nutrient recovery by processing anaerobic digestate · products through digestate processing. with...
TRANSCRIPT
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Nutrient recovery by processing anaerobic digestate
Bernhard Drosg Werner Fuchs
Teodorita Al Seadi Bernd Linke
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IEA BIOENERGY Task 37 – Energy from Biogas
IEA Bioenergy aims to accelerate the use of environmentally sustainable and cost competitive bioenergy that
will contribute to future low-carbon energy demands. This report is the result of the collaboration between IEA
Bioenergy Task 37: Energy from Biogas and IEA Bioenergy Task 36: Integrating energy recovery into solid
waste management systems.
The following countries are members of Task 37, in the 2012-2015 Work Programme:
Austria
Brazil
Denmark
European Commission (Task Leader)
Finland
France
Germany
Ireland
Netherlands
Norway
Sweden
Switzerland
South Korea
UK
Bernhard DROSG [email protected]
Günther BOCHMANN [email protected]
Cícero JAYME BLEY [email protected]
José Geraldo de MELO FURTADO [email protected]
Jeferson Toyama [email protected]
Teodorita AL SEADI [email protected]
David BAXTER [email protected]
Jukka RINTALA [email protected]
Outi PAKARINEN [email protected]
Olivier THÉOBALD [email protected]
Guillaume BASTIDE [email protected]
Bernd LINKE [email protected]
Jerry MURPHY [email protected]
Mathieu DUMONT [email protected]
Roald SØRHEIM [email protected],
Tobias PERSSON [email protected]
Nathalie BACHMANN [email protected]
Ho KANG [email protected]
Clare LUKEHURST [email protected]
Charles BANKS [email protected]
Written by
Bernhard Drosg
IFA-Tulln
Konrad Lorenzstrasse 20, A-3430 Tulln
Austria
Werner Fuchs
IFA-Tulln
Konrad Lorenzstrasse 20, A-3430 Tulln
Austria
Teodorita Al Seadi
BIOSANTECH
Lerhøjs Allé 14, DK-6715, Esbjerg
Denmark
Bernd Linke
Leibniz-Institut für Agrartechnik Potsdam-Bornim
Max-Eyth-Allee 100, 14469 Potsdam
Germany
Date of publication:
Edited by
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This first draft mainly contains the raw input from Bernhard and Werner Fuchs,
concerning the digestate processing technologies. The present draft was not
yet reviewed by any of the other co-authors. The contributions to different
parts of the report, submitted by some of the other co-authors, are likewise not
yet included in the present first draft. A more elaborated second draft is
expected to be ready by June 2014.
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Table of contents
1.1 What is digestate processing? ...................................................................................... 5
1.2 When does digestate processing make sense? ............................................................. 5
1.3 Overview on digestate processing technologies .......................................................... 5
2 Characteristics of digestate ................................................................................................. 9
3 Solid-liquid separation – the first step in digestate processing ........................................ 11
3.1 Screw press ................................................................................................................ 12
3.2 Decanter centrifuge .................................................................................................... 14
3.3 Belt filters .................................................................................................................. 16
3.4 Discontinuous centrifuge ........................................................................................... 17
3.5 Enhanced solids removal ........................................................................................... 18
3.5.1 Precipitation/Flocculation .................................................................................. 18
3.5.2 Flotation ............................................................................................................. 19
3.5.3 Screens and filters .............................................................................................. 19
4 Fibre/Solids processing .................................................................................................... 21
4.1 Composting ................................................................................................................ 21
4.2 Drying ........................................................................................................................ 21
5 Liquor processing ............................................................................................................. 23
5.1 Nitrogen recovery ...................................................................................................... 23
5.1.1 Ammonia stripping ............................................................................................. 23
5.1.2 Ion exchange ...................................................................................................... 25
5.1.3 MAP-Precipitation ............................................................................................. 25
5.2 Nitrogen removal ....................................................................................................... 27
5.2.1 Nitrogen removal by biological oxidation ......................................................... 27
5.3 Nutrient concentration and water purification ........................................................... 28
5.3.1 Membrane technologies ..................................................................................... 28
5.3.2 Evaporation ........................................................................................................ 30
6 Marketing possibilities – a main limitation for nutrient recovery .................................... 33
6.1 Legal limitations ........................................................................................................ 33
6.2 Market limitations and incentives .............................................................................. 33
6.3 Conditioning/Standardising ....................................................................................... 33
6.4 Economics of digestate processings .......................................................................... 34
7 Conclusions and future trends .......................................................................................... 36
8 References ........................................................................................................................ 37
9 Glossary of terms ............................................................................................................. 39
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1.1 What is digestate processing?
Digestate processing involves the application of different technologies to the digestate – the effluent
from anaerobic digesters. The technologies applied in digestate processing are mostly comparable to
existing technologies from manure processing, sewage sludge treatment or wastewater treatment.
Digestate processing can be approached in two ways. The first one is digestate conditioning (or
enhancement), which aims at producing standardized biofertilisers (solid or liquid) where the quality
and marketability of the digestate is improved. The second one can be described as digestate treatment,
similar to wastewater treatment, applied in order to remove nutrients and organic matter from the
effluent and to allow discharge to the sewage system, to the wastewater treatment plant on site or to a
receiving stream. In most cases it will be necessary to fulfil both approaches (digestate conditioning
and digestate treatment) in order to establish a viable digestate processing concept.
In order to distinguish the different fractions in digestate processing the following terminology will be
used in this publication: „whole digestate‟ or „digestate‟ refers to the untreated residue obtained from a
biogas plant, „fibre/solids‟ refers to the solid fraction after solid–liquid separation and „liquor‟ refers to
the liquid fraction.
1.2 When does digestate processing make sense?
In general, after its removal from the digester, the digestate can be applied as fertiliser without any
further treatment on agricultural area. This is the standard approach in most of the existing biogas
plants. However, storage, transport, handling and application of digestate as fertilizer results in
significant costs for farmers, compared with its fertilizer value; this is due to the large volume and low
dry matter content. The costs increase further because of the investments in slurry storage capacities,
required by national environmental regulations in countries like Denmark, Germany and France,
where the period of fertilizer application is limited to the growing season and the amount of nutrients
applied per unit of agricultural land is restricted by pollution control regulations. Also at EU level, the
European Nitrate Directive 91/676/CEE limits the annual nitrogen load which can be applied to
agricultural land. As digestate has a high content of easily plant available nitrogen this influences the
amount of digestate that can be applied. Such strict legislative frameworks, which seek to protect the
environment, may necessitate transport and redistribution of nutrients away from the intensive areas.
These conditions make digestate processing attractive.
1.3 Overview on digestate processing technologies
Digestate processing can be partial, targeting usually volume reduction, or it can be complete
processing, refining digestate to pure water, fibres / solids and concentrates of mineral nutrients. The
first step in digestate processing is to separate the solid phase from the liquid. The solid fraction can
subsequently be directly applied as fertiliser in agriculture or it can be composted or dried for
intermediate storage and enhanced transportability. For improving solid-liquid separation,
flocculation- or precipitation agents are commonly applied.
While partial processing uses relatively simple and cheap technologies, for complete processing
different methods and technologies are currently available, with various degrees of technical maturity,
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requiring high energy consumption and high costs. For nutrient recovery, membrane technology, such
as nano- and ultra- filtration followed by reverse osmosis, are used (Fakhru'l-Razi 1994, Diltz et al.
