FI130795B1

FI130795B1 - A method for recovery of ammonia within a biogas plant and a biogas plant

Info

Publication number: FI130795B1
Application number: FI20225820A
Authority: FI
Inventors: Kerttu Koskenniemi; Hanna Venäläinen
Original assignee: Ductor Oy
Priority date: 2022-09-22
Filing date: 2022-09-22
Publication date: 2024-03-25
Also published as: WO2024062163A1; FI20225820A1

Abstract

The invention intends to lower FOS/TAC ratio of material to be stripped thus enabling higher pH and enhancing stripping conditions of it within an ammonia recovery unit (4) of a biogas plant wherein the biogas plant has at least one ammonification reactor (1) for treating fed fresh feedstock to pretreated feedstock and at least one biogas reactor (2) for fermenting the pretreated feedstock to biogas and to fermented digestate and the fermented digestate is separated to solid and liquid fractions within a fermented digestate separator (5). At least a part of the separated liquid fraction is directed as returned liquid having a low FOS/TAC ratio via a returned liquid conduit (10) to a pretreated feedstock mixing arrangement (12) for mixing the returned liquid with the pretreated feedstock having a higher FOS/TAC ratio and/or to a liquid mixing arrangement (13) for mixing the returned liquid with separated liquid of the pretreated feedstock before feeding the mixed material having an intermediate FOS/TAC ratio to the ammonia recovery unit (4).

Description

A METHOD FOR RECOVERY OF AMMONIA WITHIN A BIOGAS PLANT AND A

BIOGAS PLANT

Object of the invention

The present invention relates to ammonia recovery from pretreated feedstock from an ammonification reactor of a biogas plant. The biogas plant is a two-phase fermentation plant which has at least one ammonification reactor and at least one biogas reactor.

Background of the invention

Biogas production is generally operated in one-stage units where the four phases of anaerobic digestion (hydrolysis, acidogenesis, acetogenesis, methanogenesis) — are carried out in the same digester. Biogas production can also be carried out in two-stage units: the first two phases of anaerobic digestion that occur in acidic conditions are operated in a separate hydrolyser-reactor that feeds a second reactor, the anaerobic digester, where the last two phases are carried out in near neutral conditions. Determination of volatile organic acids (FOS) and total inorganic carbon (TAC) is an easy way to monitor biogas fermentation process in a biogas plant. US8759052 describes measuring and controlling of FOS/TAC ratio, carbonate content and pH level in a biogas reactor. EP3517505 describes stripping of ammonia from fermented digestate discharged from a biogas reactor and how release of gaseous carbon dioxide elevates the pH value of stripped separated & 25 liquid. Reducing ammonia content by stripping allows recirculation of liquid to

N diluting feedstock without enrichment of ammonia within the process and facilitates = production of ammonia/nitrogen fertilizers.

Al j A biogas producing system which operates in two fermentation stages and types of

N 30 reactors is disclosed in publication WO2015151036. First stage is performed in an

N ammonification reactor which pretreats nitrogen rich feedstock in near neutral

R conditions. The ammonification fermentation phase converts nitrogen of feedstock to ammonia. Pretreated feedstock is then separated to liguid and solids parts. The solid component is fed to biogas producing reactor of the biogas fermentation stage.

Ammonia is stripped and recovered from separated liquid by a stripper of an ammonia recovery unit.

Summary of the invention

Typically, nitrogen rich biomass feedstock, which has carbon to nitrogen (C/N) ratio below 15 to 20, depending on process conditions and feedstock, at least a part of the feedstock should be first fermented in a pretreatment ammonification stage to prevent ammonia inhibition of the second, biogasification phase. In the ammonification stage most of organic nitrogen content of the feedstock is converted to ammonia and ammonium ions by microbes. If ammonia content of the pretreated feedstock is reduced by ammonia recovery, the subsequent biogas fermentation stage will not have excess ammonia content. The ammonification phase takes place in a first reactor that is preferably continuously or sequentially fed and discharged. Feasible average pretreatment time duration is from 3 to 10 days at thermophilic conditions over 41 °C. Ammonification will also occur at mesophilic conditions from 20 to 45 °C within about the same period.

