DE102005002066B4 - Process for obtaining phosphate salts, in particular magnesium ammonium phosphate - Google Patents

Abstract

Process for obtaining phosphate salts, in particular magnesium ammonium phosphate (MAP struvite), from stabilized sludge (S) of a sewage treatment plant, characterized by the steps:
a) dewatering the stabilized sludge (S);
b) extracting phosphate-containing sludge water (P-SW) by introducing ammonia water (A) and / or hot process water (PW ₂ ) into the dewatered stabilized sludge (S); and
c) precipitation of phosphate salts from the phosphate-containing sludge water (P-SW) with the addition of a metallic compound (eg MgX, CaX).

Description

The The invention relates to a method and a plant for the production of Phosphate salts, such as. For example, magnesium ammonium phosphate, from stabilized sludge a sewage treatment plant.

Magnesium ammonium phosphate MAP is also known under the name Struvit and will in particular as a fertilizer used in agriculture.

The recovery of magnesium ammonium phosphate from sewage sludge is well known. In Thomas Buer, Hermann Stepkes: Recovery of phosphorus from the aqueous phase, in "recovery of phosphorus from sewage sludge and sewage sludge ash", 66th Darmstadt Seminar-Abwassertechnik, 07.11.2002, Darmstadt, TU Darmstadt, FB 13, 2002, page 33-54 describe the precipitation of struvite in a by-pass process in which, following an increased biological phosphorus elimination, a crude sludge acidification is carried out with a phosphorus redissolution by mixing mechanically thickened phosphorus-containing excess sludge with precleaning sludge in a stripper and redissolving reactor After separation of thickened sludge As well as phosphorus-containing supernatant, the dissolved phosphates are precipitated by the addition of magnesium and ammonium (NH ₄ ⁺ ) as magnesium ammonium phosphate Optionally, the recovery of phosphate from the centrate of digested sludge treatment of biophosphate sludges is described so that the Rü burden on the biological stage can be reduced.

In Paul Roeleveld, Patricia Loeffen, Hardy Temmink, Bram Klapwijk: Dutch analysis for P-recovery from municipal wastewater, in Ödegard, H: "Wastewater Sludge Resource "; Proceedings of IWA Specialist Conference, Trondheim, Norway, 2003, pages 263 to 270 is the phosphate recovery below Production of magnesium ammonium phosphate from the mud of anaerobic or aerobic treatment and from the stabilized mud after digestion described. The stabilized mud is drained.

Farther is from Norbert Jardin: Phosphorus balance in sewage and sewage sludge treatment, in "recovery of phosphorus from sewage sludge and sewage sludge ash ", 66th Darmstadt Seminar - Wastewater Technology, 07.11.2002, Darmstadt, TU Darmstadt, FB 13, 2002, pages 17 to 31 known that the recovery potential of phosphorus from stabilized sludge and sewage sludge ash is much bigger, as if the phosphorus recovery from the main wastewater stream or the excess sludge in the secondary stream he follows.

The Problem with the known methods is the economy, which is affected by the chemicals and energy to be used.

task It is therefore an object of the invention to provide an improved process for recovery of phosphate salts, such as. As magnesium-ammonium-phosphate, from stabilized Mud of a sewage treatment plant to accomplish.

• a) Entwässern des stabilisierten Schlamms;
• b) Extrahieren von phosphathaltigem Schlammwasser durch Einbringen von Ammoniakwasser und/oder heißem Prozesswasser in den entwässerten stabilisierten Schlamm; und
• c) Ausfällen von Phosphatsalzen aus dem phosphathaltigen Schlammwasser unter Zufuhr einer metallionenhaltigen Verbindung.

The object is achieved by the generic method by the steps:

• a) dewatering the stabilized sludge;
• b) extracting phosphate-containing muddy water by introducing ammonia water and / or hot process water into the dewatered stabilized mud; and
• c) precipitation of phosphate salts from the phosphate-containing sludge water with supply of a metal-ion-containing compound.

The Introducing ammonia water and / or hot process water instead the conventional used caustic soda or other bases has the advantage that the introduced substances are available in the process anyway and already warmed up are. In addition, no complicated neutralization of the added substances necessary in the further course of the process.

Ammonia water is an aqueous solution with 25 mass% ammonia, a pH greater than 11 and a density of 0.9 g / cm ³ .

