1. Introduction:
Fruit and vegetable wastes (FVW) constitute one of the major organic
wastes production from different sources and in Western Australia,
around 30% of production end up as waste 1. The
disposal of FVW in large quantities in landfills is difficult due to its
high biodegradable organic content and subsequent environmental impacts2,3. On the other hand, anaerobic digestion has been
proven to be an efficient technology for the treatment of organic solid
wastes and energy recovery. Therefore, one of the alternatives to
landfills of these large quantities of FVW is utilization and
valorization of the residue in the production of biogas and high-quality
organic-rich digestate through anaerobic co-digestion process. Anaerobic
digestion involves biochemical degradation of organic wastes by
microorganisms in an anoxic environment with the production of methane
and carbon dioxide as major gases. In recent years, various researches
are undertaken to address the challenges and process optimization of
anaerobic digestion for its efficient application in waste management4. Anaerobic digestion involves four major biochemical
reactions, viz, hydrolysis, acidogenesis, acetogenesis and
methanogenesis; separation of respective microbial communities and
facilitating them with optimum conditions help to reduce the limitations
of anaerobic digestion 5.
Fermentation of FVW in anaerobic digestion faces severe acidification
issues that inhibit the growth of methanogenic population due to its
high sugar content and volatile solids (VS) fraction6. Moreover, the high C/N ratio of FVW leads to
nitrogen depletion in anaerobic digestion and requires an additional
substrate that can sustain fermentation efficiency 3.
For these reasons, co-digestion of FVW with complementary substrates
such as kitchen waste, agricultural waste and sewage sludge have been
tried out 3. The co-digestion of sewage sludge with
FVW have shown improvement in biogas yield and agronomic digestate
quality 7,8. Furthermore, various authors have
investigated two-stage anaerobic digestion under mesophilic conditions
to improve the biogas production and sustain high OLR (organic loading
rate) 9–11. Phase separation of microbial population
offers better control over the respective phases, increases the specific
activity of methanogens and also improves the suspended solids removal
efficiency 12. Besides, different reactor
configurations for phase separation of the acidogenic and methanogenic
population have also been studied 13,14. In most of
the studies, an acidifier (pre-methanizer) reactor helps in
stabilization of methanogenic population in methanizer reactor while
handling solid wastes with high organic content. And, the overall
retention time can also be reduced in such two-phase systems compared to
conventional anaerobic digestions.
Besides the novelties of reactor configurations and co-digestions,
various pre-treatments of solid wastes have also been studied to
overcome the rate-limiting hydrolysis step and enhance anaerobic
digestion performance 15. Pre-treatments breakdown
macromolecular structures and facilitate sludge hydrolyzation. In the
case of polysaccharide-rich solid wastes (FVW), the lignin barrier
present in skins and seeds makes bacterial hydrolysis difficult16. Among various FVW pre-treatments, thermal
hydrolysis has been established as a prominent pre-treatment technique
with enhanced sludge solubilisation, digestion performance and improved
dewaterability 17,18. Recently, microwave enhanced
advanced oxidation process has gained huge research attention as a
pre-treatment technique for its effectiveness in hydrolyzation, biogas
production and effluent digestate quality 19,20. The
synergetic effect of microwave and hydrogen peroxide
(MW-H2O2) have been shown to improve
sludge solubilisation and also significantly affect biogas production
from activated sludge samples 20–22. Apart from the
initial sludge solubilisation through thermal effect, microwave enhanced
advanced oxidation process (MW-H2O2)
also imparts oxidative stress on the microbial population mediated by
reactive oxygen species (ROS) 22,23. During the
oxidation process, highly reactive oxidative intermediates such as
hydroxyl [OH] and superoxide radicals
[\(O_{2}^{-}]\) are generated, that oxidize the organic
pollutants and also intermediate intracellular oxidative stress in the
microbial population 23. Evaluation of such oxidative
intermediates helps us to understand the effect of hybrid
(MW-H2O2) pre-treatment on microbial
sludge activity and expands the knowledge on oxidative stress in sludge
microenvironment.
Previous studies on hybrid (MW-H2O2)
pre-treatment were mainly focused on anaerobic digestion of activated
sludge samples under mesophilic condition. The effects of hybrid
(MW-H2O2) pre-treatment differs
dependent on the nature of the feedstock. And also, hybrid treatment of
activated sludge under acidic conditions has shown improved methane
production in mesophilic anaerobic digestion 20. The
acidic nature of FVW has the potential to enhance the effects of hybrid
pre-treatment in anaerobic co-digestion with activated sludge. Hence, in
the present study, the impact of hybrid
(MW-H2O2) pre-treatment on anaerobic
digestion efficiency of FVW co-digested with mixed activated sludge is
studied. Besides, a two-stage anaerobic digester was operated to
separate hydrolysis-acidogenesis in phase I (thermophilic condition) and
methanogenesis in phase II (mesophilic condition). Two different mixing
ratios of FVW and mixed activated sludge were evaluated based on
biomethanation and process stability, followed by the studies of hybrid
(MW-H2O2) pre-treatment. The impact of
hybrid (MW-H2O2) pre-treatment on
anaerobic digestibility of co-digestion is evaluated based on biogas
production, biogas quality, sludge hydrolyzation and process stability.
In addition, the effects of pre-treatment on the oxidative status of
anaerobic digestion mediated by superoxide radicals were studied along
with sludge bioactivity to establish knowledge on reactive oxygen
species in anaerobic digestion.