DE102016112300A1 - Process for the treatment of phosphate-containing sewage sludge - Google Patents

Abstract

The invention relates to a method for the treatment of phosphate-containing sewage sludge (10) with the steps of providing phosphate-containing sewage sludge (10), the release of the phosphate bound in the phosphate-containing sewage sludge (10) to obtain a digested phosphate-containing sewage sludge (12), and the education of the digested phosphate-containing sewage sludge (12) of a microfiltration to obtain a first liquid phosphate-containing filtrate (14) and thickened sewage sludge (16).

Description

The invention relates to a process for the treatment of phosphate-containing sewage sludge, e.g. within a sewage treatment plant. Furthermore, the invention relates to an apparatus for carrying out this method.

The treatment of sewage sludge generates about two million tonnes of sewage sludge dry matter from municipal sewage treatment plants every year. Making these amounts of sewage sludge economically usable is of great interest.

In addition to plant nutrients such as phosphorus and nitrogen, sewage sludge also contains organic pollutants such as hormonally active substances, heavy metals and pathogenic organisms. Since the pollutants hinder or even prevent a possible use of these nutrients, the challenge of the waste management industry is precisely to remove the pollutants and at the same time to maintain the nutrients.

One way to further utilize the sludge is thermal recovery, where energy can be recovered in the form of heat. The sludge is burned, for example, in monoclaved sludge incineration plants or in cement and coal power plants. Another possibility is material recycling in agriculture. The nutrients contained in sewage sludge are used as fertilizer.

The use of sewage sludge as a fertilizer is regulated by applicable fertilizer regulations. So sewage sludge, which are approved as fertilizer, meet certain requirements. This is necessary because an accumulation of pollutants in the soil should be prevented. One problem with the use of sewage sludge as a fertilizer is the actual availability of nutrients to the plants. Although sewage sludge contains a variety of nutrients, these are often in the form of inorganic or organic compounds. However, plants can usually only absorb nutrients that are present in soluble form in the soil.

The actual plant availability of nutrients is influenced by several factors, such as the pH of the soil and sewage sludge, or the levels of iron and aluminum in the sewage sludge. An unfavorable phosphate-iron ratio can greatly reduce the plant availability. The existing challenge in the production of fertilizers from sewage sludge is therefore to make the nutrients as available as possible to the plants.

A versatile fertilizer derived from sewage sludge is known as struvite. These are magnesium ammonium phosphate (MgNH ₄ PO ₄ .6H ₂ O); also known by the abbreviation MAP. The production of MAP within a sewage treatment plant achieves a particularly high level of nutrient recovery. The obtained MAP is a low-emission product that is used as a nitrogen-phosphorus fertilizer because of its good plant availability.

From the prior art, for example D. Stumpf: "Phosphorus recycling through MAP precipitation in municipal sewage sludge" Umweltbundesamt Berlin, 2007 , various processes and apparatus for the separation of phosphate by precipitation of magnesium ammonium phosphate from sewage sludge are known. One of these methods is the so-called "Stuttgart method".

• 1. Rücklösung des Phosphates aus ausgefaultem Klärschlamm,
• 2. Filtratgewinnung, und
• 3. MAP-Fällung.

The previously known Stuttgart method is fundamentally based on the following three process steps:

• 1. Resorption of the phosphate from digested sewage sludge,
• 2. Filtrate recovery, and
• 3. MAP precipitation.

In order to obtain the highest possible yield of MAP product, it is first necessary to re-dissolve phosphates from the sewage sludge. For example, acids, bases, heat or pressure are used. In this case, the bound phosphate is brought into solution. This is followed by a solid-liquid separation. In this case, the phosphate-rich liquid phase is separated from the remaining solid phase. In a final step, phosphate recovery from the liquid phase is accomplished by precipitation or crystallization and separation of the MAP product.

The problem with the Stuttgart method is that due to the fluctuating quality of sewage sludge dewatering (in step 2), the filtrate contains variable proportions of residual organics. These organic residues affect the subsequent process steps up to the dewaterability of the MAP product. The neutralization to set a favorable precipitation milieu (in step 3) is also disturbed by the buffering capacity of the organic residues. As a result, the crystallization of the MAP product is also strongly influenced.

In order to separate the interfering organic residues from the sewage sludge, an intermediate ultrafiltration is currently used for the filtrate obtained in sewage sludge dewatering. Due to the small pore diameter (<0.1 μm), a clear solids-free filtrate is obtained. Without the disturbing shares of Restorganik are the subsequent steps improved.

The problem here, however, is that as a result of the selective action of the membranes, a concentration of the ingredients, which are retained by the membrane, occurs at the membrane. With suspended contaminants, this leads to a cover layer formation on the membrane, which can lead to stubborn blockages. When operating the plant, it was found that blocking may not be formed if certain amounts of complexing agents, such as citric acid, are added to the sludge prior to concentration. The complexing agent can be added directly before the ultrafiltration or during the acid digestion. In both cases, no or a significantly reduced blocking of the ultrafiltration membranes was found.

The blocking can be both the so-called "fouling", as well as the so-called "scaling". In fouling, especially biological substances, colloids or metal oxides increase the membrane. In contrast, during scaling, the membrane mainly binds due to poorly soluble inorganic compounds, such as calcium carbonate or sulfate. The usual cleaning agents and cleaning cycles for cleaning the ultrafiltration unit do not show the desired success and the required use of special cleaners increases the operating costs enormously.

