WO2016005771A1

WO2016005771A1 - Treatment method for fats, oils and/or greases

Info

Publication number: WO2016005771A1
Application number: PCT/GB2015/052009
Authority: WO
Inventors: Stewart MANNING; Sophie RANDALL
Original assignee: Breathe Ad Limited
Priority date: 2014-07-10
Filing date: 2015-07-10
Publication date: 2016-01-14
Also published as: GB2528111A; GB201412322D0

Abstract

A method (10) of aerobically treating fats, oils and/or greases (FOGs), which comprises the steps of: (a)(i) diluting the FOGs with solvent to form a mixture with a viscosity lower than the undiluted FOGs, and (ii) adding hydrolytic reagents (26, 106) to the FOGs to mediate FOG hydrolysis; (b) heating the mixture to a temperature substantially in the range 30°C to 60°C, to substantially liquefy solid FOGs and optimise the rate at which FOGs are hydrolysed; and (c) controlling at least one of the following in relation to the mixture for a period of substantially up to 48 hours: pH (24, 108), M-alkalinity (108), subsequent temperature (54), aeration (22, 112), nutrient dosing (20), supplemental hydrolytic reagentaddition (106), volatile fatty acid extraction; thereby modulating the extent of FOG hydrolysis and generating fatty acids with a predetermined range of chain lengths.

Description

TREATMENT METHOD FOR FATS, OILS AND/OR GREASES

The present invention relates to a method of aerobically treating fats, oils and/or greases (FOGs) prior to anaerobic digestion, and particularly to a method for aerobically treating FOGs which creates a stable mixture suitable for high-rate anaerobic digestion and the subsequent production of either biogas or biodiesel.

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 into the sewer networks, which can become corroded or blocked as a result. Technology exists that can be used to produce biogas or biodiesel from such waste, using anaerobic bacteria to break down the input material. Conventional and high-rate anaerobic digestion (AD) plants are commercially available to process organic waste, but unlike conventional AD plants in which decomposition occurs over a period of around 28 days, high-rate AD plants require the input material to be in predominantly liquid form to enable accelerated decomposition (over a period of around 24 hours).

Used cooking oils are one source of FOGs which tend to have few contaminants, as they can be collected directly from the source instead of from sewer networks. However, many FOGs (including cooking oils) do not dissolve substantially in water as the non-polar fatty acid chains of a given fat, oil or grease molecule preferentially associate with neighbouring fatty acid chains of similar molecules, as opposed to the polar water molecules. Mixtures of poorly-dissolved FOGs have less soluble chemical oxygen demand (COD) available and are also digested more slowly and less completely than well-dissolved FOGs. This inhibits the activity of the anaerobic bacteria and leads to a concurrent reduction in the volume of biogas evolved as a result.

FOGs may also be collected from sewer networks or at sewage treatment works. However, these tend to be more heavily contaminated, which can negatively affect anaerobic digestion of the waste. To facilitate more efficient and economic transport of waste to AD facilities, for example, water can be substantially drained from organic waste designated for biogas production to create a sludge. However, FOG sludge is unsuitable for use in high-rate AD plants, which require predominantly liquid waste, restricting the potential resources available for such plants. Such plants therefore rely predominantly on the costly transport of large volumes of wastewater (containing relatively little in the way of FOGs, when compared with sludge) in order to generate energy, affecting the commercial viability of high-rate AD plants. Currently, high quality FOGs are transported large distances for biodiesel production. Notably, whilst conventional AD plants are able to process organic waste with a wide range of viscosities, the rate of bacterial decomposition is reduced for waste with a substantial solid component (for example, the aforementioned sludge) compared to mostly fluid waste. This is because mixing is inhibited, thereby limiting the rate of reaction, with the additional problem that the some of the bacterial colonies can become bound within the dense FOG sludge and subsequently removed with the digestate (digested sludge). This reduces the rate at which the plant can digest new organic waste as there are fewer bacteria remaining. FOG sludge also contains a number of inhibitors, with the rate of reaction being further reduced where FOGs with longer fatty acid chains are used, as the supply pipes of the reaction vessel can become blocked by solid waste. Furthermore, during FOG digestion, medium and long chain fatty acids can inhibit methanogenesis in both conventional and high-rate AD plants, as they bind to the biomass and reduce its activity, and may be removed from the digester.

In terms of biodiesel production, current processes require low levels of contaminants in the source material, meaning that cooking oils and FOGs with low levels of contaminants command high prices, restricting the commercial viability of biodiesel production. By comparison, using highly contaminated FOGs collected from sewage treatment plants is not cost-effective because the biodiesel produced from them is impure and low in yield. Biodiesel production from highly contaminated FOGs is also more chemically intensive than for less contaminated FOGs, further increasing costs.

It is an object of the present invention to reduce or substantially obviate the aforementioned problems. STATEMENT OF INVENTION

According to a first aspect of the present invention, there is provided a method of aerobically treating fats, oils and/or greases (FOGs), which comprises the steps of: (a)(i) diluting the FOGs with solvent to form a mixture with a viscosity lower than the undiluted FOGs, and (ii) adding hydrolytic reagents to the FOGs to mediate FOG hydrolysis; (b) heating the mixture to a temperature substantially in the range 30°C to 60°C, to substantially liquefy solid FOGs and optimise the rate at which FOGs are hydrolysed; and (c) controlling at least one of the following in relation to the mixture for a period of substantially up to 48 hours: pH, M-alkalinity, subsequent temperature, aeration, nutrient dosing, supplemental hydrolytic reagent addition, volatile fatty acid extraction; thereby modulating the extent of FOG hydrolysis and generating fatty acids with a predetermined range of chain lengths. Advantageously, waste FOGs can be treated via this method to produce treated waste containing fatty acids with a range of predetermined chain lengths. The range of chain lengths can be predetermined by varying the duration of the treatment (up to around 48 hours from initiation), such that the treated waste is suitable for either biodiesel production or for biogas production (via anaerobic digestion). After around 24 hours of hydrolysis, the treated waste is generally suitable for anaerobic digestion as practically all of the fatty acid chains having been hydrolysed as completely as possible. For highly contaminated or viscous waste, a period of up to 48 hours may be required to properly treat the waste. This reduces the total number of reactions that must take place in an AD plant in order to evolve biogas from the FOG mixture, as hydrolysis of the fatty acid chain has already taken place. Having been diluted into a substantially liquid state, the treated waste is suitable for high-rate anaerobic digestion in particular.

