WO2024047030A1

WO2024047030A1 - Sludge processing

Info

Publication number: WO2024047030A1
Application number: PCT/EP2023/073650
Authority: WO
Inventors: Abdelghafour ZAABOUT; Galina SIMONSEN
Original assignee: Sintef Tto As
Priority date: 2022-08-30
Filing date: 2023-08-29
Publication date: 2024-03-07
Also published as: GB2622009A; GB202212541D0

Abstract

Disclosed herein is a sludge processing system comprising a reactor; wherein: the reactor comprises a molten salt/base; the reactor is arranged to receive sludge; the reactor is arranged to support reactions between at least some of the constituents of the received sludge and the molten salt/base so as to recover phosphorus based products and/or Nitrogen based products from the sludge; and the reactor is arranged to thermally decompose at least some of the constituents of the received sludge.

Description

SLUDGE PROCESSING

Field

[0001] The present disclosure relates to the processing of sludge. In addition to water recovery and fuel generation, embodiments may recover usable elements from the sludge. The generated fuels may include one or more of liquid, gas and solid fuels.

Background

[0002] Aquaculture sludge comprises solid waste filtered out of the water from fish farms. Aquaculture sludge may mainly comprise soluble metabolic products and solids originating from fish excrements and waste feed.

[0003] There is a general need to improve on known techniques for processing aquaculture sludge.

Summary of the invention

[0004] Aspects of the invention are set out in the appended independent claims. Optional aspects are set out in the dependent claims.

List of figures

[0005] The invention is described below, by way of example only, with reference to the drawings, in which:

[0006] Figure 1 schematically shows an implementation of a sludge processing system according to a first embodiment;

[0007] Figure 2 schematically shows an implementation of a sludge processing system according to a second embodiment;

[0008] Figure 3 comprises a table providing the composition of a saline aquaculture sludge sample; and

[0009] Figures 4A and 4B respectively show linear and logarithmic results that demonstrate the effect of varying the ratio of KOH to saline aquaculture sludge in a process according to an embodiment. [0010] The following description is merely exemplary in nature and is not intended to limit the scope of the present invention, which is defined in the claims. It should be understood that throughout the drawings, corresponding reference numerals indicate like or corresponding parts and features.

Description

[0011] Sludges from freshwater fish production are similar in composition to other livestock wastes and it is known for them to be directly used as agricultural fertilizer or to produce biofuels in biogas plants. However, a number of challenges arise when attempting to efficiently use saline aquaculture sludge (SAS) from a land-based recirculating aquaculture system (RAS). In particular, in a RAS, the water is mostly reused in order to reduce the environmental impact. This elevates the concentration of potentially toxic substances in the SAS.

[0012] There may be significant variations in the specific compositions of SASs but all SASs are typically rich in organic fractions (e.g. carbohydrates and fats) that are convertible to biofuels, potentially valuable elements (e.g. N, Ca, and P), polluting heavy metals (e.g. Zn, Cd, Ni, As, Pb, Hg), chemicals (e.g. flocculants, coagulants, thickening agents or even steroids), and pathogens. The amount of pathogens, or organic pollutants, is typically low and is not considered to be an environmental risk. However, the presence of heavy metals (in concentrations up to about 2 mg/kg), polyacrylamide and up-concentrated salts are a challenge due to potentially toxic effects on plants and living organisms.

[0013] In addition to detoxifying the sludge, it is also highly desirable to recover the valuable nutrients, including non-renewable P. Current microbial conversion methods allow for biofuel production. However, the high Na and Cl levels present in SAS are known to inhibit methane generation and this is a limiting factor for biogas production from SAS.

[0014] Embodiments provide a new technique for sludge processing. Embodiments are particularly advantageous in the processing of SAS. Advantages of embodiments may include improved reduction of the toxicity of SAS and improved recycling of the contents of the SAS. Embodiments may allow one or more of bio-fuel production, the recovery of elements, and the separation of heavy metal fractions from SAS. The SAS processing system according to embodiments may receive raw SAS and use it to generate bio-oil, char, minerals and fertiliser salts.

[0015] The SAS processing system according to embodiments comprises a pyrolysis reactor for preforming a pyrolysis process on the SAS. The pyrolysis reactor comprises a molten salt/base medium. The salt/base medium may be KOH, or other mixtures. Within the pyrolysis reactor, the SAS is thermally decomposed and/or chemically converted to other products.