Especially it is advantageous if the dewatering of the stabilized sludge and extracting phosphate-containing muddy water from the dehydrated one stabilized sludge takes place in a cycle process. there becomes phosphate depleted, i. H. low-phosphate mud from the Withdrawn cycle process. By the repeated drainage and repeated extraction with the associated repeated Use of ammonia water or hot process water can be the Effectiveness of Extraction process can be significantly increased.

The separation of sludge water from the stabilized sludge is carried out in step a), preferably with a mechanical dewatering unit, for example in the form of a filter press, wherein the ammonia water or the hot process water in step b) in the Entwässerungsag gregat is introduced during the dewatering for the extraction of phosphate-containing sludge water. Thus, the drainage effect of existing in the filter press filter cake is exploited due to the increased drainability.

Especially It is advantageous if an entry of water vapor in the precipitation process in step c). This is a stirring effect on the precipitation process exercised by the steam and at the same time causing steam stripping of ammonia. By Condensing the vapors Ammonia water can then be obtained from the steam stripping.

Preferably becomes a partial flow of the steam stripping and condensation-heated ammonia water in the filtered off in step a) sludge water for the extraction of recycled phosphate-containing sludge water.

The Extraction of phosphate-containing sludge water in step b) takes place preferably at a temperature in the range of 40 to 80 ° C and especially preferably at a temperature in the range of 60 ° C with a tolerance range of ± 10 ° C and still preferably a tolerance range of ± 5 ° C.

The at the precipitation supplied by phosphate salts metal ion-containing compound MeX may be magnesium oxide, magnesium sulfate, Magnesium chloride, calcium oxide, calcium hydroxide, calcium chloride or a combination of it.

Especially it is advantageous if a thermal hydrolysis of raw sludge and fuming the hydrolyzed raw slurry to obtain stabilized mud takes place. The thermal hydrolysis additionally accumulating energy (biogas) can generate water vapor and electric power are used, which then for the extraction in the step b) and / or steam stripping in step c) are used. By the thermal hydrolysis will not only make the digestion about 30 to 50 %, which leads to an OTS degradation rate of approx. 60 to 65% (oTS = organic dry matter), but also the digested sludge volume flow reduced by the factor of about 3 and more, as well as the drainability of the sludge significantly increased.

• – ein mechanisches Entwässerungsaggregat mit einem Einlauf für stabilisierten Schlamm, einem Auslauf für Schlammwasser und einem Auslass für entwässerten Schlamm, wobei ein Spüleinlass zum Einleiten von Ammoniakwasser und/oder heißem Prozesswasser zur Extraktion phosphathaltigen Schlammwassers in das Entwässerungsaggregat vorgesehen ist, und
• – einen Fällungsreaktor mit einem Einlauf für das phosphathaltige Schlammwasser, einem Einlass zum dosierten Einbringen einer metallionenhaltigen Verbindung, einem Auslass für ausgefälltes Phosphatsalz und einem Auslass für Prozesswasser.

The problem is further solved by the generic system by

• A mechanical dewatering unit having a stabilized sludge inlet, a muddy water spout and a dewatered sludge outlet, wherein a scavenging inlet for introducing ammonia water and / or hot process water for extracting phosphate-containing sludge water into the dewatering aggregate, and
• - A precipitation reactor with an inlet for the phosphate-containing sludge water, an inlet for the metered introduction of a metal ion-containing compound, an outlet for precipitated phosphate salt and an outlet for process water.

With Help the rinsing inlet in the dewatering unit for introducing ammonia water and / or hot process water, the Extraction economically with available and heated materials in the process respectively. At the same time, with the help of the rinsing inlet after extraction a neutralization with dilute Acid, such as For example, sulfuric acid, take place. This has the advantage that the remaining ammonia water chemically is converted into a salt, reducing the ammonia odor of the mud eliminated and thus the further treatment is not restricted.

Of the precipitation reactor preferably has a crystallization zone for the crystallization of precipitated Phosphate salt. This allows the removal of the phosphate salt and the subsequent drainage and drying can be simplified.

Preferably has the precipitation reactor a stripping column for steam stripping with an inlet for water vapor and an outlet for vapors from the steam stripping, with the inlet for steam arranged so is that by the steam a stirring effect on the precipitation process exercised becomes. This will not only ammonia with the help of steam stripping but at the same time improves the precipitation process.

Preferably is a condenser to the outlet of the stripping column for recovery connected by ammonia water. The capacitor is then preferred to the dewatering unit for Introduction of a partial flow of the recovered ammonia water in the dehydration unit connected.

