NL2033778B1

NL2033778B1 - Methode voor het produceren van biogas in een anaerobe slibbehandeling

Info

Publication number: NL2033778B1
Application number: NL2033778A
Authority: NL
Inventors: Mirzaee Parastoo; Giesen Andreas; Marika Scherrenberg Sigrid; Koornneef Eddie
Original assignee: Haskoningdhv Nederland Bv
Priority date: 2022-12-21
Filing date: 2022-12-21
Publication date: 2024-06-27

Abstract

The invention relates to a method for producing biogas in an anaerobic sludge treatment, wherein an organic waste stream is first subjected to a (semi-)continuous anaerobic thermal pretreatment wherein temperature is controlled between 40 and 80 °C, retention time t is controlled between 1 to 50 hrs, and pH is controlled between 4.5 and 7, and wherein temperature, retention time and pH are selected within the aforementioned ranges in order to, during pre-treatment: (i) collect at least 10 mg hydrogen gas per gram volatile solids (VSremoved), and (ii) prevent or limit methanogenesis to below 6% v/v methane gas, wherein the pre-treated organic waste stream during pre-treatment is subjected to hydrolysis and acidogenesis, and wherein the treated organic waste stream is then fed to an anaerobic digester optionally after subjecting the treated organic waste stream to pasteurization, and wherein essentially all produced hydrogen gas which is collected from the pre-treatment is fed to the organic waste stream in the anaerobic digester, to obtain biogas with increased methane concentration.

Description

Methode voor het produceren van biogas in zen anaerobe sijbbehandeling

Field of invention

The invention is in the field of anaerobic degradation of sludge, and biogas production.

Background of invention

Biogas is considered an important renewable energy source and it is produced in the process of anaerobic degradation of organic material such as municipal sewage waste. Biogas is typically produced as a mixture of mainly CHa (50-75% v/v), CO: (25-50% v/v), and a small fraction of H2S, moisture and siloxanes. However, the high CO: content of biogas makes biogas less attractive as an energy source.

Stll, due to environmental issues caused by fossil fuel consumption, great amount of attention has been drawn to biogas production through anaerobic digestion (AD) in these recent years.

There is report of many attempts to purify biogas and remove CO:, and to lower the amount of acidic acid, referred to in the field as biogas upgradation. For biogas plants in order to be able to inject the biogas in (natural) gas grid or use it as a fuel for transportation, a purity of at least 95% CHa wv is needed. Biogas upgrading systems focus on either removing CO: or converting CO2 to CHa. Biogas upgrading can be done through physical, chemical, and biological processes. Physical and chemical upgrading biogas technologies consist of water washing, pressure swing adsorption, polyglycol adsorption and chemical treatment to remove CO: from biogas. However, upgrading gas by means of chemical and physical processes is costly, due to high energy and chemicals consumption. Another important sustainability disadvantage of current technologies is that the captured CO: is wasted and, while removing CO2, part of the CH. is also removed. US2022/0127646 discloses a biological process in which hydrogen gas is injected into the bioreactor to reduce a portion of the CO: into methane and thus lower the CO: content of the biogas. However, production of hydrogen is energy-intensive and reduces overall sustainability of the produced biomethane.

In the field, ‘green’ hydrogen is considered being an attractive alternative to fossil fuels. This is due to the fact that hydrogen has greater energy density that other fuels (141.9 MJ/kg for Hz while for CHa is 50 MJ/kg) and also the only product of hydrogen combustion is water vapor. The most common and cheapest hydrogen production is based on fossil fuels, however, this is now contributing to 70 to 100 million tons of carbon dioxide on a yearly basis. Therefore, developing and improving green technologies for hydrogen production is of utmost importance. These alternative routes for hydrogen production are renewable resources-based technologies (electrolysis of water with electricity from solar and wind plants), and biological processes (e.g. photo-fermentation, and dark fermentation).

In recent years, dark fermentation (DF) of biomass, e.g., sewage sludge, is on spotlight as a green technology to produce hydrogen. In dark fermentation, through hydrolysis and acidogenesis the biomass is transformed into volatile fatty acids (VFAs) and hydrogen. DF however is a complicated process and biohydrogen yield is affected by many parameters, making it troublesome to efficiently be used in 1 practical conditions. For example, parameters like temperatures, organic loading, hydraulic retention time, and pH have significant impact on hydrogen production, and due to fluctuation in the feed source and flows large variations in hydrogen production are observed. Tena ef al (2021) observed that the highest yield of hydrogen from dark fermentation of mixed organic substrate (sewage sludge and wine vinasse) was achieved at an HRT of 0.5 days.

Another important factor that can significantly affect hydrogen production in DF is hydrogen partial pressure. When hydrogen partial pressure increases, the metabolic pathways change to lactate, ethanol, acetone, and butanol production and negatively affect Hz production. To counteract this, sparging with an inert gas can be used to decrease the partial pressure of Hz in the substrate and shifts the reactions to more Hz production (Akhlaghi & Najafpour-Darzi, 2020). However, purging gas needs high capital cost for the equipment and its maintenance.

US 2015/011 1273 discloses an attempt to tackle the problem of low hydrogen yield and costly gas purging system using the Le Chatelier principle. In the Le Chatelier principle, for a reversible reaction, if the product is removed continuously, reactions go to the right. Therefore, if CO: is removed, reaction goes more forward, and more hydrogen can be produced. Glucose degradation is possible through two pathways which are not thermodynamically favorable, but they are possible if CO: is removed from head space (Jackson et al, 2002). In these pathways, butyrate and propionate are consumed and the products are Hz and acetate. US 2015/011 1273 teaches to install a CO: trap on top of the reactor to capture CO2 from the headspace. This CO: trap is a hydroxide in solid form and most preferably 100% potassium hydroxide or sodium hydroxide pellets. However, use of such chemicals and increased generation of

CO: strongly reduce the sustainability level of the produced biomethane. Also, the organic acids produced in DF can often not be harvested and beneficially utilized. In dark fermentation of sewage sludge, only a small fraction of the energy contained in the biomass is effectively transformed into Ha.

