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ENVIRONMENTALIMPACTOFWASTEBASED

BIOGASDIGESTATESASINFLUENCEDBY

POST-TREATMENTMETHODS

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·JUNE2015

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Humboldt-UniversitätzuBerlin

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ENVIRONMENTAL IMPACT OF WASTE BASED BIOGAS DIGESTATES

AS INFLUENCED BY POST-TREATMENT METHODS

Ross, C.-L.; Wragge, V.; Nielsen, K.; Sensel-Gunke, K.; Bechstein, F.

Institute of Agricultural and Urban Ecological Projects affiliated to Berlin Humboldt University (IASP)

Philippstraße 13, 10115 Berlin, Germany

[e-mail], [e-mail], [e-mail], [e-mail]-

berlin.de, [e-mail]

ABSTRACT: Biowastes contain valuable plant nutrients. After being processed, they could be used as fertilizers. Yet
therefore among other things their environmental impacts should be known. The impacts of biowaste digestates -
composted after fermentation and processed in three different ways - were tested using two bio-tests with earthworms
and two with plants. None of the tests with earthworms led to negative effects within the tested ranges. The tests with
plants showed a slightly negative response especially of monocotyledonous plants (Avena sativa in the test) towards
the pure compost but not towards the further worked up products. For dicotyledonous plants (here represented by
Brassica rapa) no negative effects were found within the tested ranges. The results of the conducted tests indicate
that bio-wastes from households, if treated in an appropriate way, could be used as fertilizers without negative
impacts to plants and earthworms.
Keywords: organic waste, digestate, environmental impact, earthworms, biomaterial

1 INTRODUCTION

Since January 2015 the separate collection and

recovery of organic wastes from households (“biowaste”)
is a basic statutory obligation in Germany. Biowaste is
particularly suited as substrate for biogas plants, since it
can be digested effectively under both mesophilic and
thermophilic conditions

[9].

Consequently more biogas

plants using organic household wastes as feedstock are
being built. Under the aspect of nutrient recycling an
agricultural use of the resulting digestates as fertilizers on
arable land is desired

[2]

. However so far not enough

knowledge exists about the ecological aspects of
biowaste application. High amounts of impurities like
heavy metals or pieces of plastic and glass could cause
environmental problems and make the products
unpopular for farm use. Two ecotox-tests (one with
earthworms and one with plants) were therefore carried
out to estimate environmental impacts of biowaste
application.

2 MATERIAL AND METHODS

2.1 Production of Digestate Products
The tested digestate products are obtained by
anaerobic digestion, done with a batch high solids
anaerobic digestion process of the company GICON
(Germany) with separated hydrolysis and methanization
utilizing percolation, and subsequent composting of
organic wastes. In a second step the thus obtained solid
digestate is further processed, resulting in three different
products (Fig. 1):

1. “Compost fine”: sieved mechanically (20 mm) and

thus ridded of rough impurities like bigger
plastic particles

2. “Pellet”: Compost fine pressed into Pellets of 20

mm in diameter and differing in length

3. “Agglomerate”: moistened Compost fine formed into

small round agglomerates through treatment
with a mixer of the company Eirich (Germany)

Figure 1: The three tested digestate products, a) Compost
fine, b) Agglomerate, c) Pellet

2.2 Avoidance-Response Tests with Earthworms
Earthworms feed of the organic matter in the soil.
This and the fact that they are in permanent contact with
the soil, its water content and therein dissolved chemicals
as well as their sensitivity against chemicals, makes them
an ideal organism for testing potentially toxic soil
conditions

[4, 8]

. The bio-tests with earthworms were

conducted as an Avoidance-Response Test in accordance
with the International Standard Norm ISO 17512-1
(2008). The composts and two reference fertilizers, cow
manure and calcium ammonium nitrate (CAN), were
tested in two concentrations according to a nitrogen
application of 200 and 400 kg ha

-1

. The tests were

conducted with two-section vessels (1 liter volume) with
Eisenia fetida and lasted 48 hours. In each vessel ten
worms had the choice between an untreated control soil
and soil mixed with compost or reference fertilizer. Each
treatment was tested in four replications. Two tests were
carried out with individual batches of the composted bio-
wastes, which differed only slightly in their composition.
In test 1 the soil was completely untreated, whereas in
test 2 the soil in the control and in the treated variants
was mixed with a small amount of dried lucerne green
meal. The soil was adjusted to 45 % (1

st

test) and 50 %

(2

nd

test) of the water holding capacity. After 48 hours the

number of worms in each section was counted. On the
assumption of a 50:50-distribution the avoidance of the
treated soil by the earthworms was calculated.

An

avoidance of > 80 % of the treated soil would be
evaluated as a risk potential for earthworms [6].

2.3 Phytotoxicity Test with Oat and Turnip rape

Different studies have shown that the composting

process and its duration may have a significant influence
on the tolerance of plants against the composts and their
suitability as

fertilizers

[1, 5].

Therefore the

phytotoxicity of the different products was tested in a pot
experiment with dicotyledonous and monocotyledonous
plant species (Brassica rapa and Avena sativa) following
the International Standard Norm ISO 11269-2 (2012).
The test detects effects on the emergence and early
growth of plants. For each pot 600 g soil were mixed
with one of the three compost-products or calcium
ammonium

nitrate

as

reference

fertilizer

in

concentrations according to 100, 200, 400, 600 and
900 kg N ha

-1

. Untreated soil was used as control.

