, organic acids, aldehydes, azides, pesticides, aromatics, substituted hydrocarbons and the like from wastewater in addition to metals. No anaerobic conditions need be maintained and no shielding of the adsorption room from the air is necessary, thanks to the absence of anaerobic micro-organisms in the sludge, and this is advantageous from a structural point of view. Thus, the invention provides a method of purifying wastewater which comprises the following steps: contacting wastewater with sludge of an oxidative biological water purification process that has been subjected to a complete anaerobic fermentation step followed by a dying-off period to render it substantially free of viable micro-organisms, in order to adsorb metals, colorants, detergents, phenols and the like from the wastewater onto the sludge, and separating the sludge from the wastewater after contact. The adsorbent employed by the invention is a by-product of the normal oxidative biological water purification processes. During such purification processes, a high amount of sludge from micro-organisms is formed under aerobic conditions and such sludge may be stabilized after separation from the water by subjecting it to fermentation under anaerobic conditions in closed containers. A complete fermentation will need a relatively long residence time, e.g. from 20 to 30 days at a temperature of 30―35°C and a pH of about 8. Thereupon, the stabilized product is usually stored in drying beds in the open air for some time (e.g. for one or two weeks) resulting into a certain dehydration and thickening of the product and into dying-off of the anaerobic micro-organisms. The resulting end product becomes available in high amounts but up till now, it has found only a few utilisations, meaning that it is combusted or discarded frequently. In accordance with the invention, however, it appears to have a high adsorption capacity for metals, colorants, and the like and this offers new utilisation possibilities. In the next part of the specification, the adsorbent employed by the invention will often be indicated as an "anaerobically fermented sludge". The adsorptive activity of the anaerobically fermented sludge may be ascribed to the presence of extracellular polymers in the sludge. Sludge that has been withdrawn directly from an aerobic water purification process and sludge that has been withdrawn from an anaerobic fermentation process will already comprise a certain amount of extracellular polymers, but sludge that has been subjected to a complete anaerobic fermentation followed by a dying-off period by means of drying beds or the like will have a much higher content of such polymeric substances and this will contribute to the high adsorption capacity. It is remarked that the invention should not be restricted, however, by such an explanation. The method of the invention may be effected in a simple way by contacting anaerobically fermented sludge with wastewater and separating it from the water after a certain period of contact. The contacting period need only be short, e.g. from 5 to 20 minutes while 10 minutes is preferred. The mixture of wastewater and sludge may be agitated continuously by means of agitating or mixing means during contact. Separating the sludge from the water is preferably effected by sedimentation and the sedimentation time will be dependent from the degree of loading of the sludge with adsorbed substances. Thus, the sedimentation time may vary, e.g. between 10 and 30 minutes with an optimum at 20 minutes. Further, a separation by filtration or centrifuging is also possible. Sedimentation of the sludge may be accelerated by employing polyelectrolytes, i.e. polymeric substances carrying charged functional groups. Non-ionic and anionic polyelectrolytes are preferred. Such polyelectrolytes may be introduced in any desired stage of the process, e.g. at the start of the sedimentation stage or during the stage of contact or earlier. Perhaps, it might be possible to add the polyelectrolytes already to the sludge in the drying beds. The concentration of polyelectrolytes in the mixture of wastewater and sludge may vary between 1 and 10 ppm with an optimum value at 5 ppm. In this way, the sedimentation efficiency may be improved by 75%. The whole process, from the addition of the anaerobically fermented sludge up to the separation of the sludge, may be effected in a continuous or a discontinuous way. The separated sludge may be reused one or more times until a point of saturation has been reached and may be processed later on for recovering metals, colorants, detergents and the like. The separated wastewater may be discarded, stored or further purified. Further purification of the wastewater after separation of the anaerobically fermented sludge from it may be effected in principle in different ways. The most suitable purification method for this purpose will mostly be selected in function of the contaminants still present, in the special case that the wastewater still contains a high percentage of dissolved organic substances derived from normal domestic wastewater, an oxidative biological purification process can be selected advantageously for the further purification. The total process will comprise at least two steps then, viz. a first step for the removal of metals, colorants, detergents, and the like with the aid of anaerobically fermented sludge and a second step for the digestion of dissolved organic substances with the aid of oxidative-biological methods, e.g. in oxidation beds, biodiscs, an active sludge plant or an oxidation ditch. This embodiment has the advantage that all substances capable of inhibiting the second step, will be adsorbed and removed now by the anaerobically fermented sludge in the first step such that a distinct improvement in the efficiency of in the oxidative biological process of the second step can be achieved. Moreover, if the inhibitory substances were toxic products such a metals, then an additional advantage is that the sludge resulting from the second step is not contaminated with toxic substances and may be used without any objection for manuring purposes in agriculture. The method of the invention employing anaerobically fermented sludge may be used for removing various different substances from wastewater. Examples thereof are cationically dissolved substances such as metal cations, cationic colorants and cationic detergents, and moreover non-ionically dissolved and semidissolved