In the art, there is a need for improving biogas production through anaerobic digestion, reducing the

CO: footprint, and improve sustainability.

SUMMARY OF THE INVENTION

In order to improve biogas production efficiency, the inventors found that a thermal pre-treatment of the sludges to be digested, e.g. sludge from a waste water treatment plant like waste activated sludge (WAS) or waste granular sludge (WGS) should be applied, while controlling pH, retention time and temperature. This thermal pre-treatment of WAS and/or WGS is advantageously be used prior to anaerobic digestion (AD) achieving improved degradability, increased biogas production, and better dewaterability. With the controlled thermal pre-treatment, it is possible to achieve hydrogen production rates of at least 10 mg per g volatile solids removed

In the art, thermal pre-treatment alone is improving efficiency of the breakdown of complex organic structures, what results in a higher biogas production in the digester. Where thermal pre-treatment is known, such thermal pretreatment involving temperatures higher than 100°C are regarded as thermal 2 hydrolysis pretreatment (THP); these are characterized by limited retention times, and the short retention times in THP are typically associated with an increased risk for formation of recalcitrant materials which are difficult to be degraded. High-temperature THP comes with an energy cost and the high temperature also puts restraints on the materials used.

Instead, thermal pre-treatment in a lower temperature range of 60 to 80°C is advocated in NL2009007, which method is used in the so-called Themista™ process. These mild conditions were also found to be effective in terms of solubilization of organic compounds. The advantages of low temperature thermal pre-treatment are low investment cost since it can be done in ambient pressure and results in a minimal formation of recalcitrant materials. NL2009007 discloses the highest yield in terms of cumulative methane production at 70°C with a retention time of 30 minutes; hydrogen peroxide is used to reduce particle size and to add hydroxyl radicals to the mixture. However, NL2009007 is silent on hydrogen production or collection of hydrogen anywhere, and whether and how a THP could affect the hydrogen production.

While NL2009007 is silent on the process being batch, semi-batch, or continuous, from reports and articles related to NL2009007, it is clear that the skilled person would select a batch process; reference is made to Stowa report 2016-34. However, the inventors found that at the conditions according to

NL2009007, including batch operation, a lack of pH control during pretreatment will lead to low hydrogen concentrations, of the order of 10-100 mg based on volatile solids removed. Reference is made to the comparative example. It is the inventors’ insights that these low hydrogen concentrations of produced biogas in NL2009007 are believed not significant because of the combination of a very short retention time, lack of pH control and a batch-wise treatment, i.e. the tanks were emptied completely after each treatment. The short retention times of 30 minutes advocated in NL2009007 are not enough for hydrogen-producing bacteria to thrive, and also emptying the whole tanks in a batch configuration removes essentially all of the grown population which negatively affects the hydrogen production, since that would have to start up each and every time again.

The inventors made a number of changes to the original set-up of NL2009007, applying the pre- treatment in a continuous operation, with a retention time of 15 hours for the whole pre-treatment process, while controlling acid accumulation by keeping the pH between 4.5 and 7. Under these conditions, the growth of hydrogen-producing bacteria is optimized. An increase in pH would favor acetate over butyrate and also stimulate methanogenic bacteria to grow; on the other end, without pH control, acid accumulation occurs, and the pH would drop below pH 4.5, at which point hydrogen- producing bacteria activity would be inhibited. With that combination of parameters, the inventors found that a significant amount of biohydrogen could readily be generated (i.e. at least a factor 100-1000x increase) and that the biohydrogen production could be controlled and fluctuations minimized, all to improve the overall sustainability of the integrated sludge treatment process. The method significantly increases the sustainability of sludge digestion and removes drawbacks of prior art methods. 3

Hence, in a first aspect of the invention, the invention relates to a method for producing biogas in an anaerobic sludge treatment, wherein WAS or WGS is first subjected to a (semi-) continuous anaerobic thermal pretreatment wherein temperature is controlled between 40 and 80 °C, the retention time is controlled between 1 to 50 hours, and the pH is controlled between 4.5 and 7, wherein temperature, retention time and pH within these ranges are selected in order to (i) collect at least 10 mg hydrogen gas per gram removed volatile solids (VS) ['VSremoved'] and (ii) prevent or limit methanogenesis to below 6% v/v methane gas during pre-treatment, wherein the pre-treated WAS/WGS, optionally after pasteurization, is subjected to hydrolysis and acidogenesis, and wherein the treated WAS/WGS is then fed to an anaerobic digester, and wherein essentially all (i.e. at least 90%), preferably all of the hydrogen gas which is collected from the pre-treatment is fed to the organic matter in the anaerobic digester, to obtain biogas with increased methane concentration. In order to maximize biohydrogen and methane production from waste treatment, (pH, T, t)-controlled thermal pretreatment is combined with dark fermentation. This was found to work particularly well because of the maximized hydrogen gas production during pre-treatment, which is a consequence of appropriate control of temperature, retention time and pH during the pre-treatment.

The inventors found that — regardless the source and quality of the WAS/WGS, it is possible to produce at least 10 mg hydrogen gas per gram VSremoved. The pretreatment not only increases the biodegradation of sludge hydrolysis with mild temperature, but also decrease the CO: footprint of sludge treatment line by adding the produced gas to the digester for the purpose of biomethanization.

