DE102019112513A1 - Wastewater treatment plant and process for the treatment of phosphorus-containing sewage sludge - Google Patents

Abstract

A method for treating phosphorus-containing sewage sludge is described. The process has the following steps: a) adding complexing agent (K) to the phosphorus-containing sewage sludge to form complexes of metal ions and the release of orthophosphates; b) separation of a phosphorus-containing liquid phase (F) from the in step a) with complexing agent (K) mixed phosphorus-containing sewage sludge; c) separating a phosphorus component from the phosphorus-containing liquid phase (F); d) supplying the process water (P) remaining in step c) which contains a proportion of the complexing agent (K) supplied in step a) and around the phosphorus component which has been separated off is reduced, to an anaerobic treatment stage of the sewage sludge.

Description

1. a) Zuführen von Komplexiermittel zu dem phosphorhaltigem Klärschlamm zur Komplexbildung von Metallionen und der Freisetzung von ortho-Phosphaten,
2. b) Abtrennen einer phosphathaltigen Flüssigphase aus dem im Schritt a) mit Komplexiermittel versetzten phosphorhaltigem Klärschlamm; und
3. c) Abtrennen eines Phosphoranteils aus der phosphorhaltigen Flüssigphase. Die Erfindung betrifft weiterhin eine Abwasserbehandlungsanlage zur Behandlung von phosphorhaltigen Klärschlämmen mit einem solchen Verfahren mit
   • - einer Mischvorrichtung, die zum Zuführen von Komplexiermittel zu dem phosphorhaltigen Klärschlamm zur Bildung von Metallionen und der Freisetzung von ortho-Phosphaten eingerichtet ist,
   • - einer Trennvorrichtung, die zum Abtrennen einer phosphorhaltigen Flüssigphase aus dem in der Mischvorrichtung mit Komplexiermittel versetzten phosphorhaltigen Klärschlamm eingerichtet ist,
   • - und einer Abtrennvorrichtung, die zum Abtrennen eines Phosphoranteils aus der phosphorhaltigen Flüssigphase eingerichtet ist.

The invention relates to a method for the treatment of phosphorus-containing sewage sludge with the following steps:

1. a) supply of complexing agent to the phosphorus-containing sewage sludge for complex formation of metal ions and the release of orthophosphates,
2. b) separating a phosphate-containing liquid phase from the phosphorus-containing sewage sludge to which complexing agent has been added in step a); and
3. c) Separation of a phosphorus component from the phosphorus-containing liquid phase. The invention further relates to a waste water treatment plant for treating phosphorus-containing sewage sludge using such a method
   • - a mixing device which is set up to supply complexing agent to the phosphorus-containing sewage sludge for the formation of metal ions and the release of orthophosphates,
   • - a separating device which is set up to separate a phosphorus-containing liquid phase from the phosphorus-containing sewage sludge mixed with complexing agent in the mixing device,
   • - and a separation device which is set up to separate a phosphorus component from the phosphorus-containing liquid phase.

The mixing device, the separating device and the separating device are connected directly to one another in the process direction and are connected in series or indirectly connected in series with other interposed units.

DE 10 2016 112 300 A1 discloses a method for treating phosphorus-containing sewage sludge, in which a complexing agent is added to a filtrate to dilute the precipitation. Such complexing agents also reduce blocking prior to ultrafiltration. To extract phosphate from excess sludge, it is anaerobically treated and stabilized in a digestion tower and then digested in a further treatment stage so that the bound phosphate is released.

When disposing of sewage sludge, limit values for a phosphorus content must no longer be exceeded. Incineration of digested sludge together with other waste is only permitted if the phosphorus content in relation to the dry residue is less than 2%. Otherwise, mono-incineration is required to recover the remaining phosphorus from the ash with a yield of 80%. This is complex and requires sufficient capacity from mono-incineration plants.

