Treatment Methods

Wastewater Treatment

Application of untreated OOMW on soils and crops as a fertiliser is a common practice, due to the organic matter and nutrients contained that could improve arid soils. However, the disposal problem of OOMW is not completely solved since the effluents produced cause adverse impacts to aquatic life, change of the colour of natural waters, toxicity and odours. A balanced disposal method requires controlled land application of properly characterised wastewaters. Another important issue is that pre-treatment of OOMW should improve its quality as well as remove most of its toxicity (Saviozzi et al. 2001; López-Piñeiro et al., 2007; Saadi et al., 2007). Several treatment options have been considered including physical, physicochemical, biological, thermal technologies and combinations thereof, as well as other combined approaches that could improve decontamination efficiency (Zaharaki and Komnitsas, 2009).

- Physical treatment, which involves the separation of different phases through mechanical means. When used, so far, for the treatment of OOMW are unable to reduce alone the organic load and toxicity of the wastes to acceptable limits.

- Physico-chemical treatment, which involves use of additional chemicals for their neutralization, flocculation, precipitation, adsorption, chemical oxidation and ion exchange.

- Biological treatment, employing the use of microorganisms to break down biodegradable chemical species present in OOMW, is considered environmentally friendly and cost effective. The actual type of microorganisms depends on the treatment option, i.e. anaerobic or aerobic.

- Thermal treatment involves the concentration of OOMW by reducing their water content and total volume. Three main treatment options are used:

  1. physico-thermal (evaporation - distillation of OOMW and drying of olive cake),
  2. irreversible thermo-chemical (combustion and pyrolysis), which require expensive facilities and entail possible emission of toxic substances into the atmosphere and
  3. combined physical and biological, such as lagooning where the sun’s energy accelerates evaporation and drying of OOMW followed by partial degradation over long time periods.

- Combined treatment includes the pre-treatment before the application of the selected process for the effective management of OOMW, which can be hardly achieved by the adoption of a single process.

- Other emerging treatment options. Integrated systems employing treatment of OOM effluents with sand filters (Achak et al., 2009), co-composting of olive mill sludge with poultry manure or sesame bark (Hachicha et al., 2009a; 2009b) or mixing them with calcareous soil and incubation under aerobic conditions (De la Fuente et al., 2008) are able to achieve a very high degree of treatment.

Summary of Wastes Management Methods as described in Niaounakis, M. nad Halvadakis, C.P. Olive Processing Waste Management, Volume 5, 2nd Edition: Literature Review and Patent Survey, 2006

Establishment of indicators for the evaluation of OMW treatment technologies

This section is included in the paper: “K. Komnitsas and D. Zaharaki, Establishment of weighted indicators for the evaluation of agricultural waste treatment technologies, 4th International Conference of Hellenic Solid Waste Management Association (HSWMA) on Solid Waste Management in Crisis: New Challenges and Perspectives (, 30 November - 1 December 2012, Athens, Greece, p. 629-636”.

According to EPA (1996), indicators should present information, measure pressures or stressors that degrade environmental quality and evaluate society's response at improving environmental conditions. Indicators can be used to manage information in a simple and clear way so that future actions, such as those related to OMW treatment and reuse, can be critically assessed and communicated to decision makers and relevant stakeholders. When indicators are established and the most appropriate are selected, decision makers can assess progress towards the achievements of a treatment technology (Azapagic et al., 2003; Arendse and Godfrey, 2010; Hak et al., 2012).

In any case, appropriate indicators should comply with the following general criteria (OECD, 2001):

  • be simple, easy to interpret and able to show trends over time
  • be responsive to changes in the environment and related human activities
  • provide a representative view of environmental/technical/economic conditions and pressures
  • provide a basis for international comparisons
  • be theoretically well founded in technical and scientific terms
  • be based on international standards
  • provide a threshold or reference value so that users can assess their significance

In order to assess the efficiency of OMW treatment technologies, established indicators can be grouped as technical, environmental, economic and socio-cultural.

Technical indicators

  • By-products production
    •  Compost (kg/m3 waste)
    • Treated liquid waste for irrigation (m3/m3 waste)
    • Energy (thermal kJ/m3 waste, electrical kWh/m3 waste)
    • Purified polyphenols (g/m3 waste)
  • Co-utilization of OMW with other agricultural or industrial wastes
  • Ease of application of the treatment technology
  • Transferability/flexibility of the treatment technology
  • Long-term sustainability

Environmental indicators

  • Decrease of soil/water/air contamination, %
  • Decrease of phytotoxicity, %
  • Decrease of ecotoxicity (freshwater, terrestrial, marine), kg 1,4-dichlorobenzene eq (DBeq)/year
  • Decrease of human toxicity, kg 1,4-dichlorobenzene eq (DBeq)/year
  • Improvement of biodiversity, %
  • Reduction of Global Warming Potential (GWP), kg CO2eq/year
  • Reduction of eutrophication potential, kg PO4eq/year
  • Reduction of acidification potential, kg SO2eq/year
  • Decrease of ozon layer depletion, kg chlorofluorohydrocarbons eq, CFC11eq/year
  • Energy savings, %
  • Reduction of water consumption, m3/year
  • Increase of cultivated land, ha 
  • Increase of crop yield, kg/ha
  • Reduction of photochemical smog, kg C2H4eq /year
  • Reduction of radioactivity, Bq/m3 waste
  • Reduction of odours, m3 of affected air
  • Reduction of noise, dB

Economic indicators

  • Capital investment cost, €
  • Operating cost, €/m3 treated waste
  • Payback period, y
  • Direct revenues, €/year
  • Indirect revenues, €/year
  • Contribution to sectoral growth and Gross Domestic Product (GDP), %

Socio-cultural indicators

  • Compliance with environmental legislation, %
  • Public acceptance of treatment technology, %
  • Public behaviour, %
  • Compatibility with institutional requirements, %
  • Employment growth and development, %
  • Required expertise of personnel
  • Socio-economic risk
  • Modernization level
  • Equity concern

Treatment technologies for OMW

So far, many projects aiming at the development of OMW treatment technologies have been funded within European Funding schemes and especially LIFE, as seen in Table 1. A total of 20 funded projects have been identified through searching of all relevant and available databases (LIFE, Sciencedirect, Scopus, Cordis, Google etc.) by Technical University Crete. Technologies are grouped by level of development and coordinating country; information regarding duration, funding scheme, budget, beneficiaries as well as a short description of the developed technology are also given. More details can be found on the websites of the projects, where available.

All of the projects have focused on the development of innovative technologies for OMW treatment as well as on the recovery of useful by-products and energy, minimization of the environmental impacts and production of treated wastes for safe disposal. Apart from European research/scientific communities, some technologies to treat OMW have been developed by private funding, aiming at improving quality of the final products, minimizing waste volume and thus environmental degradation caused by their disposal (


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