LIST OF FIGURES

Figure 1 gives a schematic overview of thermal pretreatment, hydrogen and methane gas production, including a setup with two pre-treatment reactors in series, and with all reactors performing in anaerobic conditions; organic waste sludge a is fed to a first reactor 1 which works at an increased temperature

T1 (e.g. 55 °C), after which the fermented organic matter b is fed to a second reactor 2 operating at a temperature T2 which is higher than applied in the first reactor (e.g. 70 °C). The two reactors together carry out the pre-treatment as described in the invention, the average retention time is the sum of the retention times in reactors 1 and 2; pH is monitored and controlled by acid or base addition (not shown) in both reactors independently. From each of the reactors, gas d can be collected, and means 6 can be provided to capture CO: therefrom. The mixture of hydrolyzed sludge and gas e is forwarded to the anaerobic digester 3 applying methanogenesis-favored conditions, optionally after mixing with primary sludge g. The digester is provided with means for collecting biogas h and an outlet for the digested sludge f; part of the digested sludge can be recirculated j and mixed with stream e to the digester 3.

Optionally external hydrogen gas can be added prior to the hydrolyzed sludge c before collecting the gas and capturing COz2 (k1) and/or to the gas produced in the digester 3 (k2).

Figure 2 shows a setup in which the incoming sludge a is divided over 3 reactors 1a, 1b, and 1¢, working in parallel, and wherein the fermented organic matter b is then fed to a setup of 2 parallel reactors 2a and 2b. 4

Figure 3 represents a schematic lab setup of a full-scale Themista®.

LIST OF EMBODIMENTS

1. A method for producing biogas in an anaerobic sludge treatment, wherein an organic waste stream is first subjected to a {semi-)continuous anaerobic thermal pretreatment wherein - temperature is controlled between 40 and 80 °C, - retention time t is controlled between 1 to 50 hrs, and - pH is controlled between 4.5 and 7, and wherein temperature, retention time and pH are selected within the aforementioned ranges in order to, during pre-treatment: (iYcollect at least 10 mg hydrogen gas per gram volatile solids (VSremoved), and (ii) prevent or limit methanogenesis to below 6% v/v methane gas, wherein the pre-treated organic waste stream during pre-treatment is subjected to hydrolysis and acidogenesis, and wherein the treated organic waste stream is then fed to an anaerobic digester optionally after subjecting the treated organic waste stream to pasteurization, and wherein essentially all produced hydrogen gas which is collected from the pre-treatment is fed to the organic waste stream in the anaerobic digester, to obtain biogas with increased methane concentration. 2. The method according to embodiment 1, wherein methane gas production in the anaerobic digester is monitored, and wherein fluctuations in methane gas production are minimized within 25% of the average hydrogen production at the end of the pre-treatment by controlling the hydrogen gas production during the pre-treatment through adjustment of temperature, retention time and/or pH within the set ranges. 3. The method according to embodiment 1 or 2, wherein the pre-treatment involves subjecting the organic waste steam first to a temperature T1 and a retention time t1, followed by a temperature T2 > T1 and a retention time t2, wherein t = t1 + t2, and wherein T1 is preferably in the range of 40 to 60°C and T2 is preferably in range of 60 to 80°C. 4. The method according to any one of the preceding embodiments, wherein the hydrogen gas subjected to the anaerobic digester is mixed with the pre-treated sludge using a gas-liquid pump at high pressure, preferably 1 — 2 bar. 5. The method according to any one of the preceding embodiments, wherein the methane concentration in the biogas increases in range of 5 to 75% compared to the concentration achieved in a corresponding anaerobic sludge treatment without the corresponding anaerobic thermal treatment. 6. The method according to any one of the preceding embodiments, wherein the impact on methane gas production levels by fluctuations in the hydrogen gas output from the pre-treatment is buffered by collecting part of the hydrogen gas from the pre-treatment, for temporary storage and/or addition to the organic matter in the anaerobic digester and/or addition to the methane gas produced by the digester, to obtain biohythane gas. 5

7. The method according to any one of the preceding embodiments, wherein part of the hydrolyzed and fermented sludge obtained at the end of pre-treatment is recycled to the start of the pre- treatment, in order to increase the amount of hydrogen-producing bacteria in the pre-treatment reactor(s). 8. The method according to any one of the preceding embodiments, wherein the organic waste stream subjected to the pre-treatment comprises waste activated sludge from municipal or industrial biological activated sludge wastewater treatment processes and/or waste granular sludge from industrial or municipal aerobic granular sludge wastewater treatment processes. 9. The method according to any one of the preceding embodiments, wherein the organic waste stream subjected to the pre-treatment has a carbohydrates:protein weight ratio of > 2. 10. The method according to any one of the preceding embodiments, wherein plug-flow hydraulics is used for the pre-treatment. 11. The method according to any one of the preceding embodiments, wherein the anaerobic digester receives in addition to hydrogen gas from the pre-treatment also hydrogen from a source external to the process.

DESCRIPTION OF THE PROCESS AND EMBODIMENTS

The invention relates to a method for maximizing biohydrogen and methanization along with minimizing the CO: footprint of sludge treatment. Biogas with high methane content is generated from waste activated sludge or waste granular sludge in a two stage biomethane production process, wherein hydrolysis and acidification of organic waste generates VFAs including acetic acid, propionic acid, and butyric acid, dissolved in an aqueous phase, and the aqueous phase comprising VFAs is then converted to biogas by methanogenic microbial cultures under anaerobic conditions.

In the context of the invention, the hydrogen gas that is produced by using the method of the invention is called biohydrogen gas, thus reflecting the nature of the method for obtaining the hydrogen gas.

With ‘biogas’ it is understood a gaseous mixture comprising predominantly methane, CO:2 and a small fraction of H2S, moisture and siloxanes.

The pre-treatment according to the invention is a semi-continuous or a continuous treatment. Batch processes are excluded, since due to the complete removal of treated feed from a batch reactor, the minimum requirement of 10 mg Hz/gram VSremoved could not be achieved, and hydrogen concentration in the gas would remain unsatisfactory low. The minimum requirement of 10 mg Hz/gr VSremoved was still found possible if a semi-continuous treatment was applied provided that at least 10%, more preferably atleast 25%, even more preferably at least 50%, most preferably 50 — 90% of working volume remains in the reactor for the next batch; with that, a significant amount of hydrogen-producing bacteria remain present in the reactor when a next batch of activated sludge is fed to the reactor.

