WO2016005770A2

WO2016005770A2 - Mobile digestion plant

Info

Publication number: WO2016005770A2
Application number: PCT/GB2015/052008
Authority: WO
Inventors: Stewart MANNING
Original assignee: Breathe Ad Limited
Priority date: 2014-07-10
Filing date: 2015-07-10
Publication date: 2016-01-14
Also published as: GB201412320D0; GB2528110A; WO2016005770A3

Abstract

A mobile anaerobic digestion plant (10) for the high-rate anaerobic digestion of organic waste to evolve bio gas comprising: a reactor system (13), a control system, and a heating system (33), mounted on or in a vehicle or towable trailer; the reactor system (13) including two or more reactor tanks (16, 20), operable in series or in parallel, with a gas collection system in at least one reactor tank; the control system including a piping manifold (24) with means for controlling the flow of fluids in the reactor system (13); and the heating system (33) being adapted to control the temperature of at least one reactor tank.

Description

MOBILE DIGESTION PLANT

The present invention relates to a mobile digestion plant, and particularly to a mobile high-rate anaerobic digestion plant which is capable of generating electricity from substantially liquid organic waste, including (but not limited to): fats, oils and/or greases (FOGs).

BACKGROUND TO THE INVENTION Household and commercial organic waste and/or wastewater often contain energy-rich components like fats, oils and greases (FOGs), contributing to the volume of material being sent to landfills or sewer networks, which can become corroded or blocked as a result. Technology exists that can be used to produce biogas from such waste, using anaerobic bacteria to break down the input material. A conventional anaerobic digestion (AD) plant produces biogas with a methane content of between 50% and 60%, typically resulting in chemical oxygen demand (COD) removal of 70% from the input material. However, the conventional AD process takes place over a period of 28 days, and requires a large plant area. In comparison, a high-rate AD plant can produce biogas with a methane content of between 70% and 87% over the course of 24 hours, which can achieve COD removal of over 90%, requires less space than a conventional AD plant and is more efficient. Biogas produced in either process can subsequently be utilised to generate electricity, essentially acting as a source of renewable energy.

However, both conventional and high-rate AD plants treat waste within a single reaction vessel, meaning that reaction conditions (including temperature, pH, M- alkalinity and Volatile Fatty Acid (VFA) concentration) must be compromised to balance the rates of reaction and yields from the multiple strains of bacteria present in the reaction vessel, resulting in non-optimal conditions for all of the bacteria. Consequently, a significant proportion of FOG compounds remain unreacted, leaving digestate (digested sludge) in the reaction vessel which must be disposed of, increasing operating expenses and not realising the maximum potential yield. The resultant biogas is also of low quality because it is contaminated by compounds evolved due to the non- optimal reaction conditions. Such compounds can include hydrogen sulphide, which is odorous, corrosive, and poisonous if inhaled, making it extremely hazardous to health.

Furthermore, high capital investment is needed to construct an AD plant, with both conventional and high-rate systems also requiring that waste is transported to the plant, which raises operating expenses and increases pollution due to the energy requirements of transporting large volumes of waste between different sites. Due to the duration of transportation, many of the wastes involved begin to degrade prior to being received at the AD plant. These factors act as barriers to the widespread commercial adoption of AD plants, with excess FOGs continuing to pollute the environment as a result.

It is an object of the present invention to reduce or substantially obviate the aforementioned problems. Note that the terms "granular biomass" and "biomass" are used interchangeably, and they both refer to cultured anaerobic bacteria and/or enzymes which facilitate anaerobic digestion.

STATEMENT OF INVENTION

According to the present invention, there is provided a mobile anaerobic digestion plant for the high-rate anaerobic digestion of organic waste to evolve biogas comprising: a reactor system, a control system, and a heating system, mounted on or in a vehicle or towable trailer; the reactor system including two or more reactor tanks, operable in series or in parallel, with a gas collection system in at least one reactor tank; the control system including a piping manifold with means for controlling the flow of fluids in the reactor system; and the heating system being adapted to control the temperature of at least one reactor tank. Advantageously, it is easy to transport the plant as it is mounted on a vehicle or towable trailer, allowing the plant to be demonstrated at the source of the waste or in remote locations without restriction. The plant can then demonstrate the efficiency of high-rate anaerobic organic waste digestion with an on-site feasibility study, confirming the results of small scale laboratory trials and indicating the commercial viability of investing in a tailored full scale industrial plant. The reactor system allows high waste throughput where it is operated in parallel, or superior methane evolution and chemical oxygen demand removal where it is operated in series, using the control system piping manifold to re-direct waste to one or both reactors as needed. Some waste, such as glycerine, can be digested more thoroughly by operating the reactor system in series, making this particularly advantageous. Conversely, digesting wastes comprised of simple compounds (often with low molecular weights) may not see any increase in methane evolution or COD removal, and so it is more advantageous to digest such waste by operating the reactor tanks in parallel, also reducing costs.

Collecting the biogas as it evolves via a gas collection system maintains the internal tank equilibria and reduces scaling of the biofilm, allowing the biogas to be used immediately rather than waiting for the digestion process to complete before collection. The control system allows the process to run coherently by controlling the flow of waste through the reactor system, whilst the heating system enables digestion to proceed at an optimum rate.

The reactor system may include a conditioning tank, a first reactor tank, a first break tank, a first recirculation loop, a second reactor tank, a second break tank, and a second recirculation loop, where the first reactor tank may be connected to the first break tank via the first recirculation loop, and the second reactor tank may be connected to the second break tank via the second recirculation loop, where at least one reactor tank may be connected to the conditioning tank, and the second reactor tank may also be connected to the first reactor tank.

A multi-tank system is advantageous because it separates different aspects of the anaerobic digestion process and allows them to be optimised. The conditioning tank can beneficially store waste destined for each reactor tank, allowing feedstock to acclimatise before it is transferred to the reactor tanks. Using two reactor tanks which can each connect to both the conditioning tank and each other permits the plant to operate the reactor tanks in series or in parallel, which offers greater flexibility when deciding how to treat waste in a given scenario. Specifically, waste can be transferred directly from the conditioning tank to either reactor tank, and between each reactor tank as needed. Connecting each reactor tank to its own break tank with a recirculation loop ensures that the waste being treated does not stagnate, and prevents the backflow of partially digested waste into previous tanks.

The control system may include a pH correction system which may allow the pH within at least one reactor tank to be controlled by at least one of the following: alkaline addition, M-alkalinity buffer addition.