2007). Membrane filtration produces a nutrient concentrate and purified process water (Castelblanque
and Salimbeni 1999, Klink et al. 2007). The liquid digestate can also be purified through aerobic
biological wastewater treatment (Camarero et al. 1996). However, because of the high nitrogen
content and low biological oxygen demand (BOD) an addition of an external carbon source may be
necessary to achieve appropriate denitrification. A further possibility for concentrating digestate is
evaporation with waste heat from the biogas plant. For reducing the nitrogen content in the digestate,
stripping (Siegrist et al. 2005), ion exchange (Sánchez et al. 1995) or struvite precipitation (Uludag-
Demirer et al. 2005, Marti et al. 2008) have been proposed. Whatever process is applied, advanced
digestate processing in most cases requires high chemical- and energy inputs. Together with increased
investment costs for appropriate machinery, considerable treatment costs may result. An overview of
viable digestate processing technologies is given in Figure 1.
Figure 1 Overview of viable options for digestate processing.
Overview of applied processes at industrial scale
A very broad range of technologies are currently being applied for digestate processing, depending on
the boundary conditions. Up to now, no key technology has evolved. The most abundant approach is
solid-liquid separation of digestate, where depending on the consistency of the digestate screw presses
or centrifuges are applied. Solid-liquid separation can be improved by the addition of precipitating
agents. A solid-liquid separation step can also be the first step of particle removal before subsequent
treatment of the liquid fraction is carried out. The distribution of existing large-scale digestate
processing facilities in Germany, Switzerland and Austria is shown in Figure 2.
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Figure 2: Overview of the distribution of industrial-scale applications for further treatment of
the liquid fraction of digestate in Germany, Austria and Switzerland, Status from 2009 (based
on Fuchs and Drosg, 2010)
Among the technologies for further treatment of the liquid effluent, membrane purification is the only
process which can achieve a degree of purification of the digestate so it can be directly discharged into
receiving waters. However, it is the most expensive and in large-scale applications still a high
potential for optimization could be detected. If waste heat is available digestate evaporation is an
interesting option. Currently in Germany digestate processing technologies using heat (e.g.
evaporation, drying) are being used more frequently due to the subsidies for waste heat utilization at
biogas CHPs. Digestate evaporation is a rather robust technology, however, if digestate contains
considerable amounts of fibrous material it is necessary to remove these beforehand to avoid clogging
in the heat exchangers.
Digestate is sometimes also treated in aerobic wastewater processes (“biological treatment”). This is,
however, often expensive and problematic due to the low amounts of residual COD compared to the
high nitrogen concentrations. Therefore, for total nitrification/denitrification the addition of external
carbon source is necessary which is expensive and leads to high amounts of aerobic sludge
accumulated in the process. An alternative is the co-treatment with other wastewaters rich in COD;
this possibility is, however, often very limited. In addition, recalcitrant COD and colour of aerobically
treated digestate will often demand a subsequent treatment (e.g. membrane treatment), before direct
discharge qualities can be reached. Other technologies which are rather applied in solitary approaches
are NH3-stripping, ion exchange, solar drying of digestate, etc.
Residue management in digestate processing
Another very important issue, especially in large-scale applications, is the accumulation of by-
products through digestate processing. With the example of a membrane treatment process (see Figure
3), it is shown, that only approximately 50% of the treated digestate will become purified water. The
rest will accumulate as by-products/residues in the process. For these fractions economically viable
utilisation concepts have to be established. If additional treatment costs occur, this will affect
economics of digestate processing decisively. However, these fractions normally contain higher
Comment [B1]: Should this better be placed at the membrane processes?
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nutrient concentrations (e.g. concentrate from reverse osmosis), so their market value should be
higher. Nevertheless, further treatment can be necessary for commercialisation.
Figure 3: Side streams and residues in membrane purification of digestate (Fuchs and Drosg,
2010)
Solids: 15 %
Concentrate in microfiltration: 20 %
(recirculation to the biogas plant)
Concentrate in reverse osmosis : 15 %
(utilisation)
Permeate: 50 %
Digestate: 100 %
Solids: 15 %
Concentrate in microfiltration: 20 %
(recirculation to the biogas plant)
Concentrate in reverse osmosis : 15 %
(utilisation)
Permeate: 50 %
Digestate: 100 %
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2 Characteristics of digestate The composition of whole digestate is mainly influenced by the input materials. In Europe,
predominant feedstocks are renewable raw materials (e.g. maize whole crop silage), biogenic waste
(food waste, municipal organic waste, etc.) and agricultural/livestock by-products (manure). Other
feedstocks are by-products from food industry (animal by-products from slaughter houses, brewers‟
spent grains, etc.) and residues from bioethanol or biodiesel production. Characteristic differences of
the whole digestate deriving from the fermentation of renewable raw materials in comparison to
biogas plants treating organic waste or industrial by-products were identified and are shown in Figure
4 with regard to nitrogen concentration. It can be seen that nitrogen concentrations in energy-crop
digestion plants are quite similar, whereas in biogas plants which treat organic wastes the nitrogen
concentration varies strongly. The reason for this is mostly the different nitrogen concentration in the
corresponding feedstock. In addition, the process design, e.g. the amount of fresh water and
recirculation effluent in use, can influence the total nitrogen concentrations. In the monofermentation
of industrial by-products the influence of nitrogen concentration in the feedstock can be seen clearly.
Figure 4 Examples of total nitrogen concentration (TN) in the digestate of biogas plants with different
feedstock types (in kg per ton fresh matter (FM)). Horizontally striped columns
indicate digestate from typical agricultural plants, diagonally striped columns indicate digestate from
monodigestion of industrial by-products, and unstriped columns indicate digestate from typical waste
treatment plants.
The used feedstocks have a high influence on the digestate composition. This influence factor is
summarised in Table 1. The fact that the feedstock has an influence on digestate composition is quite
obvious, it is clear for example that a digestate from energy crop digestion will be different from a
digestate from whey. As a consequence the ideal digestate processing technology will also be
different. However, the process conditions can also have an influence on the digestate composition
which is demonstrated in Table 2.
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Table 1: Feedstock parameters influencing digestate composition
Feedstock Parameters Impact on Digestate Composition
Energy crops high Total Solids (TS) content
high percentage of organics in TS (VS/TS)
Organic wastes low Total Solids (TS) content
low percentage of organics in TS
High amount of manure
very low TS
high nitrogen concentration
high percentage of NH4+ in Total Nitrogen
High amount of slaughterhouse
waste
high nitrogen concentration
high percentage of NH4+ in Total Nitrogen
Table 2: Process parameters influencing digestate composition
Process Parameters Impact on Digestate Composition
High amount of
fresh water
high amount of digestate accumulated
low salt/ammonia concentration
low dry matter (TS) content
High amount of
recirculation liquid
low amount of digestate accumulated
high salt/ NH4+ concentrations
high Total Solids (TS) content
Short retention time
high VFA (volatile fatty acids) concentration
high percentage of organics in TS
low percentage of NH4+ in Total Nitrogen
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3 Solid-liquid separation – the first step in digestate processing Most frequently solid-liquid separation is the first step in digestate processing. Only in very few cases
the whole digestate is processed without a prior solid–liquid separation step (e.g. drying of whole
digestate). The principle of solid-liquid separation is shown in Figure 5.