[0016] The molten salt/base medium may provide an excellent environment for fast heat transfer to the SAS for the pyrolysis process. This may facilitate the rapid thermal decomposition of the SAS to produce bio products (such as bio-oil, bio-gas & char), and also recovers the metal and other elemental components (e.g., phosphorus) in a number of different forms. The use of hydroxide salts/bases stimulates the collection of different non-organic materials from the SAS, either attached to subordinates or in the form of hydroxides. Segregation or evaporation of the non-organic products may occur enabling their collection. The specific nature of the segregation or evaporation of the non-organic products may depend on the specific composition of the SAS, the composition of the molten salt/base, and the nature of reactions that take place in the pyrolysis reactor. A number of cyclones may be provided downstream of the pyrolysis reactor for recovering elutriated products. [0017] Embodiments may also include heat integration steps to improve the overall efficiency of the SAS processing system. In particular, heat integration may be provided at the dewatering, drying, heating and pyrolysis stages. A main source of heat may be the combustion of the gaseous fuel products generated in the pyrolysis. The number of heat integration steps and their locations will greatly depend on different factors such as the molten salt/base composition, the pyrolysis temperature, the desired products that the system is designed to produce, the composition of the raw SAS, etc.

[0018] Embodiments include a number of different implementations of a SAS processing system. [0019] Figure 1 schematically shows an implementation of the SAS processing system according to a first embodiment.

[0020] There is a supply conduit 103 that supplies raw SAS to the SAS processing system. The raw SAS may have a water content of about 80%. The supplied SAS by the supply conduit 103 may be heated in a first heat exchanger 106. A conduit may transfer the heated SAS to a dewatering apparatus 111. The SAS may be heated, or otherwise dewatered, in the dewatering apparatus 111. A number of known dewatering apparatuses 111 may be used. The dewatering apparatus 111 may alternatively, or additionally, be a dehumidifying apparatus.

[0021] A stream of water and/or steam is output from the dewatering apparatus 111 to a second heat exchanger 112 where the water and/or steam may be cooled. The cooled water may flow out of the water output conduit 117. In particular, the water in the water output conduit 117 may be recycled by supplying it back to the fish farm that is the source of the SAS. The second heat exchanger 112 may comprise a coolant input conduit 119 and coolant output conduit 120 for providing a flow of coolant. The coolant may be, for example, a flow of cold water. [0022] A stream of substantially dewatered SAS may be output from the dewatering apparatus 111 to a third heat exchanger 113 where the SAS is heated. The heated SAS may then be supplied to a dryer 114 for further drying of the SAS. The dried SAS may then flow through a fourth heat exchanger 115 where it is further heated. The SAS may then be supplied to a pyrolysis reactor 101. [0023] The pyrolysis reactor 101 comprises a molten salt/base, as described above. The processes that occur within the pyrolysis reactor 101 chemically convert and/or thermally decompose the constituents of the SAS.

[0024] A first output conduit 121 of the pyrolysis reactor 101 may support a flow of solid reaction products out of the pyrolysis reactor 101. The first output conduit 121 of the pyrolysis reactor 101 may supply the solid reaction products to the fourth heat exchanger 115 where they are cooled by heat exchange with the SAS supply to the pyrolysis reactor 101. An output conduit 116 of the SAS processing system may comprise the cooled solid reaction products received from the fourth heat exchanger 115. The cooled solid reaction products may include char, ash, metals and metal compounds.

[0025] A second output conduit 122 of the pyrolysis reactor 101 may support a flow of gaseous reaction products out of the pyrolysis reactor 101. The second output conduit 122 may supply the gaseous reaction products to a cyclone separator 102 where some of the particles within the gaseous reaction products may be extracted by the cyclone separation process. The extracted particles may be supplied to an output conduit 105 of the SAS processing system. The extracted particles may comprise metal hydroxide powder and other solid products not recovered by the flow through the first output conduit 121. The cyclone separator 102 may be a known cyclone separator 102.

[0026] The gaseous reaction products that flow through the cyclone separator 102 may be supplied to the first heat exchanger 106 where they may be cooled by heat transfer with the SAS supply to the SAS processing system. The cooling may condense some of the gaseous reaction products to liquids.

[0027] The cooled reaction products may then be supplied to another cyclone separator 104 where some of the particles within the reaction products may be extracted by the cyclone separation process. The extracted particles may be supplied to an output conduit 107 of the SAS processing system. The extracted particles may comprise metal hydroxide powder, as well as other solid products not recovered by the flows through the first output conduit 121 and the output conduit 105. The cyclone separator 104 may be a known cyclone separator 104.

