GB2459881A

GB2459881A - Treating solid waste material

Info

Publication number: GB2459881A
Application number: GB0808381A
Authority: GB
Inventors: John Clare Boyce; John David Winders
Original assignee: BIOPLEX TECHNOLOGIES Ltd; C H Dobbie & Co Ltd
Current assignee: BIOPLEX TECHNOLOGIES Ltd
Priority date: 2008-05-09
Filing date: 2008-05-09
Publication date: 2009-11-11
Anticipated expiration: 2028-05-09
Also published as: GB0808381D0; GB2459881B

Abstract

A process for treating solid waste material comprises: a) preparing a slurry comprising particles of the solid waste material in a liquid carrier, the particles having sizes less than a predetermined maximum size; b) subjecting a decomposable fraction of the solid waste material to a fermentation process in a solids digestion vessel to produce a compostable 10 material and a fermentation gas comprising mainly carbon dioxide; c) before the fermentation gas from the slurry contains as much as 1% of methane by volume, removing liquid from the solids digestion vessel to leave a compostable fermented solid waste material; d) removing at least some of the compostable solid waste material from the solids digestion vessel; e) feeding at least some of the liquid from step (c) to a liquid digestion vessel; and f) subjecting the liquid in the liquid digestion vessel to anaerobic bacterial digestion to produce a liquid fraction containing active anaerobic bacteria and a methane-rich gas fraction. Also claimed is an apparatus for digesting a solids waste material comprising a mixing vessel, a solids digestion vessel, and a liquid digestion vessel. The mixing vessel is connected to the solids digestion vessel by a slurry pump. The pump may be a chopper pump, a self-cleaning pump or a macerating pump. As, little or no methane-producing anaerobic digestion occurs in the solid digestion vessel, there is no danger that of an explosive mixture being formed in the event that air is drawn into the solid digestion vessel.

Description

WASTE TREATMENT

BACKGROUND

a. Field of the Invention

Abattoirs, farms, food processing plants and the like regularly produce solid organic waste which needs to be treated in order to render it suitable for discharge into the environment. The present invention relates to a process and apparatus for treating solid waste materials.

b. Related Art One method (the Dobbie Process") which is known for digesting such waste is described in British patent number 2 230 004. This document describes a two stage digestion process which uses an installation comprising a fluids digestion vessel and a solids digestion vessel which are connected together. The solids digestion vessel is in the form of a tower or other fixture which is located in or on the ground, and the fluids digestion vessel is an adjacent tank. Bacterially active waste is fed from the fluids digestion vessel into the solids digestion vessel where it percolates through the solid content. The solid waste is subjected to anaerobic bacterial digestion for seven to 14 days to produce a methane rich gas fraction and a sohd fraction which is environmentally more acceptable than the feed solids.

The digested solids material may be used a soil conditioner.

The retention time of up to two weeks for solid waste in the solids digester limits the throughput of the apparatus for the Dobbie Process. A potential problem is that, in the event of a system failure, air may be drawn in to the methane-rich gas phase, forming a potentially explosive mixture in the confined space of the solids digestion vessel.

SUMMARY OF THE INVENTION

Aspects of the invention are specified in the independent claims. Preferred features are specified in the dependent claims.

We have found that by introducing solid waste into the solids digestion vessel in the form of a slurry with controlled particle size, and by subjecting that waste to fermentation in the solids digestion vessel, an environmentally acceptable compostable material can be produced within one to five days. The moist, compostable material can be removed for composting. At least some of the liquid phase is transferred to the liquid digestion vessel where it undergoes anaerobic digestion.

The process of the invention allows much faster throughput because the solids digestion vessel is ready to accept a new batch of solids waste material within five days.

Because little or no methane-producing anaerobic digestion occurs in the solids digestion vessel, there is no danger of an explosive mixture being formed in the event that air is drawn in to the solids digestion vessel. Moreover, the gaseous product of the fermentation is carbon dioxide, which can be vented to atmosphere.