The pH correction system maintains an ideal pH for the bacteria, improving the proportion of methane in the biogas and reducing the need for further chemical treatment of the biogas, also minimising the associated costs. By carefully controlling the pH with alkaline or M-alkalinity buffer addition, the rate of reaction can be optimised against the preferred environmental acidity or alkalinity requirements of the relevant bacteria. Alkaline addition can include lime dosing or sodium hydroxide dosing, depending on the specific requirements of the bacteria, and the degree of pH change required. This encourages anaerobic digestion without causing internal (or externally released) bacterial enzymes to denature, which would slow the digestive process and potentially kill the bacteria. Controlling the pH suitably also reduces the volume of hydrogen sulphide evolved, thereby minimising the mass of ferric chloride required to precipitate iron sulphide and reducing operating expenses.

The heating system may allow the temperature within at least one reactor tank to be controlled.

By carefully controlling the temperature, the rate of reaction can be optimised against the preferred thermal environment of the relevant bacteria, encouraging anaerobic digestion without causing internal (or externally released) bacterial enzymes to denature, which would slow the digestive process and potentially kill the bacteria. Controlling the temperature suitably also reduces the volume of hydrogen sulphide evolved, thereby minimising the mass of ferric chloride required to precipitate iron sulphide and reducing operating expenses.

When two or more reactor tanks are operating in series, the first reactor tank may include granular biomass and biofilm which may facilitate one or more of the following types of reaction: hydrolytic, acidogenic, acetogenic. In this case, the contents of the first reactor tank may be maintained substantially in the temperature range 39°C to 40°C, with a pH range which may be substantially 5.8 to 6.8, and with an M-alkalinity buffer which may be substantially in the range 1500 mg/L to 2500 mg/L. These conditions are optimised to promote feedstock hydrolysis, acidogenesis and acetogenesis, maximising the overall rate which feedstock is converted to acetic acid. However, these conditions inhibit methanogenesis, and as such methanogenic bacteria are purposefully isolated in the second reactor tank. Excessively high temperatures denature enzymes, but lower temperatures slow the rate of reaction, so an optimum temperature of 39°C to 40°C is maintained to maximise the rates of hydrolysis, acidogenesis and acetogenesis without denaturing the enzymes. The temperature is slightly higher in the first reactor tank compared to the second reactor tank as the granular biomass and biofilm in the first reactor tank are less sensitive to ambient conditions and can operate well at the higher temperature range used (increasing the rate of each reaction), and at a slightly acidic pH. The M-alkalinity buffer concentration is also lower than the subsequent reactor(s) to maintain the mixture at a slightly acidic pH, accounting for the production of acetic acid during the process.

When two or more reactor tanks are operating in series, the second reactor tank may include granular biomass and biofilm which may facilitate one or more of the following types of reaction: methanogenic. In this case, the contents of the second reactor tank may be maintained substantially in the temperature range 37°C to 38°C, with a pH range which may be substantially 7.2 to 7.8, and with an M-alkalinity buffer which may be substantially in the range 2500 mg/L to 3500 mg/L.

With predominantly methanogenic reactions occurring in the second reactor tank, conditions are optimised to promote methane production, maximising the rate which acetic acid is consumed. Having a slightly lower temperature than the preceding reactor tank(s) allows the methanogenic processes to dominate relative to any sulphate- reducing processes, minimising the production of hydrogen sulphide whilst maximising methane production. Running the reactor tank under slightly alkaline conditions further promotes the decomposition of acetic acid to methane whilst also reducing the percentage of carbon dioxide in the biogas evolved. Nutrient addition also ensures that the methanogens have the trace nutrients required to perform at their optimum rate, efficiently digesting VFAs in addition to maintaining optimum pH and M-alkalinity.

The second reactor tank may contain an M-alkalinity buffer of substantially 3300 mg/L.

Operating the plant in series and using a buffer concentration similar to or at the above value can promote the evolution of biogas containing as much as 87% methane. It can also maintain the pH at its current value without the concentration of buffer becoming toxic to the biomass. In particular, it counteracts the increase in acidity associated with the creation of intermediary acetic acid by capturing hydrogen ions released by the dissociation of the acetic acid.

When two or more reactor tanks are operating in parallel, each reactor tank may include granular biomass and biofilm which may facilitate one or more of the following types of reaction: hydrolytic, acidogenic, acetogenic, methanogenic. In this case, the contents of at least one reactor tank may be maintained substantially in the temperature range 38°C to 39°C, with a pH range of substantially 6.8 to 7.2, and with an M-alkalinity buffer of substantially 2500 mg/L. This allows the entire anaerobic digestion process to occur under conditions which maximise the overall rate of conversion from feedstock to biogas within a single reactor tank, balancing the competing pH and temperature demands of the different bacteria. This maximises the throughput of organic waste in a multi-tank scenario at the expense of overall chemical oxygen demand (COD) conversion efficiency, which is advantageous where treating a larger volume of waste is preferable to more extensive COD conversion. Operating in parallel is therefore preferable where the feedstock contains relatively simple compounds, often with low molecular weights, as large volumes of waste can be treated without compromising on the extent of COD conversion. The relatively neutral pH and moderate temperature range limits the combined activity of the hydrolytic, acidogenic and acetogenic processes, but allows the methanogenic processes to still compete sufficiently with sulphate-reducing processes to produce biogas of good quality.

The M-alkalinity buffer may be predominantly sodium bicarbonate. Sodium bicarbonate can be added to the waste, dissolving to release hydrogencarbonate anions that are advantageously in equilibrium with carbonate ions (CO3^2") and aqueous carbonic acid (H2C0₃(aq)), which can further evolve carbon dioxide. This makes sodium carbonate an effective M-alkalinity buffer, balancing the addition and/or removal of hydrogen ions through its equilibrated forms. The M-alkalinity buffer may instead be generated by adding sodium hydroxide to the waste, which reacts with carbon dioxide to generate hydrogencarbonate ions in situ, increasing the proportion of methane in the biogas evolved. The positive counter-ion may instead be ammonium where waste with high nitrogen content is being digested.

The reactor system may digest organic waste with a sodium content of substantially 1000 mg/L or less, up to a maximum sodium content of substantially 3000 mg/L. Excessive levels of sodium can adversely affect the biomass (being toxic in very high concentration) and can therefore inhibit the rate of anaerobic digestion. Equally, low levels of sodium may adversely affect the growth or health of the biomass. Restricting the sodium concentration as above helps to sustain an optimal rate of reaction, although the plant can operate at higher levels of sodium in the waste once the bacteria have acclimated.

