GB2529804A

GB2529804A - Method of pre-treating a feedstock for anaerobic digestion

Info

Publication number: GB2529804A
Application number: GB1414712.8A
Authority: GB
Inventors: Mike Williams
Original assignee: C Tech Innovation Ltd
Current assignee: C Tech Innovation Ltd
Priority date: 2014-08-19
Filing date: 2014-08-19
Publication date: 2016-03-09
Also published as: GB201414712D0

Abstract

A method of pre-treating a feedstock intended for anaerobic digestion, the method comprising: (1) providing an organic waste at a temperature of from 80 to 200 Â°C and a first pressure of greater than 1 bar; and (2) flash evaporating volatiles from the organic waste by reducing the pressure of the organic waste from the first pressure to a second pressure lower than the first pressure. Suitably, the organic waste comprises animal waste, such as intestinal waste and/or bacterial contaminated tissue. One or more of water soluble inorganic salts, water soluble organic salts and water soluble nitrogen-containing compounds may be added to the organic waste prior to step (2). A method of producing biogas is also claimed which comprises pre-treatment of a feedstock as described above; and passing the feedstock to an anaerobic digester to produce biogas. In another aspect, a feedstock for anaerobic digestion is claimed, wherein the feedstock is in a liquid state and has a total viable count (TCG) according to ISO 4833-1:2013 of less than 5, the feedstock being obtainable according to the method described above.

Description

Method of Pre-treating a Feedstock for Anaerobic Digestion The invention relates to a method of pre-treating a feedstock intended for anaerobic digestion.

Anaerobic digestion is a collection of processes by which microorganisms break down biodegradable material in the absence of oxygen. The process is used to manage waste and/or to produce fuels.

Anaerobic digestion typically comprises four stages: hydrolysis, acidogenesis, acetogenesis and metbanogenesis. In the hydrolysis stage, particulates are solubilised and large polymers are converted to simpler monomers via a chemical reaction. During the second stage, acidogenesis, a biological reaction occurs in which simple monomers are converted into volatile fatty acids. The acetogenesis stage involves a biological reaction in which volatile fatty acids are converted into acetic acid, carbon dioxide and hydrogen. In the final methanogenesis stage, a biological reaction occurs in which acetates are converted into methane and carbon dioxide, while hydrogen is consumed.

Anaerobic digestion is widely used as a source of renewable energy. The process produces a biogas, consisting of methane, carbon dioxide and traces of other contaminant" gases. This biogas can be used directly as fuel, in combined heat and power gas engines or upgraded to natural gas-quality biomethane. The nutrient-rich digestate also produced can be used as fertilizer.

Contaminated animal waste, such as category 2 and category 3 waste stipulated by EC Regulation 1069/2009 Article 13, has little commercial value and cannot normally be disposed of without extensive pasteurisation. While such pasteurisation may be achieved by subjecting the waste to elevated temperatures and/or pressures, the resulting pasteurised waste is typically in the form of a friable solid, which may be unsuitable for transport to, or use in, an anaerobic digestion plant.

The present invention seeks to tackle at least some of the problems associated with the prior art or at least to provide a commercially acceptable alternative solution thereto.

The present invention provides a method of pre-treating a feedstock intended for anaerobic digestion, the method comprising: (1) providing an organic waste at a temperature of from 80 to 200 °C and a first pressure of greater than 1 bar; and (2) flash evaporating volatiles from the organic waste by reducing the pressure of the organic waste from the first pressure to a second pressure lower than the first pressure.

Each aspect or embodiment as defined herein may be combined with any other aspect(s) or embodiment(s) unless clearly indicated to the contrary. In particular, any features indicated as being preferred or advantageous may be combined with any other feature indicated as being preferred or advantageous.

The term "animal waste" used herein encompasses any material derived from an animal, for example animal tissue, animal digestive tracts and/or faeces. The term also encompasses waste derived from fish and shellfish. Animal waste may encompass food waste, for example comprising meat and/or fish.

The term "flash evaporating" used herein encompasses subjecting a liquid stream to a reduction in pressure to cause vapour to be emitted therefrom.

