WO2020043607A1 - Method of improving the dewatering of sewage sludge - Google Patents

Abstract

In order to provide an improved method of dewatering sewage sludge, with which sewage sludge having a minimum residual moisture content can be obtained, a method of dewatering sewage sludge is proposed, in which (a) sewage sludge is suspended in water in order to obtain an aqueous suspension of the sewage sludge, (b) carbon dioxide is introduced into the suspension generated in stage (a), in order to produce a carbon dioxide-enriched suspension, (c) elevated pressure is applied to the carbon dioxide-enriched suspension and (d) a dewatering step for separating water from the carbon dioxide-enriched suspension is conducted, characterized in that stages (b) and (c) are performed in a continuously operated reactor until the ratio of monovalent/divalent cations, expressed by the following formula [c (Na+) + c (K+) + c (NH4 +)] : [c (Ca2+) + c (Mg2+)] , in the liquid component of the carbon dioxide-enriched suspension that remains after the solids have been removed has decreased by at least 10% compared to the liquid component of the suspension obtained in stage (a) after the solids have been removed.

Description

 Process to improve sewage sludge dewatering

OBJECT OF THE INVENTION

The present invention relates to a method for improving the dewatering of sewage sludge and a process plant with which this method can be carried out.

BACKGROUND AND PRIOR ART

During wastewater treatment in sewage treatment plants, sewage sludge accumulates as waste and is disposed of. The sewage sludge is digested at most sewage treatment plants. This takes place in the absence of air for about 30 days.

Sewage sludge is thin in the initial state and has a dry matter content of approx. 3-5% after digestion. The rest of the mass is water. Before the sewage sludge is disposed of, many sewage treatment plants carry out treatment steps that reduce the water content and increase the dry matter content. The background to this is the associated savings in disposal costs. This is because they relate to the amount actually disposed of (original substance = OS) and not to the dry matter.

Various measures are known from the prior art for increasing the dry matter content in the OS. The most common measure used in sewage treatment plants is mechanical drainage, e.g. Decanters, centrifuges, belt presses, chamber & bag filter presses and screw presses. Depending on the sewage sludge, a dry matter content of up to 30% can be achieved, which results in a significant reduction in the amount of OS.

Flocculants are often used to support the mechanical dewatering of the sewage sludge. These are usually polymers that promote the formation of flakes in the sewage sludge, since these can then be better separated off during mechanical dewatering.

In the next step, drying can take place, for example using a belt dryer or solar drying. A dry matter content between 75 and 90% can be achieved. To reduce the amount of sewage sludge even further, it can be burned. Based on the dry matter before combustion, approx. 50% mineral constituents remain in the form of ash.

Both the drying and the combustion are time and energy consuming processes and there is therefore a need for a method with which one can achieve an even higher degree of dewatering before the disposal of the sewage sludge than is possible with the previously known methods.

TASK

Against this background, the object of the present invention was to provide an improved process for dewatering sewage sludge with which sewage sludge with the lowest possible residual moisture content can be obtained.

DESCRIPTION OF THE INVENTION

This task is solved by a process for the dewatering of sewage sludge, in which one

(a) sewage sludge suspended in water to obtain an aqueous suspension of the sewage sludge,

(b) introducing carbon dioxide into the suspension produced in step (a) in order to produce a suspension enriched with carbon dioxide,

(c) applying increased pressure to the suspension enriched with carbon dioxide and

(d) performs a dewatering step to separate water from the carbon dioxide-enriched suspension, characterized in that steps (b) and (c) are carried out in a continuously operated reactor until the ratio of monovalent / divalent cations is determined by the following Formula is expressed

[c (Na ⁺ ) + c (K ⁺ ) + c (NH ₄ ⁺ )]: [c (Ca ²⁺ ) + c (Mg ²⁺ )] in the suspension enriched with carbon dioxide has decreased by at least 10% compared to the suspension obtained in step (a).

The “sewage sludge” to be dewatered in accordance with the invention denotes the mixture of water and organic and mineral substances, which arises as waste from the treatment of wastewater in wastewater treatment plants (sewage treatment plants).