The reactor system may digest organic waste with a nitrogen content of substantially 1000 mg/L or less, up to a maximum nitrogen content of substantially 3000 mg/L. Diluting the waste can reduce the proportion of nitrogen present per unit volume of the waste. Restricting the nitrogen content of the organic waste prevents waste by-products (such as ammonia) being created which can contaminate the reactor tank(s) and liquid output. Controlling the nitrogen content as above helps to sustain an optimal rate of reaction as it can prove to be toxic to anaerobic bacteria, although the plant can operate in the presence of higher levels of nitrogen in the waste once the bacteria have acclimated. The reactor system may digest organic waste with a chemical oxygen demand (COD) greater than substantially 3000 mg/L, and preferably between substantially 3000 mg/L and 15000 mg/L. Up to 95% of COD in organic waste mixtures in the ranges above can be digested by the biomass available in the reactor tank(s) in the expected time frame, producing biogas with a composition of greater than 70% methane. Feedstock with any COD level can be treated, including mixtures with a COD higher than 50000 mg/L, given sufficient dilution and recirculation. However, the reactor system is ideally fed with a mixture having a COD between 3000 and 15000 mg/L, as this maximises the extent of COD conversion. If the COD content is less than 3000 mg/L, the waste can still be digested but, being so dilute, may not be efficient and or economically viable to process.

The reactor system may operate substantially in the range 20 to 40 mbar.

Operating at reduced pressure helps maintain anaerobic conditions, essential to the digestion of the feedstock. It also increases the rate of biogas production by establishing an equilibrium that encourages gas evolution without causing excessive boiling of the solvent, and without being low enough to pose an excessive risk of implosion.

Organic waste may be decomposed into volatile fatty acids (VFAs), where the concentration of VFAs in the first reactor tank may be maintained at substantially 10 g/L or less, and where the concentration of VFAs in the second reactor tank may be maintained at substantially 1 g/L or less.

A VFA concentration of 10 g/L or less in the first reactor tank helps maintain the pH at an acidic value, without inhibiting the decomposition of further feedstock in VFAs. Maintaining the VFA concentration at 1 g/L or less in the second reactor tank ensures that the VFAs do not overly acidify the second reactor tank and so the bacteria are able to evolve biogas from the VFAs at a sustainable rate, without VFAs building in quantity and inhibiting methanogenesis.

At least one reactor tank may contain an injection manifold. The injection manifold confers the advantage of controllably introducing waste to each reactor tank, allowing the rate of injection to be modulated depending on the type and properties of the waste being treated. The upflow velocity is set to maintain the biomass in suspension and to assist provide adequate mixing, ensuring contact between the feedstock and biomass.

Each reactor tank may include one or more strains of bacteria seeded on packing media and in a sludge bed. The inclusion of packing media (for example, Veolia 1140 Cascade™ Filterpak™) for biofilm and a sludge bed to host granular biomass reduces the degree to which said biofilm and biomass are lost in liquid output from each reactor tank, as these provide support structures for the biofilm and biomass to colonise. This reduces the need to re- colonise the reactor system between treatment cycles, increasing plant throughput and aiding separation of the liquid and biogas phases. Colonising biofilm on the packing media can also remove up to an additional 10% of the COD content, as compared to digestion without colonised biofilm.

At least one reactor tank or break tank may contain one or more gas separators as part of a gas collection system, where each gas separator may be disposed inside the upper half of each reactor tank or break tank.

Using one or more gas separators allows biogas to be treated separately to liquid output, so filtration can optimised for the different phases. The one or more gas separators are able to collect a portion of the biogas evolved, reducing turbulence and gas velocity within each tank. This in turn prevents scouring of the biomass film from the packing media in each reactor tank, thereby contributing to the retention of biomass and optimal rates of reaction, also preventing carryover of biomass to any subsequent tanks. The majority of the gas is collected from the top of each tank as the gas lies above the liquid feedstock, although some may be collected from gas separators disposed around the middle of each tank as well.

Each break tank may have one or more drain mechanisms or valves which may be disposed at the bottom of the break tank to remove precipitates. Precipitates such as iron sulphide (created by the removal of hydrogen sulphide with ferric chloride) can accumulate in each break tank. Drain mechanisms or valves located at the bottom of each break tank aid the removal of these precipitates without unduly disturbing the biomass in the base of the reactor or the biofilm on the packing media in the relevant reactor tank. In particular, valves can maintain anaerobic conditions by preventing air from entering when precipitates are removed.

Each injection manifold may be disposed at the bottom of each reactor tank.

It is advantageous to locate each injection manifold at the bottom of each reactor tank because it ensures optimal mixing, combining the input flow with the recirculation flow to ensure contact between the mixture, the biomass bed and the biofilm. The chances of biomass being stripped from the packing media are reduced by taking this approach. The injection velocity is key to mixing biomass and ensuring that the individual seeded biomass remain in suspension, ensuring optimum contact with the feedstock whilst minimising carryover of biomass from each reactor tank and preventing separation of the biofilm from the packing media. The conditioning tank may have an inline conditioning tank mixer, and at least one reactor tank may have an inline mixer. Preferably, each reactor tank has its own inline mixer.

By providing inline mixers for the conditioning tank, first reactor tank and second reactor tank, the contents of each tank and any added chemicals are adequately mixed together and distributed evenly during the anaerobic digestion process. This maintains an optimal rate of reaction in each reactor tank by ensuring unreacted feedstock reaches the bacteria located in the sludge bed and on the packing media, as well as fully distributing other compounds and any enzymes located in situ. It also ensures that organic waste does not settle in the tanks, which would impair their operation.

There may be an inline filter disposed before each injection manifold, which may restrict the total mass of suspended solids in the organic waste received by each reactor tank to substantially 500 mg/L or less. Placing an inline filter before each injection manifold allows the conditioned feedstock to be filtered prior to reaching the reactor tanks, which cannot operate efficiently with excessive levels of suspended solids. By limiting the total mass of suspended solids in the organic waste to 500 mg/L or less, a predominantly liquid feedstock is provided to the reactor tanks, which is a pre-requisite for efficient high-rate anaerobic digestion. The retention time of the waste in each reactor tank is therefore minimised as the reaction can proceed more quickly with liquid waste. It also enables better mixing of the feedstock with the biomass, concurrently increasing the rate of digestion.

Each reactor tank may have a water trap to prevent biogas leaving the tank by means other than the gas collection system.