The term "total viable count" used herein refers to the amount of colony forming units (cfu) per gram of the sample. Total viable count (TVC) may be determined using the method specified in ISO 4833-1:2013. In such a method, a series of ten-fold dilutions are prepared from an initial dilution. A 1 ml aliquot of each dilution is then dispensed into petri dishes, to which is added 12-15 ml of a molten, non-selective culture medium, such as plate count agar (FCA), at a temperature of 44-47 00. The plates are then rotated to mix the molten medium and the sample dilution, and allowed to solidify before incubation at 30 °C for 72 hours. After incubation the visible colonies on selected plates (those showing more than 15 and less than 300 colonies) are counted and the result can then be calculated on the basis of the count and the dilution factor.

The inventors have surprisingly found that the method of the present invention provides a sterile, homogenous liquid feedstock which is especially suitable for anaerobic digestion.

Without being bound by theory, it is considered that the flash evaporation modifies the physical and chemical properties of the organic waste. The physical form of the organic waste, for example animal waste, may be influenced by thermal and/or hydrolytic denaturation of proteins and peptides (reduction of molecular weight and de-crosslinking). The presence of amino acids, inorganic nutrients and other natural products or side products in the pre-treated organic waste may also enhance the biological conversion efficiency of a subsequent anaerobic digestion process and biogass generation.

The pre-treatment may result in the organic waste being in the form of a mobile liquid substantially free of solids, which is readily transferrable to an anaerobic digestion plant. This offers the possibility of faster plant throughput, greater production capacity and increased biogas production. This is to contrast to conventional methods of pre-treating organic waste involving slow cooling of the heated waste, which typically result in the formation of a friable solid.

The temperature and first pressure may result in the substantial sterilisation of the organic waste, thereby facilitating easy handling and transport, and reducing the risk of contamination. For example, the pre-treated organic waste may exhibit a total viable count (TVC) according to ISO 4833-1:2013 of less than 5 cfu/g, typically less than 3 cfu/g, more typically about 2 cfu/g. Such low cfu/g values enable the organic waste to be handled safely. Such levels of sterilisation make the method suitable for use on category 2 and category 3 waste as stipulated by EC Regulation 1069/2009 Article 13. Such material cannot normally be disposed of without extensive pasteurisation followed by incineration. Accordingly, the method of the present invention may be advantageously used to form a useful anaerobic digestion feedstock from a waste product that would otherwise have little commercial value.

The temperature and first pressure of step (1) may result in hydrolysis of the organic waste. Optionally, water may be added to the organic waste prior to step (1). Following flash evaporation, the bydrolysate may be passed to an anaerobic digester.

The volatiles that may be flash evaporated from the organic waste include, for example, water and volatile organics.

Flash evaporation may be carried out, for example, by passing the liquid through a throttling valve or other throttling device. The flash evaporation is typically carried out in using a flash vessel.

Flash evaporation typically results in cooling of the organic waste. If the pre-treated waste is not intended to be passed directly to an anaerobic digester, then further cooling may be carried out in order to reduce the temperature of the organic waste to a temperature suitable for transportation and/or storage, typically ambient temperature.

The organic waste may be provided in the form of a slurry in order to improve the ease of handling. The organic waste may be macerated prior to step (1). This may serve to increase the surface area of the organic waste, thereby increasing the efficiency of the method. The provided organic waste may have a moisture content of from 40 to 90%, more typically from 60 to 85 %, even more typically about 80%.

The organic waste may comprise, for example, one or more of plant waste (i.e. fruit, vegetable, seaweed and/or cereal waste), animal waste, mineral oils, food waste and biodegradable synthetic polymers (such as, for example, polyvinylacetates, starches and carboxymethyl cellulose). The organic waste preferably comprises one or more of vegetable waste and animal waste.

In a preferred embodiment, the organic waste comprises, or is, animal waste.

The animal waste may be obtained from, for example, poultry, beeL pork or game. A particularly preferred animal waste is chicken waste. The organic waste is preferably category 2 and category 3 waste as stipulated by EC Regulation 1069/2009 Article 13.

The animal waste may comprise intestinal waste and/or bacterial contaminated tissue. Such waste may contain high levels of bacteria (for example cfu/g values of greater than about 10000) and may therefore have little commercial value.