The treated waste may instead be suitable for biodiesel production where the waste undergoes treatment for potentially less than 24 hours. The treated waste is also chemically stable and has a higher quantity of soluble COD than wastewater (particularly as it contains greater quantities of volatile fatty acids (VFAs)), making it much more cost efficient for transport irrespective of whether it is intended for use in biodiesel or biogas production. Heating the FOG mixture to between 30°C to 60°C is advantageous as it maximises the activity of the hydrolytic reagents, increasing the overall rate of hydrolysis. Temperatures around 60°C are advantageous for mixtures which are particularly viscous at room temperature, as they become fluid enough for treatment at higher temperatures. Equally, controlling the pH, M-alkalinity and subsequent temperature of the mixture allows the rate of hydrolysis to be optimised, as does controlling aeration of the mixture (and therefore the concentration of dissolved oxygen) and any nutrient dosing to it, as needed to facilitate the activity of the hydrolytic reagents. Supplemental hydrolytic reagent addition is also of benefit to replace any of the initial hydrolytic reagents which are no longer active in the mixture. By extracting volatile fatty acids (VFAs) from the mixture, hydrolysis is encouraged by increasing the relative quantity of untreated waste to treated waste (promoting the forward reaction and preventing the build-up of products), whilst also providing partial control over the pH of the mixture. It also allows the VFAs to be utilised in other processes without requiring treatment of the entire mixture to have been completed.

The method may be enacted wherein during step (b) the mixture may be heated substantially to 50°C; further during step (b) the mixture may be cooled to and maintained substantially in the range 30°C to 40°C, after which nutrients and additional hydrolytic reagents may be added to the mixture; and the method may further comprise a final step of: (d) halting FOG hydrolysis by adjusting the parameters of step (c), thereby modulating the extent of FOG hydrolysis and generating fatty acids with a predetermined range of chain lengths. Heating the mixture to around 50°C is advantageous in that any solid FOGs are melted, which greatly increases the rate at which the constituent waste can be hydrolysed. Cooling the mixture to between 30°C and 40°C and then maintaining it at this temperature maximises the rate of hydrolysis without the risk of substantially denaturing the hydrolytic reagents, which would negatively impact the rate of reaction. Adding nutrients supports biological processes in the hydrolytic reagents, and adding further hydrolytic reagents after reducing the temperature replaces any reagents which have become inactive due to the elevated temperature, ensuring that the mixture can still be hydrolysed. Being able to actively and selectively halt hydrolysis of the FOG mixture is also highly advantageous, as the treated waste will not decompose further and can be isolated as a stable product. Where the treated waste is needed for biodiesel production, the range of fatty acid chain lengths in the mixture is preserved by halting hydrolysis. Otherwise, the hydrolytic reagents would continue to break down FOGs in the treated waste en route to the biodiesel production plant, rendering it unfit for purpose. It also halts minor side reactions, such as the production of methane, which would continue to decompose the treated waste and reduce its utility for biogas or biodiesel production. Where the treated waste is needed for high-rate anaerobic digestion, hydrolysis can be allowed to run to completion, or it can be halted to create a mixture with properties which best complement the capabilities of the anaerobic digestion plant it is destined for. For instance, some plants might work most efficiently with treated waste that is not fully hydrolysed, and so the treatment can be tailored as needed. The means and conditions of transporting the treated waste will also need to ensure that the mixture does not react further.

FOG hydrolysis may be halted or substantially halted by taking at least one of the following actions: increasing the pH of the mixture to a value of substantially 7 or greater; ceasing to mix the mixture; ceasing to heat the mixture; or ceasing to aerate the mixture.

Making the pH of the FOG mixture neutral or alkaline significantly inhibits the rate at which the fatty acid chains are broken down by the hydrolytic reagents, since the treatment is acid-catalysed. When mixing of the mixture is stopped, the contents are not circulated as effectively which reduces contact between the hydrolytic reagents and the unreacted FOGs, slowing the overall rate of hydrolysis.

Without continuing to heat the reaction, the mixture cools and loses energy, such that the proportion of the mixture constituents with enough energy to overcome the activation energy barrier for hydrolysis is reduced, which slows the rate of decomposition. If the mixture is no longer aerated, oxygen levels will not be replenished in the mixture, and so aerobic degradation of the FOGs reduces as oxygen is used. In combination, these four actions will greatly reduce and effectively halt FOG hydrolysis. The solvent may be predominantly composed of water. Preferably, the solvent may contain the hydrolytic reagents, and the hydrolytic reagents may include one or more types of at least one of the following: bacteria, free enzymes.

Water is a protic solvent and can help catalyse hydrolysis of the FOGs through the efficient transfer of hydrogen ions. Furthermore, water is conducive to the survival and activity of some hydrolytic reagents compared to other solvents which would dissolve long chain FOGs much more readily, and mixing can disperse FOGs effectively into water. A balance must be struck between suitability for dissolving the initial FOGs, the reagents and the treated waste (comprising volatile fatty acids), hence water is used to provide an optimal reaction medium.

It is advantageous to use bacteria to hydrolyse the FOGs as they are self-sustaining and able to adapt to the hydrolysis of many different FOGs, and they may also excrete hydrolytic enzymes. Different bacteria can be used in combination to degrade FOGs even more quickly. Using free enzymes allows certain reactions to be selectively accelerated, preventing some steps in the degradation from being rate-limiting. A combination of bacteria and free enzymes may enable hydrolysis to proceed at its quickest.

Each type of bacteria may be taxonomically classifiable under one of the following genera: Mycobacterium, Candida, Pseudomonas, Bacillus. Furthermore, each type of bacteria may be chosen from one of the following: Mycobacterium fortuitum, Mycobacterium simiae, Candida rugosa, Pseudomonas aeruginosa, Pseudomonas cepacia, Bacillus circulans.

