GB2313116A - Treatment of wastes - Google Patents

Abstract

In a sequential means of anaerobic digestion of biodegradable material a quantity of fresh degradable material is introduced in a phased manner into a previously part-digested medium and some of the final digestate resulting from this process may be transferred to enhance further subsequent decomposition cycles. The decomposition takes place in an appropriately constructed container or vessel, equipped with the means for heating, agitating and moving the material, which is presented in a suspension. Together with the use of chemical buffering, the aim of this is to provide optimised conditions for the growth and activities of the micro-organisms responsible for the breakdown of organic material in the absence of oxygen.

Description

An Improvement in the Treatment of Wastes This invention relates to waste management. In this context, waste refers broadly to domestic, household, commercial and non-hazardous industrial refuse, together with certain agricultural or horticultural materials.

Waste, as defined, would normally contain biodegradable material, such as food scraps, cellulosic and other garden refuse, paper and wood; manures, farm slurries and sewage, for example, could also be covered by this definition.

The breakdown of such kinds of wastes by processes of biological/biochemical decomposition is well and commonly appreciated. Such decomposition, effected by certain fungi, bacteria and other micro-organisms, collectively termed decomposers, occurs in traditional landfill disposal arrangements as well as more widely throughout the natural world. When the local conditions are anaerobic, that is, lacking in oxygen, the said kinds of organic wastes are well known to produce quantities of biogas, as a consequence of the biochemical changes within the decomposing material. This biogas (sometimes called landfill gas) has a composition which usually consists largely of methane and carbon dioxide (typically approximately 60% CH4; 40%C02). Though the use of this methane to power electrical generators and its simple flaring at landfill sites are both well known, in many cases methane production is viewed as a necessary inconvenience in the running of the site, with methane migration to other areas and explosive gas build up on site being widely recognised potential hazards.

A closely linked environmental problem is the production of quantities of leachate, that is liquid which has become contaminated with various diverse substances arising from the processes of waste breakdown, or directly from the waste itself. Pollution of groundwater by said leachate is a potentially major problem at waste management sites.

The principle of providing idealised, controlled conditions artificially to bring about the managed decomposition of wastes, as defined, and the regulated evolution of biogas is well established. A number of such systems, termed Anaerobic Digesters, exist which bring about said controlled decomposition in various ways. Our earlier PCT Patent Application, W093/01456 (GB 92/01193) makes known the provision of a reservoir formed by a cavity or vessel, covered with an impervious flexible membrane and containing biodegradable material and PCT Application W095/10596 (GB94/02256) makes known a significant improvement in this kind of system (though not necessarily incorporating a reservoir), in which additional energy is supplied to the system. This accelerates the reaction and enables more careful control over biogas production, especially in the start-up phase.

Other applications disclose different methods to achieve a similar kind of waste decomposition. The present invention seeks to provide a system which represents an improvement over the prior art.

In general terms, all previous Anaerobic Digesters, can be categorised in terms of their operating criteria.

Leaving aside the criteria of solid : liquid ratio and the operating temperature range (usually either c.350C (mesophilic) or c.550C (thermophilic)) which further sub-divide the systems, Digesters fall into one of two groups: "batch" or "continuous." As their respective names suggest, batch systems are filled with all the material to be treated, sealed and then permitted to digest to completion without further interference, whereupon they are emptied and a new batch is consigned for treatment. Our application GB94/02256 is an example of a batch system. Continuous systems receive their waste feed little by little, spread over time, so that the digestion process takes place uninterrupted, having no natural end point. All previously known commercial digesters tend to fall into one or other of these broad categories.

Now a major problem with batch processes, as compared with continuous processes, is that at the start of any given batch the environmental conditions for a high activity digestion process will not be present so that digestion will start slowly, and optimum conditions for high activity will not be encountered until some way into the process.

A further problem of batch processes is that not only do the environmental conditions in relation to temperature and composition of the batch of raw material have to be brought to an optimum for the process to work effectively, but also the content of the mix at the start will usually have a low level of bacteria and micro-organisms which are essential to a digestion process.

On the other hand a continuous process has other disadvantages. In particular it will be necessary to keep the process going continuously and therefore to have a continuous stream of raw material ready for feeding into the system, and this may not be convenient or the stock of raw material available may fluctuate. Moreover more complex monitoring and control may be necessary.

The present invention is concerned with these problems and in particular with the problem of making a batch waste digestion process more effective both in terms of throughput and in optimum usage of a given sized digestion cell.

Thus according to a first aspect of the invention a process for the treatment of biodegradable waste is provided in which a first quantity of biodegradable waste is introduced into a reaction vessel which is then sealed, so forming a bioreactor cell and subjected to a process of digestion in the absence of oxygen, a second quantity of biodegradable waste is introduced into the vessel at a selected time after commencement of said process, and then said process of digestion is continued so as to produce a quantity of a resultant digestate material.

