GB2407088A - Anaerobic waste treatment process and apparatus - Google Patents

Waste Treatment Process and Apparatus The present invention relates to a

waste treatment process for treating waste material, preferably to a process for treating waste material from livestock and to apparatus for carrying out the process.

Farms, abattoirs, food processing plants and the like regularly produce organic waste that must be treated to render it acceptable for discharge into the environment.

One method for treating such waste is digestion and a known process 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 until at least some of the solids have been sufficiently digested to be environmentally acceptable for discharge. The fluid is returned to the fluids digestion vessel and the digested solids are then removed, perhaps for use as a soil conditioning agent, and a further batch of solids waste is added to the solids digestion vessel.

This is a batch process and typically the digestion stage takes over 5 days to proceed to a stage at which the waste has been rendered acceptable for discharge. It has also been noted that the drained treated solid waste still has a substantial liquid waste content which may contain liquid waste that has not been completely digested.

It is an object of the present invention to provide a process for digesting solid waste material, and apparatus for use in carrying out the process that addresses some of the above issues.

According to the invention there is provided a process for treating solid waste, the process comprising: a) transferring the solid waste to at least one solids digestion vessel; b) providing a series of liquid digester tanks including a first liquid digester tank and at least one subsequent liquid digester tank, the series of liquid digester tanks containing a source of organic material undergoing anaerobic bacterial digestion in a fluid phase digestion stage to produce a first fluid fraction containing active bacteria in at least the subsequent liquid digester tank; c) feeding at least a portion of said fluid fraction containing active bacteria into the solids digestion vessel to begin a solids digestion phase to create treated solid waste; d) draining the liquid from the solids digestion vessel to end the solids digestion phase; - 3 e) transferring the treated solid waste from the solids digestion vessel to a de-watering unit in which further liquid is removed from the treated solid waste to create de-watered treated solid waste; f) the further liquid removed from the treated solid waste in the de-watering unit is transferred to a liquid digester tank containing a source of organic material undergoing anaerobic bacterial digestion; and g) drawing from a lower portion of the subsequent liquid digestion tank a second fluid fraction for use as a fertilizer.

Such a process provides two useful and potentially commercially valuable products, a liquid product for use as a fertiliser and a solid product being the dried treated solid waste that can be used as a fuel or as a soil treatment. Preferably there is also a gaseous product, a methane and carbon dioxide gas which is known as biogas that can be used as a fuel that is generated in the liquid digester tanks during liquid waste digestion.

The liquid may be used as a liquid fertiliser, or it may be dried and powdered or pelletised for use as a solid fertilizer.

The solid and gaseous products are preferably used as fuels on site to generate heat and power that may be used to provide at least some of the heat or power for the process or sold to third parties.

The production of at least two separate products from the waste treatment process increases the commercial viability of such processes as more income may be generated, or running costs lowered. It is preferred that the solid and gaseous products are used on site as a fuel to provide at least some of the heat and power for the process and that the liquid product is sold as a fertilizer. It is further preferred that the solid product is used as a soil treatment and/or as a fuel to generate power and heat in an energy yielding device such as a pyrolysis processor, in which the solids are pyrolysed to create heat, power and a carbon product which can be sold. The energy yielding device could also be a gasifier or a boiler which raises steam or hot fluids.

It should be understood that the term solid waste is used herein to include waste that has solid components and this could include waste or residues from agriculture, horticulture, aquiculture, food industry, housing or other waste having digestible components.

The liquid digester tanks are preferably conventional liquid digesters, for example completely stirred tank reactors. There are preferably at least 3 tanks and more preferably at least 4 tanks in the series of liquid digester tanks. This is to ensure that liquid waste material, or leachate, from the solids waste material, which is preferably passed to the first tank in the series has a residence time within the series of liquid digester tanks that permits digestion of the liquid waste to an - 5 environmentally more acceptable state before discharge from the series of tanks. It is also preferred that the subsequent tank in the series is the final tank in the series, but it should be understood that this need not be the case as liquid may be passed through the series of tanks in a plurality of different ways, for example the series may include means for recycling some liquid backward along the series. It should also be understood that the series of tanks may be separate vessels, or they could be within one or more vessels and the tanks may be defined by baffles or other means.