Based on this, it is the object of the present invention to provide an alternative method for the treatment of sewage sludge, with which the above-mentioned disadvantages of the treatments known in the prior art can be overcome.

According to the invention, the object is achieved by a process for the treatment of phosphate-containing sewage sludge comprising the following steps: a) providing phosphate-containing sewage sludge, b) releasing the phosphate bound in the phosphate-containing sewage sludge to obtain an digested phosphate-containing sewage sludge, and c) subjecting the digested phosphate-containing sewage sludge to decomposition a microfiltration to obtain a first liquid phosphate-containing filtrate and thickened sewage sludge, dissolved.

Further, the object is achieved by the use of a microfiltration unit in sewage treatment plants for treating sludge containing released sludge to obtain a liquid phosphate-containing filtrate and thickened sewage sludge.

With the method according to the invention, a sewage sludge treatment takes place, with which a first liquid phosphate-containing filtrate is recovered. As a result of the release of the phosphate bound in the sewage sludge, this filtrate is rich in dissolved or hydrated phosphate. In addition, this filtrate is free of solids due to microfiltration. Such a filtrate can be used in versatile ways within a sewage treatment plant; for example, for a nutrient recovery, in particular a phosphate and / or nitrogen recovery.

With the method according to the invention also takes place a sewage sludge treatment, which can run in continuous operation, and does not necessarily have to be carried out by a more costly batch process.

According to the invention, "phosphate-containing sewage sludge" means a sewage sludge which contains phosphate in any desired form. According to the invention, "phosphate" is understood to mean not only phosphate in the sense of PO ₄ ^3- , but rather any phosphorus compound within the sewage sludge. This phosphate may be bound, for example within cells and / or in the form of iron and / or aluminum phosphate and / or dissolved or hydrated in the sewage sludge.

In contrast to "phosphate-containing sewage sludge", the term "digested phosphate-containing sewage sludge" is understood as meaning sewage sludge which has a high dissolved phosphate content. The two sewage sludge thus differ in the proportion of phosphate, which is dissolved, but not in the total amount of phosphate, ie the sum of bound and hydrated phosphate. According to the invention, the phosphate-containing sewage sludge may contain a dissolved phosphate fraction of about 10 to 50 mg / L, whereas the digested phosphate-containing sewage sludge may have a dissolved phosphate fraction of about 100 to 800 mg / L.

According to the invention, "release" means any process with which the phosphate bound in the phosphate-containing sewage sludge can be brought into solution in any form. In the process of release, for example, cells of microorganisms can be destroyed, releasing the phosphate that is inside the cells. Furthermore, the release can also take place by chemical reactions. Thus, phosphates, which are bound, for example, as metal phosphates, are hydrated by shifting the pH.

According to the invention, a "microfiltration" is understood to mean the use of a low-pressure membrane to remove solid constituents from a liquid Phase separate. The solid constituents include micro- or macro particles with a particle size of greater than 1 μm, as well as bacteria with a size greater than 0.1 μm. According to the invention, a microfiltration is therefore not an ultrafiltration in which molecules are separated off to a particle size of 0.1 μm to 0.01 μm. According to the invention a microfiltration also has a pressure difference between the filter surfaces of the inlet and outlet between 1 and 3 bar, which differs from the ultrafiltration. Here, the pressure difference between inlet and outlet is up to 10 bar.

According to a preferred embodiment of the method according to the invention, the thickened sewage sludge has a dry matter content of 1 to 10 wt .-%, preferably from 3 wt .-% to 10 wt .-%, in particular from 6 wt .-% to 8 wt .-% on.

The term "dry matter content" is understood according to the invention to mean the dry matter contained in a volume in relation to the starting volume.

This embodiment has the advantage that the thickened sewage sludge clearly loses volume in comparison with the phosphate-containing sewage sludge with the aid of microfiltration. Subsequently, the thickened sewage sludge can be used versatile in a sewage treatment plant operation, for example in a digestion. Due to the reduced volume and the lower phosphate content, the anaerobic degradation in the digestion tower is significantly improved. This also increases the amount of digested gas and the amount of self-generated or heat that can be obtained from it.

Alternatively, the thickened sewage sludge can be dehydrated in a further step so as to lose volume again. This would further facilitate disposal of sewage sludge.

In a further embodiment of the invention, the thickened sewage sludge has a phosphate content of not more than 2% by weight in the dry substance.

According to the invention, the 2% by weight relates to the total phosphate content (dissolved and undissolved). This measure has the advantage that the thickened sewage sludge only has a very low phosphate content. This in turn means that the thickened sewage sludge, for example, in a subsequent drainage, can be simplified dewatering. When drainage of sewage sludge with a high phosphate content, there is an increasing deterioration of the sludge dewatering, which entails an increase in the amount of flocculant used.

According to a preferred embodiment of the method, the following further process step d) is carried out: dewatering the thickened sewage sludge to obtain a second liquid phosphate-containing filtrate.

Due to the liberation of the bound phosphate in step b), the phosphate is present predominantly in solution. However, the microfiltration in step c) does not completely liberate the thickened sewage sludge from its liquid constituents; but only thickened. This thus still contains a proportion of dissolved phosphate, so that a second liquid phosphate-containing filtrate is obtained by dewatering the thickened sewage sludge. The second phosphate-containing filtrate can also be used subsequently for nutrient recovery.