Figure 5 Solid-liquid separation step in digestate processing (Fibre/solids; Liquor)
In order to establish the best solid-liquid separation process, the focus has to lie on finding the right
technology (or technology combination) for an efficient but cost-effective solids separation step.
Especially for consecutive membrane treatment, but also for evaporation, the right separation degree
of the solids/fibres from the digestate is essential (for enhanced solids removal see 3.5).
Figure 6 Distribution of the principal constituents after solid–liquid separation (data based on
own investigations and various references; adapted after Bauer et al. (2009)). (DM: dry
matter; oDM: organic dry matter; TN: total nitrogen.)
Typical ranges for the distribution of the main constituents between the fibre/solids and the liquor are
provided in Figure 6. The separated fibre/solids can be applied directly for agricultural purposes, with
the advantage of considerably lower transport costs due to the reduced water content. Another
advantage is that the fibre/solids can be stored under much simpler conditions. As an alternative to
Processing of the
liquid phase
Processing of the
liquid phase
Liquid
fraction
Solid fraction
Digestate
Land applicationLand application
Solids stabilisation(Composting, Drying)
Solids stabilisation(Composting, Drying)
Recirculationeffluent
Recirculationeffluent
Solidsseparation
Processing of the
liquid phase
Processing of the
liquid phase
Liquid
fraction
Solid fraction
Digestate
Land applicationLand application
Solids stabilisation(Composting, Drying)
Solids stabilisation(Composting, Drying)
Recirculationeffluent
Recirculationeffluent
Solidsseparation
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direct land application further stabilisation and transformation into a marketable product can be
achieved through drying or composting. Typically the obtained end-products are used as a solid
fertiliser. Another application, the production of pellets for heating purposes, is currently the subject of
investigations. However, with regard to the high N content and the associated increased NOX
emissions the suitability of the pellets for thermal recovery is not sufficiently clarified.
The major fraction deriving from the separation step is the liquor. Depending on the characteristics of
the whole digestate and the efficiency of solids removal, its composition is subject to a large variation.
Frequently, part of the liquor is recycled to adjust the dry matter concentration of the input feedstock
(Resch et al., 2008). For the remaining liquor, there are a variety of recovery and treatment options. In
the simplest case, it is spread on agricultural land. Here the advantage of solid–liquid separation can be
an improved storage and residues management logistics. Nevertheless, in most cases further treatment
with the aim of volume reduction and recovery of nutrients is applied. In most cases, these objectives
will be achieved only through a sequence of several steps. As a general rule, the necessary procedures
are relatively complex and therefore expensive.
3.1 Screw press
Figure 7 Screw press separator
Screw press separators (see Figure 7) are often used if the digestate contains high fibre content. In
Figure 8 the detailed set-up of a screw press separator is shown. A screw presses the fibres against a
cylindrical screen. The liquid fraction leaves the separator through the sieve. Because of the increasing
diameter of the screw the pressure is increased when the fibres advance in the separator. Finally, the
solid fibre fraction exits at the end of the separator, where the resistance can be adjusted by
mechanically. The degree of the solids separation can be influenced by the mesh size of the screen,
smaller particles (diameter of 0.5-1 mm) will always remain in the liquid (Weiland, 2008).
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Figure 8 Detailed set-up of a screw press separator
Unlike decanter centrifuges, screw press separators cannot separate sludge fractions from the
digestate. If the digestate contains mainly fibre fractions the amount of solid fraction which will
accumulate is dependent on the dry matter content of the digestate. Bauer et al. (2009) found a
correlation between dry matter content in digestate and the amount of solid fraction accumulated
(Figure 9).
Figure 9 Relation between dry matter content of the digestate and the amount of solid fraction
accumulated (Bauer et al., 2009)
The separation efficiency of different components in the digestate was investigated by
KTBL (2008). In Table 3, an overview of the observed separation efficiency is given. As mentioned above the
separation efficiency will always depend on the dry matter and fibre content in the digestate. The
advantages of a screw press separator compared to the decanter centrifuge are the low investment costs
(approx. 20,000 € for a 500 kWel plant, Bauer et al, 2009) and low energy consumption (0.4-0.5 kWh
/ m³, Fuchs and Drosg, 2010).
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Table 3 Typical separation performance of screw press separators (KTBL, 2008)
Percentage of
fresh matter
[%]
Degree of separation [%]
TS VS COD NH4-
N
TN PO4-P K
Screw press separator
Solid fraction 10.0 48.1 56.3 48.8 9.2 17.0 21.8 10.0
Liquid fraction 90.0 51.9 52.4 51.2 82.0 83.0 78.0 90.0
3.2 Decanter centrifuge
Figure 10 Decanter centrifuge
Decanter centrifuges (see Figure 10) are frequently applied in digestate processing. They are in use to
separate also small particles and colloids from the digestate. In addition, they can be used to separate
the majority of the phosphorus contained in digestate with the fibre/solids fraction (Møller H B, 2001).
There are several commercial brands of decanter centrifuges utilized today for digestate separation,
with similar performances. In Figure 11 the detailed set-up of a decanter centrifuge is shown. The
digestate enters the centrifuge via a central inlet and is applied in the centre of the centrifuge.
Depending on particle size, difference in density and viscosity the particles can be separated by the
centrifugal force. The separated particles accumulate on the walls of the cylinder and are transported
and further compressed by a screw. On the final outlet (right-hand side, see Figure 10), the solids leave
the decanter. On the other side, the clarified liquid leaves the decanter. Energy consumption is rather
high (3-5 kWh/m³, Fuchs and Drosg, 2010), compared with other solid-liquid separation technologies.
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Figure 11 Detailed set-up of a decanter centrifuge
In Table 4 and Table 5, technology test results of the GEA Westfalia decanter centrifuge are shown
(DANETV, 2010). The test was made on five batches of minimum four hours each, with a fixed start
and end time for each batch. For each batch the weight or volume of input digested biomass, liquid
output fraction and solid output fraction was measured and concentrations of solids and nutrients were
determined by analysing representative samples of the inlet and the two outlet flows. During the 5
batches the decanter centrifuge treated 283 m3 of digestate, corresponding to an average capacity of
13.72 m3 biomass treated per hour.
Table 4 Digestate separation by decanter centrifuge - average content of total solids, ashes,
volatile solids, suspended solids and pH over 5 batches. Adapted after DANETV (2010) Fraction Total solids
%
Ash content
%
Volatile solids*
%
Suspended solids
mg/l
pH
ppm
Input digestate
4,85 1,46 3,39 35.000 7,64
Liquid
output fraction
2,31 0,82 1,49 8.400 7,94
Solid
output fraction
27,66 6,46 21,20 Not relevant 8,12
* Values for volatile solids are not measured but calculated as the difference between total solids and ash
content.