The second fluid fraction is drawn from a lower portion of the subsequent liquid digester tank as liquid rich in nitrates, phosphates and other fertilizing nutrients has been found to concentrate in the subsequent liquid digester tank and to be denser than the liquid fraction in which the active bacteria exist and therefore the second liquid fraction is drawn from a bottom portion of the subsequent liquid digester tank. It is preferable to separate the nutrient rich liquid further by means of a centrifuge, in which the denser, nutrient rich liquid is forced radially outward and collected, while less dense fluid is drawn from near the centre of the centrifuge and returned to the series of liquid digester vessels.

It is preferred that the fluid fraction containing active bacteria that is transferred to the solids digestion vessel is drawn from an upper portion of the subsequent liquid digester tank as this reduces the likelihood that - 6 - potentially valuable nutrient rich fluid will be fed to the solids waste digester.

Different types of bacteria predominate at different temperatures in anaerobic digestion of organic waste.

Mesophilic bacteria operate in a relatively low temperature range, usually less than 40 C, and they have a relatively slow rate of digestion. Thermophilic bacteria work effectively in a higher temperature range, around 40 to 60 C (the thermophilic range). Digestion in the higher temperature thermophilic range occurs more rapidly than in the lower temperature mesophilic range, and it gives better control or elimination of pathogens and parasitic organisms.

The solids digestion vessel may be operated in the temperature range of between 20 C to 80 C and is preferably operated in the temperature range of between 50 C to 60 C during the solids digestion phase.

The liquid digester tanks are preferably operated at a temperature of between 30 C and 40 C for mesophilic digestion and between 50 C and 75 C for thermophilic digestion.

It has been found that pre-treatment of the solid waste material reduces the required retention time in the solids digester before the solids waste has been sufficiently treated to become environmentally acceptable for discharge.

The pre-treatment can be through several methods and these methods may break down the solid waste to release digestible components upon which the bacteria may act, for example vibrating the solid waste, shocking the waste with physical or electrical shocks, comminution, squeezing, freezing and crushing. These actions may all increase the surface area upon which the bacteria may act. For example, freezing increases the volume of the water as it forms ice within the cells rupturing the cell walls.

The pre-treatment may also or alternatively sterilise the solid waste as this means that the bacteria that will digest the waste have reduced competition for food during the solids digestion phase and can therefore multiply and act faster. The pre-treatment may also or alternatively inactivate pathogens and prions within waste, such as from the meat industry and catering. Methods of sterilizing the solids waste include autoclave heating to high temperatures and/or pressures sufficient to sterilize the waste, steam heating, electrical induction and electromagnetism.

The solids digestion vessel preferably includes means for recirculating at least a portion of the fluid fraction containing active bacteria within the solids digestion vessel during the solids digestion phase to agitate the solids waste. The recirculated fluid is preferably drawn from one of a top portion or a bottom portion of the solids digestion vessel and returned to the other of the top portion or bottom portion. The fluid is more preferably returned to the solids digestion vessel through one or more inlets at or adjacent a floor of the vessel and preferably though a sparger to maximise agitation of any solids material that may settle during the solids digestion phase. Preferably the means for recirculating at least a portion of the fluid fraction comprises a pump and more preferably a chopper pump which will mechanically break down any solid material passing therethrough.

The material within the solids digestion vessel often includes a solid layer floating upon a liquid layer. This occurs during the solids digestion phase as the bacterial action produces gas bubbles that adhere to solid material, typically fibrous material, causing it to become buoyant.

The solids material that floats above the liquid layer is not in continuous contact with active bacteria in the liquid and so this floating material is digested at a slower rate than if it was submersed in the liquid. It is therefore preferable to include submerging means by which at least a part of the floating layer may be forced into the fluid layer below. Such means preferably includes an apertured member that is used to force the floating solids down as this allows fluid to flow upwards through the solid material and through the apertures in the apertured member so that the fluid substantially completely covers the solid material. It is preferred that the submerging means do not continuously force the solid material into the fluid, but cause the floating material into the fluid and then allow it to rise back up as this increases the - 9 transfer of heat and the percolation of nutrient and bacteria rich fluid through the floating layer.

The apertured member is preferably an apertured drum that rolls over the solid layer forcing it into the fluid layer and then allowing the solid layer to float back upwards as the drum rolls on. The drum could move continuously up and down the solids digestion vessel or move according to a schedule in which periods of movement and non-movement occur. There could also be more than one drum in each vessel. The aperture size of the apertured member will depend upon the nature of the solid waste to be treated and can readily be determined by trial and error.