By venting the solids digestion vessel to atmosphere, possible build-up of an explosive gas mixture is prevented.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be further described, by way of example only, with reference to the following drawings in which: Figures 1 and 2 are schematic views of apparatus for digesting solid waste in accordance with embodiments of the invention; Figure 3 illustrates part of an apparatus for digesting solid waste according to another embodiment of the invention; and Figure 4 is a schematic plan view according to a further embodiment of the invention.

DETAILED DESCRIPTION

The apparatus shown in Figure 1 comprises a mixing chamber, a solids digestion vessel, a liquid digestion vessel, a charge tank and associated connecting members for heating, fluid passage, and drainage.

The mixing chamber receives a feedstock of solids waste materials and liquor. In this example the liquor is fed from the liquid digestion vessel via the charge tank; however any other suitable source of bacterially active liquid could be used, either for make-up of a slurry in the mixing vessel or added to the slurry in the solids digestion vessel. The solids waste material may be any suitable waste, for example vegetable, kitchen, meat and garden waste, agricultural waste or autoclaved fibre.

The solids and liquor are mixed together and the solid materials are finely shredded using suitable means, for example a slurry tank wall mixer or specialist comminuter, to produce rotation and mixing of the contents. Alternatively, if the waste to be treated is finely chopped, a chopper pump or a self-cleaning or macerating pump could be used to mix the contents. The resulting slurry is pumped into the solids digestion vessel by a slurry pump. The slurry pump limits the maximum size of solid particles that can be pumped, in this example to a maximum size of about 15 mm, with a typical size range from about 5-15 mm. The slurry typically has a solids content of about 50-70% by weight.

The solids digestion vessel in this embodiment is designed to hold slurry to the brim and to provide access for loading and unloading. The sides, floor and roof are preferably fully insulated to retain heat. The roof may comprise an insulated panel or a floating sheet; optionally the top may be open to atmosphere, for example by use of a fabric or porous roof which allows evolved gases to escape.

The solids digestion vessel may be heated, for example by means of an internal or external heat exchanger, to ensure an optimum temperature for the fermentation process. The temperature may be thermostatically controlled. By heating the contents appropriately, the retention time needed to convert the waste into an environmentally acceptable condition may be reduced to as little as two days, although fermentation may be continued for up to five days.

In an example process, during Day 1 the mixing vessel is filled with 100 tonnes of comminuted solid waste material and 50-1 00 tonnes of liquor. At the end of Day 1, the slurry pump is started, to pump the slurry with a predetermined maximum particle size, into the solids digestion vessel. In this example the particle sizes range from about 5-15 mm. On Day 2, the contents of the solids digestion vessel are heated up. For food waste, UK DEFRA regulations require maintaining the waste at 70°C for one hour during digestion, or 57°C for seven hours.

Substantially no methane is produced during the solids digestion phase. Acid hydrolysis occurs at about pH 6.5, and fermentation which evolves carbon dioxide gas. The pH may optionally be altered to facilitate acid hydrolysis and speed up the process, for example by addition of acetic acid. The fermentation process may be continued for from one to five days, after which methanogenic anaerobic digestion will start to occur.

At the end of Day 2, or no later than when the concentration of methane in the gas evolved in the solids digestion vessel approaches 1% methane, the liquid is drained or pumped from the solids digestion vessel. At least some of this liquid is fed into the adjacent liquid digestion vessel where it will undergo methanogenic anaerobic digestion in a manner well known per Se. Methane (biogas) from the liquid digestion vessel may be stored, for example, in a gasometer or used as a local fuel source. In a preferred embodiment the biogas is burned to heat liquid for the mixing vessel, the solids digestion vessel, or the liquid digestion vessel. The liquid in the liquid digestion vessel is preferably maintained at about 35�3°C. The liquid in the liquid digestion vessel typically has a pH from about 7-8, notably about 7.5. In addition to a fluid inlet from the solids digestion vessel, the liquid digestion vessel has an outlet for overflow liquid to pass to the charge tank. Both the inlet and the outlet are open to the air to prevent siphoning.

