Method and Apparatus for Disintegrating Organic Substrates

Abstract

In a method for disintegrating organic substrates, an alkaline solution is added as pH-altering solution to the substrate and said substrate is then treated with steam, or steam is added, for heating to a temperature below 100 C. Under pressureless conditions, the heated substrate admixed with alkaline solution is subjected to a residence time. Preferably, the organic substrates are sludges from wastewater treatment plants.

Claims

1. Method for disintegrating an organic substrate, wherein said substrate is treated with steam for heating said substrate to a temperature below 100 C., and wherein said substrate is subjected to rest for a residence time under pressureless conditions.

2. Method according to claim 1, wherein a pH-altering solution is added to said substrate.

3. Method according to claim 1, wherein said organic substrate is sludge from wastewater treatment plants or is a substrate which is introduced into a biogas plant.

4. Method according to claim 3, wherein said sludge has a solids content between 2% and 20%.

5. Method according to claim 2, wherein said pH-altering solution is alkaline.

6. Method according to claim 5, wherein said pH-altering solution is sodium hydroxide solution.

7. Method according to claim 5, wherein 1 litre to 5 litres of sodium hydroxide solution per m.sup.3 of substrate are added.

8. Method according to claim 1, wherein said steam is saturated steam.

9. Method according to claim 8, wherein said steam is low-pressure saturated steam.

10. Method according to claim 1, wherein said substrate is heated to a temperature between 40 C. and 95 C.

11. Method according to claim 10, wherein said substrate is heated to a temperature between 70 C. and 75 C.

12. Method according to claim 1, wherein said residence time is between 0.5 h and 3 h.

13. Method according to claim 1, wherein a mixing of said substrate is provided during said residence time.

14. Method according to claim 1, wherein said method is carried out in batch mode.

15. Method according to claim 1, wherein said substrate is situated in at least one residence vessel during said residence time.

16. Method according to claim 15, wherein said substrate is situated in two or more of said residence vessels during said residence time, wherein said residence vessels are alternately charged with said substrate.

17. Method according to claim 1, wherein a preheating of said substrate by a return flow of an already heated substrate volume is provided before a treatment of said substrate with said steam.

18. Method according to claim 1, wherein a reheating of said substrate is provided during said residence time.

19. Method according to claim 1, wherein said substrate is subjected to a digestion after said residence time.

20. Method according to claim 1, wherein an at least partial cooling of said substrate is provided after said residence time.

21. Apparatus for carrying out a method according to claim 1, comprising at least one unit for treating said substrate with said steam and comprising at least one residence vessel intended for said steam-treated substrate to rest in said vessel for said residence time.

22. Apparatus according to claim 21, comprising at least one metering unit for metering a pH-altering solution that has been added to said substrate.

23. Apparatus according to claim 21, wherein said residence vessel is designed for a residence time of said substrate admixed with pH-altering solution and treated with said steam.

24. Apparatus according to claim 21, wherein at least one unit for admixing said steam opening into an inflow of said residence vessel or into said at least one residence vessel is provided for said treatment of said substrate with said steam.

25. Apparatus according to claim 21, wherein at least one pump for a recirculation of said substrate from said residence vessel to said opening point of said unit for admixing steam is provided.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Further features and advantages of the invention are revealed by the following description of exemplary embodiments in conjunction with the drawings. Here, the individual features can in each case be realized separately or in combination with one another. In the drawings:

[0034] FIG. 1 shows a block diagram to illustrate a first embodiment of the proposed method;

[0035] FIG. 2 shows a block diagram to illustrate a further embodiment of the proposed method;

[0036] FIG. 3 shows a block diagram to illustrate a further embodiment of the proposed method;

[0037] FIG. 4 shows a schematic representation of a plant for carrying out a first embodiment of the proposed method; and

[0038] FIG. 5 shows a schematic representation of a plant for carrying out a further embodiment of the proposed method.

DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

[0039] FIG. 1 shows a block diagram to illustrate an exemplary configuration of the proposed method. The method is used for the disintegration of thickened excess sludge 1 in this example, and the excess sludge 1 can, for example, be thickened to a solids content of 4% to 8% or up to 16% in order to save an appropriate volume in the subsequent digestion 2. The thickened excess sludge 1 is admixed here with an alkaline solution 3 as pH-altering solution, preferably 50% sodium hydroxide solution. Depending on the proportion of organic material in the excess sludge, a quantity of 1 to 5 litres per m.sup.3 of sludge, usually approx. 1.5 litres per m.sup.3, is necessary for this purpose. This depends on the desired degree of disruption, and is appropriately explicitly ascertained for the particular sludge. What is customary is a degree of disruption of approx. 40-50% based on the chemical oxygen demand. Although the addition of the alkaline solution or a pH-altering solution in general is advantageous, it is not imperative. Even in the absence of a pH-altering solution, solely the increase in temperature brings about a disintegration, albeit admittedly only to a lesser extent.

