=Paper=
{{Paper
|id=Vol-2030/HAICTA_2017_paper20
|storemode=property
|title=Processing Digestate from Agricultural Biogas Plant to Obtain Financial and Environmental Benefits
|pdfUrl=https://ceur-ws.org/Vol-2030/HAICTA_2017_paper20.pdf
|volume=Vol-2030
|authors=Wojciech Czekała
|dblpUrl=https://dblp.org/rec/conf/haicta/Czekala17
}}
==Processing Digestate from Agricultural Biogas Plant to Obtain Financial and Environmental Benefits==
https://ceur-ws.org/Vol-2030/HAICTA_2017_paper20.pdf
Processing Digestate from Agricultural Biogas Plant to
Obtain Financial and Environmental Benefits
Wojciech Czekała1
1
Institute of Biosystems Engineering, Poznan University of Life Sciences,
Poznań, Poland e-mail: wojciech@up.poznan.pl
Abstract. At European Union, as well in Poland, renewable energy sources
(RES) are an important alternative to fossil fuels. Solid biomass is mainly used
for the production of renewable energy. The anaerobic digestion process
creates biogas, and digestate. The latter is a residue from the fermentation
process, consisting mainly of non-digested organic and mineral components.
The market offers many devices for the management of the digestate. Among
them the most important are the separators that separate the raw material into
solid and liquid fractions. The aim of the research was to discuss the possibility
of improving the management efficiency of biogas plant by using the digested
pulp and its fractions. For a designed 1 MWel biogas plant installation, modern
technological solutions have been proposed. It can be concluded that a proper
digested pulp economy can bring additional profits to the biogas plant.
Keywords: digestate, digested pulp, biogas production, waste management.
1 Introduction
With the increase in energy demand, it is necessary to seek new technologies for its
production. It should be cheap, efficient and what is most important, safe for the
environment. Biofuels are renewable energy sources that are produced from biomass.
Agricultural biogas is produced in biogas plants that use agricultural substrates to
produce energy (Budzianowski, 2016). It is a mixture of gases with high methane
content, so it can be used for energy production (Cieślik et al., 2016). Another
product, often overlooked substrate from the anaerobic digestion process of
agricultural products is digested pulp (Börjesson and Berglund, 2007). Its
composition consists mainly of non-digested organic substances, minerals and
microbial biomass.
The most common way to use the digested pulp is use it directly as fertilizer
(Tampio et al., 2016). This is due to the ease and high efficiency of its application in
the soil. In many countries in Europe and for example in China digestate is perceived
as a valuable fertilizer. This is a desirable substrate and apparently presents no
environment concerning related to its management. In Poland, there is some
resistance about the use of digestes as fertilizers mostly due a misperception and
understanding about the biogas and the use of digested pulp. Nowadays, biogas
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plants often have problems for selling the digestes, even for free. However, slowly,
with the understand of the benefits of digestate, the perception of its application as
fertilizer by farmers is favorably shifting, and digestate started to be treated as a
valuable fertilizer.
One possible solution for this problem is the separation (dehydration) of the
digestate (Wu et al., 2017). The effect of such action is to obtain a minimum of two
significantly differentiated fractions. This will allow easier management of the
product after fermentation process. Another advantage will be the possibility of
increase the revenue of the biogas plant. The technology which can even increase the
fertilizing value of digestate solid fraction is composting. Because of intensive
temperature growth during composting, the obtained material is characterized by
high fertilizing value, sanitary safety, lack of odor and chemical stability (Waszkielis
et al., 2013; Wolna-Maruwka and Dach, 2009;). Another possibility of digestate
management is to use it as feeding medium for algae production as the substrate for
biogas plant (Cerbin et al., 2012). The experimental installation (greenhouse of 3000
m2) for algae production with usage of digestate solution for microalgae nutrition
was built in Central Poland in 2015. However, the real scale investment has shown
much lower microalgae production (mainly because of colder climate and technical
problems) than promising results obtained in laboratory scale (Lewicki et al., 2013).
The aim of the research was to discuss the possibility of improving the
management and economic revenue of biogas plant by further use the digested pulp.
During the implementation of the work, an apparatus for drying and a separation of
digestate was utilized.
2 Production of the Digestate, Essence of separation and
Digestate Drying
The biogas plant is an installation that produces energy in a recovery process,
while manage waste (Dach et al., 2016; Kozłowski et al., 2016). The most common
criteria for division of an installation is the substrate used. Therefore, one may
distinguish biogas plants in: agricultural, landfills or municipal waste and sewage
treatment plants. Agricultural biogas plants are safer from the environmental
perspective because the substrates are from agricultural origin, often undergo through
high sanitation standard procedures as it is designed for human consumption
(Czekala et al., 2015).