After draining off the excess of liquid, the remaining solids are a moist, fermented, compostable material. The example material weighs about 120 tonnes including moisture. The treated solids may be unloaded by pump, conveyor and/or a loading shovel (eg a suitably sized JCBTM) via a ramp or doors from the solids digestion vessel. The unloaded fermented solids material may then be taken to beds or windrows for composting, typically for a further 18 days or more (Days 3-21). The resulting composted material is a biofertiliser with a substantial quantity of retained nitrogen. It is suitable for use as organic fertiliser-rich fibre for soil conditioning and as a growing medium.

In the embodiment illustrated in Figure 2, liquor from the liquid digestion vessel is fed into the solids digestion vessel without being used to make up the slurry which is fed to the solids digestion vessel. The liquor provides an inoculum of suitable bacteria to ensure that the fermentation process occurs. To ensure that the inoculum is suitably dispersed, it is preferred that the solids digestion vessel is provided with agitation means (not shown); for example a paddle or a pump for recycling fluid from the bottom to the top of the vessel.

Such agitation means are also useful for other embodiments of the invention, for preventing or reducing separation or stratification of the contents of the solids digestion vessel. Typically, unless the contents are agitated, a scum builds up at the top end and the efficiency of fermentation is reduced.

In the embodiment shown in Figure 3, a solid, waste feedstock from a hopper is mixed with make-up water (typically from the liquid digestion vessel) and broken up in a comminutor. The slurry from the comminutor is held in a holding tank and pumped to the solids digestion vessel by a slurry pump. The solids digestion vessel has a side opening near the top, through which fermented solid waste material may be removed for composting, optionally after dewatering in a centrifuge or press. The opening is covered by a moveable weir during the fermentation process. A moveable curtain is located in the solids digestion vessel, and in this example depends from a float. Suitable means (in this example, a winch) is provided for moving the curtain across the solids digestion vessel towards the exit opening to push fermented solids waste material out of the solids digestion vessel. A pump is located at the bottom end of the solids digestion vessel for pumping liquid to the liquid digestion vessel. Solids will be collected via a cyclone trap or gulley. Thus all three components: fermented fibrous material, liquor, and grit particles, may be removed separately.

Here, the floating compostable solids may be removed prior to removing liquid from the solids digestion vessel. Alternatively, at least some of the liquid may be drained prior to removal of the solids via a weir, providing that a suitably sealed side exit is provided at the level where the solids are to be removed from, the seal being removable when solids are to be removed. It will therefore be understood that these process steps (removal of at least some liquid and removal of at least some of the compostable solids) may be carried out in either order.

After the floating compostable material has been removed via the weir, the solids digestion vessel typically has a semi-liquid phase on top of solid material at the bottom of the vessel. In a preferred embodiment, some or all of this remaining solid material may be removed by recirculating the liquid so as to reform a slurry with the solid material, and using a pump at the base of the solids digestion vessel to pump this slurry from the vessel, preferably to a de-watering station prior to composting.

The process may be operated with any number of solids digestion vessels and/or liquid digestion vessels. The system illustrated in Figure 4 uses two solids digestion vessels and a liquid digestion vessel. Typically, the solids digestion vessels will be made of insulated steel or concrete. The operator ensures that all the right connections are in place and starts the process on a control panel in the Control/Plant Room. In this example, liquid is pumped from the charge tank into the solids digestion vessels and mixed with a slurry of macerated, comminuted solid waste material. Agitation means may be operated within the solids digestion vessels to ensure that heat, microbes and nutrients are thoroughly distributed, and to help break up any agglomerated particles in the slurry. If biogas is available from the liquid digester or biogas storage gasometer, this will be used as a primary source of fuel for heating water; otherwise other heating means will be used. The system may switch between alternative heating means as and when required during the process.

Hot water is supplied to a heat exchanger (not shown) within the solids digestion vessels until the contents reach a predetermined temperature. Typically this will be 57°C and maintained for at least seven hours, or 70°C and maintained for at least one hour. The temperature in each vessel is monitored at various locations to ensure that all contents have reached the desired temperature; for example the temperature may be monitored by three temperature sensors per vessel. After heating the contents of one solids digestion vessel it will be possible to disconnect the heating water and liquid digester heating circuits and reconnect to a further solids digestion vessel to commence the heating cycle there. The control panel will indicate this status and temperatures will be monitored and logged within the control system. Thus the fermentation reactions in each solids digestion vessel may be completed together or they may be operated alternately for a more even production of corn postable material.

