DE102018120117A1 - Process, plant and system for digestate treatment - Google Patents

Abstract

It describes a process, an installation, a system and the use of these for the treatment of fermentation residues that are produced during the fermentation for biogas production. The resulting fermentation residues of an upstream fermentation device are fed to a further Nachgäreinrichtung, wherein a post-fermentation of digestate takes place. Thereafter, these fermentation residues are fed to a separating device in which the solids are separated from a centrate of the fermentation residues. The centrate then passes into a first aeration tank in which the centrate is biologically purified by an activated sludge process using bacteria. In a vented phase of the cleaning is oxygen and in an un-aerated phase, an external carbon source is supplied. Thereafter, the activated sludge is fed to a first secondary settling tank. The clear filtrate is discharged therefrom and fed to a small-scale revitalization system with an integrated second secondary settling tank, wherein an additional biological purification and thorough mixing of the clear filtrate with addition of oxygen takes place by means of a further activated sludge process by means of bacteria. Subsequently, the clarified wastewater is fed to a third secondary clarifier, in which at least one precipitant and / or reagent are fed to the treated wastewater.

Description

It describes a process, a plant and a system for digestate treatment and the use of these for the treatment of fermentation residues, which occur during the fermentation for biogas production.

background

Biogas is a gas mixture that mainly consists of methane. The methane-containing gas is formed when biomass is decomposed under oxygen occlusion. Microorganisms, so-called archaebacteria, are responsible for this.

The biogas can be used in gas burners or engines for the provision of heat, electricity and power. Another option is biomethane treatment plants, which raise the quality of the biogas produced to the level of natural gas and feed it into the natural gas grid.

The necessary process technology is widely available and is in use on numerous plants. The adoption of the Renewable Energy Sources Act (EEG) created the incentive for the construction of güllebasis agricultural biogas plants. The same applies to waste fermentation plants, which are also specifically promoted via the EEG.

The biogas and waste fermentation plants use different biomasses for fermentation.

Energy crops such as maize, sugar beets, grass, cereal crop silage (GPS), cereals, etc. are primarily fermented in the biogas plants. In addition, animal excreta such as pork or cattle manure, dung, turkey dung or poultry droppings are allowed for the production of biogas and are also used to obtain the so-called manure bonus.

In the waste fermentation plants waste from municipal waste disposal companies, such as contents of the green or brown bin, as well as food and / or slaughterhouse waste are accepted.

These so-called substrates (especially energy crops and wastes) represent a great challenge for digestate treatment solely on account of their different composition. In addition to the different consistency of the substances, the relevant parameters for classification as farm manure or as digestate are also far apart. Especially here are the elements nitrogen, phosphorus and many heavy metal compounds.

State of the art

• - Separation - Trennung der festen Phase vom Gärrest und somit Erzielung einer Volumenreduzierung der flüssigen Lagerung
• - Bandtrockner - Trocknung der separierten Feststoffe und somit Minimierung der festen Gärreste
• - Solares Verfahren - Trocknung der separierten Feststoffe durch Sonnenenergie
• - Stripverfahren - Reduzierung der Stickstofffrachten im flüssigen Gärrest
• - Vakuumverdampfung - Verdampfung der flüssigen Phase des Gärrestes
• - Membranverfahren - Komplettaufbereitung der flüssigen Phase des Gärrestes

At present, the following processes are known for the treatment of the fermentation residues occurring in the biogas production, in particular for the minimization or processing of digestate residues:

• - Separation - separation of the solid phase from the digestate and thus achieving a volume reduction of the liquid storage
• - Belt dryer - Drying of the separated solids and thus minimization of the solid digestate
• - Solar process - Drying of the separated solids by solar energy
• - Strip process - Reduction of nitrogen loads in the liquid digestate
• - Vacuum evaporation - Evaporation of the liquid phase of the digestate
• - Membrane process - Complete treatment of the liquid phase of the digestate

However, the known methods have various disadvantages. On the one hand, the processes allow only a partial treatment, leaving an untreated residue. On the other hand, the procedures are very time-consuming and energy-intensive.

Membrane processes also have the disadvantage that the membranes used - depending on the degree of contamination of the fermentation residue to be cleaned - often need to be replaced.

In order for the wastewater obtained from the treatment to be discharged, for example into a sewer system of a municipality, various regulations and directives must be observed and implemented. These include the Water Framework Directive (WFD), the Water Resources Act (WHG), the Groundwater Ordinance (GrwV) and the discharge values of the Waste Water Ordinance (AbwV).

Initiated by the amendment to the EEG 2014, continued by the EEG 2017 and taking into account the Fertilizer Ordinance 2017, the biogas sector is undergoing change. For many biogas and waste fermentation plants, economic operation under these conditions is only possible if they can achieve full utilization of the resources available to them.

• - Lagerkapazität von 6 Monaten für flüssige Wirtschaftsdünger und Gärprodukte
• - Lagerkapazität von 9 Monaten ab dem Jahr 2020 bei > 3/ha GV oder ohne eigene Aufbringungsflächen
• - Verkürzung der Sperrfrist ab Ernte der Hauptkultur (mit Ausnahmeregelungen aber auch mit Begrenzung max. Aufbringmengen von Ammoniumstickstoff bzw. Gesamtstickstoff)
• - Sperrfrist für Grünland und mehrjährigem Feldfutterbau vom 01.11. bis zum 31.01.
• - Betriebliche Aufbringungsgrenze von 170 kg N/ha

The focus of many plants will be on the utilization of the fermentation products produced during biogas production. The following cornerstones have been created with the Fertilizer Ordinance 2017, which directly affect the operation of a biogas plant:

• - Storage capacity of 6 months for liquid manure and fermentation products
• - Storage capacity of 9 months from 2020 at> 3 / ha GM or without own application areas
• - Reduction of the retention period from harvest of the main crop (with exceptions but also with limitation of maximum application rates of ammonium nitrogen or total nitrogen)
• - Blocking period for grassland and perennial field food production from 01.11. until 31.01.
• - Operational application limit of 170 kg N / ha

• - Veredelung der Gärreste zu Gärprodukten (Wertschöpfung durch Vermarktung von Düngemitteln)
• - Reduzierung der Gärrestmengen hinsichtlich der bestehenden Flächenknappheit
• - Reduzierung der Gärrestmengen hinsichtlich Lagerung, Transport und Ausbringung
• - Verringerung der Umweltbelastung (z.B. Nitratgehalt im Grundwasser)