The resultant digestate material can then, after suitable treatment, be disposed of in a landfill site within a lower volume and without the leachate problem previously referred to. Alternatively it can be used in other ways.

Also, a greater quantity of biogas is likely to arise in the process to be collected and utilised.

In a preferred form of the invention a portion of said resultant digestate material can be used in a mixture with a new quantity of waste to begin the digestion process again, thus forming a semi-continuous sequence.

In a practical application of the invention to process a relatively large volume of organic waste, either by way of a total treatment or as one part of a multi-stage approach, a series of such bioreactor cells can be employed to bring about the desired result, namely the direct, inter-cell transfer of quantities of digested material as an inoculant medium, that is to say, a medium rich in the bacteria and other micro-organisms useful to seed the next treatment reaction, and the phased introduction of fresh waste into partially digested material.

The calculation of the correct time at which the addition of the second quantity of waste is made is based on established representations and scientific models relating to the production of biogas under anaerobic conditions. Such models define the production of these gases in generalised stages, since the exact timings for the onset of each, and their specific duration, are variable and depend on a number of factors, such as temperature, moisture content, characteristics of the waste, rate of material mixing, speed of bacterial culture establishment, rapidity of oxygen exclusion and so on. These variables are chiefly related to the digestion system used and the precise operational procedures followed.

A number of authorities have sought to establish models for the prediction of biogas production, ranging from the very simplistic to the highly sophisticated. Generally, it is widely accepted that the gas evolution / cellulose decomposition curves can be characterised as having five main phases, of which the main points relevant to the present discussion are: Phase I Maximum cellulose loadings; oxygen content drops to near zero; nitrogen, and carbon dioxide at atmospheric levels (20% and 78% respectively).

Phase II Carbon dioxide, hydrogen and free fatty acids levels rise to peak values; nitrogen levels fall to around 10%; cellulose breakdown begins.

Phase III Carbon dioxide decreases to plateau at around 40%; methane production commences and achieves plateau at around 60%; free fatty acids decrease to hold at minimal levels; cellulose breakdown continues at a linear rate with respect to time; nitrogen falls to near zero.

Phase IV Plateau phase with carbon dioxide at c.40% methane at c.60% and free fatty acids at less than 5%; cellulose declines steadily throughout this phase.

Phase V Cellulose becomes fully decomposed, resulting in the tail off to zero of methane and carbon dioxide; oxygen and nitrogen regain atmospheric levels. (20% and 78% respectively).

For the purposes of the preferred practical embodiment of the present invention, the addition of the second quantity of waste is timed to make maximum advantage of the physical and biochemical changes taking place within the digesting mass. This is around the time of the boundary interface between phases III & IV, when cellulose breakdown is already under way and free fatty acids are rapidly reducing in concentration as methane production rates increase to steady state.

Looking at the process of anaerobic digestion more specifically, large organic molecules are converted, chiefly, into methane and carbon dioxide by the action of bacteria in the absence of free oxygen. The reality of this breakdown at the microscopic level is chemically very complex, involving a great many potential intermediary reactions and compounds, many further needing specific synergistic chemicals, catalysts or enzymes. However, there are three main stages to anaerobic digestion: 1. Hydrolysis 2. Acidogenesis 3. Methanogenesis During hydrolysis, complex insoluble organic polymers, such as carbohydrates, cellulose, proteins and fats, are broken down and liquified by the extracellular enzymes produced by hydrolytic bacteria. This makes them more easily available for use by the acidogenic bacteria of the next stage. The liquefraction of complex compounds, and especially cellulose, to simple, soluble substances is often the rate limiting step in digestion.

Acidogenesis is characterised by the production of acetic acid from the monomers released in the preceding stage and volatile fatty acids (VFAs) which are derived from the protein, fat and carbohydrate components of the feedstock. During this stage, the pH falls as acetic, lactic and proprionic acid levels increase.

Methanogenesis involves the production of methane from the raw materials produced in the previous stage. Acetic acid or acetate are the most important of these, since around 75% of the methane produced is thus derived (CH3COOH - > CH4 + C02). Methane forming bacteria may also use methanol or carbon dioxide and hydrogen (CH30H + H2 - > CH4 + H20 and C02 + 4H2 - > CH4 + 2H20 respectively).

This stage also naturally helps to buffer the acid/base equilibrium, since methanogens effectively act to reduce any trend towards an increase in VFA concentration.

There are known to be four main groups of bacteria involved in the stages of AD (anaerobic digestion): (i) Hydrolytic fermentative bacteria (e.g.

