Thermophilic Digestion Research Articles

Effects of sludge properties in a combined process of mesophilic anaerobic digestion and thermophilic aerobic digestion

The combination of mesophilic anaerobic digestion and thermophilic aerobic digestion (MAN-TAD) has been recognized as a suitable technology to deal with conventional activated sludge. The MAN-TAD system can make sludge achieve better stability. The retention time of anaerobic digestion (AD-time) had an important role in the sludge stabilization process. With the extension of anaerobic digestion retention time, MAN-TAD system can make sludge achieve better stability. In comparison, volatile/total solids ratio (VS/TS) at AD-time of 38 d presented the lowest values among the three different AD-time with the value of 0.32. Long retention time of anaerobic digestion resulted in high amounts of soluble organic compounds (SCOD), and extracellular polymers (EPS) in the supernatant. Such high amount of organic compounds caused deterioration in sludge dewaterability. Compared with sludges of AD-time 17 d and 24 d, sludge of AD-time 38 d became extremely difficult to dewater due to its high capillary water absorption time (CST) at the end of digestion.

Антимикробные свойства биоцида на основе четвертичных аммонийных соединений и полигексаметиленгуанидина и потенциальные способы его дезактивации

Biocidal agents (BA) are widely used in environmentally safe toilet complexes (ESTC) of passenger railcars to suppress microbial activity in fаecal sludge (FS). Subsequent disposal of BA-containing FS at municipal sewage treatment facilities adversely affects their work due to the loss of activated sludge. The antimicrobial properties of BA, based on quaternary ammonium compounds (QAC) and polyhexamethylene guanidine (PHMG), as well as methods for its neutralization, have been studied. It was confirmed that BA based on QAC and PHMG has an antimicrobial effect on various groups of bacteria, reducing their number by 10-100 or more times. It was found that FS contains two groups of microorganisms with different sensitivity to BA. Methods for deactivation of the BA antimicrobial action in FS were tested using: (1), a deactivating agent; (2), incubation with thermophilically digested wastewater sludge as a source of the microbial methanogenic community; and (3), chemical deactivation by acidification or alkalization. The highest efficiency was shown by the method of deactivation of BA via thermophilic anaerobic digestion with pretreatment of FS with a strong acid biocide, fаecal sludge of ESTC of passenger railcars, antimicrobial activity, quaternary ammonium compounds, polyhexamethylene guanidine, deactivator, methanogenic microbial community This work was financially supported by the Ministry of Science and Higher Education of the Russian Federation.

Enhancement of Thermophilic Digestion of Food Waste (FW) via Trace Element Supplementation

In this study, the role of trace element (TE) supplementation and performance characteristics of a thermophilic anaerobic digester fed by food wastes (FW) is investigated in the long run, and a representative operational data set for field application is reported over the whole experimental period. Continuous feeding of food wastes with a dry matter of 5% for 150 days was carried out using a 100 L pilot-scale CSTR type anaerobic digester under thermophilic operation conditions. Hydraulic retention time (HRT) and organic loading rate (OLR) were kept around 28 days and less than 3.0 kg oDM m3 day-1, respectively. Volumetric biogas production values were reported to be 0.32 m3 m-3 day-1 during the period where there is no TE supplementation; on the other hand, biogas production was doubled (0.69 m3m-3day-1) following TE supplementation. This corresponds to an average unit biogas production of 317 and 443 L kg-1 oDM, during TE supplementation and no TE supplementation periods, respectively. Statistical analysis indicated that Co, As, Se, and Al were the most significant trace elements affecting the digester performance.

Benchmarking the sustainability of sludge handling systems in small wastewater treatment plants

A benchmarking strategy was developed to assess all aspects of sludge handling in small wastewater treatment plants and tested on a cross-section of Ontario facilities. Using operational data and on-site measurements, sustainability metrics that addressed energy consumption, chemical use, biosolids quality and disposition, and greenhouse gas (GHG) emissions were estimated. Electricity consumption for sludge handling ranged from 0.9 to 3.9 kW-hours per dry kilogram of raw sludge (kWh/dry kg) with thermo-alkali hydrolysis and auto-thermal thermophilic aerobic digestion (ATAD) processes consuming the least and most electricity for stabilization, respectively. Mechanical dewatering processes consumed between 2 and 5% of total sludge handling electricity, however, associated polymer dosages were higher than literature values in some cases. Disposition fuel requirements for plants with dewatering were up to 85% lower than facilities without dewatering. Biosolids contaminant (pathogen/metals) contents were observed to be substantially below Non-Agricultural Source Material (NASM) requirements. The copper content of the hauled biosolids exhibited the highest concentration relative to the NASM limit among all plants studied, ranging from 14 to 37% among facilities practicing land application of biosolids. Four biosolid products met Class A requirements for E. coli content, including one product generated via long-term storage. Carbon emissions ranged from −119 to 299 kg CO2 equivalents per dry tonne of raw sludge (g CO2 eq./kg). Six facilities that practiced land application exhibited net-negative GHG emissions; the carbon credits gained from fertilizer production avoidance outweighed emissions associated with sludge processing and transportation. The results provide evidence that this practice is sustainable from a GHG emissions standpoint. The benchmarking approach developed and information gathered is beneficial to plant owners and operators seeking to better understand how their utility is performing relative to peers, identify areas of need and further investigation, and improve the sustainability of their operations.