Thus, new and long-term safe ways to use the resulting digestate must be found. The following conditions should be considered:

• - refining digestate into fermentation products (adding value by marketing fertilizers)
• - Reduction of the digestate quantities with regard to the existing area shortage
• - Reduction of digestate quantities with regard to storage, transport and application
• - Reduction of environmental impact (eg nitrate content in groundwater)

The fulfillment of the requirements of the aforementioned regulations and guidelines is a prerequisite for the effluent to be discharged. However, the methods already mentioned do not or only insufficiently meet the specifications.

task

In plants for the production of biogas, it would therefore be desirable to provide a way to work up the resulting digestate so that in addition to a solid phase, which can be used, for example. As fertilizer, dischargeable wastewater is provided. The resulting during the treatment liquid phase as the final product should therefore be prepared so far that all requirements are met with respect to the pollution.

It is therefore the object to provide a Gärrestaufbereitungskonzept in which liquid fermentation residue (wastewater) prepared for flooding and the dehydrated residues can be sold as a fermentation product.

It is also an object to improve the digestate treatment over known processes and in particular to reduce the nitrogen loads and to provide a plant and a system which serve for Gärrestaufbereitung.

solution

• - Zuführen der anfallenden Gärreste aus einer vorgelagerten Gäreinrichtung zur Biogaserzeugung einer weiteren Nachgäreinrichtung,
• - Nachgären der anfallenden Gärreste in der weiteren Nachgäreinrichtung,
• - Zuführen der Gärreste aus der weiteren Nachgäreinrichtung einer Separiereinrichtung,
• - Trennen der Feststoffe von dem Zentrat der Gärreste in der Separiereinrichtung,
• - Zuführen des Zentrats einem Gärrestlager,
• - Zuführen des Zentrats einem ersten Belebungsbecken,
• - biologische Reinigung des Zentrats durch einen Belebtschlammprozess mittels Bakterien, wobei eine Nitrifikation und Denitrifikation bei einer intermittierenden Belüftung erfolgen, und wobei in der belüftete Phase Sauerstoff und in der unbelüfteten Phase eine externe Kohlenstoffquelle zugeführt werden,
• - Zuführen des Belebtschlammes einem ersten Nachklärbecken, in dem eine Trennung des Belebtschlamms durch Separation erfolgt,
• - Abführen des Klarfiltrats,
• - Zuführen des Klarfiltrats einer Kleinbelebungsanlage mit integriertem zweitem Nachklärbecken, wobei durch einen weiteren Belebtschlammprozess mittels Bakterien eine biologische Reinigung und Durchmischung des Klarfiltrats unter Zuführung von Sauerstoff erfolgt,
• - Zuführen des geklärten Abwassers einem dritten Nachklärbecken, in dem wenigstens ein Fällungsmittel und/oder Reaktionsmittel dem geklärten Abwasser zugeführt werden.

The above object is achieved by a process for the treatment of fermentation residues, which are incurred during the fermentation for biogas production, comprising at least the following process steps:

• Feeding the resulting digestate from an upstream fermentation device for biogas production of a further post-fermentation device,
• - post-fermentation of the accruing fermentation residues in the further Nachgäreinrichtung,
• Feeding the fermentation residues from the further subsequent fermentation device of a separating device,
• Separating the solids from the centrate of the fermentation residues in the separating device,
• Feeding the centrate to a digestate store,
• Feeding the centrate to a first aeration tank,
• biological purification of the centrate by an activated sludge process by means of bacteria, whereby nitrification and denitrification take place with an intermittent aeration, and wherein in the aerated phase oxygen and in the non-aerated phase an external carbon source are supplied,
• Feeding the activated sludge to a first secondary settling tank in which the activated sludge is separated by separation,
• Removal of the clear filtrate,
• Feeding the clear filtrate of a small-scale revitalization system with an integrated second secondary settling tank, whereby biological purification and thorough mixing of the clear filtrate with supply of oxygen takes place by means of a further activated-sludge process by means of bacteria,
• - Supplying the clarified wastewater to a third secondary sedimentation tank, in which at least one precipitant and / or reactant are supplied to the treated wastewater.

The above process steps allow the processing of fermentation residues by the sequence, wherein as one of the end products einleitfähiges wastewater is provided.

The addition of the carbon source and the precipitating agent and / or the reaction aid the treatment and are essential for the supply of wastewater.

The individual process steps are already known in a similar form from the prior art. However, the order of the individual cleaning and treatment steps allows an unpredictable improvement in the quality of the treated wastewater with respect to the removal of carbon compounds and nutrients such as nitrogen and phosphorus.

In particular, a method is provided which allows a treatment of digestate and thereby manages without mechanical filter devices and membrane. This eliminates the frequent replacement of filters or membranes and / or the cleaning of such facilities.

The dehydrated residues that are solid in the process can be used as fertilizer or fed to agriculture or composting.

In the fermentation residue storage, the digestate absorbed therein may linger for a period of several days. Care must be taken to ensure adequate mixing so that no solid deposits and digestion processes in the digestate store occur.

The separating device can be a centrifuge in which the solids of the fermentation residue are separated from the liquid phase, the centrate, by rotation.

In the case of the 2-phase separation via the separating device, the liquid phase and the solid phase are separated from one another. The solid particles can collect here due to the higher density of a drum wall and are transported by means of a screw conveyor to discharge openings. At the same time, the clarified liquid can flow along the screw into the liquid discharge zone.

The activated sludge process in the first aeration tank is a biological wastewater treatment process. The centrate is freed by the metabolic activity of aerobic chemoorganoheterotrophic microorganisms, the so-called activated sludge, as far as possible from organic contaminants, so purified. Advantageous are the general applicability and the good cleaning action for the centrate for reducing the contents of suspended solids, the chemical oxygen demand (COD), the biochemical oxygen demand (BOD5) and the nitrogen compounds (N).

The process is a low load cleaning with a focus on nitrification and denitrification to the largest possible nitrogen elimination. This type of purification results from the excess of nutrients compared to available carbon compounds.

During the biological purification in the first aeration tank, an alkaline auxiliary can be supplied against the drop in the pH of the centrate to be purified and to increase the buffering capacity. For example, caustic soda may be supplied as alkaline adjuvant.