The drainage can be carried out for example by means of filter presses or centrifuges. The filter press can be batched, whereas the centrifuge can be used in continuous operation.

The drainage is not necessarily according to the invention in one step. According to the invention, in a multi-stage dewatering, preferably a filter press and an ultrafiltration can be used. Depending on the dewatering measure, the first and second liquid phosphate-containing filtrates differ in their compositions. By dewatering, the second liquid phosphate-containing filtrate, for example, have an increasingly high polymer content. Polymers can be used as auxiliaries within the drainage. On the other hand, the second phosphate-containing filtrate may have a smaller proportion of substances smaller than 0.1 μm compared to the first liquid phosphate-containing filtrate.

When dewatering the thickened sewage sludge, a dewatered sewage sludge in the form of a filter cake is obtained in addition to the second liquid phosphate-containing filtrate. This can, in contrast to the thickened sewage sludge, be disposed of better and, above all, cheaper due to its smaller volume. A disposal option offers the controlled thermal utilization. Too high a water content is extremely disturbing.

In a further preferred embodiment, the method for the treatment of phosphate-containing sewage sludge has the following further Process step e), which takes place after step c) and / or step d), precipitates the phosphate from the first and / or second liquid phosphate-containing filtrate as magnesium ammonium phosphate to obtain a magnesium, ammonium and phosphate-containing suspension.

This embodiment offers the advantage that by precipitating magnesium ammonium phosphate both phosphate and ammonium are recovered as nutrients. The precipitation can be carried out both with the first, and only with the second liquid phosphate-containing filtrate, as well as with both filtrates or a mixture thereof, since both filtrates have sufficient phosphate and ammonium ions.

In the precipitation, it may be advantageous if initially a complexing agent is added to the filtrate. One possible complexing agent is citric acid. Due to its complex-forming properties, it binds to metal ions such as Fe ³⁺ , Al ³⁺ or Ca ²⁺ . These are also in competition with Mg ²⁺ in the subsequent magnesium ammonium phosphate precipitation (MAP precipitation). By adding the complexing agent, the formation of unwanted metal phosphates is prevented. The addition of the amount of complexing agents is carried out by a person skilled in the art in such a way that an underdosing and thus an undesired competing reaction as well as an overdosing and thus a complex formation of the existing magnesium are prevented.

The MAP precipitation is carried out by precipitant addition of magnesium compounds, such as magnesium chloride (MgCl ₂ ) or magnesium oxide (MgO). Magnesium chloride can be obtained in liquid form, which considerably simplifies handling in operational practice. In contrast, magnesium oxide is preferably obtained in solid form and metered in as a dry product. Since the product has a lower water solubility, it is used stoichiometrically overdosed. A preferred stoichiometric ratio of 1.1 (magnesium) to 1 (phosphate) has been found to be sufficient.

In order to achieve the highest possible rate of precipitation, it may be advantageous, after the addition of the magnesium compounds, to first regulate the pH and bring it to a desired value. This regulation can be done by adding lye; for example, sodium hydroxide solution. The consumption of sodium hydroxide solution depends on the metered amount of complexing agents, the amount of dissolved magnesium compounds and the pH set for the precipitation.

After the start of precipitation by pH regulation, magnesium ammonium phosphate (MAP) is formed. This arises from dissolved magnesium, ammonium and phosphate ions when the equilibrium of the solubility product shifts to the side of the MAP product as the pH increases. The precipitation takes place according to the following equation: Mg ²⁺ + NH ^{4 +} + HPO ₄ ²⁺ OH ^- + 5H ₂ O ⇌ MgNH ₄ PO ₄ · 6H ₂ O

Crystallization nuclei initially form during precipitation, which in time grow into microscopically visible crystals, which in turn agglomerate into larger crystals (mature). The larger the crystals, the better the product can be separated and dried in a possible subsequent step.

According to a preferred embodiment, the precipitation in step e) is carried out in a pH range from 7 to 10, preferably from 8 to 9.

Since the precipitation of magnesium ammonium phosphate depends on the pH value, the said pH range offers a particularly good precipitation in terms of the yield. The pH can be regulated by adding lye, preferably sodium hydroxide solution.

Already in the pH range of 7 precipitation occurs. An increase in the pH allows a better precipitation yield, but it may lead to an increase in the salt content of the filtrate. One skilled in the art will produce an optimum balance between yield and possible salification.

According to a further embodiment, the method for the treatment of phosphate-containing sewage sludge on a further process step f), which takes place after step e). In this case, the magnesium, ammonium and phosphate-containing suspension is dehydrated to obtain a magnesium, ammonium and phosphate-containing recyclate.

This step allows the recovery of phosphate and ammonium as a magnesium, ammonium and phosphate-containing recyclate (MAP recyclate), which can be used agriculturally as a fertilizer, for example. The drainage takes place after an appropriate crystallization time. The drainage can be done in one step or in several consecutive steps.

Preferably, the drainage takes place within a settling tank. The filtrate is separated from the precipitate or the MAP recyclate. The resulting sludge can in a further drainage, for example by means of a filter press or centrifuge, further dehydrated. By subsequent drying in air or by the supply of heat, the MAP recyclate is obtained as a gray, sandy powder.

