Optimisation of a pulp treatment method

Abstract

Disclosed is a method for treating pulp, particularly a method for treating sludge from wastewater treatment plants, for producing energy and/or organic materials that have undergone hygienization, including at least the following steps: a step of aerated or non-aerated thermal hydrolysis of the pulp, a digestion step, a dehydration step and a step of recirculating part of the dehydrated pulp into the step of thermal hydrolysis.

Claims

1. A method for treating pulp, in particular a method for treating purification plant sludge, for producing energy and/or organic matter that has been rendered hygienic, comprising i. a step of aerated or non-aerated thermal hydrolysis of the pulp, ii. optionally a step of dilution of the pulp hydrolyzed in step i), iii. a digestion step of mesophilic or hermophilic type either of the pulp hydrolyzed in step i), or of the pulp hydrolyzed and diluted in step ii), iv. a step of dehydration of the pulp resulting from the step iii) and v. a step of recirculation of part of the dehydrated pulp resulting from step iv) to the thermal hydrolysis step i), wherein the part of the dehydrated pulp resulting from step iv) which is recirculated to the thermal hydrolysis step i) is controlled by the degree of loading in order to keep it as constant as possible and close to 1 and wherein the rest of the dehydrated pulp resulting from step iv) is evacuated.

2. The method as claimed in claim 1, further comprising a step of calculation of the degree of loading of the thermal hydrolysis i) or of the digestion step ii) or of both, wherein: the rate of loading of the thermal hydrolysis is calculated from the measurements of the flow rate (20) and of the concentration (27) at the inlet of the thermal hydrolysis and of the residence time in the thermal hydrolysis, to which it is added the measurements of flow rate (24) concentration (28) at the return of the dehydration (15), the rate of loading of the digestion step is calculated from the measurements of flow rate (21) entering into the digester, the nominal residence time and the concentration of the sludge at the inlet (25) of the digester.

3. The method as claimed in claim 1, further comprising a step of injection of base or of acid before the thermal hydrolysis step i) in order to control the pH during the digestion step ii) and to increase the hydrolysis during step i).

4. The method as claimed in claim 3, wherein several hydrolysis reactors are operating in parallel and for which the injection of base or of acid is carried out exclusively in one of the reactors in order to greatly increase the hydrolysis in this reactor.

5. The method as claimed in claim 4, wherein the hydrolysis reactor receiving acid or base is the one receiving the effluents of step v) and wherein: in case of an increase in free NH.sub.3, without an increase in pH or in acidification in the digester as detected by the pH and ammonium analyzer (23), the addition of base in the hydrolysis reactor is performed to help the hydrolysis of the organic matter, and in the case of an increase in pH as detected by the pH and ammonium analyzer (23), the addition of acid in the hydrolysis reactor is performed to help to hydrolyze the organic matter.

6. A facility for treating pulp, in particular purification plant sludge, for carrying out a method as claimed in claim 1, said facility comprising a means of aerated or non-aerated thermal hydrolysis (13) communicating with a means of digestion (14) of the hydrolyzed pulp, said hydrolysis means communicating with means for conveying the pulp (11) resulting from the pre-dehydration means (12) and means for conveying said pulp recycled from a means of dehydration (15) of the pulp resulting from the digestion means (14) and means for evacuating biogas (16) originating from said digestion means (14) and a system for controlling (30, 31) the rate of loading of the thermal hydrolysis means (13) or of the digestion means (14) or of both, by controlling the amount of return originating from the dehydration (15) to the thermal hydrolysis (13), wherein the rate of loading of the thermal hydrolysis means is compensated by the sensor (13b) and the rate of the digestion means is compensated by the sensor (14b).

7. The facility as claimed in claim 6, further comprising a means of regulating the pH (41) in the hydrolysis reactor by injecting an acid or a base so as to improve the hydrolysis kinetics.

Description

EMBODIMENT ACCORDING TO FIG. 1

(1) The sludge (11) is pre-dehydrated in the element (12) in order to obtain sufficient dryness to minimize the size of the thermal hydrolysis reactor (13). This step is carried out by bringing the sludge to temperature and placing it under pressure. In accordance with the invention, it is possible to hydrolyze only part of the sludge. The type of hydrolysis technology is not taken into account here.

