DIGESTATE MANAGEMENT IN FLANDERS: NUTRIENT REMOVAL VERSUS
NUTRIENT RECOVERY
Viooltje Lebuf
Flemish Coordination centre for Manure processing (VCM), address: Abdijbekestraat 9 – 8200,
Bruges – Belgium - Tel: +32 (0) 50 - 40 72 03 - Fax: + 32 (0) 50 – 40 74 89 - e-mail:
Viooltje.Lebuf@pomwvl.be
Sara Van Elsacker
Flemish Coordination centre for Manure processing (VCM), e-mail: sara.vanelsacker@vcmmestverwerking.be
Frederik Accoe
Flemish Coordination centre for Manure processing (VCM)
Celine Vaneeckhaute
Ghent University, Faculty of Bioscience Engineering, Laboratory of Applied Analytical and Physical
Chemistry, Coupure Links 653, 9000 Ghent, Belgium, e-mail: celine.vaneeckhaute@ugent.be
Erik Meers
Ghent University, Faculty of Bioscience Engineering, Laboratory of Applied Analytical and Physical
Chemistry
Evi Michels
Ghent University, Faculty of Bioscience Engineering, Laboratory of Applied Analytical and Physical
Chemistry
ABSTRACT
Intensive livestock production combined with limited availability of land for manure disposal and
fertilisation restrictions by the EU-legislation make Flanders (Belgium) a 100% Nitrate Vulnerable
Zone. The Flemish Manure Decree has been implemented in order to take measures against nitrate
and phosphate pollution in water, resulting from the produced nutrient excess.
From the 39 anaerobic digestion plants operational in Flanders, most of the installations are codigestion plants that process an input mixture of animal manure, organic waste streams and energy
crops. According to the Manure Decree however, digestate from co-digestion plants that take in
manure also has the status of animal manure, and application on arable land is limited by the 170 kg
N/ha/y restriction. For this reason, digestate from anaerobic digestion plants competes with manure for
nutrient disposal on arable land, which forms a serious hinder for the biogas sector to develop in these
regions.
Hence, one of the biggest challenges for anaerobic digestion plants in a region like Flanders, is to find
cost-effective and sustainable ways for digestate processing or disposal.
In the framework of the ongoing Interreg NWE project ARBOR, VCM, Ghent University and Inagro
investigate the options for nutrient recovery from digestate. This paper gives an overview, both of the
currently applied techniques for digestate processing in Flanders, as well as the techniques that
enable nutrient recovery from digestate. It also focuses on the physicochemical characteristics of the
end-products, and the potential constraints for successful valorisation.
KEY WORDS: Digestate processing, nutrient recovery.
INTRODUCTION
The region of Flanders (Belgium) has a high nutrient pressure because of intensive livestock
production combined with limited availability of land for manure disposal, due to the defined fertilization
restrictions. The Flemish Manure Decree has been implemented in order to take measures against
nitrate and phosphate pollution in water, resulting from this nutrient excess. Since 2007, Flanders has
been designated as a 100% Nitrate Vulnerable Zone according to the European Nitrates Directive,
which implies a fertilization restriction of maximum 170 kg N/ha/y for animal manure in entire Flanders
(except for some derogation zones).
One of the policy measures implemented to cope with this high nutrient pressure is the obligation for
farmers to process a percentage of their manure surplus on farm level. According to the Manure
Decree, this processing obligation can be met by treating the manure in such a way that i) the manure
products can be exported out of Flanders, ii) the nutrients are removed from the manure (e.g. by
conversion of NH4 to N2-gas) or iii) the nutrients are converted into a mineral fertilizer. Export of
untreated manure is an option for poultry or horse manure. Various techniques have been developed
and implemented for manure processing, and currently about 20% of the net N manure production in
Flanders is processed (VCM, 2012).
Currently, there are 39 anaerobic digestion plants operational in Flanders, with a total input capacity of
about 2.000.000 tons/year (Biogas-E, 2012). Most of these installations are co-digestion plants that
process an input mixture of animal manure, organic waste streams and energy crops. According to the
Manure Decree however, digestate from co-digestion plants that take in manure also has the status of
animal manure, and thus application on arable land is limited by the 170 kg N/ha/y restriction. For this
reason, digestate from anaerobic digestion plants competes with manure for nutrient disposal on
arable land, which forms a serious hinder for the biogas sector to develop in these regions. Hence,
one of the biggest challenges for anaerobic digestion plants in a region like Flanders, is to find costeffective and sustainable ways for digestate processing or disposal.