The substrate used in the agricultural biogas plant is subject to numerous
transformations. The most important is distribution of particles into more easily
accessible compounds. Regarding elemental composition, the greatest changes occur
in relation to carbon. Organic compounds are degraded, consequently the amount of
organic matter decreases (Neugebauer and Sołowiej, 2017). The result of the process,
among others, is methane generation. In relation to macroelements, another relevant
aspect is the conversion of organic matter into mineral compounds. Minerals are
then, available for plants, so that digestates are characterized by better parameters
than the unprocessed substrate e.g. slurry.
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The intensity of all those transformations depends primarily on the type of
substrate used (Czekała et al., 2016a). In addition, the process conditions are an
important element, for example the parameters: type of technology used, process
temperature, substrate size and type and intensity of the process.
It is estimated that about 90% of the initial substrate matter still remain in the
digestate. These values are the assumed values often used in calculation in Poland.
Consequently, a typical biogas plant with 1 MW electric power produces, daily, a
few tens of tons of digestate. Such a considerable amount of digestate often becomes
an liability for biogas plants operating in Poland. Ideally, part of the digestate could
be used directly to fertilize the fields that provide substrate to the biogas plant. This
will keep the nutrients within the production chain of the biogas plant. Another
advantage is related to economic and environmental aspects. The direct use of
digestate as fertilize can decreases the cost of crop production. While, the production
of mineral fertilizers, in some cases, can pose treat to the environment. Therefore,
this alternative is economical and environmentally ideal.
Digestate is characterized by a high degree of hydration, due to hydraulic
requirements the pumping operation should take place directly in the agricultural
biogas plant. Thus, the dry matter content of the digestate is rather low, about 6%.
Dehydration is carried out by means of separation or drying (Kaparaju and Rintala,
2008). In case of separation, solid and liquid fraction are the products which can be
used for different purposes. Dry matter content in solid fraction is usually 20-35%
while, liquid fraction is about 1.5 to 4%. One advantage of drying is the efficiency of
the process because it allows the substrate to be processed to a specific humidity
level. However, the drawback is the need to supply a significant amount of energy in
the operation. Moreover, in case of use of digestates as fertilizes it is recommended
to maintain certain level of humidity that can favor the absorption of nutrients by
plants (tab. 1).
Table 1. Averaged composition of solid and liquid fractions of digestate from selected Polish
biogas plants [Kowalczyk- Juśko and Szymańska 2015].
Analyzed feature Solid fraction Liquid fraction
Dry matter [%] 22 - 27 2,7 - 4,3
Dry organic matter [%] 89 - 94,5 58 – 62
Total nitrogen [%] 0,4 - 0,8 0,2 - 0,75
N-NH4 [%] 0,08 - 0,52 0,28 - 0,38
P [%] 0,1 - 0,28 0,03 - 0,05
K [%] 0,12 - 0,69 0,5 - 0,62
Ca [%] 0,22 - 0,43 0,05 - 0,07
Mg [%] 0,06 - 0,17 0,01 - 0,02
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In Europe, as the market for separators and digestate driers is developed, it is
easier to use or improve agricultural biogas installations to include such technologies.
For example, separators of slurry, originally used for sewage sludge, were installed in
agricultural biogas plants.
3 Digestate Processing
Regarding the process of digestion pulp, the direction of its further use should be
carefully considered. If the digestate is used as fertilizer, it is not necessary to further
process it. The only requirement is to mix it beforehand, to equalize the nutrient
content. However, in case you separate the digestate in liquid and solid fraction, the
former, can be either used as fertilizer in the fields or to decrease the dry matter
content of the used substrate to align it with the feed hydraulic parameters of the
biogas plant, (Sigurnjak et al., 2017). The solid fraction biomass can be used as well
as fertilizer or to energy purposes (Czekała et al., 2017; Obidziński, 2012). Direct
distribution on the surface of the fields will enrich the organic matter content of the
top soil. And, it is also possible to use the solid digestate fraction for compost, both
as a stand-alone substrate or as an additive in combination with other ingredients.
Generally, the selection of elements required for waste management should be
preceded by:
1. analysis of the scientific literature and legislation in force in the country,
2. industry market analysis of available equipment and technologies,
3. calculating the amount of digestate available,
4. determining the preferred directions of using the digestate,
5. development of a biogas effluent processing plant that is compatible with the
biogas plant.
