Fertiliser

Abstract

A method of producing a solid fertilizer where residual from an anerobic digestin process is contacted with a sulfate salt. The digestate may be a sludge or liqour with a dry matter content up to 40 wt %. The ratio of digestate to sulfate between 10:90 and 97:3 digestate to sulfate. The sulfate is may be an alkali or alkiline earth metal salt, preferably calium sulfate. The CaSO4 may be supplied as gypsum. The mixing of the digestate and sulfate may be performed at atmospheric pressure, at 40 °C or less and for at least 90 minutes. The mixing process may be performed as a batch process. The fertilizer product may comprise at least 10 wt % dry matter.

Description

The present invention relates to a method of producing a fertiliser from a digestate obtained from anaerobic digestion plants, and the fertiliser perse. The term "anaerobic digestion" relates to a collection of processes by which microorganisms break down biodegradable material in the absence of oxygen. Anaerobic digestion plants use methanogenesis or biomethanation to produce low cost, low carbon renewable energy, often by recycling food waste. Organic material which is to be processed in an anaerobic digestion plant is shredded and pasteurised before it is fed into digesters which provide a completely sealed oxygen free environment. The material is digested by microorganisms within the digesters at around 55°C for about a month to produce biogas and a digestate. The biogas is extracted and burned to generate electricity and the digestate is given to farmers to use as a fertiliser. In the UK this process is used to dispose of human food and plant matter from food producers. However, there are a number of problems associated with the digestate. Firstly, the digestate produced is a liquid, typically containing about 4 % (m / m) dry matter. In contrast, traditional fertilisers are provided as solid pellets, and fertiliser spreaders are configured to be used with these. Accordingly, farmers are unable to spread the digestate with their existing machinery. Furthermore, the digestate will continue to produce highly flammable methane. Accordingly, the digestate would need to be transported in vented containers to prevent pressure build-up. The vented containers would release the methane, which is a greenhouse gas, into the atmosphere, which is undesirable from an environmental point of view. The inventor has attempted to address the problems associated with the prior art. In accordance with a first aspect of the invention, there is provided a method of producing a solid fertiliser, the method comprising contacting a digestate with a sulphate (SO42) to obtain the solid fertiliser. Advantageously, the solid fertiliser may be shaped into pellets and used with existing fertiliser spreaders. Furthermore, the solid fertiliser is stable and so can be stored and transported safely. Additionally, all of the beneficial components of the digestate are retained in the final product. Furthermore, since the solid fertiliser comprises sulphate it may also be used to remediate salt damage and help to release "locked nitrogen" and replace other elements that are depleted in the soil. The term "digestate" can refer to a digestate produced from an anaerobic digestion process. Preferably, the digestate is a methanogenic digestate, and may be in the form of a sludge or liquor. The term "methanogenic digestate" can refer to a digestate produced from a methanogenesis or biomethanation process. Preferably, the digestate has been produced from food and / or a food material product. Preferably, the digestate has not been produced from any other material. The food may be human food. The food material product may be a plant material. The plant material may be a waste product from a food producer. The digestate may comply with the British Standard Institution's Publicly Available Specification (BSI PAS 110). It may be appreciated that in the UK PAS110 approved digestate may be applied to agricultural land without a permit. The digestate may comprise less than or equal to 1,000 CFU / g of E. coli. The amount of 5. coli may be determined using the method set out in ISO 16649-2:2001. The digestate may not comprise Salmonella spp. The digestate may comprise less than or equal to 0.43 COD / g VS of volatile fatty acids. The digestate may comprise less than or equal to 0.25 l / g VS of residual biogas potential. The amount of the volatile fatty acids may be determined using gas chromatography. The residual biogas potential may be determined as set out in the "Residual biogas potential test for digestates" Final Report, dated 7 January 2010 (https: / / www.ktbl.de / fileadmin / user upload / Allqemeines / Download / Rinqversuch-Bioqas / Residual-Bioqas-Potential.pdf), a copy of which is incorporated herein by reference. COD may be understood to refer to chemical oxygen demand. VS may be understood to refer to volatile solids. Volatile solids may be understood to be solids which are lost on ignition of the dry solids at 550°C. Preferably, the digestate is a liquid, a suspension or a sludge. Preferably, the digestate contains less than 50 % (m / m) dry matter. More preferably, the digestate contains less than 40 % (m / m) dry matter, less than 30 % (m / m) dry matter, less than 20 % (m / m) dry matter or less than 10 % (m / m) dry matter. Most preferably, the digestate contains less than 9 % (m / m) dry matter, less than 8 % (m / m) dry matter, less than 7 % (m / m) dry matter, less than 6 % (m / m) dry matter or less than 5 % (m / m) dry matter. The digestate may comprise at least 0.1 % (m / m) dry matter, at least 0.5 % (m / m) dry matter, at least 1 % (m / m) dry matter, at least 2 % (m / m) dry matter, at least 3 % (m / m) dry matter, at least 3.5 % (m / m) dry matter or at least 4 % (m / m) dry matter. The digestate may comprise between 0.1 and 40 % (m / m) dry matter, between 0.5 and 30 % (m / m) dry matter, between 1 and 20 % (m / m) dry matter, between 1.5 and 10 % (m / m) dry matter, between 2 and 9 % (m / m) dry matter, between 