This embodiment also has the advantage that the back load within the treatment plant is lowered by process water, which is obtained in the treatment of phosphate-containing sewage sludge. The process water is such water which is obtained in step f) in the dewatering and can be reused within the treatment of sewage sludge. By removing phosphorus and nitrogen in the form of MAP recyclate, the return load of the wastewater treatment plant with phosphorus is reduced by more than 90% and with nitrogen by about 10%.

In a further embodiment, the first liquid phosphate-containing filtrate and / or the thickened sewage sludge are mixed with complexing agents, preferably citric acid.

This embodiment has the advantage that complexed heavy metals, but also iron and aluminum and calcium are complexed. These ions are in a possible MAP precipitation in competition with the magnesium ions, so that it comes without complexing agents increasingly unclean MAP recyclates. By complexing with complexing agents, these ions are prevented from further reactions.

In addition to the complexation of these ions, the addition of complexing agents additionally has the advantage that a subsequent dehydration is facilitated because there is no formation of sparingly soluble compounds that could block the pores of a drainage membrane.

According to a preferred embodiment, the thickened sewage sludge is mixed with an alkali.

This embodiment has the advantage that a neutral to slightly basic thickened sewage sludge is obtained by the addition of the liquor, which can be used further improved. A possible subsequent digestion of the sewage sludge is pH-dependent. A neutral to slightly basic pH may provide better digestion results.

Furthermore, this embodiment has the advantage that a possibly subsequent drainage is significantly simplified. If the sludge to be drained has an acidic pH, drainage can cause problems. Following the possible dewatering a neutral filter cake is obtained, which can be used, for example, thermally.

In a further preferred embodiment, the phosphate-containing sewage sludge is enriched by phosphate-containing compounds.

The "phosphate-containing compounds" according to the invention, for example, animal or meat bone meal from animal carcass utilization, sewage sludge ash from the mono-incineration of sewage sludge, HTC coal or other phosphate-containing residues understood. By HTC coal is meant such coal, which is obtained by hydrothermal carbonation, for example from sewage sludge. These substances are rich in phosphate and can be used in the treatment of phosphate-containing sewage sludge for nutrient recovery.

This embodiment has the advantage that the phosphate content within the sewage sludge is increased. This in turn has the advantage that a much higher yield can be achieved with a possible phosphate recovery by MAP recyclate, since phosphate is a limiting factor in the MAP precipitation. The other required ingredients such as magnesium and ammonium are added to the first and / or second liquid phosphate-containing filtrate or are present in excess.

By adding the phosphate-containing compounds, it is also preferable to convert more ammonium to a MAP recyclate. In this way, there is an increased nitrogen removal of the process water, which can be reused within the treatment of phosphate-containing sewage sludge. If this process water is returned, for example, to the activation plant of the sewage treatment plant, this relieves the nitrogen elimination due to the lower nitrogen content.

In addition, this embodiment has the advantage that in a possible dewatering the drainage properties of the sludge can be improved by the mineral ash. In a drainage often drainage aids, for example, various polymers used. These can be saved by this embodiment. This is not only less expensive, but also avoids contamination of the drainage products by polymers.

According to a further embodiment, the phosphate-containing sewage sludge provided in step a) is digested sludge and / or excess sludge or activated sludge and / or primary sludge.

This measure has the advantage that sewage sludge, which differ in their composition, can be used. An increased phosphate content has sludge because it was previously enriched by digestion with phosphate. Activated sludge, also referred to as surplus sludge, has a substantially lower phosphate content, but can also be used in the process of the present invention.

Primary sludge is the untreated sewage sludge. This, like the other sludges, has a number of nutrients which can be solubilized by the said process and thus be available for recovery.

According to the invention, the sewage sludge can also be used in a mixture of the respective sludges in the process. It will be clear to the person skilled in the art which mixture or mixing ratio is advantageous for the process.

In a further preferred embodiment, the release of the bound phosphate in step b) takes place by chemical hydrolysis and / or by thermal and / or physical and / or thermal-chemical digestion.

According to the invention, "hydrolysis" or "digestion" refers to a process in which bound phosphate is digested and in this case hydrolyzed. The process may also be sewage sludge disintegration. Here, the comminution of sewage sludge is effected by the action of external forces, for example physically, chemically or thermally. The action of the forces leads to a dissolution of the floc structure of the sludge and to a digestion of the microorganisms contained in the sludge. Within the process, long-chain organic molecules are broken down into monomeric building blocks, destroying cell walls. In this case, cell ingredients, such as phosphate, released. Furthermore, by this process metal-bound nutrients, such as metal phosphates, can be hydrated.

By "chemical hydrolysis" is meant a digestion process according to the invention, which is carried out with the addition of strong acids or bases, for example HCl, H ₂ SO ₄ or NaOH. In addition to technical chemicals, waste acids, eg from batteries, can also be used. In this case, the hydrolysis rate is accelerated proportionally to the H ⁺ or to the OH ^- concentration. In the digestion complex molecules such as sugar, starch or protein are split into monomers. Furthermore, the cell wall structure of microorganisms is dissolved, so that the cell content and the bound phosphate becomes available and goes into solution. In addition, changes in pH such as metal compounds are digested, so that in addition to the nutrients increasingly metals can be brought into solution. In order to achieve better digestion results, the process can also be carried out at elevated temperature.

