Abstract
The research aimed at implementing biogas-refinery from livestock effluents, by defining innovative/adapted technologies and processes for the nitrogen control, which could be easy to be apply in the alpine context and directly manageable by the farmer. Those processes should be practical, feasible, easy, efficient and inexpensive. The autonomous Province of Bolzano (Italy) shows the absence of “vulnerable” zones-ZVN (Directive 91/676/EEC) (Peratoner & Stimpfl, 2012; Bottarin & Tappeiner, 2010). The geographical and climatic conditions have enabled the development of widespread animal husbandry, which is often the only form of efficient use of territories at higher altitudes. Cattle are the main bred animals. Proportionally to area and available UAA, the South Tyrol cattle density and the N-load have the same order of magnitude as the neighboring territories, which actually have ZVN defined.
A sustainable management of the Province’s livestock sector implicates the research of solutions for reducing polluting emissions. The role of manure for biogas production from anaerobic digestion is an emerging significant topic, which is related both to energetic and environmental issues. Several aspects support this statement: reduction of CH4 and CO2 emissions, dejection of unpleasant odours, hygienisation of the manure, and increase of the digestate fertilizing capacity. Biogas is also a versatile energy source, enhancing agricultural development and energy self-sufficiency (Dinuccio et al., 2010; Börjesson & Berglund, 2010). Specifically, N-flows can be monitored and managed, by providing adequate recovery or removal treatments, integrated with the biogas facilities.
The research provided previously an estimation of nitrates-criticalities from the livestock sector. We firstly analyzed the animal husbandry sector and define “typical farm” categories in terms of manure, feeding and housing management; objectives of the analysis were estimation of the actual N-load from livestock sector on the Province and calculation of the N-balance for each farm. Five municipalities and several farms belonging to their areas were identified as presenting a potentially critical N-load on soil. We then focused on treatments for the N-reduction from livestock digestate. Both pilot and lab-scale experiments were carried out, in order to identify the best condition for each process line (Fig.1). Digestate from small (<20 UBA), medium (50 UBA) and large (>50 UBA) cattle farms with an operative biogas plant were used for the experiments. Three innovative systems were tested and integrated to identify two process lines: i) Solid-Liquid separation by means of drainage bags, after induced flocculation with specific polyelectrolytes (lab and pilot scale); ii) Ammonia Stripping of the liquid fraction without pH modification, working at high temperature (90°C) (lab-scale) and iii) optimized Nitro-Denitro in SBR working on the liquid fraction (lab-scale). The processes showed a total (sum of conservative and effective) N-removal efficiency from 70 to 90%, depending on the type of digestate, on the type of process and on the longest period of maintenance into the treating facility. We identified the Solid-Liquid separation and the Nitro-Denitro processes as suitable in small-medium farms, while the Stripping technique as more appropriate for larger farm or consortia biogas plants.