In addition, during the biological treatment in the first aeration tank the Zentrat example. Phosphoric acid are supplied to achieve a certain ratio of BOD to phosphorus - depending on the nature of the digestate - and maintain. For example. For example, the ratio of biochemical oxygen demand (BOD) to phosphorus can be 100: 1. If the centrate is low in phosphorus, then phosphorus, for example in the form of phosphoric acid, must be supplied.

In the secondary clarifier, the solids are separated by sedimentation. The substances dissolved by the liquid phase settle at the bottom of the secondary clarifier.

• - Optimale Abtrennung der Feststoffe (Schlamm) vom gereinigten Wasser, geringe Konzentrationen im Ablauf (Feststoffe),
• - Trennung des Belebtschlamm-Wasser-Gemisches,
• - Eindickung des Schlammes, um einen hochkonzentrierten Rücklaufschlamm und Überschussschlamm bereitzustellen, und
• - Speicherungsfunktion zur Vermeidung des Ausschwemmens der Biomasse aus dem System, insbesondere bei erhöhter hydraulischer Beschickung.

The first secondary settling tank is part of the activated sludge process and fulfills the following tasks:

• - Optimum separation of the solids (sludge) from the purified water, low concentrations in the effluent (solids),
• Separation of the activated sludge-water mixture,
• Thickening of the sludge to provide a highly concentrated return sludge and excess sludge, and
• - Storage function to prevent the purging of biomass from the system, especially at elevated hydraulic feed.

The secondary clarifier is based on the principle of separation. The sewage activated sludge mixture passes from the aeration tank into the secondary clarifier. The sludge contained in the water settles (sedimentation) and is collected at the bottom and thickened.

The clear filtrate may additionally be fed a precipitating agent before and / or during the introduction into the small-scale aeration system, which accelerates the settling of the substances.

The resulting in the biological treatment in the first aeration tank, in the first secondary sedimentation tank, in the second secondary sedimentation tank and in the third secondary sedimentation sludge can supplied upstream fermenter. Subsequently, this excess sludge is reintroduced into the process, wherein a separation of the solids from the centrate takes place in the separating device and then the further process steps are run through. There is no residue left that can not be further processed.

The digestate may be supplied to the Zentrat a flocculant prior to separation of the solid phase, which supports the separation.

In addition, dilution water can be supplied in the separation of the solid phase from the centrate. The dilution water can come from upstream processes of biogas production and is advantageously used further. An upstream process may, for example, include the washing of biogas production feedstocks. Furthermore, the dilution water can come from a rainwater retention basin, which collects the precipitate of a biogas plant and / or digestate treatment plant.

In intermittent aeration, the aerated phase and the unaerated phase may each be maintained for 1 to 2 hours, preferably 1.5 hours.

Aluminum chloride (AlCl ₃ ) or polyaluminium hydroxide chloride (PAC) can be added as precipitant. Additionally or alternatively, polymers may be added as flocculants. Polyaluminum hydroxide chloride (PAC) is particularly suitable as a combined flocculant and precipitant.

In particular, glycerol can be supplied as the carbon source.

• - eine weitere Nachgäreinrichtung,
• - eine Separiereinrichtung zur Trennung der festen Phase vom Zentrat des Gärrests,
• - ein Gärrestlager,
• - ein erstes Belebungsbecken zur biologischen Reinigung des Zentrats,
• - ein erstes Nachklärbecken zur Trennung des Belebtschlamms,
• - eine Kleinbelebungsanlage mit integriertem zweiten Nachklärbecken zur weiteren biologischen Reinigung, und
• - ein drittes Nachklärbecken zur Absetzung des Belebtschlamms.

The above-mentioned object is also achieved by a plant for the treatment of fermentation residues, wherein the plant is designed and designed to carry out a method for digestate treatment according to one of the preceding methods, at least comprising

• - another post-fermentation facility,
• a separating device for separating the solid phase from the centrate of the fermentation residue,
• - a digestate storage,
• - a first aeration tank for the biological purification of the centrate,
• - a first secondary clarifier for the separation of the activated sludge,
• - A small revitalization plant with integrated secondary sedimentation tank for further biological purification, and
• - a third secondary sedimentation tank for sedimentation of the activated sludge.

In addition to the essential components for digestate treatment, the plant can also contain components that are used to produce biogas.

The above object is also achieved by a system for the treatment of digestate, which has components for carrying out the method steps of the above methods.

Key components of the digestate treatment form at least the separating device, which for example comprises at least one decanter centrifuge, for the common dehydration of the fermentation residues and / or excess sludge, and the activated sludge process in the intermittent driving mode for removing mainly the remaining dissolved carbon compounds and nutrients (at least N and / or P) ,

The solid dewatered residues can, for example, be collected in a container and fed to agriculture or composting. The clarified liquid digestate (wastewater) is discharged into the receiving waters.

The separating device represents a key technology in this concept. By means of the separating device, the digestate is separated into solid and liquid. The solid can be obtained in earthy form with a dry matter content of, for example, 25-35% by weight. This solid fermentation residue can be used directly as organic fertilizer or refined by composting, drying and pelleting.

The liquid phase, the so-called centrate, is then passed on to biological wastewater treatment. The "purity" of the centrate, expressed by the degree of separation, is crucial to the success of wastewater treatment. In the separation process, a variety of main nutrient contents such as total nitrogen, ammonium nitrogen, phosphorus, etc., are reduced by solids discharge up to 25% by weight. The nutrients then no longer need to be treated, but remain in the solid digestate as a fertilizer component.

The clean centrate is buffered in the digestate storage and fed to the subsequent wastewater treatment in the aeration tank. The centrate from the Digestate storage can be pumped with a continuous amount in the aeration tank.

The activated sludge process with the two phases nitrification and denitrification takes place in a container in which the so-called intermittent aeration process takes place.

In the aerated phase, the ammonium nitrogen is converted according to the reaction to the nitrite and further to the nitrate nitrogen: NH ₄ ⁺ + O ₂ + 2HCO ₃ ^- -> NO ₃ ^- + 2CO ₂ + H ₂ O

In the non-aerated phase, the addition of an external C source (eg glycerine) and intensive mixing of the effluent (centrate) activated sludge mixture is carried out so that a denitrification can take place according to the following equation: 2NO ₃ ^- + 2H ⁺ + 2.5C organic -> N ₂ + H ₂ O + 2.5 CO ₂

The addition of glycerol compensates for the carbon deficit required for denitrification in the ratio of BSB5 / NH4-N = 4.

