Development of a digestate treatment’s scheme for nutrients recovery and re-use to apply on alpine small/medium size agricultural plants
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One of the problems still open in many anaerobic digestion (AD) projects regards digestate’s management, in order to ensure the environmental sustainability of the production cycle. Digestate flows, as well as the original materials (biomass), are characterized by high concentrations of nutrients (NPK). Presence of areas with high livestock activity; particular characteristics of agricultural land, as slope or altitude; establishment of more restrictive regulations (EU Nitrates Directive) are contingent elements, which threaten agricultural and livestock activities in Alpine areas. Nitrogen recovery is a desirable option, but its sustainability and the possibility of large-scale applications have yet to be demonstrated, both from the technical, environmental, administrative/regulatory, and finally, economic aspects. The economic feasibility is closely linked to the market for recovered materials and the ability to compete with traditional chemical fertilizers. Since the mere intervention on diet hardly solves alone the problem of total nutrients surplus’ abatement, it is essential to plan proper treatments directed on the digestate. These treatments should be calibrated based on the specific needs of each farm or group of homogeneous farms, in order to optimize performance and ensure economic sustainability. The implementation of a biogas plant simplifies the existing manure management, but does not automatically solve the problem of N over-loads. The integrated optimization of the AD process and of the downstream phases of N-disposal - both in terms of reduction, recovery or re-use of nutrients in concentrated solutions as fertilizer - has considerable potential in terms of economic (thanks to revenues from the sale of renewable electricity) and technique (thanks to the plants synergies and to the availability of electricity and thermal energy) sustainability. This project aimed at evaluating and experimenting two scenarios of digestate (from agricultural and livestock waste) treatment, through implementation, monitoring and optimization of two integrated lines of post- treatment, with an objective and detailed evaluation of the process characteristics, including technical, managerial and economical aspects. The results of this analysis have been used to develop a treatment scheme calibrated on small/medium size agricultural installations, which can be easily handled by unskilled personnel. The project was divided into two sub-projects, developed in parallel. The first concerned physical-chemical post- treatment, using an innovative Stripping column without preliminary solid/liquid separation of the digestate, with pH modification based on Calcium addition, working at high temperature (85°C). The second concerned biological post- treatment, using the innovative Anammox process in plant DEMON. DEMON combines SHARON and ANAMMOX processes, by alternating aerobic and anaerobic phases in a SBR reactor. In the aerobic phase, the concentration of oxygen is limited to 0,3 mg/l, a value that inhibits the growth of ammonium-oxidizing bacteria (then favoured in the anaerobic phase). The processes showed N-removal efficiency from 70 to 90%, depending on: the type of digestate, the type of process, and the longest period of maintenance into the treating facility. We identified both the Stripping technique and the DEMON process as suitable in medium farms or consortia biogas plants.