However, such materials may be sufficiently sterilised by the method of the present invention. In addition, such materials may be transformed into a feedstock particularly suitable for the production of biogas using anaerobic digestion.

Prior to anaerobic digestion, suitable enzymes and/or chemical additives may be added to the organic waste to aid anaerobic digestion and/or allow the anaerobic digestion to be carried out at a lower temperature.

The temperature of step (1) is preferably from 100 to 180 °C, more preferably from 120 to 180 °C. Lower temperatures may result in lower levels of sterilisation of the organic waste. Higher temperatures may increase the cost of the process without any corresponding benefit and may also result in charring of the organic waste.

Prior to step (1) the organic waste is preferably heated to the temperature of from to 200 °C (preferably from 100 to 180 °C, more preferably from 120 to 180 °C) using ohmic heating. In contrast to conventional heating methods, ohmic heating, sometimes known as joule heating, may result in reduced levels of charring and solid products. Charring may give rise to blockages and restrict flow in apparatus used to carry out the method. Charring may also reduce heat transfer efficiency and lead to unfavourable sediments forming when carrying out subsequent anaerobic digestion.

The first pressure is preferably from 2 to 20 bar, more preferably from 2 to 15 bar, even more preferably from 3 to 10 bar, still even more preferably from 6 to 10 bar. Lower pressures may result in only minimal levels of volatiles being removed from the organic waste during flash evaporation and/or lower levels of sterilisation. Higher pressures may increase the cost of the process without any corresponding benefit and may unfavourably affect the physical properties of the organic waste.

During step (1), the organic waste is preferably subjected to the temperature and first pressure for at least 1 minute, preferably from 2 to 30 minutes, more preferably from 3 to 10 minutes. Shorter times may result in lower levels of sterilisation. Longer times may increase the cost of the process without any corresponding benefit.

In a preferred embodiment: the temperature of step (1) is from 120 to 180 °C; the first pressure is from 3 to 10 bar; during step (1) the organic waste is subjected to the temperature and first pressure for from 3 to 10 minutes; and prior to step (1) the organic waste is heated to the temperature of from 120 to 180 °C using ohmic heating.

Such a method provides high sterilisation and results in the organic waste exhibiting favourable physical and chemical properties that make it particularly suitable for use as a feedstock for anaerobic digestion.

Step (2) preferably comprises passing the organic waste to a chamber at the second pressure. Such a chamber is typically known as a flash vessel. Carrying out step (2) in such a vessel may allow easy recovery of the organic waste and/or suitable containment of the volatiles.

Step (2) preferably further comprises controlling the temperature of the organic waste to control the amount of volatiles evaporated therefrom. This may allow the final physical and chemical properties of the organic waste to be controlled.

The method may be a batch method or a continuous method. Preferably, the method is a continuous method.

The method may further comprise: (3) recovering the organic waste.

The recovering may comprise, for example, filtration, centrifugation and/or decanting.

Preferably, the method further comprises adding one or more of water soluble inorganic salts, water soluble organic salts and water soluble nitrogen-containing compounds to the organic waste prior to step (2). Suitable inorganic salts include, for example, group 1A and 2A alkaline metal salts such as, for example, sodium and/or potassium phosphates, sulphates, nitrates and halides. Suitable organic salts include, for example, organic carboxylates and carboxylic acids, preferably C2 to C6 carboxylic acids or carboxylates, such as, for example, acetates, propionates, butanoates, maleates, fumarates, for example sodium or potassium acetate. Suitable nitrogen-containing compounds include, for example, synthetic or naturally occurring nitrogen compounds such as, for example, urea, guanidine, ammonium phosphates and ammonium sulphates.

In a further aspect, the present invention provides a feedstock for anaerobic digestion, the feedstock being in the liquid state and having a total viable count (TCG) according to ISO 4833-1:2013 of less than 5, the feedstock obtainable according to the method as described herein.

In a further aspect, the present invention provides a method of producing a biogas, the method comprising: pre-treating a feedstock using the method as described herein; passing the feedstock to an anaerobic digester to produce biogas; and recovering the biogas.

The invention will now be described, by way of example, with reference to the following non-limiting drawings, in which: Figure 1 is a flow chart of the method of the present invention.