Allowing biogas to escape into the atmosphere untreated would be odorous, hazardous, and a loss of a potential resource, so the water trap prevents this by forcing the biogas to transit through the gas collection system. This also allows the liquid output to be separated from the biogas (gaseous output), enabling optimal treatments to be applied to each output individually. Each reactor tank may have one or more pressure-relief safety mechanisms, where the one or more pressure-relief safety mechanisms may include one or more valves which may be disposed at the top of each reactor tank.

This advantageously prevents the build-up of pressure within one or more of the reactor tanks, which would initially slow the evolution of biogas and inhibit the reaction rate, and if continued, would pose a serious safety risk as the affected reactor tank(s) could explode. Placing valves at the top of the tank(s) means that in circumstances where pressure builds up, the relief valve will only release gas when it activates rather than alternate lower placement of the valve(s) which would spill partially digested waste that would need to be cleaned up.

The pressure within each reactor tank may be regulated by a generator and/or a flare which burns excess biogas. Using a generator and/or a flare provides confirmation of the successful production of biogas, indicating that the plant is operating normally.

Biogas output from the reactor system may pass through a biogas scrubber to remove contaminants. The biogas scrubber may contain activated carbon cells to remove hydrogen sulphide and moisture, and biogas leaving the biogas scrubber may be utilised by the generator to produce energy which may power the plant.

By using a biogas scrubber to remove contaminants, such as hydrogen sulphide, from any biogas evolved, it prevents odorous emissions being released by the plant, which mitigates the Health and Safety hazards associated with hydrogen sulphide. Additionally, by removing hydrogen sulphide from the biogas before it is potentially used in the generator prevents further sulphurous emissions being released, as well as preventing damage to the generator, whilst removing moisture allows more efficient combustion of the biogas in a generator. Utilising biogas evolved in situ to generate energy means that there is no need to store the biogas produced, and so there are no operational risks associated with the storage of biogas. The biogas is also a renewable resource, and energy generated from it can potentially be supplied to power local facilities (including the plant itself) or the national grid, for example, thereby providing an additional return for treating waste.

Each reactor tank may be capable of treating up to substantially 200kg of soluble chemical oxygen demand (COD) within a 24-hour period, where the maximum flow rate within the reactor system may be substantially 600 L/hr.

The limit of 200kg of soluble chemical oxygen demand (COD) per reactor tank optimises the small-scale treatment of waste on site, as a demonstration of the viability of treating waste by anaerobic digestion, whilst still permitting the anaerobic digestion plant to be transportable. The flow rate limit of 600 L/hr in turn optimises the throughput of waste whilst mitigating the likelihood of turbulence in the reactor tanks, which would otherwise cause the biomass to separate from the packing media and lead to carryover of the biomass. With these parameters, COD removal of up to 90% is achieved, producing biogas with a methane content of greater than 70%. The control system piping manifold may have one or more valves for controlling the flow of fluids between each component of the reactor system.

Using valves restricts the flow of fluids to the desired direction, preventing reflux through the reactor system. This enables the control system to effectively balance the flow of fluids (and hence maintain efficient circulation) in the reactor system.

The control system may allow the controlled addition of one or more of the following to the reactor system: ferric chloride, sodium bicarbonate, lime, nutrients.

Controlling the addition of ferric chloride to one or more of the tanks allows for careful modulation of the removal of hydrogen sulphide from the feedstock, particularly where waste material with high sulphate concentrations is being treated. As a result, levels of hydrogen sulphide in the biogas are limited to 200ppm and below, and limited to less 0.2 mg/L in liquid output (which compares favourably with the industry standard of less than 1 mg/L). An additional benefit is that ferric chloride can enhance the degree of granulation in the biomass by providing iron in situ.

It is equally advantageous to control the addition of sodium bicarbonate to one or more of the tanks, which can act as a buffer to maintain the pH at a consistent value given that acidic intermediates are generated in situ, and modulate the M-alkalinity of the mixture. Lime may also be used to buffer the mixture. Nutrients are also controllably added to one or more of the tanks to ensure that the bacteria in the biomass have the trace elements they require to function optimally when anaerobically digesting the waste. Nutrient dosing requirements to the feedstock depend on its COD, which is affected by the relative ratios of carbon, nitrogen and phosphorus, typically in the range 250:5: 1 to 500:5: 1.

The control system may allow the controlled addition of sodium hypochlorite to the liquid output of one or more of the reactor tanks to remove hydrogen sulphide.

The use of sodium hypochlorite to treat the liquid output from each reactor tank removes all detectable traces of hydrogen sulphide, meaning that the treated liquid is not odorous and that there are none of the Health and Safety issues associated with hydrogen sulphide, which can otherwise prove corrosive and poisonous.

The heating system may include one or more heat exchangers to maintain an optimum temperature in each reactor tank, one or more temperature probes which may monitor the temperature of each reactor tank, and a means of supplying energy to the heat exchangers.

Using heat exchangers to heat the reactor tanks allows for an even distribution of thermal energy, in contrast to directly heating the feedstock mixture which would potentially generate much greater thermal gradients in the fluid. Note that this would cause convection currents and eddies to mix the feedstock which, whilst mixing may be desirable, would not be easily controlled. Therefore it is more advantageous to heat each reactor tank using heat exchangers to more uniformly heat the feedstock and recirculated material, employing other means to facilitate mixing. The heat exchanger may take the form of a plate heat exchanger, with the incoming flow being heated to keep the reactor at the optimum temperature through heat transfer.

The one or more heat exchangers may include one or more heating loops which may each contain an energy transfer medium, which may consist of water.

The use of heating loops maximises the surface area of the heat exchangers, improving the rate of energy transfer to each tank. As an energy transfer medium, water is efficient at transferring heat since it is a fluid with a very high specific heat capacity.

Each temperature probe may be disposed inside each reactor tank to monitor the internal temperature.

Constantly monitoring the temperature inside each reactor tank via one or more temperature probes ensures that the user or control system can check that each reactor tank is operating at its optimum temperature, maximising the associated reaction rates and hence the overall efficiency of the plant. It also enables the user or control system to identify whether reactor tanks are above or below their optimum temperatures and thus take appropriate measures to prevent, for example, the reactions running too slowly (at overly cool temperatures) or the bacterial cultures dying (at overly hot temperatures).

The means of supplying energy to the heat exchangers may be a boiler fuelled with heating oil.

Heating oil can be burned quickly and efficiently to supply energy to the heat exchangers by heating the water in the heating loops, allowing the heat exchangers to reach the desired temperature without delay.

The total volume of the reactor system may be no greater than 250 cubic metres.