Ammonia and ammonium ions are mainly suspended in liquid phase of the pretreated feedstock. Resulting content of ammonia in the liquid phase may be too high for methanogenic microbes to effectively and stably convert carbon content of the pretreated feedstock to methane in the second reactor i.e. biogas reactor where the ammonia content may rise to an inhibiting level. Amount of ammonia in the pretreated feedstock or the liquid separated from it should in such cases be reduced & 25 in an ammonia recovery unit before the pretreated feedstock or the separated liquid

N can be supplied to the biogas reactor. Without ammonia reduction, the separated = liguid also cannot be used to dilute fresh feedstock to be fed to the biogas plant

N without ensuing enrichment of ammonia within the ammonification reactor and/or fz the biogas reactor. Before recovering ammonia by e.g. stripping in the ammonia recovery unit, liquid and solid phases of feedstock may be separated and ammonia

N is recovered or stripped from the liquid phase. Ammonia stripping is a method for

R transferring of volatile ammonia of a liguid into gas phase. Ammonia stripping happens most effectively at high pH levels and elevated temperatures. Ammonia reduction and recovery from the pretreated feedstock may also be performed in a biomass stripper without liquid/solid separation.

Methane production is normally low or completely inhibited during the short pretreatment in the ammonification reactor, but other first phase transformations such as formation of ammonia and fatty acids will happen in the continuously or sequentially operated ammonification reactor. The separately performed ammonification stage enables efficient and reliable biogas fermentation, but removal of ammonia from the pretreated feedstock may face problems related to the two-stage fermentation process.

It has now been discovered that fatty acids formed during ammonification strongly hinder vaporization of ammonia in the stripping phase. The fatty acids lower the pH of the pretreated feedstock and further hinder ammonia stripping by binding ammonium ions. Reducing the ammonia/ammonium content of a material by stripping in the ammonia recovery unit has previously only been feasible by adding sufficient amounts of pH elevating additives such as sodium hydroxide or potassium hydroxide to cause transformation of the soluble ammonium ion to the more volatile ammonia form. Addition of pH elevating chemicals leads to high operating costs that reduce economic availability of this environmentally effective technology that can produce energy and fertilizers from organic waste and manures. Sodium from sodium hydroxide would also be enriched in water recirculation of the process and is an unwanted material in the fermented digestate which is used as a fertilizer. One aim of the ammonia recovery is that nitrogen fertilizer product formed from the & 25 recovered ammonia has increased value if it can be verified to be organic fertilizer

N for organic cultivation. Introducing non-organically created additives to the = processes of the biogas plant will destroy the organic status of not only the nitrogen

N containing fertilizer byproduct but also the status of potassium and phosphorus rich fz digestate discharged from the biogas reactor.

N In our extensive studies concerning stripping of ammonia from pretreated feedstock

R discharged from an ammonification reactor, it has surprisingly been found out that

FOS/TAC ratio of a material to be stripped correlates with removal rate of ammonia at stripping phase. The FOS/TAC ratio, also known as FOS/TAC value, determines the ratio of free organic acids (FOS) to total inorganic carbonate (TAC). The FOS value indicates content of volatile fatty acids, while the TAC value is a measure for the buffering capacity of the sample. Herein, we report that the FOS/TAC ratio has an even more significant correlation to removal rate of ammonia than pH value that is conventionally used to assess and enhance stripping efficiency.

It was also found out that the FOS/TAC ratio in the pretreated feedstock and/or the liquid separated from pretreated feedstock can be adjusted to a desired level by mixing with liquid separated from the fermented digestate of the biogas reactor. At low FOS/TAC ratio levels, recovery rate of ammonia at an elevated temperature of for example 80 degrees Celsius may be about 80 % without any pH elevating additives such as sodium hydroxide. A FOS/TAC ratio of 1.3 or below enables efficient stripping of ammonia without further additives. The lower the ratio is, the better the recovery ratio of ammonia. Also, a low ratio of 1.3 or below facilitates using a lower stripping temperature and thus a smaller heat input can be used.

Calculated initial FOS/TAC ratio or measured FOS/TAC ratio without decarbonization of mixed stripped materials is around 0.5, enabling excellent stripping results. As used herein, initial FOS/TAC ratio refers to the FOS/TAC ratio in liquid phase of the fermented digestate of the biogas reactor. In an embodiment, an optimum FOS/TAC value for material to be stripped is 0.8 or below.

