IE20120554A1 - Apparatus for achieving pathogen reduction in solid state anaerobic digestate utilizing process heat - Google Patents

Abstract

This invention allows for the use of heat that is generated during the combustion of the biogas produced during anaerobic digestion to effect a targeted reduction in the pathogen concentrations of the resultant solid state digestate under controlled conditions in a separate static sealed vessel. Heat from the flue gas and hot water that is emitted from the combined heat and power generators and/or boilers connected to the anaerobic digestions system is used directly or indirectly to facilitate a targeted and specific increase in the temperature evenly throughout the static digestable product in a controlled manner. This results in a controlled and measureable reduction in pathogen concentrations in the static solid matrix. Typically, the hot water and hot exhaust are applied in combination to heat the whole of the solid state digestate without pumping and/or stirring of the solid substrate. The heating apparatus is automatically adjusted to control the amount of heat that is utilised in achieving target temperature development within the substrate. This heating is sustained for prescribed periods and/or until pathogen concentrations are reduced down to within prescribed limits or by a prescribed amount. The sanitised substrate can be certified as treated or sanitised and can be released for use.

Description

APPARATUS' FUR ACAib'miG PATHOGEN REDUCTION IN SOLID STATE ANAEROBIC DIGESTATE UTILISING PROCESS HEAT FIELD OF THE INVENTION The present invention relates to pathogen control in material produced during biological waste processing. In particular the invention relates io a new method of sanitizing the output from anaerobic digestion.

BACKROUND The present invention relates to heal treatment resulting in a measurable pathogen reduction associated with, but not limited to, anaerobic digestion where a solid state (non-pumpable) residue is produced. Solid state biological substrates of animal and vegetable origin such as bio-waste and manure contain pathogens of concern to human, animal and plant health, e.g. E. coli, Salmonella, foot and mouth and club root disease. When these biological substrates are anaerobically digested, there are concerns that the products of this process may still contain pathogens at concentrations that can represent an environmental health risk. As a result, mechanisms must be used to reduce or remove the pathogens from the material to allow for safe subsequent use. This typically includes heat treatment in accordance with the time and temperature criteria dictated by the environmental laws of the Country where the facility is located. Examples include the EU animal by-product regulation (ABPR), the RAL standards in Germany, the PAS 100 and 110 British Standards in the UK and the F.PA 503 rule in the USA. Specifically, the system can be configured to achieve specific logarithmic reductions in microbial pathogen concentrations and indicator organisms such as faecal coliforms and faecal streptococci etc and or to achieve a specific pathogen concentration concentration that is considered to be safe for subsequent use.

In the field of anaerobic digestion, while evidence has been presented that the digestion process itself can result in the reduction in pathogen concentrations when operated at mesophyllic temperature, i.e, <40°C, (e.g. US20100297740), this is typically not accepted in many jurisdictions where additional heat treatment is typically prescribed. In addition, even those anaerobic digestion systems that operate in the thermophyllic range, i.e. up to Sb’G, will not be acceptable where the temperature requirement is above this, e.g. the 70°C set point as is the case of the EU ABPR. Therefore, as anaerobic digestion methodology itself cannot always ensure the requisite destruction of pathogens, the substrate must either be heat treated before anaerobic digestion or thereafter. - r r ί·.τ 1 Σ7 J. ..........

C.O 2_ Ct < ιΑ oo (fa tel 2 0554 The standard mechanism of sanitizing biological material prior to anaerobic digestion is to liquefy the substrate into a suspension and heat the suspension as a pumpable fluid within a vessel using an external heating jacket while agitating the material through either stirring or pumping such as described for example in US 4511370 & US 8110106. Thereafter, the heat treated material can be pumped to the anaerobic digestion tank where biogas production is facilitated* This is widely practiced at 'wet anaerobic digestion' facilities where pathogen control is required for pumpable fluid substrates such as slurry and sludge.

High solid substrates such as municipal solid waste, co-mingled food and green waste (bio-waste) and animal dung mixed with bedding (straw/wood shavings) are typically non-pumpable and arc not directly suited to standard sanitization techniques as described. Therefore, they must be rendered pumpable by the addition of water allied to the removable of non-pumpable elements e.g. wood, plastic, glass etc. This is not ideal as a large volume of water may need to be added and sophisticated energy intensive pre-treatment may also be required to remove the contaminants that would otherwise block the pumping systems.

One option that can be applied in such circumstances is to facilitate high temperature (thermophyllic) pre-composting of these solid, non-pumpable substrates prior to anaerobic digestion. This biogenie self-heating or auto-thermic aerobic pre-treatment method is a well-recognized method of bringing pathogens under control as seen in US4139640, for example, where high temperatures can be readily achieved in most solid state biological materials. However, this is not ideal as this high rate aerobic step invariably results in a significant loss in the biogas potential of the substrate in the subsequent anaerobic digestion process.

