THIS INVENTION relates to treatment of organic waste materials. More particularly, the invention relates to anaerobic digestion of organic waste materials. The invention provides a method of operating an anaerobic digester system. The invention also provides an anaerobic digester system. Anaerobic digestion is a biological process which involves the breakdown or biodegradation of organic material(s), in the present case organic waste materials such as sewage, by microorganisms, such as bacteria, in the absence of oxygen. Generally, anaerobic digestion of an organic material proceeds along four biological stages comprising hydrolysis, acidogenesis, acetogenesis, and methanogenesis. In each of these stages the organic feedstock is progressively broken down or biodegraded into simpler or more basic products. More particularly, hydrolysis involves the breakdown of insoluble organic polymers into sugars and amino acids; acidogenesis involves the conversion of the sugars and amino acids into organic acids with carbon dioxide, hydrogen and ammonia being released as by products; acetogenesis involves the conversion of the organic acids into acetic acid with carbon dioxide, hydrogen and ammonia again being released as by products; and methanogenesis involves the conversion of the acetic acid into biogas, such as methane, and carbon dioxide. The anaerobic digestion of organic material thus produces a solid-based anaerobic digestate, usually of lesser volume than the original feedstock, and also produces, in addition to other by-products, biogas, such as methane. The biogas which is produced by anaerobic digestion of organic materials, particularly by the methanogenesis stage thereof, is known to be useful as a renewable energy source, more particularly as a fuel, or for generating electricity, heating, and the like. Although anaerobic digestion is used commonly in the treatment of organic waste materials, such as sewage, anaerobic digestion treatment processes are not, in the Applicant's experience, optimized and exploited for their production of biogas, particularly not in rural areas where these processes may find particular application and provide welcome relief, considering that appropriate supporting infrastructure, such as a reliable electricity supply, is not readily available in such areas. It is the Applicant's experience that this lack of exploitation of anaerobic digestion treatment processes for their production of biogas is, at least partly, due to the complex nature of anaerobic digestion as a biological process, requiring highly skilled operators to exercise accurate control over the various stages of the process in order to optimize biogas production and render the biogas production commercially viable. Most notable of these is related to the production of volatile fatty acids (VFA's) during acetogenesis. The VFA's are required by the methanogenic microorganisms, or bacteria, in order to produce the biogas. The acetogenic bacteria, producing the VFA's during acetogenesis, are, however, more efficient at producing VFA than the methanogenic bacteria are at producing methane gas. Accordingly, the supreme activity of the acetogenic bacteria typically results in an increase of the acidity, and thus a drop in the pH, of the anaerobic digestion process. This has a negative effect on the production of biogas as the methanogenic bacteria are less active at lower pH. This feature of anaerobic digestion illustrates the requirement for constant hands-on monitoring and control of anaerobic digestion. The requirement of pH control in particular is therefore an obstacle to unskilled exploitation of anaerobic digester systems.

The requirement of highly skilled operator presence renders the expansion of anaerobic treatment plants, such as sewage plants, into biogas production facilities economically and practically unattractive, particularly for rural areas. The present invention seeks to address this requirement.

The present invention also seeks to maximise the useful outputs from an anaerobic digestion process, particularly as regards biogas production and anaerobic digestate recovery, without impacting negatively on biogas production/collection efficiency.

In accordance with one aspect of the invention, there is provided a method of operating an anaerobic digestion system, the method including

feeding, under gravity, fresh organic material feedstock to an anaerobic digestion stage along an organic material feedstock feed line;

subjecting the feedstock to anaerobic digestion in the anaerobic digestion stage; withdrawing biogas along a digestion stage biogas transfer line from the digestion stage;

passing withdrawn biogas to and collecting withdrawn biogas in a biogas collection stage;

withdrawing, under gravity, digestate generated in the anaerobic digestion stage from the anaerobic digestion stage;

transferring the withdrawn digestate to a digestate phase separation stage along a digestate transfer line; and

allowing the digestate to settle and separate into phases thereof in the digestate phase separation stage.

The anaerobic digestion system may be a system as is hereinafter described. The method may include pre-loading the anaerobic digestion stage, preferably to operating capacity thereof, with at least one of pre-loaded, or start-up, donor organic material and pre-loaded, or start-up, organic material feedstock, prior to commencing feeding fresh organic material feedstock into the anaerobic digestion stage. Preferably, the method includes pre-loading the anaerobic digestion stage with both pre-loaded donor organic material and pre-loaded organic material feedstock, typically in a ratio of about 2:1 , by volume. In this regard, it is to be noted that although the fresh organic material feedstock and pre-loaded organic material feedstock may typically be similar in nature, they are respectively designated as "preloaded" and "fresh" for the purposes of indicating their respective roles in the method of the invention. Feeding fresh organic material feedstock to the anaerobic digestion stage, under gravity, and withdrawing digestate generated in the anaerobic digestion stage from the anaerobic digestion stage, under gravity, may be conducted synchronously. Feeding organic material feedstock into the anaerobic digestion stage may be conducted incrementally, i.e. by feeding predetermined quantities or volumes of fresh organic material feedstock to the anaerobic digestion stage at predetermined intervals, thus constituting a predetermined incremental fresh organic material feedstock feed regime. The method may therefore include metering the quantity of feedstock that is fed to the anaerobic digestion stage, preferably over a predetermined time period. Typically, the organic material feedstock may be fed to the anaerobic digestion stage at a rate of 12 to 16 litres per day, preferably being broken up in feed increments of 0.5 to 0.66 litres every hour or 1 to 1 .22 litres every 2 hours or, more preferably, 3 to 3.96 litres every 6 hours.

Incremental feeding of fresh organic material feedstock to the anaerobic digestion stage may be controlled by means of a fresh organic material feedstock feed regulator, such as a valve. Prior to commencing incremental feeding of organic material feedstock to the anaerobic digestion stage, the flow rate of organic material feedstock, under gravity, would typically be determined experimentally so as to determine how long the valve should remain open so as to deliver the desired incremental volume of organic material feedstock thereto. Alternatively, incremental feeding of fresh organic material feedstock to the anaerobic digestion stage may involve use of a flow sensor, provided to measure the quantity of fresh organic material feedstock that is delivered to the anaerobic digestion stage. With feeding of fresh organic material feedstock to the anaerobic digestion stage and withdrawal of digestate from the anaerobic digestion stage being synchronised, withdrawal of digestate from the anaerobic digestion stage and feeding thereof to the digestate phase separation stage may thus also be metered. It will be appreciated that metered quantities or volumes of fresh organic material feedstock and digestate respectively being fed to the anaerobic digestion stage and to the digestate phase separation stage may be equivalent, i.e. more or less equal. Through synchronised feeding to and withdrawal from the anaerobic digestion stage, it is sought to achieve a synchronized and constant flow of matter through the anaerobic digestion system, thereby to prevent the forming of pressure differences, considering that the system operates under anaerobic conditions.