Up to now, the technical approach for digestate processing in most of the co-digestion plants is similar
to and adopted from the approach for manure processing. This means that the goals are nutrient
removal or treating the digestate in such a way that the products can be exported. However, we are
now facing the challenge to evolve to more sustainable approaches which focus on nutrient recovery,
rather than nutrient removal, both for manure and digestate processing.
In the framework of the ongoing Interreg NWE project ARBOR, VCM, Ghent University and Inagro
investigate the options for nutrient recovery from digestate. There is a diverse range of techniques that
can be applied for digestate processing, but certainly not all of them can be considered as a nutrient
recovery technique. There is no straightforward definition of a nutrient recovery technique. In this
paper we consider techniques that (1) create an end-product with higher nutrient concentrations than
the raw digestate or (2) separate the envisaged nutrients from organic compounds, with the aim to
produce an end-product that is fit for use in the chemical or fertiliser industry or as a mineral fertiliser
replacement, as a nutrient recovery technique. This makes it possible to re-use the present nutrients
locally and close the nutrient cycle.
In Figure 1 an overview of the existing digestate treatment options is given. The techniques that are
delineated as a nutrient recovery technique are indicated in grey.
Figure 1: Schematic overview of digestate processing techniques (grey boxes indicate nutrient
recovery techniques).
In general, mechanical separation is a first processing step. Separation results in a solid fraction
containing the major share of the initial phosphorus (60-70%) and organic matter content (69-73%),
next to a liquid fraction that contains almost all soluble nutrients (N, K) and salts. As the solid fraction
offers the opportunity to transport a more concentrated amount of nutrients and organic matter, it is
often transported to nutrient deficient regions. European legislation (ER 1069/2009), however, obliges
that a pasteurization treatment of 1 hour at 70°C is executed before export. To realize this and to
further reduce transport volumes, the solid fraction is often dried or composted before being exported.
Nevertheless, it could be more preferable if phosphorus could be extracted from its organic matrix and
be re-used as a secondary raw material for industrial processes, as the phosphorus extracted from
phosphate rock is becoming scarce.
The most commonly applied post-treatment technique in Flanders, for the liquid fraction of the
digestate, is biological nitrification/denitrification process which converts ammonium to nitrogen gas.
Currently, 5 of the 39 digestion plants in Flanders apply this technique for treatment of the liquid
fraction. Notwithstanding, from an environmental point of view, the biological treatment poses a
paradox: while nitrogen gas is removed from the digestate and released into the air, the fertilizer
industry captures nitrogen gas to produce artificial N-fertilizers through the energy-consuming HaberBosch process.
These issues have put many researchers to think how nutrients (N, P and K) could be recovered from
digestate in a sustainable way. Available techniques for the recovery of nitrogen are pressurized
membrane filtration, ammonia stripping and scrubbing or thorough separation using polymers
(coagulants and flocculants). A promising technique for phosphorus recovery is struvite crystallization.
NUTRIENT RECOVERY TECHNIQUES FOR LIQUID FRACTION
Pressurized membrane filtration
The input stream for membrane filtration is either the liquid fraction of the digestate or a pre-processed
stream, such as the condensate of the evaporator. The input stream is forced through the membrane
by varying pressures. Several types of membranes are used in manure/digestate processing: MF(microfiltration, pore-size > 0,1 µm, 0,1-3 bar), UF- (ultra-filtration, pore-size > nm, 2-10 bar) and ROmembranes (reversed osmosis, semi-permeable, 10-100 bar). In a MF-concentrate suspended solids
are retained, while in a UF-concentrate also macromolecules are retained. Both filtration steps can be
used as a pre-treatment for reversed osmosis, in order to prevent that either suspended solids or
macromolecules block the RO-membrane. Another technique that can be used prior to RO is dissolved
air flotation (DAF), a technique that consists of blowing small air bubbles through the liquid fraction,
entraining suspended solids to the surface where they form a crust. This crust is then scraped off.
When using DAF coagulants and flocculants are often added.
The permeate of RO, which consists mainly of water and small ions, can be discharged, if necessary
after a ‘polishing’ step, or re-used as process water.