• - die Klarwasserzone
• - die Trennzone
• - die Speicherzone
• - die Eindick- bzw. Räumzone.

The secondary clarifier can be divided into 4 zones:

• - the clear water zone
• - the separation zone
• - the storage zone
• - the thickening or clearing zone.

The sludge from the thickening zone is withdrawn and returned as return sludge to the first aeration tank. The clarified water is withdrawn from the clear water zone using various techniques and forwarded to the next purification stage (small-scale aeration plant).

As a final purification stage, the third secondary settling tank (precipitation tank) is provided, wherein a precipitant is added to the treated wastewater to further reduce the COD value and possibly the phosphorus content. The resulting precipitation sludge settles in the tank and is removed by pumps from the tank.

Furthermore, it is possible to insert additional process steps or downstream of the digestate or wastewater treatment. For example. Additional measures can be taken if the values determined are not reached after the separation of the solids (for example by means of a decanter). In this case, a separate process stage for the removal of phosphorus can be used, since the precipitation of the phosphorus pure precipitant can no longer be practicable.

In order to make available the phosphorus in the liquid phase for agriculture, for example, the so-called AirPrex® method can be used.

The AirPrex® process can improve biological phosphate elimination. The fermentation residue is fed to a fermenter in an AirPrex® reactor system and subjected to air stripping there. By outgassing of CO ₂ , the pH of the sludge increases significantly. At the same time, the addition of magnesium precipitation salts leads to the formation and precipitation of magnesium ammonium phosphate (MAP). The crystals integrate homogeneously into the mud matrix. The recovered MAP, also known as struvite, can then be used as fertilizer.

Phosphorus fertilizers can thus be easily obtained, on the one hand, the phosphorus processed from the digestate and on the other hand, the ever-decreasing, available amounts of phosphorus are taken into account.

The AirPrex® process can significantly increase the profitability of a plant. The complete solution for the processing of phosphorus (struvite problem) ensures smooth processes and good results in the phosphate elimination. The following improvements can be achieved with this method:

- Less phosphate chargeback

The reduction of ortho-phosphates in the sludge reduces the potential for further crystallizations and reduces the reclamation of a plant.

- Less polymer consumption

The reduction in ortho-phosphates can reduce the consumption of additional flocculants by up to 30 percent.

- Lower disposal costs

The lower water binding increases the dry matter content and thus reduces the disposal volume and thus the disposal costs of digestate.

- Better process quality

Ortho-phosphate, which is redissolved in a fermenter, can be more than 90 percent in the MAP be bound. This will improve sludge dewatering and drainage quality.

- Lower operating costs

The MAP crystallization can be carried out in a specially designed reactor and thus preferred. Thus, malfunctions in the subsequent systems can be excluded, which reduces operating costs.

The methods, equipment, and systems described herein have a variety of advantages. For example, the general applicability and good cleaning efficiency of effluents for reducing particulate matter, chemical oxygen demand (COD), biochemical oxygen demand (BOD5) and nitrogen compounds (N) are considered to be beneficial. In addition, a simple operation and a high degree of automation are available. The nutrients are concentrated in the solid digestate, reducing transport and application costs. Solid fermentation residues can also be further refined as needed, e.g. as agricultural fertilizer, bedding, compost, etc. The restrictive provisions of the Fertilizer Ordinance 2017 can be complied with and there is a reduction of the basic and surface water pollution by nitrogen, nitrate and phosphorus. The storage of fermentation residue is additionally reduced, with up to 80% of the digestate becoming clean water and thus are flooding. The concept described herein is further suitable for the full or partial cleaning of animal manure, digestate and industrial wastewater, with low specific cleaning costs for large quantities of digestate.

Further advantages, features and design options will become apparent from the following description of the figures of non-limiting embodiments to be understood.

list of figures

• 1 eine schematische Darstellung der Schritte im Vergährungsprozess;
• 2 ein schematisches Ablaufdiagramm der Reinigungs- und Aufbereitungsschritte;
• 3 eine schematische Darstellung von Komponenten einer Anlage zur Gärrestaufbereitung;
• 4 eine schematische Darstellung eines Reaktorsystems zur Durchführung eines Airplex®-Verfahrens;
• 5 eine schematische Darstellung der Umsätze bei der Gärrestaufbereitung;
• 6 eine schematische Schnittansicht einer Zentrifuge;
• 7 eine beispielhafte Darstellung einer Kleinbelebungsanlage (KBA);
• 8 ein Blockschaltbild eines Nachgärers 3;
• 9 - 28: Tabellen 1 - 20.

In the drawings shows:

• 1 a schematic representation of the steps in the Vergährungsprozess;
• 2 a schematic flow diagram of the cleaning and processing steps;
• 3 a schematic representation of components of a plant for digestate treatment;
• 4 a schematic representation of a reactor system for carrying out an Airplex® process;
• 5 a schematic representation of sales in the digestate treatment;
• 6 a schematic sectional view of a centrifuge;
• 7 an exemplary representation of a small revitalization plant (KBA);
• 8th a block diagram of a Nachgärers 3 ;
• 9 - 28 : Tables 1 - 20.

Parts indicated by like reference characters in the figures substantially correspond to each other unless otherwise specified. Furthermore, it is not intended to show and describe components that are not essential to the understanding of the teachings disclosed herein. Furthermore, the reference numbers are not repeated for all elements already introduced and illustrated, provided that the components themselves and their function have already been described or are known to a person skilled in the art.

Detailed description of an embodiment

In the following, the concept of a plant for digestate treatment as well as the process steps carried out during the digestate treatment will be described by means of examples.

For this purpose, three existing biogas plants will be used to show the starting point of fermentation residues with regard to their components and concentration. In the following, an example is given of how a system has to be set up in order to achieve the required purity of dischargeable water.

In the 2 to 8th is a plant for the production of biogas and for the treatment of the resulting digestate with essential components shown. The 9 to 28 show tables, which will be referred to in the following to illustrate the principle of digestate processing by way of examples.

In 9 the results of fermentation residue analyzes from three typical biogas plants (BGA Storkow, BGA, Neulewin II, BMA Schuby) are shown in the table shown.