Whilst it is advantageous to treat as much waste as possible during each anaerobic digestion cycle, the plant must also be small enough to still be mobile. As such, a limit of 250 cubic metres allows the plant to still be transportable whilst minimising restrictions on its waste-processing throughput.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made by way of example only to the accompanying drawings, in which:

Figure 1 shows a first embodiment of the mobile anaerobic digestion (AD) plant;

Figure 2 shows part of the control system and piping manifold of the mobile AD plant in Figure 1 ;

Figure 3 shows the pH controls of the control system in Figure 2;

Figure 4 shows part of the heating system of the mobile AD plant in Figure 1; Figure 5 shows a schematic of the mobile AD plant of Figure 1; and Figure 6 shows a flow diagram indicating the anaerobic digestion process facilitated by the mobile AD plant.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring firstly to Figures 1 to 4, a first embodiment of the mobile anaerobic digestion (AD) plant is indicated generally at 10, being mounted on a towable trailer 11 in this embodiment. The control and heating systems are located inside a trailer compartment 12, next to which is the reactor system (indicated generally at 13) comprising: a conditioning tank 14, a first reactor tank 16, a first break tank 18, a second reactor tank 20, and a second break tank 22. The conditioning tank 14 is used to hold and prepare waste for anaerobic digestion, which begins in the first reactor tank 16 and ends in the second reactor tank 20. The first and second break tanks 18, 22 prevent the reflux of waste in the reactor system and each reactor tank 16, 20. The trailer has the trailer compartment 12 at the front of the trailer, with a door on the right-hand side.

Further back, through the remainder of the trailer, the waste is processed in sequence from the conditioning tank 14 near the front to the second break tank 22 near the back of the trailer. The trailer has a moveable curtain 15 which covers the sides of the trailer during transport in a conventional manner. Suitable input and output means are provided to supply incoming and outgoing waste; for example, a transfer pump to bring waste from an intermediary bulk container (IBC) into the reactor system through an inbound pipe, and a discharge pipe for treated waste exiting the reactor system. The same plant can operate the reactor system in series or in parallel, depending on user requirements, with waste being directed from the conditioning tank to each reactor tank individually if operated in parallel, or with the output waste of the first reactor tank being the input waste for the second reactor tank if operated in series.

The interior of the trailer compartment 12 contains a control system which controls the piping manifold 24 and two control panels 26. Compartment 12 also contains the heating system 33 and hot water storage tank 39. The control system itself may initially be operated from a 13 A 240V electrical supply, but can subsequently also be powered from energy provided by the on-board biogas generator. The piping manifold 24 can be seen to have valve isolation switches 27 for different sections of pipe, allowing the flow of fluid between the different components of the reactor system 13 to be carefully controlled. Pressure gauges 25 are also provided to indicate the operating pressures at various points, allowing a user to check whether the values are within acceptable parameters. For instance, if abnormal pressure readings are observed in the first break tank 18, the respective shut-off valves can be used to safely isolate the first break tank 18 until the problem can be further investigated.

The control panels 26 allow a user to monitor the contents of each reactor tank, and take remedial action if it is identified that the conditions are no longer optimal for the rate of digestion. Digital displays 28 and 30 on pH controllers correspond to the first and second reactor tanks 16 and 20 respectively, in the present embodiment. The inputs 32 and 34 to each control panel permit the user to scroll through information on each of the digital displays 28 and 30, and make an informed choice as to whether the any of the conditions (such as pH or temperature) in either tank require intervention, although they are also automatically regulated. The user can adjust the pH through the control panel inputs 32 and 34, which sends a signal to the pH controller to adjust the pH, corresponding to adjustments in the first reactor tank 16 or second reactor tank 20 respectively. This may involve the addition of ferric chloride (to remove hydrogen sulphide), sodium bicarbonate, sodium hydroxide or lime (to increase the pH), and/or nutrients (to supply the biomass with trace elements). Ferric chloride can also improve the supply of iron to the biomass, aiding the digestive process and assisting in the production of granular biomass.

Another part of the interior of the trailer compartment 12 show some of the piping manifold 24 (with associated valve isolation switches 27 and pressure gauges 25) next to the heating system, indicated generally at 33. This includes a boiler 36 fuelled with heating oil, which can heat water in the heating system to supply thermal energy to the reactor system 13. On start-up, the boiler 36 is required to bring the reactor system 13 up to its optimum operating temperature and maintain optimum conditions.

Figure 5 is a schematic which illustrates the interconnectivity between the various components of the reactor system, including the conditioning tank 14, the first reactor tank 16, the first break tank 18, the second reactor tank 20, and the second break tank 22. To begin the anaerobic digestion process, waste is first added to the conditioning tank 14, and pump mixing or a paddle mixer 62 disposed inside the conditioning tank 14 ensures that the waste is uniformly mixed with the diluting solvent prior to digestion in the first reactor tank 16, if dilution is required. After the waste is conditioned, it is pumped incrementally to the first reactor tank 16 via a first heat exchanger 37. The heating system boiler 36 maintains the hot water tank 39 at 60°C, pumping the incoming flow in a loop via the first heat exchanger 37, with heat transfer occurring via the heat exchanger plates to heat the feedstock or recirculated material.

The first reactor tank 16 has a pH probe 42 and temperature probe 40 with a feedback mechanism to the pH correction system 48 and heating system 33 respectively to allow optimal conditions to be maintained. The ferric chloride dosing system 44 and nutrient dosing system 46 also allows ferric chloride and nutrients respectively to be dosed into the tanks 14, 16. Pressure-relief safety valves 60 atop each reactor tank 16, 20 and each break tank 18, 22 are present to ensure that if pressure builds up inside any of the tanks, it can be released in a controlled manner.

The contents of the first break tank 18 can be sent to the second reactor tank 20, which recirculates waste through the second break tank 22. A second heat exchanger 38 can heat the second reactor tank 20 waste during circulation, which then cools slightly as it returns to the relevant reactor tank, reaching the desired temperature for optimal reaction rates. The second reactor tank 20 also has a pH probe 52 and temperature probe 50, again with a feedback mechanism to the pH correction system 48 (for alkaline or M-alkalinity buffer addition, or nutrient dosing) and heating system 33 respectively to allow optimal conditions to be maintained.

Gas collected from each tank (16, 18, 20, 22) constitutes the final biogas product, although methane in the biogas is almost wholly evolved within the second reactor tank 20. Any biogas collected is burned in the flare 56. A water tank 58 (water trap) is utilised to collect condensate, and can be used to set reactor system pressures and as a system fail-safe.