The method may include controlling, preferably automatically, operating conditions in the anaerobic digestion stage, including at least controlling anaerobic digestion stage operating temperature and controlling anaerobic digestion stage operating pH, by means of operating condition regulators, each including at least one anaerobic digestion stage operating condition control component. The operating condition regulators may therefore include at least an anaerobic digestion stage operating temperature regulator and an anaerobic digestion stage operating pH regulator. It is to be noted that, in this specification, the term "regulator" is used broadly, designating an anaerobic digester system component or arrangement of components capable of exercising control over or, more particularly, of effecting change in or adjustment of an anaerobic digestion stage operating condition. Controlling anaerobic digestion stage operating temperature may include passing a heat transfer medium through the anaerobic digestion stage, in a heat transfer relationship with the organic material being subjected to anaerobic digestion in the anaerobic digestion stage. Of course, this control would be exercised by the anaerobic digestion stage operating temperature regulator, by means of its operating temperature control component.

Controlling anaerobic digestion stage operating pH may include feeding or withholding organic material feedstock to/from the anaerobic digestion stage. In this regard, it is to be noted that an increase in the quantity of fresh organic material feedstock in the anaerobic digestion stage would generally result in an increase in the quantity of volatile fatty acids being produced by the acetogenic bacteria. Thus, should the anaerobic digestion stage operating pH be too high, the method may include feeding additional fresh organic material feedstock to the anaerobic digestion stage, thereby to increase acetogenic bacteria activity and cause an increase in volatile fatty acid production and thereby lowering the anaerobic digestion stage operating pH. Such pH controlling feeding of fresh organic material feedstock to the anaerobic digestion stage would, of course, typically not be in accordance with the predetermined incremental fresh organic material feedstock feed regime. Conversely, the quantity of volatile fatty acids which are generated by a smaller quantity of fresh organic material feedstock present in the anaerobic digestion stage would reduce as acetogenesis nears completion for the particular quantity of fresh organic material feedstock. Accordingly, should the operating pH of the anaerobic digestion stage be too low, the method may include withholding one or more particular fresh organic material feedstock feed increments from the anaerobic digestion stage, which feed increments are due in accordance with the organic material feedstock feed regime, thereby effectively starving the anaerobic digestion stage of fresh organic material feedstock and not only preventing a further decrease in pH due to an increase in acetogenic activity, but also effecting a progressive increase in the operating pH of the anaerobic digestion stage due to the consumption of volatile fatty acids by methanogenic bacteria in producing biogas. Again, such pH controlling withholding of fresh organic material feedstock from the anaerobic digestion stage would, of course, typically also not be in accordance with the predetermined incremental fresh organic material feedstock feed regime. Typically, pH control would including retarding or advancing, as appropriate, a particular feed increment due in accordance with the fresh organic material feedstock feed regime.

Controlling operating conditions in the anaerobic digestion stage may be carried out, at least in part, automatically by an electronic anaerobic digestion stage operating condition control system. The control system may include an electronic controller. The electronic controller may comprise, in particular, a computer-based data-capturing programmable controller and may operate by means of a combination of analogue and digital inputs and outputs. The electronic controller may have a database of desired anaerobic digestion stage operating condition values, and may exercise control by

measuring values of operating conditions of the anaerobic digestion stage, including at least anaerobic digestion stage operating temperature and anaerobic digestion stage operating pH; communicating measured operating condition values including at least a measured anaerobic digestion stage operating temperature value and a measured anaerobic digestion stage operating pH value, to the electronic controller;

comparing the measured operating condition values, including at least the measured anaerobic digestion stage operating temperature value and the measured anaerobic digestion stage operating pH value, to their corresponding desired operating values; and

curing any discrepancy between a measured operating condition value and a corresponding desired operating condition value by communicating a control signal to the associated operating condition regulator, causing said operating condition regulator to adjust its associated operating condition.

Communication of the control signal to the associated operating condition regulator may, in fact, be communication to the operating condition control component of the operating condition control regulator.

The method may further include mixing or agitating contents of the anaerobic digestion stage, typically by means of a mixer. In particular, the method may include agitating the contents of the anaerobic digestion stage while fresh organic material feedstock is being fed to the digestion stage, as well as when pH or temperature control is being exercised on the anaerobic digestion stage.

Operation of the mixer may also be controlled by the anaerobic digestion stage operating condition control system, in particular by the electronic controller forming part thereof. The method may also include mixing or agitating the fresh organic material feedstock, prior to feeding thereof to the anaerobic digestion stage, thereby preventing stratification thereof.

The method may also include withdrawing biogas generated in the phase separation stage, due to residual methanogenic activity in the phase separation stage, from the phase separation stage along a phase separation stage biogas transfer line and transferring the so-withdrawn biogas to the biogas collection stage.

The method may also include generating electricity for operating electrical system components, such as the electronic controller and other system components. Preferably, the method includes generating electricity from a renewable energy source, such as solar energy and/or wind energy. The method may also include storing generated electrical energy, typically in one or more batteries.

In accordance with another aspect of the invention, there is provided an anaerobic digester system, which includes

a fresh organic material feedstock feeder;

an anaerobic digestion stage which is located operatively lower than the organic material feedstock feeder;

an organic material feedstock feed line which leads from the fresh organic material feedstock feeder to the anaerobic digestion stage and along which fresh organic material feedstock can be passed, under gravity, from the fresh organic material feedstock feeder to the anaerobic digestion stage and which is provided with a fresh organic material feedstock feed regulator;

a digestate phase separation stage, which is located operatively lower than the anaerobic digestion stage;

a digestate transfer line leading from the anaerobic digestion stage to the phase separation stage, along which digestate transfer line anaerobic digestate generated in the anaerobic digestion stage can be passed, under gravity, from the anaerobic digestion stage to the digestate phase separation stage, the digestate transfer line being provided with a digestate withdrawal feed regulator;

a biogas collection stage; and

an anaerobic digestion stage biogas transfer line which leads into the biogas collection stage from the anaerobic digestion stage and along which biogas generated in the anaerobic digestion stage can be passed from the anaerobic digestion stage to the gas collection stage.