Table 5 Digestate separation by decanter centrifuge - average concentrations of nutrients
over 5 batches. Adapted after DANETV (2010) Fraction Total
Nitrogen
kg/t
Ammonium
Nitrogen
kg/t
Organic
Nitrogen*
kg/t
Total
Phosphorous
kg/t
Total
Sulphur
kg/t
Input digestate
4,08 2,87 1,21 0,94 0,42
Liquid output
fraction
3,49 2,63 0,86 0,31 0,29
Solid output
fraction
8,15 4,50 3,65 6,52 1,56
* Values for organic nitrogen are not measured but calculated as the difference between total-N and ammonium-
N.
Feststoff
Gärrest
Zentrifugat
Verteilkopf
Transportschnecke
Einlaufrohr
Trommel
Überlaufwehr
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A specific example of the effect of digestate separation by decanter centrifuge is given in Table 6.
Table 6 Separation of digestate by decanter centrifuge. Adapted after Jørgensen P J (2009)
3.3 Belt filters
Belt filters can be used for digestate processing. There exist two types: belt filter presses and vacuum
belt filters. A belt filter press can be seen in Figure 12. It consists of a closed loop of texture which is
wound around cylinders. Digestate is applied continuously at the start of the belt filter. The first pre-
dewatering happens by gravitation. As next step the material is pressed between two filter belts.
Subsequently varying mechanical forces are applied so that the filter cake is dewatered further. Finally
the dewatered cake is removed from the filter belt by a mechanical device. The filter belt is then
cleaned by spray-washing (where often the filtrate is used) and is then used again for filtration. The
second option is a vacuum belt filter, as illustrated in Figure 13. In vacuum belt filters the digestate is
applied on a filter below which a vacuum is applied. By the low pressure water is sucked through the
filter and the filter cake remains on the belt. Flocculating and precipitating agents are mixed into the
digestate prior to application on the filter.
Figure 12 Scheme of a belt filter press
Produktaufgabe
Abspritzeinrichtung
Filtratabzug
Filtratabzug
Vorentwässerung
Presszone
keilförmige Verdichtungszone
entwässerteFeststoffe
17
Figure 13 Vacuum belt filter
In belt filters the addition of precipitating and flocculating agents (see section 3.5.1) is indispensible in
order to improve solids separation. Factors which influence separation efficiency are: characteristics of
digestate, amount and type of precipitating and flocculating agents and mesh size of the filter. The
advantages of the belt filter are a higher solids separation efficiency compared to the screw press and a
lower energy demand (1.5 - 2 kWh/m³) than a decanter centrifuge. A drawback is, however, the high
amount of precipitating / flocculating agents which is two to three times higher than in a decanter
centrifuge.
3.4 Discontinuous centrifuge
Apart from decanter centrifuges, also discontinuous centrifuges (see Figure 14) can be used for
digestate processing. These centrifuges are operated batch wise, which means that in subsequent
cycles a certain amount of digestate is centrifuged. In these cycles whole digestate is fed to the
centrifuge continuously and also the supernatant leaves the centrifuge continuously. The separated
solids remain in the centrifuge and are removed at the end of the cycle. Then a new cycle can be
started.
Figure 14 Scheme of a discontinuous centrifuge
Energy demand and efficiency are comparable with decanter centrifuges; however, a slightly higher
total solids concentration of the solids can be achieved. Although a discontinuous centrifuge can be
operated fully automated, it can show higher risks of process failure due to batch wise operation. In
M M
M
Feststoffe
Absaugeinrichtung
Flüssigphase
Mischbehälter
Gülletank
Dosierstationen für Fäll- und Flockungshilfsmittel
Zulauf
bewegliches SchälmesserZentrifugat
abzentrifugierteFeststoffe
18
practice, discontinuous centrifugation of digestate is not wide-spread, so little experiences are
available.
3.5 Enhanced solids removal
The following processes are not the main separation processes, as described above, where the majority
of the solids are removed. They either enhance the main separation processes (precipitation /
flocculation) or polish the liquor by a subsequent solids removal step. The importance of enhanced
solids removal depends on the overall digestate processing concept. Enhanced solids removal is
indispensable, if the liquor is treated, for example, in a membrane process. Another issue is if e.g. high
phosphorous removal is demanded. Although, in general, phosphorous is concentrated in the
solids/fibre fraction in any solid-liquid separation process (see Figure 6), the separation efficiency can
be increased drastically (> 90% total separation) by adding precipitating / flocculating agents.
3.5.1 Precipitation/Flocculation
Precipitating agents and flocculants can be added to digestate in order to increase separation efficiency
of e.g. suspended solids or phosphorous in practically any solid-liquid separation process. As can be
seen in Figure 15 small suspended particles in digestate are often negatively charged and therefore
remain in solution. Here is where precipitating agents and flocculants come into play. Positively
charged ions aggregate around the particles and thereby produce larger particles (coagulation). Finally
by flocculation much larger particles are formed which can be separated more easily. Organic
polymers (e.g. acrylamide) may be added to increase the linkage of the flocks and therefore
flocculation performance. Precipitating agents which are most commonly applied are aluminium
sulphate (Al2 (SO4)3), ferric chloride (FeCl3), ferric sulphate (Fe2(SO4)3)) and lime (Ca(OH)2). The
dosage of the precipitating agents or flocculants can either be done separately in mixed tanks prior to
solid-liquid separation or in-line which means that they are injected directly into the pipes where there
are static-mixing systems integrated in order to achieve sufficient turbulence.
Figure 15 Simplified illustration of the different phases in flocculation I: suspended colloids,
II: destabilization of colloids by flocculation agents, III: linkage and increase of flocks by
flocculation agents
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3.5.2 Flotation
Flotation is an efficient process, which is, however, rarely applied in digestate processing. The
principle of flotation is that the lifting force of suspended particles is increased by the attachment of
small gas bubbles to them. Consequently, they are lifted to the surface where they produce a floating
layer which can be harvested. In general, flotation plants need 30-50% less space than standard
sedimentation plants as the lifting force is generally much higher than the sedimentation force. Two
different flotation processes exist: flotation by decompression or by gassing. In the first process
pressurized water which is air saturated is injected (see Figure 16), while in the second process directly
air is injected, where special nozzles are applied to produce small gas bubbles. The first process
produces smaller bubbles and is more commonly applied in wastewater treatment. For any efficient
flotation process the addition of flotating agents is necessary, which are comparable to precipitating /
flocculating agents (see section 3.5.1). Apart from increasing flock size and volume, also the ability of
the gas bubbles to attach to the flocks is enhanced.
Figure 16 Scheme of flotation
3.5.3 Screens and filters
Vibrating screens (see Figure 17) and vibrating curved screens (see Figure 18) are commonly applied
in digestate processing. The liquor is applied on the screen and the screenings remain on the screen
(and are constantly removed), whereas the liquid passes through it. In order to prevent rapid clogging
of the screens, they are operated under vibration. Typical screen sizes are 150-250µm for vibrating
screens and 100-300µm for vibrating curved sieves. Too small screen sizes can lead to rapid clogging
and in addition the amount of screenings will increase. Apart from screens also security filters are
found in digestate processing, they have the function of retaining larger particles e.g. before a
membrane system which have accidentally passed previous solid-liquid separation steps. As they have
a different function than the described screens, the retained material is not constantly removed.