The BGA Storkow is primarily fed with maize silage and a very high proportion of dry chicken manure (HTK) and duck manure. It represents the classic type of NawaRo plant (renewable raw materials), which ensures a "liquid manure bonus" according to the EEG on the use of HTK.

The BGA Neulewin is fed with a very diverse mix of renewable raw materials and cattle manure and manure. This facility belongs to the group of biogas plants, which are widespread in Germany.

The BMA Schuby is fed exclusively with sugar beet, a so-called mono-fermentation, and represents another component of the biogas plants in Germany.

The BMA Schuby serves as the basis for the presentation of the concept of fermentation residue processing described herein, wherein it is shown below how to build the plant to provide dischargeable wastewater.

The analyzed liquid phases of the three biogas plants show the large parameter differences in the nutrient composition, in particular with regard to phosphorus, nitrogen and COD (chemical oxygen demand), which are usually present in biogas plants. Waste fermentation plants can be assumed to have the same ratios.

In the present composition, the liquid phases accumulating as digestate in the three biogas plants can not easily be introduced into the environment, so that a treatment of the liquid phases must take place. There are prerequisites for discharging into public waters.

It must therefore be examined under which conditions it is possible to introduce treated wastewater from the liquid phase of the fermentation residue from biogas production. In particular, the WFD of the European Union, as well as its implementation into national and provincial law in the Water Resources Act and possibly existing exemptions of the Waste Water Ordinance must be observed.

The digestate treatment concept described here is essentially influenced by two points, namely the substrate mix used for biogas production, which strongly influences the composition of the fermentation residue and is decisive for the final result of the digestate treatment and the limit values that are required for digestate processing accumulating wastewater must be respected for discharge.

The concept described herein therefore contains different building blocks, which allow flexible adaptation to the prevailing conditions and thus ensure the economic success of biogas plants with digestate treatment and digestate treatment plants in the respective environment. For this purpose, adaptation to the general conditions of biogas plants and digestate treatment plants must also be carried out.

In the design and layout of the individual components and process steps analysis results of each present digestate are used and it will determine the boundary conditions for the biogas plant, as well as the eventual discharge values for the receiving water.

Illustration of an exemplary plant

The BMA Schuby is used as an example to illustrate the concept of digestate processing disclosed herein. The concept can also be transferred to other biogas plants, such as the biogas plants Storkow and Neulewin, and other Gärrestaufbereitungsanlagen, with appropriate modifications to the components and the process steps are required. For example, an adaptation of the supplied substances to the respective nutrient content (for example nitrogen, phosphorus) and the carbon compounds of the fermentation residue must take place. In addition, the amounts of digestate are essential for the design of a digestate treatment plant.

biogas production

The BMA Schuby are fed sugar beet as a starting substrate for biogas production. Instead of sugar beets, other plants that implement or have the concept described herein may be supplied with other substrates for biogas production.

1 shows schematically the individual steps in the fermentation process.

For fermentation, beets are used as substrate, whereby a classical mono-fermentation takes place. The biogas plant Schuby can, for example, be designed for 100,000 or 274 tons of substrate per year.

Beet picking - Washing water handling

Although the degree of contamination of sugar beets, which are already pre-cleaned at the edge of the field, is already greatly reduced with regard to possibly sandy and low-clay producer areas, these are still processed for beet reception.

In the beet acceptance, the beets are freed of dry earth and foreign bodies, processed in several purification steps and finally crushed or painted.

The dirt content of delivered beet may, for example, between 3% and 8%, for example, about 5%, accept the delivered mass. In the course of a daily several hours of operation of Beet acceptance, the delivered sugar beets can be freed from adhering soil, mud, stones and other foreign matter. The separated substances can be collected.

In a scrubber, the actual washing process can be carried out of the washing water tank and washed dry adhering soil. The washed beets can be fed to the crusher or muser via a rod conveyor, in which a portion of the adhering water and sludge drips off, with an adhering residual moisture content of about 1% of the total mass and about 1% adhering residual contamination.

The washing water, the dripping water and sludge can flow into a two-part washing water tank. In the first part, initially floating solids, such as beet rape, which is obtained in the course of mechanical cleaning, and other solids, such as turnip greens, can be removed by means of a solids separator. By means of a free overflow, the wash water can flow into the second part of the wash water tank. Here, fine particles settle in the form of sludge and can be removed and disposed of at regular intervals with an external device.

The beet leaves and beet rape taken off from the washing water tank can also be fed to the existing beet shredder via one, two or more delivery units.

The wash water from the wash water tank can be recycled as much as possible for washing. If necessary, wash water can be supplied from a settling container or, if there is only an insufficient amount of stored washing water, via a service water well. Any excess washing water can be pumped through a shaft into a pumping station during the beet acceptance campaign into the sedimentation containers or, if necessary, into the secondary digester 3 be lowered.

In the settling container washing water can be kept as Vorlagereserve for the beet laundry and fed in case of need beet wash. For this purpose, for an excess washing water from the washing water tank and during the beet acceptance campaign rainwater from the beet receiving surface can be temporarily stored via a pumping station. In case of overfilling, the automatic lowering of the washing water in the secondary fermenter 3 respectively.

Outside the beet campaign, the beet receiving area can be cleaned and the unpolluted rainwater can be directed via the pumping station into a rainwater retention basin.

In the campaign, the sugar beets are transported from the edge of the field to the biogas plant site and recorded on the existing scales, dumped and conveyed into the fixed beet preparation. By preparing the beets, the beets pre-cleaned on the field are once again pitted, then washed and painted. The beet tops and small portions of beet resulting in beet reception can be returned to the beet processing system via conveyor systems. This recirculation considerably reduces the residues to be disposed of from the beet cleaning. About at least one pump, preferably at least one revenge funnel pump, the beet is in storage tanks 1-8 ( 3 ) pumped. The throughput capacity of the beet processing can be 60 tons per hour, so that 900 tons of sugar beet can be processed per day.

The beet silage can subsequently be fed to fermenters via redundant pumping stations. The fermentation advantageously has two plant lines, which are each constructed of a high fermenter and an associated secondary fermenter. Over at least one disintegration process step, a treatment in the high voltage field, existing residual gas potential is further exhausted.