Throughout this process, the liquid in the liquid digestion vessel is preferably maintained at its optimum operating temperature range (35�3°C).

The program will be complete when the process has continued for a predetermined time after achieving the target temperature, for example three days overall and the contents of the solids digestion vessels have cooled to a predetermined safe handling temperature, for example 40-50°C. The control panel will indicate when the program is complete.

Following completion of the temperature cycle, the liquid content of the solids digestion vessel(s) will be pumped to the liquid digestion vessel. The fermented solids material will be removed, optionally de-watered using a centrifuge or press, and composted.

In one preferred embodiment, the control system will store the following data for each processed load: * date * program/waste type * temperature data log * operation of agitator * state of level switches * any warnings that occur during the process.

Solids Digestion Vessel(s) In one embodiment, the or each solids digestion vessel will incorporate some or all of the following features: * based upon bulk material storage bunkers * watertight door or ramp access * removable insulated floating roof * base could include pipe heat exchanger * pipes set in the walls angled downwards will provide a method of washing down any remaining solids to the pumps; water may be provided by a pump * electric driven agitator or chopper pump or pto-driven agitator depending on the material to be treated * inset into the vessel at suitable locations shall be pockets for integrated temperature sensors (3 probes); the probes may be removable and supplied separately * an appropriate level sensor (or mechanical switch) will be incorporated.

Liquid Digestion Vessel(s) In one embodiment the or each liquid digestion vessel will incorporate some or all of the following features: * preferably situated below ground to provide structural support and insulation * 3m3 capacity per 1 m3 solids digestion vessel volume * mild steel with twin pack epoxy lining; stainless steel, concrete or fibreglass could be used * built in level sensor * built in temperature sensor * heat exchanger liquid overflow tank (0.5rn3 per I rn3 solids digestion vessel volume * external connectors: -liquid fill -10- -liquid return -heating water send -heating water return -control cable (sensors).

Plant Room In one embodiment the plant room will incorporate some or all of the following features: * housed in a standard 20 ft or 40 ft ISO container for ease of transportation and loading and unloading * cargo doors to be modified to include a pedestrian access door in one cargo door leaf * bunded oil tank (10,000 L, approx 3 days max heating) * oil fired boiler or boiler fired by biogas from the process * hotwater pump * hot water tank/heat exchanger * boiler flues * interconnecting plumbing including heating water pumps, manifold and valves etc * hot water expansion tank * liquid digestion vessel liquid pump (stainless steel mono pump for secondary heating circuit) * a piped washdown water pumped system * external connectors: -mains power in -mains power out -liquid digestion vessel liquid in (secondary heating circuit) -liquid digestion vessel liquid out -heating water send -11 - -heating water return -control cable (sensors) * the external connections to the solids digestion vessel could be moved manually between units between cycles * the control panel will be designed to be simple to operate with the operator interface being limited to a series of selection switches, push buttons and status indicator lamps * interior lighting * mains sockets with RCCD protection Gasometer In one embodiment the gasometer (biogas storage container) will incorporate some or all of the following features: * lOOm3 capacity * "floating cylinder" construction or flexible membrane * operating pressure 10 inches water gauge * low/high volume sensor(s).

The invention provides a new pre-composter and anaerobic fermenter for the treatment of a wide range of waste materials including farm waste, food and catering wastes, trade, garden and local authority wastes. The process and plant are designed to be easy to operate, low maintenance, relatively odourless, clean and compact. The vessels are preferably enclosed to prevent cross-contamination and disease risk from birds, rodents and insects.

The by-product of the process is a fermented and compostable solid material which, when composted, can be used as an organic fertiliser-rich fibre for soil conditioning and as a growing medium. The biogas produced during he process may be recycled and can represent a large proportion of the total energy -12-requirement for the system.

The articles, a' and an' are used herein to denote at least one' unless the context otherwise dictates.

It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately, or in any suitable combination.

It is to be recognized that various alterations, modifications, and/or additions may be introduced into the constructions and arrangements of parts described above without departing from the ambit of the present invention as specified in the claims.