Clostridium. Eubacterium and Peptococcus) (ii) Acetogenic bacteria (e.g. Desulfovibrio, Syntrophobacter and Syntrophomonas) (iii) Acetoclastic methanogens (e.g. Methanosarcina and Methanothrix) (iv) Hydrogenotrophic methanogens (e.g.

Methanobacterium and Methanobrevibacterium) Calculating the correct moment to introduce the second quantity of waste, which is at the boundary interface between phases III and IV of the biogas evolution/cellulose model previously described, can be made by reference to measurement of the directly determinable effects of these groups of bacteria and the related stages of AD.

The requirement to know that the cellulose material (which accounts for the main bulk of the organic fraction of waste as it typically presents) is being broken down, may be inferred from the presence of related catabolic products in the slurry liquid of the digester.

Throughout the digestion process in a full scale, commercial operation, monitoring of the chemical composition of said slurry is desirably carried out routinely, with the specific objective of identifying the key substances required for proper breakdown and gas production. Certain of these, for example VFAs, will be investigated by direct extractive sampling from within the body of the digester, thereby reflecting very accurately the situation within the decomposing slurry matter. The presence of these indicates that cellulosic material within the waste has begun to break down, since, as was previously stated, (a) such chemicals derive from the breakdown products of complex organic molecules, of which group cellulose forms the majority in organic waste and (b) the breakdown of cellulose is the rate limiting step in the stages of AD.

The routine monitoring of VFAs also enables the second of the phased addition criteria to be determined, that is, that free fatty acid concentration is rapidly decreasing. Clearly, with records showing VFA levels on a strictly regulated, periodic basis, it is a simple matter to establish their decrease.

During the start up and initial phases of running a digester, before truly anaerobic conditions are established, a variety of bacteria, fungi and other micro-organisms play their part. Thus a succession will be observed from aerobic organisms, through facultative anaerobes to, ultimately, obligate strict anaerobes.

Accordingly, the by-products of their respiratory activities will mirror this change. Once a truly anaerobic environment exists, the stages of AD will take place and methanogenesis will begin. The rate of methane production and the concentration of methane within the derived biogas would also be measured routinely. This will, again, enable the determination of the onset of the plateau or steady state of methane production which further indicates the readiness of the system to accept a further phased waste input, in accordance with the invention.

A specific embodiment of the invention will now be described by way of example and with reference to the accompanying drawing, in which: Figure 1 is a side view of a single bioreactor vessel, in accordance with the invention; Figure 2 is a sectional side view of the single bioreactor vessel of Figure 1; Figure 3 is a schematic of the system, showing four linked bioreactor cells, in accordance with the invention; and Figure 4 is a flow diagram of the process.

As previously described, the invention relates to the cyclic addition of a known amount of fresh waste at a calculated moment in the digestion process, optionally combined with the use of a controlled quantity of material which has been digested to form a seed culture of bacteria for the next quantity of waste to undergo treatment.

Referring now to the drawings, a vessel or container 1 is provided, which provides a void space 7 into which waste containing organic material is deposited, with the possible addition of sewage, manures or other biodegradable materials. An upper cover 5 is then sealed to ensure that air ingress is prevented and that the biogas produced is contained within the system. In a large scale application of the invention, a number of these units is employed, and these units are linked to each other by a series of transfer pipes 2.

The waste within the vessel 1 has been prepared as a slurry or suspension of optimum humidity / water content for high solid digestion. A heater input 9, applies heat to raise, and maintain, a temperature appropriate for the facilitation of microbial activity to bring about the required anaerobic breakdown of the material. The manner of this heat addition, its purpose and control is disclosed in our earlier PCT Application GB94/02256.

Within the void space 7 there is provision for the collection of the biogas evolved from the decomposing waste within the system. The gas produced is removed for use, providing heat and/or electricity, or for disposal, via an extraction pipe 6 which exits the structure of the digester vessel 1.

Also within the void space 7 there is provision for a pump/agitator 8, or series of said devices, to maintain circulation and agitation of the material to enhance and facilitate the microbial action within the decomposing waste and effect adequate and proper distribution of the heat supplied to the digesting matter.

At a calculated time after the first amount of waste has been placed within the void space 7 and has been subject to anaerobic conditions, a controlled second amount of fresh waste is added in to the part-decomposed original suspension, either in the form of another suspension, in its raw state, or otherwise.

As has been previously stated, the correct moment for the addition of the second quantity of waste is calculated based on established models of biogas production under anaerobic conditions. This second quantity of waste is added at a time calculated to permit maximum advantage to be gained from certain physical and biochemical changes taking place within the digesting mass.

Figure 4 shows a flow diagram of the various phases of gas evolution/cellulose breakdown during the process, as specifically described earlier in this Specification. As can be seen, the second quantity of waste is introduced between phases III and IV.