Ten

seeds were sown and later thinned out to five plants per
pot. Dry matter yield was determined after 20 days. The
test was conducted with four replications per treatment.



3 RESULTS AND DISCUSSION

3.1 Tests with Eartworms
In the tests all three soils treated with compost-
products and also soil mixed with cow manure were
preferred compared to untreated control soil (Fig. 2, 3). A
significant

difference

between

the

two

tested

concentrations was found only for agglomerate in test 2.
An avoidance response of the earthworms was only
detected for the reference substance calcium ammonium
nitrate.

Figure 2: Avoidance of earthworms (E. fetida) in
reaction towards different digestate products and their
concentration; test 1 with completely untreated control
soil; n = 4

The slightly higher preference of the control soil (lower
negative avoidance of the treated soil) in test 2 is
probably due to the added dried lucerne, which gives the
worms an alternative feeding substrate. However, the
general tendency of the worms to prefer the variants with
digestates over the control soil was not affected by this.
These results confirm with those found by Moreira et al.
[7] who also didn´t observe negative effects of composts
to earthworms, but a preference of treated soils.

Figure 3: Avoidance of earthworms (E. fetida) in
reaction towards different digestate products and their
concentration; test 2 with soil treated with dried lucerne;
n = 4; significant differences between low and high
concentration are marked with *, Tukey-Test with α =
0.05, n = 4


3.2 Phytotoxicity Test
Regarding the digestate-products there were no
significant reductions of dry matter yield compared to
untreated control soil for dicotyledonous plants. A slight
negative effect was observed for sieved compost on
monocotyledonous plants (Fig. 4).

Figure 4: Dry matter yield [g per pot] of oat and turnip
rape in dependence of application of different digestate
products in five concentrations compared to untreated
control soil; n = 4; bars of one fertilizer product marked
with the same letters do not differ significantly, Tukey-
Test with α = 0.05, n = 4

In most cases there was no clear dose response either.
Only the reference fertilizer calcium ammonium nitrate
lead to heavy responses with significant yield reductions
at very high (and for field application unusual) dosages.
Since composted digestates are slow releasing sources of
nutrients tests with longer duration are needed to fully
assess their fertilizing qualities or e.g. possible hormesis-
effects [3].

In contrast to the sieved compost the two further
processed digestates had no negative influence towards
the apparently slightly more sensitive monocotyledonous
plants. This could be an indicator for growth inhibiting
chemical components, which get released easier from the
pure compost than from the pressed or agglomerated
products with lower surface area and higher density.
Further investigation in this area is recommended.


4 CONCLUSIONS

Within the conducted bio-tests also very high
application rates of the tested biowaste digestates did not
lead to significantly negative impacts to earthworms or
plants. Only the reference fertilizer calcium ammonium
nitrate, which is a high potential nitrogen fertilizer, lead
to negative impacts at highest application rates. The
composted bio-wastes can therefore be considered as an
environmentally save long term available nutrient source.
Further tests should assess the fertilizing values of the
products as well as their impacts to other soil organisms
and the potential release of harmful chemical compounds.


5 REFERENCES


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HALLETT,

P.D.,

WHEATLEY,

R.

(2008):

Investigating the effects of anaerobic and aerobic
post-treatment on quality and stability of organic
fraction of municipal solid waste as soil amendment.
Bioresource Technology, 99 (18) 8631-8636.

[2] BMELV (2012): Das Erneuerbare-Energien-Gesetz.

Daten und Fakten zur Biomasse – Die Novelle 2012.
Bundesministerium für Ernährung, Landwirtschaft
und Verbraucherschutz.

[3] Erhart E., Hartl W., Putz B. (2005): Biowaste

compost affects yield, nitrogen supply during the
vegetation period and crop quality of agricultural
crops. European Journal of Agronomy 23, 305-314.

[4] FRÜND, H.-C.; BUTT, K.; CAPOWIEZ, Y.;

EISENHAUER, N.; EMMERLIN, C.; ERNST, G.;
POTTHOFF, M.; SCHÄDLER, M.; SCHRADER, S.
(2009): Using earthworms as model organisms in the
laboratory: Recommendations for experimental
implementations. Pedobiologia 53 (2010), 119–125.

[5] FUCHS, J., BAIER U., BERNER A., MAYER J.,

TAMM L., SCHLEISS, K., (2008). Potential of
different composts to improve soil fertility and plant
health. Paper presented at the International workshop
“The future for anaerobic digestion of organic waste
in Europe”, 16th -17th January 2008, Nuremberg,
Germany, 11 pp. (online publication).

[6] HUND-RINKE K., WIECHERING H. (2001):

Earthworm Avoidance Test for Soil Assessments.
Journal of Soils and Sediments 1(1), 15-20.

[7] MOREIRA R., SOUSA J. P., CANHOTO C. (2008):

Biological testing of a digested sewage sludge and
derived composts. Bioresource Technology 99, 8382-
8389.

[8] RATHKENS, K.; V.D.TRENCK, K.T. (2006):

Schwermetalle

in

Regenwürmern

Baden-

Württembergs. Dokumentation zum Ergebnisbericht.
LUBW Landesanstalt für Umwelt, Messungen und
Naturschutz Baden-Württemberg.

[9] ZHANG, C.; SU, H.; BAEYENS, J.; TAN, T. (2014):

Reviewing the anaerobic digestion of food waste for
biogas production. Renewable and Sustainable
Energy Reviews 38 (2014), 383–392.