The method for producing biogas in the downstream anaerobic digester may be batch, semi-continuous or continuous, but it is key that for producing hydrogen that the pre-treatment itself is semi-continuous 6 or continuous. After-pretreatment, the treated waste stream may be subjected to pasteurization (which is carried out batch-wise). An example of a suitable pasteurization step is described in GB2496723, its contents herein incorporated by reference. This pasteurization step is not a part of the pre-treatment since the pasteurization conditions would hinder the hydrogen-producing bacteria from producing hydrogen in the pre-treatment, because pasteurization should be always a batch process with a certain relation between exposure time and exposure temperature.

The organic waste stream subjected to the pre-treatment preferably comprises waste activated sludge from municipal or industrial biological activated sludge wastewater treatment processes and/or waste granular sludge from industrial or municipal aerobic granular sludge wastewater treatment processes.

Throughout the application the terminology ‘activated sludge’, ‘waste-activated sludge’ (WAS) and waste granular sludge (WGS) are used interchangeably. The WAS suitable for use in this invention may be, but is not limited to, sewage sludge from municipal wastewater treatment plant (WWTP), biological wastewater treatment plants in industry, and/or sewage sludge high in sugars such as wine, vinegar, sugar, food, alcohol, etc. manufacturing industries. It is preferred to avoid or minimize the amount of feed material high in protein, since the pathways of hydrogen production from proteins lead to lower hydrogen production. Waste streams which are relatively high in sugar content (characterized preferably by a carbohydrates: protein weight ratio of > 2) are preferred since carbohydrates pathways lead to higher hydrogen production.

The pre-treatment is carried out under anaerobic conditions. The skilled person is well capable of controlling the anaerobic process conditions so desired.

Large amounts of primary sludge (PS) from primary settling tanks in a wastewater treatment process are preferably avoided since thermal pretreatment was found to have limited effect on hydrolysis and solubilization there. PS is generated from chemical precipitation, sedimentation, and other primary processes, and distinct from secondary sludge which is the waste biomass resulting from biological treatment, typically using indigenous, water-borne micro-organisms in a managed habitat, e.g. WAS and

WGS. By limiting the contribution of PS, preferably below 10 vol% of the sludge stream subjected to the thermal pre-treatment, both energy consumption and volume of organic matter throughput can advantageously be limited. Instead, it is preferred that primary sludge is directly forwarded to the anaerobic digester to be treated, where it can be conveniently applied to cool down the pre-treated secondary sludge. pH, temperature and retention time during pre-treatment are controlled to collect (in stream d) at least 10 mg hydrogen gas per gram VSremoved, preferably at least 12 mg hydrogen gas per gram VSremoved, more preferably at least 20 mg H2/g VSremoved, more preferably at least 50 mg H2/g VSremeved; and prevent or limit methanogenesis to below 6% v/v methane gas produced by the sum of all pre-treatment reactor(s). While the proof of concept in the examples showed a 12 mg/g VSremovea, it is believed that the amount can be further increased by further optimization of pH, t and T. It is preferred that between 7

10 and 100 mg H2/g VSremoved is produced. The VS (or volatile suspended solids [VSS]) is well-known in the art; these VS measurements are executed as described in Standard Methods (PHA, 2017.

Standard Methods for the Examination of Water and Wastewater Method, 23 edn. American Public

Health Association, Washington, DC, USA).

During pre-treatment, the organic waste stream undergoes hydrolysis and acidogenesis.

The temperature range of pretreatment is between 40 to 80°C and more preferably in range of 45 to 75°C, even more preferably in the range of 50 and 70 °C, even more preferably in the range of 50 and 60 °C, most preferably between 50 and 55 °C, for example as demonstrated in example 1. The optimum temperature within the above temperature range depends on the type of organic matter that is used for feeding the system. For a sewage sludge, keeping the temperature between 55 and 70°C can guarantee higher solubilization and thus more substrate availability for hydrogen-producing microorganisms.

However, for a more readily biodegradable material such as food waste, the temperature is preferably controlled in range of 40 to 45°C.

While the retention time of the pretreatment step is within the range of 1 to 50 hours, it is more specifically between 4 to 30 hours and most preferably 5 to 15 hours. Within the range, the retention time can be reduced if methane concentration in gas increases, in order to wash away the methanogens since they cannot keep up with the growth rate of hydrogen-producing bacteria. With the retention time an average retention time is intended. A person skilled in art, can regulate the average retention time to remove methanogens from the reactors by considering its effect on hydrogen production.

It rests within the skilled person's ambit knowledge to correlate and optimize the combination of temperature and retention time during the pre-treatment; as a rule of thumb, the lower the temperature, the longer the retention time is preferred. It would be unduly restrictive to name a specific combination of temperature and retention time here, since it for example strongly depends on the source and quality of the sludge. However, the appropriate combination of temperature and retention time within the aforementioned ranges can be directly and positively verified by the skilled person who can simply monitor the amount of hydrogen gas that is produced per gram VSremoved, and the vol% methane gas that is produced during the monitoring test. Adjustment of temperature and/or retention time is required if the production of hydrogen and methane gas gets below resp. above the required thresholds, and it does not require any undue experimentation for the skilled person to continuously and (semi-

Jautomatically monitor the production of hydrogen, methane and VS in an anaerobic digestion of sludge.