Figure 6 indicates the general high-rate anaerobic digestion process as operated in the mobile AD plant (10), describing the sequence of anaerobic digestion as a flow chart. The description of the plant in operation below describes the steps in greater detail. In use, the waste designated for treatment (usually stored in an IB C) is first sampled to analyse the chemical oxygen demand (COD), pH, composition and concentration of FOGs and volatile fatty acids, sodium concentration and the presence of other metal ions, using a range of techniques which may include spectroscopy, mass spectrometry, and liquid chromatography. These measurements then inform the subsequent dilution of the waste (if required), as well as the feed rate to the reactor tanks 16 and 20. The waste is preferably diluted to have a COD of less than 50000 mg/L, although waste with values higher than this may still be treated, but ideally the waste should contain COD of between 3000 mg/L and 15000 mg/L to ensure maximum COD conversion to biogas. Generally, for a given mass of feedstock COD, approximately the same mass of biomass is required to digest it.

Sodium levels must initially be less than 3000 mg/L, although they may be higher after than this once the biomass has acclimated, but ideally there is less than 1000 mg/L of sodium. Equally, nitrogen levels must be less than 3000 mg/L, although they may be higher after than this once the biomass has acclimated, but ideally there is less than 1000 mg/L of nitrogen content. The reactor tanks 16 and 20 are pre-heated to 39-40°C and 37-38°C respectively before the waste digestion process is initiated, and pH and temperature are further modulated upon deciding whether to operate the reactor system 13 in series or in parallel. Sodium bicarbonate is added to achieve the desired M- alkalinity in each tank, which is 3300 mg/L in the second reactor tank 20 when being operated in series, as this maximises methane production. Conditions in the reactor tanks 16 and 20 are monitored and maintained by internal pH and temperature probes (40, 42, 50 and 52), which are linked to the control system, allowing the temperature and pH to be adjusted automatically as needed by the heating system 33, ferric chloride dosing system 44, nutrient dosing system 46 and pH correction system 48. A pH controller controls the pH in one embodiment, and the temperature probes directly control the temperature in another embodiment. For example, the first reactor tank 16 preferably operates at a temperature of 39°C to 40°C and at a pH of between 5.8 and 6.8 when the plant is operating the reactor tanks 16, 20 in series. The pH is automatically adjusted by the control system, with the pH probes 40, 42 monitoring the pH within the reactor tanks 16, 20 and sending data back to control system which controls each of the systems 44, 46, 48 as needed to maintain an ideal pH value. The M-alkalinity buffer is maintained in the range 1500 to 3500 mg/L as required. The temperature is also automatically controlled, with a signal from the temperature probes 50, 52 causing the supply of hot water to the relevant tank to increase or decrease as needed to maintain the desired temperature.

The conditions above promote the hydrolysis, acidogenesis and acetogenesis digestion steps for the types of bacteria which enable the associated reactions. The second reactor tank 20 then preferably operates at a temperature of 37°C to 38°C and at a pH of between 7.2 and 7.8 when the plant is operating the reactor tanks 16, 20 in series, as these conditions promote methanogenesis, producing biogas with fewer contaminants such as hydrogen sulphide, as sulphate-reducing bacterial processes are slow relative to methane-producing bacterial processes under these conditions. Sulphate-reducing bacteria prefer conditions where the pH is between 6.6 and 7.4, and the temperature between 35-37°C for optimum rates of reaction, but the methanogenic bacteria out- compete them in the conditions used in the second reactor tank.

If operating the first and second reactor tanks 16 and 20 in parallel, the tanks each operate at a temperature of 38°C to 39°C and at a pH of between 6.8 and 7.2, balancing the reaction conditions to enable all of the aforementioned types of reaction to take place. The reactor system 13 is operated at reduced pressure of around 30 mbar, driving the relevant equilibria towards the evolution of gaseous products, maximising biogas production. Importantly, operating at reduced pressure also maintains anaerobic conditions as the partial pressure of oxygen is negligible, allowing anaerobic processes to occur preferentially. For safety, pressure-release valves 60 are present on each reactor tank 16, 20 and each break tank 18, 22 preventing any build-up of gas due to reactor failure from causing an explosion.

After the plant has been set up, waste is pumped into the conditioning tank 14 from intermediary bulk containers (IBCs) or the waste source, at which point the waste is then diluted (which may be achieved via the recirculation flow) and mixed to achieve a liquid of uniform viscosity. This ensures the waste has a low total mass of suspended solids per unit volume. The mixture may also be pre-heated in the conditioning tank 14 to ensure that the rate of is optimal from the point at which the mixture is pumped to the first reactor tank 16. Ferric chloride and/or nutrients may be added during the mixing to achieve the ideal composition. If not already heated, the waste is then heated by first and second heat exchangers 37 and 38 (and continually maintained at temperature by recirculation through the reactor system 13 and first and second heat exchangers 37 and 38), and the M-alkalinity adjusted by adding sodium bicarbonate or lime, if not already mixed in when the waste was in the conditioning tank 14. The recirculated waste is heated to maintain ideal conditions for the corresponding bacteria.