If the hydrolysis is alkaline, saponification of the cell wall is additionally achieved. In this case, more organic substances are brought into solution than for example in acid hydrolysis.

By "thermal digestion" is understood according to the invention a process which allows increased cell disruptions by increasing the temperature of the phosphate-containing sewage sludge. According to the invention, both the low-thermal process (below 100 ° C.) and the thermal disintegration, which is carried out above 100 ° C., are understood. The higher the temperature is set, the better the digestion, but the energy consumption is correspondingly greater. One skilled in the art will understand which temperature is best for which particular phosphate-containing sewage sludge.

By way of example, the thermal digestion can be carried out by the so-called Cambi method. Here, the digestion temperature is 160 to 170 ° C. Within this process, the phosphate-containing sewage sludge is first dehydrated and then hydrolyzed with steam. In practice it has been shown that this method has a particularly advantageous effect on a subsequent digestion.

By "physical digestion" is meant according to the invention a method which unlocks cells, for example of bacteria or microorganisms, by the action of external shear forces. According to the invention, the physical digestion can be carried out, for example, by means of stirred ball mills, ultrasonic homogenizers or high-pressure homogenizers.

The term "thermal-chemical digestion" refers to a process according to the invention which is carried out both with an increase in temperature and by the addition of chemicals. An example is the Pondus method. In this case, the cells are destroyed by release of heat in a basic environment to release the nutrients. By the addition of lye can the Digest already at a temperature between 60 and 70 ° C.

The release of the bound phosphate can be carried out according to the invention in one or more steps. A two-stage or multi-stage release may be beneficial.

It will be clear to a person skilled in the art which digestion is most suitable or most economical. This depends on the nature of the treated phosphate-containing sewage sludge and the structural conditions within the sewage treatment plant, as well as a possible further treatment of the liquid phosphate-containing filtrate and the thickened sewage sludge.

A further subject of the present invention relates to a device for treating phosphate-containing sewage sludge by the method described above, the device comprising: a container for receiving and / or treating phosphate-containing sewage sludge for releasing the bound phosphate to obtain digested phosphate-containing sewage sludge, and a microfiltration unit for treating the digested phosphate-containing sewage sludge to obtain a first liquid phosphate-containing filtrate and thickened sewage sludge.

The features, advantages and properties of the method according to the invention apply correspondingly to the device according to the invention.

According to the invention, the "container" is a vessel which, in its design, is adapted to the release conditions. In question, for example, comes a pressure vessel. In addition, the container may have a mixer, so that the phosphate-containing sewage sludge can be mixed during the ongoing release. The container may also preferably have an inflow and an outflow to pump phosphate-containing sewage sludge through the inflow into the container and remove after treatment as digested sewage sludge through the drain. This allows the process to run continuously. Furthermore, the container may have various measuring instruments that can measure the pH, the conductivity, as well as the level.

According to the invention, the "microfiltration unit" is a filter unit with which a solid / liquid separation of the digested phosphate-containing sewage sludge is carried out. The microfiltration unit according to the invention can be connected to the container, which allows a continuous flow.

According to a further embodiment, the microfiltration unit has a rotary disk filter.

According to the invention, a "rotary disk filter" is a filter with rotatable disk-shaped filter elements. Such a filter is for example from the EP 1 444 025 A1 known. The rotary disk filter can be used in continuous operation, so that the obtained first liquid phosphate-containing filtrate can be further used within a sewage plant operation.

In a further embodiment, the microfiltration unit has a membrane pore diameter of 0.1 μm to 1.0 μm.

Because of this membrane pore diameter, the first liquid phosphate-containing filtrate is free of solids since they are stopped by the membrane structure. In addition to the solids, any particles as well as molecules having a diameter larger than the pore diameter are retained. The retained substances are in the thickened sewage sludge after microfiltration.

According to a preferred embodiment, the device additionally has a dewatering unit for dewatering the thickened sewage sludge and / or the magnesium, ammonium and phosphate-containing suspension.

According to the invention, the "dewatering unit" is a device with which a solid / liquid separation can be carried out. For example, the dewatering unit may comprise an ultrafiltration, a centrifuge, a filter press or a settling tank. According to the invention, a plurality of drainage units can also be connected in series.

In a preferred embodiment, the device has an additional treatment tank for magnesium ammonium phosphate precipitation.

According to the invention, the "treatment tank" is a vessel which, in its design, is adapted to the conditions of magnesium ammonium phosphate precipitation. For example, the pool is particularly acid-resistant.

In addition, the basin may have a mixer which can mix the magnesium, ammonium, and phosphate containing suspension during magnesium ammonium phosphate precipitation. The vessel advantageously has Dosing devices with which the addition of, for example, the complexing agent, the magnesium compounds and the alkali can be regulated. Furthermore, in a preferred embodiment, the treatment tank can have measuring devices by means of which the pH value, the conductivity and the fill level can be measured.

Further advantages will become apparent from the figures and the following description of preferred embodiments.

It is understood that the features mentioned above and those yet to be explained below can be used not only in the particular combination given, but also in other combinations or in isolation, without departing from the scope of the present invention.