It is to be noted that the term "feed regulator" is used broadly, designating an anaerobic digester system component that is capable of allowing and disallowing feed/flow of the material with which it is associated. The feeder may comprise a feed vessel or tank, e.g. a conical bottom hopper tank and may be provided with a feeder mixer or agitator, acting to prevent stratification of the fresh organic material feedstock in the feeder, prior to being fed to the anaerobic digestion stage. The anaerobic digestion stage and the digestate phase separation stage may each be single stage in nature, respectively comprising a single anaerobic digestion vessel or tank and a single digestate density separation vessel or tank. Both the anaerobic digestion vessel and the digestate density separation tank may be sealed so as to provide anaerobic conditions. The inside of the anaerobic digestion vessel, however, may be rendered accessible by means of an air-tight lid which closes an opening of the anaerobic digestion vessel and through which lid the fresh organic material feed line may, possibly, lead. The digestate phase separation stage may, in particular, be a digestate density separation stage.

The organic material feedstock feed regulator and digestate withdrawal feed regulator may be operatively associated with each other for synchronous regulation of feeding organic material feedstock to and withdrawing digestate from the anaerobic digestion stage. Thus, the organic material feedstock feed regulator and the digestate withdrawal feed regulator may be configured such that feeding a particular quantity or volume of fresh organic material feedstock to the anaerobic digestion stage occurs synchronously with withdrawal of an equivalent quantity or volume of digestate from the anaerobic digestion stage. The organic material feedstock feed regulator and digestate withdrawal feed regulator may, in particular, each comprise a valve, particularly a solenoid valve, provided inline respectively in the fresh organic material feedstock feed line and the digestate withdrawal line. The anaerobic digestion system may further include a plurality of anaerobic digestion stage operating condition regulators. Such anaerobic digestion stage operating condition regulators may include at least an anaerobic digestion stage operating temperature regulator and an anaerobic digestion stage operating pH regulator, configured to control associated anaerobic digestion stage operating conditions. The same consideration outlined above in relation to the use of the term "regulator" applies here. Each operating condition regulator may include an operating condition control component, the anaerobic digester system including at least an anaerobic digestion stage operating temperature control component and an anaerobic digestion stage operating pH control component.

The anaerobic digestion stage operating temperature regulator may, in particular, comprise a heat exchanger including a heat source and a heat exchange element. The heat exchange element may be arranged in a heat transfer relationship with the anaerobic digestion stage. In a preferred embodiment of the invention, the heat exchanger may operate by means of a heat transfer fluid, typically being water. In such a case, the heat source may comprise a water heater, preferably a solar water heater, and the heat exchange element may comprise coiled tubing which is in flow communication with the solar water heater such that heated water can be passed through the coiled tubing from the solar water heater. The heat exchange element, i.e. the coiled tubing, may typically be provided inside the anaerobic digestion stage, or vessel for that matter, thus being in contact with the contents of the digestion stage. Alternatively, the tubing may be provided around, i.e. externally to, the digestion vessel. The anaerobic digestion stage operating temperature control component may, in particular, be a heat transfer medium pump, capable of initiating and maintaining heat transfer medium flow through the heat exchange element and of discontinuing such flow.

The anaerobic digestion stage operating pH regulator may comprise the fresh organic material feedstock feed regulator, for regulating anaerobic digestion stage pH by delivering/withholding fresh organic material feedstock to/from the anaerobic digestion stage. In such a case, the anaerobic digestion stage operating pH control component would, of course, also be the fresh organic material feedstock feed regulator.

The digestate phase separation stage, or at least the digestate phase separation stage vessel, may be provided with a plurality of digestate phase outlet lines, being vertically spaced from each other. Such provision of digestate phase outlet lines may enable separate or discreet withdrawal, from various levels in the digestate density separation vessel, of phases into which the digestate separates in the digestate phase separation stage. Alternatively, the digestate phase separation stage, or vessel, may be provided with a single outlet line, typically a bottoms outlet line, such that the various phases of the digestate can be withdrawn sequentially from the settling tank along the same outlet line. Generally, anaerobic digestate may separate, under quiescent conditions, into four phases. These phases comprise, from least to most dense: scum, supernatant, active sludge, and stabilized sludge. It will be appreciated that, whilst the stabilized sludge has been digested completely, the active sludge may still be undergoing anaerobic digestion even in the digestate phase separation stage. In such a case, the digestate phase separation stage may typically include separate phase outlet lines only for the scum, supernatant, and stabilized sludge respectively. This will render it possible for the active sludge to be retained in the digestate phase separation stage so as to stabilize. The active sludge is typically more dense than the supernatant, but less dense than the stabilized sludge. It will be appreciated that the active sludge will thus typically separate as a layer between the supernatant and the stabilized sludge, thus being subjected to anaerobic conditions, resulting in further anaerobic digestion which produces at least some biogas in the digestate phase separation stage. Biogas may thus also be recoverable from the digestate phase separation stage. Accordingly, the anaerobic digestion stage may include a phase separation stage biogas transfer line, along which biogas which may have been generated inside the phase separation stage due to the presence of activated sludge in the phase separation stage, may be transferred to the gas collection stage.

The anaerobic digester system may also include mixing or agitation means, such as an anaerobic digestion stage mixer or agitator having a mixing impeller, that is provided in the anaerobic digestion stage, more particularly in the anaerobic digestion vessel or tank. The anaerobic digestion stage mixer and the feeder mixer may be configured for synchronous operation.

The anaerobic digester system may also include an anaerobic digestion stage operating condition automatic control system. The anaerobic digestion stage operating condition automatic control system may include

a number of anaerobic digestion stage operating condition sensors arranged to measure values of measured operating conditions, including at least an anaerobic digestion stage operating pH sensor and an anaerobic digestion stage operating temperature sensor; and

an electronic controller, containing or being programmed with desired anaerobic digestion stage operating condition values, configured to receive operating condition measurement signals from the operating condition sensors and to communicate operating condition control signals to the operating condition regulators, or to their associated operating condition control components, causing the operating condition regulators to adjust their associated anaerobic digestion stage operating conditions. The anaerobic digestion stage operating condition sensors may be configured to take measurements, continuously or intermittently, i.e. periodically, of respective anaerobic digestion stage operating conditions and to communicate, typically by means of measurements signals transmitted along communication lines, measured values of measured anaerobic digestion stage operating conditions to the electronic controller. It will be appreciated that the communication lines may comprise fixed lines, the method thus involving fixed-lined communication. Alternatively, the communication lines may be hypothetical lines, with the communication of measured values to the electronic controller being done wirelessly, e.g. by means of radio waves. The anaerobic digestion stage operating condition sensors may, optionally, include, in addition to the anaerobic digestion stage operating pH sensor and the anaerobic digestion stage operating temperature sensor, a fresh organic material feedstock feed rate sensor that is provided in the fresh organic material feedstock feed line, thereby to measure a feed rate of fresh organic material feedstock into the anaerobic digestion stage. It will be appreciated that the feed rate sensor would communicate feed rate measurements to the electronic controller along a feed rate communication line and may serve to provided data to be used in incremental loading of the fresh organic material feedstock. Preferably, however, the anaerobic digester system does not include such a sensor, with incremental loading of fresh organic material feedstock rather being effected by controlled opening and closing of the fresh organic material feedstock feed regulator for a predetermined incremental feed period, based on the rate of uninhibited flow of fresh organic material feedstock from the feeder, under gravity. The anaerobic digestion stage operating condition sensors may also include heat transfer medium inlet and outlet temperature sensors.