In ammonification phase, fresh feedstock is normally diluted to 8% to 15% (weight/volume) total solids. This means that a significant amount of liquid needs to be stripped and a large amount of heat energy is reguired to increase the & 25 temperature for stripping to achieve sufficient reduction of ammonia. Further

N increasing the amount of liguid in the material to be stripped is contrary to = economical operation principle of an ammonia recovery unit as more liguid to be

N stripped means larger and more expensive installations are reguired with higher fz capacity and higher consumption of heat and electric energy. Even though a biogas plant often has a combined heat and power (CHP) unit for producing electricity and

N heat, the amount of heat available for the processes of the plant is often restricted.

R Any generated heat energy would also have more value if it could be utilized externally. Still, the reduced or omitted need of pH adjusting additives leads to more economic process and more valuable fertilizer products for sale. The mixing of the liquid from biogas reactor to the material to be stripped also avoids nitrogen enrichment in the internal circulation of water within the biogas plant and reduces the need to add fresh water and to clean the effluent discharged from the plant. 5 Here, a novel solution is developed to solve the problems related to recovery of ammonia formed during the ammonification phase. The purpose of the invention is achieved when a biogas plant and/or a method for recovery of ammonia within a biogas plant are implemented as defined in the independent claims. Preferred embodiments of the invention correspond to dependent claims.

The solution of the invention is based on that separated returned liquid that is the liquid phase of fermented digestate discharged from the biogas reactor is mixed with pretreated feedstock discharged from the ammonification reactor and/or with liquid separated from the pretreated feedstock. Fatty acids are decomposed during biogas fermentation, so their content in the fermented digestate is very low. Liquid separated from the fermented digestate instead has a high carbon dioxide content that is formed in connection with biogas fermentation in the biogas reactor. Thus, the separated liquid from the fermented digestate has very low initial FOS/TAC ratio. Initial pH of the fermented digestate is also normally above neutral and may be higher than pH of the pretreated feedstock. Mixing the returned liquid separated from the fermented digestate with the pretreated feedstock and/or liquid fraction separated from the pretreated feedstock will lead to a remarkably better ammonia recovery rate and/or ability to use lower stripping temperature without introducing pH elevating additives. Notably, feeding the returned liquid to the ammonification & 25 reactor as dilution water does not lead to the same stripping enhancing effect of the

N pretreated feedstock. In order to improve stripping conditions, the returned liguid = must be mixed i) with the pretreated feedstock after it is discharged from the

N ammonification reactor and/or ii) with the liguid fraction separated from the fz pretreated feedstock before feeding the mixed material(s) into the ammonia recovery unit for stripping. Mixing fresh water instead of the returned water does

N not help either as this will not change the FOS/TAC ratio. The improvement of

R stripping conditions is based for example on releasing gaseous carbon dioxide which leads to raised pH of the material to be stripped.

Mixing at least a part of the returned liquid with the pretreated feedstock results in lower ammonia content of solid fraction separated from the pretreated feedstock.

Then, the ammonia content of ammonia reduced liquid discharged from the ammonia recovery unit can advantageously be higher in order to keep the combined ammonia feed to the biogas reactor below a safe value that does not cause inhibition of biogas production. This enables use of lower stripping temperature. The returned liquid also contains ammonia, and it can be simultaneously recovered. Stripping all the returned liquid would not be economically feasible. If at least part of the returned liquid is used to dilute fresh feedstock, its ammonia content will be recovered from the pretreated feedstock later in internal circulation of fluids.

Optimum or minimum mixing ratio of the returned liquid with the pretreated feedstock and/or the separated liquid can be adjusted during production of the biogas plant to create desired ammonia recovery from material to be stripped within the ammonia recovery unit. A mixture controller and/or to a controlling system of the biogas plant may be configured to adjust mixing ratio of the pretreated feedstock mixing arrangement and/or the liquid mixing arrangement according to measured properties of mixed materials. The samplings and/or measurements may be taken from streams of mixed materials before and/or after the mixing arrangements and from discharged material from the ammonia recovery unit. The preferred measured properties are FOS/TAC ratio, pH, temperature, ammonia content, and recovered ammonia from the ammonia recovery unit. An important value is also recovery rate of ammonia which rate is calculated from the ammonia content of fed and & 25 discharged materials of the ammonia recovery unit. Initial FOS/TAC ratios should

N be measured from the pretreated feedstock and the fermented digestate before any = treatment including separation, if separation allows escape of carbon dioxide. Then

N the targeted FOS/TAC ratio and corresponding mixing ratio is not affected by fz optional decarbonization of materials to be stripped downstream of the sampling points. The targeted FOS/TAC ratio defining the mixing ratio of mixed materials

N should be between 0.5 to 1.2. The mixing ratio can be linearly calculated from the

N initial FOS/TAC ratios. Measured flow rates of mixed liguids are easy to use for adapting and controlling the mixing ratio.