According to the invention, an aqueous suspension of the sewage sludge is treated with carbon dioxide under increased pressure before the final dewatering step takes place, and an important advantage which arises in the execution of the dewatering process according to the present invention is a strong relative reduction in the ratio of the sum of the concentrations of the monovalent cations Na ⁺ , K ⁺ and NH ⁺ to the sum of the concentrations of the divalent cations Ca ²⁺ and Mg ²⁺ (in the following ratio monovalent / divalent cations) in the liquid fraction of the suspension treated with carbon dioxide after separation of the solids compared to that after separation of the solids, the remaining liquid portion of the untreated starting suspension. This is because the inventors of the present application have found that a significant improvement in drainage is achieved if the ratio of monovalent / divalent cations is reduced by more than 10%. Even more preferred is a reduction in the ratio of monovalent / divalent cations by 15% and more. A reduction in the ratio of monovalent / divalent cations by 20% and more is particularly preferred.

The above-mentioned advantage is achieved in particular by introducing carbon dioxide into the sewage sludge to be treated in stages (b) and (c) of the process according to the invention and allowing it to act. According to the invention, these process steps are carried out in a continuously operated reactor. In this regard, the use of a tubular reactor has proven to be particularly suitable.

The “continuously operated” feature is to be understood as follows in the sense of the present invention. In continuous operation, the reactor is continuously filled with new sewage sludge suspension at one point and the suspension treated with carbon dioxide is continuously removed at another point. In contrast, in batch operation, a reactor is filled once with new sewage sludge suspension, and once the reaction has ended, the suspension treated with carbon dioxide is removed once.

For the purposes of the present invention, the term “continuously operated” can also include “quasi-continuously” operated embodiments in which no completely continuous operation is carried out. but only an almost uninterrupted operation is realized. In particular embodiments of the present invention, the “continuously” operated reactor is either a reactor that is operated continuously in the narrow sense, ie continuously, or a reactor that is operated “quasi-continuously”.

In the context of the present invention, the term “tubular reactor” denotes a tubular reactor that works according to the “plug flow” principle, in which the suspension treated with carbon dioxide is from one end (proximal end) of the tube, to the sewage sludge suspension that has not yet been treated with carbon dioxide is introduced into the reactor, is transported to the other end (distal end) of the tube, from which sewage sludge treated with carbon dioxide is removed from the reactor. This is where the material conversions take place along the transport route through the reactor tube. The tubular reactor is characterized in particular by a tubular reactor space, the length of which is substantially greater than its diameter.

The “diameter” of a tube reactor in the sense of the present invention is to be understood as the average inside diameter of the reaction space of the tube reactor. The term “reaction space” denotes the area in which the reaction conditions required for the reaction to take place are present.

In certain embodiments of the invention in which a tubular reactor is used, the diameter of the tubular reactor is in the range from 5 to 100 cm, preferably in the range from 8 to 50 cm and more preferably in the range from 10 to 25 cm.

In certain embodiments of the invention, the ratio of the length of the reactor interior to the diameter of the reactor interior is at least 50: 1, at least 100: 1, at least 200: 1, at least 300: 1, at least 400: 1 or even at least 500: 1. Some embodiments have a length to diameter ratio of up to 1,000: 1, but some particular embodiments can also have a ratio of up to 2,000: 1, up to 5,000: 1 or even up to 10,000: 1.

In certain embodiments of the invention, mixing devices are provided for the transport and / or the mixing of the suspension treated with carbon dioxide through the reactor. For example, in certain embodiments, an eccentric screw pump is provided at the proximal end of the reactor. As an alternative or in addition, mixing paddles arranged longitudinally through the reactor and / or a conveyor screw running longitudinally through the reactor and / or static or dynamic mixers can also be provided. To a certain extent, a homogeneous mixture of the suspension can also be achieved in that the tube has several turns in a tube reactor. In certain embodiments of the invention, the tube of the tube reactor therefore has at least 5, at least 10, at least 20 or even at least 30 turns, in which the course of the tube in the transport direction describes an angle in the range from 90 ° to 180 °. Embodiments with as many turns as possible through 180 ° can be particularly advantageous, for example at least 5, at least 10, at least 20 or even at least 30 turns through 180 °.

In certain embodiments of the invention, either gaseous or liquid carbon dioxide is introduced. The inventors of the present application have found that, in certain embodiments of the invention, a higher yield can be achieved if not gaseous carbon dioxide is introduced into the sewage sludge suspension for dewatering, but liquid carbon dioxide.

The decisive factor for this characteristic is the state of aggregation of the carbon dioxide in the feed line through which the carbon dioxide is led, i.e. up to the point in time at which the carbon dioxide enters the reactor through the opening of the feed line. Depending on the specific process conditions in the reactor (in particular pressure and temperature), the state of matter of the introduced carbon dioxide can change after it has entered the reactor space. In further embodiments of the invention, an even higher yield can be achieved if both gaseous and liquid carbon dioxide are introduced into the sewage sludge suspension for dewatering.