As an alternative, the substrates can be sanitised after anaerobic digestion to ensure pathogen destruction while conserving the energy in the biogas. Methods of achieving this sanitization again typically involve standard post-composting of the high solids digestate as seen in WO201206234fl & CA 2592214Al where the increase in temperature can result in pathogen reductions. However, just as pre-composting removes energy from biological material prior to anaerobic digestion, pre-digestion substantially reduces the capacity of the output digestate to generate auto-thermic biogenic heat thereafter. Specifically, the biogenic self-heating potential of the digestate is greatly diminished as a result of the previous anaerobic digestion step that removes much of the energy from the biological material. Consequently, additional fresh biological material is typically added to ensure that sufficiently high temperatures can be attained and sustained in order to meet the statutory time and temperature criteria. This however results in material by-passing the anaerobic IE 1 2 0 5 5 4 digestion phase with a consequential loss of biogas potential. Alternatively, additional biogenic materials must be imported thus creating further energy inefficiencies.

Many examples of enclosed or in-vessel composting systems that rely on biogenic self-heating to achieve pathogen destructive temperatures are evident in tbe literature. Examples in the field of waste management would include where there is a complete reliance on biogenic heat to effect sanitization while in the field of mushroom compost production (US 4233266) external steam generation is utilised to achieve sterilization in accordance with the specifies of mushroom growing substrate criteria. None of these apply to anaerobic digestion systems where biogas derived heat is utilised to achieve a targeted and measured reduction in the pathogen concentrations.

Therefore, an alternative is required to provide dependable, measurable and controlled sanitization of the non-pumpable digestate that is generated from anaerobic digestion systems. These solid state non-pumpable digestates are either generated through the de-water ing of pumpable substrates from wet anaerobic digestion systems or derived in whole from dry anaerobic digestion systems where the material is digested as a non-pumpable solid.

This invention, which is not yet practiced, will utilize the excess heat that is generated during the combustion of the biogas produced during the anaerobic digestion process for the specific purposes of sanitizing the solid state digestate output within an adjacent static sanitization vessel. This heat can be derived from the flue gas and / or hot water that are emitted from either the combined heat and power (CHP) generators or boilers that are connected to the anaerobic digestion apparatus. This heat can be used directly or indirectly via heat exchangers to facilitate a homogenous and targeted increase in the temperature of the solid state digestate in accordance with the time / temperature requirements necessary to effect a logarithmic reduction in pathogen concentrations as prescribed by the regulations within the Country of operation or as required by the operator.

In Ihe specific field of anaerobic digestion, there are two examples where it has been claimed that heated air from a biogas system can be used to modify the digestate output. In both cases (EP2275763 & DE102008047411AI) the claims are restricted to the use of indirectly heated air to specifically remove moisture from the waste digestate to effect bulk reduction. However, neither of these embodiments is configured or claims to achieve controlled pathogen reduction through the IE 1 2 0 5 54 attainment and control of precise temperature targets throughout the solid substrate mass using both a combination of process gases and heated water.

The current invention represents a significant innovation in the utilisation of heat from the combustion of biogas derived from anaerobic digestion for the sole purpose of achieving controlled and targeted pathogen destruction in die resultant solid state digestate. Accurate computer control of the ratio and volume of harvested heated fluids (water and process gases) will achieve consistently high temperatures throughout the substrate as prescribed by the competent authorities, e.g. 70°C for 1 hour (EU ΑΒΡΕ/ 65°C for 7 days (UK PAS 100) or 55°C for 72 hours (USEPA 503 rule) or as required by the operator, legislator or risk assessment. This precise temperature control throughout the static substrate within the confines of fhe solid state sanitization vessel will result in measurable reductions in target pathogen or indicator organism concentrations to below set standards, e.g. <1,000 MPN (most probable number) of E. coli and/or reducing Salmonella to undetectable levels. Alternatively, the apparatus can be configured to achieve other end-point pathogen concentration targets or logarithmic reductions required. These end points can be achieved because this apparatus and method, unlike prior art versions, is neither a thermal drying system nor a composting system but a dedicated optimised pathogen reduction system designed and built for the specific purposes of sanitising solid state (non-pumpable) digestate from anaerobic digestion systems in a static vessel.

IE ? 20 5 5 4 BRIEF DESCRIPTION OF THE PROCESS Biogas that is produced during the anaerobic digestion process is typically used to fuel a device such as a boiler and/or a CHP plant for the purposes of generating renewable energy in the form of electricity and or heat. In this invention, excess hot flue gas and hot water from this process are conveyed via insulated gas and water pipes to a control centre adjacent to a sanitization vessel as described.