2.5 and 8 % (m / m) dry matter, between 3 and 7 % (m / m) dry matter, between 3.5 and 6 % (m / m) dry matter or between 4 and 5 % (m / m) dry matter. It may be appreciated that the term "dry matter" may be used interchangeably with the term "total solids". The percentage dry matter may be calculated as described in the examples. Preferably, the weight ratio of the digestate to the sulphate is between 10:90 and 90:10. More preferably, the weight ratio of the digestate to the sulphate is between 20:80 and 80:20 or between 30:70 and 75:25. Most preferably, the weight ratio of the digestate to the sulphate is between 40:60 and 70:30, between 45:55 and 65:35 or between 50:50 and 60:40. In a most preferred embodiment, the weight ratio of the digestate to the sulphate is about 59:41. Preferably, the weight ratio of the digestate to the sulphate is between 10:90 and 97:3. More preferably, the weight ratio of the digestate to the sulphate is between 30:70 and 95:5 or between 40:60 and 93:7. Most preferably, the weight ratio of the digestate to the sulphate is between 50:50 and 92:8, between 60:40 and 90:10 or between 80:20 and 88:12. In a most preferred embodiment, the weight ratio of the digestate to the sulphate is about 85:15. Preferably, the sulphate is a metal sulphate. More preferably, the sulphate is an alkali metal sulphate or an alkaline earth metal sulphate. Accordingly, the sulphate may be lithium sulphate (Li2SO4), sodium sulphate (Na2SO4), potassium sulphate (K2SO4), rubidium sulphate (Rb2SO4), caesium sulphate (CS2SO4), beryllium sulphate (BeSO4), magnesium sulphate (MgSO4), calcium sulphate (CaSO4), strontium sulphate (SrSO4) or barium sulphate (BaSO4). Most preferably, the sulphate is an alkaline earth metal sulphate, i.e. a group (II) metal sulphate. In a preferred embodiment, the sulphate is calcium sulphate (CaSO4). In an even more preferred embodiment, the sulphate is in the form of gypsum. Accordingly, the method may comprise contacting the digestate with gypsum. It may be appreciated that the term "gypsum" can refer to calcium sulphate dihydrate (CaSO4-2H2O). Preferably, the weight ratio of the digestate to the gypsum is between 10:90 and 90:10. More preferably, the weight ratio of the digestate to the gypsum is between 20:80 and 80:20 or between 30:70 and 70:30. Most preferably, the weight ratio of the digestate to the gypsum is between 35:65 and 65:35, between 40:60 and 60:40 or between 45:55 and 55:45. In a most preferred embodiment, the weight ratio of the digestate to the gypsum is about 50:50. Preferably, the weight ratio of the digestate to the gypsum is between 10:90 and 95:5. More preferably, the weight ratio of the digestate to the gypsum is between 30:70 and 92:8 or between 50:50 and 90:10. Most preferably, the weight ratio of the digestate to the gypsum is between 60:40 and 90:10, between 70:30 and 88:12 or between 75:25 and 85:15. In a most preferred embodiment, the weight ratio of the digestate to the gypsum is about 80:20. Preferably, the digestate and the sulphate are contacted at a pressure of between 1 kPa and 10,000 kPa. More preferably, the digestate and the sulphate are contacted at a pressure of between 10 kPa and 1,000 kPa, between 50 kPa and 500 kPa, between 60 kPa and 400 kPa, between 70 kPa and 300 kPa, between 80 kPa and 200 kPa or between 90 kPa and 200 kPa. Most preferably, the digestate and the sulphate are contacted at about atmospheric pressure, i.e. about 101 kPa. Preferably, the digestate and the sulphate are contacted at a temperature of less than 100°C. More preferably, the digestate and the sulphate are contacted at a temperature of less than 75°C or less than 50°C. Most preferably, the digestate and the sulphate are contacted at a temperature of less than 40°C, less than 30°C or less than 20°C. Preferably, the digestate and the sulphate are contacted at a temperature of between 0°C and 100°C. More preferably, the digestate and the sulphate are contacted at a temperature of between 2°C and 75°C or between 3°C and 50°C. Most preferably, the digestate and the sulphate are contacted at a temperature of between 5°C and 40°C, between 7.5°C and 30°C or between 10°C and 20°C. Advantageously, since the process may be carried out under ambient conditions, very little energy is required to produce the solid fertiliser. Preferably, the digestate and the sulphate are contacted for a time of at least 1 minute, at least 5 minutes or at least 10 minutes. More preferably, the digestate and the sulphate are contacted for a time of at least 10 minutes, at least 20 minutes, at least 30 minutes, at least 40 minutes, at least 50 minutes or at least 60 minutes. Most preferably, the digestate and the sulphate are contacted for a time of at least 70 minutes, at least 80 minutes or at least 90 minutes. In some embodiments, the digestate and the sulphate are contacted for a time of at least 2 hours, at least 4 hours, at least 6 hours, at least 12 hours, at least 18 hours or at least 24 hours. Preferably, the step of contacting the digestate with the sulphate comprises stirring the resultant mixture for at least 1 second. More preferably, the step of contacting the digestate with the sulphate the method comprises stirring the resultant mixture for at least 10 seconds, at least 20 second or at least 30 seconds. Most preferably, the step of contacting the digestate with the sulphate the method comprises stirring the resultant mixture for at least 40 seconds, at least 50 second or at least 60 seconds. The step of contacting the digestate with the sulphate may comprise a batch process, a semi-batch process or a continuous process. In a preferred embodiment, the step of contacting the digestate with the sulphate comprises a batch process. The method may comprise solidifying, pelletising and / or granulating the resultant mixture. Solidifying the resultant mixture may comprise feeding the resultant mixture into a prilling machine. Accordingly, the solid fertiliser may comprise prills. Solidifying the resultant mixture may comprise forming a jet of the