The biogas produced is withdrawn through at least one compressor station, passed through at least one biogas cooling for the separation of condensate and conveyed to the biogas upgrading plant, for example via one or more pumping devices and / or suction devices. The biogas treatment prepares the biogas via the separation of the remaining water vapor and the bad gas components (CO ₂ , H ₂ , H ₂ S) and transfers the remaining biomethane with a volume flow of 1070 Nm ³ / h with a methane content of at least 97% CH ₄ to the biogas methane feed station.

fermentation residues

The fermentation residues accumulating in the fermentation process are then treated by the digestate treatment plant in such a way that the resulting wastewater can be discharged into a receiving water. The digestate treatment plant is part of the concept of the modified biogas plant, with individual components of the entire plant being used both for biogas production and for digestate treatment.

The amount of wastewater introduced is, for example, accounted for at 80,300 m ³ / a. Due to operational fluctuations (variation of material properties sugar beets, discontinuity precipitation for washing water retention), the annual amount of wastewater discharged may change slightly.

The digestate disposal must be ensured at all times for reasons of operational safety of industrial production and represents a significant ecological and economic aspect in the operation of the biogas plant.

Exemplary biogas plant "Schuby"

For example, the biogas plant Schuby can produce 220 m ^{3 of} fermentation residues with a dry matter content of between 2 and 3% every day. To work up the digestate, the system is modified, with 3 schematically shows the concept of a digestate treatment plant with the essential components of the entire system. Some components of the overall system serve both the biogas production and the digestate treatment.

11 Table 1 shows the discharge values to be achieved for the wastewater by the digestate treatment.

The values shown represent limit values which must not be exceeded so that the waste water can be discharged into public waters.

The origin of non-degradable substances can be found, for example, in the dry matter of sugar beet. These include so-called lignocelluloses and hemicelluloses. Both are components and structural elements of plant cell walls whose matrix consists of fibrillar, partially amorphous cellulose. Their biological transformation products in wastewater technology are humic acids, which cause a wastewater coloration from slightly yellow to dark brown. This nature means that after the biological purification a persistent, non-degradable fraction as COD (chemical oxygen demand) remains in the treated wastewater. The wastewater always has a certain chemical oxygen demand after treatment. The limit value for the discharge of the wastewater is a COD inlet value of 200 mg / l.

The in the table of 12 The values shown represent the basis of assessment for the further procedure described below. In a regular operation of the biogas plant, a digestate with the stated values remains. The digestate must be treated in such a way that the limit values listed in Table 1 are reached.

Preparation of digestate: Separation of excess dirty water from the digestate

Fermentation as part of beet mono-fermentation produces a nutrient-poor fermentation residue with a low dry matter (TS) content. Despite the high degree of degradation of the fermentation remains a small proportion of biodegradable carbon in the substrate. Further digestion takes place through the disintegration and repeated biodegradation in the post-fermenter 3 , This leads to a further reduction of the organic load and the nutrient load continues to decrease and a mechanical dewatering of the digestate is possible.

The digestate from the post-fermenter 3 consisting of digestate, rain water, wash water and excess sludge from the digestate treatment, with a total amount of, for example. 104,230 m ³ with a maximum TS content of 4% is via a feed pump of the decanter system for separation of the feedstock in liquid phase, the so-called centrate, and supplied to the solid phase. In order to achieve the best possible separation of the fine solid particles, flocculation aids (polymers) are supplied. For solid-liquid separation of the digestate from the dirty water can be in the table of 25 specified amount of flocculant can be used.

The fermentation substrate passes with the advantageously mixed flocculant suspension via an inlet pipe into the likewise rotating feed chamber. There, the suspension is accelerated in the direction of rotation and passed through openings in the likewise rotating drum. In the first section of the drum, the cylindrical part, the solid moves under the influence of centrifugal force to the drum wall. The centrifuged solid is transported by the screw via the cone to the discharge openings of the drum and is transferred to a screw conveyor system, which distributes the solid alternately on two waste containers. With a degree of separation of 98%, in the exemplary embodiment shown, for example, a total of about 10,030 t of solid fermentation residue can be produced, which can subsequently be applied to the areas of origin as fertilizer.

This process is used to remove the suspended and fibrous solids (TS = 1.5 to 2.5%, max 4%) and is carried out by means of a decanter centrifuge with the aid of the flocculation aids.

The table in 13 shows the nature of the digestate, which after flocculation and centrifugation in a first Set fermentation treatment step. For the determination, experiments were carried out.

For the separation of solids, a centrifuge can be used. In the table of 14 information on the design of the centrifuge are reproduced. The table of 15 shows the material balance of the centrifuge.

The centrate produced in the decanter system drains away in the drum via an overflow edge and flows into a subsequent digestate shaft with pumping station. The pumping station conveys this into the digestate tank (digestate storage - see 3 ), which serves as a storage tank for the subsequent aeration tank.

During starting and stopping of the decanter system, the unit is flushed with fresh water. The resulting fresh water is directed via the outlet on the screw conveyor into its own waste water shaft. This takes in addition the resulting contaminated rainwater from the solid loading area on. The polluted water is subsequently the secondary fermenter via a submersible pump 3 fed.

The centrate from the digestate solids separation is fed to the digestate storage. The volume of the fermentation residue storage is, for example, V = 1,800 m ³ . Due to the large volume of the content is homogenized with any concentration fluctuations. The calculated residence time of the dirty water is t = 6 d. It is important to ensure sufficient mixing in order to avoid solid deposits and digestion processes.

Preparation of digestate: Treatment of the liquid digestate in the processing of digestate

From the digestate storage, the liquid digestate is transported to the aeration tank (see 3 ). In the aeration tank with the help of specialized bacteria biocenosis (activated sludge process) the dirty water is largely purified biologically. Here the intermittent nitrification and denitrification take place. In the nitrification phase, oxygen is added via a blower and the biological conversion process begins. In the denitritification phase, nitrate degradation takes place under oxygen deficiency on addition of a carbon source. For the intermittent method, for example, a change interval of 1.5 hours ventilated to 1.5 hours unventilated used. The required ratio C / N for extensive denitrification is, for example, 3.5. Since the need for easily available carbon compared to the provided cargo in the digestate for an adequate supply of bacteria is not sufficient, the supply of a carbon source is required, which is supplied in the form of glycerol (see 26 ). In other biogas plants or fermentation treatment plants, other carbon sources can be supplied.

The process is a low load cleaning with a focus on nitrification and denitrification for extensive nitrogen elimination. This type of purification results from the excess of nutrients (N) compared to available carbon compounds.