The controller can also learn these relationships and adjust retention time by monitoring the butyric-to- acetic acid weight ratio in the activated sludge during pre-treatment; the ratio is an excellent indicator of the efficiency of the pre-treatment; an increase in butyric-to-acetic acid ratio indicates improved hydrogen production. It is preferred to keep the butyric-to-acetic acid weight ratio above 2.5 to favor the hydrogen production pathway. Reference is made to Ghimire(2015), its contents herein incorporated by reference. 8

It was found possible to reach the minimum of 10 mg H2/g VSremovea within the (T, t, pH)-controlled pre- treatment of the invention in a one-step pre-treatment in one or parallel-operating reactors working at the same increased temperature. However, there is a risk of burning sludge and also, in full-scale design, a AT of more than 20°C for a heat exchanger is not feasible, and hence the thermal pretreatment is preferably continuous to increase the sludge temperature in multiple, preferably at least two, steps, within the aforementioned temperature range. Hence, in a preferred embodiment, within the set temperature range, the waste can first be subjected to a temperature T1 and a retention time t1, followed by a temperature T2 > T1 and a retention time t2, wherein the sum of the retention time in the series of reactors t1 + {2 is within the aforementioned retention time range. T1 is preferably in the range of 40 to 60°C and T2 is preferably in range of 60 to 80°C. Step-wise temperature control is realized using a series of reactors, wherein the organic matter in a pre-treatment reactor is subjected to a temperature which is increased from the temperature applied in the preceding reactor, in order to stimulate hydrolysis and solubilization, to decrease viscosity - which is beneficial for downstream processes following the pre-treatment — and also to harvest the gas entrapped in substrate more straightforwardly, due to a decrease in gas solubility in liquid phase with higher temperature. While a range of 3 or more reactors in series with step-wise temperature increase for executing the pre-treatment can be envisaged, for practical reasons a series of two reactors applying a step-wise temperature increase going from the first to the second reactor is preferred.

It is preferred to carry out pre-treatment such that hydraulically plug flow or plug-flow alike conditions are obtained, in order to reduce the retention time within the set time range and in order to get to the preferred temperature of at least 70 °C. Plug flow during pretreatment helps better biomass degradation when the temperature is increased from the first to the next reactor.

The optimum pH during pre-treatment is dependent on the type of organic matter feed but is always in the range of 4 to 7, preferably in the range of pH 4.5 to 6, most preferably pH 5 to 5.5. pH control is an essential parameter for maximization of hydrogen production, and it is preferred to (continuously) monitor pH during pre-treatment. When the pH increases to 7, it is key to acidify and reduce methanogenic activity. When the pH drops to the minimum value, hydrogen-producing bacterial activity can get affected negatively. For sewage sludge the pH value is preferably kept at least at 5.5. The relatively short retention time and low pH used in the pre-treatment of the invention are selected because of the sensitivity of hydrogen-producing bacteria to pH and prolonged periods at increased temperature outside the claimed ranges.

The flow rate of streams acids and/or biosolids originating from an external source into the anaerobic digester(s) is preferably controlled in order to optimize carbon dioxide levels. 9

In a preferred method, in the pretreatment step, the WAS or WGS is fed to two continuously stirred tank reactors (CSTR) reactors in which hydrolysis and acidogenesis takes place. In this pretreatment, the biohydrogen production is controlled to reach Thauer limit (1 mole of glucose -> 4 mole of hydrogen).

The parameters temperature, pH, and HRT are continuously monitored, controlled and predicted for the most optimum performance of the system. Samples can be taken for extensive analysis of VFAs and particularly acetic acid concentrations therein, by a gas chromatographer (GC); key for maximizing hydrogen production is to achieve high acetic acid concentration. Through advanced control, by using daily recorded data, relationships can be learned between different operational parameters in order to maximize biohydrogen production. In the first reactor, a considerable part of organic compounds is solubilized, and the viscosity of the sludge/sludge mixture is decreased which eases further processing.

All the mentioned parameters are controlled by hand or (semi)automatically, in order to keep the hydrogen production above at least 10 mg H2/g VSremoveg and the hydrogen concentration in gas in range of 10% to 90%, more preferably in range of 20% to 80% and most preferably in range of 30% to 70%.

The gas produced in the reactor is continuously collected from the headspace to reduce hydrogen partial pressure and promote hydrogen production.

Also, in one embodiment, after methanogenesis a part can be recycled to the pre-treatment in order to boost the hydrogen-producing microbial community.

In one embodiment, making use of Le Chatelier principle, the CO2 that is produced in the pre-treatment can be captured (in the gas header of the reactors) to improve the biohydrogen production.

In a preferred method, pH, T and t are controlled during pre-treatment not only to collect at least 10 mg, preferably at least 12 mg, hydrogen gas per gram VSremoves and prevent or limit methanogenesis to below 6% v/v methane gas during pre-treatment, but also to minimize variations in the methane gas quality at the digester outlet; it is preferred to maintain the hydrogen concentration at the end of the pre- treatment stable i.e. preferably allowing for fluctuations in hydrogen concentrations of less than 25% of the average hydrogen production, by controlling the hydrogen gas production during the pre-treatment by amending temperature, retention time and pH during pretreatment. This average can be lower or higher depending on the substrate and the operational parameters. However, once an increased hydrogen gas output is achieved according to the requirements of the invention, it is preferred to find a balance by monitoring and controlling T and t to reduce fluctuations in the hydrogen concentration.

In addition, the impact on methane gas production levels by fluctuations in the hydrogen gas output from the pre-treatment can be buffered by collecting part of the hydrogen gas from the pre-treatment, for temporary storage and/or addition to the organic matter in the anaerobic digester. In one embodiment, the above-average hydrogen gas output from the pre-treatment is added to the methane gas produced by the digester, to obtain biomethane gas. 10

After pre-treatment, the hydrolyzed and fermented organic matter can advantageously be mixed with

PS, before feeding it to the anaerobic digester. In this way, the temperature of the pretreated organic matter can be decreased to the mesophilic temperature, suitable for the anaerobic digester. The pretreated organic matter could also be cooled down using a heat exchanger or other conventional heat transfer equipment, for example when no or limited PS is available.