• - eine Phosphor-Fällung, wobei zumeist ein Eisen- oder Aluminiumfällmittel mit dem Ziel der Ausfällung als Eisenphosphat (FePO4) oder Aluminiumphosphat (AlPO4) verwendet wird. Präzise werden diese ausgefällten Phosphatanteile auf Eisenorthophosphat bzw. Aluminumorthophosphat genannt. Die Fällung wird vor den Faulbehältern, in der Regel in der aeroben biologischen Behandlung durchgeführt. Die gebildeten Metallsalz-Phosphate sind chemisch sehr stabil und eine Rückgewinnung des Phosphoranteil ist technisch eigentlich nur nach einer Verbrennung aus der Asche möglich.
• - eine vermehrte biologische Phosphor-Elimination. Hierbei wird die Zelle zu einer vermehrten Phosphataufnahme veranlasst, mehr als sie eigentlich für die Energiegewinnung benötigt (Luxury uptake). Bei diesem Verfahren wird der Phosphor z.B. als Polyphosphat in der Zelle und den äußeren Polymerschichten gespeichert. Bewirkt wird dies durch Stress, hervorgerufen durch Über- und Unterversorgung mit Sauerstoff. Eine solche organische Phosphatbindung durch die biologische Phosphor-Elimination ist wesentlich labiler, als die chemische Bindung. Den größten Teil des Phosphors beinhaltet bei beiden herkömmlichen Verfahren der aerobe Belebtschlamm. Dieser wird als sogenannter Überschussschlamm dem System entnommen und gemeinsam mit dem Primärschlamm aus der Vorklärung einer anaeroben Behandlung (Faulung) zugeführt. Die Organik wird hier zum großen Teil zu Methan, Kohlendioxid (CO2) und Waser abgebaut. Bei diesem anaeroben Prozess werden die organisch gebundenen Phosphorverbindungen zum Teil wieder als ortho-Phosphate zurückgelöst, sofern ein biologischer Prozess gemäß des oben genannten zweiten Verfahrens durchgeführt wurde. Die Rücklöserate bezogen auf den gesamten Phosphoranteil liegt zwischen 20 und 35%.

In municipal sewage technology, there are mainly two methods of phosphorus elimination in water treatment. These are:

• - a phosphorus precipitation, mostly an iron or aluminum precipitating agent with the aim of precipitation as iron phosphate (FePO4) or aluminum phosphate (AlPO4) is used. These precipitated phosphate components on iron orthophosphate or aluminum orthophosphate are called precisely. Precipitation is carried out in front of the digesters, usually in the aerobic biological treatment. The formed metal salt phosphates are chemically very stable and the recovery of the phosphorus content is technically only possible after incineration from the ash.
• - an increased biological phosphorus elimination. This causes the cell to take up more phosphate than it actually needs for energy generation (luxury uptake). In this process, the phosphorus is stored in the cell and the outer polymer layers, for example as polyphosphate. This is brought about by stress, caused by excess and insufficient supply of oxygen. Such an organic phosphate bond through biological phosphorus elimination is much more labile than the chemical bond. The aerobic activated sludge contains most of the phosphorus in both conventional processes. This is taken from the system as so-called excess sludge and fed to an anaerobic treatment (digestion) together with the primary sludge from the primary treatment. The organic matter is largely broken down into methane, carbon dioxide (CO2) and water. In this anaerobic process, the organically bound phosphorus compounds are partially dissolved back again as orthophosphates, provided that a biological process according to the above-mentioned second process has been carried out. The redissolution rate based on the total phosphorus content is between 20 and 35%.

In sewage sludge containing phosphorus, there is usually always a mixture of chemically bound phosphorus and organically bound phosphorus components. This also applies if metal salt precipitation has predominantly been used. Oxic zones and anoxic zones are also formed in the process of nitrogen elimination. These lead at least to a low organic phosphorus absorption with a proportion of about 10 to 20% of the bound phosphorus. The organically bound phosphorus components with their property of anaerobic redissolution as orthophosphate cause problems during and after the digestion caused by the undesired precipitation of metal salt phosphate (e.g. magnesium-ammonium-phosphate MAP) and by the constant return of phosphates into the aerobic biological level.

For phosphorus depletion in sewage sludge treatment, it is necessary to break the chemical phosphorus bond. This is achieved by adding a complexing agent, such as citric acid (CitH3), which leads to extensive iron complexation and thus to the release of phosphorus as a precipitable orthophosphate. However, there is the problem that the complexing agent remains in the separated process water after the depletion of the phosphorus content, which is thus burdened with an extremely high biological and chemical oxygen demand (BOD, COD). In addition, the breakdown of the complexing agent creates a large amount of new activated sludge, which in turn has to be treated. There is also the risk of highly toxic compounds, such as Hydrogen sulfide (H2S). In addition, heavy metals that were previously bound can be dissolved by the complexing agent. The complexing agent also has an extremely negative effect on the effect of flocculants used in sewage sludge treatment and thus the separation behavior. Furthermore, the complexing agent also stabilizes suspensions, which is disadvantageous in the subsequent treatment.

Based on this, it is therefore an object of the present invention to provide an improved method for treating phosphorus-containing sewage sludge and an improved wastewater treatment plant for treating phosphorus-containing sewage sludge, in which a sufficiently large proportion of phosphorus is removed from the phosphorus-containing sewage sludge while avoiding the above-mentioned disadvantages.