In the embodiments in which both gaseous and liquid carbon dioxide are supplied, the molar ratio of the carbon dioxide can be varied according to the invention in these two states of matter. In certain embodiments, the molar ratio of supplied gaseous to supplied liquid carbon dioxide is in the range from 90:10 to 10:90. The percentage of molar amount of liquid carbon dioxide based on the total amount of carbon dioxide supplied is preferably at least 30%, at least 40%, at least 50%, at least 60%, at least 70%, or even at least 80%.

The introduction of both gaseous and liquid carbon dioxide has the advantage that a more homogeneous distribution of the carbon dioxide used for the treatment of the sewage sludge can be achieved in the sewage sludge suspension. Without wishing to be bound by this, one possible explanation could be that liquid carbon dioxide is better distributed in the sewage sludge suspension than gaseous carbon dioxide. As an alternative or in addition to the measures described above, a more homogeneous distribution of the carbon dioxide in the sewage sludge suspension can also be achieved in certain embodiments in that the suspension treated with carbon dioxide is subjected to ultrasound (frequency> 16 kHz). In certain embodiments, the ultrasonic frequency is in the range of 20 kHz to 1 GHz. In certain embodiments, it may be preferred that the ultrasound frequency is a maximum of 300 kHz or only 250 kHz.

The present invention encompasses both those embodiments in which the introduction of the liquid carbon dioxide and the gaseous carbon dioxide takes place via one and the same feed line, and also such embodiments in which the introduction of the gaseous carbon dioxide and the introduction of the liquid carbon dioxide via separate feed lines lines done.

In certain embodiments of the invention, the gaseous carbon dioxide is introduced via at least a first supply line and the liquid carbon dioxide is introduced via at least a second supply line. In some embodiments of the invention, the first lead is in the process direction, i.e. in the direction in which the suspension treated with carbon dioxide moves through the reactor, in front of the second feed line. In an alternative embodiment, the second feed line lies in the process direction before the first feed line. In certain embodiments, the at least one first feed line and the at least second feed line can also be at the same height in relation to the process direction.

In particular embodiments of the invention, there are a plurality of first and a plurality of second lines, which are arranged either in groups or alternately. In certain embodiments, the first and / or second feed lines are all arranged on the top or bottom of a horizontally oriented reactor in the longitudinal direction. In other embodiments, the first and / or second feed lines are distributed over the circumference of the cross-sectioned reactor.

In certain embodiments, the supply line or the supply lines in the longitudinal direction of the tubular actuator is only at the beginning, i.e. arranged within the first 10% of the reactor interior length. In certain embodiments, first and / or second feed lines for carbon dioxide are found only in the first quarter, only in the first third, only in the first half or in the first two thirds of the reactor interior length.

In certain embodiments, each first and / or second feed line has its own quantity regulator with an inlet valve controlled thereby. In other embodiments, the amount controlled on supplied carbon dioxide via the first and / or second feed lines via a common flow controller. In some embodiments, a plurality of first and / or second feed lines are assigned in groups to a common flow regulator.

The sewage sludge is treated under increased pressure (stage c)). In certain embodiments, the increased pressure in the reactor is in the range from 2 to 100 bar, preferably in the range from 3 to 50 bar, more preferably in the range from 5 to 25 bar and particularly preferably in the range from 5 to 16 bar. In special embodiments, the dewatering step takes place at a pressure in the reactor of at least 6 bar or even at least 8 bar. In certain embodiments, the maximum pressure in the reactor is 25 bar, 20 bar or even only 16 bar. The specific selection of a specific pressure depends in particular on the specific properties of the sewage sludge to be digested, the exact composition of the selected suspending agent, the process temperature and the molar ratio of gaseous to liquid carbon dioxide.

The treatment of the sewage sludge with carbon dioxide takes place at a temperature in the range from 5 to 100 ° C. In special embodiments, the treatment is carried out at a temperature in the reactor of at least 10 ° C. or even at least 15 ° C. The maximum temperature in the reactor is 75 ° C, 50 ° C or 40 ° C in certain embodiments. A particularly preferred temperature range is from 20 to 35 ° C. The specific choice of a specific temperature depends in particular on the specific properties of the sewage sludge to be digested, the exact composition of the selected suspending agent, the process temperature and the molar ratio of gaseous to liquid carbon dioxide.

