- Introduction
Since the time of the industrial revolution, the utilization of fossil
fuels has severe environmental pollution which was the direct cause of
global warming (GW) because of the emission of greenhouse gases
(GHG). Therefore, the ‘road map’ indicates that by 2050,
according to internal energy agencies (REN21 report, 2013), about 75%
of the global primary energy supply ought to be renewable. Agro-food
wastes provide huge amounts of biodegradable materials which can be
recycling to recover energy or commercial products as bio-refinery
processes. In china alone, for example, the annual production of food
waste and crop straw are about 30 and 600 million tons, respectively
(Chen et al., 2012; Dongyan et al., 2014). Numerous negative
environmental aspects, such as aquatic life toxicity, altered soil
quality, phyto-toxicity, GHG and mal odor may be emerged because of
improper practices of such wastes (Nayak and Bhushan 2019). As a
consequence, it is crucial to develop a specific, efficient and
sustainable approach for treatment of agro-food wastes.
Anaerobic digestion, a microbial-based process where the microorganisms
play a pivotal role in degrading organic pollutants to biogas, is one of
the most efficient waste management strategies worldwide and thus
“two-in-one” advantage of waste disposal and energy production could
be achieved. AD is divided to four interdepended processes namely,
hydrolysis, acidogenesis, acetogenesis and methanogenesis where the
products of one stage are the substrate of the other stage until the
biogas is produced. The syntrophic relationship among the microorganisms
(bacteria and archaea) involved in these stages is the key for the
process stability.
Mechanisms of microbial assemblage in anaerobic digestion process remain
unclear. However, temperature, substrate composition, OLR and HRT
represented most important operating conditions, affecting structure of
anaerobic microbiome (Cho et al., 2017; Nag et al., 2019). The
disturbances in such parameters were reported to play a crucial role in
the microbial profiles shaping as a result of accumulation of some
intermediate products such as VFAs. The methanogens, for instance, are
the most sensitive to process disturbances and thus result in unbalance
between VFAs producers and consumers. As a matter of fact, there are
three basic behaviors could be distinguished in the microbial dynamics
under harsh conditions, namely, 1) resistance in which microbes can deal
with changes and thus no composition variation will take place, 2)
resilience in which microorganisms able to recover after upset and 3)
redundant in which new microbes have the ability to replace the
disturbed populations (Carballa et al., 2015). Even though
microorganisms involved in different AD stages are functionally
redundant (De Vrieze et al., 2017; Zhang et al., 2019), the distinct
members that are able to replace each other upon operational
disturbances need more investigation.
Since operational disturbances such as temperature fluctuation or over
feeding may accidently occur during the industrial-scale operation.
Therefore, it is essential to clarify the effect of the disturbances in
process performance and link it to microbial population. Indeed, the
impacts of operating parameters disturbances on process stability were
frequently investigated with regard to their effects on biodegradability
efficiency and biogas production (Mahdy et al., 2015). A sharp drop in
methane yield and high VFAs concentration (9000 mg/L) were observed when
OLR increased up to 6 g VS/ L/d (Li et al., 2015). However, kinetics
synergies among interdependent reactions and its correlating to
microbial dynamic and function under stable and harsh conditions still
so far unclear and require more investigation. Some investigations have
revealed microbial community composition in many healthy anaerobic
digesters to enhance the process management (Li et al., 2016; Li et al.,
2019). Other studies have even linked harsh conditions (extreme ammonia,
HRT, temperature and OLR disturbances) with microbial profiles in
healthy AD process. For instance, Tian et al., (2018) studied population
dynamics in digesters with step-wise increase in ammonia concentration
up to 10 g NH4/L with stable methane production (more
than 95% on uninhibited phase). Jiang et al. (2019) revealed the
stability of process performance and microbial structure under ambient
temperature. Mahdy et al., (2019a) demonstrated microbial community
shifts in the digesters with different OLRs and HRT under stable-state
conditions. A very few studies have taken the deteriorative phase into
accounts. Furthermore, although it is well accepted that high loading
rate with short retention time, for instance, could increase the process
capacity of the plant, the processes under such conditions have to be
handled carefully and the entire process management including the
response of the AD microbiome should be fully assumed.
Therefore, this study aimed to compare the process performance and the
population profiles that were stablished at optimal OLR and HRT in AD
process fed with agro-food wastes with that attained under organic
overloading and finite digestion time. To achieve this goal, 5 different
OLRs were investigated and meanwhile HRTs were shortenings down to 1.5
day, during which methane yield, methane production and major
intermediates were evaluated as well as phylogenic analyses targeting
16S rRNA sequences and quantitative polymerase chain reaction (qPCR)
were performed to monitor microbial communities of each state. Overall,
the objective of this study was the highlighting attempt to reveal how
processes respond to varying exterior effects and how the performance of
AD microorganism can be reacted and thus the comparison between
well-performed and disturbed microorganism could be used for
knowledge-based process control.