The bacteria specified above have been found to readily break down FOGs in aerobic conditions. Some of these bacteria thrive at mesophilic temperatures, and others thrive best at thermophilic temperatures. This is particularly applicable when melting solid FOGs at around 50°C or 60°C, a temperature at which many other bacteria would not survive. Similarly, some of these bacteria thrive at moderately acidic pH values, thereby increasing the rate of FOG hydrolysis without being killed. Each type of free enzyme may be one of the following: lipase, beta-oxidase.

These enzymes can act as optimum catalysts for FOG hydrolysis in the reaction conditions provided.

The mixture may have a dynamic viscosity of substantially between 300 and 800 centipoise. Preferably, a dilution ratio of solvent to FOGs of or substantially of 5: 1 may be used to form the mixture. More preferably, a dilution ratio of solvent to FOGs of or substantially of 3 : 1 may be used to form the mixture, where the FOGs are comprised of or substantially of oils.

The mixture is diluted to attain a dynamic viscosity of between 300 and 800 centipoise, which optimises the fluidity of the mixture for mixing during the treatment process. A mixture with this initial viscosity will also produce treated waste which is sufficiently fluid for use in high-rate anaerobic digestion, or for use in biodiesel production if stopped early. Using a dilution ratio of 5: 1 for FOGs from sewage waste ensures that the desired viscosity is achieved, depending on the average densities of FOGs in a given scenario. Where the waste is substantially comprised of oils, its viscosity is comparatively lower than sewage waste, and it may contain fewer contaminants. Therefore a reduced dilution ratio of 3 : 1 may be used, resulting in an increased concentration of FOGs in such cases.

The ratio of hydrolytic reagents (ml) to FOGs (kg) may be substantially in the range 1 : 1 to 5: 1.

Depending on the composition of the FOG mixture (including the chain lengths of the constituent triglycerides), between 1 ml and 5 ml of hydrolytic reagents will be added for every kilogram of FOGs. Dilute FOG mixtures may be fully treated with a minimum of 1 ml of hydrolytic reagents, whereas concentrated FOG mixtures may be fully treated with a maximum of 5 ml of hydrolytic reagents. This ensures that mixtures with variable quantities of FOGs can be fully treated, whilst reagents are not wasted unnecessarily by using excess. Nutrients may be added to the mixture, and these may include a source of at least one of the following elements: boron, magnesium, potassium, calcium, iron, cobalt, nickel, copper, zinc, selenium, molybdenum, tungsten. These elements may be needed by the hydrolytic reagents (particularly the bacteria) in trace amounts to facilitate hydrolysis, either directly or indirectly; for example, keeping other bacterial processes functioning. Adding biologically compatible sources of these elements will therefore promote hydrolysis. The mixture may be maintained with a pH substantially in the range 6.2 to 6.8, and there may be at least one pH sensor which monitors the pH of the mixture.

Acidic conditions promote FOG hydrolysis, but overly acidic conditions will denature the hydrolytic reagents. Therefore, slightly acidic conditions with a pH around 6.2 to 6.8 optimise the overall rate of reaction. A pH sensor enables the pH of the mixture to be constantly monitored, such that any deviation from the ideal pH range is known, and subsequent corrective action can be taken to maintain the rate of FOG hydrolysis.

The pH of the mixture may be controlled by adding at least one of the following: an alkaline chemical, an M-alkalinity buffer.

The pH value of the mixture may need to be raised as the reaction proceeds with fine control of the pH informed by pH sensor measurements. As the mixture is slightly acidic, an alkaline chemical is needed to raise its pH value in such cases. An M- alkalinity buffer allows the mixture to stay within an ideal pH range for hydrolysis whilst treating the FOGs, balancing any changes in pH due to the extent of acid dissociation as shorter chain fatty acids are produced.

The alkaline chemical may be sodium hydroxide, or preferably a solution of sodium hydroxide, and more preferably a solution of sodium hydroxide with a concentration of or substantially around 5.5 M.

Sodium hydroxide is a strongly alkaline chemical, and by using a solution, it can mix with the FOGs more readily than by adding solid crystals. The exothermic reaction created by dissolving the crystals is unfavourable as it can kill the bacteria and denature the enzymes, so it is preferable to add a solution of sodium hydroxide. Supplying a solution of around 5.5 M requires that only small volumes of solution be added to increase the pH value of the mixture. This also allows the pH to be changed without significantly diluting the mixture, as might occur for weaker bases. The associated low level of dissolved sodium from solutions around this concentration will not inhibit the activity of the hydrolytic reagents.

The M-alkalinity buffer may be sodium bicarbonate.

Sodium bicarbonate can dissolve to form hydrogencarbonate anions which are advantageously in equilibrium with carbonate ions (CO3^2") and aqueous carbonic acid (H2CC"3(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 mixture may be maintained with a mass ratio of dissolved molecular oxygen to FOGs substantially in the range 3 : 1 to 5: 1. Preferably, there may be at least one oxygen sensor which monitors the mass ratio of dissolved molecular oxygen to FOGs in the mixture, and the mass ratio may be maintained by aerating the mixture. More preferably, the mixture may be periodically or constantly aerated. The mixture may also be periodically or constantly mixed.

Having a minimum mass ratio as given above ensures that the FOG waste can be fully hydrolysed by the hydrolytic reagents. Having a maximum mass ratio as given above prevents excess oxygen from promoting the formation of sludge, which would make the mixture unsuitable for anaerobic digestion.

Periodically aerating the mixture is advantageous in that it ensures the hydrolytic reagents have sufficient oxygen to hydrolyse the FOGs, the optimum periodicity of aeration being derived from oxygen sensor measurements to ensure finer control of the degree of aeration. The mixture may be constantly aerated with smaller volumes of air, or periodically aerated with larger volumes, so as to ensure that the mass ratio of dissolved molecular oxygen to FOGs is maintained in the desired range. Periodically or constantly mixing the mixture ensures that hydrolytic reagents are dispersed to hydrolyse unreacted FOGs, rather than becoming surrounded in a local sphere with fully hydrolysed FOGs, which would impede the rate of reaction. According to a second aspect of the invention, there is provided apparatus for use with a method in accordance with the first aspect of the invention the method, which may comprise: at least one limpet tank reactor which may hold the FOG mixture during hydrolysis; at least one recirculation line which may recirculate the FOG mixture in the at least one limpet tank reactor; a first input means which may transfer FOGs to the at least one limpet tank reactor; a second input means which may transfer solvent and hydrolytic reagents (separately or jointly) to the at least one limpet tank reactor; and a control system which may communicate with: a heating system which may have one or more integrated heating coils, and which may have at least one thermal sensor; a reagent dosing system which may have at least one pH sensor; an aeration system which may have at least one oxygen sensor; and a nutrient dosing system.