The measured operating values will in practice stay quite constant as normal operation of a biogas plant should be very stable. Thus, active sensors located at the measurement locations may not be needed for taking the measurements.

Measurements can be taken for example daily, weekly or even monthly from samples of the materials. The installation should have sample collection ports for taking the samples. Adjustable valves and pumps for adjusting desired mixture may not need to be remote controlled. Automated controls could still enable remote control of the whole plant with less or no local employees. For example, the mixing ratio should be between 50 % and 20 % of the returned liquid from the biogas reactor within combined flows of liquid to the ammonia recovery unit. A larger ratio that the range may lead to too high capacity and cost of the installations. A lower ratio may not lead to relevant changes to stripping conditions.

Ammonification is a quite a rapid process, and the controlling system of the biogas plant should be configured to adjust average retention time of the pretreated feedstock within the ammonification reactor between 4 to 10 days. More preferably the retention time is between 4 to 7 days.

Evaporation of gaseous carbon dioxide i.e. decarbonization during stripping results in higher pH value of stripped material and enables efficient ammonia removal.

Carbon dioxide evaporates from the stripped material during stripping, but it can also be removed before stripping. If carbon dioxide of material to be stripped is mainly released before entering the ammonia recovery unit, decarbonization of the stripped material may be performed at lower temperature than ammonia stripping. & 25 Some decarbonization may also happen within extended time period during storage

N of the returned liguid. Preferably, decarbonization is performed to the returned = liguid. Decarbonization of the separated liguid or the mixed materials may also be

N performed. Decarbonization liguids before stripping can be beneficial for designing 7 an optimum arrangement of an ammonia recovery unit. Cheaper lower temperature heating media sources can be utilized for the decarbonization phase. The

N decarbonization may be boosted by ultrasonic waves or other agitation means

R within a decarbonization vessel. Decarbonization before stripping is very beneficial in enhancing stripping phase. Then there is less need to vent out released carbon dioxide gas in order to keep the partial pressure of the carbon dioxide of recirculated stripping gas adequately low.

At least a part of exhaust gas from absorber of the ammonia recovery unit may be led to a preheater of stripping gas and/or to the reactors for recovery of water and heat of the exhaust gas. The absorption within the absorber is exothermic and the applied heat should be recovered to processes of the biogas plant, preferably to preheat fresh stripping gases.

List of drawings

In the following, examples of the embodiments of the invention are disclosed in more detail with reference to the appended drawing.

Fig. 1 illustrates a biogas plant according to an embodiment of the invention.

Detailed description of the invention

Fig. 1 illustrates a biogas plant according to an embodiment of the invention. The biogas plant stores feedstock materials in storages. Feeding means conveys and comminutes and dilutes and feeds the feedstock to an ammonification reactor 1. A part of the feedstock may also be fed directly to a biogas reactor 2 wherein main methane production takes place. Conversion of most of carbon of the feedstock to methane happens typically within 30 days within the biogas reactor. The biogas plant may have several ammonification reactors 1 and biogas reactors 2. Nitrogen © 25 rich feedstock which contains too high portion of nitrogen versus to the portion of

N carbon (too low C/N molar ratio) is mainly fed for pretreatment to an ammonification 3 reactor 2 for converting the nitrogen to ammonia by ammonification fermentation.

N The ammonification fermentation is accomplished by biological organisms. Such

E process is disclosed in detail in publication WO2015151036. The ammonification 9 30 fermentation is preferably performed in thermophilic anaerobic conditions. 00

S

O FOS/TAC ratios of the pretreated feedstock and the fermented digestate are measured by titration. Automatic titrators are available for the task. Initial FOS/TAC ratio of the pretreated feedstock depends on supplied feedstock and process conditions. The is FOS/TAC ratio normally above 1,5 when pretreating chicken manure. Initial FOS/TAC ratio of the fermented digestate is normally from 0,1 to 0,5.