It was also found that the hydrogen-enriched gas produced in the pre-treatment should be mixed well with the organic matter either before or in the anaerobic digester. A mere inlet to the digester was found to yield unsatisfactory mixing with the hydrolyzed and fermented sludge in the digester. It is preferred that the hydrolyzed and fermented organic matter is mixed with gas from a pretreatment step (outside the digester) by a gas-liquid mixing pump, at a relatively low pressure between 1 and 2 bar, depending of the height of the digester, in order to ensure sufficient gas-liquid interaction and allow for maximization of biomethanization in the digester. Mixing can advantageously be enhanced by recirculation of part of produced biogas in the digester (see figure 1, stream m).

If for disposal, or beneficial use of the sludge that remains after downstream anaerobic digestion, it is required to reduce pathogen in a pasteurization step, the activated sludge pretreated in the method according to the invention can be exposed to a batch-operated pasteurization system, e.g. as disclosed in GB2496723, before subjected to anaerobic digestion.

The pre-treatment can be combined with any anaerobic digester suitable for (semi-)continuous operation, but preferably a CSTR is used due to relatively easy operation. The operational parameters for controlling the digester performance and biomethanization are organic loading rate (OLR), VS and

TS in and out of the digester, pH, temperature, retention time, CH4 concentration, CO2 concentration, hydrogen partial pressure, VFAs concentration, alkalinity, and amount of volatile fatty acids (FOS)/buffer capacity (TAC) ratio. pH value in the digester can be in range of 6 to 8.5 but more preferably in range of 6.5 to 8 for an optimum performance of anaerobic microorganisms. pH control can be achieved using conventional acids and bases. The increase of pH is expected due to removing CO: from the media, bicarbonate consumption and reduced buffer capacity of the digester. Therefore, for a stable and optimum performance, pH is continuously monitored and controlled. The retention time can be in range of 5 to 40 days, but more is preferably in range of 8 to 25 days. The operational temperature of the digester is controlled to be between 20 to 50°C and more preferably between 30 to 40°C and most preferably between of 35 to 38°C. There is preferably a recirculation stream over the digester to keep the reactor temperature in mesophilic range by warming up the part of the digested organic matter with a heat exchanger and sending it back to feeding line of the digester.

The hydrogen flow to the digester can be adjusted based on the output of CO: in the biogas that is produced by the digester. The maximum theoretical volume of hydrogen to react with all the CO: molecules in the biogas is 4 times higher than the volume of CO: of the biogas. The hydrogen production in pretreatment is controlled to be in range of 5% to 90%, more preferably 10% to 80% of the maximum 11 theoretical hydrogen. The increase in methane concentration v/v is set to be in range of 5% to 75%, more preferably 10 — 50% in comparison of the situation with only conventional digester. The increase depends on the volume of hydrogen added to the digester. While the aim is to increase the amount of hydrogen in the first hydrolysis/fermentation step to maximize methanization in the subsequent anaerobic digestion step, if needed an incidental amount of external hydrogen-enriched gas can be provided to the digester (k2) in order to shift the balance from CO: to CHa.

Different options for biomethanization can be used. A non-limiting list is described here. Bringing gas in contact to liquid for an optimum biomethanization can also be achieved through reactor configuration, high pressure vessel digester, diffusion systems or hollow-fiber membranes. As it can be seen in figure 1, there is an alternative route to insert the gas-liquid mixture in the reactor. This alternative route is through the recirculation stream of digested organic matter into the digester. There is also a possibility to insert hydrogen gas from an external source. The hydrogen from an external source can be from any kind of renewable hydrogen production such as wind and solar energy or any other process generating hydrogen. External hydrogen can be inserted in either gas line from pretreatment step or the biogas recirculation line of digester. A plug flow configuration can be also used for anaerobic digestion instead of CSTR to further improve the organic matter degradation in shorter retention time with more biogas production. For instance, in case of using a plug flow configuration such as Ephyra®, the same biogas production can be achieved in a shorter retention time of 6 to 8 days. Anaerobic digestion can be implemented both in mesophilic (20 to 45°C) and thermophilic {50 to 80°C) temperature. There is a possibility to add CO: from an external source for both regulating the pH in the digester and increase the biomethanization in case of low CO2 production from the digester itself. Other possibilities for pH regulation are acid/base additives and/or blending the feed to the digester with an acidic organic waste.

The latter one has the benefit of additional food for the microbes.

EXAMPLES

Example 1

During Themista® operation in one of the treatment plants, a thermal pre-treatment was tested in full- scale (Figure 2) in the form of continuous operation, part of a wastewater treatment plant (WWTP) which treated municipal wastewater of 324,000 population equivalent (p.e.) with primary and secondary treatment and nitrogen and phosphorus removal.

The effect of pretreatment was studied by comparing the in- and output from a conventional Themista® wherein the WAS after 2-3 hours settling in a buffer tank was fed to the anaerobic digester, with

Themista® wherein additional pre-treatment steps were incorporated. After settling in a Themista® buffer tank (2-3 hours), the WAS was fed to the first pre-heating step with a temperature of 55°C. As can be seen in figure 2, three reactors were applied in parallel each with 28m? working capacity. The reason for having three reactors in parallel instead of one reactor with bigger volume was from a practical point, simplifying construction, warming up, and operation; the same could have been achieved using only a single reactor instead. The WAS stream was heated up to 55°C and the retention time in this step 12 was 5.5 hours. Afterwards, the WAS stream entered a second heating step to increase the temperature to 70°C for further hydrolysis of the organic matter. The retention time of this step was 3.5 hours, rendering a total pre-treatment retention time of 9 hours. The warming up of the sludge was designed in two heating steps due to practicality reasons since it was easier and more efficient to heat up the sludge stepwise and also, in full-scale, the maximum allowed AT for the heat exchanger was 20°C only.