Advantageously, the at least one limpet tank reactor may be heated by one or more integrated heating coils which transfer heat evenly throughout the tank and does not become clogged with waste. A conventional heating system utilising internal heat exchangers, however, would become clogged by the FOG mixture during repeated use, reducing the efficiency of the heating system and promoting the retention of waste within the tank, which would need to be cleaned. Recirculating the mixture from the lower half to the upper half of the at least one tank through the recirculation line aids mixing. This ensures that the FOG mixture remains relatively homogenous in the at least one limpet tank reactor, optimising the distribution of hydrolytic reagents with hydrolysed and unreacted FOGs. Transferring the solvent and hydrolytic reagents separately gives a greater degree of control over FOG dilution and the volume of hydrolytic reagent added. That said, transferring the solvent and hydrolytic reagents together reduces the time taken to begin the reaction as two separate steps are not required.

The control system allows control over the main parameters of the FOG treatment by controlling each of the subordinate systems for heating, pH, aeration and nutrient dosing. Manipulating the temperature of the limpet tank reactor through the heating system allows the reaction to be optimised for the particular hydrolytic reagents used, where some species of bacteria may be more mesophilic than thermophilic, or vice versa. Adjusting the pH through the reagent dosing system maintains an optimal pH as the reaction proceeds and the properties of the contents of the limpet reactor tank change. Varying the degree of aeration through the aeration system balances the oxygenation of the FOG mixture so that the hydrolytic reagents have sufficient oxygen for aerobic hydrolysis, but not such an excess that sludge formation becomes problematic. Thermal, pH and oxygen sensors all monitor their respective parameters to provide informed manual or automatic adjustment of each via the control system.

The at least one limpet tank reactor and/or recirculation line may include one or more of the following: the at least one oxygen sensor, the at least one pH sensor, the at least one thermal sensor. By disposing the sensors in the at least one limpet tank reactor and/or the recirculation line, the sensors can provide measurements indicative of internal conditions, which then inform adjustment of the associated parameters.

The at least one limpet tank reactor may have at least one carbon filter for odour control.

Using at least one carbon filter ensures that any odorous compounds arising from the treatment process are absorbed, preventing the area in which the treatment process is conducted from becoming odorous. The second input means may include at least one tank to store solvent and hydrolytic reagents (either separately or jointly), and which may have a controllable internal temperature for culturing the hydrolytic reagents.

Using at least one tank with a controllable internal temperature allows the solvent to be warmed, improving its ability to dissolve FOGs. It also optimises the activity of the hydrolytic reagents (for enzymatic activity and for culturing bacterial growth), maximising the rate of FOG hydrolysis as soon as the reagents are added. Storing the solvent and hydrolytic reagents separately gives a greater degree of control over FOG dilution and the volume of hydrolytic reagent added. That said, storing the solvent and hydrolytic reagents together allows the reagents to acclimate, and can require less volume to store them as having at least two separate chambers is not a requirement.

The heating system may control the temperature of the FOG mixture by heating the one or more integrated heating coils.

Heating the limpet tank reactor via one or more integrated heating coils ensures that the coils do not become clogged with FOG mixture, which would reduce their heating efficiency. The coils also ensure that the tank is heated evenly, so that the FOG mixture is maintained at the same temperature in each part of the limpet tank reactor, whereas internal coils could have a greater thermal gradient between the coil(s) and tank wall. Using information from the thermal sensors enables finer control of the temperature.

The nutrient dosing system may dose nutrients to the mixture. The reagent dosing system may also dose at least one of the following to the mixture via the at least one recirculation line: an alkaline chemical, an M-alkalinity buffer, hydrolytic reagents. Preferably, each system may dose to the mixture via the at least one recirculation line.

This allows the nutrients and chemicals (including hydrolytic reagents) to be dosed controllably, gradually and evenly through the FOG mixture, as they are automatically mixed into the FOGs as the mixture flows through the recirculation line, rather than requiring dedicated homogenisation of the mixture.

The aeration system may include at least one of the following to aerate the FOG mixture: at least one Venturi pump, a compressor and distribution system.

Aerating the mixture with at least one Venturi pump forces air into the mixture at high- pressure, which both aerates the mixture and contributes to mixing it by inducing eddies. A compressor and distribution system can also force compressed air into the mixture, and may do so at multiple points in the limpet tank reactor, increasing the degree of mixing. 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 is a flow diagram indicating one embodiment of the method according to a first aspect of aerobically treating fats, oils and/or greases (FOGs); and Figure 2 shows a schematic of the apparatus according to a second aspect of the invention of Figure 1.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring firstly to Figure 1, a first embodiment of the method for aerobically treating fats, oils and/or greases (FOGs) to prepare them for anaerobic digestion is indicated generally at 10, with the dotted boundary indicating the steps of the method. Flow diagram components outside the dotted line are not essential to the treatment process, but do facilitate its operation in this aspect. Feedstock 12 comprising FOG waste may be stored in tanks, such as a skip 14 or a first Intermediate Bulk Container (IBC) 16, prior to FOG pre-treatment 18. After suitably diluting the waste, FOG pre-treatment 18 involves: nutrient addition 20, aeration 22, pH correction 24 and the addition of hydrolytic reagents 26, each of which is monitored and adjusted as needed. Adding nutrients 20 provides trace sources of various elements to the hydrolytic reagents, including at least one of but not limited to: boron, magnesium, potassium, calcium, iron, cobalt, nickel, copper, zinc, selenium, molybdenum, and tungsten. These are needed to ensure that the hydrolytic reagents used are able to successfully hydrolyse the FOGs in the mixture, facilitating the synthesis of relevant enzymes by any bacteria present.

Aerating the mixture 22 (either periodically or constantly, depending on oxygen consumption) ensures that aerobic hydrolysis occurs. Examples of aeration systems include Venturi pumps, and compressor and distribution systems. The action of forcibly aerating the mixture also helps to mix the contents of the tank periodically or constantly, depending on the desired aeration process.