In certain embodiments of the invention, the pressure increased in step c) is reduced between steps c) and d). The pressure reduction has the advantage that under reduced or normalized pressure for the subsequent dewatering, those methods can also be used which can be better carried out under non-elevated pressure conditions. In some embodiments, the pressure is reduced to a pressure in the range below 1 bar. The concrete selection of the pressure takes place depending on which method for drainage follows.

The pressure reduction can be done either at the end of the process in the last section of the actuator or in a separate flash tank that connects to the reactor. The pressure can be reduced in one step or in two or more stages in an expansion vessel arranged after the reactor or in two or more expansion vessels arranged in succession. In a flash tank, devices for foam destruction are provided, since in particular the carbon dioxide introduced into the suspension in gaseous form outgasses under reduced pressure and this results in a considerable foam formation. Examples of devices for foam destruction are, without limitation, nozzles for spraying liquid (for example water), mixing devices (stirrers), supply lines for surface-active agents, special fittings (for example foam screens) and ultrasound sources.

In stage d) of the process according to the invention, dewatering takes place in which undissolved solids are separated from the suspension. In one embodiment of the invention, the undissolved solids are separated off by means of at least one sieve and / or at least one filter (e.g. filter press, vacuum belt filter). In an alternative embodiment, the undissolved solids are separated from the suspension by centrifugation.

In certain embodiments, various methods for separating undissolved solids from the suspension are combined. In such cases, at least one centrifugation and at least one filtration are therefore preferably carried out. A first filtration step is particularly preferably carried out, followed by centrifugation and finally a second filtration step. The second filtration step is preferably a microfiltration (pore size> 100 nm to 100 pm), ultrafiltration (pore size 2 - 100 nm) or nanofiltration (pore size <2 nm).

Various pretreatment methods can also be used to optimize dewatering, in particular to disintegrate the sewage sludge, disintegration of the sewage sludge means that the cells contained are disrupted by microorganisms.

Basically, the disintegration of the cell structures can take place both before and after the sewage sludge digestion, and various chemical and / or physical methods can be implemented, such as:

Grinding the sewage sludge, e.g. in a wet mill (pearl mill),

 Treatment of sewage sludge with pressure (in the range 10 - 120 bar),

 thermal pretreatment at temperatures between 30 and 150 ° C,

 Pretreatment at a pressure between 10 and 120 bar and temperatures between 30 and 250 ° C (thermal pressure hydrolysis (TDH® process)) and / or

Ultrasonic treatment of the sewage sludge before or during the addition of C0 ₂ . Methods to relax the sewage sludge after the C0 ₂ treatment can also lead to disintegration of the cells in the sewage sludge. Sudden relaxation and directing the sewage sludge against a baffle plate are particularly noteworthy in this regard, as this effectively destroys the cell structures (disintegration / hydrolysis).

The inventors of the present invention have found that the residual moisture in the sewage sludge treated by the process according to the invention is surprisingly low, which can be regarded as very advantageous from the point of view of the sewage treatment plant operator, since the weight of the sewage sludge to be disposed of is thus significantly lower than with higher residual moisture content . Since the disposal costs depend on the weight of the sewage sludge to be disposed of, a lower residual moisture means lower disposal costs.

In addition, for the particularly effective dewatering of the sewage sludge using the process according to the invention, significantly smaller amounts of flocculant have to be used, which brings a further significant cost saving for the plant operator. Examples of flocculants which can be used in the removal of solids from the sewage sludge are known to the person skilled in the art. Flocculants based on starch are particularly preferred according to the invention.

The present invention also includes the provision of a process plant for the dewatering of sewage sludge by treatment with carbon dioxide, which is suitable for carrying out the process according to the invention. In particular, the plant has the following facilities for this: a reactor for treating the sewage sludge suspension with carbon dioxide, a facility for introducing carbon dioxide into the reactor, a facility for adjusting an increased pressure in the reactor, a facility for separating solids from the extracted suspension, characterized in that the reactor is a reactor suitable for continuous operation and is connected to at least one measuring station with which the ratio of monovalent / divalent cations of the suspension in the reactor can be determined during operation. The carbon dioxide introduced can either be “fresh” carbon dioxide or come from secondary sources. Secondary sources are, for example, power plant waste gases, such as waste gases from combined heat and power plants, which are very often used in sewage treatment plants to burn the methane produced during sewage sludge digestion. The exhaust gases consisting mainly of carbon dioxide must be compressed for use in the process and, if necessary, cleaned and dried. State of the art units can be used for this. According to the invention, the process plant is particularly characterized in that the reactor, which is suitable for continuous operation, at least one measuring station being arranged on the reactor, with which the ratio of monovalent / di-valent cations of the suspension in the reactor is in operation can be determined.