Farther it is advantageous to have a muddy water storage between the mechanical dehydration unit and the precipitation reactor to provide for the processes of drainage and extraction of the precipitation and steam stripping time to decouple and a buffer line provide. The same applies to the intermediate storage of ammonia water between the condenser and drainage unit.

The Invention will be exemplified with reference to the accompanying drawings explained in more detail. It shows:

1 - Block diagram of the process for the recovery of phosphate salts in the form of magnesium ammonium phosphate in the sludge stream of a sewage treatment plant.

The figure shows a block diagram of a sludge stream in a sewage treatment plant, in which primary sludge PS and excess sludge ÜS and co-substrates are conducted into a storage tank and in a thickening apparatus 2 , For example, a mechanical dewatering unit such. B. a filter press to be thickened This process water PW ₁ is returned. The thickened raw sludge RS is subsequently subjected to thermal hydrolysis 3 subjected. It is not only in the subsequent digester 4 Improved digestion by approx. 30 to 50% compared to digestion without previous hydrolysis (OTS degradation rate approx. 60 to 65%), but also reduces the digested sludge volume flow by a factor of approx. In addition, the drainability of the digested sludge is significantly increased to about 40% dry residue TR with a chamber filter press. The digested sludge from the digester 4 is in a mud store 5 stored and further processed as stabilized sludge S.

This is done in a mechanical dewatering unit 6 , such as B. a filter press in a step a) the dewatering of the stabilized sludge S and in a step b) by supplying hot process water PW ₂ and / or energy E z. B. in the form of water vapor from the hydrolysis 3 and / or ammonia water A is an extraction of phosphorus P from the dewatered sludge. For this purpose, the ammonia water A or the hot process water PW ₂ from the precipitation reactor 8th in the dewatering unit 6 so initiated that the drainage effect of the dewatered sludge in the dewatering unit 6 is being used. After step b), the phosphate-depleted sludge is optionally neutralized with a weak acid. The Phosphor P-enriched extract in the form of phosphate-containing sludge P-SW is then stored in a sludge tank 7 stored. In addition, less phosphate, dehydrated and. optionally neutralized sludge ES an outlet on the dewatering unit 6 taken.

The phosphate-containing sludge water P-SW is from the sludge water storage tank 7 in a precipitation reactor 8th passed in the phosphate salts, such as. B. magnesium ammonium phosphate MAP (struvite) can be precipitated. The precipitation reactor 8th Water vapor is supplied, so that the phosphate-containing sludge water P-SW is stirred in the precipitation.

Furthermore, the precipitation reactor has 8th an inlet for the metered introduction of a metal ion-containing compound MeX, for example magnesium oxide MgO, magnesium sulfate MgSO ₄ or magnesium chloride MgCl ₂ or calcium compound such as calcium oxide CaO, calcium hydroxide Ca (OH) ₂ , calcium chloride CaCl ₂ , magnesium oxide MgO being preferred. The precipitation reaction requires about 1.29 kg MgO per kgP.

Furthermore, it is optional in the precipitation reactor 8th a stripping column and a crystallization zone are provided. With the help of the stripping column is carried out at the same time a stripping of ammonia NH ₃ in the introduction of water vapor, because the pH increases by the parallel stripping of dissolved carbon dioxide CO ₂ skyrocketed. The ammonia NH ₃ is introduced into a condenser 9 passed, at the output of ammonia strong water A is obtained. Ammonia Strong Water is an aqueous solution with an ammonia content of 25% by weight, a density of 0.9 g / cm ³ and a pH (10% in water) of more than 11. Strong Ammonia A is a colorless, clear liquid with a pungent Odor.

The crystallization zone in the precipitation reactor 8th is used for the production of nuclei and formation of phosphate-containing crystals of a certain size in different zones of the precipitation reactor 8th taking advantage of the different hydrodynamic conditions. The crystallization is improved by the formation of a fluidized bed by means of the steam input.

By the recovery of phosphate salts, such as. B. magnesium ammonium phosphate MAP, and Ammoniakstarkwasser A is in addition to the Phosphorentnahme simultaneously taken a significant amount of nitrogen from the sludge S, so that also the efficiency of a sewage treatment plant with respect to the essential Aspect of nitrogen removal can be significantly improved.

From the precipitation reactor 8th Therefore, low-phosphate and low-nitrogen process water PW _{2 is returned} to the sewage treatment process.