The pretreated feedstock discharged from the ammonification reactor 1 is fed to a pretreated feedstock separator 3 via a feedstock conduit 7. The liquid conduit 7 may be led through a pretreated feedstock mixing arrangement 12. Separated pretreated solid feedstock from the pretreated feedstock separator 3 is transferred via another feedstock conduit to the biogas reactor 2. Separated liquid from the pretreated feedstock separator 3 is fed through a liquid conduit 8 to ammonia recovery unit 4 for reducing ammonia content of the separated liquid in elevated temperature. The liquid conduit 8 may be led through a liquid mixing arrangement 13. Ammonia reduced liquid is pumped via liquid conduit 9 to the biogas reactor 2 and/or to the ammonification reactor 1 in order to dilute fed the pretreated feedstock — or fresh feedstock. Recovered ammonia gas from a stripping chamber of the ammonia recovery unit 4 may be directed to react with an acid in within absorber of the ammonia recovery unit 4 for creating ammonia salt fertilizer or it is concentrated to ammonia water. Citric acid is normally produced organically, so ammonium citrate can be an organic fertilizer product from the ammonia recovery unit 4.

Discharged fermented digestate from the biogas reactor 2 is transferred to digestate separator 5. Solids fraction from the digestate separator 5 is transferred to biomass storages for further transport optionally through a dryer. Liquid fraction of the fermented digestate i.e. returned liquid is transferred via a liquid conduit 10 to the & 25 pretreated feedstock mixing arrangement 12 and/or the liquid mixing arrangement

N 13, optionally via a returned liguid storage 16. Within the pretreated feedstock = mixing arrangement 12 the pretreated feedstock from conduit 7 is mixed with liguid

N from liguid conduit 10. Within the liguid mixing arrangement 13 the separated liguid fz from liquid conduit 8 is mixed with liquid from liquid conduit 10. Within the mixing arrangements 12 and/or 13 materials to be stripped from the reactors 1 and 2 are

N mixed in order to enhance stripping conditions. The mixing arrangements 12, 13

R may comprise adjustable pumps and/or valves for controlling mixing ratio of mixed materials. The mixing ratios of mixing arrangements 12 or 13 may be controlled by a mixture controller 14 or they may be controlled by a controlling system 15 or operator of the biogas plant. The liquid conduit 10 may lead the returned liquid through a decarbonization vessel 17 and/or storage vessel 16 for releasing carbon dioxide from the liquid. The pretreated feedstock may not contain relevant amount of carbon dioxide for relevant decarbonization. The decarbonization may also take place after mixing arrangements 12, 13 in a decarbonization vessel 17 before ammonia recovery unit 4. The separators 3 and 5 may be any type of filters and separators dependent of the separated material. A preferred type, decanter centrifuge may release carbon dioxide during operation.

The pretreated feedstock separator 3 may be omitted, and ammonia of the pretreated feedstock can be recovered within a biomass stripper of the ammonia recovery unit 4. The invention would be highly useful also with that embodiment.

The mixture controller 14 or the controlling system 15 of the biogas plant may be configured to optimize or minimize the mixture ratio so that there is no or minimal need to add pH raising additives. The recovery ratio of ammonia may also be maximized. Minimized liquid feed to the ammonia recovery unit is often preferable as higher liquid throughflow will cause higher need of heat energy of the ammonia recovery unit 4 and heat availability may be limited. The optimization tasks may be based on for example measurements of FOS/TAC ratios of mixed materials and/or percentage of recovered ammonia versus amount of inputted ammonia and/or measured ammonia content of ammonia reduced liquid exiting the ammonia recovery unit 4. Operating temperature of the stripping part of the ammonia recovery unit 4 is also a critical parameter that affects to the optimal mixing ratio of & 25 the fluids and to reaching required low ammonia content of the ammonia reduced

N liguid. For example, ammonia content of the pretreated feedstock may be around 6 = g/liter and targeted ammonia recovery should result to 2 g/l in ammonia reduced

N liguid. An optimal situation may also be overall economic optimization of the overall fz biogas production plant. That may mean for example optimized direction of different temperature fluids an optimized operation temperatures of all units of the biogas