Embodiments of the invention are illustrated in the drawings and are explained in more detail below. Show it:

1 Block diagram of a first embodiment of the method for the treatment of phosphate-containing sewage sludge;

2 Block diagram of a second embodiment of the method for the treatment of phosphate-containing sewage sludge;

3 Block diagram of a third embodiment of the method for the treatment of phosphate-containing sewage sludge; and

4 Sketch of a first embodiment of a municipal wastewater treatment and sewage sludge plant for the treatment of phosphate-containing sewage sludge.

Functionally equivalent elements are identified in all figures, even in different embodiments, with the same reference numerals.

1 shows a block diagram of a first embodiment of a method for the treatment of phosphate-containing sewage sludge. This is phosphate-containing sewage sludge 10 in a first container 18 added. The mud 10 may be in the form of digested sludge and / or excess sludge or activated sludge and / or primary sludge. In a first step, the mud 10 , with liberation of the bound phosphate. The release can take place by chemical hydrolysis and / or by thermal and / or physical and / or thermal-chemical pulping. This so-called digestion results in an open-minded phosphate-containing sewage sludge 12 receive. In this phosphate is preferably dissolved before. After release microfiltration of the digested phosphate-containing sewage sludge takes place 12 by means of a microfiltration unit 20 , In this case, a separation into the first liquid phosphate-containing filtrate 14 and in thickened sewage sludge 16 , The first liquid phosphate-containing filtrate 14 is particularly rich in dissolved phosphate.

In 2 is another block diagram of a second embodiment of the method for the treatment of phosphate-containing sewage sludge recognizable. This block diagram includes the block diagram 1 and the features described there. In the block diagram in 2 can the products of microfiltration 14 and 16 be treated in subsequent steps.

The thickened sewage sludge 16 can into a drainage unit 28 transferred and dehydrated there to obtain a second liquid phosphate-containing filtrate 22 , The dewatering may be carried out within one or more process steps by means of one or more dewatering units. Preferably filter presses, centrifuges or ultrafiltrations are used. To improve the product quality of the resulting magnesium, ammonium and phosphate-containing recyclate 26 can the thickened sewage sludge 16 For example, be first drained in a filter press. The resulting filtrate may then be subjected to ultrafiltration to remove the organic ingredients. The filter press together with the ultrafiltration are in this case as dehydration unit 28 summarized.

The drainage within the drainage unit 28 has the advantage that the dissolved nutrients, which are still in the thickened sewage sludge, are made accessible and the overall yield of nutrient recovery is increased. Furthermore, the drainage offers the advantage of volume minimization. So here is next to the filtrate 22 obtained a filter cake, which can be disposed of better or less cost due to its small volume (not shown).

The first and second liquid phosphate-containing filtrate 14 and 22 can subsequently be in a treatment basin 32 be transferred. Because the two filtrates 14 and 22 may differ in their composition, it may be beneficial to the filtrates 14 and 22 to treat separately from each other. Alternatively, they may also be treated together or in mixtures. The different composition results from the different continuous solid / liquid separations within a microfiltration unit 20 and a drainage unit 28 , Both filtrates 14 and 22 However, they have a variety of nutrients that can be recovered in subsequent stages.

The precipitation of phosphate as magnesium ammonium phosphate in the treatment tank 32 proceeds according to a chemical reaction in which the phosphate is converted into a solid with the addition of precipitants. The previously dissolved phosphate is eliminated from the solution or removed. Suitable precipitants are, for example, magnesium compounds, preferably magnesium chloride or magnesium oxide. The addition of these forms a magnesium, ammonium and phosphate-containing suspension 24 , In this suspension is magnesium ammonium phosphate in the form of crystals in solution. In the precipitation, a basic pH is preferably set, thereby achieving a particularly high precipitation yield. It has been found that especially in a pH range of 7 to 10, the best yields can be achieved. The pH can be adjusted and regulated by means of lye, preferably sodium hydroxide solution.

After a crystal maturity of several hours, the suspension 24 in another drainage unit 30 be dehydrated, so that a magnesium, ammonium and phosphate-containing recyclate 26 is won. The drainage can be carried out by means of various drainage units; for example, by another filter press, a centrifuge, a drying plant, a hydrocyclone or a settling tank. It may be the drainage unit 30 around the same drainage unit as 28 or 20 act.

In 3 is another block diagram of a third embodiment of the method for the treatment of phosphate-containing sewage sludge shown. This block diagram includes the block diagram 1 and 2 and the features described there. In addition, this block diagram has further treatment options for the phosphate-containing sewage sludge 10 on.

The phosphate-containing sewage sludge 10 can preferably by means of phosphate-containing compounds 38 be enriched. Phosphate-containing compounds 38 are preferably animal and / or meat bone meal from carcass utilization, sewage sludge ash from the mono-incineration of sewage sludge or HTC coal or other phosphate-containing residues. The addition increases the phosphate content of the phosphate-containing sewage sludge 10 clear. This has positive effects on the overall yield of the magnesium, ammonium and phosphate-containing recyclate 26 , as well as the treatment steps of the drainage within the drainage unit 28 and the microfiltration unit 20 ,

In 3 It can also be seen that the first liquid phosphate-containing filtrate 14 and / or the thickened sewage sludge 16 with complexing agents 34 can be offset. A preferred complexing agent is citric acid. Due to its chemical structure, it has the property of complexing certain ions. For example, the ions are heavy metal ions, iron ions, aluminum ions and calcium ions. It has been shown that these ions have a particularly disturbing effect on the following treatment steps. This will improve the drainage of the thickened sewage sludge 16 within the drainage unit 28 by adding complexing agents 34 reached.