As already mentioned at the beginning, a continuously operated reactor of the type claimed here is associated with the advantages which have already been described in detail above.

In order to ensure that the ratio of monovalent / divalent cations in the suspension which is optimal for the dewatering of the sewage sludge has already been reached (see above), it is proposed according to the invention that at least one measuring station is arranged on the reactor, with which in operation one or more measuring points the ratio of monova lente / divalent cations of the suspension in the reactor can be determined. At least one measuring point is expediently located at the distal end of the reactor or at a distal end of a reactor section.

The measuring station is used to check whether in the suspension at the distal end of the reactor or at a distal end of a reactor section the reduction in the ratio of monovalent / divalent cations (at least 10%) was achieved by the method according to the invention. If this is the case, no changes need to be made to the procedure. If the target value has not yet been reached, either the procedure must be changed or the response time must be extended.

To determine the ratio of monovalent / divalent cations, the measuring station includes a device for separating the solids from the sewage sludge suspension (e.g. filter, centrifuge) as well as analytical devices for determining the concentration of the cations relevant for the ratio of monovalent / divalent cations in the after separating the solids the liquid remains (eg AAS = atomic absorption spectroscope).

If the reaction time has to be extended, either the flow rate through the reactor can be slowed down or the length of the distance that the carbon dioxide was added to Suspension must be replaced must be extended, and for the latter two different specific embodiments of the invention are proposed.

The first special embodiment provides that the suspension, the ratio of monovalent / divalent cations of which has not yet reached the desired threshold value, is at least partly returned to the proximal section of the reactor in order to run through the reactor again at least in part. For this purpose, a return circuit is provided in the corresponding embodiments of the plant according to the invention on the reactor, through which at least part of the suspension is returned from the distal end of the reactor to the section at the proximal end of the reactor. The recycle circuit preferably has a cross section which corresponds to at least half, more preferably at least two thirds, even more preferably at least 90% of the cross section of the reactor. A return pump is particularly preferably arranged on the return circuit, with which the suspension to be returned is pumped through the return circuit.

There are various options for returning the suspension treated with carbon dioxide to the proximal end of the reactor. In certain embodiments, the suspension treated with carbon dioxide is returned to the area of the tubular reactor which is immediately after the inlets for the introduction of carbon dioxide. In other embodiments, the suspension treated with carbon dioxide is returned to the area of the reactor which is immediately in front of the inlets for the introduction of carbon dioxide. The decision as to which of the two recirculation options mentioned depends in particular on how far away the ratio of monovalent / divalent cations of the suspension treated with carbon dioxide deviates from the target and to what extent this requires the introduction of additional carbon dioxide.

In a further special embodiment, the reactor has at least two merging sections in the longitudinal direction, the suspension, the ratio of which monovalent / di-valent cations at the end of the first section has not yet reached the desired threshold value, in the subsequent second section of the reactor is forwarded to pass through this second section of the reactor. If, on the other hand, the desired threshold value has been reached at the end of the first section, the suspension can be passed on for further processing according to stages c) and d) to the downstream system parts in which these process stages are carried out. For this purpose, an outlet is provided at the end of the first reactor section through which the suspension treated with carbon dioxide can be passed into a bypass circuit. In certain variants of these embodiments, a third reactor section is arranged at the proximal end of the second reactor section and then optionally a fourth, fifth or nth reactor section. If the desired threshold value is reached at the end of one of these additional sections, the suspension can also be passed on via a bypass circuit for further processing according to stages c) and d) to the downstream system parts in which these process stages are carried out. If this is not the case, the suspension goes into the following section. In order to be able to increase the proportion of carbon dioxide again, further feed lines for carbon dioxide are provided in certain embodiments at the proximal end of the second, third, fourth, fifth or nth reactor section.

In certain embodiments, the principle of the recycle circuit is combined with the principle of the merging reactor sections, each of which has outlets at their ends, via which the suspension treated with carbon dioxide can be passed into a bypass circuit.

In certain embodiments, for purposes of better homogenization of the carbon dioxide treated suspension, i.e. in particular for better distribution of the carbon dioxide in the suspension treated with carbon dioxide, mixing devices. Examples of mixing devices suitable for this purpose are known to the person skilled in the art. In certain embodiments, the mixing devices provided are paddles and / or one or more conveyor screws and / or static or dynamic mixers.

