Production of organic fertilizers from residues of the olive oil processing industry

The olive oil processing industry generates large amounts of liquid and solid residues (olive-mill wastewater, OMWW and olive mill solid waste, OMSW, respectively). OMWW is mainly collected in storage ponds and evaporated during summer leading to serious environmental problems. OMSW is further processed in central pomace oil plants to recover the residual oil by chemical extraction. After this extraction a final residue, deoiled solid cake (DOC), is produced. Currently, DOC is composted, used directly as soil improver or sold as fuel. In previous studies, it has been demonstrated that anaerobic digestion can be a promising alternative to treat these residues. This process has the advantage that the residues are converted to energy in form of methane and that the digestate can be dried and used as a stabilized organic soil improver. In fact, land application of stabilized digestate is an excellent practice to increase soil fertility. However, in regions with intensive livestock production, land application is not always possible because of the already high nutrient content of the soil. In those regions, digestate must be either transported to other areas with nutrient demand or stored for long periods. Drying and pelletizing digestate could (1) stabilize the digestate, (2) reduce the transportation costs and storage requirements, and (3) facilitate handling and commercialization of the product. The objective of this study was to convert digested OMWW and DOC to a compact organic soil improver to create a commercial product, which could be stored, transported and spread with common fertilizer spreaders used in agriculture.

Two types of digestate: (A) from the co-digestion of OMWW and dairy wastewater, and (B) from the co-digestion of DOC and manure, were collected from two anaerobic reactors (100 L) at our laboratories. A solid-liquid separation was performed and the solid fraction was dried at 100°C until the sample had a total solid content (TS) of 85%. The dried solid was then milled and pelletized. The water content and plant nutrients concentration (TN, NH₄, TP, K, Mg, Ca, S and Fe) were measured before and after each process to compute a complete water and nutrient balance of the whole process. To determine whether the obtained product could be used as an organic fertilizer relevant plant nutrients and potential pollutants (Tl, As, Pb, Cd, Cr, Ni, Hg, Cu, Zn) were measured.

 

 

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