The object is achieved by the method with the features of claim 1 and by the waste water treatment plant with the features of claim 12. Advantageous embodiments are described in the subclaims.

It is proposed that the process water remaining in the step of separating off a phosphorus portion from the phosphorus-containing liquid phase, which contains a portion of the added complexing agent and is reduced by the separated phosphorus portion, is fed to an anaerobic treatment of the sewage sludge.

A phosphorus portion or phosphorus content is understood to mean the portion or content of phosphorus, salts of phosphorus (phosphate) or other phosphorus compounds. This means that phosphorus does not have to be in its pure form.

According to the teaching of the present invention, the process water mixed with complexing agents is not then treated biologically aerobically, but first treated anaerobically, for example in a digester. This has the advantage that the complexing agent is broken down quickly with an increase in the gas yield (in particular methane and CO2), without the complexing agent having a disadvantageous effect on the other treatment stages of the sewage sludge. In the anaerobic treatment of the sewage sludge together with the process water containing the complexing agent, e.g. the iron ions are made available again, which combine with undesired sulfur components to avoid hydrogen sulfide and bind heavy metals as metal sulfite (MeS). The complexation can significantly increase the proportion of the separated phosphorus portion from the dry matter, so that at the end of the process the phosphorus content in the remaining dewatered sewage sludge is far below 2% P / t DR.

It is advantageous if the solid phase remaining in step b) and reduced by the phosphorus-containing liquid phase and an anaerobic treatment of this hydrolyzed solid phase together with the process water are carried out, the process water remaining in step d) being fed to this anaerobic treatment. The hydrolysis, which can be thermal, chemical or mechanical hydrolysis or a combination thereof, releases the biologically bound phosphorus content, increases the rate of dissolution of phosphorus content and improves the energy balance through additional COD digestion and higher gas production (especially methane production).

The sewage sludge mixed with complexing agents can be, for example, excess sludge. It is also conceivable, however, that the sewage sludge to which complexing agent has been added is the solid phase of excess sludge and / or primary sludge that remains after a liquid phase has been separated off. The sewage sludge mixed with complexing agent can, however, also be primary sludge and / or excess sludge which has been subjected to anaerobic treatment. It is also conceivable that the sewage sludge mixed with complexing agent is a phosphorus-containing sludge from wastewater or industrial processes.

It is advantageous if a solid phase of primary sludge and / or excess sludge is a Is subjected to hydrolysis and this hydrolyzed phosphorus-containing sewage sludge is then anaerobically pretreated. This anaerobically pretreated sewage sludge can then be mixed with complexing agents in step a) and then reduced by the phosphorus content. Such an anaerobic pretreatment of the phosphorus-containing sewage sludge before the addition of complexing agents is particularly suitable when the phosphorus is predominantly chemically bound and a phosphorus depletion through the complexation of iron ions is sufficient to achieve the desired phosphorus depletion. The process water remaining after the phosphorus fraction has been separated off can then be fed to an anaerobic pretreatment and / or an anaerobic aftertreatment in order to achieve the fastest possible degradation of the complexing agent, for example citrate ligands, and to increase the gas yield. Here, too, the combination with a hydrolysis treatment before and / or after the anaerobic pretreatment is conceivable and has further advantages in order to better release the organically bound phosphorus.

• - anaerobe Behandlung von Primärschlamm und/oder Überschussschlamm;
• - Zuführen von Komplexiermitteln gemäß Schritt a) zum anaerob-behandelten phosphorhaltigen Klärschlamm;
• - Hydrolysieren des komplexierten Klärschlamms;
• - Abtrennen einer phosphorhaltigen Flüssigphase gemäß Schritt b) aus dem hydrolysierten Klärschlamm;
• - Abtrennen eines Phosphoranteils gemäß Schritt c) aus der phosphorhaltigen Flüssigphase; und
• - Rückführen des nach dem Abtrennen des Phosphoranteils verbleibenden Prozesswassers zu der anaeroben Behandlung des Primärschlamms und/oder Überschussschlamms.

Diese Rückführung hat den Vorteil, dass das Komplexiermittel und die freigesetzten Eisen-Ionen wieder zur Verfügung stehen und Komplexier- und Fällmittel eingespart werden können.Process management with the following process steps is conceivable:

• - Anaerobic treatment of primary sludge and / or excess sludge;
• - feeding of complexing agents according to step a) to the anaerobically treated phosphorus-containing sewage sludge;
• - hydrolyzing the complexed sewage sludge;
• - Separation of a phosphorus-containing liquid phase according to step b) from the hydrolyzed sewage sludge;
• - Separation of a phosphorus component according to step c) from the phosphorus-containing liquid phase; and
• - Returning the process water remaining after the removal of the phosphorus fraction to the anaerobic treatment of the primary sludge and / or excess sludge.