Out 3 it can also be seen that the thickened sewage sludge 16 additionally with a lye 36 can be offset. The lye used is preferably sodium hydroxide solution. By adding the lye, the following treatment step of the drainage is positively influenced. In this case, however, no phosphate-containing filtrate is obtained.

4 FIG. 2 illustrates a sketch of a first embodiment of a municipal wastewater treatment and sewage sludge plant for the treatment of phosphate-containing sewage sludge. FIG.

In a first treatment tank, the primary clarifier 40 , can be untreated sewage 39 be subjected to a first cleaning. This is a mechanical purification of the wastewater from coarse undissolved substances. The substances deposited on the base of the pool, which predominantly consist of organic materials, form the so-called primary sludge. This can be done in a further step directly within the digestion tower 52 be treated further (not shown).

The pre-cleaned wastewater 42 may be following the primary treatment in an aeration tank 44 be treated further. This is pre-purified wastewater 42 biologically by the metabolic activity of aerobic, anoxic and anaerobic microorganisms, the so-called activated sludge, largely freed from organic and inorganic impurities and thus purified.

In the secondary clarifier 48 becomes the supplied activated sludge 46 by, for example, sedimentation of the treated wastewater 47 separated. The sedimented activated sludge 49 will be back in the aeration tank 44 recycled. The sedimented activated sludge 49 contains the unneeded biomass, the so-called surplus sludge 50 , The excess sludge 50 becomes the secondary clarifier 48 taken. This will determine the biomass concentration within the aeration tank 44 kept constant.

Within the treatment in the treatment basin 40 . 44 and 48 For example, precipitation of the phosphate may preferably be carried out (not shown). In this case, iron or aluminum salts, such as iron chloride, iron sulfate or aluminum sulfate, and, if necessary, polymers are added to the respective sludge. This results in a precipitation reaction of the phosphate, so that subsequently predominantly as sparingly soluble metal phosphate compound in the supplied activated sludge 46 , in sedimented activated sludge 49 or in excess sludge 50 is present.

The excess sludge 50 can both within a digester 52 as well as in a second digestion basin 80 be treated further. In the treatment of excess sludge 50 By digestion, the sludge is stabilized by anaerobic fermentation. The organic compounds can be broken down. This will produce a stabilized digested sludge 54 receive.

The stabilized digested sludge 54 can be analogous to excess sludge 50 be digested in a next treatment step so that the bound phosphate is released. The release takes place in a digestion tank 56 respectively. 80 , Under certain circumstances it can be at the digestion tanks 56 and 80 to trade the same vessels.

If, within the primary treatment or the activated sludge plant, the phosphate was precipitated by means of iron and aluminum salts as iron and aluminum phosphate, the phosphate is preferably redissolved by acid hydrolysis. In this case, by adding acids, preferably sulfuric acid, at a pH of about 2 to 5 and a reaction time of 2 to 4 h, a maximum rate of redissolution can be achieved.

After release of the phosphate into the digestion tank 56 and 80 Preferably, the microfiltration of the digested phosphate-containing sewage sludge 58 respectively. 82 , The microfiltration takes place in the microfiltration unit 60 respectively. 84 , Preferably, a rotary disk filter is used here, which preferably has a membrane pore diameter of 0.1 μm to 1.0 μm. In this solid / liquid separation, a phosphate-containing filtrate 62 respectively. 86 and a thickened sewage sludge 72 respectively. 88 receive. In the microfiltration units 60 respectively. 84 it may be the same microfiltration unit, depending on the design of the treatment plant.

Under certain circumstances, some of the phosphate-containing filtrates 62 respectively. 86 after microfiltration back into the digestion tank 56 respectively. 80 be returned (shown by the dashed line). By this recirculation chemicals are mainly saved and a concentration of the phosphate-containing filtrate can take place.

The thickened sewage sludge 88 can in a subsequent treatment step together with the excess sludge 50 a digestion within the digester 52 be subjected. Under certain circumstances, an addition of lye 36 to the mud 88 be preferred so as not to affect the digestion by low pH.

The thickened sewage sludge 72 can in a drainage unit 74 be separated into its solid and liquid components. It has been found that drainage can be carried out with the aid of a filter press, in particular with a chamber filter press, and subsequent ultrafiltration. Particularly good drainage results can be achieved when the thickened sewage sludge 72 previously with lye 36 is neutralized. This will be a neutral filter cake 76 obtained, for example, can be thermally utilized. Next to the filter cake 76 also becomes another phosphate-containing filtrate 78 receive.

The filtrates 62 . 78 and 86 All can be used analogously within a nutrient recovery because each filtrate is rich in nutrients such as phosphate. In a treatment basin 64 Furthermore, a precipitation reaction is carried out. In this case, first of all a complexing agent, preferably citric acid, can be added to the filtrate in order to complex the so-called interfering ions. Furthermore, the addition of magnesium compounds, such as magnesium chloride (MgCl ₂ ) or magnesium oxide (MgO) to adjust the molar ratio required for the precipitation. A preferred stoichiometric ratio of about 1.1 (magnesium) to 1 (phosphate) has been found to be sufficient. Thereafter, a favorable pH for the precipitation can be adjusted by means of lye, so that the precipitation of magnesium ammonium phosphate starts. After a certain treatment time, the magnesium, ammonium and phosphate-containing suspension 66 in a drainage system 68 dewatered. The dewatering plant may preferably have a settling tank or a filtration in which the magnesium, ammonium and phosphate-containing recyclate 70 can be deducted.