The pH of the mixture is corrected to remain within a range with a lower value of 6.2 and an upper value of 6.8. This maximises the rate of FOG hydrolysis without denaturing the enzymes or killing the bacteria, and is achieved by dosing sodium hydroxide, known as 'caustic' or caustic soda, into the mixture. An M-alkalinity buffer of sodium bicarbonate can also be used to offset small changes in the pH of the mixture as hydrolysis proceeds.

The hydrolytic reagents 26 may include bacteria (such as those classified by the genera Mycobacterium, Candida, Pseudomonas, or Bacillus) and/or free enzymes (such as lipases, beta-oxidases). Preferably, one or more of the following bacteria may be utilised: Mycobacterium fortuitum, Mycobacterium simiae, Candida rugosa, Pseudomonas aeruginosa, Pseudomonas cepacia, Bacillus circulans.

The treatment process essentially converts triglycerides to short-chain fatty acids, using bacteria and/or enzymes as catalysts. The first step involves breaking down each triglyceride into glycerol and long chained fatty acids, with the second step of beta oxidation incrementally reducing the chain lengths of each fatty acid until acetic acid is formed. Where hydrolysis is run to completion (and the FOGs are predominantly decomposed to acetic acid), the output from FOG pre-treatment 18 is sent to a waste reception tank 28 with an internal volume of 70 cubic metres. The treated waste then passes through a screening unit 34, with screened contaminants sent to a waste skip 36. In conjunction with an anaerobic digestion plant, the treated waste is fed into a conditioning tank 38, where further nutrient addition 40, pH conditioning 42, and aeration (and mixing) 44 occur to prepare the mixture for anaerobic digestion. The anaerobic digestion process 46 may occur in series or parallel, depending on which will maximise decomposition of the FOG waste used, but biogas 48 produced can be used to run a Combined Heat and Power (CHP) unit 50 where the pre-treatment process is utilised in conjunction with an anaerobic digestion plant. Excess power 52 generated by the CHP unit 50 can be utilised as needed (e.g. fed into the national grid), but otherwise the CHP unit 50 provides heat via the heating and cooling system 54, which runs the boiler 56. The CHP engines require cooling, which is achieved by using plate heat exchangers to transfer energy from the engine to water in hot water storage tanks. The tanks hold water at 90°C and 60°C, storing the heat provided by the engines until it is circulated to heat the mixture. The cooled water is then returned and provides cooling for the engines, allowing the cycle to continue.

The boiler 56 also heats water contained in the heating and cooling system 54, which circulates thermal energy to the FOG pre-treatment process 18, optimising the rate of hydrolysis. Hot water 58 is also routed to the screening unit 34 to pre-heat and dilute the feedstock (treated waste) as required, and ensures optimum performance of the screening unit 34, which is beneficial for the anaerobic digestion processes which then take place.

Where FOG pre-treatment 18 is stopped at a suitable point prior to completion, achieved by adjusting the pH to a neutral or alkaline value (pH 7 or greater), and/or ceasing to aerate and/or heat the mixture, the contents of the tank will have a range of fatty acid chain lengths which make the mixture suitable for biodiesel production 60. Glycerine 62 arising from the treatment process may be separated from the mixture and sent to the waste tank 28 for further use in anaerobic digestion, whilst the biodiesel precursors may be put up for sale 64.

If the user requires waste to bypass the pre-treatment, waste 30 which has been brought in by tanker can be added directly to the waste reception tank 28. In an alternate embodiment, untreated glycerine can be added from a second IBC 32 directly to the waste reception tank 28. This can then be sent for anaerobic digestion when desired.

In a further alternate embodiment, the FOG pre-treatment process 18 hydrolyses FOGs and the treated waste is then placed in storage containers. The mixture is stable as the contents (such as acetic acid) are miscible with the solvent used (water), and any remaining material forms a stable colloidal suspension. The treated waste can be sold and/or transported to another site for use in anaerobic digestion plants, without the need to have FOG treatment conducted at the same site as the AD plant. Figure 2 illustrates an apparatus configured to enact the method of Figure 1, indicated generally at 70. FOG waste may be provided from a variety of sources, typically housed in waste tanks 80 prior to beginning the treatment process. Such sources may include screenings, recovered FOGs (such as those from wastewater), FOGs washed off from solids, depackaged washings, and skimmed FOGs. The FOGs may include a variety of compounds, or may in an alternate embodiment be provided as relatively uncontaminated cooking oils directly from commercial sites. The nature of the FOGs added will affect the specific measures taken in the method to treat the waste, as discussed below.

The waste is first diluted with water to reduce its viscosity to between 300 and 800 centipoise, which in turn ensures the treated mixture is suitably liquid for later anaerobic digestion, if desired. As water is a protic solvent, it facilitates the transfer of hydrogen ions which catalyse the reaction, further improving the rate of hydrolysis. Generally, a typical dilution ratio of solvent to FOGs of 5: 1 will be used to dilute the mixture, such as where the FOGs originate from sewage. Where the FOGs are composed substantially of oils, a dilution ratio of 3: 1 is more appropriate, as the mixture is less viscous to begin with (as fats and greases each tend to have greater viscosities).

The solvent used to initially dilute the FOGs may contain the hydrolytic reagents, or the hydrolytic reagents may be added separately. The ratio of hydrolytic reagents (ml) to FOGs (kg) is between 1 : 1 and 5: 1, depending on the properties of the FOG mixture, to optimise the rate of hydrolysis without wasting reagents. Further dilution of the mixture (or feedstock added later) may be achieved by dilution with effluent from an attached AD plant, or by further addition of hydrolytic reagents.

The combined FOG waste is transferred to limpet tank reactors 82 and 84 (each having an internal volume of ten cubic metres in this instance). Each tank 82, 84 has a carbon filter 89, 90 respectively, which controls odours associated with the FOG waste and its hydrolysis. There is a vent on top of each tank 82, 84 which maintains atmospheric pressure inside each tank, with each vent connected to an odour scrubbing system. The total volume of FOG waste added to each tank 82, 84 is regulated by using ultrasonic controls 86 and 88, disposed near the top of the tanks 82 and 84 respectively. The ultrasonic controls 86, 88 provide negative feedback to the FOG waste transferral mechanism which stops the tanks from being overfilled. Should the tanks 82, 84 be overfilled anyway, a limpet tank reactor overflow mechanism 91 is included which directs excess waste to an overflow pumping station (not shown) which redirects excess waste to the waste reception tank (28). In an alternate embodiment, the waste may be transferred undiluted, and then diluted with solvent within each limpet tank reactor 82, 84.

