“Factors affecting quality of compost produced from
agricultural waste: assessment of risk”
Kostas Komnitsas, Dimitra Zaharaki, Despina Vamvuka
Dpt. Mineral Resources Engineering, Technical University Crete,
Chania, Crete, Greece

Selected parameters such as optimal conditions for
successful composting, application rates of compost on
soil and limit values for concentrations of heavy metals in
soil, are defined

The risk for humans exposed to various recalcitrant
compounds contained in treated or untreated agricultural
waste (AW) can be assessed
2

The most important AW produced in the Mediterranean region
include olive mill and winery wastewater, animal waste, rice
and wheat straw, as well as wastes from fruit and vegetable
processing

AW are characterized by seasonal production and exhibit often
a substantial contamination potential

Uncontrolled disposal of untreated AW on soil or water bodies
can cause various adverse effects such as decreased soil
permeability, inhibition of crop growth, eutrophication of
water bodies and contamination of drinking water sources
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
Olive mill wastewater (OMW) is characterized by high organic
load (BOD5: 20-120 g/L; COD: 25-240 g/L), high content of
phenolic compounds (0.5-24 g/L) and rather low pH (4-6)

Wine production generates large volumes of solid wastes
(grape stalks, grape marc and wine lees) and wastewater (pH
3.5-7, 4-7 g/L BOD, up to 10 g/L COD, salinity 3-4 mS/cm, high
content in sulphide compounds)

Animal wastes include mainly manures, poultry and
slaughterhouse waste

Other AW include rice straw, garden, and agroindustrial wastes
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
Composting is a process of biodegradable materials
decomposition under controlled conditions and may be carried
out either aerobically or anaerobically

Compost can replace fertilizer in many applications such as in
commercial greenhouses, farms and land remediation
contributing also to fertilizer cost reduction

Application of compost on soil improves soil fertility and
physical properties, increases soil organic matter and nutrients
content and enhances crop yield
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
The main factors that need to be taken into consideration
during composting of AW, include:
the chemical composition of the waste
 the porosity of the pile
 the population of organisms involved
 pH (optimum 6.5-8) and electrical conductivity (optimum <8
mS/cm)
 C:N ratio (optimum 25:1 to 35:1)
 moisture content (optimum 45-60 % w/w)
 other parameters (temperature, cation exchange capacity, organic
matter, N, P, K content etc.)

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
The following Table shows the average, high and maximum application
rates of fertilizers, in kg/ha, used for the calculation of metal addition
to soil to improve the growth of crops, vegetables and fruits; it is
assumed that all fertilizers are applied annually.
Fertilizer type (active
ingredient)
Phosphate (P2O5)
NPK applied for phosphorus (P)
NPK applied for nitrogen (N)
Potash (K2O)
Sulfur (nutrient)
Sulfur (pH)
Lime (CaCO3)
Gypsum
Iron
Boron
Manganese
Zinc
Micronutrient mixes
50th percentile (average),
kg/ha
94
85th percentile
(high), kg/ha
194
95th percentile
(maximum), kg/ha
282
94
194
282
139
115
22
897
4483
2242
11
2
4
6
8
231
198
45
2242
897
4483
22
3
11
11
not available
464
599
67
2802
16812
8967
34
4
20
22
not available
8

According to the Directive 86/278/EEC (amended by the Directive
91/692/EEC) aiming to prevent adverse effects on soil, vegetation, animals
and humans, limit values for concentrations of heavy metals in soil, sludge
as well as for the maximum annual quantities of heavy metals applied on
soil, have been defined
Parameter
Cd
Cu
Ni
Pb
Zn
Hg
Cr
Limit values for
Limit values for
concentrations of heavy
concentrations of heavy
metals in sludge for
metals in soil (mg/kg of dry
agricultural use (mg/kg of
matter of soil, pH 6-7)
dry matter)
1-3
50-140
30-75
50-300
150-300
1-1.5
not defined
20-40
1000-1750
300-400
750-1200
2500-4000
16-25
not defined
Limit values for the maximum
quantities of heavy metals
added in agricultural land,
based on a 10-year average
(kg/ha/y)
0.15
12
3
15
30
0.1
not defined
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
The general framework of the ecological risk assessment
involves three major phases: I) problem identification, II)
analysis and III) risk characterization

Other parameters to be taken into consideration include the
fate of contaminants in soil, the geographical characteristics of
the implementation area i.e. agricultural land use, climate data,
characteristics of receptors i.e. body weight and inhalation rate
for humans, as well as exposure routes and duration
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
Recalcitrant compounds such as metals present in AW, may
pose substantial risk to humans, mainly farmers, as well as
ecosystems in case of excessive compost application on soil

The relevant exposure routes for humans are:
 direct ingestion of the compost during application
 ingestion of amended soil
 inhalation of particles and vapors present in the air during
and after compost application
 ingestion of plant, vegetables and fruits produced on
amended soil as well as grazing animals
 ingestion of fish from streams adjacent to amended fields
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
Toxicity values or health benchmarks include: reference doses
(RfDs), reference concentrations (RfCs), cancer slope factors
(CSFs) and unit risk factors (URFs)

RfDs and RfCs, are used to evaluate non-cancer effects for
ingestion or inhalation exposure, respectively

CSFs and URFs are used to evaluate cancer effects for ingestion
or inhalation exposure, respectively

Health benchmark values for metals present in fertilizers were
proposed by U.S. EPA and are presented in the following Table
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RfD
(mg/kg/d)
RfC
(mg/m3)
CSF oral
(mg/kg/d)-1
Inhalation URF
(μg/m3)-1
As
0.0003
NA
1.5
0.0043
Cd
0.001
NA
NA
0.0018
Cr
0.003
0.0001
NA
0.012
Cu
NA
NA
NA
NA
Pb
NA
NA
NA
NA
0.0001
0.0003
NA
NA
Ni
0.02
NA
NA
0.00024
V
0.007
NA
NA
NA
0.3
NA
NA
NA
Metal
Hg*
Zn
NA: not available
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
The careful and according to guidelines application of compost produced
from treated or untreated AW, as soil amendment, improves physical
properties of soil, enhances crop growth and contributes to minimization
of risk for soil, water, ecosystems and human health

The main factors that affect the quality of compost include composition of
waste, pH, EC, temperature, moisture content, content of organic matter,
N, P, K and potentially hazardous elements

Careful compost application rates, compliance with limit values for
concentrations of heavy metals in soil and determination of the fate of
contaminants in soil, water, plant and animal tissue are required in order to
safeguard its use, maximize benefits, eliminate adverse effects and thus
improve the overall sustainability of the agricultural sector
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Technical University of Crete
Dpt Mineral Resources Engineering
Research Unit “Management of mining/metallurgical wastes
and rehabilitation of contaminated soils”
www.mred.tuc.gr/p013215_UK.htm
Ackowledgement
Financial support of LIFE+ Environment Policy and Governance in
the framework of the LIFE10 ENV/GR/594 WasteReuse project
“Best practices for Agricultural Wastes (AW) treatment and
reuse in the Mediterranean countries (WasteReuse)”, duration
01/09/11 - 31/08/15, www.wastereuse.eu
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