Proceedings

Abstract

Anaerobic digestion as a treatment of Municipal Solid Wastes (MSW) has become a new clean technology based on energy recovery from waste to produce biogas. In order to accelerate the hydrolysis process, which is commonly the limiting step in the degradation of solids, two pretreatments (ultrasound and thermal hydrolysis) were tested in laboratory scale using the organic fraction of MSW. Biochemical Methane Potential (BMP) batch tests were performed and physical parameters from digestates were also characterized to study hydrodynamic properties and assess the implementation of pre-treatment in a continuous operation.

Ultrasound was tested using a 1kW device at different sonication times and two power levels. BMP tests showed no improvements in terms of biogas production with respect to the nonpretreated sample (320 mLCH4/gVSadded). Similarly, thermal hydrolysis was tested at three temperature levels for different operation times and resulted in a better pretreatment than ultrasound, increasing the methane potential by 58% for the optimal conditions (120ºC, 15 minutes) and improving the kinetics of the process. However, inhibitory behaviour (lag-phase) for the hardest conditions for both pretreatments indicates the presence of recalcitrant compounds possibly due to Maillard reactions. Therefore, there are interesting perspectives for a further study.

Keywords

Anaerobic digestion, hydrolysis, municipal solid waste, pretreatment, solubilisation, ultrasounds

Introduction

Municipal Solid Wastes (MSW) is considered to arise from activities in urban areas carried out in homes, offices, shops or other services. In modern society, the treatment of wastes generated as a result of a high consumption of products has become a significant problem. It is estimated that each person in the cities generates a kilo of waste per day and between 15 and 20% of the state budget is dedicated to the collection, transport and treatment of MSW. This problem increases yearly due to factors such as the rapid population growth, the concentration of population in the cities or the use of material goods with rapid ageing. This situation presents at the same time an opportunity to develop new, clean technologies based on energy recovery from these wastes, such as anaerobic digestion. Several companies have already invested in such technologies, designing and operating different commercial technologies of continuous anaerobic digestion reactors of MSW such as Valorga, Cambi, Kompogas, Dranco, Arrowbio, Waasa, BTA… (Karagiannidis 2009)

Anaerobic digestion is a biological process in which the organic matter is transformed into biogas by the action of specific bacteria in the absence of oxygen. The biogas produced contains 50-70% methane and is available for energy recovery through combustion. On the other hand, digestion results in a fraction that has not been degraded, called digestate, which, after an appropriate treatment, can have agricultural applications as fertilizer. (Hilkiah Igoni et al. 2008) The anaerobic digestion process consists of several consecutive biological processes. Thus, the degradation rate of the overall process is limited by the slowest step, in this case hydrolysis (especially when dealing with solid wastes). Hydrolysis is the first stage in which complex organic matter (proteins, lipids, carbohydrates…) becomes simple soluble matter (amino acids, sugars, fatty acids…) more easily assimilated by bacteria in further steps. Therefore, this stage is crucial for the whole process success and for a fast and profitable biogas generation.

Nowadays, it is desirable to accelerate and improve the hydrolysis process by pre-treatment technologies, which take place before anaerobic digestion. These technologies are based on the solubilisation of organic matter by mechanical processes: grinding (Izumi et al. 2010), ultrasounds (Cesaro et al. 2012), microwaves (Shahriari et al. 2012); chemicals: reagents such as NaOH, HCl (López Torres and Espinosa Lloréns 2008); thermal processes: high temperature (Liu et al. 2012) and high pressure (Cuetos et al. 2010); or biological: enzymatic treatment (Fdez.-Güelfo et al. 2011). Pre-treatments can also deal with some other problems that take place in anaerobic digesters such as ammonia inhibition, Volatile Fatty Acids (VFA) accumulation, mixing problems, digestate minimization and stabilization…

All these pre-treatment technologies have been widely tested with sewage sludge (Pérez-Elvira et al, 2006) and even applied in real scale continuous processes in several waste water treatment plants (WWTP). Nevertheless, in the area of MSW they have hardly been studied and need further research for its implementation in MSW treatment plants.

The aim of this study is to evaluate the effect of two of these pretreatments (ultrasounds and thermal hydrolysis) in the anaerobic digestion process of MSW in a laboratory scale by the application of batch biodegradability tests and hydrodynamic tests.