Comparison of Single-Stage and Two-Stage Thermophilic Anaerobic Digestion of SS-OFMSW During the Start-Up Phase

The accumulation of volatile fatty acids during the anaerobic digestion of biodegradable substrate such as Source Sorted Organic Fraction of Municipal Solid Waste (SS-OFMSW) is associated with an inhibitory effect on methane producing biochemical pathways. While inconclusive at times, the usage of two-stage digesters has been argued as a potential solution for this issue. In this study single-stage versus two-stage thermophilic digesters, fed with SS-OFMSW, were examined during the startup phase while increasing the loading rate from 0.5 to ~ 2 g VS L−1 d−1. While both systems exhibited a stable performance, with neutral pH and low intermediate-to-partial alkalinity, the two-stage digester exhibited better effluent quality with 79 and 57% lower average total chemical oxygen demand (TCOD) and soluble COD (SCOD), respectively. Also, upon reaching steady-state conditions, the two-stage system showed a superior performance with an overall methane content of 54%, compared to an average of 45% in the single-stage system.

Analysis of the implementation of biogas plants: pluses and minuses of the thermophilic digestion

Analysis of world practices in the implementation of biogas plants is presented for the growth energy efficiency in Russia. The object of study is biogas technologies used for the disposal of animal waste products. The subject of the study is a biogas plant for processing animal waste products, implemented in Russia. The purpose of the study is to develop a technological scheme for producing biogas for farms. The advantages of the thermophilic mode of fermentation include: an increased rate of decomposition of raw materials and, therefore, a higher biogas yield, as well as the almost complete destruction of pathogenic bacteria contained in the substrate. The disadvantages of thermophilic digestion include are the need to supply a significant amount of energy to heat the raw material in the digester and maintain its temperature, the increased sensitivity of the process to minimal temperature fluctuations and a slightly lower quality of the obtained fertilizers compared to the mesophilic regime, which maintains a higher amino acid composition of the fertilizer.

Occurrence and fate of antimicrobial triclocarban and its transformation products in municipal sludge during advanced anaerobic digestion using microwave pretreatment

This study, for the first time, investigated the impact of microwave pretreatment on the fate of the pervasive antimicrobial triclocarban (TCC) that was already present in municipal sludge, before and during advanced anaerobic digestion (AD) under thermophilic and mesophilic conditions. A range of microwave temperature (80 and 160 °C) and exposure duration (1 and 30 min) configurations were studied by employing ten bench-scale anaerobic digesters fed with mixed sludge at three different solids retention times (SRTs) including 20, 12, and 6 days. Seasonal changes influenced the levels of TCC in municipal sludge sampled from a plant employing the biological nutrient removal. Initial batch pretreatment studies showed that microwave irradiation itself can achieve TCC removal efficiencies up to 30 ± 4 and 64 ± 5% at 80 and 160 °C, respectively. The control digesters utilizing un-pretreated mixed sludge showed limited TCC removals, between 18 and 32% and 11–26% respectively, under thermophilic and mesophilic temperatures. On the other hand, the highest TCC elimination (78 ± 2%) was obtained from the thermophilic digester utilizing microwaved sludge at 160 °C for 30 min at SRT of 12 days. The non-chlorinated carbanilide (a transformation product of TCC) was detected and quantified for the first time during conventional and microwave-pretreated anaerobic sludge digestion. The formation of carbanilide in biosolids through reductive dechlorination could be an indicator of efficient and complete TCC transformation. This research demonstrated that AD coupled with microwave pretreatment can be used to reduce environmental concentrations of TCC in municipal sludge and biosolids.