substances such as dispersion and reaction colorants, aldehydes, aromatics and substituted hydrocarbons. The anaerobically fermented sludge may be used without any special pretreatment for all these substances and the adsorption mechanism is presumably based upon an interaction with charged portions of the extracellular polymers from the sludge and upon occlusion phenomena. Anionic substances (chromates, anionic colorants, anionic detergents, phenols, organic acids, azides, pesticides, and the like) may also be adsorbed by the anaerobically fermented sludge provided that it has been subjected to a pretreatment with a cationic detergent such as e.g. benzalkonium chloride or cetyltrimethylammonium bromide. The cationic detergent adheres presumably to the sludge in such a way that its cationic portion will remain freely available, thus permitting adsorption of anionic substances from the wastewater. The anaerobically fermented sludge treated with cationic detergents can be separated still easier than untreated anaerobically fermented sludge from the aqueous medium by means of sedimentation or the like. The pretreatment of the anaerobically fermented sludge with cationic detergent may simply be effected by contacting the sludge with the detergent. The required amount of cationic detergent is dependent from many factors such as e.g. the nature and amount of the anionic substances to be removed and can best be determined experimentally. Lower amounts than the optimum amount have insufficient effects and higher amounts than the optimum amount have the disadvantage that the sedimentation capacity of the sludge will deteriorate then. Moreover, account should be taken of the fact that part of the amount of cationic detergent is not adsorbed by the sludge but will remain in dissolved state near to the sludge. This part of the cationic detergent will mix with the waterwater to be purified and may be removed from it later on in a separate operation. The adsorption capacity of the anaerobically fermented sludge for anionic substances may be improved further by employing aluminum (III) ions or iron (III) ions during pretreatment of the sludge with cationic detergents. The invention is further illustrated by the drawing, showing some embodiments in a schematic way. Figure 1 shows a simple arrangement for removing metals, colorants, detergents, phenols etc. from wastewater. Figure 2 shows a two-step arrangement: a step for removing metals, colorants etc. with anaerobically fermented sludge and a step for further purifying the wastewater with oxidative biological methods. Figure 3 shows an arrangement adapted for sequentially removing cationic and anionic metals as well as metal complexes, colorants, detergents and the like from wastewater. Figure 4 shows a variant to the arrangement of Figure 3. The arrangement of figure 1 may serve generally for removing metals, colorants, detergents, phenols and the like from wastewater. In the flow direction of the wastewater (from left to right) there can be found in sequence a mixing station 1 with agitator 2 and a sedimentation station 3. The wastewater is supplied through conduit 4 to the mixing station 1 and will contact there an amount of anaerobically fermented sludge supplied through conduit 5. After a contact period of sufficient length, the mixture arrives in the sedimentation station 2 where the sludge is separated by sedimentation and discharged through conduit 6. Should the wastewater as supplied have a content dissolved metals in it, then the sludge discharged through conduit 6 is loaded with such metals. It may be reused one or more times for the same purpose or it may be subjected to a process for recovery of the metals. The wastewater discharged through conduit 7 is substantially free from metals. The arrangement may not only be used for removing metals, but also for removing colorants, detergents, phenols and the like from wastewater. Should any anionic substances be involved, then the anaerobically fermented sludge supplied through conduit 5 should have been pretreated with a cationic detergent. Figure 2 shows an arrangement adapted for the purification of domestic wastewater in two stages. When seen in the flow direction of the water (from left to right) there are in succession a presedimentation station 8, a mixing station 1, a sedimentation station 3, an aeration station 9 and a resedimentation station 10. The wastewater supplied through conduit 11 is freed from suspended substances in the presedimentation station 8 whereupon such substances are discharged through conduit 12. Anaerobically fermented sludge from conduit 5 is admixed to the water in mixing station 1 in order to remove metals, colorants, detergents etc. from it. After a certain contact period, the sludge is allowed to sediment in station 3 and withdrawn through conduit 6. This sludge has been loaded then with metals, colorants, detergents or the like, as far as such substances were present in the wastewater. Activated sludge is added to the water in station 9 through conduit 13 and the mixture is allowed to pass this station under continuous aeration (via 14). Next, the active sludge is separated from the water in the resedimentation station 10 and discharged through conduit 16 whilst purified water leaves the arrangement through conduit 15. Part of the active sludge (which in the meantime has increased in amount) is recycled to aeration station 9 through return conduit 13. Another part is passed to a sedimentation station 18 where it is completely fermented and stabilized under anaerobic conditions. The fermented sludge is discharged through conduit 19 while part thereof is passed to mixing station 1 through return conduit 20 in order to adsorb metals, colorants, etc. from freshly supplied wastewater. The arrangement of figure 3 is adapted for sequentially removing cationically and anionically dissolved metals as well as metal complexes, colorants, detergents and the like from wastewater. When seen in the flow direction of the wastewater (from left to right) there are in succession a mixing station 21, annex agitator 22, a sedimentation station 23, a second mixing station 24 with agitator 25 and a second sedimentation station 26. The wastewater from conduit 27 is contacted in the fist mixing station 21 with anaerobically fermented sludge supplied through conduit 28. After a certain contact period, the mixture is allowed to sediment in station 23 whereupon the sludge (loaded