The electronic controller may be in communication with the regulators or their control components along respective control lines, such that the electronic controller controls operation of the regulators either directly or by means of their control components by means of control signals transmitted along the control lines.

In relation to the operating temperature control component, more particularly constituted by the heat transfer medium pump, the electronic controller may be configured to activate the pump for a predetermined control period, regardless of the value of the measurements taken by the anaerobic digestion stage operating temperature sensor whilst the pump is activated. Alternatively, the electronic controller may be configured to exercise dynamic control by keeping the pump activated and until the temperature has been restored to the desired level. In relation to the operating pH control component, more particularly being constituted by the fresh organic material feedstock feed regulator valve, the controller may be configured to open or close the valve thereby respectively to advance or retard a particular fresh organic material feedstock feed increment, depending on the nature of the operating pH measurement communicated to the controller.

The mixer may be operatively associated, typically by means of the electronic controller, with the fresh organic material feedstock feed regulator such that the mixer is activated when fresh organic material feedstock is being fed to the digestion vessel. Activation of the mixer when feedstock is fed to the anaerobic digestion vessel may be for a predetermined mixing time period, regardless of whether feeding is ceased during said time period or not.

The heat transfer control component pump may also be operatively associated, typically also by means of the electronic controller, with the mixer provided in the digestion vessel, such that the mixer is activated when heat transfer medium is passed through the heat transfer element. Activation of the mixer may again be for a predetermined time period, regardless of whether the passing of heat transfer medium through the heat transfer element has ceased.

The system may also include electricity generation means for generating electricity to power the pump and the electronic controller. Typically, the system includes renewable energy electricity generation means, utilizing renewable energy sources such as wind energy and solar energy. Preferably, the system includes at least one photovoltaic cell and, optionally, a wind turbine. The system may also include one or more batteries for storing generated electricity.

The invention will now be described by way of example only with reference to the accompanying diagrammatic drawing, which is a block flow-diagram representing an anaerobic digester system in accordance with the invention.

In the drawing, reference numeral 10 generally indicates an anaerobic digester system in accordance with the invention.

The anaerobic digester system 10 includes an organic material feedstock feeder 12, an anaerobic digestion stage 14, a digestate phase separation stage 16, and a gas collection stage 18. The digestate phase separation stage constitutes a density separation stage.

Although not illustrated in such detail, the feeder 12 comprises a feeder vessel. Similarly, the anaerobic digestion stage 14 comprises an anaerobic digestion vessel and the phase separation stage 16 comprises a density separation vessel. Each of the anaerobic digestion vessel and the density separation vessel is sealed, the anaerobic digestion vessel having an opening that is closed by an air tight lid, to create anaerobic operating conditions.

The gas collection stage 18, similarly to the anaerobic digestion stage 14 and the phase separation stage 16, comprises a gas collection vessel. It will be appreciated that, comprising only one vessel each, the anaerobic digestion stage 14 and the phase separation stage 16 are both single stage in nature, with essentially all the anaerobic digestion phases of hydrolysis, acidogenesis, acetogenesis and methanogenesis occurring in the anaerobic digestion vessel and separation of the digestate into its phases occurring in the density separation vessel.

As conceptually illustrated, if the drawing is regarded as a side view representation of the anaerobic digester system 10, the anaerobic digestion stage 14, and thus the anaerobic digestion vessel, is vertically spaced from and located lower than the feeder 12. Similarly, the phase separation stage 16, and thus the density separation vessel, is vertically spaced from and located lower than the anaerobic digestion stage 14. The anaerobic digester system 10 accordingly has a generally cascaded or step-like layout which allows, as is explained in more detail hereinafter, for material to be transferred from the feeder 12 to the anaerobic digestion stage 14, and from the anaerobic digestion stage 14 to the phase separation stage 16, under gravity.

From the feeder 12, a fresh organic material feedstock feed line 20 leads to the anaerobic digestion stage 14. Fresh organic material feedstock is fed along the feed line 20 to the anaerobic digestion stage 14 from the feeder 12 under gravity. The fresh organic material feedstock feed line 20 would typically comprise a pipe.

From the anaerobic digestion stage 14, a digestate transfer line 24 leads to the digestate phase separation stage 16. Digestate is transferred along the digestate transfer line 24 from the anaerobic digestion stage 14 to the digestate phase separation stage 16, under gravity. The digestate transfer line 24 also typically comprises a pipe. The phase separation stage 16 has three digestate phase outlet lines

26.1 , 26.2, 26.3. The outlet lines 26.1 , 26.2, 26.3 are vertically spaced from each other, so as to allow for separate or discrete withdrawal of different digestate phases at different levels of the density separation vessel in the phase separation stage 16. More particularly, outlet line 26.1 is an overhead or overflow outlet line, outlet line 26.2 is an intermediate outlet line, and outlet line 26.3 is a bottoms outlet line.

An anaerobic digestion stage gas transfer line 22 and a digestate phase separation stage gas transfer line 28, along which biogas streams can be withdrawn respectively from the anaerobic digestion stage 14 and the digestate phase separation stage 16, lead from the anaerobic digestion stage 14 and the digestate phase separation stage 16 to a gas stream mixing point 30, from where a single combined biogas product stream is passed to the gas collection stage along biogas product line 34. The feedstock feed line 20, digestate transfer line 24, and digestate outlet lines 26.1 , 26.2, 26.3 are each provided with feed regulators, in the form of respective solenoid valves 36A - 36E. It will be appreciated that the solenoid valve 36A constitutes an organic material feedstock feed regulator and the solenoid valve 36B constitutes a digestate withdrawal feed regulator. The anaerobic digestion stage 14 is provided with a mixer 38, comprising an impeller, which is powered by an electrical motor (not illustrated).