Simulation of Heat-integrated Autothermal Thermophilic Aerobic Digestion System Operating under Uncertainties through Artificial Neural Network

The aim of the study is verification of a stochastic optimization approach for redesign of heat-integrated two-stage Autothermal Thermophilic Aerobic Digestion (ATAD) bioreactors system for municipal wastewater treatment operating under uncertainties. It was implemented through simulation of the ATAD system operation using feed-forward Artificial Neural Networks (ANN) for bioreactors modelling of the both stages and heat integration model of ATAD system with one heat storage tank. For the simulation purpose the design parameters values of the heat integration equipment associated with the obtained optimal solution of the stochastic approach were used. The ANN models were applied for prediction of the thermal shock occurred in the first bioreactor stage at incoming of each new portion of raw sludge, the expected temperature of the sludge at the end of the process and the volatile solids reduction at constant parameters of the inlet flows. They were included in two sequentially linked modules for simulation of the bioreactors operation. An appropriate data transfer between the modules, simulating the bioreactors and the Heat Integration module was provided. The simulation was carried out for two 15 d winter and summer periods. The simulation results have shown that applying the heat-integration of the process can lead to an increase in the temperatures of the inlet raw sludge about 6-8 °? and a decrease in the depth of thermal shock in both bioreactors about 5-7 °?. It also can provide higher and sustainable temperatures of the hot treated sludge at the end of the process which get much closer to the optimal of 55 °? for the first bioreactor and 65 °? for the second bioreactor and higher volatile solids reduction in both bioreactors about 1.5 wt.%.

Application of Flexibility Index Approach for Sustainable Operation of Heat-integrated Autothermal Thermophilic Aerobic Digestion System

The present study proposes a decision-making approach based on flexibility index calculation of already obtained solutions of a stochastic multi-scenario optimization model for redesign of heat-integrated two-stage Autothermal Thermophilic Aerobic Digestion (ATAD) bioreactors system for municipal wastewater treatment operating under uncertainties. Obtained solutions with minimal costs for redesign were presented in Vladova et al. (2018). Aiming to find the solution which provides sustainable operation of the heat-integrated ATAD system within the widest range of variation of its uncertain parameters, the Flexibility Index (FI) approach should be applied. A fast approach for evaluation of the flexibility index by presenting it as an inscribed a hyper-rectangle in the scaled hyper-space of variation of the uncertain parameters so as to ensure no violation of the process feasibility constraints is proposed. An optimization problem for determining the maximal flexibility index is formulated and solved for each solution obtained in Vladova et al. (2018). Decision making is carried out to choose most sustainable solutions for redesigned ATAD system. Analyzing the results obtained, it can be concluded that not always the solutions with the minimal annual costs for redesigning provide the sustainable operation of the ATAD system.

Responses of antibiotics, antibiotic resistance genes, and mobile genetic elements in sewage sludge to thermal hydrolysis pre-treatment and various anaerobic digestion conditions

Sewage sludge from wastewater treatment plants (WWTPs) harbours large amounts of antibiotics, antibiotic resistance genes (ARGs), and mobile genetic elements (MGEs), the variation and fate of these emerging pollutants during sludge treatment processes must be thoroughly studied to reduce their potential risks to human health. In this study, 7 pilot-scale CSTR anaerobic digesters were established with the same seed sludge and fed with the same thermal hydrolysis pre-treated sewage sludge, while operating under different conditions. High-throughput quantitative PCR, UPLC-MS/MS and Illumina Hiseq-sequencing were used to systematically evaluate the responses of antibiotics, ARGs, and MGEs in sewage sludge to thermal hydrolysis pre-treatment and various anaerobic digestion (AD) conditions. The results showed that thermal hydrolysis effectively reduced the abundance (>94%) of almost all subtypes of ARGs and MGEs, and it was a powerful technology for reducing tetracyclines, macrolides, and lincosamides. Besides, the abundance of ARGs and MGEs in thermophilic digesters was lower than that in mesophilic digesters, suggesting that thermophilic digesters could be used to avoid the ARGs rebounding. In addition, the thermophilic system further reduced the concentrations of quinolones. For the digesters operated under the mesophilic conditions, a longer hydraulic retention time (HRT) facilitated the removal of antibiotics, ARGs, and MGEs. Furthermore, the microbial community and MGEs had important effects on the persistence and proliferation of ARGs in AD process. The findings of this study provide effective clues for controlling the spread of antibiotic resistance and suggest the optimal operating conditions of digesters.

Air-side ammonia stripping coupled to anaerobic digestion indirectly impacts anaerobic microbiome.