The treated solid waste may be transported from the solids digestion vessels to the de watering unit manually or mechanically and is preferably transported by mechanical propulsion or conveyance such a conveyer belt. The treated solid waste may be transported by an enclosed elevator to a large heated insulated tank, where over a period of several days, further gas may be given off and collected.

An auger screw located into the bottom of the tank may be used to take product to the de-watering unit. It is also preferable that the solids digestion vessels include emptying means for automatically forcing the contents from the vessel to a location from which they are to be transported to the de watering unit.

The emptying means may be a ram which pushes the contents out of an opening in the side of the vessel, or the entire - 10 vessel may be tilted so that the material falls from the vessel under gravity. In the currently envisaged process, a ram forces the treated solids waste from the vessel onto a conveyer belt that transports the material to the de- watering unit.

The pre-treatment operations and the submersion means are applicable to any solids digestion vessel and to any material that may be treated by the above process and not only to a solids digestion vessel employed in the above process.

Accordingly the invention also provides a solids digestion vessel for use in a process as described above, the solids digestion vessel comprising means by which solid waste may be introduced into the vessel, means by which a liquid fraction containing active bacteria may be introduced into the vessel, means for draining liquid from the vessel and submerging means for forcing at least a portion of a floating layer of material within the vessel into a liquid layer upon which the floating layer is floating.

The liquid fraction transfreered into the vessel may be heated to transfer heat into the vessel. The vessel preferably includes a means of sealing the vessel to prevent release or transfer of heat, odours, aerosols, vapours and moisture. The means of sealing also preferably prevents the ingress of vermin, birds and the like. The vessel may include a means of heating or cooling of the contents of the vessel and also a means of agitation, - 11 mixing or circulation of the contents.

The invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows a schematic plan view of a plant for carrying out a process according to the invention; Figure 2 shows a cross section through a solids digestion vessel according to the invention; and Figure 3 shows a cross section through the last liquid digester tank of Figure 1.

Figure 1 shows a schematic plan view of a plant 1 for carrying out a process according to the invention. Waste (not shown) is collected and stored in holding vessels 2.

The material may be subjected to Material from the holding vessels 2 is drawn through a feed pipe 4 by a chopper pump 6 which mechanically breaks down at least some of the solid waste and is fed into one or more solids waste digestion vessels 100 through an overhead feed manifold 8.

The plant 1 includes a series of liquid digester tanks 10 which includes a first liquid digester tank 12 and a last liquid digester tank 14 in the series 10. The series of liquid digester tanks 10 contain a source of organic material undergoing anaerobic bacterial digestion in a fluid phase digestion stage to produce a first fluid - 13 fraction containing active bacteria in at least the last liquid digester tank 14.

A portion of the first fluid fraction containing active bacteria is fed from a top portion of the last liquid digester vessel 14 into the solids digester vessel 100 to initiate a solids digestion stage.

After the solids digestion phase has proceeded to a point at which the solids waste has become treated solid waste and/or a product which is environmentally more acceptable for discharge and the fluid within the solids digester vessel is drained and transferred to the first liquid digestion tank 12.

The liquid in the series of liquid digester tanks 10 is gradually transferred from tank to tank along the series.

There may be some recycling of liquid between the tanks such that liquid in the last tank is environmentally more acceptable for discharge, that the last tank 14 includes first fluid fraction containing active bacteria and that the last tank 14 contains a second fluid fraction that can be drawn off and used as a fertilizer. The second fluid fraction is passed through a centrifuge 16 to concentrate the nutrients in a heavy fraction of the fluid, with a light fraction of the fluid being returned to the last liquid digester tank 14. The heavy fraction is passed to a storage vessel 18 for storage before being either dried for use as a solid fertiliser, or used directly as a liquid fertiliser.

The drained treated solid waste in the solid digester vessel is emptied onto a conveyor belt 20 and transferred to a storage hopper 22. The storage hopper feeds the treated solids waste to a de-watering unit 24 in which liquid in the drained treated solid waste is removed and returned to the last liquid digester vessel 14 in which it will undergo further digestion. The de-watered treated solids waste is passed to a compression device such as a pelletiser 28 in which it is compressed into pellets, and the pellets are then fed into an energy producer such as a pyrolysis unit 26 in which the pellets are pyrolysed to produce heat, a combustible gas and carbon. The carbon is stored in a carbon storage vessel 30 for later sale or use. It should be understood that the term 'pellets' as used herein is used to include briquettes and the like.