More specifically, this is when cellulose breakdown is already under way and methane production rates have increased to steady state, with free fatty acids concentration rapidly decreasing.

At this time the second quantity of "fresh" waste is introduced into the void space 7 via an access pipe 4.

This new waste is mixed into the partially digested suspension already occupying the void space 7 by means of the pump/agitator 8. The digestion process continues throughout for the derived mixture of "new" and "old" wastes.

During the digestion process, careful sampling of the acid/alkali balance is employed and lime or other suitable alkali substances added to buffer the acidity naturally occurring as a result of the activities of certain of the bacterial types required to bring about the breakdown of the waste. These bacteria, known as acetogenic bacteria, begin the decomposition of waste, resulting in the production of volatile fatty acids. A second group of bacteria, called methanogens, change these organic compounds into biogas. If the internal environment within the void space 7 becomes too acidic, the bacterial activity is inhibited and may, ultimately, cease. Buffering will be employed when the pH value drops below 6.5, the aim being to re-establish a value slightly above this.

When the final required level of decomposition of the waste contained within the void space 7 has been achieved, a given volume of the resultant digestate is transferred via transfer pipes 2 to another of the vessels, this flow being directed, controlled and regulated by means of a valve 3 or similar device. This acts to seed the new vessel with both an appropriate bacterial culture and certain key biochemical substances which facilitate the establishment and start up of anaerobic conditions more speedily than would otherwise be achieved. This has the effect of reducing the period of the earlier phases of digestion, that is, the phases previously discussed which are necessary before effective digestion and gas production can take place.

Figure 4 shows the feeding of a proportion of the digestate output from a first cell of a first batch process to the start of a new batch process. Subsequent batches can also be seeded in the same way.

Alternatively, the seed material may remain to perform the same function within the original vessel itself when a new batch of raw material is introduced into that vessel to start a new batch process.

The remaining part of the resultant digestate is removed either for use or disposal. In the preferred embodiment of the invention, it is dewatered, that is, treated by a mechanical device such that a large proportion of the liquid content is expelled from the fibrous, solid matter. This liquid may be reused directly within the system to help form a new waste suspension for digestion, or removed for use, disposal or treatment elsewhere.

The volume of the fibrous material arising is very much smaller than that of the volume of waste originally introduced into the vessel 1.

This is a characteristic of anaerobic digestion and means that disposal in a landfill site at this point is both less wasteful of land and less environmentally harmful, since the potential for leachate and methane production has been very greatly reduced. Alternatively, the material could be further treated for other end uses.

The void space 7 now made empty by the evacuation of digestate material as described is available for reuse and can be recharged with a new first suspension of waste and the cycle previously explained repeated.

Claims (7)

1. A Process for the treatment of biodegradable waste in which a first quantity of biodegradable waste is introduced into a reaction vessel which is then sealed and subjected to a process of digestion in the absence of oxygen, a second quantity of biodegradable waste is introduced into the vessel at a selected time after commencement of said process, and then said process of digestion is continued so as to produce a quantity of a resultant digestate material.

2. A Process according to claim 1 in which a portion of said resultant digestate material is mixed with a further quantity of biodegradable material and said process is repeated.

3. A Process according to claim 1 or claim 2 in which said second quantity of biodegradable waste is introduced at a time when methane production is increasing to a steady state.

4. A Process according to claim 1 or claim 2 in which said second quantity of biodegradable waste is introduced at a time when cellulose breakdown is under way and free fatty acids are rapidly reducing in concentration.

5. A Process according to any preceding claim in which said process of digestion includes the application of heat, chemical buffering and agitation to optimise the conditions of microbial and biochemical decomposition.

6. A Process according to any preceding claim in which said digestion process comprises five successive phases of gas production and cellulose breakdown defined as follows: Phase I Maximum cellulose loadings; oxygen content drops to near zero; nitrogen, and carbon dioxide at atmospheric levels; Phase II Carbon dioxide, hydrogen and free fatty acids levels rise to peak values; nitrogen levels fall to in the region of around lOt; cellulose breakdown begins; Phase III Carbon dioxide decreases to plateau at in the region of 40%; methane production commences and achieves plateau at around 60%; free fatty acids decrease to hold at minimal levels; cellulose breakdown continues at a linear rate with respect to time; nitrogen falls to near zero; Phase IV Plateau phase with carbon dioxide at approximately 40% methane at approximately 60% and free fatty acids at less than 5%; cellulose declines steadily throughout this phase; Phase V Cellulose becomes fully decomposed, resulting in the tail off to zero of methane and carbon dioxide; oxygen and nitrogen regain atmospheric levels; and that said second quantity of waste material is introduced during a period between phases III and IV.

7. A process according to any preceding claim in which biogas produced by said digestion process is extracted for use elsewhere.