1. 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.