The anaerobic digester system 10 further includes an anaerobic digestion stage temperature regulator in the form of a heat exchanger 40, which includes a heat source, in the form of a solar water heater 42, and a heat exchange element which comprises coiled copper tubing 44. The coiled copper tubing is provided in a heat transfer relationship with the anaerobic digestion stage 14, more particularly with the contents thereof, by being provided inside the anaerobic digestion vessel of the anaerobic digestion stage 14. In order to apply heat to the anaerobic digestion stage 14, heat transfer medium, in the form of heated water, is passed along a heat transfer medium flow circuit from the solar water heater 42 through the copper tubing 44 by operation of an anaerobic digestion stage operating temperature control component in the form of a heat transfer medium pump 48. The water, having been heated in the solar water heater 42, is thus passed in a heat transfer relationship with the anaerobic digestion vessel, or at least the contents thereof, thereby to exercise temperature control thereon.

The fresh organic material feed regulator 36A also constitutes an anaerobic digestion stage operating pH regulator, as described in more detail hereinafter, and thus serves as anaerobic digestion stage operating pH control component.

The anaerobic digester system 10 is also provided with an anaerobic digestion stage automatic operating condition control system, generally being indicated by reference numeral 60, for controlling operating conditions of the anaerobic digester system 10, and more particularly of the anaerobic digestion stage 14.

The anaerobic digestion stage operating condition control system 60 includes a electronic controller 62 which includes, or is programmed with, desired operating condition values, including at least a desired anaerobic digestion stage operating temperature value, a desired anaerobic digestion stage operating pH value and a desired fresh organic material feedstock feed rate, constituting a fresh organic material feedstock feed regime.

The anaerobic digestion stage operating condition control system 60 further includes an anaerobic digestion stage operating temperature sensor 64 and an anaerobic digestion stage operating pH sensor 66, which are provided in the anaerobic digestion stage 14 so as to take measurements of temperature and pH in the anaerobic digestion stage 14. The sensors 64, 66 communicate temperature and pH measurements to the electronic controller 60 by means of respective temperature and pH measurement signals along respective anaerobic digestion stage operating temperature and anaerobic digestion stage operating pH communication lines 64.1 , 66.1 .

The electronic controller 62 is provided in a control relationship with the heat transfer pump 48 and the solenoid valve 36A, being connected to and in communication with the heat transfer medium pump 48 and solenoid valve 36 respectively along an anaerobic digestion stage operating temperature control line 48.1 and along a combined anaerobic digestion stage operating fresh organic material feed rate and anaerobic digestion stage operating pH control line 36A.1 , along which control lines control signals can be transmitted by the electronic controller, selectively to activate or deactivate the heat transfer medium pump 48, and selectively to open or close or prevent opening or closing of the solenoid valve 36A.

The anaerobic digestion stage operating condition control system 60 further, optionally, includes a solar water heater temperature sensor 68, as well as inlet and outlet heat transfer medium water temperature sensors 70, 72. The sensors 68, 70, 72 are connected to and communicate with the electronic controller 62 along respective communication lines 68.1 , 70.1 , 72.1 .

The anaerobic digestion stage operating condition control system 60 also includes a fresh organic material feedstock flow sensor 74, arranged to measure the quantity of fresh organic material feedstock which is fed to the anaerobic digestion stage 14 along the fresh organic material feedstock feed line 20. The flow sensor 74 is connected to and communicates with the electronic controller 62 along communication line 74.1 .

The electronic controller 62 is also provided in a control relationship with the solenoid valves 36B - 36E, being in control communication with the valves 36B - 36E along branched control line 36B.1 .

The electronic controller 62 is configured to receive measurement signals from the sensors 64, 66, 74 along the respective communication lines 64.1 , 66.1 , 74.1 and to compare the measured values to associated desired anaerobic digestion stage operating condition values. When the measured operating conditions are those of anaerobic digestion stage operating temperature and anaerobic digestion stage operating pH, measured by means of the sensors 64, 66, and a discrepancy exists between the measured values and the desired values, more particularly if the temperature is lower than the desired operating temperature and/or the pH is lower/higher than the desired operating pH, the electronic controller 62 communicates a control signal to the pump 48 and/or the solenoid valve 36A, activating the pump 48 or preventing opening of or opening the valve 36A. By activation of the pump 48, heated water, as heat transfer medium, is pumped through the coiled tubing 44 and transfers heat to the anaerobic digestion stage 14, thereby to increase the digestion temperature.

Similarly, opening or prevention of opening of the solenoid valve 36A results in pH control being exercised on the anaerobic digestion stage. More particularly, as hereinbefore described, opening the solenoid valve 36A for the purpose of pH control results in fresh organic material feedstock being introduced into the anaerobic digestion stage 14. This causes an increase in acetogenic bacteria activity, which in turn causes in an increase in the volatile fatty acid content in the anaerobic digestion stage, thus decreasing the pH. Conversely, preventing opening of the solenoid valve 36A for the purposes of pH control prevents such an increase in acetogenic bacteria activity, resulting in a progressive slowing in the production of volatile fatty acids and allowing for consumption of produced volatile fatty acids by the methanogenic bacteria. Thus, by effectively starving the anaerobic digestion stage of fresh organic material feedstock, an increase in anaerobic digestion stage pH is caused. Accordingly, the electronic controller 62 may exercise pH-directed control over the solenoid valve 36A. Should the anaerobic digestion stage operating pH be too high, the electronic controller 62 may open the solenoid valve 36A, thereby to feed additional fresh organic material feedstock to the anaerobic digestion stage 14. Conversely, when the anaerobic digestion stage operating pH is too low, the electronic controller may exercise pH-directed control over the valve 36A by retaining the valve closed, and thus starving the anaerobic digestion stage 14 of fresh organic material feedstock, even when opening of the valve is required in accordance with the fresh organic material feedstock feed regime set for feeding fresh organic material feedstock to the anaerobic digestion stage 14 in an incremental fashion.