This technique is developed on full-scale, but is not implemented frequently yet. In Flanders 4 biogas
plants use RO for digestate treatment. The produced mineral concentrates cannot be used as mineral
fertilizer (yet) because current European legislation (NitDir en ER 2003/2003) doesn’t allow production
of mineral fertilizers from animal manure.
In the Netherlands a large research project is ongoing since 2008 on the RO-concentrate of 8 different
manure/digestate processing installations. This project received an exceptional permission of the
European Commission to use the mineral concentrates as a mineral fertilizer (for the duration of the
project) to investigate the agronomic, economic and environmental effects of the production and use of
mineral concentrates as mineral fertilizer replacement (Velthof, 2011).
The average composition varies between installations. This can be partially explained by differences of
the input type of slurry. The pretreatment probably has an effect on the composition of the concentrate
as well. The installations using a combination of a centrifuge and ultra-filtration and the ones using a
combination of a sieve belt press and flotation tend to have higher nutrient contents in their
concentrate than the ones using a screw press and flotation (Velthof, 2011).
Ammonia stripping and scrubbing
Ammonia is stripped by blowing air or steam through the liquid fraction in a packed tower. The
stripgas, which is charged with ammonia and volatile organic matter, is then put in contact with a
strong acid solution (H2SO4), which produces ammonium sulphate (see acid air washers further in the
paper).
Ammonia stripping is developed on a full-scale but is not yet frequently used for digestate and manure
treatment. In the ongoing Flemish MIP-project Nutricycle, testing of ammonia stripping from digestate
on a pilot scale is implemented. The results of the test will give information on the influence of pH
change during the process and how scaling can be avoided. The goal of the project is to find the most
suitable type of ammonia stripping for digestate treatment, and how process parameters can be
optimized.
A combination of the ammonia stripping technique and struvite precipitation (see phosphorus
precipitation further in the paper) was studied by Quan et al. (2010). Both processes were taking place
simultaneously in a water sparged aerocyclone reactor (WSA).
The Dutch company Dorset developed another type of ammonia stripping system for manure and
digestate without air recirculation or ventilation. The system consists of rotating disks that are partly
submerged in either the liquid manure or the receiving sulphuric acid solution. The rotating disks are
close to each other so the ammonia coming from the gas phase is absorbed at the other disc with the
sulphuric acid (Dorset GM).
Phosphorus precipitation
Several ions can be added to a solution containing soluble phosphate (orthophosphate) to induce a
precipitation reaction forming phosphate salts. Addition of calcium to a phosphate solution will result in
the formation of calcium phosphate. By adding magnesium or potassium and adjusting pH to 9-11,
MgNH4PO4 (MAP or struvite), KMgPO4 (Potassium Magnesium Phosphate) or K2NH4PO4 (potassium
struvite) precipitates. Struvite is considered to be a slow-release fertilizer.
Researchers at the Fraunhofer Institute for Interfacial Engineering and Biotechnology in Germany
have patented an electrochemical process to precipitate struvite without the addition of salts or bases.
The mobile pilot plant consists of an installation with a magnesium anode and a metallic cathode. The
electrolytic process splits the water molecules into negatively charged hydroxyl ions at the cathode. At
the anode an oxidation takes place: the magnesium ions migrate through the water and react with the
phosphate and ammonium in the solution to form struvite.
Struvite is mostly formed by adding MgO but adding MgCl2 is also a possibility. Main advantage of
MgCl2 is that its production requires less energy than that of MgO. Main disadvantages are a slower
and less complete reaction as well as the presence of chloride ions in the remaining solution. This
implies that this solution can only be valorised as a fertiliser for crops that are tolerant for chloride ions,
e.g. grass (Sanders, 2010).
Besides from the addition of Mg or K, Ca(OH) 2 can also be added. Because of pH and temperature
increase ammonia is stripped out of the solution and should be scrubbed with an acid air washer.
Quan et al. (2010) examined the coupling of CaNH4PO4.4H2O precipitation and ammonia stripping in a
water sparged aerocyclone reactor on lab scale.
The amount of P that is bound to the organic fraction can be released by using an initial hydrolysis
step (Schoumans et al., 2010) or by using an acidification step combined with solid/liquid separation.
Current use of struvite precipitation is mostly limited to treatment of industrial and municipal
wastewater. There is one full-scale system operating on calf manure in the Netherlands. A pilot plant is
installed at research centre De Marke (NL) by Fermtech Systems bv for the treatment of cattle slurry
digestate. The liquid fraction of the digestate goes to a crystallisation reactor where struvite is formed
and a NK-effluent remains which can be used as a fertiliser on the dairy farm (van Zessen, 2012).