In the drainage 68 falls process water 90 at. This can be in the aeration tank 44 be initiated. In the process water may be a greater amount of iron and Aluminum ions are located. These can be inside the aeration tank 44 react again as precipitant. As a result, additional precipitant can be saved. Due to the recirculation of the process water 90 from 68 to 44 the return load of the wastewater treatment plant is positively influenced. In addition, the process water has an increased content of organic matter (expressed by the COD) due to the addition of citric acid, which can be used in a denitrification as a carbon source.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• EP 1444025 A1 [0079]

Cited non-patent literature

• D. Stumpf: "Phosphorus recycling by MAP precipitation in municipal sewage sludge" Umweltbundesamt Berlin, 2007 [0008]

Claims (19)

Process for the treatment of phosphate-containing sewage sludge ( 10 ) comprising the following steps: a) Provision of phosphate-containing sewage sludge ( 10 ), b) release of the in the phosphate-containing sewage sludge ( 10 ) bound phosphate to obtain a digested phosphate-containing sewage sludge ( 12 ), and c) subjecting the digested phosphate-containing sewage sludge ( 12 ) microfiltration to obtain a first liquid phosphate-containing filtrate ( 14 ) and thickened sewage sludge ( 16 ). Process according to claim 1, characterized in that the thickened sewage sludge ( 16 ) has a dry matter content of 1 to 10 wt .-%, preferably from 3 wt .-% to 10 wt .-%, in particular from 6 wt .-% to 8 wt .-%. The method of claim 1 or 2, characterized in that the thickened sludge ( 16 ) has a maximum phosphate content of 2% by weight in the dry matter. Method according to one of claims 1 to 3, characterized in that after step c) the following further process step is carried out: d) dewatering the thickened sewage sludge ( 16 ) to obtain a second liquid phosphate-containing filtrate ( 22 ). Method according to one of claims 1 to 4, characterized in that after step c) and / or step d) the following further process step is carried out: e) precipitation of the phosphate from the first and / or second liquid phosphate-containing filtrate ( 14 . 22 ) as magnesium ammonium phosphate to obtain a magnesium, ammonium and phosphate-containing suspension ( 24 ). A method according to claim 5, characterized in that the precipitation in step e) in a pH range of 7 to 10, preferably from 8 to 9 is performed. A method according to claim 5 or 6, characterized in that after step e) the following further process step is carried out: f) dewatering of the magnesium, ammonium and phosphate-containing suspension ( 24 ) to obtain a magnesium, ammonium and phosphate-containing recyclate ( 26 ). A method according to any one of claims 1 to 7, characterized in that the first liquid phosphate-containing filtrate ( 14 ) and / or the thickened sewage sludge ( 16 ) with complexing agents ( 34 ), preferably citric acid, is added. Method according to one of claims 1 to 8, characterized in that the thickened sewage sludge ( 16 ) with a lye ( 36 ). Method according to one of claims 1 to 9, characterized in that the phosphate-containing sewage sludge ( 10 ) by phosphate-containing compounds ( 38 ) is enriched. Method according to one of claims 1 to 10, characterized in that it is in the provided in step a) phosphate-containing sewage sludge ( 10 ) is digested sludge and / or excess sludge or activated sludge and / or primary sludge. Method according to one of claims 1 to 11, characterized in that in step b) taking place the release of the bound phosphate by chemical hydrolysis and / or by thermal and / or physical and / or thermal-chemical digestion. Device for the treatment of phosphate-containing sewage sludge ( 10 ) by the method according to one of the preceding claims, comprising: - a container ( 18 ) for receiving and / or treating phosphate-containing sewage sludge ( 10 ) to release the bound phosphate to form digested phosphate-containing sewage sludge ( 12 ), and - a microfiltration unit ( 20 ) for treating the digested phosphate-containing sewage sludge ( 12 ) to obtain a first liquid phosphate-containing filtrate ( 14 ) and thickened sewage sludge ( 16 ) to obtain. Apparatus according to claim 13, characterized in that the microfiltration unit ( 20 ) has a rotary disk filter. Apparatus according to claim 13 or 14, characterized in that the microfiltration unit ( 20 ) has a membrane pore diameter of 0.1 .mu.m to 1.0 .mu.m. Device according to one of claims 13 to 15, characterized in that the device additionally comprises a drainage unit ( 28 . 30 ) for dewatering the thickened sewage sludge ( 16 ) and / or the magnesium, ammonium and phosphate-containing suspension ( 24 ) having. Device according to one of claims 13 to 16, characterized in that the device additionally a treatment basin ( 32 ) for a magnesium ammonium phosphate precipitate. Use of a microfiltration unit ( 20 in sewage treatment plants for treating sewage sludge containing released phosphate to obtain liquid phosphate-containing filtrate and thickened sewage sludge. Use according to claim 18, characterized in that the microfiltration unit ( 20 ) has a rotary disk filter.

Images