This recycling has the advantage that the complexing agent and the released iron ions are available again and complexing and precipitating agents can be saved.

The phosphorus fraction can be separated off, for example, by precipitation, in particular metal salt precipitation. Precipitation of magnesium ammonium phosphate or calcium phosphate through the addition of magnesium or calcium salts is conceivable here.

However, it is also conceivable that the phosphorus component is separated off by filtering phosphorus ions in solution. This has the advantage that with phosphoric acid, for example, a much higher quality product than the precipitated magnesium ammonium phosphate or calcium phosphate can be obtained. The solution with the metal salt complex obtained by complexation is then passed back into the anaerobic treatment.

Citric acid, sodium citrate, salicylic acid, ascorbic acid or the like, for example, can be used as complexing agents.

• 1 - Blockdiagramm einer ersten Ausführungsform einer Abwasserbehandlungsanlage;
• 2 - Blockdiagramm einer zweiten Ausführungsform der Abwasserbehandlungsanlage;
• 3 - Blockdiagramm einer dritten Ausführungsform der Abwasserbehandlungsanlage;
• 4 - Blockdiagramm einer vierten Ausführungsform der Abwasserbehandlungsanlage;
• 5 - Blockdiagramm einer fünften Ausführungsform der Abwasserbehandlungsanlage.

The invention is explained in more detail below using exemplary embodiments with the accompanying drawings. Show it:

• 1 - Block diagram of a first embodiment of a wastewater treatment plant;
• 2 - Block diagram of a second embodiment of the wastewater treatment plant;
• 3 - Block diagram of a third embodiment of the wastewater treatment plant;
• 4th - Block diagram of a fourth embodiment of the sewage treatment plant;
• 5 - Block diagram of a fifth embodiment of the wastewater treatment plant.

1 shows a block diagram of a first embodiment of a wastewater treatment plant for the treatment of phosphorus-containing sewage sludge. Thereby, excess sludge is OSS in a first separation device 1 thickened and separated into a solid phase and a liquid phase. The liquid phase is fed to a biological treatment B.

With the separating device, the excess sludge OSS is thickened, for example, to a proportion of 4% -10% dry residue TR. Optionally, the process can also be carried out with primary sludge PS only or with a combination of excess sludge OSS and primary sludge PS.

The one with the first separator 1 Thickened sewage sludge is then digested with a complexing agent K in a mixer before the anaerobic treatment 2 mixed. The mixing device 2 can, for example, have a stirring unit for stirring the mixture.

Citric acid or the like, for example, can be used as complexing agent K. The mixture remains in the mixing device for as long 2 until the metal complexation (eg Fe or Al complexation) is complete.

The sludge mixture then goes to another second separator 3 fed, in turn, a solid phase and a liquid phase are separated from each other. The first and second separators 1 , 3 can for example be designed as a centrifuge or belt filter press or the like. The liquid phase contains a high proportion of ortho-phosphates, so the aim is to use the second separation device 3 to separate as high a proportion of water as possible. On the other hand, the sludge of the solid phase must still be pumpable and later miscible again with the aqueous (liquid) phase depleted of phosphorus, including phosphorus compounds or phosphorus salts (phosphate).

The separated metal-complexed liquid phase F becomes a separation device 4th supplied in order to separate a portion of phosphorus from the liquid phase F there. The phosphorus-containing liquid phase of the sewage sludge contains only a small amount of COD from the sewage sludge, which essentially originates from the complexing agent, but a high proportion of dissolved phosphates. The severing device 4th can, for example, be a precipitation container in which the liquid phase F is mixed with a metal salt and the phosphorus salts are precipitated and separated. For example, a calcium precipitating agent (eg Ca (OH) ₂ ) can be used to precipitate calcium phosphate. Calcium phosphate is a good and approved fertilizer.

With the addition of an ammonium component, it would also be conceivable to precipitate the phosphate components of magnesium-ammonium-phosphate by adding magnesium. This ammonium component can be introduced, for example, by adding primary or digested sludge PS.

It is also conceivable to feed a portion of ammonium NH ₄ from the anaerobic treatment or from sludge water to the dewatering after the anaerobic treatment of the separation device in order to provide a sufficient portion of the ammonium for the MAP precipitation.

