EU legislation and framework – Proposed indicators for OMW application on soils

This section is included in the paper: “Komnitsas K. and Zaharaki, D. 2012. Pre-treatment of olive mill wastewaters at laboratory and mill scale and subsequent use in agriculture: Legislative framework and proposed soil quality indicators, Resour Conserv Recy 69, 82– 89”.

Application of OMW in high rates, even after treatment may severely affect soil properties, plant growth and water quality. Apart from pre-treatment, which due to the scattering of olive mills in all Mediterranean countries should be better considered at mill scale, other important factors such as optimum OMW application rates, frequency of application, soil type, type of cultivated plants and depth of aquifer should be taken into account prior to application. Specific care should be taken if OMW are applied on sandy soils or sensitive water bodies. OMW are highly toxic to microorganisms as well as to invertebrates (e.g. D. magna, D. longispina, B. calyciflorus), bacteria (e.g. V. fischeri), plants and algae affecting thus the sustainability of receiving systems. Polyphenols, present in OMW in high concentration, may affect enzymatic activity and fish reproduction via inhibition of neurotransmitter receptors and enzymes, as well as may lead to membrane dysfunction and cause necrotic cell death or neuronal degenerative diseases. It has to be mentioned though that no thresholds exist at EU level to assess phytotoxicity of phenols (Mekki et al., 2008; Basu et al., 2009; Justino et al., 2012). The application of a battery of biomarkers to assess the cytotoxic and genotoxic effects of OMW on aquatic invertebrate species (Mytillus galloprovinciallis) and aquatic animal species, respectively, when exposed to 0.1 and 0.01% v/v diluted OMW is strongly recommended (Danellakis et al. 2011). However, no ecotoxicological data are available for the evaluation of toxicity of OMW on edaphic invertebrate species after soil irrigation with OMW.

One of the most important environmental issues associated with olive cultivation is impact to soils and waters. Intensified olive farming results in serious soil erosion, thus reducing the productive capacity of the groves and potentially causing other impacts such as desertification and run-off of top-soil into water courses. The main aspect of the Soil Thematic Strategy was the proposal, by the European Commission, for a Soil Framework Directive (COM (2006) 232). However, Member States have so far been unable to agree on the Soil Framework Directive, as well as to systematically identify damaged soils and prevent/combat soil erosion, salinity, OM decline, compaction and landslides. Regarding water usage and quality, it is known that olive production does not require such high inputs of water as arable crops or crops such as lettuce or tomatoes, but expansion and intensification of olive production has caused water shortages in several Mediterranean regions. The Water Framework Directive (2000/60/EC) deals with water management in the broad sense, with the aim of achieving by 2015 an appropriate ecological and chemical status for surface waters, as well as an acceptable chemical and quantitative status for groundwater. The Groundwater Directive (2006/118/EC), which is a daughter directive of the Water Framework Directive, sets 50 mg/L as acceptable level of nitrate. The earlier Nitrates Directive (91/676/EEC), defines the general objective of protecting EU waters against excessive nitrates from agricultural sources, and plays an important role in olive grove cultivation. It is known that in extreme cases the nitrogen inputs in the most intensive, irrigated olive farming can reach levels as high as 350 kg per hectare (European Commission, 2010).

The Waste Framework Directive (2008/98/EC) includes rules on hazardous waste and waste oils and requires Member States to recycle at least half of their household and general waste by 2020. According to the hierarchy, waste should be dealt with first by prevention, then re-use, recycling, recovery and finally, disposal. During recovery, waste is either converted into usable forms or is incinerated so that energy is recovered. Disposal, meaning landfilling in most cases, as outlined in the Landfill Directive (99/31/EC), can only be done once the previous four steps have been exhausted. Liquid wastes from olive oil production fall under the Urban Waste Water Treatment Directive (91/271/EEC). This involves collection, treatment and discharge of urban wastewater and treatment and discharge of wastewater from certain industrial sectors, including olive oil production. Furthermore, the application of techniques to increase productivity of olive groves may cause in certain cases detrimental effects on wildlife. The EU’s key biodiversity policy instruments related with these issues are the Birds Directive (79/409/ EEC) and the Habitats Directive (92/43/ EEC).