Ablauf
Druck-luft
Gärrest
DrosselDrossel
Druckbegasung
20
Figure 17 Vibrating screen
Figure 18 Vibrating curved screen
Entnahme gesiebter Gärrest
Unwuchtantrieb
Siebfläche
Feder-lagerung
Bodenplatte
Grobstoff (Überkorn)-entnahme
21
4 Fibre/Solids processing
The fibre/solids fraction which accumulates in solid-liquid separation shows TS concentrations around
20-30%. This fraction is partially stabilized so that appropriate storage and direct application as
fertilizer or soil improver on agricultural areas is possible. Nevertheless, this fraction still contains
microbially available material, so that microbial activity can happen and odor emissions occur. If it is
desired to obtain a stable and marketable product, further processing is necessary, which can be either
composting or drying.
4.1 Composting
In the composting process microbes degrade and transform the organic material under aerobic
conditions to compost, which is stabilised organic matter, containing humic substances. Compost is an
ideal fertilizer as it slowly releases nutrients and shows a good performance as soil improver.
However, as the fibre/solids fraction from digestate is wet and already partially degraded the addition
of bulking material (such as woodchips) is necessary for a stable composting process. The bulking
material helps that air can enter the compost heap and aerobic conditions are maintained. Depending
on the local availability of bulking material it may be advantageous to do the composting at a
centralized composting facility. A special application of composting is vermiculture using
earthworms. In general, composting of the solid fraction increases the concentration of nutrients in the
solid fraction, but also may result in nitrogen loss.
Figure 19 Composting facilities in an open (left) or closed (right) environment
4.2 Drying
Processes for drying of digestate aim at stabilising the digestate as well as reducing the total mass of
the digestate and by that increasing nutrient concentration. If electrical power is produced at the biogas
plant in e.g. a CHP unit, the waste heat can be utilised for drying of the digestate. Apart from drying
the fibre/solids fraction it is also possible to dry the whole digestate (without prior solid-liquid
separation). However, as waste heat is not sufficient to dry all the digestate, drying of the fibre/solids
fraction is more frequently applied.
22
Figure 20: Principles of drying processes, drying by convection (left) and drying by contact
(right)
The principles of the drying process are illustrated in Figure 20. As drying technologies the following
can be applied: belt dryer, drum dryer, feed and turn dryer and fluidised bed dryer. For digestate
applications the belt dryer (see Figure 21) is more commonly applied. As an alternative also solar
drying systems are applied for digestate (see Figure 22), these systems can be supported by waste heat
from a CHP unit. As the exhaust air of the digestate dryers contains dust, ammonia and other volatile
substances (e.g. volatile acids) exhaust gas cleaning systems have to be applied in order to reduce
emissions. Such systems contain a dust filter as well as washer units.
Figure 21: Scheme of a belt dryer
Figure 22: Solar drying of digestate
The dried digestate can be marketed as it is or is pelletised for better marketability. Such products are
already available as biofertilisers on horticulture or gardening markets e.g. in Germany. The material
can be used also in nurseries or for special cultivation systems, such as mushroom production.
Heißluft
Heißluft
Heißluft mit Wasserdampf
Wasserdampf
Luftstrom
Wärme (über Heizmedium)
Heißluft mit Wasserdampf
Wasserdampf
zur Abluftreinigung
Trocknungsluft
Produktaufgabe
getrocknete Feststoffe
Zusatz-Bodenheizung (optional)
Wendesystem
Lufterwämung
Belüftungsklappen
23
5 Liquor processing
The liquid fraction (liquor) after solid liquid separation still contains considerable amounts of
suspended solids and nutrients. The exact concentrations depend on the feedstock, as well as the
separation technology and any applied enhanced nutrient removal. It is never possible to achieve a
liquor which can meets sufficient environmental standards so that it can be directly discharged to
receiving streams. Part of the liquor can be used for the mashing of the feedstock. This amount
depends on the one hand on the water content of the feedstock, and on the other hand on the
concentration effect of ammonia nitrogen and salts in the process. In any case, at least a partial
reutilisation as process water is recommended as this reduces the treatment effort for the liquor. If
other facilities are also near the biogas plant the liquor can also be used to moisturise compost heaps or
bio filters. In these cases the reduction of the ammonia concentration will probably be necessary in
order to reduce ammonia emissions.
Further liquor processing approaches can have different driving forces: First, the recovery of nitrogen
from the liquor, in order to produce a nitrogen-rich product. Then, the removal of nitrogen by
biological processes in order to be able to meet limits for discharge of the liquor or processes where a
nutrient rich (N, K) concentrate is produced as well as widely purified water.
5.1 Nitrogen recovery
5.1.1 Ammonia stripping
Gas stripping is a process where volatile substances are removed from a liquid by a gas flow through
the liquid. In digestate processing it is aimed at removing/recovering ammonia from the liquid. The
volatility of ammonia in an aqueous solution can be increased by increased temperature and increased
pH (as indicated in Figure 23). So in digestate processing waste heat can be used for heating up the
digestate and the pH can be increased by degassing of CO2 or addition of base.
Figure 23: Dependence of the volatility of ammonia in water on temperature and pH
0%
20%
40%
60%
80%
100%
4,0 5,0 6,0 7,0 8,0 9,0 10,0 11,0 12,0
pH [-]
Am
mo
nia
kan
teil
[%]
120°C
100°C
40°C
60°C
20°C
80°C
24
Figure 24: Ammonia air stripping including CO2 removal and ammonia recovery by
sulphuric acid scrubbers
For ammonia stripping in digestate mainly two processes are applied: air stripping and vapour
stripping. In air stripping (see Figure 24) the heated digestate enters a stripping column, as a pre-
treatment CO2 is removed, which lowers the buffer capacity. Then in a stripping column, which is full
of packing material to increase the surface for the ammonia mass transfer, ammonia is transferred
from the liquid digestate to the stripping air. In a subsequent step the ammonia is recovered from the
air by a sulphuric acid scrubber and ammonium sulphate is produced. The cleaned air can be reused in
the stripping column. For vapour stripping a much higher temperature is needed to produce the
vapour. The setup can be comparable to Figure 24, only that there is no need for a final scrubber, as
the ammonia can be directly condensed together with the vapour to produce ammonia-water of up to
25-35% ammonia.
A big problem for the stripping of digestate is the usage of packed columns, as residual solids can clog
the column. As a consequence a good solid-liquid separation is necessary beforehand. In addition, a
high maintenance and cleaning effort may be necessary. As an alternative, promising results have been
obtained with a stripping method performed in simple stirred tank reactors (see Figure 25). A first
large-scale facility using such a type of process principle is already in operation (Bauermeister et al.
2009). To what extent the above-mentioned method can meet the targeted benefits will emerge from
further practice.
The big advantage of ammonia stripping is that a standardized nitrogen fertilizer product can be
recovered. In addition, such a fertilizer liquid can be used to enrich other digestate fractions in
digestate processing to a standardized nitrogen concentration, which can increase their marketability.
Ablauf
Wäscher-kolonne
Strippluft-umwälzung
NH -Strippkolonne
3
Säuredosierung(H SO )2 4
Lauge-dosierung(NaOH )4 Auffangbehälter
Ammoniumsulfat
Gärrestvorlage
CO -Strippung
2
25
Figure 25: Details of a simplified in-vessel stripping process without stripping columns
(Bauermeister et al., 2009)
5.1.2 Ion exchange
The principle of ion exchange processes is that charged particles (Na+ in Figure 26) in a resin can be
replaced by other equally charged particles (e.g. NH4+ in the case of digestate) and by that their
concentration in the liquid is reduced. Such ion exchange resins contain high amount of cavities, so
that a high contact and exchange area is possible. As ions are replaced stoichiometrically, after a
certain time the ion exchange resin is fully loaded and has to be regenerated by e.g. sodium chloride.