In certain embodiments, the system for better distribution of the carbon dioxide in the suspension treated with carbon dioxide has devices for at least partial exposure to the reactor with ultrasound. Ultrasound is preferably applied in particular in the region in which the carbon dioxide is passed into the reactor.

Depending on the embodiment, the plant according to the invention can also have one or more of the following components: a device for disintegrating the sewage sludge,

a filtration unit for the separation of sewage sludge particles from the centrate obtained by centrifugation. Since the process according to the invention is in any case carried out under elevated pressure conditions in the process stage in which the carbon dioxide is allowed to act on the sewage sludge suspension, the reactor of the process plant claimed here must be suitable at least for operation under a pressure in the above-mentioned ranges.

For introducing carbon dioxide into the reactor, in some embodiments of the invention, either devices for introducing gaseous carbon dioxide or for introducing liquid carbon dioxide are arranged on the reactor. In order to also be able to carry out the embodiments of the claimed method according to the invention, in which both gaseous and liquid carbon dioxide are introduced into the sewage sludge suspension, certain embodiments of the plant according to the invention have a device which is used both for introducing gaseous carbon dioxide and for introducing liquid carbon dioxide is suitable in the reactor.

In certain embodiments, the reactor is characterized in that it has separate feed lines for introducing the gaseous carbon dioxide on one side and for introducing the liquid carbon dioxide on the other side. For this purpose, at least one first feed line for introducing the gaseous carbon dioxide and at least one second feed line for introducing the liquid carbon dioxide are provided. Various embodiments for the number and arrangement of the feed lines have already been described above in connection with the method according to the invention.

It has also already been described in detail above that, in certain embodiments of the process plant used for this purpose, an expansion tank separate from the reactor space is provided for carrying out the process according to the invention by lowering the pressure prevailing in the reactor interior. In certain embodiments of the invention, devices for foam destruction are provided in this expansion tank, as have already been described in detail above.

In certain embodiments of the invention, at least one sieve, a filter and / or a centrifuge is provided for separating the undissolved solids from the treated suspension after the carbon dioxide treatment. For the optional separation of the phosphates precipitated from the filtered liquid, a settling funnel, a sieve, a filter, a centrifuge or a hydrocyclone or combinations thereof is provided in certain embodiments in the plant according to the invention.

Further advantages, features and possible uses of the present invention will become apparent from the following description of certain specific embodiments. For example, FIG. 1 shows an R&I flow diagram of the reactor area of a process plant which is suitable for dewatering a sewage sludge suspension by the process according to the invention.

A tubular reactor 9 is shown schematically here, at the distal end 1 of which the sewage sludge suspension to be dewatered is introduced. This sewage sludge suspension is transported by a screw pump 2 through the pipeline 3 and mixed with carbon dioxide there. For this purpose, injectors 6, 7 are provided, via which either gaseous carbon dioxide 4 or liquid carbon dioxide 5 can be fed into the pipeline 3. From the point at which the first injector for carbon dioxide (here injector 6 for gaseous carbon dioxide 4) is arranged in the flow direction of the suspension to be extracted, the actual tubular reactor 9 begins, in the further course of which a static one in the embodiment shown here Mixer 8 is arranged.

The tubular reactor shown here is a so-called loop reactor, which has several turns by 90 ° 10, 1 1 and in particular a whole series of turns by 180 ° 12, 13, 14, 15. In the embodiment shown here, a total of 16 turns by 180 ° are realized, which are particularly important for the purposes of mixing the suspension treated with carbon dioxide. The loops of the reactor shown here are divided in the longitudinal direction into three merging sections, each of which makes up about a third of the total length of the loops of the reactor. In the embodiment shown here, an ion measuring point 16, 18, 20 is arranged at the respective distal end of the respective section, via which the ratio of monovalent / divalent cations in the suspension treated with carbon dioxide, which has passed the previous section of the loop reactor, is determined can be. If the suspension treated with carbon dioxide already has the desired ratio of monovalent / divalent cations at the said point, the shut-off devices 29 and 30 can be opened so that the closed suspension devices 17 and 19, respectively, the suspension treated with carbon dioxide via the bypass circuit 28 , ie bypassing the other sections of the tubular reactor in the form of the suspension sufficiently treated with carbon dioxide from the reactor part 31 of the plant.