resultant mixture with a cooling medium. The method may comprise causing the resultant mixture to form prills. Prills may be understood to be substantially spherical solids. The jet may be fed into the top of the prilling tower. The cooling medium may be fed into the bottom of the prilling tower. The cooling medium may be a gas stream. The gas stream may be an airstream or a nitrogen stream. Pelletising and / or granulating the resultant mixture may comprise using a pan granulation method. The method may comprise feeding the resultant mixture into a pan, and rotating the pan to cause the resultant mixture to pelletise or granulate. The pan may comprise a base. The base may be disposed at an angle which is offset from horizontal. Preferably, the solid fertiliser contains at least 10 % (m / m) dry matter. More preferably, the solid fertiliser contains at least 20 % (m / m) dry matter, 30 % (m / m) dry matter or 40 % (m / m) dry matter. Most preferably, the solid fertiliser contains at least 50 % (m / m) dry matter. Preferably, the solid fertiliser has a pH of between 4 and 11. More preferably, the solid fertiliser has a pH of between 5 and 10 or between 6 and 9. Most preferably, the solid fertiliser has a pH of between 7 and 8. Prior to contacting the digestate with the sulphate, the method may comprise capture methane from the digestate. The methane may be combusted and used to provide power. The inventor believes that the solid fertiliser obtained by the method of the first aspect is novel per se. Accordingly, in accordance with a second aspect, there is provided a solid fertiliser obtained or obtainable by the method of the first aspect. All features described herein (including any accompanying claims and abstract), and / or all of the steps of any method or process so disclosed, may be combined with any of the above aspects in any combination, except combinations where at least some of such features and / or steps are mutually exclusive. Example 1 As discussed above, the inventor has attempted to address the problems with the prior art by developing a process for producing a solid fertiliser. An embodiment of the process is discussed in detail below. Digestate was obtained from an anaerobic digestion plant. Analysis of the digestate found that it had a pH of 8.8 and contained 4.1 % (m / m) dry matter. The percentage dry matter is calculated by placing a sample with a known mass in a drying oven set at 105°C until the residue appears dry, typically the sample is left overnight. The sample is then cooled in a desiccator and weighed. After having been weighed, the sample is returned to the oven and dried for a further hour before being cooled and weighed again. This process is repeated until the measured mass does not differ by more than 0.5% of the previous value or 2 mg, whichever is greater. The percentage dry matter is then calculated as the mass of the dried sample divided by the mass of the sample prior to drying and then multiplying the amount by 100. 1 kg of digestate and 1 kg of gypsum were mixed in an open vessel at 15 °C for one minute. The inventor noted that directly after mixing the product became a slurry and lost the very strong odour associated with the digestate. The sample was then left for 90 minutes at which point the product was observed to be solid which was easily breakable into any particle size required. The product was analysed and the pH of a 10% suspension was found to be 7.9. The product contained 58.4 % (m / m) dry matter, which was calculated as explained above. It should be noted that this is higher than the dry matter content in commercially available fertilisers. Elemental analysis of the product was also carried out and the results are given in Table 2 below: Table 2: Elemental analysis of resultant product Element Amount (mg / kg) Total nitrogen 3460 Ammonical nitrogen 1607 Phosphorus 665 Potassium 4372 Magnesium 14180 Iron 6333 Sodium 97=1 Sulphur 87840 Copper 18 Chromium 21 Cadmium <0.1 Lead <0.1 Nickel 15 Zinc 23 Mercury <0.1 Chloride 1785 The product was found to be stable, even at high moisture and organic content, and may be stored without decomposing. It will be appreciated that the ratio of digestate to gypsum may be varied. The inventor has found that the optimum ratio is 80 wt% digestate to 20 wt% gypsum. The inventor has also found that while a ratio of more than 50 wt% gypsum may be used this is not desirable as it dilutes the beneficial components of the digestate without any other benefits being obtained. Example 2 The inventors mixed digestate and gypsum as explained in example 1. 15 mins after mixing the slurry was moved onto a stockpile. 24hrs later, the stockpile was granulated using the pan granulation method. The inventors note that they intend to capture the gas produced from the raw digestated liquid storage buffer tanks prior to using the liquid in the mix. This gas may be used to power the process. Conclusions The process developed by the inventor may be used to produce a solid fertiliser suitable for use with existing fertiliser spreading machinery. The product is stable and so can be stored and transported safely. Additionally, all of the ammonia and other beneficial components of the digestate are retained in the final product. Furthermore, the gypsum advantageously will also remediate salt damage and help to release "locked nitrogen" and replace other elements that are depleted in the soil. This is particularly advantageous as a digestate made from recycled food waste is likely to have a high salt concentration. Accordingly, the sulphate in the solid fertiliser prevents this from being problematic. Applying the final product will give the benefits of both fertiliser and gypsum in one application. Finally, since the process may be conducted at atmospheric pressure and at ambient temperature, the process requires very little energy to produce the product. Furthermore, the process can be used to provide material in the form of pellets or granules. Accordingly, the material could be spread using pre-existing equipment.