N plant. Average treatment times within the ammonification reactor and/or biogas

N reactor may also affect the FOS/TAC ratio and pH of the material to be stripped as well as economy of the biogas plant and they are also relevant parameters to be optimized. Also, recovery cost and price of recovered ammonia salt or water will affect to the financial optimization. The critical value for the operation of the biogas plant is the combined ammonia content of ammonia reduced liquid from liquid conduit 9 and the pretreated feedstock fed to the biogas reactor in order to keep the ammonia content of the biogas reactor at desired optimal level. Mentioned other parameters should fulfill that initial task and other adaptations are just economical and other adjustments to cope with other operational limitations. The mixture control can also be manually adjusted.

The ammonia stripping phase within the ammonia recovery unit 4 may be performed by any known stripping process, for example air, steam or flash stripping or by distillation. If the gases flown out of an ammonia absorbing part of the ammonia recovery unit 4 are returned to the stripping phase, there should be means to keep partial pressure of carbon dioxide of the circulated gas so low that it does not hinder releasing of carbon dioxide from the material to be stripped. Effective releasing of carbon dioxide from the material to be stripped or prior decarbonization is required for the stripping enhancement effect. At least a part of exhaust gas from absorber of the ammonia recovery unit may be led through preheater 18 of stripping gas and then to the reactors 1, 2 for recovery of water and heat of the exhaust gas.

The decarbonization temperature is preferably from 45 to 55 degrees. Air bubbles may be blown to the vessels 16, 17 for enhancing the decarbonization. Within so low temperature much of ammonia will not be released. The decarbonization vessel 17 may be vented out to the biogas reactor 2. Throughput time of the decarbonization vessel 17 should be below 30 minutes. & 25 Generated biogas from ammonification and biogas reactors 1, 2 may fed to a CHP

N unit 6 for producing electricity and heat. The heat energy is mainly in forms of hot = cooling water and hot flue gas. Those fluids can be used to apply heat to any of

N process phases of the biogas plant. Excess heat can be used for other external fz heating purposes too. Biogas may also or instead be locally burned in a boiler in order to heat processes of the biogas plant. A boiler or other heat source is needed

N especially, if biogas is purified to methane fuel.

O

N

Claims

1. A method for ammonia stripping in an ammonia recovery unit (4) of a biogas plant, the method comprising: - treating feedstock to pretreated feedstock in at least one ammonification reactor (1), - fermenting the pretreated feedstock to biogas and fermented digestate in at least one biogas reactor (2), - solid/liquid separating the fermented digestate to a solid fraction and a liquid fraction in a fermented digestate separator (5), wherein the method further comprises: - directing at least part of the separated liquid fraction as a returned liquid via a returned liquid conduit (10) i) to a pretreated feedstock mixing arrangement (12) for mixing the returned liquid — with the pretreated feedstock, and/or ii) to a liquid mixing arrangement (13) for mixing the returned liquid with a liquid fraction from solid/liquid separation of the pretreated feedstock, and - feeding the mixed material(s) from step(s) i) and/or ii) to the ammonia recovery unit (4) for ammonia stripping.

2. The method of claim 1, wherein the returned liquid is led through a storage vessel (16) and/or through a decarbonization vessel (17) and operating temperature of the storage vessel (16) and/or the decarbonization vessel (17) is lower than temperature that is operating temperature of stripping phase of the ammonia & 25 recovery unit (4) and it is preferably from 45 to 55 degrees. N

? 3. The method of claim 1 or 2, wherein predefined targeted or measured FOS/TAC N ratio of the mixed materials is below 1.3 or preferably below 0.8. fz

4. The method of any of the previous claims, wherein mixing ratio of the mixed N materials within the mixing arrangements (12, 13) is adjusted according to N measured FOS/TAC ratios of initial or mixed materials and/or calculated percentage of recovered ammonia versus amount of inputted ammonia and/or measured ammonia content of ammonia reduced liguid exiting the ammonia recovery unit (4).