If operating in series, the first reactor tank 16 converts the waste to acetic acid. The conditioned waste is sent to the first reactor tank 16 via an inline filter and an injection manifold at the base of the tank. The filter is necessary to ensure that the total mass of suspended solids in the mixture is limited to 500 mg/L or less upon entering the first reactor tank 16. The feedstock is initially hydrolysed in the first reactor tank 16, followed by acidogenesis and acetogenesis to reduce the fatty acid chains to acetic acid via the granular biomass of the sludge bed and biomass cultured on the packing media inside the tank. The packing media may comprise one or more Veolia 1140 Cascade™ Filterpaks™, or more generally an inert material with a large surface area, for the purposes of culturing a thin film of bacteria. The packing media is around one metre in height, and is situated around one metre above the sludge bed inside the tanks. The waste is continually recirculated through the first break tank 18, which prevents reflux through the reactor system and also allows additional ferric chloride to be dosed as required. Iron sulphide precipitates can be removed from the first break tank 18 via a drain valve located near its base. The partially digested waste is gradually sent to the second reactor tank 20 from the first break tank 18, where it is mixed with methanogenic bacteria in the biomass sludge bed. It also comes into further contact with methanogenic bacteria on the packing media in the form of biofilm, converting the acetic acid to biogas. The waste is again continually recirculated through the second break tank 22, which prevents reflux through the reactor system and also allows additional ferric chloride to be dosed as required. Iron sulphide precipitates can be removed from the break tank 22 by a drain valve located near its base. The treated waste is then discharged through a water trap to prevent biogas escaping, with sodium hypochlorite dosing to the liquid output if required to remove any residual hydrogen sulphide. Operating the reactor tanks 16 and 20 in parallel is similar, but does not use a separate methanogenic bacterial culture. Instead, such a system operates with a sludge bed and biofilm on the packing media containing all four types of digestive bacteria. A proportion of the biogas evolved is collected by gas separators inside the reactor tanks 16 and 20, at which point it is passed through the connecting break tanks 18 and 22 to reduce turbulence within the packing section of the reactor tanks, preventing scouring of the biofilm. This also aids scrubbing of the biogas, and assists with carbon dioxide capture via the M-alkalinity buffer. The remaining biogas is collected at the top of the reactors and passes through the biogas scrubber 53, which uses activated carbon cells to remove moisture and hydrogen sulphide. This renders the biogas suitable for combustion in the generator 54 to provide energy, in addition to fuelling the external flare 56 which indicates biogas is being produced. Energy produced in this manner may be used to power the demonstration unit instead of consuming electricity, reducing the overall cost of running the plant as energy from previously digested waste can be used to warm currently digesting waste, making the whole plant energy efficient. The biogas output can be analysed by a biogas analyser to monitor its composition and confirm that the plant is operating correctly (and also acts as an indicator of plant operating performance). Overall, the mobile AD plant is capable of treating up to 400kg of soluble COD in a 24-hour period, with a maximum flow rate of 600 L/hr. With the parameters described above, the plant can achieve COD removal of 90%, producing biogas with a methane content of greater than 70%.

The above description covers one embodiment of a mobile anaerobic digestion plant, but other embodiments are also envisaged within the scope of this application. Other embodiments might include a mobile AD plant fully integrated into a lorry, van or other vehicle, as opposed to individual tanks mounted onto a trailer as in the above embodiment. Furthermore, an embodiment where an intermediary bulk container (IBC) with waste can be loaded onto the mobile AD plant vehicle, and the AD process undertaken during transit to the next waste site (perhaps harvesting energy from the biogas generator to power the vehicle or heat the heating system), is another possible embodiment. A further embodiment may include a conditioning tank which also functions as the first reactor tank, saving additional space and weight which is advantageous in terms of overall plant mobility. An additional adaptation might be the incorporation of each break tank into each reactor tank, further saving space and reducing thermal losses which would otherwise occur during waste transfer.

Another possible embodiment might involve pre-treating the waste in the conditioning tank with various chemicals, or enzymes, or bacteria, or any combination thereof, enabling the anaerobic digestion of waste which would otherwise be processed with minimal biogas yield and poor decomposition of FOGs. The collection of all gas occurs via gas separators, using gas headers which are located above the gas collection void at the top of both break tanks and reactor tanks. It may also be that another embodiment involves a storage tank for containing biogas evolved, which would be commercially beneficial relative to burning excess biogas. Furthermore, the generator which burns biogas may have a means for storing the energy generated for later discharge, such as one or more capacitors, or for the potential use of biogas within a fuel cell. The reaction conditions for each stage of the anaerobic digestion process may be adapted to accommodate alternate strains of bacteria, with different nutrients as needed. Other buffer compounds, such as phosphates, may be used to maintain the pH instead of sodium bicarbonate. Additionally, hydrogen sulphide may be removed from the input and output waste of the reactor system by means other than activated carbon cells and ferric chloride dosing, precipitating various sulphides using elements which are nontoxic to the bacteria.

In summary, it is therefore possible to provide a mobile high-rate anaerobic digestion plant which is mounted on a vehicle or towable trailer, and includes a reactor system, a control system, and a heating system. The reactor system may include two or more reactor tanks, operable in series or in parallel, with a gas collection system in at least one reactor tank. The control system may include a piping manifold with means for controlling the flow of fluids to and from each reactor tank, and the heating system may be adapted to control the temperature of at least one reactor tank.

The embodiments described above are provided by way of example only, and various changes and modifications will be apparent to persons skilled in the art without departing from the scope of the present invention as defined by the appended claims.

Claims

1. A mobile anaerobic digestion plant for the high-rate anaerobic digestion of organic waste to evolve biogas comprising:

a reactor system, a control system, and a heating system, mounted on or in a vehicle or towable trailer;

the reactor system including two or more reactor tanks, operable in series or in parallel, with a gas collection system in at least one reactor tank; the control system including a piping manifold with means for controlling the flow of fluids in the reactor system; and

the heating system being adapted to control the temperature of at least one reactor tank.

2. A mobile anaerobic digestion plant as claimed in claim 1, in which the reactor system includes a conditioning tank, a first reactor tank, a first break tank, a first recirculation loop, a second reactor tank, a second break tank, and a second recirculation loop, with the first reactor tank connected to the first break tank via the first recirculation loop, and the second reactor tank connected to the second break tank via the second recirculation loop, where at least one reactor tank is connected to the conditioning tank, and the second reactor tank is also connected to the first reactor tank.

3. A mobile anaerobic digestion plant as claimed in claim 1 or 2, in which the control system includes a pH correction system which allows the pH within at least one reactor tank to be controlled by at least one of the following: alkaline addition, M-alkalinity buffer addition.

4. A mobile anaerobic digestion plant as claimed in any one of claims 1 to 3, in which the heating system allows the temperature within at least one reactor tank to be controlled.

5. A mobile anaerobic digestion plant as claimed in any one of claims 1 to 4, in which, when two or more reactor tanks are operating in series, the first reactor tank includes granular biomass and biofilm which facilitate one or more of the following types of reaction: hydrolytic, acidogenic, acetogenic.

6. A mobile anaerobic digestion plant as claimed in claim 5, in which the contents of the first reactor tank are maintained substantially in the temperature range

39°C to 40°C, with a pH range of substantially 5.8 to 6.8, and with an M- alkalinity buffer substantially in the range 1500 mg/L to 2500 mg/L.

7. A mobile anaerobic digestion plant as claimed in claim 5 or 6, in which, when two or more reactor tanks are operating in series, the second reactor tank includes granular biomass and biofilm which facilitate one or more of the following types of reaction: methanogenic.

8. A mobile anaerobic digestion plant as claimed in any one of claims 5 to 7, in which the contents of the second reactor tank are maintained substantially in the temperature range 37°C to 38°C, with a pH range of substantially 7.2 to 7.8, and with an M-alkalinity buffer substantially in the range 2500 mg/L to 3500 mg/L.