The plant 1 is provided with a combined heat and power unit 32 which makes use of the biogas fuel produced in the liquid digester tanks 10 to provide at least some of the heat and power for the plant 1. The plant may also make use of natural or renewable forms of energy such as wind, solar or water energy.

It should be understood that connections to and from the combined heat and power unit are not shown in these drawings due to their complex nature. It should also be understood that further efficiency for the plant may be achieved through the use of heat exchangers located to transfer heat between process streams.

Figure 2 shows a cross section through a solids digestion vessel 100 in use. The vessel includes a liquid inlet 102 through which liquid may be fed into the vessel 100 to start a solids digestion stage, a liquid outlet 104 through which liquid may be drained from the vessel 100 to end the solids digestion stage and a solids inlet 106 through which solids may be fed into the vessel 100 through the manifold 8 to charge the vessel 100 before a solids digestion stage. As shown in this figure, the vessel 100 is charged with both liquid 108 and solids 110 and is shown during a solids digestion stage. The solids include floating material 112 and sunken material 114.

The liquid 108 contains active bacteria that treat the solids material to render is environmentally more

acceptable.

The vessel 100 further includes recirculating means 116 for recirculating at least some of the liquid within the vessel 100. A take off 120 is located in a floor of the vessel 100 so that the intake of the recirculating means includes at least some of the sunken material 114. The recirculating means 116 includes a chopper pump 118 which chops up any solids passing therethrough to increase the surface area upon which bacteria may act.

The chopper pump then returns the liquid and chopped solids to the vessel 100 through returns 122 and 124 at a top 128 and bottom 130 of the vessel 100 respectively. The return 122 at the top 128 may be sprayed over the floating material 112 so that liquid containing active bacteria percolates through the floating solids 112. The return in the bottom 130 may be through a sparger 126 to agitate the sunken material 114.

Means 132 are provided for forcing at least some of the floating material 112 into the liquid 108. The means 132, in this case, comprise an apertured roller 134 supported by a support member 136 such that the lowest point of the apertured roller 136 is below a liquid level 138 in the vessel. The support member 136 moves along the top 128 of the vessel 100 and the apertured roller 134 rolls along the top floating material 112 and forces it below the liquid level 138.

The sparger 126 may also be used to add air to the vessel to initiate an aerobic digestion stage in the digestion, either before or after the solids digestion stage has occurred. The sparger 126 is coupled to an air pump 140 for this purpose. The sparger 126 may have a dual role and include means for heating or cooling the contents of the vessel loo. This could be as a pre-treatment as discussed above, or could be as a temperature control means during the solids digestion stage.

The vessel 100 further includes emptying means 142 located at one side within the vessel 100. The emptying means comprise a ram 144 and a pushing plate 146. To empty the vessel 100, a door 148 is opened and the ram 144 activated. This forces the pushing plate 146 towards the - 17 door 148 and thereby forces the contents of the vessel 100 out of the door 148.

Figure 3 shows a cross section through the last liquid digestion tank 14 of Figure 1. The liquid digestion tank 14 includes an upper portion 200, a middle portion 202 and a conical lower portion 204. Liquid is fed into the tank 14 through an inlet 206 into the middle portion 202. A first fluid fraction 208 within the tank 14 contains lo active bacteria. At least some of the first fluid fraction 208 may be fed from the tank 14 through a first outlet 210 in the top portion 200.

A second fluid fraction 212 rich in nutrients collects in the bottom portion 204 of the tank 14 as it has a density greater than the liquid in the rest of the tank 14. At least some of the second fluid fraction 212 is drawn off through a second outlet 214 and passed to a centrifuge 16.

During the centrifuge process the denser nutrient rich liquid is concentrated and drawn off through a product outlet 216. Less dense fluid is returned from the centrifuge to the middle portion 202 of the tank 14 through a recycle 218. It should be understood that the fluid fractions may not separate as shown in the figure, there may be more fractions within the tank, or there may be a gradual transition from the first fraction 208 to the second fraction 212.

It should be understood that the invention has been described above by way of example only and modification in - 18 detail may be made without departing from the scope of the invention as defined in the claims.