Claims

1. A method of producing a solid fertiliser, the method comprising contacting a digestate with a sulphate (SO42) to obtain the solid fertiliser.

2. A method according to claim 1, wherein the digestate is a methanogenic digestate and is in the form of a sludge or liquor.

3. A method according to either claim 1 or claim 2, wherein the digestate contains less than 40 % (m / m) dry matter, 30 % (m / m) dry matter, 20 % (m / m) dry matter, 10 % (m / m) dry matter, 9 % (m / m) dry matter, 8 % (m / m) dry matter, 7 % (m / m) dry matter, 6 % (m / m) dry matter or 5 % (m / m) dry matter.

4. A method according to any preceding claim, wherein the weight ratio of the digestate to the sulphate is between 10:90 and 97:3, between 30:70 and 95:5 or between 40:60 and 93:7.

5. A method according to any preceding claim, wherein the sulphate is a metal sulphate.

6. A method according to claim 5, wherein the sulphate is an alkali metal sulphate or an alkaline earth metal sulphate.

7. A method according to claim 6, wherein the sulphate is calcium sulphate (CaSO4).

8. A method according to claim 7, wherein the sulphate is in the form of gypsum.

9. A method according to any preceding claim, wherein the digestate and thesulphate are contacted at about atmospheric pressure.

10. A method according to any preceding claim, wherein the digestate and the sulphate are contacted at a temperature of less than 40°C, less than 30°C or less than 20°C.

11. A method according to any preceding claim, wherein the digestate and the sulphate are contacted for a time of at least 10 minutes, at least 20 minutes, at least30 minutes, at least 40 minutes, at least 50 minutes, at least 60 minutes, at least 70 minutes, at least 80 minutes or at least 90 minutes.

12. A method according to any preceding claim, wherein the step of contacting the 5 digestate with the sulphate comprises a batch process.

13. A method according to any preceding claim, wherein the solid fertiliser contains at least 10 % (m / m) dry matter, at least 20 % (m / m) dry matter, 30 % (m / m) dry matter, 40 % (m / m) dry matter or at least 50 % (m / m) dry matter.1014. A method according to any preceding claim, wherein the solid fertiliser has a pH of between 4 and 11, between 5 and 10, between 6 and 9 or between 7 and 8.

15. A solid fertiliser obtained or obtainable by the method of any preceding claim.1511