[0040] Subsequently, the sludge is heated with saturated steam 4 to a temperature of about 65 C. in a virtually pressureless manner. Preference is given to using saturated steam because it is better than superheated steam at condensing in the substrate or sludge and thereby releasing its heat. If, for example, there is an excess sludge thickened to 15% solids and having a volume flow of 10 m.sup.3/h at a temperature of 15 C., this requires a quantity of steam of approx. 0.9 to. The solids concentration is reduced to approx. 13.8% by the introduced quantity of water. Subsequently, the heated sludge 5 admixed with sodium hydroxide solution is subject to a residence time of 1.5 to 2 hours in a residence vessel 6. As an alternative or in addition to an introduction of the steam 4 upstream of the residence vessel 6, i.e. in the inflow, what can also be provided is that steam 14 is introduced directly into the residence vessel 6. The residence vessel 6 is preferably designed such that low mixing, but not short-circuit flows, can occur. In this way, what is achieved is that all the particles of the sludge 5 pass through the residence time effectively. To achieve this objective, the residence vessel 6 can, for example, be provided with flow-guiding fixtures. After the residence time within the residence vessel 6, the disintegrated, warm excess sludge 7 is mixed with cold primary sludge 8 or some other nontreated substrate. Optionally, what can be provided beforehand is a further cooling operation 9, which is, however, generally not absolutely necessary. The mixture 20 of cold primary sludge and disintegrated excess sludge usually has a temperature below or at the temperature of the digestion space, meaning that the entire thermal energy which is fed into the system via the steam 3 can be used in the digestion 2.

[0041] FIG. 2 illustrates a further possibility for carrying out the proposed method, the method shown being comparable with the method shown in FIG. 1 in large parts and being provided with the same reference signs. In contrast to the method as per FIG. 1, what is provided for the method as per FIG. 2 are two (or more) residence vessels 6, which are connected successively as a cascade. As a result, short-circuit flows can be minimized effectively. Especially in the case of high or very high solids concentrations, it may additionally be advantageous to provide the residence vessels 6 with stirrers 13 in order to prevent deposition.

[0042] To achieve or to be able to demonstrate compliance with legal requirements concerning a hygienization (Regulation (EC) No 1069/2009 of the European Parliament and of the Council of 21 Oct. 2009; EPA/625/R-92/013, December 1992), a batch process with a minimum temperature over the entire holding time (residence time) in a closed area is necessary. The legal requirements stipulate that a temperature of at least 70 C. must be observed over the residence time for this purpose, for at least 60 minutes according to (EC) No 1069/2009 and for least 30 minutes according to EPA/625/R-92/013. Both requirements can be readily integrated in the proposed method, especially in a configuration of the method in which primary sludge 8 is added as well to the disintegration. The conditions used for the disintegration of the organic substrate according to the proposed method simultaneously fulfil, then, the conditions for a hygienization, in which any pathogens present in the substrate are inactivated. Such a configuration of the method is illustrated in FIG. 3. In said configuration of the method, all the substrates ultimately reaching the digestion 2 are disintegrated according to the proposed method and hygienized at the same time, and so legal requirements that may exist concerning hygienization for the substrates are met. In said configuration, both the primary sludge 8 and the excess sludge 1 are treated together. Similarly as described above, the substrate mixture composed of excess sludge 1, primary sludge 8 and optionally supplementary substrates 10 is admixed with alkaline solution as pH-altering solution (e.g. 50% sodium hydroxide solution) as alkaline solution 3, i.e. approx. 1 to 5 litres per m.sup.3 of substrate. As an alternative to alkaline solution, an acid can also be added, as described above. A separate intermediate vessel 15 with stirrer 16 can be provided for the mixing and action of the sodium hydroxide solution 3. The substrate mixture 11 admixed with sodium hydroxide solution 3 is then heated with steam 4 such that the hygienization temperature is held for the required time of at least 60 or 30 minutes. Usually, the temperature is a little, for example approx. 2 K, above the hygienization temperature in order to compensate for the heat losses during the hygienization time (residence time), i.e. the substrates are heated to 72 C. If further supplementary substrates 10, which must likewise be hygienized according to the legal provisions, should be conveyed into the digestion 2, they should also be co-heated. The nature of the supplementary substrates determines whether an admixing with sodium hydroxide solution 3 appears appropriate. For example, if there are no proportions of cells in the supplementary substrates 10, a disruption may not be appropriate or necessary. If this is the case, the substrates 1 and 8 already admixed with sodium hydroxide solution 3 should remain in contact with the sodium hydroxide solution 3 for a certain time alone without the supplementary substrates. A residence time of 10 to 20 minutes is generally sufficient to this end. This can take place in the intermediate vessel 15, which is equipped here with a stirrer 16, but this is not imperative.