Claims

CLAIMS1. A process for treating solid waste material, which process comprises: a) preparing a slurry comprising particles of the solid waste material in a liquid carrier, the particles having sizes less than a predetermined maximum size; b) subjecting a decomposable fraction of the solid waste material to a fermentation process in a solids digestion vessel to produce a compostable material and a fermentation gas comprising mainly carbon dioxide; c) before the fermentation gas from the slurry contains as much as 1% of methane by volume, removing at least some of the liquid from the solids digestion vessel to leave a compostable fermented solid waste material; d) removing at least some of the compostable solid waste material from the solids digestion vessel; e) feeding at least some of the liquid from step (c) to a liquid digestion vessel; and f) subjecting the liquid in the liquid digestion vessel to anaerobic bacterial digestion to produce a liquid fraction containing active anaerobic bacteria and a methane-rich gas fraction.
2. A process according to claim 1, further comprising: subjecting the compostable solid waste material removed from the solids digestion vessel to a composting process to produce a composted material suitable for use as a soil conditioner.
3. A process according to claim I or claim 2, further comprising using at least some of the liquid fraction from the liquid digestion vessel in the preparation of the slurry.
4. A process according to any preceding claim, wherein the decomposable fraction of the solid waste material undergoes acid hydrolysis prior to or during the fermentation process.

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5. A process according to claim 4, wherein the pH of liquid in the solids digestion vessel is maintained at a value between 6 and 7.
6. A process according to any preceding claim, wherein the pH of liquid in the liquid digestion vessel is maintained at a value between 7 and 8.
7. A process according to any preceding claim, wherein the maximum size of the particles of the solid waste material is 15mm.
8. A process according to any preceding claim, wherein the slurry is prepared by mixing solid waste materials and a liquid in a mixing vessel and pumping the mixture to the solids digestion vessel using a comminuter or slurry pump which breaks up particles of the solid waste material larger than the predetermined maximum size.
9. A process according to claim 8, wherein the slurry pump is a chopper pump, or a self-cleaning or macerating pump.
10. A process according to any preceding claim, wherein most of the mass of the solid waste material comprises particles having a size in the range 5-15 mm.
11. A process according to any preceding claim, further comprising agitating the slurry in the solids digestion vessel.
12. A process according to claim 11, wherein the slurry is agitated by recycling liquid from a lower part of the vessel to an upper part of the vessel.
13. A process according to claim 12, wherein the recycling is carried out by means of a chopper pump, or a self-cleaning or macerating pump.
14. A process according to any preceding claim, wherein the weight ratio of solids to liquids in the slurry fed to the solids digestion vessel is in the range 50:50 -15-to 70:30.
15. A process according to any preceding claim, wherein the solids digestion vessel is open to the atmosphere during the fermentation process.
16. A process according to any preceding claim, wherein the fermentation step (b) is carried out for from two to three days.
17. A process according to any preceding claim, wherein the step of removing at least some of the compostable solid waste material from the solids digestion vessel comprises using a moving curtain to push floating solid waste material out of the solids digestion vessel.
18. A process according to claim 17, further comprising reforming a slurry from liquid remaining in the solids digestion vessel and solids at or near the bottom of the vessel, and pumping the reformed slurry out of the solids digestion vessel.
19. Apparatus for digesting a solids waste material, comprising a mixing vessel, a solids digestion vessel, and a liquid digestion vessel; the mixing vessel being connected to the solids digestion vessel by a slurry pump, and the solids digestion vessel having a fluid connection to the liquid digestion vessel for transferring liquid from the solids digestion vessel to the liquid digestion vessel.
20. Apparatus according to claim 19, further comprising a fluid connection for transferring liquid from the liquid digestion vessel to the mixing vessel.
21. Apparatus according to claim 19 or claim 20, further comprising a pump for recycling liquid from a lower part of the solids digestion vessel to an upper part of the solids digestion vessel.
22. Apparatus according to claim 21 wherein the pump is a chopper pump, or a self-cleaning or macerating pump. -16-
23. Apparatus for digesting a solid waste material, substantially as herein described with reference to the drawings.
24. A process for digesting solid waste material, substantially as herein described with reference to the drawings.