Claims (16)

A process for obtaining phosphate salts, in particular magnesium ammonium phosphate (MAP struvite), from stabilized sludge (S) of a sewage treatment plant, characterized by the steps of: a) dewatering the stabilized sludge (S); b) extracting sludge water containing phosphate (P-SW) by the introduction of ammonia water (A) and / or hot process water (PW ₂₎ (in the dewatered sludge stabilized S); and c) precipitating phosphate salts from the phosphate-containing muddy water (P-SW) with the supply of a metallic compound (eg MgX, CaX). Method according to claim 1, characterized in that that draining of stabilized sludge (S) and extracting phosphate-containing Sludge water (P-SW) from the dewatered stabilized sludge (S) takes place in a cycle process and phosphate depleted drained mud (S) is subtracted from the cycle process. A method according to claim 1 or 2, characterized by separating sludge water (P-SW) from the stabilized sludge (S) in step a) with a dehydrating unit ( 6 ) and introducing the ammonia water (A) or hot process water (PW) in step b) into the dewatering unit ( 6 ) for the extraction of phosphate-containing sludge water (P-SW). Method according to one of the preceding claims, characterized by a final neutralization of the phosphate-depleted sludge by rinsing with steam or an acid solution, for example with sulfuric acid, with simultaneous conversion of the remaining ammonia (NH ₃ ) into corresponding salts. Method according to one of the preceding claims, characterized by introducing water vapor (D) in the precipitation process in step c), wherein in addition steam stripping of ammonia (NH ₃ ) takes place, and condensing the vapors from the steam stripping to obtain ammonia water (A). Method according to claim 5, characterized by returning a Partial stream of ammonia water heated by steam stripping and condensation (A) in the filtered off in step a) sludge water for extraction phosphate-containing sludge water (P-SW). Method according to one of the preceding claims, characterized characterized in that the extraction of phosphate-containing sludge water (P-SW) in step b) at a temperature in the range of 40 to 80 ° C. Method according to claim 7, characterized in that that the extraction of phosphate-containing sludge water (P-SW) in step b) at a temperature in the range of 60 ° C with a Tolerance range of ± 10 ° C takes place. Method according to one of the preceding claims, characterized in that the metal ion-containing compound is a magnesium compound, for example magnesium oxide (MgO), magnesium sulfate (MgSO ₄ ), or magnesium chloride (MgCl ₂ ), a calcium compound, for example calcium oxide (CaO), calcium hydroxide (Ca (OH ) ₂ ), calcium chloride (CaCl ₂ ), or a combination thereof. Method according to one of the preceding claims, characterized by thermal hydrolysis ( 3 ) of raw sludge (RS) and digestion of the hydrolyzed raw sludge (RS) to obtain stabilized sludge (S), the thermal hydrolysis ( 3 ) additionally used energy (E) for extraction in step b) and / or steam stripping in step c) is used. Investment. for obtaining phosphate salts, for example of magnesium ammonium phosphate (MAP struvite), from stabilized sludge (S) by the process according to one of the preceding claims, characterized by a mechanical dewatering aggregate ( 6 ) with a stabilized sludge inlet (S), a sludge water spout (P-SW) and an outlet for dewatered sludge, a purge inlet for introducing ammonia water (A) and / or hot process water (PW) for the extraction of phosphate-containing sludge ( P-SW) into the dewatering unit ( 6 ), and - a precipitation reactor ( 8th ) with an inlet for the phosphate-containing sludge water (P-SW), an inlet for the metered introduction of a precipitated salt compound, an outlet for precipitated phosphate salt, and an outlet for process water (PW). Equipment according to claim 11, characterized in that the precipitation reactor ( 8th ) has a crystallization zone for crystallizing the precipitated phosphate salt. Plant according to claim 11 or 12, characterized in that the precipitation reactor ( 8th ) has a stripping column for steam stripping with an inlet for steam and an outlet for vapors from the Dampfstrippung, wherein the inlet for water vapor is arranged so that by the water vapor, a stirring action is exerted on the precipitation process. Plant according to claim 13, characterized in that a capacitor ( 9 ) is connected to the outlet of the stripping column for the production of ammonia water (A). Plant according to claim 14, characterized in that the capacitor ( 9 ) to the dewatering unit ( 6 ) for introducing a partial flow of the recovered ammonia water (A) into the dewatering unit ( 6 ) connected. Plant according to one of Claims 11 to 15, characterized by a sludge water separator cher ( 7 ) between the dewatering unit ( 6 ) and the precipitation reactor ( 8th ).