Biological co-treatment of agroindustrial wastes.

The production of organic wastes is growing world-wide, while agricultural soils are witnessing a progressive reduction in their organic content.

The biological co-treatament of organic wastes is an interesting alternative if the proper wastes are mixed. Agroindustrial wastes offer a wide variety of potential co-substrates, and many of these possibilities have not yet been explored. In the region of Navarra (Spain) there is a strong presence of agro-industries that generate a significant volume of these types of wastes. Currently, their most important destination is the feeding of cattle or the controlled landfilling. However, these applications are being limited by increasing restrictions concerning animal feeding (e.g. Regulation 1774/2002/EC) and the conditions imposed by Council Directive 1999/31/EC on the landfill of waste. In these respects, anaerobic digestion is a promising technology to optimize the recovery of energy. Meanwhile, the end product should also be managed in a sustainable way, with the possibility of its application to poor soils being necessary to complete the cycle of organic matter. This work has one main objective: to asses the effect of anaerobic digestion and composting on the co-treatment of the wastes used, as well as on the quality of the end-product.
A pilot plant has been designed for the anaerobic treatment of solid wastes. It has a mixing system and it works within the mesophilic range of temperature. The process was performed in batch mode, and an initial mixture composed of Vegetables Wastes (VW) with Meat and Bone Meal (MBM) was used. The anaerobic digestate has been mixed with wood shaving so that it can be composted in an aerobic reactor provided with an automatic aeration and mixing system. In both of the processes the agronomic characteristics of the product were analyzed, especially those related to its humification.
The beneficial effect of the co-digestion of the selected wastes has been studied: the VW compensate for the lack of moisture in the MBM and dilute the excess of ammonium released during the digestion. The MBM are highly biodegradable due to its small particle size and high content of protein and fat, which results in a high production of methane. The methane proportion reached in the biogas was over 70% and the Specific Methane Production (SMP) was 0.41 m3 CH4 kg-1 VSadded waste (SRT = 55 days), with a VS removal in the wastes used close to 85%. Post-composting has allowed not only the complete stabilization of the end product but also compliance with the sanitary requirements of the Working Document on Biological Treatment of Biowaste, 2nd draft (EC, 2001). The agronomic study has revealed a significant increase in the humification after the post-composting process.
It can be concluded that the anaerobic co-digestion of VW with MBM is beneficial due to the complementary properties of the co-substrates used. The presence of enough levels of alkalinity and the use of an adapted inoculum allow a stable start-up and performance of the digester. Moreover, the post-composting leads to a complete stabilization of the digestate, and the organic amendment obtained has a high degree of humification together with a good fertilizing capacity. The combination of both processes enables energy to be recovered from the waste and the recycling of the organic matter that it contains.

Copyright: © European Compost Network ECN e.V.
Quelle: Orbit 2008 (Oktober 2008)
Seiten: 11
Preis inkl. MwSt.: € 9,00
Autor: Dr. Enrique Aymerich Soler
José María García-Mina
Dr. L. Sancho

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