The anaerobic digester system 10 further includes electricity generation means in the form of a photovoltaic cell 80 which includes a 24V battery bank (not illustrated) for storing electrical energy generated by the cell 80. Alternatively, or additionally, the anaerobic digester system 10 may include a wind turbine for generating electricity. Electricity is provided, from the photovoltaic cell 50 or the battery bank (not illustrated), to the mixer 38 along electrical supply line 82 and to the electronic controller 62 along electrical supply line 84. As is evident from the description of the invention hereinbefore, the anaerobic digester system 10 includes two main operating stages, namely the anaerobic digestion stage 14 and the digestate phase separation stage 16. By providing these two stages, quasi-continuous or semi-batch operation of the anaerobic digester system 10, and thus semi-batch anaerobic digestion of the organic material that is fed from the feeder 12, is enabled. It is regarded as particularly advantageous that density separation of the digestate into its various constituent strata or phases does not need to be conducted in the anaerobic digestion stage 14, as is conventionally the case when anaerobic digestion is conducted in simple batch fashion. As will be appreciated, conducting density separation of the digestate into its various phases in the anaerobic digestion stage 14 would inevitably impact negatively on available anaerobic digestion stage 14 digestion time (reactor time), thus impacting negatively on organic material throughput through the anaerobic digester system 10 and also on the overall time-based efficiency of biogas production. Further, cascaded arrangement of the feeder 12, the anaerobic digestion stage 14 and the digestate phase separation stage 16, minimizes energy input required for the conveyance of fresh organic material feedstock from the feeder 12 to the anaerobic digestion stage 14 and then to the digestate phase separation stage 16 as well as for the conveyance of pH control substance from the pH control substance reservoir 50 to the anaerobic digestion stage 14.

The anaerobic digester system 10 is typically set up for anaerobic digestion of a predetermined daily load of organic waste material feedstock (also referred to as "sludge" hereinafter), which load is fed to the anaerobic digestion stage 14 according to an incremental feeding regime, which involves feeding predetermined incremental volumes of fresh organic material feedstock to the anaerobic digester stage 14 in a semi-batch or quasi-continuous fashion over a 24 hour period at a desired feed rate, i.e. duly feeding due predetermined set increments of fresh organic material feedstock to the anaerobic digestion stage 14 at predetermined set intervals. It will be appreciated that the suitable feed rate for a particular configuration of the anaerobic digester system 10 would depend largely on the volume of the anaerobic digested vessel in the anaerobic digestion stage 14, as well as on the required retention time of the feedstock in the digestion vessel in the anaerobic digestion stage 14. The retention time is largely dependent on the chosen or calculated desired operating conditions of temperature and mixing. More particularly, it is expected that the feed rate for a particular configuration of the anaerobic digester system 10 would be calculated by means of either equation (1 ) or equation (2): mass of feed sludge (kg. d) volatile solids

Loading (kgVS.m ) = — x ...(1 )

available digester volume (m ) 100% available digester volume (I) , ₀ .

Loading = ^■■■

sludge retention time (SRT) (days)

As to equation (1 ), it will be appreciated that the volatile suspended solids content of the sludge (i.e. fresh organic material feedstock) would, typically, be unknown. This content represents the actual organic matter contained in the sludge that can be broken down / digested in the anaerobic digester stage 14. It also represents, to a certain extent, the quantity of material that can potentially be converted to biogas. The volatile suspended solids content can be determined by experimental methods known in the art of the invention.

As indicated hereinbefore, particularly in order to prevent over-feeding of the anaerobic digestion stage 14, the desired fresh organic material feedstock feed rate, calculated as indicated above, must be adhered to. Incremental loading of the total feedstock load at the desired feed rate, either in a continuous or semi-batch fashion, is essential, as loading the total organic material feedstock volume into the anaerobic digestion stage 14 at once could cause an unnecessary delay in biodegradation, i.e. anaerobic digestion, of the feedstock. Incremental feeding of the total fresh organic material feedstock volume is further beneficial for even distribution of bacteria through organic material feedstock fed to the reactor.

It is to be borne in mind that reference to "fresh organic material feedstock" is in relation to the operational content of the feeder 12. As indicated hereinbefore, at start-up of the anaerobic digester system 10, the anaerobic digestion vessel in the anaerobic digestion stage 14 will be filled to capacity with at least one, and preferably a mixture, of pre-loaded donor sludge and pre-loaded organic material feedstock. Feeding fresh feedstock from the feeder 12 into the anaerobic digestion stage 14 then commences at the desired / calculated incremental or continuous feed rate in accordance with the predetermined fresh organic material feedstock feed regime. An incrementally supplemented supply of nutrients for the bacteria used in the anaerobic process is thus provided.

Incremental loading of fresh organic material feedstock into the digestion tank of the anaerobic digestion stage 14 is, as described above, achieved by means of the fresh organic material feedstock regulator, comprising the solenoid valve 36A. The solenoid valve 36A is activated by means of the electronic controller 62 in accordance with the predetermined fresh feedstock feed regime, which transmits an activation control signal to the valve 36A along control line 36A.1 . Activation and opening of the valve 36A allows fresh organic material feedstock to flow and be fed to the anaerobic digestion stage 14 under gravity. As mentioned, incremental feeding of organic material feedstock from the feeder 12 to the anaerobic digestion stage 14 may be checked by the feed sensor 74, which transmits measurements to the controller 62 which retains the valve 36A open until the desired volume of fresh organic material feedstock has been fed to the anaerobic digestion stage 14. More preferably, however, incremental feeding of fresh organic material feedstock to the anaerobic digestion stage 14 may be effected in the absence of such a flow sensor 74, with the controller 62 being programmed to open the valve 36A for a predetermined time period, depending on the rate of flow of fresh organic material feedstock from the feeder 12 under gravity.

The influx of fresh organic material feedstock into the anaerobic digestion vessel may cause at least some dilution of bacteria in the tank, if the fed fresh organic material feedstock is left unmixed. Accordingly, as also indicated above, when the solenoid valve 36A is activated, and thus opened, the mixer 38 is also activated. Mixing of the feedstock in the digestion tank is essential to ensure an even distribution of the bacteria through the feedstock. Preferably, the mixer is activated for a specified time period, typically 5 minutes. Importantly, with activation of the solenoid valve 36A, the solenoid valve

36B, constituting the digestate withdrawal feed regulator, is also activated, considering that these two valves are operatively related, and controlled by the electronic controller, for synchronous operation. The solenoid valves 36A and 36B thus operate synchronously, thereby to prevent pressure differences from occurring in the anaerobic digestion system 10, due to the movement of material therein. It will be appreciated that the creation of such pressure differences is a possibility, considering that the system 10 operates under anaerobic conditions, with the anaerobic digestion stage 14, in particular, comprising a sealed vessel. In the anaerobic digestion stage 14, and thus in the anaerobic digestion vessel, the four basic stages of anaerobic digestion, namely hydrolysis, acidogenesis, acetogenesis, and methanogenesis take place.