The activated sludge process with N / DN takes place in a tank and requires precise control of the aeration and mixing times, ie intermittent aeration. In the aerated phase, the ammonium nitrogen is converted according to the reaction to the nitrite and further to the nitrate nitrogen: NH ₄ ⁺ + O ₂ + 2 HCO ₃ ^- -> NO ₃ ^- + 2 CO ₂ + H ₂ O

In the non-aerated phase, the addition of the external C source (glycerol) and intensive mixing are carried out so that a denitrification can take place according to the following equation: 2NO ₃ ^- + 2 H ⁺ + 2.5C organic -> N ₂ + H ₂ O + 2.5 CO ₂

The addition of glycerine compensates for the carbon deficit required for denitrification in the ratio BSB4 / NH4-N = 4. The dosed amount of glycerine as well as its specification is in the table of 27 shown.

The in the 16 and 17 The tables shown represent exemplary values of biological purification as well as quantities.

Since the fermentation residue does not have a sufficiently high acid capacity, the pH may decrease below 7.0 due to nitrification and denitrification. In this pH range, however, both the nitrification rate and the oxygen utilization rate are severely impaired.

A prerequisite for high nitrification conversion rates is that the acid ions formed are rapidly absorbed by the bicarbonate buffer of the wastewater. For example, 0.14 mmol (1 kg NH4-N - 140 mol) of acid capacity are consumed per mg of degraded NH4-N. During denitrification, the nitrate formed by the nitrifying bacteria is broken down into elementary nitrogen. In contrast to nitrification, denitrification consumes acid ions (H +). Theoretically, denitrification returns 50% of the acid capacity previously consumed in nitrification. 0.03 mmol / l acid capacity per mg of degraded NO3-N won. The actual acid capacity depends on the buffering capacity of the particular wastewater.

In the fermenters, for example, TAC values (carbonate buffer capacity) may be around 2000 mg / l, which corresponds to an acid capacity of about 20 mmol / l. The arithmetic requirement for acid capacity on the basis of such values and, for example, an assumed effluent capacity of 1.5 mmol / l from the 1st activation stage is 37.5 mmol / l.

Since the buffering capacity of the digestate may be insufficient, measures must be taken to increase the buffer and to decrease the pH. This is done via the addition of sodium hydroxide as an alkaline excipient.

Since the digestate, depending on the nature of the feed can also be low in phosphorus and the required ratio of BOD to phosphorus is 100: 1, this must be added to the process stability phosphoric acid.

The table of 18 gives the results of a control of biological purification.

From the first aeration tank, the digestate is conveyed to a first secondary clarifier in concrete construction. The first secondary sedimentation tank is used to separate the activated sludge and the clarified wastewater after the activated sludge process.

In general, the area is decisive for the actual separation of solids, but the height of the basin also plays a role for efficient separation. In the vertical direction, various zones are formed in the process of clarification: Clear water zone with min. H = 0.5 m, separation zone and storage and thickening zone, the extent of which depends, inter alia, on the sludge properties (SVI). In the tables of 19 exemplary details of a dimensioning of the first secondary settling tank for the digestate treatment are shown.

The basin with a diameter of, for example, 6.0 m and a height of 4.0 m has a filling height of 3.5 m and a filling volume of about 100 m ³ . The digestate is fed centrally from above. By gravity, sludge particles settle on the bottom of the container and are sucked off via a slowly rotating round suction reamer with central drive. The extracted sludge is returned to the aeration tank to increase the active bacterial mass for nitrification and denitrification (about 1.8-2.0 m ³ / h) and on the other hand, the excess sludge in the secondary fermenter 1 pumped for biodegradation as part of the fermentation. Floating sludge in the pool is withdrawn and also the Nachgärer 1 fed. The thus purified liquid fermentation residue (about 220 m ³ / d), the so-called clear filtrate, flows into the clear water channel and becomes a small purification plant with integrated final clarification (KBA with secondary clarifier NKB) for further purification 2 ) pumped.

Sewage / digestate treatment: treatment of the remaining wastewater from the digestate treatment

The small-scale revitalization plant can be designed as a compact small-scale plant in container construction with integrated final clarification (KBA with integrated NKB 2 ) be formed. This activation step combines the activated sludge and immersion method. In addition to the activated sludge, a sessile biofilm biocenosis develops on rotating submerged bodies. An advantage of this combination lies in the high degree of specialization of the biofilm for the degradation of hardly degradable substrates.

In the system, all the above-mentioned purification steps and procedures are repeated except for the denitrification. In order to be able to precipitate and separate any excess phosphorus, which can occur depending on the nature of the feedstock, a clearing agent, such as aluminum chloride, is added to the clear filtrate in the feed to the KBA. The resulting sludge is withdrawn and the Nachgärer 1 fed. The remaining waste water is in the clarification zones of the secondary clarification NKB 2 deducted and the secondary clarifier NKB 3 fed.

Due to the expected cleaning performance of the previous stages, this further purification stage serves to further reduce the "heavy" COD mainly by specialized biofilm bacteria. Since the small-scale activation system relatively few substrates (see 5 ), the internal recirculation of the activated sludge is not necessary. The excess sludge produced is deducted at the low point of the post-clarification sections of the KBA and the post-fermenter 1 or post-fermenter 2 fed to the secondary fermentation.

All during the biological purification process of the dirty water after the separation of digestate at the corresponding stages resulting excess sludge can in the Nachgärer 1 or in the post-fermenter 2 and then on to the post-fermenter 3 be directed. The arithmetic quantity of sludge accumulating in secondary sedimentation tanks is shown in the tables of 22 . 23 and 24 shown.

The excess sludge is partially anaerobically degraded in the secondary fermenters and the remaining residues together with the solids from the fermentation process from the post-fermenter 3 fed to the centrifuge for drainage. For the amount of remaining solids after the Drainage, which are disposed of externally, the material balance of the centrifuge, as in the table of 15 specified.

Waste water treatment: Treatment of the remaining waste water from the digestate treatment (further wastewater treatment)

As a further purification stage for the final reduction of the COD value and, if necessary, the correction of the P _ges. -Content, the sewage is the NKB 3 fed, which is designed as a settling tank with integrated Lamellapacket (see 21 ). The purpose of the lamellapacket is to increase the clarification area through inclined plates and to improve the retention of the fine flakes from the precipitation process. An effective precipitant is added to the feed (see 28 ) to correct the values required for the initiation (CSB, Pges.) if necessary.