Biomass production and harvest
González-Fernández et al. (2011) inoculated four open ponds with microalgae-bacteria consortia to
treat anaerobically digested pig slurry to observe nitrogen transformations in the ponds under realistic
conditions of light and temperature. When digestate was fed to the ponds, nitrification followed by
biomass uptake and denitrification were the main nitrogen transformations. In ACRRES (part of
Wageningen UR) lab tests were performed where liquid fraction digestate (containing 5 g N/l) was
added to growing media containing algae. A high concentration of the liquid fraction reduced the algal
growth capacity significantly (R. Schipperus, pers.comm.).
Besides algae, macrophytes have also been studied to recover nutrients from digestate. Xu and Shen
(2011) studied the use of duckweed (Spirodella oligorrhiza) for nutrient recovery from anaerobically
digested pig slurry. During the growing season, the duckweed was capable of removing 83.7% and
89.4% of total nitrogen and total phosphorus respectively in eight weeks at a harvest frequency of
twice a week.
The produced algae/macrophytes can serve as feedstock for chemical industry and biofuel industry or
could be used as animal feed or spread out as a fertilizer on the fields. For bulk products the cost of
producing algae is too high in comparison with other types of biomass (Muylaert and Sanders, 2010).
Nor in Flanders nor in the Netherlands there are commercial scale ponds operational at the moment
that treat digestate or manure.
In ACRRES there is a pilot algae pond installed which is currently fed with artificial fertiliser. However,
if future legislation allows the marketing of algae fed on digestate in feed industry, pilot scale
experiments could be performed at that site. A first legislative bottleneck to be tackled will be the
acceptance in the feed safety database of GMP+. For algae and duckweed there are no restrictions for
use in feed, still if they are grown on a medium containing animal manure, the biomass will also be
defined as animal manure, unless it can be marketed free of manure particles (A. Kroon and R.
Schipperus, pers.comm.).
Other techniques
During the last couple of years there has been an increased interest in forward osmosis as opposed to
reversed osmosis. In forward osmosis there is also a semi-permeable membrane, but no external
pressure. The water flow is obtained by imposing an osmotic pressure by means of a draw solution
such as NaCl. Forward osmosis can be an interesting technique for use in wastewater treatment, food
processing and seawater desalination. More research is necessary to investigate if this technique
could also be used for concentration of digestate sludge.
During electrodialysis ammonia in the diluate solution is transferred by electromigration to an
adjacent solution by an ion-exchange membrane under the driving force of an electrical potential. This
means that the main ionic compounds in the liquid digestate (in the diluate cells) i.e. NH4+, K+ and
HCO3- are transferred and concentrated. In the Netherlands a pilot plant was installed at Dairy
Campus in Leeuwarden, where digestate is treated by means of electrodialysis. An ammonium
concentrate (10%) is produced as well as a potassium concentrate (10%). These nutrients are
captured in a gas scrubbers by means of CO2 as carbonates. The remaining nitrogen will be captured
in an acid air washer. In 2013 extensive research will be performed on the fertilising value of the endproducts (van Zessen, 2012).
Transmembranechemosorption (TMCS) is used in pig slurry treatment systems in the Netherlands,
where the ammonia is stripped and removed using TMCS. Ammonia is brought in the gaseous phase
by means of a pH increase. The ammonia diffuses through a hollow-fibre membrane with gas-filled
pores and is captured at the other side of the membrane in a sulphuric acid solution (www.sustec.nl).
NUTRIENT RECOVERY TECHNIQUES FOR SOLID FRACTION
Phosphorous extraction
Phosphorus extraction has been tested extensively for dried or dewatered sludge and ashes from
sludge incineration. However, tests on dried fraction, ashes or biochar from digestate are absent in
literature.
Digestate is considered a waste stream that is eligible for recycling as soil conditioner, which makes it
not eligible for conversion to energy by combustion according to Flemish waste legislation. On the
other hand, animal manure, which is not subject to the waste legislation, can be combusted, taking
into account the emission standards (Art. 4.5.2., VLAREMA, 2012). The goal of combustion could be
to produce electricity from the released energy and to recover nutrients (mainly P) from the ashes.