The remaining process water P depleted of the phosphorus portion is added to the solid sludge portion S. In the exemplary embodiment shown, this is in the second separating device after the solid-liquid separation 3 remaining solid sludge fraction S was previously hydrolyzed and digested with a hydrolysis device. The remaining process water P is added to this hydrolyzed sludge fraction S. This mixture is put into a digester 6th fed for anaerobic treatment by digestion. The upstream hydrolysis brings about an additional redissolution of the biologically bound phosphorus components and the COD is further broken down. This results in a higher phosphorus depletion and also a more complete COD digestion, which leads to a higher methane production (CH ₄ ).

In the digester 6th gas G is produced, in particular methane gas. The digested sludge is then placed in a third separator 7th again divided into a solid phase (residual sludge RS) and a liquid phase (residual water RW). The liquid phase, that is to say the residual water RW, is then fed to the biological treatment B and the solid residual sludge fraction RS is disposed of.

By adding the compounding agent K before the anaerobic treatment in the digester 6th a very rapid anaerobic degradation of the complexing agent, for example of citrate ligands, and an associated release of iron ions, in particular Fe _II or Fe _III ions, is ensured.

2 shows a block diagram of a second embodiment of the wastewater treatment plant. The two anaerobic treatment stages which are often available in conventional sewage treatment plants and which are connected in series and fed with raw sludge either in series or in parallel are used.

In this embodiment, a mixture of primary sludge PS and excess sludge OSS is fed to an upstream second digester 8th supplied and anaerobically pretreated. This anaerobically pretreated phosphorus-containing sewage sludge is then used in the mixing device 2 supplied and mixed there with complexing agent K. Here too, citric acid CitH _{3 is} added to the anaerobically pretreated digested sludge. After a sufficient reaction time, the Fe ions are released from the FePO ₄ bond, for example as iron citrate, and phosphorus is present as orthophosphate. The complexation can take place either in a separately stirred container or inline in a pipeline, possibly with an inline mixing device.

Other salts, such as sodium citrate, salicylic acid, ascorbic acid, etc., are also conceivable as complexing agents. Citric acid has the advantage that it is very inexpensive, is available in large quantities and works well together with FeCl ₃ (iron chloride) as a common precipitant. However, it is also conceivable to use other metal salts, such as Al salt, as a precipitant.

After the complexation and the associated release of the phosphorus ions, the separation device 3 again a separation of the phosphorus-containing liquid phase F from the solid sludge phase S is reached. Here again a suitable dewatering unit, for example in the form of a centrifuge, belt filter press, screw press or the like can be used. The dry content in the solid sludge phase S should be chosen so that on the one hand there is good water separation and thus good phosphorus separation and on the other hand the solid sewage sludge S can be mixed again with the remaining process water in the further course. The use of synthetic or natural flocculants is conceivable in each case.

The phosphorus-containing liquid phase F in turn becomes the separation device 4th fed. Here, too, it can be a precipitation container in which magnesium-ammonium-phosphate MAP or calcium-phosphate or the like is precipitated by adding metal salts, such as magnesium salts or calcium salts, as precipitation reagent. Of course, the necessary process conditions, i.e. temperature, pH value, etc., must be set. The phosphorus salt formed is processed further after the precipitation. It can be separated off in a suitable manner by sedimentation, filtration, centrifugation or the like and passed on for further use.

The remaining process water P freed from the phosphorus content is mixed with the solid, thickened sludge phase S and the second anaerobic treatment stage is mixed with the digester 6th fed. Gas G, in particular methane gas, is then produced again there. The anaerobically treated sewage sludge is then in a further separation device 7th separated into a solid phase (residual sludge RS) and a liquid phase (residual water RW), the liquid phase RW being fed to the biological treatment B.

3 shows a block diagram of a third embodiment of a wastewater treatment plant in which phosphorus-containing raw sludge is fed. This raw sludge is usually a mixture of excess sludge OSS and primary sludge PS. This raw sludge can initially be in a first separation device 1 be divided into a solid sludge phase and a liquid phase, the liquid phase can be fed to the biological treatment B. The solid sludge phase is first subjected to an upstream hydrolysis in a hydrolysis device 9 subjected. Optionally, however, it is also conceivable that not the sludge mixture (PS + OSS) but only the excess sludge OSS is hydrolyzed. The upstream hydrolysis results in a faster and more complete anaerobic conversion (digestion) in the downstream digester 8th achievable and a more complete conversion of the organically bound phosphorus to orthophosphate is achieved. The anaerobic treatment in the digester 8th can be for a statistical residence time of 12-18 days, which is sufficient.