To reduce the persistent COD content, another precipitant, such as PAC (polyaluminum hydroxide chloride), is added to the wastewater (see 28 ). The resulting sludge is separated in the secondary clarifier by gravity and the Nachgärer 1 deducted.

Alternatively, other chemicals are also suitable for precipitation / flocculation in advanced wastewater treatment for safe compliance with the COD / P inlet concentrations.

After passing through the cleaning or digestate treatment, for example, about 80,300 m ^{3 of} wastewater can be ready for the flood with a TS content of 35 mg / l and a COD content of 200 mg / l. The introduction can take place via a pump shaft into a running through the plant rainwater channel. Due to operational fluctuations (variation of material properties sugar beets, discontinuity of precipitation to wash water retention, increase of water supply decanter system, increase return excess sludge, etc.), the amount of sewage discharged annually may change.

Another treatment step involves the precipitation of magnesium ammonium phosphate (MAP) by means of a so-called AirPrex® process ( 4 ). This can improve the biological phosphate elimination, the digestate after a fermenter in a in 4 shown reactor system out there and is subjected to an air flow. By supplying air is a outgassing of CO ₂ whereby the pH value of the digestate sludge increases significantly.

In addition, magnesium chloride (MGCL ₂ ) is added resulting in the formation and precipitation of magnesium ammonium phosphate (MAP). The resulting MAP crystals can grow in the reactor and settle to the bottom of the reactor system due to sedimentation. Subsequently, the MAP crystals can be fed via a conveyor system to a washing plant for processing the crystals, for example as fertilizer.

Measures to avoid wastewater

During the beet acceptance campaign, contaminated rain water from the beet reception area can be temporarily stored in the sedimentation container and used for beet washing.

Accumulating excess polluted rainwater can enter the post-digester 3 be lowered.

At the end of the campaign, the area is cleaned. Subsequently, the Absenkweg direction rainwater retention basins open and the unpolluted rainwater can be initiated.

Components of the digestate processing plant

In the 7 and 8th components of the digestate treatment plant are presented in detail 7 an exemplary embodiment of a small scale revitalization plant (KBA) in container construction is shown.

In 8th is a block diagram of the postgraver 3 shown. The post-fermenter 3 In the operation of the biogas plant with additional digestate treatment both dirty water, including rainwater, from the settling containers, rinse water from the decanter system and rainwater from the loading area of the decanter and fermentation residues from the Nachgärern supplied. The post-fermenter 3 , which serves as a precursor for digestate treatment, are therefore supplied to all accumulating digestate and liquids.

For the buffering of both excess sludge and beet pulp are storage tanks 1-8 available (see 3 ). The storage tanks are connected via pipes with the secondary clarifier, small aeration plant, secondary fermenter, fermenter / biogas plant (BGA) and the supply of beet pulp and with each other and can be switched as required via slide valves, pumps and valves so that either excess sludge or Rübenmus stored or is outsourced. A mixing of the two substances in the storage tanks does not take place.

The structure shown in the figures of the entire plant for digestate treatment and biogas production is characterized by the fact that ready-to-flood wastewater is provided as the end product. The provision of the water is carried out by the above-described purification steps, wherein the dosage and selection of the added agent is given by the embodiment of sugar beet in the tables. In the case of substrates other than starting materials for digestate treatment, the amount of added substances may also differ from the examples.

Claims (11)

Process for the treatment of fermentation residues, which occur during the fermentation for biogas production, comprising at least the following process steps: Feeding the resulting digestate from an upstream fermentation device for biogas production of a further post-fermentation device, - post-fermentation of the accruing fermentation residues in the further Nachgäreinrichtung, Feeding the fermentation residues from the further subsequent fermentation device of a separating device, Separating the solids from the centrate of the digestate (liquid constituents) in the separating device, Feeding the centrate to a digestate store, Feeding the centrate to a first aeration tank, biological purification of the centrate by an activated sludge process by means of bacteria, whereby nitrification and denitrification take place with an intermittent aeration, and wherein in the aerated phase oxygen and in the non-aerated phase an external carbon source are supplied, Feeding the activated sludge to a first secondary settling tank in which the activated sludge is separated by separation, Removal of the clear filtrate, Feeding the clear filtrate of a small-scale revitalization system with an integrated second secondary settling tank, whereby biological purification and thorough mixing of the clear filtrate with supply of oxygen takes place by means of a further activated-sludge process by means of bacteria, - Supplying the clarified wastewater to a third secondary sedimentation tank, in which at least one precipitant and / or reactant are supplied to the treated wastewater. Method according to Claim 1 , wherein the clear filtrate before and / or during the introduction into the small-scale aeration plant is supplied. Method according to Claim 1 or 2 , wherein the excess sludge resulting from the biological purification in the first secondary clarifier, in the second secondary clarifier and in the third secondary clarifier is supplied to the upstream fermenter. Method according to one of Claims 1 to 3 in which a coagulant is added to the digestate before separation of the solids phase from the centrate. Method according to one of Claims 1 to 4 in which, in the separation of the solid phase from the centrate dilution water is supplied. Method according to one of Claims 1 to 5 wherein the aerated phase and the non-aerated phase are each maintained for 1 to 2 hours, preferably 1.5 hours. Method according to one of Claims 1 to 6 in which aluminum chloride (AlCl ₃ ) or polyaluminum hydroxide chloride (PAC) and / or polymers are added as precipitating agents. Method according to one of Claims 1 to 7 , wherein glycerol is supplied as the carbon source. Plant for the treatment of fermentation residues, wherein the plant is designed and constructed, a method according to one of Claims 1 to 8th to carry out, at least - a further reclamation device, - a separating device for separating the solid phase from the centrate of the fermentation residue, - a digestate storage, - a first activated sludge tank for biological purification of the centrate, - a first secondary clarifier for separating the activated sludge, - a small resuscitation plant with integrated second secondary sedimentation tank for further biological purification, and - a third final sedimentation tank for sedimentation of the activated sludge. System for processing fermentation residues, comprising components for carrying out the method steps of Claims 1 to 8th , Using a system after Claim 9 or a system Claim 10 for the treatment of digestate and / or wastewater treatment.

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