BioCycle February 2005, Vol. 46, No. 2, p. 64
Problems with managing residuals lead to a composting plant and successful application of finished material.
Maria L. Cayuela, Miguel A. Sánchez-Monedero, Juan Molina and Asunción Roig

OLIVE orchards are emblematic crops in Mediterranean countries. They represent their most traditional agricultural activity and the most typical feature of their farming landscape. Thus, olive growing has cultural, economical and ecological importance in this region.
The main use of olive fruits is the extraction of olive oil, the principal source of nutritional fats in Mediterranean cuisine. The European Community produces 75 percent of the olive oil in the world. Spain is the main producer followed by Italy, Tunisia, Turkey and Greece.
For centuries, the extraction of oil was made by pressing the olives with big conical stones but in the last decades this traditional method has been replaced by centrifugation systems. There are two centrifugation technologies, called three-phase and two-phase systems depending on the fractions obtained. Thus, the three phase system generates oil, waste water and husk, while the two-phase system produces olive oil and a semisolid by-product called two-phase olive mill waste (TPOMW).
In Spain, the new two-phase centrifugation system has recently replaced the older and more problematic three-phase technique. The production of this new residue (TPOMW) in great quantities has led to serious management problems due to its peculiar consistency and composition. Some experts have recommended cocomposting of TPOMW with other agricultural by-products but there is limited information regarding management strategies or field experiments showing successful use of this residue as fertilizer.
In our field experiment, TPOMW was recycled by composting with goat manure and grape stalks on an organic farm in Murcia, South Spain. The end product was applied as organic fertilizer in the olive orchard, aiming at the sustainable use of resources.
Main activities at the farm in Murcia are agriculture (olive orchard, vineyards, other crops and fallow land), livestock (goats) and processing activities such as production of olive oil and wine. These activities produce a wide range of organic residuals including TPOMW, goat manure (GM), grape stalks (GS), pruning wastes, etc. TPOMW can cause the largest environmental impact due to its phytotoxicity and physical properties. A composting plant was designed to transform TPOMW, and other organic wastes generated on site, into a valuable compost to be used as soil amendment on the organic farm.
Composting Pile Ratios
Six composting piles were built with different proportions of TPOMW, GM and GS. The proper proportions of materials were calculated considering the C/N ratio of the starting materials to prepare mixtures with an initial C/N around 30. Also, the texture of the mixture was considered to ensure correct aeration. No shredding of raw wastes was necessary since the original particle size of GM and GS was suitable for the composting process. Furthermore, no initial addition of water was necessary since the original moisture of TPOMW is quite high, usually around 65 percent.
TPOMW management is a problem because of its semisolid consistency that makes it difficult to handle during windrow preparation. In order to standardize the process and make handling easier, two low cost and simple methodologies were developed for successful mixing with bulking agents such as green waste; direct mixing was done with a front-loader tractor or a semi-automatic mixing system by using a mixer truck.
For the direct mixing system, the windrows were built following these steps: After weighing the appropriate amount of GM/GS, the bulking agent was spread on the composting pad forming a volcano shape. After that, TPOMW was collected in a truck from the storage tank and put into the GM/GS base. Finally, the wastes were mixed thoroughly with a front-loader tractor, forming a trapezoidal windrow 18 m (length), 3 m (width) and 1.5 m (height).
For the semiautomatic alternative, the farm should be equipped with a mixer truck. In this case, windrows were prepared as follows: TPOMW was collected in the mixer drum directly from the storage tank. The quantity of bulking agents was calculated, weighed on a scale and placed into a storage bin. Bulking agents were inserted into the mixer drum by a conveyor belt. After that, the drum was activated and the residues homogeneously mixed for a period of about 15 minutes and then placed in the composting pad forming trapezoidal windrows.
The peculiar characteristics of the TPOMW also determine the composting system to be used. The most suitable aeration method is by windrowing and occasional turning. Forced aeration has been previously tested for TPOMW but it presented several drawbacks because of TPOMW physical consistency. Thus, when forced aeration was applied, this material tended to get dry forming aggregates that slow down the microbial activity and, consequently, paralyzing the composting process. The proposed method consisted of turning by a front-loader tractor, twice during the first month and then monthly until the end of the process.
Process Evolution And Compost Characteristics
All the piles had a common pattern that corresponded to a typical composting process as shown by the evolution of the main monitoring parameters (T, pH, moisture, organic matter, etc.). The main characteristic is that all piles had a long duration of the thermopile phase that is characteristic of this kind of material due to its lignocellulosic nature and high lipid concentration.
The final composts, regardless of starting proportions and materials, reached a good degree of humification and a chemical composition suitable to be used as compost or humic conditioner according to the Spanish legislation. However, not all the composts could be commercialized as organic fertilizer, because this denomination means a concentration of total nitrogen greater than two percent, which is not always achieved.
Addition Of Composts To Olive Orchard
According to Regulation EEC 2092/91 on organic production, soil fertility must be kept through any of these three different methods: Cultivation of legumes, green manures or deep-rooting plants in an appropriate multi-annual rotation program; Addition of manure from organic livestock production (maximum 170 kg N/ha); and Addition of any organic material from organic production. However, when necessary, other supplements, specified in Annex II of this Directive, could be used, such as composts, poultry manures, bone meals, dairy by-products, phosphate rocks, etc.
According to organic farming regulations, the composts obtained were applied on the olive tree plantation. Addition rate was 25/30 kg per tree and caused a significant increase in P and K in plants compared to nonfertilized trees. No differences were observed regarding nitrogen. The increase in organic matter concentration and cation exchange capacity were observed in amended soil even after 12 months of the application. The experiments performed for three years showed that the fertilizer obtained may be used in accordance with organic farming regulations, contributing to the revalorization of residues and the maintenance of the ecosystem.
Maria L. Cayuela, Miguel A. Sánchez-Monedero and Asunción Roig are with the CSIC Department of Soil and Water Conservation and Organic Waste Management in Murcia, Spain. Juan Molina is with the organic farm, Casa Pareja in Jumilla, Murcia, Spain.