Moreover, in the embodiment shown here, there is the possibility of returning the suspension treated with carbon dioxide after the tube reactor 9 has passed through the return circuit 22 from the distal end of the tube reactor to the proximal end of the tube reactor. This happens when a ratio of monovalent / divalent cations is measured at the measuring station 20, which does not correspond to the target. The shut-off device 32 is then closed while at the same time the shut-off device 21 is opened so that the Pump 23 can pump the carbon dioxide-treated suspension back to the proximal end of the reactor.

There are two return options 24 and 26 for returning the suspension treated with carbon dioxide to the proximal end of the reactor. The first return option 24 leads the suspension treated with carbon dioxide with the shut-off device 25 open back into the area of the tubular reactor immediately after the two injectors 6 and 7 for the introduction of carbon dioxide. The second return option 26 returns the carbon dioxide-treated suspension with the shut-off device 25 closed and the shut-off device 27 open to the area of the reactor which is immediately in front of the injectors 6 and 7 for the carbon dioxide. The decision as to which of the two mentioned return options is selected depends in particular on how far away the ratio of monovalent / diva cations measured by the measuring station 20 deviates from the target and to what extent this requires the introduction of additional carbon dioxide.

If the measurement at the ion measuring points 20 indicates that the suspension treated with carbon dioxide has the desired ratio of monovalent / divalent cations, with the shut-off device 21 closed and with the shut-off device 32 open, the suspension which has been sufficiently treated with carbon dioxide and treated with carbon dioxide can be conducted via the discharge line 31 the further treatment steps. Immediately following the reactor region shown in FIG. 1, the pressure is first reduced in a flash tank and then the undissolved solids are then removed, the corresponding devices not being shown in the flow diagram shown in FIG. 1.

EXAMPLES

Digested sludge samples from the sewage treatment plant in Mainz (KA Mainz) and the sewage treatment plant in Itzehoe (KA Itzehoe) were examined. For this both samples of the conventional, i.e. not examined with carbon dioxide-treated municipal digested sludge as well as samples of the same digested sludge treated with carbon dioxide according to the method according to the invention.

To determine the ratio of monovalent / divalent cations, the concentrations of the monovalent cations ammonium, sodium and potassium and that of the divalent cations calcium and magnesium were determined.

1. Carrying out tests at the sewage treatment plant in Mainz The ratio of monovalent / divalent cations in the liquid fraction of the digested sludge from KA Mainz (reference) remaining after the solids had been removed was 25.40. This value was reduced to 19.40 by the treatment process (see Table 1 below), which corresponds to a 23.6% decrease in the ratio of monovalent / divalent cations.

The average dry residue for sewage sludge not treated with CO2 is 25.2% TR. In the case of sewage sludge treated with CO2, the mean dry residue is 28.3% TR. The difference between the sewage sludge not treated with CO2 and the sewage sludge treated with CO2 is thus 3.1%, which corresponds to a relative increase in the dry residue by 12.3%.

2. Conducting experiments at the Itzehoe wastewater treatment plant

The ratio of monovalent / divalent cations in the liquid fraction of the digested sludge from KA Itzehoe (reference) remaining after the solids had been separated off was 50.54. This value was reduced to 19.90 by the treatment process (see Table 1 below), which corresponds to a decrease in the ratio of monovalent / divalent cations by 60.6%.

With sewage sludge not treated with CO2, the average dry residue is 17.0% TR. In the case of sewage sludge treated with CO2, the mean dry residue is 20.4% TR. The difference between the sewage sludge not treated with CO2 and the sewage sludge treated with CO2 is thus 3.4%, which corresponds to a relative increase in the dry residue of 19.5%.

Table 1: Results of the tests at the sewage treatment plants in Mainz and Itzehoe

Bezugszeichenliste:

Reference symbol list:

1 Introduction of the sewage sludge suspension to be dewatered

2 screw pump

 3 pipeline

 4 Introduction of carbon dioxide (gaseous)

 5 Introduction of carbon dioxide (liquid)

 6 injector

 7 injector

 8 Static mixer

 9 tubular reactor

 10 turn by 90 °

 11 90 ° turn

 12 turn by 180 °

 13 180 ° turn

 14 180 ° turn

 15 turn by 180 °

 16 ion measuring point

 17 shut-off device

 18 ion measuring point

 19 shut-off device

 20 ion measuring point

 21 shut-off device

 22 feedback loop

 23 return pump

 24 Feedback option 1

 25 shut-off device

 26 Feedback option 2

 27 shut-off device

 28 Bypass circuit

 29 shut-off device

 30 shut-off device

 31 Derivation of suspension treated with carbon dioxide

Claims

Patent claims 1. Process for the dewatering of sewage sludge, in which one (a) sewage sludge suspended in water to obtain an aqueous suspension of the sewage sludge, (b) introducing carbon dioxide into the suspension produced in step (a) in order to produce a suspension enriched with carbon dioxide, (c) applies increased pressure to the suspension enriched with carbon dioxide and (d) performs a dewatering step to separate water from the carbon dioxide-enriched suspension, characterized in that steps (b) and (c) are carried out in a continuously operated reactor until the ratio of monovalent / divalent cations is determined by the following Formula is expressed [c (Na ⁺ ) + c (K ⁺ ) + c (NH ₄ ⁺ )]: [c (Ca ²⁺ ) + c (Mg ²⁺ )] in the liquid fraction of the suspension enriched with carbon dioxide after separation of the solids in the Compared to the remaining liquid fraction after separation of the solids of the suspension obtained in stage (a) has decreased by at least 10%. 2. The method according to claim 1, characterized in that the suspension enriched with carbon dioxide, if its ratio of monovalent / divalent cations in the liquid fraction remaining after separation of the solids at the distal end of the reactor has decreased less than 10%, at least partly in the proximal Section of the reactor is recycled to run through the reactor at least partially again. 3. The method according to any one of claims 1 and 2, characterized in that the suspension enriched with carbon dioxide, if its ratio of monovalent / divalent cations in the liquid portion remaining after separation of the solids in the distal End of the reactor has decreased less than 10%, is forwarded to a subsequent two th section of the reactor in order to also pass through this second section of the reactor. 4. The method according to any one of the preceding claims, characterized in that either gaseous or liquid carbon dioxide is introduced in step a) or both gaseous and liquid carbon dioxide is introduced. 5. The method according to any one of the preceding claims, characterized in that gaseous and liquid carbon dioxide in a molar ratio 90:10 to 10:90 to each other are introduced in step a). 6. The method according to any one of the preceding claims, characterized in that stage c) is carried out under a pressure in the range from 2 to 100 bar, preferably in the range from 3 to 50 bar, more preferably in the range from 5 to 16 bar. 7. Process plant for dewatering a sewage sludge suspension by treatment of the sewage sludge with carbon dioxide, the plant comprising a reactor for treating the sewage sludge suspension with carbon dioxide, a device for introducing carbon dioxide into the reactor, a device for setting an increased pressure in the reactor and one Device for separating solids from the extracted, characterized in that the reactor is a reactor suitable for continuous operation and is connected to at least one measuring station with which the ratio of monovalent / divalent cations of the suspension present in the reactor is in operation can be determined. 8. Plant according to claim 7, characterized in that an ion measuring point is provided at the distal end of the reactor, at which suspension can be removed from the reactor and passed to the measuring station, the plant additionally having a recycle circuit the at least part of the suspension can be returned from the distal end of the reactor to the proximal end of the reactor. 9. Plant according to one of claims 7 and 8, characterized in that the reactor has in the longitudinal direction at least two merging sections, at the respective distal end of which an ion measuring point is provided, taken from the suspension at the suspension and to the measuring station can be directed. 10. Plant according to claim 9, characterized in that further supply lines for carbon dioxide are provided at the proximal end of the second, third, fourth, fifth or nth reactor section. 1 1. Plant according to one of the preceding claims, characterized in that the device for introducing carbon dioxide into the reactor is either suitable for introducing gaseous carbon dioxide or for introducing liquid carbon dioxide or both for introducing gaseous carbon dioxide and for introducing liquid Carbon dioxide is suitable. 12. Plant according to one of the preceding claims, characterized in that the ratio of the length of the reactor interior to the diameter of the reactor interior is in the range from 50: 1 to 10,000: 1. 13. Plant according to one of the preceding claims, characterized in that the reactor is in the form of a tube with 5 to 30 turns in the range from 90 ° to 180 °. 14. Plant according to one of the preceding claims, characterized in that it a) a flash tank downstream of the reactor space,  b) a sieve, a filter and / or a centrifuge for separating the undissolved solids present after the carbon dioxide treatment from the suspension and / or  c) a settling funnel, a sieve, a filter, a centrifuge or a hydrocyclone in the precipitation reactor  having. 15. The method according to any one of claims 1 to 6, characterized in that it is carried out with a system according to one of claims 7 to 14.