The anaerobically treated sewage sludge is fed into the mixing device to release the chemically bound phosphorus components 2 fed. A complexing agent K (preferably CitH ₃ or Na citrate) is mixed in there. The mixing device 2 can be designed as a container with a stirring device. Inline mixing in a pipeline is also conceivable. In the mixer 2 The iron ions are complexed as completely as possible and phosphorus is released as a result. Due to the upstream hydrolysis and the previous anaerobic conversion, a large part of the biologically bound phosphorus is also present as orthophosphate.

It is advantageous if, in the upstream hydrolysis, thermal hydrolysis is carried out at about 130 ° C.-165 ° C. with steam injection. In this way, the raw sludge can be pre-thickened up to 16% dry residue TR using a centrifuge. So that is in the digester 8th additionally introduced water content is reduced and there is a longer residence time with an improved substrate conversion.

In one of the mixing devices 2 downstream separation device 3 the complexed digested sludge is dewatered and thickened. The separated phosphorus-containing sludge water becomes the separation device 4th supplied in order to separate the phosphorus components there.

The separator 3 can in turn be designed as a centrifuge, belt filter press, screw press or the like. The separated liquid phase F should be as low in solids as possible. A degree of separation of more than 96% of the aqueous, i.e. the liquid, portion is advantageous.

The severing device 4th can for example be designed as a precipitation reactor. There the desired precipitant is then added, such as magnesium salt or calcium salt. Depending on the precipitant, the corresponding salts of phosphorus are precipitated, such as magnesium ammonium phosphate or calcium phosphate. The operating conditions required for this, in particular the pH value, must be set accordingly. The salts of the phosphorus are then further processed and processed in order to be used as valuable material.

In all embodiments, the separation device 4th be designed as a two-stage precipitation tank in order to carry out the precipitation and sedimentation one after the other. The crystals can can also be separated from the water by filtration or centrifugation. Other separation devices for separating the phosphorus fraction from the liquid phase F are conceivable.

The process water P, which has been depleted of the phosphorus fraction that has been separated off, is mixed back into the thickened digested sludge S and subjected to further anaerobic treatment in the digester 6th fed.

In this and in other exemplary embodiments, it is conceivable that the mixture of digested sludge and process water P is first passed through a heat exchanger and the desired digestion temperature (mesophilic, thermophilic) is set. Heat recovery is advantageous for cooling or heating processes.

The sludge mixture is then placed in the digester 6th Treated anaerobically (digestion), producing gas G that can be recycled. The digested sludge is then treated with another separator 7th divided into a liquid phase (residual water RW) and a solid phase (residual sludge RS), the liquid phase RW is fed to the biological treatment B and its solid phase RS is disposed of.

4th shows a fourth embodiment of a waste water treatment plant. Here, too, the raw sludge (for example a mixture of primary sludge PS and excess sludge OSS) is first placed in a digester 8th subjected to anaerobic pre-treatment. The phosphorus-containing sewage sludge pretreated in this way is then fed into the mixer 2 supplied and complexed by adding complexing agent K. After this complexation of the iron ions, for example, a separating device is used 3 made a solid / liquid separation. In the separated sludge water, that is to say the liquid phase F, most of the released phosphorus ions are found in the separation device 4th be separated. This can in turn be done by precipitation with the addition of metal salts and setting the respective precipitation conditions.

The after that from the separator 3 remaining solid sludge phase S is in a hydrolysis device 5 given and subjected to hydrolysis there. Thermal hydrolysis by direct steam introduction at about 130-165 ° C. and about 20-40 minutes, preferably 30 minutes, is again advantageous.

As a result of this hydrolysis, the viscosity of the previously dewatered sludge drops sharply.

The process water P depleted of the separated phosphorus content is removed from the separation device 4th , mixed with this hydrolyzed sludge again. As a result, the iron complex (eg Fe-citrate) formed by the complexation, together with the sludge, is returned to the digester after a subsequent anaerobic treatment (digestion) 6th forwarded. The iron complexes then break down there with the formation of gas G. Again, the iron ions are released and the gas production is increased by this procedure.

In a subsequent separation device 7th the liquid phase is in turn separated and fed to a biological treatment. The solid residual sludge phase RS can be disposed of. The upstream hydrolysis also improves sludge dewatering, since the hydrolysis causes a very complete anaerobic breakdown of the inorganic substances.