[0028] The main flow of reaction products from the cyclone separator 104 may be supplied to a gas-liquid separator 108. The gas-liquid separator 108 may be a known gas-liquid separator 108. [0029] Some, or all, of the gaseous output from the gas-liquid separator 108 may be supplied to a combustor 110 where it is combusted. The heat generated by the combustion process may be used for the pyrolysis reaction in the pyrolysis reactor 101. In particular, the hot combustion products may flow through thermally conductive pipes within the pyrolysis reactor 101 so as to heat the contents of the pyrolysis reactor 101. The hot combustion products that have flowed through the pyrolysis reactor 101 may flow through the third heat exchanger 113 where they may be cooled by heat transfer with the SAS flow. The combustion products that have flowed through the third heat exchanger 113 may be carried within an output conduit 118 of the SAS processing system. The combustion products in the output conduit 118 may, for example, be vented to the atmosphere or supplied to a carbon capture process.

[0030] The liquid output from the gas-liquid separator 108 may be supplied to an output conduit 109 of the SAS processing system for liquid bio-fuel. Although not shown in Figure 1, further processes, such as a fuel upgrading process, may be performed on the liquid bio-fuel in the output conduit 109.

[0031] Figure 2 schematically shows another implementation of the SAS processing system according to a second embodiment.

[0032] There is a supply conduit 103 that supplies raw SAS to the SAS processing system. The raw SAS may have a water content of about 80%. The supplied SAS by the supply conduit 103 may first be supplied to a dewatering apparatus 111. The SAS may be heated, or otherwise dewatered, in the dewatering apparatus 111. The dewatering apparatus 111 may comprise a heat exchanger for using the heat from hot reaction products from the pyrolysis process to heat the SAS. A number of known dewatering apparatuses 111 may be used. The dewatering apparatus 111 may alternatively, or additionally, be a dehumidifying apparatus.

[0033] A stream of water and/or steam is output from the dewatering apparatus 111 to a first heat exchanger 202 where the water and/or steam may be cooled. The cooled water may flow out of the water output conduit 117. In particular, the water in the water output conduit 117 may be recycled by supplying it back to the fish farm that is the source of the SAS. The first heat exchanger 202 may comprise a coolant input conduit 119 and coolant output conduit 120 for providing a flow of coolant. The coolant may be, for example, a flow of cold water.

[0034] A stream of substantially dewatered SAS may be output from the dewatering apparatus 111 to a second heat exchanger 203 where the SAS is heated. The heated SAS may then be supplied to a dryer 114 for further drying the SAS. The dried SAS may then flow through a third heat exchanger 204 where it is further heated. The SAS may then be supplied to a pyrolysis reactor 101. [0035] The pyrolysis reactor 101 comprises a molten salt/base, as described above. The processes that occur within the pyrolysis reactor 101 chemically convert and/or thermally decompose the constituents of the SAS.

[0036] A first output conduit 121 of the pyrolysis reactor 101 may support a flow of solid reaction products out of the pyrolysis reactor 101. The first output conduit 121 of the pyrolysis reactor 101 may supply the solid reaction products to the fourth heat exchanger 204 where they are cooled by heat exchange with the SAS supply to the pyrolysis reactor 101. An output conduit 116 of the SAS processing system may comprise the cooled solid reaction products received from the fourth heat exchanger 204. The cooled solid reaction products may include char, ash, metals and metal compounds.

[0037] A second output conduit 122 of the pyrolysis reactor 101 may support a flow of gaseous reaction products out of the pyrolysis reactor 101. The second output conduit 122 may supply the gaseous reaction products to a cyclone separator 102 where some of the particles within the gaseous reaction products may be extracted by the cyclone separation process. The extracted particles may be supplied to an output conduit 105 of the SAS processing system. The extracted particles may comprise metal hydroxide powder. The cyclone separator 102 may be a known cyclone separator 102.

[0038] The gaseous reaction products that flow through the cyclone separator 102 may be supplied to the dewatering apparatus 111 where they may be cooled by heat transfer with the SAS supply to the SAS processing system. The cooling may condense some of the gaseous reaction products to liquids.

[0039] The cooled reaction products may then be supplied to another cyclone separator 104 where some of the particles within the reaction products may be extracted by the cyclone separation process. The extracted particles may be supplied to an output conduit 107 of the SAS processing system. The extracted particles may comprise metal hydroxide powder. The cyclone separator 104 may be a known cyclone separator 104.

