Biohydrogen and Biomethane production from organic residues in Two-Stage, batch and continuous, Anaerobic Digestion

Nowadays, environmental pollution and fossil fuel depletion have become issues of considerable global concern that encourage efforts for finding alternative energy sources and pollution control. One of the most attractive ways to achieve both goals is to recover energy and resources from waste streams through bioconversion processes. In a society characterized by the population growth and huge consumptions, removal of solid Organic Waste (OW) that is produced daily by households, offices, industries, and agricultural sectors as well as by food productions has become an ecological problem. In this context, Anaerobic Digestion (AD), an environmentally sustainable technology, represents the best alternative to manage the OW worldwide. AD producing biogas and fertilizers is a well established biotechnological process used in practice for over a century for the disposal of waste-water and solid OW. A two-stage AD (TSAD) process, combining H2 and CH4 production, was firstly proposed in 1971 by Pohland and Ghosh, (1971) and now has taken quite attention thanks to the possible increase of the overall efficiency of the process. The present thesis work in intend to deepen this hypothesis. After an introduction on the basis of the so called TSAD, many experimental tests are described in order to demonstrate the feasibility of TSAD over One-Stage AD (OSAD), either in batch and continuous mode. The effect of main engineering parameters affecting the process will be elucidated as well as the whole experimental results. On the other hand, pretreatment methods for improving the AD performances have been the focus of a large number of scientific studies over the past two decades. In the light to improve the energy production, however, the energetic sustainability of different pretreatments is an issue that has not yet been addressed; the majority of studies conducted up to date focus on the evaluation of the increase of energy production. In this study, instead, a comparison from the energetic point of view was done to enlighten their energetic sustainability. The Thesis offers substantial and suitable information from experimental tests that have usually a principal objective. To accomplish this, several research studies have been carefully reviewed in-depth and hence mentioned in these pages to better understand the performance of the TSAD. Each paragraph is completed with the description of experimental tests performed according to the subject reviewed. In detail, in CHAPTER I there is a complete overview about the most relevant notions of AD, making emphasis on the differences between OSAD and TSAD process. Furthermore, the theoretical maximum efficiency that can be obtained with an AD process is described and compared with experimental efficiencies. CHAPTER II summarizes several pretreatment processes applied in order to improve the overall AD efficiency. Physical, chemical, thermal, ultrasonic and a combination of them were evaluated and compared. Three different kinds of substrates were used, Sweet Product Residue (SPR), Organic Waste Market (OWM) and Coffe Seed Skin (CSS). CHAPTER III is intended to the valorisation of a particular organic waste, SPR, via TSAD. SPR represent the substrates with high content of sugars and low lignocellulosic material concentration. Moreover, the SPR was particularly treated with an extrusion process at 200 atm in a pilot plant, and its energetic sustainability was evaluated. CHAPTER IV is dedicated to the valorisation of the OWM refuse, that it is one of the most abundant wastes in the world, representing in turn the substrates with mean concentration of lignocellulosic material. Two types of pretreatments were evaluated from an energetic point of view. In CHAPTER V is described a study of the CSS refuse, the last kind of subtrate that represents the waste with high lignocellulosic material content. The performance of CSS was evaluated highlighting the importance of a pretreatment step for this kind of refuse. The CSS yields were campared with the yield of the most easy biodegradable molecule, the glucose. The CHAPTER VI focus its attention in a direct energy balance of the AD toward sustainability, since AD is a tecnology intended to produce usefull energy for society. The Energy Sustainability Index (ESI) is proposed, and represents the first step of a detailed energy sustainability evaluation procedure. In CHAPTER VII a Continuous Anaerobic Digestion (AD) process is studied and described in order to establish the best conditions to run this technology with the highest possible energy efficiency, with the purpose of demonstrate the feasibility of Two-Stage AD (TSAD), and to explain the main affecting parameters. The OWM was used as substrate. Finally, to close this thesis, a CONCLUSION CHAPTER is provided with a general discussion of the obtained results and depicts future perspectives about the real application of this promising technology. REFERENCES Pohland F.G., Ghosh S., 1971. Developments in anaerobic stabilization of organic wastes – the two-phase concept. Environ. Lett. 1: 255–266.