As also mentioned hereinbefore, acetogenesis involves the production of volatile fatty acids (VFA's). The VFA's are required by methanogenic bacteria in order to produce the biogas through methanogenesis. Acetogenic bacteria are, however, more efficient at producing VFA's than methanogenic bacteria are at producing biogas, resulting in VFA's being produced faster than the methanogenic bacteria can convert the VFA's to biogas. This may result in an increase of acidity, and thus a decrease in pH, in the digestion tank. As indicated above, neutralization of increased acidity is achieved by preventing feeding of fresh organic material feedstock to the anaerobic digestion stage 14. In this regard, the pH sensor 66 is configured to take measurements of the anaerobic digestion stage operating pH and to transmit measurement value signals to the electronic controller 62 along communication line 66.1 . The electronic controller 62 compares the measured anaerobic digestion stage operating pH values to the desired anaerobic digestion stage operating pH value in the database of desired operating condition values. Typically, the desired anaerobic digestion stage operating pH is 7, i.e. neutral. When the measured anaerobic digestion stage operating pH value exceeds an acceptable deviation from the desired anaerobic digestion stage operating pH, typically being 0.2 points or more, the electronic controller 62 exercises pH control over the anaerobic digestion stage 14 in accordance with the pH control regime set out above. Thus, should the anaerobic digestion stage operating pH be too high, the electronic controller 62 opens the solenoid valve 36A, thereby advancing a particular feed increment and thus feeding additional fresh organic material feedstock to the anaerobic digestion stage 14 than what is due in accordance with the set fresh organic material feed regime. Conversely, when the anaerobic digestion stage operating pH is too low, the electronic controller may exercise pH-directed control over the valve 36A by retaining the valve closed, and thus retarding feeding of a particular feed increment and starving the anaerobic digestion stage 14 of fresh organic material feedstock, even when opening of the valve is required in accordance with the fresh organic material feedstock feed regime set for feeding fresh organic material feedstock to the anaerobic digestion stage 14 in an incremental fashion.

It is important to note that such organic material feed-based pH control would only commence once a buffered pH state has been achieved in the anaerobic digestion stage. Such a state would typically, in use, be achieved during start-up by user intervention with an alkaline substance. During the start-up phase of the system 10, acetogenesis, of course, occurs prior to commencement of methanogenesis, leading to a decrease in the operating pH of the anaerobic digestion stage 14. This increased acidity is managed by the introduction of the alkaline substance, preferably calcium hydroxide, into the anaerobic digestion stage 14 until methanogenesis is occurring aggressively enough for the pH to even out at about 7. Once this has been achieved, organic material feedstock feed-based pH control can be exercised.

Temperature control of the anaerobic digestion stage 14 is important, since activity of bacteria in the biological process is sensitive to temperature changes. Once a desired anaerobic digestion stage operating temperature for a specific load and fresh organic material feedstock feed regime for the anaerobic digester system 10 has been identified, it is important to maintain the temperature to within an allowable deviation, being at least 1 ^° C, more preferably 0.5 ^° C, of the desired anaerobic digestion stage operating temperature. Typically, the desired anaerobic digestion stage operating temperature may a mesophilic temperature of between about 30 °C and about 38 °C, preferably about 35 ^° C. This temperature is maintained by using the anaerobic digestion stage temperature regulator, comprising the heat exchanger 42 for the heating of the digestion tank in the manner hereinbefore described.

If a temperature constituting a deviation in desired anaerobic digestion stage operating temperature of 0.01 ^° C is communicated to the electronic controller 62 by the temperature sensor 64, the pump 48 is activated and heated water is conveyed or pumped from the solar water heater 42 through the heat exchange element 44. The heat exchange element 44 is constructed of a coil of copper tubing. Heat from the heated water is conveyed through the copper tubing into the contents of the anaerobic digestion vessel. Activation of the pump 48 is combined with the activation of the mixer 38. Activation of the mixer 38 allows for complete and even distribution of heat through the contents of the anaerobic digestion vessel. In one embodiment of the invention, the pump 48 and mixer 38 will remain activated until the anaerobic digestion stage operating temperature is restored to the desired anaerobic digestion stage operating temperature. Preferably, however, the pump 48, at least, remains active until the operating temperature sensor communicates an operating temperature measurement to the electronic controller 62 that is 0.5 ^° C in excess of the desired operating temperature.

The electronic controller 62 also employs measurements from the sensors 68, 70, 72 in exercising control of the temperature in the anaerobic digestion stage 14. The sensors 70, 72 serve to measure the temperature of the heat transfer fluid when it enters and again when it exits the anaerobic digestion vessel. The energy transferred to the contents of the anaerobic digestion vessel can therefore be determined from such measurements. If the temperature measured by the sensor 70 is below the desired anaerobic digestion stage operating temperature, the electronic controller 62 may deactivate the pump 48 and the mixer 38. Typically, activation of the pump 48 and the mixer 38 will be at least for an initial period of 1 minute to allow for cold water in the element 44 to be replaced by heated water from the solar water heater 42. However, if the temperature measured by the sensor 70, after elapse of the 1 minute time period, is below the desired operating temperature of the anaerobic digester system 10, the pump 48 and the mixer 38 are deactivated for a predetermined period, typically 45 minutes, to allow time for the water in the solar water heater to be sufficiently heated to at or above the desired operating temperature. It will be appreciated that passing water at a temperature below the desired operating temperature through the element 44 may have to effect that the feedstock in the anaerobic digestion vessel is either cooled or retained at a temperature below the desired anaerobic digestion stage operating temperature. Such an effect is avoided by allowing time for the solar water heater 42 to heat the water to at or above the desired operating temperature.

As indicated hereinbefore, mixing of the contents of the anaerobic digestion vessel in the anaerobic digestion stage 14 by means of the mixer 38 is typically conducted when fresh organic material feedstock is fed to the anaerobic digestion vessel, when the pump 48 is activated to supply heat to the contents of the anaerobic digestion vessel. Additionally, or alternatively, the mixer 48 may be configured, typically through the electronic controller 62, to be activated on a fixed-time or periodic basis, e.g. for a specific time period per hour, so that the contents of the anaerobic digestion vessel are adequately mixed whilst being subjected to anaerobic digestion.

The anaerobic digestion process requires a specific period of time for the complete anaerobic digestion of organic material. This retention time is dictated by the environment in which bacteria operate, i.e. by the operating temperature and pH, with the fresh organic material feedstock feed rate being selected accordingly. If the temperature, pH and feed rate is not controlled at the desired values for which the anaerobic digester system 10 is set up, it can cause delays in / impact negatively on the production of biogas, i.e. methane. The anaerobic digester system 10 of the invention and the control of the operating conditions therein is allows the residence time to be minimized, due to automated control of operating conditions and separate digestate handling, thus allowing for the construction of a smaller anaerobic digestion stage to deal, in semi-batch or even in continuous fashion, with an equivalent volume of organic waste feedstock which may be treated by standard non-automated anaerobic digester in simple batch fashion. Anaerobic digestion stage residence time is, importantly, further reduced by provision of the digestate phase separation stage 16, which allows for discrete digestate product recovery on a semi-batch or quasi- continuous, or even fully continuous basis separately from the anaerobic digestion stage 14. The manner in which the system 10 according to the invention is arranged therefore provides for a dedicated anaerobic digestion stage 14 and a dedicated digestate product recovery stage 16.