[0040] The main flow of reaction products through the cyclone separator 104 may be supplied to a gas-liquid separator 108. The gas-liquid separator 108 may be a known gas-liquid separator 108. [0041] Some, or all, of the gaseous output from the gas-liquid separator 108 may be supplied to a combustor 110 where it is combusted. The heat generated by the combustion process may be used for the pyrolysis reaction in the pyrolysis reactor 101. In particular, the hot combustion products may flow through thermally conductive pipes within the pyrolysis reactor 101 so as to heat the contents of the pyrolysis reactor 101. The hot combustion products that have flowed through the pyrolysis reactor 101 may flow through the second heat exchanger 203 where they may be cooled by heat transfer with the SAS flow. The gaseous combustion products that have flowed through the second heat exchanger 203 may be carried within an output conduit 118 of the SAS processing system. The gaseous combustion products in the output conduit 118 may, for example, be vented to the atmosphere or supplied to a carbon capture process.

[0042] The liquid output from the gas-liquid separator 108 may be supplied to an output conduit 109 of the SAS processing system for liquid bio-fuel. The liquid bio-fuel in the conduit 109 may be supplied to a fuel upgrade apparatus 201 where one or more processes may be performed for upgrading it.

[0043] Accordingly, the SAS system according to embodiments may be implemented in a number of different ways. An advantage of the above-described first and second embodiments is that the heat integration improves the overall efficiency of the SAS processing system.

[0044] A main advantage of embodiments is that they enable the recovery of P (i.e. phosphorus ) and/or N (i.e. Nitrogen) from sludge. In addition, heavy metal impurities may also be separated, either by being dissolved in the molten salt/base or by evaporating them in the form of metal hydroxides.

[0045] In both of the above-described first and second embodiments, the temperature of the hot combustion products from the combustor 110, that provide heat for the pyrolysis reaction in the pyrolysis reactor 101, may be about 400°C to 600°C when they exit the pyrolysis reactor 101. [0046] The dewatering of the SAS by the dewatering apparatus 111 may comprise heating the SAS at a temperature of about 40°C to 60°C. This helps to prevent the sludge forming into a jelly/glue like state.

[0047] To illustrate the functionality of the SAS processing system, thermodynamic calculations were made for pyrolysing a SAS sample. The SAS sample had the composition shown in Table 1, as provided in Figure 3. The presence of the elements was determined by Inductively coupled plasma mass spectrometry (ICP-MS). The molten salt/base in the pyrolysis reactor was KOH. It should be noted that there may be a lot of variation in the composition of SAS. The specific SAS composition shown in Table 1 is exemplary and embodiments include SAS with other compositions.

[0048] For simplification of calculation, only elements with concentration higher than 100 ppm were considered. The calculation was completed for a pyrolysis temperature of 600°C and atmospheric pressure, by varying KOH/SAS ratio from 1 to 10. The results are depicted in Figures 4A and 4B, that respectively show the results with linear and logarithmic scales. [0049] For a KOH/SAS ratio below 5.5, a considerable amount of solid K2CO3 forms with many other elements causing consumption of the molten KOH. This may therefore result in a large formation of slags.

[0050] With a KOH/SAS ratio that is larger than 5.5, the formation of K2CO3 is substantially, or entirely, inhibited. The mixture within the pyrolysis reactor is therefore easier to melt.

[0051] Many other substances were produced. An important substance that is produced is K3PO4 which is a fix of phosphorous on potassium. This is present in a solid form. It can be made at very low KOH/SAS ratio reaching a maximum amount which is limited by the maximum content of phosphorous in the SAS. The produced K3PO4 may be used as the feedstock of processes for making potassium-phosphate fertiliser products, both in solid and liquid forms, and may also be used as a catalyst for several industrial chemical processes (e.g., Alkylation).

[0052] Another important substance that is produced is KCN in solid form. This starts being present from a KOH/SAS ratio of 2, reaching similar mass fraction as K3PO4. This is a highly toxic element but it has direct industrial applications, such as in raw gold mining.

[0053] The gaseous reaction products from the pyrolysis process comprised over about 70% hydrogen and about 20% CH4. The rest of the gaseous reaction products were impurities that mostly comprised evaporated potassium that could be collected by cooling and washing the gaseous reaction products. Alternatively, or additionally, quenching may be directly applied in the pyrolysis reactor (just above the melt surface) to substantially remove the impurities from the gaseous reaction products before they are output from the pyrolysis reactor 101. This may reduce the potential damage caused by the deposition of the impurities.