[0043] The supplementary substrates not to be admixed with sodium hydroxide solution are conducted past the intermediate vessel 15 with the line 17, and are then heated together with the already pretreated substrates 1 and 8. The now warm substrate mixture 5 admixed with sodium hydroxide solution 3 reaches the first residence vessel 61. Altogether, three residence vessels 61, 62, 63 of identical construction, each having a stirrer 13, are provided. The development of cold pockets is avoided by means of the stirrers 13. The three residence vessels 61, 62, 63 are advantageously alternately charged and each change the tasks, alternatively as a cascade as depicted more here. A piping system for an alternate charging is, however, easily imaginable. In the first cycle, residence vessel 61 is charged. On residence vessel 62, which has already been charged, all inflows and outflows have been closed. The substrate mixture 5 resides in the residence vessel 62 at least for the required residence time. The residence vessel 63 is emptied in said cycle. When the residence time required for the hygienization in the residence vessel 62 has at least lapsed, the tasks of the individual residence vessels change. Residence vessel 61 has now been filled, all inflows and outflows have been closed and the substrate mixture 5 remains at least for the residence time required for the hygienization. The hygienized substrate mixture 5 situated in the residence vessel 62 is discharged for further treatment. The residence vessel 63 is empty at the start of this cycle and is now filled within this cycle with heated substrate mixture 5 admixed with alkaline solution. When the required hygienization time has lapsed, said cycle is completed. In the next cycle, residence vessel 62 is filled, the residence vessel 63 resides in the closed state over the residence time and residence vessel 61 is emptied within this time. After the residence time required for the hygienization in the residence vessel 63 has lapsed, the 3-part cycle can be started over again.

[0044] If, after the filling of a residence vessel 61, 62 or 63, the temperature required for the hygienization and/or disintegration should not be reached or not be held, the respective residence vessel can be reheated via a circulation 12, shown here by way of example for residence vessel 61. It is also possible to heat the residence vessels 61, 62, 63 directly by means of a steam injection 14, shown with a dashed line, or other above-mentioned admixing of steam, it then being necessary for the residence vessels 61, 62, 63 to be equipped with such a means of heating.

[0045] In this method variant, after the hygienization and disintegration, the outflow temperature for the charging of the usually mesophilic digestion is too high at about over 70 C. Therefore, a cooling operation 9 is appropriate in this case. A cooling operation is generally not necessary if a thermophilic digestion at this temperature level is intended. A cooling operation can, for example, also be achieved by means of a substrate or by means of the entirety of the supplied substrates, which are thereby preheated to a certain extent.

[0046] It is likewise possible that the process for disintegration and hygienization by means of pH-altering solution, especially alkaline solution, and steam admixing or steam injection also takes place in one or alternatingly two residence vessels.

[0047] FIG. 4 shows schematically a plant for the pre-treatment of sludges according to the proposed method, the sludges being intended for a digestion. The thickened excess sludge is fed into the system by means of the pump 101 and admixed in the region of a metering point 103 with sodium hydroxide solution as alkaline pH-altering solution 102, there being provided for this purpose a storage vessel 104 for the sodium hydroxide solution 102 and a metering unit 105, for example a metering pump or a metering valve. Treated feed water from a water tank 106 is converted into low-pressure saturated steam in a steam generator 107 and injected at the steam injection point 108 into the sludge admixed with sodium hydroxide solution. The sludge is conducted further into the residence vessel 109, the incubation for the disintegration of the organic sludge taking place in the residence vessel 109. After the intended residence time has lapsed, the sludge is removed from the residence vessel 109 by means of the pump 110. In a subsequent pipeline section, cold primary sludge is fed in by means of a further pump 111 and mixes with the treated excess sludge from the residence vessel 109, resulting in the sludge being cooled down to a temperature suitable for a subsequent digestion. By means of the pump 112, the sludge is pumped further to the digestion in the digestion vessel 113. The pump 112 is advantageously a circulation pump near the digestion for mixing it. The treated sludge can be concomitantly fed into this circulation, because specifically a mixing operation in the somewhat more turbulent flow and in the circulation pump is thus achieved. The digested sludge leaves the digestion vessel 113 via the outlet 114 and is conveyed to the next method steps, such as, for example, dewatering or phosphorus recovery. In this system, the entire sludge line remains pressureless. Only the steam region has a slightly positive pressure, so that the steam can be injected.