In the present study case, the energetic use of digestate was the objective. The
production of solid biofuels, i.e. briquettes and pellets was considered. Both,
briquettes and pellets are widely used in Poland for heat production (Szmigielski et
al., 2014; Kowalczyk-Jusko et al., 2015a) The design of the agricultural biogas plant
with power 1MW will use the substrates shown in table 2. The presented data are
from studies conducted in Laboratory of Ecotechnologies in the Poznań University of
Life Sciences (PULS). The research was carried out according to the modified
German standard DIN 38 414 – S8.
Table 2. Substrates use to biogas production*
Substrate Quantity DM ODM Biogas Methane
[Mg•year-1] [%] [%DM] efficiency content
[m3•Mg-1FM] [%]
Maize silage 11 000 32 95 210 55
Brewers grains 11 000 24 90 140 60
Slurry 22 000 4 70 10 60
*non-published data from Laboratory of Ecotechnologies at PULS
191
In order to calculate the amount of digestate material produced on a biogas plant it
is necessary to estimate the dry matter and water content of the substrates. The
following formulas were used (1):
Mdm = Mfm•dm [Mg•y-1] (1)
where:
Mdm- mass of dry substance mass [Mg•y-1]
Mfm - mass of fresh mass [Mg•y-1]
dm - dry matter content [%].
The most important parameter when planning a separation installation is the water
content of the raw digestate and its fractions. The quantity and characteristics of the
substrates are shown in table 2.
The mass of substrates used for biogas production in the analyzed mix was 44000
Mg (table 2). Assuming that the amount of the digestate constitutes 90% of the feed
substrate, the annual amount of digestate is calculated (2).
Mdig = Ms • n [Mg•y-1] (2)
where:
Mdig- digestate mass [Mg•r-1]
Ms- mass of input to biogas plant [Mg•r-1]
n – mass retention coefficient in the fermentation process -0.9 [%].
At the present study case, it is expected an annually production of 39,600 Mg of
digestate, which gives about 108 Mg digestate per day. The quantity is relatively
large, since half of the load is a slurry characterized by high hydration. The addressed
project is planned to use solid fractions for the production of solid biofuels. And, the
liquid fraction will be directed to the hydration of new substrates and partly for the
production of microorganism-enriched fertilizers. This is an innovative solution that
complies with, both, scientific and entrepreneur interest in relation to research on
microbial inoculation of the liquid fraction to enhance fertilizer properties. The
following assumptions were made for calculations (table 3).
Table 3. Design assumptions
Assumption Value of assumption
3 -1
Quantity of digestate production [m •y ] 39600
3
Mass of 1 m of fermentation pulp [Mg] 1
Dry matter of digestate [%] 8
-1
Amount of liquid fraction after separation [Mg•y ] 28800
-1
Amount of solid fraction after separation [Mg•y ] 10800
*non-published data based on real scale biogas plants
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The generated solid biofuel from the digestate has a slightly lower calorific value
than the wood pellet treated as the standard solid biofuel. The determined calorific
value was 16.58 MJ•kg-1 (Czekała et al., 2016b). In the analyzed example, it was
assumed that the daily production of the solid fraction is 20 Mg with a dry mass of
23%. Generally, water content can affect the strength of materials (Chocyk et al.,
2015, Kowalczyk-Jusko et al,. 2015b). Thus, to achieve appropriate level of water in
the substrate, a belt dryer will operate in the biogas plant. Then, with the resulted
solid fraction, it will be possible to produce about 4 tons of briquettes from the
digestate. Produced briquette can be sold to external buyers for 100-130 Euro•Mg-1.
Necessary installations for digestate processing in the biogas plant are as follows:
- separator EUR 44 000
- belt dryer EUR 60 000
- line for the production of briquettes (0,5 Mg•h-1) EUR 14 000.
The cost of installation of above listed separation and treatment devices is 118,000
Euros. However the biogas plant building cost is about 5 million Euros,
consequently, the above device cost seems economically acceptable.
4 Conclusion
Lack of specific regulations and social resistance are responsible for limiting the
use of digestate at field. Moreover, biogas plant owners often do not have enough
farmland to distribute the entire produced digestate. Consequently, this situation
creates barriers to the development of the biogas sector in Poland. Additionally, it is
recommended to carry out physical, chemical and microbiological analyzes of the
digestate before its application in the soil. A better characterization of the fertilizer
can increase the trustiness of farmers and favor the utilization of digestate at fields.
Investments in an installation for industrial separation and processing of digestate
creates opportunities for the use of digestate with additional revenue.
Acknowledgments. The work was carried out as part of the project: "Research on
the development of innovative organic-mineral fertilizers from digestate"; co-
financed by the European Union under the Operational Program - Intelligent
Development (Support for environment and business potential for R & D & I
activity), application POIR.02.03.02-30-0002/16.
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