| dc.description.abstract | In the last years the increasing concern toward environmental protection and
source exploitation has guided the legislative development of measures aiming to
the sustainable management of municipal solid waste, which implies increasing
material recovery and diminishing landfill disposal.
Biodegradable waste represents the prevailing fraction of municipal solid waste: in
Europe, it can reach up to 40% of the total amount of waste produced. Therefore,
its proper handling is fundamental.
In this context, anaerobic digestion has become one of the major developments in
waste treatment facilities.
The interest in this technology is mainly due to the production of methane, which
can be used as a renewable energy source in front of aerobic stabilization that
requires energy consumption. Anaerobic digestion followed by an aerobic phase
represents a recent treatment option, which allows not only biogas but also
compost production.
Although anaerobic digestion is a well-established technology, extensive research
proved that the methane yield of solid organic material is significantly affected by
both the mass transfer in each biological step and substrate availability. It is
recognized that the rate limiting step is the hydrolysis of the complex organic
matter into soluble compounds.
Therefore, a proper treatment of the organic material before the digestion stage
can promote substrate solubilisation and increase the amount of matter that is
readily available to anaerobic microbial species.
To this end, several technologies can be applied, including ozonation and
sonolysis, which have been widely investigated as sludge pretreatment for
anaerobic digestion.
The use of ozone, which is well known as a strong oxidant, besides noticeable
reduction of sludge production, was proved to modify sludge properties and
characteristics and increase biogas production from pretreated substrates.
Sonolysis relies on the use of ultrasound to provide substrate disintegration by
acoustic cavitational phenomena, which involves the formation and expansion of
micro-bubbles and their consequent collapse. As a result, both chemical and
physical reactions can occur resulting in solubilisation and digestibility
enhancement.
The analysis of scientific literature highlighted that both ozone and sonolysis
applications represent innovative pretreatment options for solid organic substrates
to be treated by means of anaerobic digestion.
Furthermore, research development has recently pointed out that the
enhancement of methane production after ultrasonic treatment is highly variable
according to the different parameters affecting the combined US/anaerobic
process.
Therefore, the research activity discussed in this work aimed to:
- the comparative evaluation of ozonation and sonolysis as pretreatment of
the organic fraction of municipal solid waste (OFMSW) for anaerobic
digestion;
- the assessment of the technical and economic feasibility of the optimal
OFMSW pretreatment option.
To this end, experimental activity was structured in two main steps:
- the first one was focused on the assessment of both ozonation and
sonolysis effectiveness in promoting solubilisation and anaerobic
biodegradability of OFMSW;
- the second part, performed on the basis of the results of the previous
phase, was focused on sonolysis effects. In particular, experimental activity
was addressed towards the definition of main parameters, in order to
assess the technical and economic feasibility of the full scale US
pretreatment.
Moreover the relation between sonication pretreatment effects and organic matter
composition was assessed.
The first part of the research, focused on the comparative assessment of
ozonation and sonolysis, was performed at the Sanitary Environmental
Engineering division (SEED) of Salerno University.
For both ozonation and sonolysis, different operating conditions were studied in
order to highlight the relation between process extent and the induced
solubilisation, which was estimated through soluble COD. The reduction in
volatile solid content, which indicates the occurrence of mineralization
phenomena, was also monitored for the investigated operating conditions.
Results show that both ozonation and sonolysis are effective in increasing
solubilisation of the organic fraction of municipal solid waste. However, higher
ozone doses resulted in the formation of by-products which are less
biodegradable than the untreated substrate.
This aspect represents a limitation to the scale up of ozonation as pretreatment of
OFMSW for anaerobic digestion, as the effectiveness of the process requires strict
operating conditions in terms of ozone doses.
Moreover, this drawback was not reported to affect other chemical pretreatments,
which can also be handled more easily.
Conversely, the ultrasound-induced disintegration determines an increase of the
soluble compounds, which results in improved biodegradability of sonicated
samples and, consequently, in higher biogas volumes from the anaerobic digestion
of pretreated substrates.
The results of the comparative evaluation of ozonation and sonolysis addressed
the research towards the in-depth analysis of sonolysis mechanisms on organic
solid matter destined to anaerobic digestion.
In this second phase, the experimental activity was performed at the Technical
University of Hamburg-Harburg (TUHH - Germany), in cooperation with
Ultrawaves GmbH (Hamburg, Germany), acknowledged since 1995 for its
valuable work on the development of new procedures for treating water,
wastewater and sludge by means of ultrasound. This work led to the design of one
of the most highly developed ultrasonic reactors, which is internationally
commercialized. The cooperation allowed the acquisition of an over ten years
experience accrued by prof. Uwe Neis and dr Klaus Nickel in the field of
sonolysis as well as the use of the basic device of their ultrasonic reactor for the
experimental activity.
The effects of the application of different operating conditions were assessed
through the quantitative and qualitative characterization of the soluble fraction of
sonicated samples as well as by means of anaerobic biodegradability tests.
Experimental results proved that, although the effects of sonolysis on organic
substrates depend on the composition of the substrate itself, ultrasound is a valid
technical option in anaerobic digestion facilities for OFMSW treatment, as it
allowed relevant solubilisation enhancement as well as the improvement of
methane production from pretreated substrates. Moreover, its proper application
can imply several operating advantages and proved to be economically feasible.
The results of this research are of relevant interest, as they highlight advantages
and limits of these innovative pretreatment technologies as well as the high
operative potential of sonolysis as pretreatment of solid waste for anaerobic
digestion. [edited by author] | en_US |