5 shows a block diagram of a fifth embodiment of a wastewater treatment plant. The raw sludge is first subjected to anaerobic treatment in the digester 6th subjected. The raw sludge can, for example, again be a mixture of excess sludge OSS and primary sludge PS. In a first separating device 1 the solid phase and the liquid phase are again separated from each other and the phosphorus-containing sewage sludge is thickened from approx. 2% - 4% DR to approx. 14% - 16% DR. The separated sludge water, i.e. the liquid phase, is fed directly into the aerobic biology due to the normal COD and phosphorus load. If this sludge water contains high proportions of phosphate due to a high P re-dissolution from the Bio-P portion, an optional additional precipitation stage can be advantageous 10 to operate. In this case, the 2-stage P-precipitation would increase the recovery of phosphorus. The thickened phosphorus-containing sewage sludge is mixed with complexing agent K and a hydrolysis device 5 fed. It is advantageous if this mixture is subjected to vapor pressure hydrolysis, as described in the previous exemplary embodiment.

The hydrolyzed sludge then goes to another separator 3 fed. The separated liquid phase F is very heavily loaded with COD due to the hydrolysis and, depending on the ratio between biological and chemical P-bond, contains most of the phosphorus or phosphates. Due to the complexation of the metal-phosphate compound in the thickened sludge before or during the hydrolysis, the iron complex redissolved in addition to the COD is separated off together with the released orthophosphate with the sludge water. This liquid phase F then becomes the separation device 4th fed to there, for example, by precipitation phosphorus salts to separate. The COD (chemical oxygen demand) is retained. The remaining process water P is then returned to the digester 8th fed for treatment. This makes it possible to reduce the phosphorus content contained in the remaining residual sludge RS to less than 1% phosphorus per tonne of TR.

The iron complexes formed during the complexation are in the anaerobic treatment in the digester 8th reduced.

In all of the above-mentioned exemplary embodiments, it is conceivable to carry out the removal of phosphorus by precipitation. However, separation by filtering is also possible. The PO4 ions released by the metal complexation and dissolved in water can be separated using membrane technology (ultrafiltration, reverse osmosis, etc.). For example, phosphoric acid (H3PO4) can be obtained for this purpose. The remaining solution with the metal salt complex is again passed into the anaerobic treatment in order to increase the gas production and to release the iron ions again.

For all embodiments, a relatively rapid degradation of citrate ligands is independent of whether the citrate used as complexing agent or residues of unconsumed citric acid are fed to the downstream digestion or an upstream digestion. This results in a higher methane yield. The iron ions are available again through the anaerobic treatment and combine with undesired sulfur components. This reduces the formation of hydrogen sulfide H ₂ S. In addition, heavy metals are bound as Me-S. After the final sludge dewatering, the excess iron ions are returned to the anaerobic biology with the separated sludge water in order to form iron (III) phosphate FePO ₄ again. The recycled iron thus brings savings in the use of precipitants.

The sewage sludge reduced by the separated phosphorus content can have a remaining total phosphorus content of less than 2% P / t DR, so that the remaining sludge can also be used in combined incinerations. The remaining residual sludge RS can also be used as secondary fuel, for example in the cement industry. In all of the processes described, the breakdown of the iron complexes in the anaerobic treatment improves the separation behavior in the subsequent solid / liquid separation. This can reduce the need for flocculants. If the complexing agent itself is still present in the residual sludge, it acts as a peptizer and stabilizes the suspensions, which is counterproductive for sludge dewatering. The degradation of the complexing agents in the anaerobic treatment also prevents this from having a negative effect in an anaerobic treatment associated with the process. The complexing agent has a complexing effect on calcium ions, which form the framework of the anaerobic bacterial flake (CaCO ₃ framework). If such a calcium compound is complexed from the sludge flake, the biomass forms a finely dispersed suspension and can no longer be kept in the clarifier. This biomass would drift off and the cleaning process would no longer be sustainable. Such an effect is prevented by the aerobic treatment of the complexed process water P from which the phosphorus component has been removed, together with the complexing agent contained in the solid sludge.

QUOTES INCLUDED IN THE DESCRIPTION

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Patent literature cited

• DE 102016112300 A1 [0003]

Claims (18)