Referring to Figure 1, the present invention provides a method of pre-treating a feedstock intended for anaerobic digestion, the method comprising: (1) providing an organic waste, preferably an animal waste, at a temperature of from 80 to 200 °C and a first pressure of greater than 1 bar; and (2) flash evaporating volatiles from the organic waste by reducing the pressure of the organic waste from the first pressure to a second pressure lower than the first pressure. The method may optionally comprise (3) recovering the organic waste.

The invention will now be described in relation to the following non-limiting

example.

Example 1

A macerated chicken slurry was produced having a moisture content of approximately 80%, a viscosity of 87,000Cps and an electrical conductivity which increased from 2.5mS/cm at room temperature to 2OmS/cm at 180°C.

Samples of the chicken slurry were subjected to different heating regimes using a small-scale batch ohmic heater before being subjected to flash evaporation. The resulting material was in the form of a liquid. The levels of sterilisation of the resulting materials were determined and the results are set out in Table 1.

Heating Sample Total Viable E Coli Anaerobe Mesophilic Conditions tested Count (cfu/g) (cfu/g) spore (cfu/g) count Raw Material* Pre-45,000 2,500 2 2 freezing 3 bar, 133°C, Control1 45,000 850 N/A 2 2omins Heated 2 2 N/A 2 6 bar, 158°C, Control1 45,000 4,500 N/A 2 lOmins Heated 2 2 N/A 2 bar, 180°C, Control1 45,000 4,500 N/A 2 6mins Heated 2 2 N/A 2 bar, 180°C, Control1 45,000 4.500 N/A 2 4mins Heated 2 2 N/A 2 Table 1. Sterilization trials (controls were defrosted samples, not subject to heating used to benchmark microbial population, tcomparative example not forming part of the present invention).

These results show that all trials provided high level microbial kill and therefore are effective at rapidly sterilizing the chicken slurry.

The foregoing detailed description has been provided by way of explanation and illustration, and is not intended to limit the scope of the appended claims. Many variations in the presently preferred embodiments illustrated herein will be apparent to one of ordinary skill in the art and remain within the scope of the appended claims and their equivalents.

Claims

Claims: 1. A method of pre-treating a feedstock intended for anaerobic digestion, the method comprising: (1) providing an organic waste at a temperature of from 80 to 200 °C and a first pressure of greater than I bar; and (2) flash evaporating volatiles from the organic waste by reducing the pressure of the organic waste from the first pressure to a second pressure lower than the first pressure.
2. The method of claim 1, wherein the organic waste comprises animal waste.
3. The method of claim 2, wherein the animal waste comprises intestinal waste and/or bacterial contaminated tissue.
4. The method of any preceding claim, wherein the temperature of step (1) is from 100 to 180 °C, preferably from 120 to 180 °C.
5. The method of any preceding claim, wherein prior to step (1) the organic waste is heated to the temperature of from 80 to 200 °C using ohmic heating.
6. The method of any preceding claim, wherein the first pressure is from 2 to bar, preferably from 2 to 15 bar, more preferably from 3 to 10 bar.
7. The method of any preceding claim, wherein during step (1) the organic waste is subjected to the temperature and first pressure for at least 1 minute, preferably from 2 to 30 minutes, more preferably from 3 to 10 minutes.
8. The method of any preceding claim, wherein step (2) comprises passing the organic waste to a chamber at the second pressure.
9. The method of any preceding claim, wherein step (2) further comprises controlling the temperature of the organic waste to control the amount of volatiles evaporated therefrom.
10. The method of any preceding claim, wherein the method is a continuous method.
11. The method of any preceding claim, further comprising: (3) recovering the organic waste.
12. The method of any preceding claims, further comprising adding one or more of water soluble inorganic salts, water soluble organic salts and water soluble nitrogen-containing compounds to the organic waste prior to step (2).
13. A feedstock for anaerobic digestion, the feedstock being in the liquid state and having a total viable count (TCG) according to ISO 4833-1:2013 of less than 5, the feedstock obtainable according to the method of any preceding claim.
14. A method of producing a biogas, the method comprising: pre-treating a feedstock using the method of any of claims ito 12; passing the feedstock to an anaerobic digester to produce biogas; and recovering the biogas.