The FOG waste contained by tank 82 can be recirculated through recirculation line 92, and the waste in tank 84 can be recirculated through recirculation line 93. From tank 82, the waste is first passed through a non-return valve 94, before a pump 98 pumps the waste through a two-way valve 102. Similarly, from tank 84, the waste is first passed through a non-return valve 96, before a pump 100 pumps the waste through a two-way valve 104.

If the waste is recirculated back to its original limpet tank reactor 82 or 84, a hydrolytic reagent dosing system 106 may be used to dose bacteria and/or enzymes to the mixture via the respective recirculation line 92, 93. This ensures that the reagents are automatically dispersed through the mixture instead of relying on subsequent mixing of the FOG mixture.

A pH and nutrient dosing system 108 can dose alkaline chemicals (such as sodium hydroxide), M-alkalinity buffer (such as sodium bicarbonate) and/or nutrients to the mixture via the recirculation lines 92, 93. This ensures that the reagents are automatically dispersed through the mixture instead of relying on subsequent mixing of the FOG mixture. The pH of the mixture in each recirculation line 92, 93 is monitored by pH sensors 110, and informs the dosing of the above chemicals. It is beneficial to supply sodium hydroxide in solution, particularly at a concentration of around 5.5 M, as this enables fine control of the pH with a minimal amount of sodium hydroxide.

An aeration system 112 uses oxygen sensors 114 in each recirculation line 92, 93 to detect levels of dissolved oxygen in the mixture, which informs subsequent aeration the mixture using a compressor and distribution system 116. This system 116 connects to a plurality of diffusers 118 and 120 at the base of each limpet tank reactor 82 and 84, each of which release air to improve oxygen distribution and more evenly mix the FOG mixture.

The aeration system operates in a manner which keeps the mass ratio of dissolved oxygen to FOGs in the mixture in the range 3 : 1 to 5 : 1. The oxygen sensor measures the level of dissolved oxygen and this affects the volume of oxygen introduced to each tank by the compressor and distribution system 116. This ensures that an oxygen deficit does not arise as the hydrolytic reagents use the oxygen present to hydrolyse the FOGs, but also that an excess of oxygen is not supplied as this would promote an increase in viscosity and form a sludge, reducing the rate of hydrolysis and rendering the mixture unsuitable for later use in a high-rate anaerobic digestion plant. If the sensor detects the dissolved oxygen concentration is too low, it signals for the aeration system 112 to increase its activity, increasing the levels of dissolved oxygen in the mixture. If the level of dissolved oxygen is too high, it signals to decrease the activity of the aeration system 112, allowing the level of dissolved oxygen to reduce as hydrolysis proceeds. In an alternate embodiment, one or more Venturi pumps in each tank achieve a similar result, sending high pressure air into each FOG mixture to aerate and mix the contents.

One or more combined heat and power (CUP) units 122 are employed to heat water in a first closed circuit 128 including a first hot water tank and pump 124. Hot water (at roughly 80°C) is pumped through a heat exchanger 126, where it transfers energy to water in a second closed circuit 134 including a second hot water tank and pump 130. Hot water (at roughly 60°C) can be pumped through the integrated heating coils 136 and 138 of each limpet tank reactor 82 and 84, with measurements of the temperature of the FOG mixture via thermal sensors 132 affecting the rate at which hot water is pumped. As the heating coils 136, 138 are integrated, they cannot become clogged with the FOG mixture and can therefore operate most efficiently. After hot water has transferred energy through the heating coils 136, 138 to the FOG mixture, the cooled water 140 is returned to the heat exchanger 126 to be re-heated and circulated again.

In this manner, the temperature of the contents of each limpet tank reactor are maintained between around 30°C and 60°C during the process. The temperature can be specifically held at 50°C to liquefy solid FOGs in the waste and achieve the desired viscosity. After this, the temperature is cooled to between 30°C and 40°C, which is the optimal temperature range for bacterial and enzymatic activity, maximising the rate of FOG hydrolysis. Whilst elevated temperatures of 50°C to 60°C would kill many bacteria, and denature many enzymes, the hydrolytic reagents used in the process are mesophilic or thermophilic, meaning that they can survive (and may thrive) in such temperatures. For any bacteria that are killed or enzymes that are denatured, additional hydrolytic reagents can be added to replenish the quantity of active reagents.

The process of hydrolysis can be controllably halted by changing the pH, aeration (i.e. oxygenation) and temperature of the mixture, as well as the degree of mixing, allowing the extent of FOG hydrolysis to be controlled, and with the overall result that fatty acids with a desired range of chain lengths can be selectively generated. To halt hydrolysis, the pH of the mixture can be made neutral (i.e. pH 7), thereby removing catalytic hydrogen ions. Ceasing to aerate the mixture means that the level of dissolved oxygen available is reduced, causing the rate of hydrolysis to quickly fall. This also has the effect of reducing the degree of mixing in the mixture. Ceasing to heat the mixture reduces the probability of any one collision having enough energy to overcome the activation energy barrier for hydrolysis, and reduces the general activity of the bacteria.

Complete hydrolysis of the FOG mixture can take up to approximately 24 hours, or up to 48 hours for highly contaminated or viscous waste. Partial hydrolysis occurs over a shorter timeframe and, if the treatment is halted, produces fatty acids with a range of chain lengths suitable as feedstock for biodiesel production, for example. Once the FOG mixture has been sufficiently hydrolysed, the treated FOG mixture 142 can be controllably passed through the relevant two-way valve 102 or 104 and transferred to a batch feed tank 144. The batch feed tank 144 acts as a holding tank where the treated FOG mixture 142 can be held prior to further processes involving anaerobic digestion. An ultrasonic control 146 ensures that the tank 144 is not overfilled by regulating the two-way valves 102 and 104, thereby controlling the transfer of treated FOG mixture 142 to the tank 144. A plurality of diffusers 148 at the base of the tank 144 keep the treated FOG mixture 142 aerated and mixed by distributing air from the compressor and distribution system 116, where required.