At the end of the anaerobic digestion process solid state digestate can be extracted from the digestion vessel and this is loaded into the sanitization vessel by loading shovel and/or conveyor through a door or hatch. This may occur immediately or after a period of mechanical de-watering, standing or composting and with or without the addition of amendment material that may be required to facilitate the passage of gases through the static mass. The material is typically formed into an even heap or loaf within the vessel. Once the door or hatch is shut the wails and floor of the vessel are heated utilising the hot water as described while the heated gases are circulated through the mass by the blowers. Excess gases will typically be discharged to the atmosphere via an odour abatement system. This application of combined hot water and process gas to the static digestate mass will result in a gradual increase in temperature where the heat flows are controlled by computer, micro-controller and/or programmable logic controllers (PLCs) using input data from oxygen, pressure and gas sensors and instruments such as temperature probes and thermometers and flow meters. When a given temperature set point is achieved across the whole of the mass, the computer will modulate the heat flow via actuated flow control valves attached to the gas and water lines to maintain the target temperature for a defined period of time. Once the required time and temperature regime has been achieved the heating can be decreased and possibly turned off to allow the mass to cool whereupon the material can be removed from the vessel. It is possible to sample the material within the vessel for pathogen concentrations or thereafter once the material has been removed to verify the pathogen reduction.

IE '> 2 Ο 5 5 4 BRIEF DESCRIPTION OFTHE DRAWINGS The invention will be more clearly understood from the following description of an embodiment thereof, given by way of example only, with reference to the accompanying drawings/ in which:5 Figure 1. A schematic side on view diagram of the physical embodiment of the design showing tire general arrangement and assembly of the main components of the invention.

Figure 2. A schematic side on view diagram showing the combined heat flow through the digestate within the sanitization vessel.

DETAILED DESCRIPTION OF THE DRAWINGS (1} The Anaerobic Digestion Vessel This is the anaerobic digestion vessel that may be of any kind that produces a solid state digestate product through either physical separation or directly. This can be a single unit or multiple vessels. (2} Biogas output The biogas that is generated by the anaerobic digestion system is directed to the biogas combustion devise, (3) Biogas combustion device (boiler or CHP plant) The biogas with a methane content of >45% is combustible. This combustibility can be utilised in a boiler to produce hot water and/or air or within a combined heat and power (CHP) plant for the generation of renewable electricity as well as hot water or air. Both biogas utilization mechanisms generate hot water and hot air. (4) Hot water output The hot water output from the biogas combustion device via a heating jacket heat exchanger will have a temperature of 80-98°C. (5) Hot exhaust output The hot gaseous exhaust will have a temperature of 150-450<λ (6) Hot exhaust to atmosphere Excess hot exhaust from the biogas combustion process is discharged to atmosphere through a stack. (7) Heat exchanger The utilisation of the combustion flue gas can either be used directly or through the utilisation of an air to air heat exchanger.

IE1 2 Ο 5 5 4 (8) Actuated valve gas input The input of the heated gas bo the sanitization vessel is controlled by an actuated valve that is operated in accordance with a computer controller operated on a temperature feed-back protocol. (9) Sanitization vessel The sanitisation vessel is typically an enclosed concrete or steel chamber or bay within which the solid state digestate is stacked. The floor of the sanitization vessel is fitted with vents or channels to allow the pressurised introduction of the heated gas. The floor and/or walls are fitted with embedded water pipes or chambers that convey the heated water under computer controi. This may be a single vessel or multiple units, i'he dimensions of the vessel will vary depending on the scale of the application. (10) Solid state digestate within the sanitization vessel The solid state digestate from the anaerobic digestion processing is stacked within the vessel for sanitization. (11) Input Blower / compressor The input blower or compressor mixes the incoming hot exhaust with recirculated exhaust from the sanitization vessel. (12) Pressurised hot gas input The blended mixture of hot and re-circulated exhaust is pressurised and blown into the interface channels / vents within the floor of the sanitization vessel. (13) Interface channel / vents The hot blended exhaust is forced into the interstitial spaces within tire digestate stacked on the ventilation floor of the sanitization vessel. This results in a transfer of heat from the input exhaust gas to the digestate. This leads to an increase in the temperature throughout the material over time. (14) Headspace of the sanitation vessel The gas that is pushed through the solid digestate substrate is captured in the headspace of the vessel. (15) Exhaust output l he exhaust in the head space of the sanitization vessel is drawn off for re-use or discharge. (16) Re-c irculation blower / compressor A second blower or compressor is involved in the vacuum removal of the exhaust gases from the headspace of the sanitization vessel. This blower / compressor directs the exhaust for re-circulation within the sanitization vessel or discharges it to atmosphere. (17) Re-circulation valve This valve controls the ratio of re-circulated exhaust to biogas combustion gas and thus the proportion vented to atmosphere. ίΕ; 2. 3 5 54 (18) Exhaust to odour abatement system / atmosphere The exhaust from the sanitization vessel is typically discharged to atmosphere via an odour abatement system such as a biofilter and / or scrubber. (19) Heating pipes in the floor and walls of the sanitization vessel.