In the second treatment step, a small amount of H202 (15mg H202/g TS) was added for further solubilization of organic matter.

After the second pre-treatment reactor, the pretreated WAS stream was mixed with primary sludge (PS).

The mixing was done based on solid content of each stream in which the stream going to the digester contained 60% WAS and 40% PS. The mixing of WAS with PS regulated the temperature of the mixed stream to be suitable for entering the mesophilic digester. The produced gas in both heating steps of

Themista® was collected in a gas header. The CH: concentration in the biogas produced in the pretreatment step (from the first and second series of reactors) fluctuated in range of 0.1 to 2% which indicated the suppression of methanogenesis activities. pH was kept in the range of 6 to 6.3.

The effect of Themista® on biogas production - without using the hydrogen produced - in the digester, are reported in table 1: Thermal pretreatment yielded on average 10% more biogas production in comparison to the reference situation without any pretreatment, which means that the biogas flow increased from 400 m%nh to 440 m?3/n. This was also reflected in the increase of 11% for TS breakdown and 8.6% for VS breakdown.

Table 1. Full-scale digester performance with and without Themista® thermal pretreatment "Anaerobic digester performance ~~ “WWTP capacity (pe) 324000 ~~ withoutpre-treatment ~~ with pre-treatment

Gas production (mh) 400 ~~ 440

CHa production (mh) 260 286

CO: production (mh) 140 154

TS breakdown (%) 27 30

VS breakdowns (%) 35 38

The actual amount of hydrogen produced with pre-treatment was not measured here above, but the 10% increase in biogas production was a clear indicator that the amount of hydrogen produced with pre- treatment was significantly higher than for the control (no pre-treatment).

In table 2, the effect of adding hydrogen to the digester was calculated for the case of the above mentioned WWTP. The maximum theoretical amount of hydrogen needed for biomethanization was calculated to be 4 times higher than the produced volume of CO: in the digester; the amount of hydrogen needed for biomethanization of biogas to reach 95% v/v CH. purity was 95% of the maximum needed 13 volume of hydrogen. As it can be seen in table 2, the 95% of maximum theoretical amount of hydrogen needed would be 585 m3h, which could reduce CO: production from 154 m3h to 21 m?/h. In this case, methane production would also increase from 260 mh to 418 m¥h. Moreover, the biogas would not require any further processing (upgrading) and could be applied directly for either injection in grid or as fuel for vehicles.

The hydrogen needed to shift the balance could be provided from Themista® biogas but also provided from external sources (see Figure 1).

Depending on the hydrogen flow rate going to the digester, the increase in CHa content of the gas is expected to vary; the increase in biogas content - and the corresponding CO: footprint reduction - can be between 8% to 44% more than the baseline.

Table 2. Hydrogen needed for biomethanization of 95% CH: in biogas for a WWTP with 324,000 p.e.

Theoretical maximum needed volume of hydrogen basedon 616 the amount of COz(m?h) “Hydrogen needed for methanization of 95% CHa (v/v) (mhr) 585

Before After biomethanization biomethanization

Gas production (m¥h) 440 440

CO: production (mh) 154 21

CHa production (mh) 260 418

Example 2

In this example, the results are shown from full-scale measurements including the pretreatment according to the invention, using the setup as described in example 1 and shown in Figure 2.

Temperature and retention time were the only controlled parameters; pH value in the reactors fluctuated between 6 and 6.3. The temperature in reactor 1 and 2 was set at 55°C and 70°C, respectively, and the retention time was controlled at 5 hours in each of the reactors, making a total (average) retention time of 10 hours (figure 2). The gas composition was measured at the gas header in which the produced gas from reactors was diluted with air. The air flow into the header pipe was 275 mh and the produced gas from reactors in total was 18 m%nh. Therefore, a dilution of 15.3 times must be considered for all the concentration readings at the end of header before entering gas treatment unit.

The results from gas measurement are presented in table 3. As it can be seen in table 3, the pretreatment configuration achieved 12.5 mg H2/g VSremoved.

Table 3. composition analysis of the gas from gas header of full-scale Themista® diluted with air without any modification and control to maximize the hydrogen production 14

~~ Fullscaleresuts “VSbreakdown (%) 3

Initial VS of the feed (g/l) 45

Gas production from pre-treatment (m3/hr) 18

Air flow into the header (m3hr) 275

Total gas flow at the measuring point (m3hr) 293

Dilution factor at measuring point 16.3

Hz in the diluted stream (ppm) 11680

Hz before dilution (ppm) 190,384 mg H2/gr VSremoved 12.5

Example 3

For a period of 2 months, the same configuration of full-scale Themista® in example 1 and figure 2, was tested in the laboratory. The schematic lab setup is depicted in figure 3. The flow to the system was 58

L/day and the working volume of each reactor was 12L. In Reactor R1, the temperature was ambient temperature (~21°C), at anaerobic conditions. The feeding was semi-continuously since every 2 hours, 4.8 L of fresh substrate was fed to the system which was equal to 58 L/day inlet to the system. The retention time in reactor 2 (55°C), and reactor 3 (70°C) was 5 hours for each, and for the whole pre- treatment, retention time was 10 hours. The operational parameters that were regulated in this experiment were temperature and retention time and were set to be representative for the full-scale

Themista® conditions. The reactors were insulated and surrounded by water bath to maintain the desired temperature. Each reactor was connected to gas bags from gas collection. The gas composition was measured by portable gas measurement equipment. For the sake of reproducibility of the WWTP situation, the WAS was taken on a weekly basis from the same WWTP in which full-scale Themista® operates.

The results from the semi-continuous lab-scale Themista® are given in table 4.