with cationic metals, colorants, detergents and the like in the meantime) is withdrawn through conduit 29. The wastewater flowing through mixing station 24 is mixed therein with anaerobically fermented sludge from conduit 30, said sludge having been pretreated with a cationic detergent. After a certain contact period, the latter sludge, loaded in the meantime with anionic colorants, detergents, phenols and the like, is separated in sedimentation station 26 and removed through conduit 31. Both types of sludge (from 23 and 26) may also contain non-ionic and semidissolved colorants, detergents and the like and may be reused for the same purpose if desired. As an alternative, the sludge from conduit 29 may be processed to recover metals, colorants, detergents and the like. Wastewater liberated from metals, colorants, detergents and the like, as far as initially present, is discharged through conduit 32. Since the water discharged from the arrangement of figure 3 will still contain some cationic detergent in dissolved state originating from the pretreated sludge introduced through conduit 30, it may be advisory to have this water further purified. The variant of figure 4 may be used for this purpose. The arrangement of figure 4 differs from that of figure 3 by the fact that the sludge pretreated with cationic detergent is prepared within the arrangement itself and by the fact that withdrawal of free cationic detergent with the wastewater is prevented. When seen in the flow direction of the wastewater (from left to right) there are in succession again a mixing station 21, a sedimentation station 23, a second mixing station 24 and a second sedimentation station 26, followed by a third mixing station 33 and a third sedimentation station 34. Stations 21, 23, 24 and 26 operate in the same way as those of figure 3. In the third mixing station, the wastewater (liberated in the meantime from cationic and anionic metals, colorants, detergents etc. as far as initially present) is mixed with anaerobically fermented sludge from conduit 35 in order to have the sludge adsorb dissolved cationic detergent from the water. The sludge pretreated with detergent in this way is separated in the third sedimentation station 34 and recycled to the second mixing station 24 through return conduit 36. An additional amount of cationic detergent is admixed to it through conduit 37. At the end of the arrangement, wastewater substantially free from cationic and anionic metals, colorants, detergents, phenols, etc. is discharged through conduit 38. It will be clear from the preceding lines that the process of the invention is excellently suitable to remove metals, colorants, detergents, phenols etc. from wastewater with high efficiency and in an economical way. Moreover, it will be clear that this process can be utilized in many fields of the art. Some examples of such utilisations are: removal of heavy or noble metals from wastewater produced by galvanizing plants, tanneries, electronic and photographic industries and non-ferro industries, optionally with recuperation of such metals, removal of metals, colorants and/or detergents from normal domestic wastewater as an introductory step to the normal purification of water with oxidative biological methods (fig. 2), removal of colorants from wastewater of textile industries, pharmaceutical industries or paper industries, removal of detergents and phenols from wastewater of cleaning stations, removal of pesticides, aromatics, azides, aldehydes, organic acids and the like from other types of industrial wastewater, removal and optional recovery of uranyl ions from wastewater of the atomic industry, detoxication of industrial wastewater which is supplied to purification systems for domestic wastewater.

Claims: A process for the purification of wastewater comprising the steps of: contacting wastewater with sludge of an oxidative biological water purification process that has been subjected to a complete anaerobic fermentation step followed by a dying-off period to render it substantially free of viable micro-organisms, in order to adsorb metals, colorants, detergents, phenols, and the like from the wastewater onto the sludge, and separating the sludge from the wastewater after contact. A method according to claim 1, wherein the wastewater is contacted with the sludge during a period of 5-20 minutes and preferably 10 minutes. A process according to claim 1 or 2, wherein the wastewater is further purified after separation of the sludge from it by subjecting it to an oxidative biological water purification process. A process according to claim 1, wherein metal cations, cationic colorants and cationic detergents are adsorbed from wastewater by contacting the wastewater with sludge from an oxidative biological water purification process, which has been subjected only to a complete anaerobic fermentation step and to a dying-off period to render it substantially free of viable micro-organisms, A process according to claim 1, wherein anionic metal compounds as well as anionic colorants, detergents, phenols and other anionically dissolved substances are removed from wastewater by contacting such wastewater with sludge from an oxidative biological water purification process that has been subjected to a complete anaerobic fermentation step and dying-off period, to render it substantially free of viable micro-organisms, and that has been subjected next to a treatment with a cationic detergent. A process according to claim 5, wherein the sludge has been treated with a cationic detergent and with aluminium (III) ions or iron (III) ions. A process according to claim 5 or 6, comprising the steps of: contacting the wastewater with sludge of an oxidative biological water purification process which has been subjected only to a complete anerobic fermentation step and a dying-off period, to render it substantially free of viable micro-organisms, followed by separation of the sludge from the wastewater after contact, and thereafter contacting the wastewater with sludge of an oxidative biological water purification process which has been subjected to a complete anaerobic fermentation step and a dying-off period to render it substantially free of viable micro-organisms, and which has been subjected next to a treatment with a cationic detergent.

Inventors: GEUENS LUC, GEUENS, LUC
Applicants: SANOTEC NV, N.V. SANOTEC
Application year: 1986
Main classification: C02F1
IPC codes: C02F9, C02F1, C02F3