Then a new cycle can be started.
In practise, ion exchange is marginally applied in digestate processing. One reason is that for the usage
of ion exchange the digestate has to be free of any particles which is only the case after membrane
processes. So, for example, ion exchange is applied for a final ammonium removal after two steps of
reverse osmosis in a membrane purification concept (see section 5.3.1).
Figure 26: Principle of ion exchange resins
5.1.3 MAP-Precipitation
Ammonium and phosphate can be removed from the digestate by struvite precipitation according to
the following formula:
Mg2+
+ NH4+ + HPO4
2- + OH
- + 5 H2O MgNH4PO4 x 6 H2O
EvakuierungsluftKühlung
Vorwärmung
Kühlung
Substratzufuhr
Substratabfuhr
Strippbehälter A
Vorlagebehälter
Stripp-behälter B Wasserzugabe
Absorptionsmittelzugabe (z.B. REA-Gips)
Ammoniumsulfat-/Kalkabzug
SO 3
-
SO3
-
SO 3
-
SO 3
-
SO
3 -
Na+
Na+
NH4
+
Na+
Na+
Na+
Na+
26
In order to achieve best nutrient recovery performance in practice, magnesium is given in excess so
that nutrient concentrations are 1.3:1:0.9 for Mg:N:P. As ammonia is practically always in excess in
digestate magnesium oxide and phosphoric acid are added to the digestate. In addition, the pH is
slightly increased to 8.5-9.0. The produced struvite is a good fertiliser as N,P,Mg are valuable plant
nutrients. As illustrated in Figure 27 the chemicals can be added either in a first step with a subsequent
separation by centrifuge or chemical addition and sedimentation of the struvite occurr in the same
vessel.
The biggest disadvantage of struvite precipitation is that the high amount of chemicals needed leads to
high costs. An alternative process can be to recover the chemicals, as struvite releases ammonium and
water after heating to 100°C. The resulting magnesium hydrogen phosphate can then be reused for
precipitating ammonium.
Figure 27: Possible process options for struvite precipitation (adapted after Lehmkuhl (1990))
Gärrestnach Feststoffabtrennung
Zentrifuge
Ablauf
H PO
MgO3 4
NaOH
MAP-Abzug
Gärrestnach Feststoffabtrennung
AblaufFließbettreaktor
Rührreaktor
Luft MAP-Abzug
H POMgO
3 4
NaOH
rückgeführtes Träger-MAP
27
5.2 Nitrogen removal
5.2.1 Nitrogen removal by biological oxidation
A standard approach to reduce the nitrogen load of a wastewater is the biological wastewater treatment
process. However, in digestate processing biological nitrogen elimination processes are a rather
unattractive option due to their significant operating expenses and high investment costs. A
fundamental problem is that biological treatment does not meet the quality criteria for direct discharge.
Thus, an additional treatment step (e.g. membrane processes) is mandatory, which further increases the
complexity of the process. The only practical option is the combined treatment with other wastewater
in municipal wastewater treatment plants, especially if the extra load is comparatively low.
The basic problem for treating digestate in a standard wastewater treatment plant is that the microbes
need degradable carbon sources in order to be able to eliminate the present nitrogen. However, after a
biogas plant most of the available carbon has been transformed to biogas. The consequence is that the
concentration of nitrogen is quite high in digestate, whereas the concentration of available organic
carbon is low. For the nitrification/denitrification process in wastewater treatment plants the relation of
BOD51 to nitrogen should be higher than 3, which is rarely the case in digestate. The consequence is
that the digestate cannot be purified enough for direct discharge. An alternative can be to add artificial
carbon source (methanol, acetic acid) to the process which increases the costs dramatically. Apart
from the nitrogen problem, residual COD and colour of the treated effluent (turbidity) will make it
difficult to meet required discharge levels.
Figure 28: Different biological processes for biological nitrogen elimination
As alternatives to the nitrification/denitrification process in standard wastewater plants there are other
biological processes which can be applied for eliminating the nitrogen load in digestate (see Figure
28). Such alternative processes are nitritation/denitration or deammonification (annamox process).
Although these processes show quite a potential, they have been rarely applied upto now.
1 Biological Oxygen Demand after 5 days
Nitrifikation/Denitrifikation
Nitritation/Denitritation
Deammonifikation
NH4+ NO2
- NO3- N2
autotroph heterotroph
NH4+ N2
autotroph heterotroph
NH4+ NO2
- + NH4+ N2
autotroph autotroph
NO2-
CSB/TKN: ~ 5,5
aerob/anoxisch
CSB/TKN: ~ 3,6
aerob/anoxisch
CSB/TKN: 0
aerob/anoxisch
28
5.3 Nutrient concentration and water purification
5.3.1 Membrane technologies
Figure 29: Principle of membrane separation
The principle of membrane processes is shown in Figure 29. It is a physical separation process where
the liquid which has to be purified (feed) passes a membrane. Depending on the pore size of the
membrane and the trans membrane pressure, particles of different characteristics are retained by the
membrane and remain in the concentrate. Other particles and the partially purified water pass the
membrane and this fraction is called permeate.
Figure 30: Overview on membrane separation processes
Membrane processes are categorised depending on their pore sizes (see Figure 30). In a microfiltration
- depending on the corresponding membrane – particles down to diameters of 0.1 µm can be separated,
whereas ultrafiltration is able to separate colloids. With nanofiltration and reverse osmosis even
dissolved salts can be separated.
Feed
Membran
Konzentrat
Permeat
Triebkraft
Membran-verfahren
Größenordnung der abtrennbaren Stoffe
Beispiele
UmkehrosmoseUmkehrosmose
MikrofiltrationMikrofiltration
Molekulargewicht[Dalton]
ungefähre Größe[µm]
NanofiltrationNanofiltration
UltrafiltrationUltrafiltration
0,001 0,01 0,1 1,0
suspendierte Feststoffesuspendierte FeststoffekolloidaleStoffe
kolloidaleStoffe
EnzymeEnzyme
VirenViren BakterienBakterien
Belebtschlamm-flocken
Belebtschlamm-flocken
200 10000 20000 100000 500000100 200 10000 20000 100000 500000200 10000 20000 100000 500000100
TensideTenside Fett- und Öl-Emulsionen
Fett- und Öl-Emulsionen
gelöste Salzegelöste Salze
Metall-ionen
Metall-ionen
29
Figure 31 Different types of membranes: porous membranes (left) and solution-difusion
membranes. (cF stands for the feed concentration of the substance which is separated in the
process and cP for its concentration in the permeate)
In general there exist two types of membranes (see Figure 31). On the one hand porous membranes are
applied where the particles are retained by size-exclusion, as they are not able to enter the pores of the
membrane. On the other hand solution-diffusion membranes are used. Here the principle of separation
is the ability of substances to dissolve in the membrane material and consequently by their different
diffusion velocity. As membrane materials either polymer-membranes are used, or ceramic
membranes. The latter are only applied in micro- and ultrafiltration and have the advantage that they
are more robust to intensive chemical cleaning.