Pre-treatment of OMW, careful application on soils, use of standardized procedures to evaluate phytotoxicity, determination of the fate of contaminants in soil and water, and finally the adoption of a Soil Framework Directive will definitely maximize OMW reuse in agriculture and minimize impacts on ecosystems. So far, information available regarding OMW application to soils is piecemeal, insufficient, and contradictory. Thus, in order to assess the long term effects of OMW application on soils appropriate soil indicators should be agreed between authorities and all involved stakeholders. These indicators should be frequently monitored in order to prevent any potential adverse effects. For this, the following issues should be taken into consideration:

  • Most olive oil producing countries in the Med region suffer from soil degradation and thus application of OMW as agricultural amendment may be considered as a suitable approach to solve disposal problems and in parallel restore soil fertility
  • There is no sufficient information regarding the long term effects of OMW application on agriculture and results derived from studies carried out so far are contradictory, mainly due to the different soil types considered and the variability of OMW quality
  • Positive effects of OMW application may include improvement of certain soil properties (e.g organic matter content, enrichment in nutrients, fertility and productivity), increase in aggregate stability, reduction of soil porosity and reduced leaching of pesticides
  • Negative effects may include modification of pH, soil structural stability, acceleration of K and P leaching, mobilization of certain heavy metals, induction of anaerobic conditions, release of phytotoxic compounds including phenols, toxicity towards nitrifying bacteria and increase of salinity increasing thus the risk of human toxicity and groundwater contamination.

By considering all these abovementioned issues the following soil indicators are proposed: soil pH (more applicable if soils are already acidic), electrical conductivity, content of organic matter, total polyphenols, total nitrogen, available phosphorous and exchangeable potassium. It is mentioned though that the effect of each indicator as well as their interactions depend mainly on the soil type, the climatic conditions and the type of plant.

More specifically, soil pH is also a reliable indicator of Al toxicity, which is in general the most limiting factor for crop production in acid soils (pH<5.5). In alkaline soils, the solubility of Al and Mn compounds is limited and thus toxicity to plants is substantially reduced (Norton et al., 1999). Soils with pH>7.0 and high content in CaCO3 have the ability to buffer OMW acidity and thus, no long term significant effect on soil pH is anticipated. OM is a critical soil property that is involved in all soil functions and affects physical, chemical and biological processes in soils. In addition, due to its high water holding capacity it may adsorb micronutrients after OMW disposal and increase soil fertility while at the same time it reduces its toxicity.

Regarding polyphenols, for which the determination of their content in soils is considered difficult and often entails a high degree of uncertainty, it is recommended that site specific thresholds are defined and used (Kavvadias et al., 2010). Several studies have reported noticeable increase in the content of phenolic compounds in soils immediately or some months after OMW application. However, it has been also reported that a healthy soil is capable to reduce the concentration of phenols to a certain extent through natural biodegradation processes (Chartzoulakis et al., 2010).

Nitrogen is the most important micro-element in soil for plant development. On the other hand excess of nitrogen, caused by fertilization or waste disposal, may result in water eutrophication or cause health problems to humans. The content of nitrates should be also frequently monitored to minimize the risk for groundwater contamination. P in soils exists in inorganic and organic form and is also considered as essential element for plant growth. However, P can be rapidly fixed in forms unavailable to plants, depending on soil pH and type as well as on the content of Al, Fe, and Ca. It is very important though to establish the relation between total and plant available P, and thus specify fertilization needs in order to maximize yield (Watson and Mullen, 2007).

Regarding potassium, it is mentioned that only a small percentage, not exceeding 1% of the total K in soils is exchangeable. Exchangeable K, which is considered the primary source of K for plant uptake, ranges from less than 100 to 2,000 mg/kg, while total K values are normally in the order of 1 to 2% w/w. In highly weathered soils or soils where the parent material contains only some K-bearing minerals, the exchangeable K can be depleted by plant removal and is replenished only by the fertilizer provided after waste application or return of K from plant residues.

Finally modeling studies may be considered to establish the rates of contaminants transport towards deeper soil horizons, as well as contaminant degradation, adsorption and transformation and thus predict the risk for all associated ecosystems.


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