(2) This reactor (or these reactors if need be) make(s) is possible: to guarantee a residence time of the organic matter under the temperature conditions and with the desired amount of oxygen, to introduce organic matter optionally under pressure, to introduce steam and to maintain appropriate pressure and temperature conditions, that is to say conditions which are sufficient to make part of the organic matter rapidly digestible. The hydrolyzed sludge at the outlet (13) is injected into the digester (14) after optional cooling (not represented) and after dilution by means of a system (43) of which the role will be described hereinafter. The sludge is then digested in the mesophilic or thermophilic digester (14), the residence time of which is adjusted to the nominal loading of the facility. This digester is proportioned with a residence time of between 10 and 25 days, preferentially 20 days in mesophilic operation, and 12 days in thermophilic operation. The digester has its own backup heating circuit (not represented on the diagram). The temperature in the digester is between 35 and 45 C., advantageously equal to 37 C., for mesophiles and between 50 and 60 C., advantageously equal to 55 C., for thermophiles. Finally, at the outlet of the digester (14), a dehydration station (15) makes it possible to make the sludge available to a treatment or a final evacuation.

(3) The tools for pretreating the sludge in order to remove the tows and the sand which can disrupt the thermal hydrolysis are also not represented.

(4) A comminuting device and a desanding device are set up if required, depending on the quality of the sludge, in order to protect the downstream pieces of equipment.

(5) Finally, the thermal fluid circuits which make it possible to heat the thermal hydrolysis and the digester are not represented.

(6) The invention comes from the setting up of a loop at the outlet of the dehydration (15) in order to pass the sludge again into the thermal hydrolysis (13) when the thermal hydrolysis/digester (14) couple is available.

(7) This is because, in the case of the maximal loading, all of the flow passes into the thermal hydrolysis (13) and also into the digester (14) with residence times in these two pieces of equipment which are limiting with respect to the proportioning of the pieces of equipment. There is thus no space to return the sludge to the inlet of the thermal hydrolysis (13).

(8) On the other hand, in the case of a lower loading at the inlet (11), sludge can be returned to the thermal hydrolysis (13) and also to the digester (14).

(9) The fact of dehydrating at the outlet of digestion makes it possible, in addition, for the ammonia to leave with the effluents. If sludge that is less loaded with nitrogen and thus less capable of releasing large amounts of ammonia into the digester is returned to the top of the digester (14), and the difference is made up with process water not containing nitrogen, the concentration of free ammonia in the digester is decreased and the operation of the latter is relieved.

(10) The return to thermal hydrolysis (13) is controlled by several sensors (13b and 14b).

(11) A flow rate measurement (20) and a concentration measurement (27) at the inlet of the thermal hydrolysis measures the degree of loading averaged on the basis of a characteristic time (typically from 0.5 to 5 residence times in the thermal hydrolysis) in order to calculate a degree of loading of the thermal hydrolysis. Added to this value are the measurement of flow rate (24) and a measurement of concentration (28) at the return of the dehydration (15). This degree of loading measurement can also be done by means of a number of fillings of the reactor (batches) or any other system which makes it possible to calculate the volume passing through the system, on the basis of the characteristic time.

(12) This degree of loading is compensated by a sensor (13b) at the level of the thermal hydrolysis which determines the availability of the machine, that is to say the number of lines in parallel actually operating (for example a line undergoing maintenance).

(13) The combination of the these two pieces of information determines an actual degree of loading (30) of the machine of between 0 and 1.

(14) A measurement of flow rate (21) entering the digester measures the hydraulic residence time of the sludge in the digester.

(15) This residence time is compensated by a piece of information originating from the digesters which determines an availability of the digesters.

(16) This compensated residence time, divided by a nominal residence time defined by the method (typically from 10 to 25 days) determines a degree of loading (31) of the digester of between 0 and 1.

(17) A measurement of concentration of the sludge at the inlet (25) completes the calculation of the degree of loading (31) by comparing the inlet loading relative to the acceptable weight loading of the digester.

(18) A modifiable parameter of amount of organic matter relative to solids can also be entered into the system in order to calculate the loading on the basis of the organic matter.

(19) Finally, the measurement of weight loading of the digester can also be carried out upstream of the thermal hydrolysis by means of a measurement of concentration at (27) and of flow rate at (20) before the thermal hydrolysis (13) completed by the measurement of the concentration (28) and of the flow rate (24) on the return loop. This is because the thermal hydrolysis has a tendency to dissolve organic matter which is therefore no longer accessible to the concentration measurement.

(20) A final degree of loading 31 is thus calculated.

(21) The maximum value which corresponds to the degree of loading of the facility is taken from the two degrees of loading 30 and 31.

(22) This piece of information controls the return of the sludge (15) so that the calculated degree of loading is always equal to 1. This flow rate is controlled by a flow rate measurement (24).

(23) A pH and ammonium analyzer (23) is located at the outlet of the digester. This sensor determines a concentration of free ammonia in the digester and a toxicity factor on the basis of this concentration.