Figure 32 Typical process steps for digestate processing by membrane purification
A membrane purification process is a complex process consisting of several steps (see Figure 32).
First a solid-liquid separation is applied. Then to the liquid fraction of digestate an enhanced solids
removal (see section 3.5) has to be performed. This is a crucial point in membrane purification
processes, besides of membrane fouling. Therefore, usually decanter centrifuges are used in the first
solid-liquid separation step, and often precipitating agents added for increased solids removal. The
Feed-seite
Feed-seite
cFcF
cPcP
Porenmembran Lösungs-Diffusions-Membran
Permeat-se itePermeat-
seite
Reinwasser (Vorfluter, Verregnung)
Flüssigfraktion
Mikro-/Ultra-filtration
3-stufige
Umkehrosmose
Konzentrat (Rückführung
in den Biogasreaktor)
Konzentrat (Verwertung)
Pressschnecke/ Dekanter
Pressschnecke/ Dekanter
Bogensieb/Flotation
Bogensieb/Flotation
Feinpartikel (Rückführung
vor die Feststoffseparation)
Feststoffe (Verwertung, Kompostierung)
Antiscalant,
H2SO4
30
next step is ultrafiltration and finally reverse osmosis is used for removal of ammonia and COD
(chemical oxygen demand). Normally, three steps of reverse osmosis are needed to reach discharge
levels for ammonia. The permeate quality depending on 2- or 3-step reverse osmosis is shown in Table
7. As an alternative, the last reverse osmosis step can be replaced by ion exchange.
A drawback of such membrane purification processes is that only a limited amount of the digestate
will be purified water, about 50% of the digestate is accumulated as by-products. The following
fractions accumulate in the process: solid fraction, ultrafiltration retentate, reverse osmosis
concentrate. In order to reduce the amounts the ultrafiltration retentate is often recycled into the biogas
plant and / or the solid-liquid separation step. Membrane purification is quite expensive and requires a
considerable amount of energy.
Table 7 Examples of permeate quality after a 2-step reverse osmosis (Schulze und Block,
2005) and a 3-step reverse osmosis (Brüß, 2009) Parameter Einheit 2-stufige Umkehrosmose 3-stufige Umkehrosmose
TS [mg/l] 0 0
CSB [mg/l] 50 - 60 < 5
NH4-N [mg/l] 300 - 320 -
TN [mg/l] 320 - 340 3,5
TP [mg/l] 53 < 0,05
5.3.2 Evaporation
The evaporation of digestate is applied at biogas plants where waste heat is available in big amounts.
This is the case in countries like Germany, where biogas is prevailingly burned in CHP units to
produce electrical power. As many biogas plants are located in rural areas efficient heat utilisation can
be problematic. In addition, in Germany biogas plants receive extra funding for heat utilisation.
Alternatively waste heat from other sources near a biogas plant can be used.
Figure 33: Forced circulation evaporator (left) and natural circulation evaporator (right)
He izmedium
Umwälzpumpe
Brüden
Wärmetausc her
Kondensatabscheider
Konzentrat
Zulauf
Heizmedium
Brüden
Wärmetausc her
aufsteigendes Dampf-Flüssigkeits-Gemisch
Kondensatabscheider
Konzentrat
Zulauf
31
As prevailing technologies in digestate evaporation forced circulation evaporators (see Figure 33) are
used, alternatively also natural recirculation evaporators (see Figure 33) are applied. In these
evaporation processes the digestate is heated beyond evaporation temperature in a heat exchanger, and
then relaxed in the evaporation vessel. In forced circulation evaporators a pump is applied to achieve
the circulation of the digestate, whereas in natural circulation evaporators, the circulation takes place
automatically as the vapour digestate mixture rises into the evaporation vessel. The reason why the
described processes are applied is that they are quite robust with regard to the solids content in the
digestate.
Figure 34 Different process steps in digestate evaporation.
Figure 35: Multistage evaporation system
In a typical digestate evaporation process (see Figure 34) first the fibre/solids fraction is removed e.g.
by a screw press and vibrating screen, in order to reduce possible clogging of the evaporators. Then
large quantities of acid (sulphuric acid) are added to degas CO2 and bind nitrogen in the digestate2.
Then the digestate is concentrated by a 3-step low pressure evaporation system, as can be seen in
2 The pH is typically reduced to around 4.5 where the equilibrium between NH3(aq) and NH4
+ lies entirely at NH4
+,
which means that during the evaporation process practically all nitrogen will remain in the concentrate.
3-stufige
Eindampfung
3-stufige
Eindampfung
EntgasungEntgasung
Flüssigfraktion
Feststoff-
separation
Feststoff-
separation
Säuredosierung
zur pH-Absenkung
Düngerkonzentrat
Festfraktion
KondensationKondensation
Gärrest
UmkehrosmoseUmkehrosmose
Reinwasser
KonzentratBrauchwasser
90°C
80°C
Konzentrat
Brüden
flüssiger Gärrest
Heizmedium
70°C 55°C
32
detail in Figure 35. As low pressure is applied waste heat at 90°C can be used for evaporation. The
vapour is condensed in the process, and as it contains low amounts of ammonia and volatile acids (see
Table 8) it cannot be directly discharged. Therefore, it is used as process water for mashing in the
biogas plant or for other usages. Alternatively it can be discharged to a wastewater treatment plant. If
direct discharge limits have to be met a post treatment like reverse osmosis or ion exchange has to be
applied. If the waste heat of a CHP unit is used, typically a volume reduction of 50% of the digestate is
obtained. Based on general experience a thermal energy demand of about 300–350 kWh is needed per
ton of water evaporated. Typical performance data of an evaporation process are provided in Table 8.
Table 8 Exemplary performance data on the performance of evaporation (Heidler, 2005,
modified according to personal communication)
DM oDM TN PO4-P COD
Digestate [%] [%] [mg/kg] [mg/kg] [mg/kg]
Inflow 3.1 1.7 3,100 300 45,000
Concentrate* 10 – 12
(max. 15) 7.5 - 9
8,000 –
10,000 800 - 1200
95,000 –
120,000
Condensate 0.05 0.05 30 – 50 0 < 1000
* depending on the concentration factor
33
6 Marketing possibilities – a main limitation for nutrient
recovery All in all, up to now practically no market exists for organic fertilisers which are produced in digestate
processing facilities. From a legal point of view the commercialisation of organic fertiliser from
agricultural feedstocks should be feasible in Austria. Yet, there is no legal support for organic
fertilisers from organic wastes. For industrial wastes the legal situation will depend on the source and
the process. Nevertheless, it is expected that in future the commercialisation of organic fertilisers from
digestate processing will increase.
6.1 Legal limitations
Local policy and markets influence the marketability of compost or dried digestate. Quality standards
and legislation on fertilizers and compost products need consideration. Especially for waste digestate,
concentrations of heavy metals and other chemical pollutants may be a barrier to the sale of digestate
products. Legal frameworks in most countries stipulate the quality conditions for the marketing of
waste based digestate products.
