Mesophilic and thermophilic anaerobic digestion of the liquid fraction of pressed biowaste for high energy yields recovery
Introduction
Anaerobic digestion is a proven and widespread technology for the management of organic waste of different origin (De Baere and Mattheeuws, 2012). There are currently more than 14,000 plants running in Europe, 28% of which are dedicated to the treatment of wastewater sludge, municipal and industrial organic waste, while the remaining 72% use agro-waste as feedstock (EBA, 2014).
With specific reference to municipal waste management, the success of this technology in recent years has been determined by the implementation of deep separate collection schemes: this determined the possibility of handling streams with a reduced amount of inert material and high moister content and biodegradability like segregated food waste (Valorgas, 2010, Bernstad et al., 2013).
Beside anaerobic digestion, the other biological process widely diffused within EU for the management of organic wastes is the aerobic stabilization, or composting: at present a compost production of around 10.5 million tonnes of organic waste is reported (European Union law http://eur-lex.europa.eu).
Noticeably, the two processes, i.e., anaerobic digestion and composting, can be integrated together since the solid fraction of digestate can be treated aerobically (Nayono et al., 2009) so to recovery both renewable energy and nutrients from organic waste. At present some 8 million tons of biowaste are anaerobically digested within EU Countries and normally the biowaste is pre-treated and prepared for the AD process by means of several mechanical steps. A large number of plants treating organic waste started their operations in the 1980s, when both the amount and the composition of biowaste were quite different from the present situation. This has resulted in the need for some modifications both in plant management and operating conditions (Di Maria et al., 2012). In fact literature showed that during conventional pre-treatment methods around 30% of the initial wet material could be rejected without any treatment (Pognani et al., 2012). The pre-treatment of the organic fraction of municipal solid waste is in fact one of the main challenges in mechanical–biological treatment plants equipped with anaerobic digesters (Romero-Güiza et al., 2014).
Recent literature highlights the observations related to the loss of biodegradable organic matter during the pre-treatment steps (Müller et al., 1998, Bolzonella et al., 2006a, Bolzonella et al., 2006b, Ponsá et al., 2010). Moreover, these steps are time and energy consuming (Tonini et al., 2014) and generally are not able to achieve high removal yields for inert materials like small pieces of plastics and fine heavy materials like crashed glass, sea shells and sand. These materials could then accumulate inside the reactor determining a reduction of the reaction volume and a possible risk of process failure (Angelidaki and Boe, 2010).
Another important aspect to be considered is then the reduction of the energy demand for pre-treatment processes and if possible enhance the biogas production of the anaerobic digestion plants that treat the municipal biowaste (de Araújo Morais et al., 2008).
In order to address all these issues an interesting option is the use of very simple pre-treatment steps like presses and extruders: in these machines the size of the organic material is reduced while inert material (mainly plastic) is eliminated.
Biowaste pressing produces two streams: one semi-liquid to be digested and a second one solid to be composted (Hansen et al., 2007). Nowadays another advanced energy saving pre-treatment approach has been developed: biowaste squeezing. This is a mechanical pre-treatment process. The advantages of mechanical pretreatment include an easy implementation, better dewaterability of the final anaerobic residue and a moderate energy consumption (Ariunbaatar et al., 2014). Pretreatment and digester design are the key techniques for enhanced biogas optimization (Shah et al., 2015).
Only few examples of such approach can be found in literature at the best of our knowledge.
Nayono et al. (2009) studied AD of pressed off leachate from OFMSW and the co-digestion of press water and food waste (Nayono et al., 2010) for improvement of biogas production. The co-digestion of press water and food residues with defibred kitchen wastes (food waste), operated at an OLR in the range 14–21 kgCOD/m3 d, reported greater biogas production rates then sole biowaste. An increment of the OLR of biowaste by 10.6% with press liquid fraction increased the biogas production as much as 18%, with a biogas production rate of 4.2 m3/m3 d at an OLR of 13.6 kgCOD/m3 d. These experimentations were conducted through laboratory scale reactors, from 1 to 8 liters working volume.
According to the scenario reported above and the evidences of recent studies, this study was dedicated to the anaerobic digestion of the liquid fraction of pressed biowaste at pilot scale so to identify bottlenecks, consumes and yields of interest for a possible process scale-up. The use of a screw press allows for the production of two streams, one liquid, clean and very biodegradable, easy to convert into biogas (thus energy), and another one semi-solid with a level of biodegradability and water content and C/N ratio suitable for composting.
Clearly, the liquid stream, because of its characteristics, is particularly suitable also for co-digestion with sludge in wastewater treatment plants.
In this study particular attention was paid to the definition of the optimal operating conditions and yields for the anaerobic reactor.
Beside this the digestate characteristics were considered in detail also in order to respond to the requirements defined in the “End of Waste Criteria” technical proposal by the Joint Research Center of Sevilla (2014). Based on suggested criteria, pathogens and metals in the digestates were analyzed in order to evaluate the necessity of further anaerobic digestate treatment, for example in a co-composting process.
Section snippets
Pretreatment strategy and experimental set up description
A pilot-scale press, specifically designed for this experimentation, was used in order to pre-treat separately collected biowaste and split it into two streams, one liquid to be anaerobically digested and a second one solid to be composted.
Door-to-door collected biowaste from Treviso area (Italy) was first shredded into a knife mill and treated in a press for solid–liquid separation. Only the liquid fraction was then sent to the anaerobic process while the semi-solid part, characterized by a
Biowaste pretreatment and composition
The biowaste collected in Treviso area and used in this experimentation showed the composition reported in Table 1: fruit and vegetable waste were typically half of the waste material, a result in line with previous studies on this topic (see Valorgas D2.1 http://www.valorgas.soton.ac.uk/deliverables.htm) while pasta/bread and meat/seafood represented another 25% of the wasted food. Some 10% of the material was un-classified (melt material).
Biowaste compositional analysis (of five samples)
Conclusions
Biowaste from door-to-door separate collection was pressed and the liquid fraction underwent to mesophilic and thermophilic anaerobic digestion. Mesophilic digestion gave an average biogas production of 0.79 m3biogas/kgVS with 66.0% methane content while in the case of thermophilic conditions the average biogas production was 0.90 m3biogas/kgVS with 68.8% methane.
The application of press systems for the separation of segregated biowaste into semi-liquid and semi-solid fractions can be beneficial
Acknowledgments
This work was carried out with the financial support of the LIFE+ ENVIRONMENT POLICY AND GOVERNMENT project “Development and implementation of a demonstration system on Integrated Solid Waste Management for Tinos in line with the Waste Framework Directive” (LIFE10 ENV/GR/000610). The hospitality of Treviso City Council and Alto Trevigiano Servizi srl are gratefully acknowledged.
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