The treated FOG mixture 142 may be used as supplemental COD (chemical oxygen demand) in other processes represented by 150, such as biodiesel production or other separate processes. An inline COD controller 152 controls the rate at which the treated FOG mixture 142 is transferred to an anaerobic digestion plant 154, depending on the rate at which it is digested by the plant. In one embodiment, the plant 154 is incorporated into a single system with the apparatus for enacting the treatment method as described above.

The above description represents an embodiment of the method of FOG treatment and the apparatus used to enact it, but other embodiments are also envisaged within the scope of this application. Other embodiments might include the apparatus as described above being partially or fully incorporated into a high-rate anaerobic digestion plant to supply treated FOG waste to the anaerobic digestion tank(s). The internal volume of the limpet tank reactor(s) may vary, being greater or lesser than the ten cubic metres in the embodiment of Figure 2. The reaction may be run at a lower or higher temperature and/or pH, if alternate bacteria and/or enzymes are used which have optimal reaction conditions that are different to the ones above. The apparatus may be operated by utilising one limpet tank reactor to treat the waste, and using the second limpet tank reactor as an overflow tank, with the overflow mechanism leading only to the second limpet tank reactor as opposed to a pumping station and waste reception tank. The hydrolytic reagents may be stored in a storage tank with a controllable internal temperature, allowing the reagents to be kept at a temperature which optimises the growth of bacterial cultures and/or the activity of the enzymes. Hydrolytic reagents may be added to the mixture prior to or along with the solvent during dilution. Hydrolytic reagents may also be dosed directly into the limpet tank reactor during the treatment process. Storage and reproduction of bacteria can take place within a separate reactor, including the reproduction of bacteria on fixed film algae.

An alternate solvent may be used to accommodate different hydrolytic reagents or improve solvation of the FOGs. It will be appreciated that alternate solvents (or combinations of solvents) may be used as needed in additional alternate embodiments, where the aim is to dissolve both FOGs and hydrolytic reagents without inhibiting hydrolysis. Where incorporated into an AD plant, effluent from the plant may be used to dilute the waste during treatment, thereby achieving the desired viscosity, and also to supplement the quantity of M-alkalinity buffer. Adding sufficient quantities of AD plant effluent can raise the pH of the mixture to around 7.2 in addition to diluting the mixture, and may be used to effectively halt the reaction. Ambient thermal energy from the outputs of the AD plant may also be recovered and used to heat the waste being treated, instead of burning biogas or heating oil to heat water in the heating system. Biogas may be used direct from the AD plant where the treatment is incorporated, as opposed to externally sourcing biogas for the CHP unit. The treatment process can also be run continuously as opposed to in batches where it is incorporated into an AD plant, with the periodic or continuous harvesting of VF As from the limpet tank reactor being used to continually feed the AD plant. Continual addition of FOGs to the treatment process would also occur alongside this. Alternate nutrients or nutrient sources to those stated previously may be added to facilitate hydrolysis (which may deliberately also affect the pH, or have a reduced effect on the overall pH of the mixture as compared to the nutrients above). Furthermore, alternate M-alkalinity buffer compounds (such as phosphates) may be used in an alternate embodiment, as may alternate alkaline chemicals at various concentrations. Independent systems may be used for dosing nutrients, alkaline chemicals and/or M- alkalinity buffer to the FOG mixture. Chemicals may also be added directly to each limpet tank reactor. It will be appreciated that a variety of alkaline chemicals and M- alkalinity buffers may be used to affect the pH of FOG mixtures, and this is not limited to the compounds of the embodiments discussed.

The dilution ratio of solvent to FOGs, the mass ratio of dissolved oxygen to FOGs, and the ratio of hydrolytic reagents (ml) to FOGs (kg) may all be changed depending on the desired output properties of the treated waste, depending on whether the desired FOG mixture needs to be hydrolysed to a greater or lesser degree.

In terms of aeration, air with different proportions of oxygen may be used to oxygenate the mixture (by blending in pure oxygen gas to create a richer air mixture), or alternatively a chemical means of oxygenation may be employed. Separately, alternate mixing means may also be used, employing a mechanical mixer instead of relying on mixing through aeration. In terms of halting the reaction, an alternate embodiment could actively cool the FOG mixture in order to stop hydrolysis, rather than simply ceasing to heat the FOG mixture which means a longer period of time where hydrolysis may still occur. Alternatively, the mixture could be aerated with an inert gas (such as nitrogen) instead of ceasing to aerate the mixture. Liquid pumping is also a mixing method that might be utilised.

In summary, it is therefore possible to aerobically treat fats, oils and/or greases (FOGs) by: (a)(i) diluting the FOGs with solvent to form a mixture with a viscosity lower than the undiluted FOGs, and (ii) adding hydrolytic reagents to the FOGs to mediate FOG hydrolysis; (b) heating the mixture to a temperature substantially in the range 30°C to 60°C, to substantially liquefy solid FOGs and optimise the rate at which FOGs are hydrolysed; and (c) controlling at least one of the following in relation to the mixture for a period of substantially up to 48 hours: pH, M-alkalinity, subsequent temperature, aeration, nutrient dosing, supplemental hydrolytic reagent addition, volatile fatty acid extraction; thereby modulating the extent of FOG hydrolysis and generating fatty acids with a predetermined range of chain lengths.

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 method of aerobically treating fats, oils and/or greases (FOGs), which comprises the steps of:

(a) (i) diluting the FOGs with solvent to form a mixture with a viscosity lower than the undiluted FOGs, and

(ii) adding hydrolytic reagents to the FOGs to mediate FOG hydrolysis;

(b) heating the mixture to a temperature sub stantially in the range 30°C to 60°C, to substantially liquefy solid FOGs and optimise the rate at which FOGs are hydrolysed; and

(c) controlling at least one of the following in relation to the mixture for a period of substantially up to 48 hours: pH, M-alkalinity, subsequent temperature, aeration, nutrient dosing, supplemental hydrolytic reagent addition, volatile fatty acid extraction;

thereby modulating the extent of FOG hydrolysis and generating fatty acids with a predetermined range of chain lengths.