(24) This analyzer will trigger an injection of base at (41) and also of dilution water at (43) by means of a slow PID (proportional integral derivative) loop in order to have optimum conditions in the digester.

(25) A measurement of concentration of the sludge (25) at the inlet of the digester and a measurement of concentration of the sludge (26) at the outlet of the digester make it possible to directly measure the degree of removal of the organic matter and thus the operating quality of the digester, and thus to correct in the long term the return of the digester.

(26) The measurement of concentration (25) controls the dilution (43) so as to not to have too high a concentration in the digester, which would pose problems with stirring and thus with having a uniformly mixed reactor, a condition which is important for having good yields.

(27) In another variant, the thermal hydrolysis is placed only on the dehydration returns (15) and is thus used only in the case of under-loading of the digester (14). This can be particularly advantageous in the case of sludges said to be easy to digest, for which the provision of a thermal hydrolysis is not useful. Indeed, in this case, the digester is always used at the maximum of its capacities, even in the case of a decrease in loading, and fluctuations in loading in the digester are thus avoided, without having to invest in a pre-dehydration (12) and while constructing only a small thermal hydrolysis system (13) which will treat only a fraction of the sludge.

(28) In a variant wherein in particular there are several thermal hydrolysis lines, in the event of under-loading, one of the lines is dedicated to the treatment of the return dehydrated sludges; this line is the one which receives the base or the acid.

(29) In this line, in the event of an increase in free NH.sub.3, without an increase in pH or in acidification in the digester, the pH which is particularly high in the hydrolysis reactor through addition of base will help to hydrolyze the organic matter, and to easily capture the return ammonia in the sludge of the hydrolysis reactor (foul gas 42).

(30) In the case of an increase in pH at (23), the addition of acid in the reactor will also make it possible to help to hydrolyze the organic matter.

(31) In the system according to the invention, the final dehydration (15) is not oversized relative to a standard operation because in any event the dehydration must be capable of treating the hydraulic peak. In other words, the returns are made only if the configuration is not that of the hydraulic peak, and thus there is never any overloading of the dehydration.

(32) Example of Calculation of Optimization of a Thermal Hydrolysis Reactor and of a Digester

(33) In the case of a sludge at 100 tSolids/d 75% MV at peak and 80 tSolids/d at peak over a course of 15 days, if a thermal hydrolysis of 1 h at a concentration of 16% and a digestion of 15 days at an input concentration of 9% are envisioned, then a reactor of: 100/0.16/241=26 m.sup.3 is obtained and a digester of 80/0.0915=13 333 m.sup.3 is obtained.

(34) The thermal hydrolysis reactor is proportioned on the basis of a daily or twice-daily peak as a function of the sludge buffer tank located upstream, while the maximum loading of the digester is that corresponding to a peak of 15-20 continuous days.

(35) It is thus possible to proportion the reactor on the basis of a peak for example of 5 continuous days, for example 85 tSolids/d. This thus decreases the reactor by 15% at constant hydraulic flow rate.

(36) Next, for passing the peak of 100 tSolids/d, the residence time in the reactor will be decreased: 51 minutes instead of 60 minutes.

(37) However, the sludge that would have been hydrolyzed only for 51 minutes, over the course of the 15 days of the residence time in the digester, may rethicken in the thermal hydrolysis since the loading will subsequently still be less than 100 tSolids/d.

(38) Thus, if proportioning is done on the basis of 51 minutes in the reactor for this entire peak, during the 4 peak days, 81.25 tSolids/d is obtained (to produce 85 t over the course of 5 days) and over the course of the next 10 days in the reactor 77.5 tSolids/d are obtained (or 19.4 t wet matter/d) (to produce 80 tSolids/d over the course of 15 days).

(39) It is thus possible to treat, for 4 days, 18.75 tSolids/d at the return, which corresponds to approximately (MV outlet 60%) 7.5 t wet matter/d;

(40) and for 10 days 22.5 tSolids/d, which corresponds to approximately 9 t wet matter/d.

(41) This thus gives, over the course of the 15 days, 75 t wet matter recirculated for 300 t wet matter injected from the outside, i.e. 25% recirculation, which means that, over this period of 15 peak days, by virtue of the recirculation and despite a decreased hydrolysis time of 51 min, the sludge will have been subjected to an actual hydrolysis time of 511.25=64 min.

(42) An actual hydrolysis of the sludge is thus obtained which is at its nominal value despite a shorter physical time.

(43) The recirculation thus makes it possible to use each of the two pieces of equipment at their nominal values for as long as possible by using the uncoupling of the residence times between the hydrolysis and the digestion.