[0048] As an alternative or in addition to the steam generator 107, the steam generation can be realized by means of a waste-heat boiler of a combined heat and power plant. Furthermore, it is possible that the steam is introduced directly into the residence vessel 109 as an alternative or in addition to the introduction in the inflow.

[0049] FIG. 5 shows schematically a further plant for the pre-treatment of sludges according to the proposed method, the sludges being intended for a digestion. In large parts, said plant corresponds to the plant illustrated in FIG. 4. The corresponding elements are provided with the same reference signs. Both the excess sludge and primary sludge are subjected to a treatment according to the proposed method in said plant, in contrast to the plant from FIG. 4, with the result that the legal requirements concerning a hygienization can be realized for all substrates which are subjected to the subsequent digestion. The primary sludge is, then, mixed into the system downstream of the metering point 103 for the sodium hydroxide solution by means of the pump 111, with the result that the primary sludge together with the excess sludge is heated to the required temperature by means of the steam injection at the steam injection point 108. To also fulfil the legal provisions, the plant is operated in batch mode, with, in contrast to the plant from FIG. 4, use of three residence vessels 119, 129, 139 which are alternately charged. The volume of the residence vessels 119, 129, 139 is determined by the required length of the residence time, wherein the length of the residence time can depend on the required extent of disintegration of the particulate components of the sludge and/or on the legal hygienization requirements. Furthermore, a circulation pump 115 is provided, by means of which the sludge can be removed from the residence vessels 119, 129, 139 and fed once more into the region of the steam injection point 108. This measure makes a reheating of the sludge in the residence vessels 119, 129, 139 possible, for example in respect of the observance of the hygienization provisions. As an alternative or in addition, a steam injection in the residence vessels 119, 129, 139 can be provided, which is not shown here. After the residence time in the respective residence vessels 119, 129, 139 has lapsed, the treated sludge is removed from the residence vessels 119, 129, 139 by means of the pump 110 and conveyed further into the digestion vessel 113 by means of the pump 112. Since the digestion in the digestion vessel 113 is often operated as a mesophilic digestion at approx. 40 C., a cooling 117 of the treated sludge is necessary. What is used as cooling medium in this plant is the wastewater treatment plant outflow, which is conveyed via the pump 116.

[0050] A comparative calculation shows the differing energy expenditure between the system of a thermal/chemical treatment with steam heating as per the method according to the invention, as illustrated in FIG. 5, and a conventional thermal pressure treatment, on the basis of an example of 10 m.sup.3/h and a sludge concentration of 15% DM (DM is dry matter) in the input. A summarization of a comparative calculation comes to the following result, even with assumption of the complete recovery of energy via a flash evaporation in the case of the thermal pressure treatment:

TABLE-US-00001 Thermal/chemical Thermal Material treatment with pressure stream Unit steam treatment Comparison Quantity of kg/h 921 1677 756 kg/h steam Steam kW 630 1147 517 kW energy Quantity of 1/h 15 0 +15 1/h sodium hydroxide solution Additional m.sup.3/h 0 39.7 39.7 m.sup.3/h cooling water demand Additional kW 0 459 459 kW cooling energy

[0051] Overall, the proposed method is thus substantially more favourable compared to a thermal pressure treatment with respect to the required quantity of steam and the steam energy and with respect to the cooling water demand and the cooling energy. Furthermore, the proposed method can be realized as a pressureless system with low maintenance using simple components typical of a wastewater treatment plant and furthermore offers in general the advantages of a thermal/chemical treatment, especially a high increase in total gas production, a resultant, distinct reduction in the quantity of solids to be disposed of, a higher solids content in the dewatering, a saving of required polymers and a reduction in the required digestion time, with the processing of highly viscous substrates, i.e. even of sludges having a relatively high solids concentration, being possible at the same time as a result of the heating by means of steam treatment.