6.2 Market limitations and incentives
6.3 Conditioning/Standardising
34
6.4 Economics of digestate processings
The successful and economically justified implementation of digestate processing is highly site
specific. Depending on the local conditions, significant differences in the individual expenses as well
as in savings, e.g. for reduced storage facilities or revenues from the marketing of the resulting
products, are achieved. Even for the similar treatment concept, high variations in the total costs may
occur.
In Table 9 a summary of the energy demand and costs for different digestate processing technologies
is given. These values are derived from the detailed calculations in the case studies of Fuchs and
Drosg (2010). The values are only estimations for demonstration and can vary considerably depending
on the boundary conditions for each biogas plant.
Table 9: Overview of energy demand and costs in digestate processing (adapted from Fuchs
and Drosg, 2010)
Energy demand Costs
Thermal
energy
Electric
energy
Investment
costs
Operating
costs
[kWhtherm/m³] [kWhel/m³] [€/m³] [€/m³]
Decanter (Total digestate) - 3.5 1.57 0.63
Screw press (Total digestate) - 0.4 0.60 0.09
Ultrafiltration (Liquid fraction) - 12 9.64 2.07
Reverse osmosis
(UF permeate) - 6 8.75 1.95
Evaporation (Liquid fraction) 170 6 7.40 2.07
Digestate storage
(Total digestate) - - 25 -
Reverse osmosis
(evaporation condensate) - 3 6.51 1.08
Dryer (Solid fraction) 500 – 600 25 43.84 5.99
Comment [B3]: Is it good to give such detailed information? What would be other options? What are the PROs and CONs?
Comment [T2]: Not a good idea. I will suggest a qualitative cost analyse instead
35
In KTBL (2008) for a specific biogas plant detailed cost calculations for different digestate processing
options were carried out. The outcome is shown in Figure 36 and Table 10. However, driving forces
for installing digestate processing are very site-specific, so which process option is sensible and
economically viable always depends on the situation.
Figure 36: Comparison of the specific digestate processing costs depending on utilised
technology KTBL (2008)
Table 10: Comparison of the specific digestate processing costs depending on utilised
technology KTBL (2008)
Gärrestausbringung Separierung Bandtrockner Membrantechnik Eindampfung Strippung
[€/m³ Gärrest]
Fixe Kosten 1,62 2,15 4,01 5,19 3,03 5,07
Energie und
Betriebsstoffe 0,29 0,30 3,74 2,77 7,03 3,42
Transport und
Ausbringung 4,42 4,77 4,53 3,17 2,82 2,21
Bruttokosten 6,33 7,23 12,28 11,13 12,88 10,70
Nährstoffe -4,40 -4,40 -4,26 -4,40 -4,40 -4,38
KWK-Bonus x x -1,23 x -2,15 -0,88
Nettokosten 1,94 2,82 6,80 6,72 6,32 5,43
1,840,95
1,47
0,66
2,012,24
2,12 1,48
1,35
1,37
2,412,53
2,411,69
1,47
0,84
-4,40 -4,40 -4,26 -4,40 -4,40 -4,38
-1,23
2,151,62
4,01 5,19
3,03
5,07
0,290,30
1,17
1,82
1,06
0,92
2,574,50
-0,88-2,15
1,942,83
6,80 6,74 6,345,45
-8
-6
-4
-2
0
2
4
6
8
10
12
14
1 2 3 4 5 6
Ko
ste
n in
€/m
³
Fixe Kosten Energie elektr. Energie therm.
Betriebsstoffe Ausbringungskosten Transport
Nährstoffe KWK-Bonus Nettokosten
Ausbringung Separierung Bandtrockner Membrantechnik Eindampfung Strippung
36
7 Conclusions and future trends
37
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Brüß U (2009) Totalaufbereitung von Gärresten aus Biogasanlagen, Gülzower Fachgespräche, Band
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Fakhru'l-Razi A (1994),” Ultrafiltration membrane separation for anaerobic wastewater treatment”,
Water Science and Technology, 30,12, 321-327.
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39
9 Glossary of terms
40
10 Brainstorming / NOT USED
10.1 Technology evaluation
The principal focus of this treatment step is the separation of the suspended solids from the digestate.
The efficiency of the solids separation step can be especially important for consecutive treatment
technologies.
The screw press is the most extensively applied technology for simple solid-liquid separation in
digestate treatment - especially in energy crop digestions. It is a simple and robust technology and is
ideal if the requested efficiency of the solids removal from the liquid phase is not very high. In
addition, the solid fraction gained by screw presses normally has a fibrous and loose consistency, so
further processing can be done more easily.
If a high degree of separation of suspended solids has to be achieved, a decanter centrifuge is normally
used. Yet, this technology has much higher investment and running costs. The great advantage of a
centrifuge is that the suspended solids concentration in the liquid effluent is much lower. The addition
of precipitating or flocculating agents can even improve the performance. All in all, by increasing the
quality of the liquid effluent the remaining solids become more difficult to handle due to the higher
moisture content. In many digestate treatment concepts decanter centrifuges are indispensable and can
be seen as practically state-of-the-art technology in digestate treatment.
By applying belt filters, in general, a better solids removal than in screw presses can be achieved.
Normally the performance is improved by addition of precipitating or flocculating agents. This high
chemical demand can be seen as one of the main drawbacks in this technology.
Table 11: Evaluation of technologies for mechanical separation of the solids
Processes for mechanical separation of the solids
Evaluation criteria Quantifier Screw press Decanter
centrifuge Belt filter
State of the art 3 ++ ++ ++
Eff
ort
/Cost
s
Energy demand 3 + -- -
Investment costs 3 + -- -
Operating resources 2-3 ++ - --
41
Need of assistance 3 ++ - +
Effort/Costs 3 ++ - -
Appli
cabil
ity
Demand for pretreatment 2 ++ + --
Logistic effort / simplicity
of the process 1 ++ + ++
Reliability of operation 3 ++ + ++
Environmental issues 3 + + +
Dependence on boundary
conditions 2 + ++ +
Applicability 2 ++ ++ +
Qu
alit
y o
f en
d p
rod
uct
Quality of the produced
solids 2 + - --
Decrease of solids
concentration in liquid
fraction
3 - ++ +
Reduction of the amount
of digestate for further
processing
2 + + +
Product value 1 - - -
In total, a decanter centrifuge can be seen as the best performing technology with the highest
efficiency in solids removal from the liquid phase. This is especially of interest, if membrane
purification of the effluent is the aim. The detailed result of the evaluation of technologies for
mechanical solids separation is summarised in Table 11.
42
10.2 Details on mass- and nutrient flows
Figure 37: Digestate membrane filtration process for a waste treatment plant – 1 MWel
Figure 38: Waste treatment plant – 1 MWel: process flows of water, organics and ashes in a
process of decanter, ultrafiltration and reverse osmosis (enhancement factors in figure: water
… 1x; organics … 5x; ashes … 5x)
Decanter
Ultrafiltration Reverse osmosis
Liquid phase
Digestate Solids
Retentate
Permeate (discharge)
Concentrate
Filtrate
Decanter
Ultrafiltration Reverse osmosis
Liquid phase
Digestate Solids
Retentate
Permeate (discharge)
Concentrate
Filtrate
43
Figure 39: Waste treatment plant – 1 MWel: process flows of organic nitrogen, ammonia
nitrogen, phosphorus and potassium in a process of decanter, ultrafiltration and reverse
osmosis