2. A method as claimed in claim 1, wherein:

during step (b) the mixture is heated substantially to 50°C;

further during step (b) the mixture is cooled to and maintained substantially in the range 30°C to 40°C, with nutrients and additional hydrolytic reagents then added to the mixture; and

furthermore, the method comprises a final step of:

(d) halting FOG hydrolysis by adjusting the parameters of step (c), thereby modulating the extent of FOG hydrolysis and generating fatty acids with a predetermined range of chain lengths.

3. A method as claimed in claim 2, in which FOG hydrolysis is halted or substantially halted by taking at least one of the following actions: increasing the pH of the mixture to a value of substantially 7 or greater; ceasing to mix the mixture; ceasing to heat the mixture; or ceasing to aerate the mixture.

4. A method as claimed in any one of the preceding claims, in which the solvent is predominantly composed of water.

5. A method as claimed in any one of the preceding claims, in which the solvent contains the hydrolytic reagents.

6. A method as claimed in any one of the preceding claims, in which the hydrolytic reagents include one or more types of at least one of the following: bacteria, free enzymes.

7. A method as claimed in claim 6, in which each type of bacteria is taxonomically classifiable under one of the following genera: Mycobacterium, Candida, Pseudomonas, Bacillus.

8. A method as claimed in claim 6 or 7, in which each type of bacteria is chosen from one of the following: Mycobacterium fortuitum, Mycobacterium simiae, Candida rugosa, Pseudomonas aeruginosa, Pseudomonas cepacia, Bacillus circulans.

9. A method as claimed in claim 6, in which each type of free enzyme is one of the following: lipase, beta-oxidase.

10. A method as claimed in any one of the preceding claims, in which the mixture has a dynamic viscosity of substantially between 300 and 800 centipoise.

11. A method as claimed in any one of the preceding claims, in which a dilution ratio of solvent to FOGs of or substantially of 5: 1 is used to form the mixture.

12. A method as claimed in any one of the preceding claims, in which a dilution ratio of solvent to FOGs of or substantially of 3 : 1 is used to form the mixture where the FOGs are comprised of or substantially of oils.

13. A method as claimed in any one of the preceding claims, in which the ratio of hydrolytic reagents (ml) to FOGs (kg) is substantially in the range 1 : 1 to 5: 1.

14. A method as claimed in any one of the preceding claims, in which the nutrients added to the mixture include a source of at least one of the following elements: boron, magnesium, potassium, calcium, iron, cobalt, nickel, copper, zinc, selenium, molybdenum, tungsten.

15. A method as claimed in any one of the preceding claims, in which the mixture is maintained with a pH substantially in the range 6.2 to 6.8.

16. A method as claimed in any one of the preceding claims, in which at least one pH sensor monitors the pH of the mixture.

17. A method as claimed in claim 16, in which the pH of the mixture is controlled by adding at least one of the following: an alkaline chemical, an M-alkalinity buffer.

18. A method as claimed in claim 17, in which the alkaline chemical is sodium hydroxide.

19. A method as claimed in claim 18, wherein sodium hydroxide is provided in solution.

20. A method as claimed in claim 19, in which the solution of sodium hydroxide has a concentration of or substantially of 5.5 M.

21. A method as claimed in any one of claims 17 to 20, in which the M-alkalinity buffer is sodium bicarbonate.

22. A method as claimed in any preceding claim, in which the mixture is maintained with a mass ratio of dissolved molecular oxygen to FOGs substantially in the range 3 : 1 to 5 : 1.

23. A method as claimed in claim 22, in which at least one oxygen sensor monitors the mass ratio of dissolved molecular oxygen to FOGs in the mixture.

24. A method as claimed in claim 22 or 23, in which the mass ratio is maintained by aerating the mixture.

25. A method as claimed in any one of the preceding claims, in which the mixture is periodically or constantly aerated.

26. A method as claimed in any one of the preceding claims, in which the mixture is periodically or constantly mixed.

27. A method of aerobically treating fats, oils and/or greases (FOGs) substantially as described herein, with reference to and as illustrated in Figures 1 and 2 of the accompanying drawings.

28. An apparatus for the method as claimed in any preceding claim, comprising: at least one limpet tank reactor to hold the FOG mixture during hydrolysis; at least one recirculation line to recirculate the FOG mixture in the at least one limpet tank reactor;

a first input means to add FOGs to the at least one limpet tank reactor; a second input means to transfer solvent and hydrolytic reagents (separately or jointly) to the at least one limpet tank reactor; and

a control system which communicates with: a heating system with one or more integrated heating coils, and at least one thermal sensor; a reagent dosing system with at least one pH sensor; an aeration system with at least one oxygen sensor; and a nutrient dosing system.

29. An apparatus as claimed in claim 28, in which the at least one limpet tank reactor and/or recirculation line include one or more of the following: the at least one oxygen sensor, the at least one pH sensor, the at least one thermal sensor.

30. An apparatus as claimed in claim 28 or 29, in which the at least one limpet tank reactor has at least one carbon filter for odour control.

31. An apparatus as claimed in any one of claims 28 to 30, in which the second input means includes at least one tank to store solvent and hydrolytic reagents (separately or jointly) with a controllable internal temperature for culturing the hydrolytic reagents.

32. An apparatus as claimed in any one of claims 28 to 31, in which the heating system controls the temperature of the FOG mixture by heating the one or more integrated heating coils.

. An apparatus as claimed in any one of claims 28 to 32, in which the nutrient dosing system doses nutrients to the mixture.

34. An apparatus as claimed in any one of claims 28 to 33, in which the reagent dosing system doses at least one of the following to the mixture: an alkaline chemical, an M-alkalinity buffer, hydrolytic reagents.

35. An apparatus as claimed in claim 33 or 34, in which the nutrient dosing system and/or reagent dosing system dose to the mixture via the at least one recirculation line.

36. An apparatus as claimed in any one of claims 28 to 35, in which the aeration system includes at least one of the following to aerate the FOG mixture: at least one Venturi pump, a compressor and distribution system.

37. An apparatus for aerobically treating fats, oils and/or greases (FOGs) substantially as described herein, with reference to and as illustrated in Figure 2 of the accompanying drawings.