[0054] It was determined that elemental carbon forms when the KOH to SAS ratio is low. However, there is substantially no elemental carbon formation when the KOH to SAS ratio is above 5, with the carbon instead being attached to KCN and CH4 as the main reaction products that contain carbon. If elemental carbon is one of the targeted products by the operator of the SAS processing system, a multi-step process may be applied. A first pyrolysis process may be performed on the SAS with a low KOH to SAS ratio so as to maximise the carbon formation. The formed carbon may then be extracted. One or more pyrolysis processes may then be performed with a larger KOH concentration to generate a melt effect. The K2CO3 and KCN formation will be substantially reduced due to the removal of the carbon. Alternatively, a molten salt/base other than KOH could be applied in the post-processing steps, as most of the target products (gaseous fuel, carbon and K3PO4) form at very low KOH/SAS ratio.

[0055] Embodiments include a number of modifications and variations to the above-described techniques. [0056] The implementations of SAS processing systems according to the first and second embodiments may comprise further components from those shown in Figures 1 and 2. For example, they may both comprise pumps, and/or other equipment, for moving the sludge and reaction products between the different processing steps. The SAS processing systems may comprise a grinder for grinding the sludge after it has been dried so that it is easier to transport. There may also be a blower for providing the flow of gaseous fuel to the combustor 110.

[0057] Although embodiments have been described with reference to the processing of SAS, embodiments may more generally be used to process all types of sludge. In particular, the processed sludge by embodiments may be non-saline aquaculture sludge from a fresh water fish farm, sludge from human waste, or sludge from animal waste.

[0058] The pyrolysis reactor 101 may be a standard reactor made from steel. To prevent corrosion occurring, the inner surfaces of the pyrolysis reactor may be treated. Alternatively, the pyrolysis reactor may be made from a corrosion resistant material.

[0059] Embodiments include the combustor 110 that generates heat for the pyrolysis process additionally, or alternatively, combusting some, or all, of liquid fuel products output from the gasliquid separator 108.

[0060] Embodiments include the combustor 110 that generates heat for the pyrolysis process alternatively being integrated within the pyrolysis reactor 101. Embodiments also alternatively include heat being transferred from the combustor 110 to the pyrolysis reactor 101 by heat pipes. [0061] Embodiments include performing a plurality of pyrolysis reactions as a series of consecutive steps. There may be a separate pyrolysis reactor 101 for each step, or more than one step may be performed in the same pyrolysis reactor. The used molten salt/base step may be the same in more than one of the steps with the salt/base ratio changed. The used salt/base may also be different between steps. The temperature used for each step may also differ. This allows the SAS processing system to be more targeted towards the formation of specific products.

[0062] Although not shown in Figures 1 and 2, the SAS processing system may comprise further apparatuses for generating the liquid bio-fuel in the conduit 109. For example, a Fisher-Tropsch based process may be performed to convert the surplus gas to liquid fuel products, such as bio-oil. This may be the main generator of liquid fuel products when the pyrolysis reactor 101 substantially only generates gaseous fuels, and not liquid ones. Fisher-Tropsch based processes may also be used to increase the production of liquid fuel products when the amount of gaseous fuel that is produced in excess of that required to provide heat for the reaction in the pyrolysis reactor 101. Fisher- Tropsch based processes may in particular be used to generate liquid fuel based products when the SAS processing system is used in locations with high availability of renewable energy in the form of electricity or solar heat. This may be used to process gaseous fuel for the production of liquid fuel products. Other typical post-processing steps may be applied for upgrading the produced liquid fuel products, such as hydrogenation and de-oxidation. The post-processing steps may be performed for increasing, and preferably maximizing, the oil energy density as well as adapting it to commercial fuel standards (such as aviation, marine or car fuel standards).

[0063] The desired liquid fuel products may include bio-diesels and bio-gasolines. However, the pyrolysis process may produce many complex heavy hydrocarbons and post-processing steps may be performed to upgrade these to bio-diesel, bio-gasoline and/or jet fuel.