5. A biogas plant comprising: - at least one ammonification reactor (1) for treating feedstock to pretreated feedstock, - at least one biogas reactor (2) for fermenting the pretreated feedstock to biogas and fermented digestate, and - an ammonia recovery unit (4), characterized in that, - an inlet of a digestate separator (5) is connected to a digestate conduit connected to an outlet of the biogas reactor (2), and - a liquid outlet of the digestate separator (5) is connected via a returned liquid conduit (10) to the inlet of the ammonia recovery unit (4) via a pretreated feedstock mixing arrangement (12) for mixing returned liquid with the pretreated feedstock from a pretreated feedstock conduit (7) which is connected to the ammonification reactor (1), and/or -the returned liquid conduit (10) is connected to the inlet of the ammonia recovery unit (4) via a liquid mixing arrangement (13) which is connected via liquid conduit (8) to liquid outlet of the pretreated feedstock separator (3) and the inlet of the pretreated feedstock separator (3) is connected to the pretreated feedstock conduit

(7).

& 6. The biogas plant of claim 5, wherein the pretreated feedstock mixing N arrangement (12) and/or the liguid mixing arrangement (13) is connected to a ? mixture controller (14) and/or to a controlling system of the biogas plant (15). Al = a 25 7. The biogas plant of claim 6, wherein the mixture controller (14) and/or the controlling system (15) of the biogas plant is configured to adapt mixing ratio of the N pretreated feedstock mixing arrangement (12) and/or the liquid mixing arrangement i (13) according to received information about percentage of ammonia within ammonia reduced liguid discharged from the ammonia recovery unit (4).

8. The biogas plant of claim 6 or 7, wherein the mixture controller (14) and/or the controlling system (15) of the biogas plant is configured to adapt mixing ratio of the pretreated feedstock mixing arrangement (12) and/or the liquid mixing arrangement (13) according to received information about measured FOS/TAC ratio of the pretreated feedstock or separated liquid from it and preferably according to measured FOS/TAC ratio of the fermented digestate or separated liquid from it and/or according to measured FOS/TAC ratio of the mixed material(s) fed to the ammonia recovery unit (4).

9 The biogas plant of any of the claims 6 to 8, wherein the mixture controller (14) and/or the controlling system (15) of the biogas plant is configured to adapt mixing ratio of the pretreated feedstock mixing arrangement (12) and/or the liquid mixing arrangement (13) according to received information about measured amount of recovered ammonia from the ammonia recovery unit (4).

10. The biogas plant of any of the claims 6 to 9, wherein the mixture controller (14) and/or the controlling system (15) of the biogas plant is configured to adapt mixing ratio of the pretreated feedstock mixing arrangement (12) and/or the liquid mixing arrangement (13) according to received information about measured and calculated ratio of recovered ammonia from pretreated feedstock or mixed liquid fed to the ammonia recovery unit (4).

11. The biogas plant of any of the claims 6 to 10, wherein the mixture controller (14) and/or the controlling system (15) of the biogas plant is configured to adapt & 25 mixing ratio of the pretreated feedstock mixing arrangement (12) and/or the liquid N mixing arrangement (13) which mixing ratio is between 50 % and 20 % of the ? returned liguid within mixed liguid fed to the ammonia recovery unit (4). Al j 12. The biogas plant of any of the claims 6 to 11, wherein the mixture controller N 30 (14) and/or the controlling system (15) of the biogas plant is configured to adapt N mixing ratio of mixed flows of the pretreated feedstock mixing arrangement (

12) R and/or the liguid mixing arrangement (13) according to measured pH of the pretreated feedstock or separated liguid of the pretreated feedstock and preferably according to measured pH of the fermented digestate or the returned liquid and/or according to measured pH of the mixed liquid fed to the ammonia recovery unit (4).

13. The biogas plant of any of the claims 5 to 12, wherein the controlling system (15) of the biogas plant is configured to adjust average retention time of the pretreated feedstock within the ammonification reactor (1) between 3 to 10 days and more preferably between 4 to 7 days.

14. The biogas plant of any of the claims 5 to 13, wherein the returned liquid conduit (10) and/or the liquid conduit (8) is led through a storage vessel (16) and/or decarbonization vessel (17) for releasing carbon dioxide from the conveyed liquid.

15. The biogas plant of any of the claims 5 to 14, wherein at least a part of exhaust gas from an absorber of the ammonia recovery unit (4) is led to a preheater (18) of stripping gas and/or to the reactors (1, 2) for recovery of water and heat of the exhaust gas. O N O N © ? N I = oo N 00 LO N N O N