The hot water pipes or chambers within the floor and walls of the vessel allow hot water to be circulated through the containment vessel and thus allows the heat to be transferred to the digestate contained therein. The passage of the hot exhaust through the digestate effects the even distribution of this heat throughout the digestate mass thus ensuring even temperature development. (20) Gas input and output monitoring The composition of the gases within circulation in the sanitization vessel is monitored for temperature, relative humidity and oxygen and other parameters as required. (21) Water input and output monitoring The temperature and flow of the hot water to the vessel is monitored. (22) Digestate temperature monitoring The temperature of the digestate within the sanitization vessel is continuously monitored at multiple locations by computer to track the development of temperatures within the vessel to record time and temperature data for regulatory compliance. (23) Computer control The interaction of the hot water and hot gas in effecting controlled temperature development within the sanitization vessel is automatically monitored and adjusted by computer so that accurate temperature development can be achieved and maintained in accordance with the required time and temperature set points. (24) Actuated valve-water input The input of the heated water to the sanitization vessel is controlled by an actuated valve that is operated in accordance with a computer controlled temperature feed-back protocol.

Claims (1)

1. CLAIMS Claim 1; An apparatus for achieving controlled logarithmic pathogen reduction in solid state anaerobic digestate utilizing anaerobic digestion process heat comprising: a) An anaerobic digestion system (1) where biogas is produced (2) and combusted to generate energy; 5 b) A boiler or CHP plant (3) for combustion of biogas produced by the anaerobic digestion system whereby heat is generated in the form of hot water and hot process gases; c) A sanitization vessel (9) into which the solid state digestate is loaded (10) for heat treatment under static conditions to effect targeted pathogen reduction; 10 d) Actuated process gas input valves (8) that control the input of heated process gases, in accordance with a computer controlled (23) temperature feed-back protocol; e) Actuated hot water intake valves (24) that control the input of heated water to hot water pipes within the walls and floor of the sanitisation vessel (9). 15 f) A dedicated computerised control system (23) that receives information from flow meters, pressure sensors, gas sensors, oxygen sensors and temperature sensors and utilises these data to control the pumps, blowers, actuated valves, compressors required to operate the sanitization vessel. Claim. 2: 20 An apparatus according to claim 1 wherein the solid state sanitization vessel (9) is deployed as a single unit of various volumes or as multiple modules that operates in parallel or scries. Claim 3: An apparatus according to either of claims 1 or 2 wherein the Sanitization vessel (9) 25 has heating pipes or chambers in the floor and walls (19) through which the hot water is conveyed. Claim 4: The apparatus according to claim 3 wherein the vessel is also ventilated utilising blowers or compressors (11) operating in series that effect the circulation of the 30 heated process gases through the digestate mass held within the vessel. ί 2 ΰ 5 -4 Claim 5: The apparatus according to any preceding claims wherein the Sanitization vessel (9) is fitted with temperature monitoring systems and with a computer control system (23) that can accurately regulate the development of temperature through the 5 targeted provision of hot water and hot process gases. Claim 6: The apparatus according to any preceding claim wherein the Sanitization vessel (9) is capable of maintaining a temperature throughout tire digestate of up to 85°C for up to 7 days. Claim 7: The apparatus according to any preceding claim wherein the Sanitization vessel (9) is capable of achieved >2.5 log reduction in the concentration of certain pathogens such as E. coli, Salmonella etc. Claim 8: A method of reducing the pathogen content of a solid state anaerobic digestate utilising process heat, the method comprising: a) The removal of solid state digestate from a digester at the end of the anaerobic 20 digestion process wherein the digestate solids are mechanically extracted, pre-stored or transferred directly to and; b) The loading of the digestate solids into the Sanitization vessel by loading shovel, conveyor or other mechanical means; c) The dosing of the Sanitization vessel door or hatch and the heating of the 25 walls and floor of the vessel utilising the hot water from the boiler/CHP plant while heated gases are circulated through the mass; d) The attainment of a sustained increase in the temperature, where the heat flow is controlled by computer; e) The modulation of the heat How via actuated flow control valves to maintain 30 the required temperature for a defined period of lime; f) The cooling of the mass whereupon the material can be removed from the vessel. g) The sampling of the material within the Sanitization vessel or when it has been removed to verify the pathogen reduction.