June 21, 2010 | General

Composting Olive Mill Pomace: The Andalusian Experience (Spain)

BioCycle June 2010, Vol. 51, No. 6, p. 31
Subsidies, need for alternatives to manage the processing by-product and benefits from compost utilization have led to a significant increase in composting operations in the Andalusian region.
J.M. Álvarez de la Puente , J. Jáuregui Arana and R. Garcia-Ruiz

INTEREST has grown in promoting carbon sequestration in soils to help mitigate increasing carbon dioxide levels in the atmosphere. The potential for C sequestration in croplands in the European Union (EU) ranges between 90 and 120 million tons/ year, in the case that all known best land management practices were implemented.
For the Andalusian region of Spain, the Action Plan for Climate 2007-2012 provides the following measures to mitigate Greenhouse Gas emissions: 1) Utilization and treatment of by-products and waste from the agriculture, forestry and aquaculture and fisheries sectors, particularly from agroindustries and livestock farming, via composting and, secondarily, valorization of by-products for energy production; 2) Promoting construction of composting infrastructure in proximity to agricultural and fisheries processing plants; and 3) Increasing the amount of soil organic C storage by promoting use of compost. This article describes the experience of olive growers and researchers in the Andalusian region working together to mitigate agriculture CO2 emissions through the adoption of environmental friendly management practices.

Built-In Feedstocks
Olive mill pomace is the main by-product (approximately 70 percent of the olive crop by weight) after the two-phase extraction procedure to produce olive oil. One to 2.5 million tons of olive mill pomace are produced in Andalusia annually during the three to four months of the olive season. This accounts for around 5 to 30 percent of all agricultural organic matter produced in the country of Spain and Andalusia, respectively, including manure and the by-products of other agricultural and fisheries industries. This source of organic matter is particularly important for Andalusia, as Mediterranean soils are characterized by relatively low organic matter content (~1%).
Until recently, disposal of olive mill pomace was an environmental problem due to its relatively high polyphenol content and biological oxygen demand, together with some degree of phytotoxicity. The most common management solution is storage in evaporative ponds located near the processing facilities. However, during the past decade the regional authorities have promoted technical innovations addressed to add value to this by-product.
The main uses for olive mill pomace include: Recovery of residual olive oil through chemical extraction of the pomace. However, its high water content (50-70%) coupled with the transport cost to extraction plants may not make economic sense (depending on required travel); Use for energy production in biomass burning plants, taking advantage of its high organic C content and thus high calorific power, with zero net CO2 emissions; and Utilization of the nutrients and organic matter.
Olive mill pomace contains approximately 70 percent of the total harvested nutrients (nitrogen, phosphorus and potassium), and has high levels of organic matter (between 30-50% dry weight) and organic carbon (up to 25%). Therefore, after composting and application to the fields, olive mill pomace is a source of nutrients, organic matter and carbon, thus reducing the chemical fertilizer bill while promoting the long-term fertility of the soil. In addition, composting olive mill pomace to increase soil fertility on arable land meets the EU agricultural policy to encourage sustainable management practices through coupling of environmental and scocioeconomic criteria. But to fully realize the value of using olive mill pomace by composting, political and technological hurdles must be overcome.

Government Support

The regional authorities (Secretariat General of Rural Affairs and Organic Production of the Regional Ministry of Agriculture and Fisheries) recognize the need for a source of organic matter to promote organic olive oil groves, the huge amount of olive mill pomace produced annually in Andalusia, and the EU’s promotion in recent years to reuse olive mill pomace after composting. The main strategies have been aimed at:
1) Explicit incorporation of the “promotion of composting plants nearby the olive oil transformation plants” in the II Andalusian Organic Agriculture Plan (2007-2013). During the past two years, 11 olive mill pomace composting plants have been financed by the Secretariat General of Rural Affairs and Organic Production of the Regional Ministry of Agriculture and Fisheries (through the European Agricultural Fund for Rural Development (EAFRD), which provides funding to support the EU’s sustainable agricultural policies).
2) Promotion of scientific-technical research for optimizing the composting procedure, assessing the economic viability of composting plants, characterizing the agrochemical features of the olive mill pomace compost and assessing the environmental benefits linked to the field application of olive mill pomace compost.
3) Implementation of an information dissemination strategy addressed to the olive tree farmers and olive oil transformation industries.
4) Development of a platform for the exchange of ideas and information.
Agroindustries applying for EU agricultural subsidies must meet certain environmental control parameters, including mitigation of surface water and aquifer contamination, gas emissions, waste production and protection of sensitive lands. Until now, this subsidy has been mainly directed to mills for olive pomace composting, but following the 2009 RFP, the aid has been extended to any agroindustry making compost from its various by-products.
Olive pomace compost is meeting the necessary Type A organic amendments requirements stated on the Spanish 824 2005 fertilizers law, making its use as a soil amendment possible without the dose or frequency limitations typically attached to municipal compost. However, like all fertilizers, it carries the limitation of less than 170 N units/hectare/year in Andalusian areas vulnerable to nitrates.

Composting Operations
Thanks to the administration initiatives thus described, the olive oil sector (olive farmers and the value-added industry) is now aware of the potential environmental and socioeconomic benefits of composting olive mill pomace and has begun implementation in that direction. Those olive-oil production plants with few possibilities for other alternatives have been taking the lead. As a result, the amount of olive mill pomace compost produced in 2009 (46,000 tons) was 30 times higher than in 2002 (1,500 tons/year). (In 2007, it was 5,350 tpy and in 2008, 18,765 tpy.)
The olive mill pomace composting plants being built are typically located in the vicinity of oil production plants. Operators create an optimal mixture of olive mill pomace and structural raw material (generally other by-products such as olive leaves, olive branch clippings and soil remains, straw and manure). The facilities utilize a low cost windrow system on a concrete hard surface with dump loader (using equipment and infrastructure already available), turning once per month during the four to seven months of mesophilic and thermophilic phases and two to three during the curing phase. The whole process takes 6 to 10 months and is fully integrated into the olive oil growing and processing season.
Most of the olive mill pomace compost produced is distributed to olive oil farmers as an alternative to chemical fertilizer. Compost is typically applied to the soil surface in between olive trees rows.
Many benefits of the regular application of olive pomace compost are directly related to the soil enrichment through its high organic matter content (30-50% dry weight). Benefits include increases in soil cation exchange capacity, soil water-holding capacity and stability of soil aggregates, reducing the potential for soil loss by erosion. In addition to being a nutrient source (especially potassium), the high carbon content of the olive mill pomace compost provides energy and the “skeleton” for the proper development of soil microorganisms.
Other environmental benefits include the potential to increase soil carbon sequestration and the significant reduction of N losses to leaching. Finally, in terms of N-P-K, the production cost of olive mill pomace compost is not only significantly lower that the cost of chemical fertilization, the cost is also more fixed and not directly linked to the petroleum industry.
The initiative of the organic olive oil sector with regard to composting olive pomace has generated the support of the regional authorities and led to an established working group (Andalusia Working Group for Organic Production Composting) to exchange experiences and build on knowledge. Also, financial assistance by subsidies is provided until 2013 for the construction of composting plants in this region.

J.M. Álvarez de la Puente, Scientific Advisor (, and J. Jáuregui Arana ( are with SGMRPE, Consj. Agr. & Fish. R. Garcia-Ruiz ( is with the Ecology section of Jaén Univ., Spain.

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