Also a strong reduction in volume is obtained and pathogens are killed. However, a thorough flue gas
cleaning system is indispensible, which makes small-scale combustion up till now not viable. The
remaining ashes after combusting digestate/manure contain up to 20-25% P2O5, next to K- , Al-, Mgand Si-compounds and possibly also some heavy metals such as Cu, Zn and Cd. Several companies
have designed different processes to extract phosphorus from the combustion ashes (Schoumans et
al., 2010). These techniques can be subdivided into thermochemical and wet-chemical techniques.
Pyrolysis exposes the digestate to a temperature of 150-900°C in the absence of oxygen. Organic
matter fractionates into syngas, bio-oil and biochar (Lemmens et al., 2006). Preliminary pyrolysis tests
on digestate revealed that oil yield and quality (very viscous) were suboptimal (K. Smets, pers.
comm.). Experiments with pyrolysis of manure cakes have been conducted. The fraction of nutrients
recovered in biochar is larger than in ashes and the plant-availability of the nutrients tends to be
higher, especially for phosphorus (Schoumans et al., 2010).
Techniques for phosphorus extraction from sewage sludge or sludge incineration ash are existing on
full scale or demonstration scale. However, techniques to recover phosphorus from digestate
ashes/biochars are less frequently mentioned.
NUTRIENT RECOVERY TECHNIQUES FOR GASEOUS STREAMS
Acid air washer
Thermal drying, composting and evaporation result in emissions of dust particles, water vapour,
ammonia and odour compounds. Air treatment is obligatory before emission to the environment. Often
an acid air washer is used, which captures the NH3 in sulphuric acid by means of a packed tower
where sulphuric acid is sprayed with nozzles over the packing material and treatment air is blown into
the tower in counterstream. Ammonium sulphate is produced and the wash water is recycled until it is
saturated and the removal efficiency of ammonia cannot be guaranteed anymore. At that point the
ammonium sulphate solution should be removed and fresh sulphuric acid added. The reject solution is
variable in N-content and pH, due to the variable efficiency of acid air washers. The supplier of the
acid air washer defines a certain flow of reject wash water that guarantees a minimal ammonia
reduction of 70%.
The (NH4)2SO4 solution contains between 30-70 kg N/ton. The pH is often acid, unless an alkaline step
is incorporated. The pH varies between 3-7 (M. Heijmans, pers.comm.; N. Van Hemelrijck,
pers.comm.).
This technique is developed on full-scale. It is frequently used in manure processing and digestate
processing activities, as well as for pig stables. In Flanders and The Netherlands ammoniumsulphate
is accepted as mineral fertilizer. However, the low pH, high variability of N-content, small volume
production at farm scale are the main bottlenecks for valorization of ammoniumsulphate as a mineral
fertilizer.
CONCLUSIONS
In nutrient rich zones it has become inevitable for anaerobic digestion plants to invest in a digestate
processing technique as only a small fraction of the digestate can be spread out on land. Because of
increased attention for nutrient recycling and the possible depletion of phosphorus, digestate should
be considered a valuable source of nutrients and treated accordingly.
Defining nutrient recovery techniques is not as straightforward as it seems. This paper proposes
following definition: techniques that create an end-product in which nutrients are present in a higher
concentration than before processing or those that separate the envisaged nutrients from organic
compounds, with the aim to produce an end-product that is fit for use in chemical or fertilizer industry
or as a mineral fertilizer replacement.
Out of the discussed nutrient recovery techniques, only acid air washers, membrane filtration plants
and ammonia stripping plants are operative at full scale at anaerobic digestion plants. Nonetheless,
they may need further technical fine-tuning, especially towards energy saving and decreasing the
addition of chemicals. A breakthrough in full-scale plants is to be expected for phosphorus
precipitation. In the long run also electrodialysis, forward osmosis, TMCS and biomass production
could become part of commonly used digestate processing techniques. The extraction of phosphorus
from ashes or biochars seems less promising, because it is questionable if combustion/pyrolysis of
digestate is a sustainable treatment option and if this should be encouraged. However, extraction
techniques could also be applied on the (dried) solid fraction of digestate.
For all techniques described it is essential to put attention on fertilizing value of the end-products or
marketing value towards industrial end-users. To be economically profitable, the price allocated to the
recovered nutrients should be in accordance to the market price of N, P and K in mineral fertilizers.
Obtaining the regulatory status of “mineral fertilizer” is thus considered to be very important to achieve
successful marketing of these products for agricultural use.
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