“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

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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

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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)

50th percentile (average), kg/ha

85th percentile (high), kg/ha

95th percentile (maximum), kg/ha

Phosphate (P2O5) 94 194 282NPK applied for phosphorus (P) 94 194 282

NPK applied for nitrogen (N) 139 231 464Potash (K2O) 115 198 599Sulfur (nutrient) 22 45 67Sulfur (pH) 897 2242 2802Lime (CaCO3) 4483 897 16812Gypsum 2242 4483 8967Iron 11 22 34Boron 2 3 4Manganese 4 11 20Zinc 6 11 22Micronutrient mixes 8 not available not available

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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

Limit values for concentrations of heavy

metals in soil (mg/kg of dry matter of soil, pH 6-7)

Limit values for concentrations of heavy

metals in sludge for agricultural use (mg/kg of

dry matter)

Limit values for the maximum quantities of heavy metals added in agricultural land,

based on a 10-year average (kg/ha/y)

Cd 1-3 20-40 0.15Cu 50-140 1000-1750 12Ni 30-75 300-400 3Pb 50-300 750-1200 15Zn 150-300 2500-4000 30Hg 1-1.5 16-25 0.1Cr not defined not defined 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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Metal RfD (mg/kg/d)

RfC (mg/m3)

CSF oral (mg/kg/d)-1

Inhalation URF (μg/m3)-1

As 0.0003 NA 1.5 0.0043Cd 0.001 NA NA 0.0018Cr 0.003 0.0001 NA 0.012Cu NA NA NA NAPb NA NA NA NAHg* 0.0001 0.0003 NA NANi 0.02 NA NA 0.00024V 0.007 NA NA NAZn 0.3 NA NA NA

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 CreteDpt Mineral Resources EngineeringResearch Unit “Management of mining/metallurgical wastes and rehabilitation of contaminated soils”www.mred.tuc.gr/p013215_UK.htm

AckowledgementFinancial 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