A method for treating phosphorus-containing sewage sludge with the following steps: a) supplying complexing agent (K) to the phosphorus-containing sewage sludge for complexing metal ions and releasing ortho-phosphates; b) separating a phosphorus-containing liquid phase (F) from the phosphorus-containing sewage sludge to which complexing agent (K) has been added in step a); c) separating a phosphorus component from the phosphorus-containing liquid phase (F) (F); characterized by : d) feeding the process water (P) remaining in step c), which contains a portion of the complexing agent (K) added in step a) and is reduced by the separated phosphorus portion, to an anaerobic treatment stage of the sewage sludge. Procedure according to Claim 1 , characterized by, hydrolysis of the solid phase remaining in step b), reduced by the phosphorus-containing liquid phase (F) and anaerobic treatment of this hydrolyzed solid phase together with the process water (P), the supply of the process water (P) remaining in step d) to this anaerobic treatment takes place. Procedure according to Claim 1 or 2 , characterized in that the sewage sludge mixed with complexing agents (K) is excess sludge (ÜSS). Procedure according to Claim 3 , characterized in that the sewage sludge to which complexing agents (K) are added is the solid phase of excess sludge (OSS) remaining after a liquid phase (F) has been separated off. Method according to one of the preceding claims, characterized in that the sewage sludge mixed with complexing agents (K) is primary sludge (PS) and / or excess sludge (OSS) which has been subjected to anaerobic treatment. Procedure according to Claim 5 , characterized in that a solid phase of primary sludge (PS) and / or excess sludge (OSS) is subjected to hydrolysis, the hydrolyzed phosphorus-containing sewage sludge is treated anaerobically and this anaerobically treated sewage sludge is treated with complexing agent (K) in step a) and then the is subjected to further steps b) to d). Procedure according to Claim 1 , characterized by - anaerobic treatment of primary sludge (PS) and / or excess sludge (OSS); - feeding complexing agent (K) according to step a) to the anaerobically treated, phosphorus-containing sewage sludge; - hydrolyzing the complexed sewage sludge; - Separation of a phosphorus-containing liquid phase (F) according to step b) from the hydrolyzed sewage sludge; - Separation of a phosphorus component according to step c) from the phosphorus-containing liquid phase (F) and - recycling of the process water (P) remaining after the separation of the phosphorus component according to step d) to the anaerobic treatment of the primary sludge (PS) and / or excess sludge (OSS). Method according to one of the preceding claims, characterized in that the phosphorus fraction is separated off by metal salt precipitation. Method according to one of the Claims 1 to 7th , characterized in that the phosphorus portion is separated off by filtration of phosphate ions in solution. Method according to one of the preceding claims, characterized in that in the step of separating off the phosphorus component, magnesium ammonium phosphate, calcium phosphate or phosphoric acid (H ₃ PO ₄ ) is obtained. Method according to one of the preceding claims, characterized by supplying a complexing agent (K) of, for example, citric acid, sodium citrate, salicylic acid or ascorbic acid. Wastewater treatment plant for the treatment of phosphorus-containing sewage sludge with the method according to one of the preceding claims with - a mixing device (2) which is set up to supply complexing agent (K) to the phosphorus-containing sewage sludge for complexing metal ions with the aim of releasing orthophosphate; - a separation device (3) which is set up to separate a phosphorus-containing liquid phase from the phosphorus-containing sewage sludge mixed with complexing agent (K) in the mixing device (2), and - a separation device (4) which is used to separate a phosphorus fraction from the at the outlet of the Separating device (3) present phosphorus-containing liquid phase (F) is set up, characterized in that - the outlet of the separating device (4) at which the process water (P) remaining during the separation of the phosphorus portion is present, which contains a portion of the added complexing agent (K) and is reduced by the separated phosphorus content, is connected to a digester (6) for anaerobic treatment of the sewage sludge in order to feed the process water (P) to the digester (6). Wastewater treatment plant after Claim 12 , characterized by a hydrolysis device (5) at the exit of the separating device (3) for the solid phase, the hydrolysis device (5) being connected to the digester (6) for supplying a hydrolyzed solid phase of the sewage sludge for anaerobic treatment. Wastewater treatment plant after Claim 12 or 13 , characterized by a second digester (8) for anaerobic pretreatment of the phosphorus-containing sewage sludge, which is connected in the process direction upstream of the mixing device (2). Wastewater treatment plant after Claim 14 , characterized by a hydrolysis device (9) for hydrolyzing the phosphorus-containing sewage sludge, which is connected in the process direction for anaerobic pretreatment of the hydrolyzed phosphorus-containing sewage sludge with its outlet to an inlet of the second digester (8). Wastewater treatment plant according to one of the Claims 12 to 15th , characterized by a second separating device (1) for separating a liquid phase of the phosphorus-containing sewage sludge fed into a biological treatment (B), the outlet of the second separating device (1) for the solid phase being connected to the inlet of the mixing device (2) in order to to mix the solid phase with the complexing agent (K). Wastewater treatment plant according to one of the Claims 12 to 16 , characterized in that the separating device (4) has a falling unit with an inlet for precipitant and an outlet for the precipitated phosphorus fraction. Wastewater treatment plant according to one of the Claims 12 to 16 , characterized in that the separation device (4) is designed as a filter unit, preferably as an ultrafiltration unit and / or reverse osmosis unit, for the separation of phosphate ions.

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