[0064] Embodiments include using other molten salt/bases in the pyrolysis reactor than KOH. Embodiment may use any salts with a subordinate that can recover the P and/or N elements in the sludge. For example, NaOH may be used instead of KOH. This may result in the production of Na3PO4, which has direct uses in food and water processing (e.g. emulsifiers, thickening and leavening agents as well as for PH control), and NaCN, which is a commercial product used in industrial gold and silver mining. Alternatively, to increase the yield of bio-oils, chloride salts may be used.

[0065] Embodiments also include performing a gasification process on the sludge instead of a pyrolysis process. This may increase the required operating temperatures and salt carbonates may be preferred. Chloride salts generally have a high melting temperature and may therefore be more suited for use in a gasification based process. In a lower temperature pyrolysis process, a mix comprising chloride salts may be required so as to lower the melting temperature.

[0066] The gasification based system may use part of the heat recovered for raising steam to as a gasification agent. The water used for steam generation can come directly from the wetness of the raw sludge.

[0067] The foregoing description of the preferred embodiments has been provided for the purposes of illustration and description. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but where applicable may be interchangeably used in combination with other features to define another embodiment, even if not specifically shown or described. The description is therefore not intended to limit the scope of the present invention, which is defined in the claims.

Claims

1. A sludge processing system comprising a reactor; wherein: the reactor comprises a molten salt/base; the reactor is arranged to receive sludge; the reactor is arranged to support reactions between at least some of the constituents of the received sludge and the molten salt/base so as to recover phosphorus based products and/or

Nitrogen based products from the sludge; and the reactor is arranged to thermally decompose at least some of the constituents of the received sludge.

2. The system according to claim 1, wherein the sludge is aquaculture sludge.

3. The system according to claim 2, wherein the sludge is saline aquaculture sludge.

4. The system according to any preceding claim, wherein the sludge is from a recirculating aquaculture system.

5. The system according to any preceding claim, wherein the molten salt/base comprises KOH, NaOH, chloride salts and salt carbonates.

6. The system according to any preceding claim, further comprising at least one heat exchanger arranged to transfer heat between hot reaction products output from the reactor and the sludge so as to heat the sludge before the sludge is supplied to the reactor.

7. The system according to any preceding claim, wherein the reactor is arranged to output solid reaction products that include one or more of char, ash and metals and or metal compounds.

8. The system according to any preceding claim, wherein the reactor is arranged to output gaseous reaction products.

9. The system according to claim 8, wherein the gaseous reaction products include hydrogen and/or methane.

10. The system according to any of claims 8 to 9, further comprising a combustor arranged to receive and combust at least some of the gaseous reaction products; wherein the combustor is arranged so that at least some of the heat generated by the combustion is used in the reactor.

11. The system according to any of claims 8 to 10, further comprising a gas-liquid separator; wherein: the gas-liquid separator is arranged to separate gaseous reaction products from condensed gaseous reaction products; and the gas-liquid separator is arranged to supply at last some of the condensed gaseous reaction products to a liquid output from the system.

12. The system according to claim 11, wherein the condensed gaseous reaction products comprise bio-fuel.

13. The system according to claim 11, when dependent on claim 10, wherein the gas-liquid separator is arranged to supply at least some of the gaseous reaction products to the combustor.

14. The system according to any of claims 8 to 13, further comprising at least one cyclone separator arranged to extract particles from the gaseous reaction products

15. The system according to claim 14, wherein the extracted particles comprise metal hydroxi de(s).

16. The system according to any preceding claim, further comprising at least one dewatering apparatus and/or dehumidifying apparatus arranged to reduce the water content of the sludge before the sludge is provided to the reactor.

17. The system according to any preceding claim, further comprising at least one dryer arranged to dry the sludge before the sludge is provided to the reactor.

18. The system according to any preceding claim, wherein the system comprises a plurality of reactors arranged to process the sludge in series; wherein: each reactor comprises a molten salt/base; each reactor is arranged to support reactions between the molten salt/base products dependent on the received sludge; and one of more of the type of salt/base, ratio of salt/base and temperature of the reaction differs between the reactors.

19. The system according to any preceding claim, wherein the reactor is a pyrolysis reactor or a gasification reactor. 0. A method of processing sludge, the method comprising: supplying sludge to one or more reactors that each comprise a molten salt/base; reacting at least some of the constituents of the sludge with the molten salt/base so as to recover phosphorus based products and/or Nitrogen based products from the sludge; and thermally decomposing at least some of the constituents of the received sludge.

21. The method according to claim 20, wherein the method is performed in a sludge processing system according to any of claims 1 to 19.