It is also an advantage of the system 10 as described that anaerobic digestion stage residence time need not be extended to allow for virtually complete digestion to occur, considering that digestion of undigested organic material can continue in the digestate phase separation stage, also under anaerobic conditions.

EXAMPLE

In a particular embodiment of the invention, the desired operating temperature is 35 ^° C and the desired pH is 7.

The volume of the anaerobic digester vessel in the anaerobic digestion stage is 300 litres.

For treating 300 litres of organic material, the digestion time for such a configuration of the anaerobic digester system 10 was calculated at 20 days, thus requiring a feed rate of approximately 15 litres of fresh organic material feedstock per day. 20 days is in the region of the normal retention time required for digestion of organic waste material at a mesophilic temperature of 35 ^° C. The 15 litres daily feed rate is spread equally over a 24 hour period, either in increments of 0.63 litres every hour, 1 .25 litres every two hours, or, more preferably, 3.75 litres every 6 hours. Such incremental feeding of feedstock to the digestion tank, as also indicated hereinbefore, prevents the anaerobic digestion process, taking place in the pre-loaded donor sludge and pre-loaded organic material feedstock, from being shocked by introduction of a cooler digestate of 15 litres into the anaerobic digester system at once.

It will be appreciated, as also discussed above, that operation of the anaerobic digester system 10 proceeds by filling the anaerobic digestion vessel with a mixture of pre-loaded or start-up donor organic material and pre-loaded or start-up organic material feedstock. More particularly, 200 litres of pre-loaded or start-up donor organic material and 100 litres of pre-loaded or start-up fresh organic material feedstock will be loaded into the digestion tank.

Once the anaerobic digestion vessel has been filled as such, heating is applied to the anaerobic digestion vessel, if required, to bring the organic material to the desired operating temperature of 35 °C. Incremental loading of the 300 litres of feedstock from the feeder 10 then commences at the desired feed rate with synchronous withdrawal of digestate along the digestate transfer line 24.

Biogas is continuously collected in the biogas collection stage 18. DISCUSSION

Conventionally, as also alluded to above, batch-type anaerobic digesters undergo a stage where digestion is halted to allow for draining of different phases of digestate into which the digestate separates digester when mixing is halted. In contrast, the employment of the digestate phase separation stage 16 in the anaerobic digester system 10 of the present invention, allows semi-batch operation. This is achieved by allowing the same volume as what is fed to the anaerobic digestion stage on a daily basis to be conveyed under gravity to the digestate phase separation stage, either continuously or in semi-batch fashion as hereinbefore described.

As also alluded to above, it is important that a volume of digestate transferred to the settling tank in the digestate phase separation stage 16 is equivalent to the volume of feedstock introduced in to the digestion tank. This will avoid the formation of pressure between the digestion tank and the settling tank. A pressure difference between the tanks could result in damage of the seals of the tanks, which could result in an influx of atmospheric air in to the anaerobic digester system 10 which may delay or halt the anaerobic digestion process. As set out hereinbefore, even conveyance of digestate to the digestate phase separation stage and feedstock to the anaerobic digestion stage is achieved by synchronized opening of the solenoid valve 36A and the solenoid valve 36B. There is no heating, pH control or mixing in the second stage. This is to allow the contents to settle into the different phases. The phases will consist from the top scum, supernatant, active sludge, and stabilized sludge. The settlement of the digestate in to different phases allows for the draining of specific by-products from the anaerobic digester system 10. The draining of the by- products is achieved, as alluded to hereinbefore, by the synchronization of the operation of the solenoid valves 36C, 36D, 36E with the solenoid valves 36A, 36B.

Biogas is mainly produced in the digestion tank. A smaller amount of biogas may, however, also be produced in the settling tank due to the presence of active sludge. A series of gas sensors or analyzers (not illustrated) are typically included in the anaerobic digester system 10 to analyze the composition of the biogas. The sensors may register the presence of CO ₂ , CO, H ₂ S, SH ₄ and H in the biogas on a percentage basis.

As will be appreciated from the description presented hereinbefore, the present invention provides a two stage autonomous automatic anaerobic digester system, and method of operating a digestion anaerobic digester system. The Applicant regards it as a particular advantage of the invention as described that the requirement of skilled operator presence is greatly reduced, and may even be obviated, particularly as the control anaerobic digester system provides for automatic control of operating conditions of the anaerobic digestion anaerobic digester system. More particularly, the continuous or semi-batch feeding of feedstock into the anaerobic digestion stage prevents the anaerobic digester system from being starved of feedstock and thus allows the anaerobic digester system to be left unattended by operators, at least whilst there is feedstock in the feeder.

The Applicant further regards it as an advantage of the invention that renewable energy sources, such as wind and solar energy, are used in powering process equipment, thus enabling operation of the anaerobic digester system of the invention autonomously and independently from electricity provision grids, thereby rendering the anaerobic digester system particularly suited for use in rural areas. The Applicant additionally regards it as an advantage of the invention that anaerobic digestion, and thus biogas production, is managed optimally in the anaerobic digestion system according to the invention by limiting organic material residence time in the anaerobic digestion stage through synchronous feeding of fresh organic material feedstock and withdrawal of digestate, as well as by providing for digestate product recovery and residual active organic material digestion separately from the anaerobic digestion stage.

As will also be appreciated from the broad problem statement at the beginning of the specification and the emphasis which the invention places on pH control, pH control is indeed an important concern in controlling and indeed also in optimising operation of an anaerobic digestion process. The Applicant believes that the manner in which operational pH control is effected according to the present invention, i.e. by means of fresh organic material feedstock feed control, is also a particular advantage of the invention, in that it does not require complex pH control substance delivery arrangements or additional sensors and control equipment for controlling pH. The manner of pH control proposed by the present invention is regarded as non-intrusive and non-disruptive in relation to the general characteristics of the anaerobic digestion process, considering that is does not involve the addition of a foreign substance in controlling pH, but relies on the characteristics of the anaerobic digestion process itself in this regard. In summary, the Applicant believes that the present invention provides for operating an anaerobic digester system in an optimal but technically simplified manner which renders it particularly suitable for application in areas with minimal supporting infrastructure and technical skill.