GB2509312A - Pathogen reduction in anaerobic digestate - Google Patents

Abstract

An apparatus for pathogen reduction in anaerobic digestate comprises an anaerobic digester 1 which produces biogas 2, a boiler or combined heat and power (CHP) plant 3 which combusts the biogas produced by the anaerobic digester and a sanitization vessel 9 which receives solid state digestate 10 from the anaerobic digester. The combustion of the biogas produces hot water and hot gases which are used to heat treat the digestate in the sanitization vessel. Valves 8 control the input of heated gas in accordance with a computer controlled protocol, whilst valves 24 control the input of heated water to pipes within the walls and floor 19 of the sanitization vessel. A computerised control system 23 receives information from a plurality of sensors, e.g. gas sensors and temperature sensors, and utilises this information to control pumps, blowers, valves and compressors which are required to operate the sanitization vessel. A method of reducing the pathogen content of a solid state anaerobic digestate is also claimed.

Description

TITLE:

APPARATUS FOR ACHIEVING 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 bioTogical waste processing. In particuTar the invention relates to a new method of sanitizing the output from anaerobic digestion.

BACKROUND

The present invention relates to heat 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. F. co/i, 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 environmentaT 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 typicaTly 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 EPA 503 rule in the USA. Specifically, the system can be configured to achieve specific Togarithmic 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. <4ft'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 56°C, 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.

The standard mechanism of sanitizing bioTogicaT 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 (bin-waste) and animal dung mixed with bedding (straw/wood shavings) are typicaTly non-pumpable and are not directly suited to standard sanitization techniques as described. Therefore, they must be rendered pumpabTe 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-p umpable substrates prior to anaerobic digestion. This biogenic self-heating or auto-thermic aerobic pre-treatment method is a well-recognized method of bringing pathogens under control as seen in U54139640, for example, where high temperatures can be readiTy achieved in most solid state bioTogical 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 sartitised 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 W02012062340 & CA 2592214A1 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 bioTogical material is tvpicaTly added to ensure that sufficientTy 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 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 the 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 specifics 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-watering 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-p umpable 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 soTid 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 boiTers 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 the 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 (FF2275763 & DF102008047411A1) 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 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 the 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 ABPR), 65°C for 7 days (UK PM 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 the solid state sanitization vessel wilT resuTt in measurable reductions in target pathogen or indicator organism concentrations to below set standards, e.g. <1,000 MPN (most probabTe number) of F. cvii and/or reducing Suitnonelia to undetectabTe 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 thermaT 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-p umpable) digestate from anaerobic digestion systems in a static vessel.

BRIEF DESCRIPTION OF THE PROCESS

Biogas that is produced during the anaerobic digestion process is typicaTly used to fuel a device such as a bofler 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 sanifization 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 materiaT 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 walls and floor of the vesseT 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 whoTe of the mass, the computer wilT modulate the heat flow via actuated flow control valves attached to the gas and water Tines 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.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more clearly understood from the following description of an embodiment thereof, given by way of exampTe only, with reference to the accompanying drawings, in which:-Figure 1. A schematic side on view diagram of the physical embodiment of the design showing the 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 vesseT 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 (CHIP) plant for the generation of renewable eTectricity 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 wilT have a temperature of 80-98°C.

(5) Hot exhaust output The hot gaseous exhaust will have a temperature of 15O450°C.

(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.

(8) Actuated valve gas input The input of the heated gas to the sanitization vessel is controTled 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 typicaTly an enclosed concrete or steel chamber or bay within which the solid state digestate is stacked. The floor of the sanitization vessel is fined with vents or channels to allow the press urised 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 control. This may be a single vessel or multiple units. The 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 re-circulated exhaust from the sanitization vessel.

(12) Pressurised hot gas input The blended mixture of hot and re-circuTated 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 the digestate stacked on the ventilation fToor 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 The exhaust in the head space of the sanitization vessel is drawn off for re-use or discharge.

(16) Re-circulation 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.

(18) Exhaust to odour abatement system! atmosphere The exhaust from the sanitization vessel is typicalTy 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 aTlows 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 vesseT is monitored.

(22) Digestate temperature monitoring The temperature of the digestate within the sartitization 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 controTled 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 vaTve that is operated in accordance with a computer controlled temperature feed-back protocol.