The Hz concentration in gas produced from reactor 2 (55°C) reached high value of 846,400 ppm and for reactor 3, it reached to 463,000. The total amount of hydrogen gas produced per gram VSremoved in reactors 2 and 3 was 17.1 mg.

Table 4. Results from the lab-scale Themista®

EE Labscaledata

Inletflow (Lh) 24

Gas production (L/h) 3

TS in (%) 6

VSITS in (%) 75

VS breakdown (%) 10 15

Hz concentration in R2 (ppm) 846,400

H2 concentration in R3 (ppm) 463,000

H2 production in total (mg/g VSremoved) 17.1

Comparative example

The process as described in NL2009007 was followed as a batch process to establish the hydrogen concentration and production there, even if NL2009007 was silent on hydrogen production. The WAS sample was taken from a treatment plant in the Netherlands and was warmed up to 70°C and subsequently heated for 30 minutes. There was no pH control during the pretreatment. Tests were done both aerobically and anaerobically using a lab-scale Themista according to example 3. The trials were done both aerobicaliy and anaerobically to see the effect on hydrogen production

The results are shown in table 5. The hydrogen concentration in the produced gas was 13 and 34 ppm {based on volatile solids) for anaerobic and aerobic conditions respectively.

Table 5. Results from batch test as described in NL2009007

Hz (ppm)

Anaerobic 13

Aerobic 34

LIST OF REFERENCES

Akhlaghi, N., & Najafpour-Darzi, G. (2020). A comprehensive review on biological hydrogen production. international Journal of Hydrogen Energy, 45(43), 22492-22512. https://doi.org/10.1016/j.ijhydene.2020.06.182

Ghimire, A, Frunzo, L., Pirozzi, F., Trably, E., Escudie, R., N.L. Lens, P., Esposito, G. (2015). A review on dark fermentative biohydrogen production from organic biomass: Process parameters and use of by- products, Applied Energy, Volume 144, Pages 73-95, ISSN 0306-2619, https://doi.org/10.1016/j.apenergy.2015.01.045

Tena, M., Perez, M., Solera, R. (2021). Effect of hydraulic retention time on hydrogen production from sewage sludge and wine vinasse in a thermophilic acidogenic CSTR: A promising approach for hydrogen production within the biorefinery concept, International Journal of Hydrogen Energy, Volume 48, Issue 11, Pages 7810-7820, ISSN 0360-3199, hitps://doi.org/10. 10184. ijhydene 2020.11.258

Visser, A., Moll, S. (2016). Toepassing van nieuwe gistingsconcepten Ephyra® en Themista®. Stichting toegepast onderzoek waterbeheer (STOWA), publication number: 2016-34, ISBN 978.90.5773.737.4 16

Claims

CONCLUSIONS

1. A method for producing biogas in an anaerobic sludge treatment, wherein an organic waste stream is first subjected to a (semi-)continuous anaerobic thermal pretreatment in which - the temperature is controlled between 40 and 80 °C, - the residence time t is controlled between 1 and 50 hours, and - the pH is controlled between 4.5 and 7, and wherein the temperature, retention time and pH are selected within the above-mentioned ranges in order to (i) collect during the pretreatment at least 10 mg hydrogen gas per gram of volatile solids (VS removed), and (ii) prevent or limit methanogenesis to less than 6% v/v methane gas, wherein the pretreated organic waste stream is subjected to hydrolysis and acidogenesis during the pretreatment, and wherein the treated organic waste stream is subsequently fed to an anaerobic digester, optionally after subjecting the treated organic waste stream to pasteurisation, and wherein substantially all of the hydrogen gas produced collected from the pretreatment is fed to the organic waste stream in the anaerobic digester, to obtain biogas with increased methane concentration.

2. The method of claim 1, wherein methane gas production in the anaerobic digester is monitored, and fluctuations in methane gas production are minimized within 25% of average hydrogen production at the end of pretreatment by adjusting hydrogen gas production during pretreatment by adjusting temperature, residence time and/or pH within set ranges.

3. A method according to claim 1 or 2, wherein the pretreatment consists of first subjecting the organic waste stream to a temperature T1 and a residence time t1, followed by a temperature T2 > T1 and a residence time t2, where t = t1 + t2 and where T1 is preferably in the range of 40 - 60°C and T2 is preferably in the range of 60 - 80°C.

4. A method according to any preceding claim, wherein the hydrogen gas subjected to the anaerobic digester is mixed with the pretreated sludge using a gas-liquid pump under high pressure, preferably 1-2 bar.

5. A method according to any preceding claim, wherein the methane concentration in the biogas increases in the range of 5 - 75% compared to the concentration achieved in a corresponding anaerobic sludge treatment without the corresponding anaerobic thermal treatment. 17

6. A method according to any preceding claim, wherein the impact on methane gas production levels due to fluctuations in the hydrogen gas production from the pre-treatment is buffered by capturing a portion of the hydrogen gas from the pre-treatment for temporary storage and/or addition to the organic matter in the anaerobic digester and/or addition to the methane gas produced by the digester, to obtain biomethane gas.

7. A method according to any preceding claim, wherein part of the hydrolysed and fermented sludge obtained at the end of the pretreatment is recycled to the beginning of the pretreatment, in order to reduce the amount of hydrogen producing bacteria in the pretreatment reactor(s).

8. A method according to any preceding claim, wherein the organic waste stream subjected to pretreatment comprises activated waste sludge from municipal or industrial biologically activated sludge, wastewater treatment processes and/or waste granular sludge from industrial or municipal aerobic granular sludge wastewater treatment processes.

9. A method according to any preceding claim, wherein the organic waste stream subjected to the pretreatment has a carbohydrate:protein weight ratio of > 2.

10. A method according to any preceding claim, wherein plug-flow hydraulics are used for the pretreatment.

11. A method according to any preceding claim, wherein the anaerobic digester receives hydrogen from a source external to the process in addition to hydrogen gas from the pretreatment. 18