Energy and material recovery from high-loaded organic substrates: A territory-oriented approach

This Ph.D. research was aimed at liquid organic substrates valorisation, by means of energy and material recovery. The mountain area of Friuli-Venezia Giulia region was selected as case-study: Cheese Whey (CW), condensate Pulp and Paper (P&P) WW, OFMSW, brewery waste (spent grain, yeast, whirlpool residue, end-of-fermentation beer) and slaughterhouse waste were selected for AD process application, as well as for resource recovery. The work started with a literature study, followed by physicochemical characterization, BMP tests, continuous UASB tests, and it was then completed with an energetic analysis, as well as with some final remarks and suggestions. The Ph.D. thesis starts with a general introduction, aimed at describing EU perspective in renewable energy and waste management. Given the increasing importance of biomethane, the currently applied technologies for biogas upgrading are briefly discussed, as well. Then, UASB anaerobic treatment, as an interesting process for energy recovery from industrial wastewater, and Tolmezzo WWTP (143,000 PE) are described. Successively, the analysed substrates are introduced, and the obtained results are presented. The results from BMP tests follow: these data were useful to estimate potential methane yields and maximum methane fluxes, as well as to introduce continuous UASB tests, that were conducted on a pilot-UASB unit. In the final chapter, some energetic and material recovery considerations are drawn, considering the actual energetic costs in selected real plants and suggesting, for each substrate, an optimization route. The results underlined that a high potential is present for biogas production in dairies: CW can be successfully digested, and, if performed at plant level, AD process can provide most of the electricity and heat needed by the process. The installation of simple digesters can allow to reduce pay-back time. In larger dairies, instead, resource recovery should be privileged, due to the extra income that could be provided by the obtained products. Ultrasound (US) pre-treatment was shown to be effective in increasing biogas yields, but only at low applied US energy. OFMSW can be separated into a liquid fraction, highly biodegradable and having good methane potential, and a solid fraction, easily stabilized through composting. Given the general low amount of available organic waste in the analysed territory, co-digestion of OFMSW liquid fraction with other substrates, such as excess sewage sludge, can be an interesting option, to increase biogas yields and obtain a co-digestion mixture having optimum characteristics. Condensate water is a highly concentrated P&P WW, and is amenable to be pre-treated using UASB technology; this can reduce organic load to the aerobic basins, leading to a significant energy saving for aeration. Brewery organic waste is characterized by a pool of different substrates, including spent grain (trub), yeast, whirlpool residue and end-of-fermentation beer. A good potential for biogas production was shown to be present mainly in spent grain and yeast. The addition of little amounts of biochar and granular activated carbon increased obtainable methane yields in a significant way (more than 35%), so a synergistic effect between biomass plants and processing plants can be achieved, improving energy production. Co-digestion of brewery organic substrates at plant level can be successfully performed. Given the high energy demand of this plants, AD process appears to be a good solution to reduce operating costs. Slaughterhouse waste, finally, is a harsh substrate, difficult to hydrolyse, and rich of proteins and fats: in order to be successfully treated using AD process, efficient pre-treatments should be investigated. Moreover, sanitary protocols must be followed, for its proper management. It could be interesting to evaluate the effects of co-digestion with complementary substrates, rich in C.