9. A mobile anaerobic digestion plant as claimed in claim 8, in which the second reactor tank contains an M-alkalinity buffer of substantially 3300 mg/L.

10. A mobile anaerobic digestion plant as claimed in any one of claims 1 to 4, in which, when two or more reactor tanks are operating in parallel, each reactor tank includes granular biomass and biofilm which facilitate one or more of the following types of reaction: hydrolytic, acidogenic, acetogenic, methanogenic.

11. A mobile anaerobic digestion plant as claimed in claim 10, in which the contents of at least one reactor tank are maintained substantially in the temperature range 38°C to 39°C, with a pH range of substantially 6.8 to 7.2, and with an M- alkalinity buffer of substantially 2500 mg/L.

12. A mobile anaerobic digestion plant as claimed in any one of claims 3 to 11, in which the M-alkalinity buffer is predominantly sodium bicarbonate.

13. A mobile anaerobic digestion plant as claimed in any preceding claim, in which the reactor system digests organic waste with a sodium content of substantially 1000 mg/L or less, up to a maximum sodium content of substantially 3000 mg/L.

14. A mobile anaerobic digestion plant as claimed in any preceding claim, in which the reactor system digests organic waste with an nitrogen content of substantially 1000 mg/L or less, up to a maximum nitrogen content of substantially 3000 mg/L.

15. A mobile anaerobic digestion plant as claimed in any preceding claim, in which the reactor system digests organic waste with a chemical oxygen demand (COD) greater than substantially 3000 mg/L.

16. A mobile anaerobic digestion plant as claimed in claim 15, in which the reactor system digests organic waste with a chemical oxygen demand (COD) between substantially 3000 mg/L and 15000 mg/L.

17. A mobile anaerobic digestion plant as claimed in any preceding claim, in which the reactor system operates substantially in the range 20 to 40 mbar.

18. A mobile anaerobic digestion plant as claimed in any one of claims 2 to 17, in which organic waste is decomposed into volatile fatty acids (VFAs), where the concentration of VFAs in the first reactor tank is maintained at substantially 10 g/L or less, and where the concentration of VFAs in the second reactor tank is maintained at substantially 1 g/L or less.

19. A mobile anaerobic digestion plant as claimed in any preceding claim, in which at least one reactor tank has an injection manifold.

20. A mobile anaerobic digestion plant as claimed in any preceding claim, in which each reactor tank includes one or more strains of bacteria seeded on packing media and in a sludge bed.

21. A mobile anaerobic digestion plant as claimed in claim 19 and 20, in which at least one reactor tank or break tank contains one or more gas separators as part of a gas collection system, where each gas separator is disposed inside the upper half of each reactor tank or break tank.

22. A mobile anaerobic digestion plant as claimed in any one of claims 2 to 21, in which each break tank has one or more drain mechanisms or valves which are disposed at the bottom of each break tank to remove precipitates.

23. A mobile anaerobic digestion plant as claimed in any one of claims 2 to 22, in which each injection manifold is disposed at the bottom of each reactor tank.

24. A mobile anaerobic digestion plant as claimed in any one of claims 2 to 23, in which the conditioning tank has an inline conditioning tank mixer.

25. A mobile anaerobic digestion plant as claimed in any preceding claim, in which at least one reactor tank has an inline mixer.

26. A mobile anaerobic digestion plant as claimed in claim 19, in which there is an inline filter disposed before each injection manifold, restricting the total mass of suspended solids in the organic waste received by each reactor tank to substantially 500 mg/L or less.

27. A mobile anaerobic digestion plant as claimed in any one of claims 19 to 26, in which each reactor tank has a water trap to prevent biogas leaving the tank by means other than the gas collection system.

28. A mobile anaerobic digestion plant as claimed in any preceding claim, in which each reactor tank has one or more pressure-relief safety mechanisms.

29. A mobile anaerobic digestion plant as claimed in claim 28, in which the one or more pressure-relief safety mechanisms include one or more valves disposed at the top of each reactor tank.

30. A mobile anaerobic digestion plant as claimed in any preceding claim, in which pressure within each reactor tank is regulated by a generator and/or a flare which burns excess biogas.

31. A mobile anaerobic digestion plant as claimed in any preceding claim, in which biogas output from the reactor system passes through a biogas scrubber to remove contaminants.

32. A mobile anaerobic digestion plant as claimed in claim 31, in which the biogas scrubber contains activated carbon cells to remove hydrogen sulphide.

33. A mobile anaerobic digestion plant as claimed in claim 31 or 32, in which biogas leaving the biogas scrubber is utilised by the generator to produce energy which powers the plant.

34. A mobile anaerobic digestion plant as claimed in any preceding claim, in which each reactor tank is capable of treating up to substantially 200kg of soluble chemical oxygen demand (COD) within a 24-hour period.

35. A mobile anaerobic digestion plant as claimed in any preceding claim, in which the maximum flow rate within the reactor system is substantially 600 L/hr.

36. A mobile anaerobic digestion plant as claimed in any preceding claim, in which the control system piping manifold has one or more valves for controlling the flow of fluids between each component of the reactor system.

37. A mobile anaerobic digestion plant as claimed in any preceding claim, in which the control system includes a nutrient dosing system to allow the controlled addition of one or more of the following to the reactor system: ferric chloride, sodium bicarbonate, lime, nutrients.

38. A mobile anaerobic digestion plant as claimed in any preceding claim, in which the control system allows the controlled addition of sodium hypochlorite to the liquid output of one or more of the reactor tanks to remove hydrogen sulphide.

39. A mobile anaerobic digestion plant as claimed in any preceding claim, in which the heating system includes one or more heat exchangers to maintain an optimum temperature in each reactor tank, one or more temperature probes to monitor the temperature of each reactor tank, and a means of supplying energy to the heat exchangers.

40. A mobile anaerobic digestion plant as claimed in claim 39, in which the one or more heat exchangers include one or more heating loops each containing an energy transfer medium consisting of water.

41. A mobile anaerobic digestion plant as claimed in claim 39 or 40, in which each temperature probe is disposed inside each reactor tank to monitor the internal temperature.

42. A mobile anaerobic digestion plant as claimed in any one of claims 39 to 41, in which the means of supplying energy to the heat exchangers is a boiler fuelled by heating oil.

43. A mobile anaerobic digestion plant as claimed in any preceding claim, in which the total volume of the reactor system is no greater than 250 cubic metres.

44. A mobile anaerobic digestion plant substantially as described herein, with reference to and as illustrated in Figures 1 to 6 of the accompanying drawings.