SummaryAir‐side stripping without a prior solid–liquid phase separation step is a feasible and promising process to control ammonia concentration in thermophilic digesters. During the process, part of the anaerobic biomass is exposed to high temperature, high pH and aerobic conditions. However, there are no studies assessing the effects of those harsh conditions on the microbial communities of thermophilic digesters. To fill this knowledge gap, the microbiomes of two thermophilic digesters (55°C), fed with a mixture of pig manure and nitrogen‐rich co‐substrates, were investigated under different organic loading rates (OLR: 1.1–5.2 g COD l−1 day−1), ammonia concentrations (0.2–1.5 g free ammonia nitrogen l−1) and stripping frequencies (3–5 times per week). The bacterial communities were dominated by Firmicutes and Bacteroidetes phyla, while the predominant methanogens were Methanosarcina sp archaea. Increasing co‐substrate fraction, OLR and free ammonia nitrogen (FAN) favoured the presence of genera Ruminiclostridium, Clostridium and Tepidimicrobium and of hydrogenotrophic methanogens, mainly Methanoculleus archaea. The data indicated that the use of air‐side stripping did not adversely affect thermophilic microbial communities, but indirectly modulated them by controlling FAN concentrations in the digester. These results demonstrate the viability at microbial community level of air side‐stream stripping process as an adequate technology for the ammonia control during anaerobic co‐digestion of nitrogen‐rich substrates.

A snapshot of microbial community structures in 20 different field-scale anaerobic bioreactors treating food waste

This study aimed to identify significant factors shaping the microbial populations in biogas plants treating food waste (FW). Twenty full-scale anaerobic acidogenic/methanogenic bioreactors, located at 11 FW treatment facilities, were compared to find patterns in their microbial community structures and potential interactions with the process parameters. Temperature, hydraulic retention time, and organic loading rate were design parameters that systematically influenced the microbial communities. The latter two clearly separated the acidogenic and methanogenic bioreactors. Lactobacillus was the dominant (69.7 ± 19.8%) bacteria in the acidogenic reactors, while hydrogen-utilizing methanogens, such as Methanoculleus (65.1 ± 33.5%), were the dominant archaea in most methanogenic digesters. Defluiviitoga was the dominant (82.7 ± 1.4%) bacteria in the thermophilic digesters, but was also the most abundant (33.1–33.6%) bacteria in dry mesophilic digesters. The two bioreactor categories had lower bacterial diversities, and also higher propionate concentrations (>5 g/L in 4 out of 5 cases), which may impose potential risks for the management of such digesters. The current ‘snapshot’ of the microbial communities suggests several bacterial and archaeal taxa as potential indicators of bioreactor categories and/or process variables.

Autothermal Thermophilic Aerobic Digestion (ATAD) for Heat, Gas, and Production of a Class A Biosolids with Fertilizer Potential.

Autothermal thermophilic aerobic digestion (ATAD) is a microbial fermentation process characterized as a tertiary treatment of waste material carried out in jacketed reactors. The process can be carried out on a variety of waste sludge ranging from human, animal, food, or pharmaceutical waste where the addition of air initiates aerobic digestion of the secondary treated sludge material. Digestion of the sludge substrates generates heat, which is retained within the reactor resulting in elevation of the reactor temperature to 70–75 °C. During the process, deamination of proteinaceous materials also occurs resulting in liberation of ammonia and elevation of pH to typically pH 8.4. These conditions result in a unique microbial consortium, which undergoes considerable dynamic change during the heat-up and holding phases. The change in pH and substrate as digestion occurs also contributes to this dynamic change. Because the large reactors are not optimized for aeration, and because low oxygen solubility at elevated temperatures occurs, there are considerable numbers of anaerobes recovered which also contributes to the overall digestion. As the reactors are operated in a semi-continuous mode, the reactors are rarely washed, resulting in considerable biofilm formation. Equally, because of the fibrous nature of the sludge, fiber adhering organisms are frequently found which play a major role in the overall digestion process. Here, we review molecular tools needed to examine the ATAD sludge consortia, what has been determined through phylogenetic analysis of the consortia and the nature of the dynamics occurring within this unique fermentation environment.

Accelerated bio-methane production rate in thermophilic digestion of cardboard with appropriate biochar: Dose-response kinetic assays, hybrid synergistic mechanism, and microbial networks analysis

The effect of biochar on the thermophilic digestion of mono-cardboard was investigated. Compared with control group (T0), the maximum rate of biomethane production was significantly improved after the addition of biochar, especially, it has been improved by 40.6% in T1 (0.77 g/gTS sludge) with the methane production of 89.28 mL/gVS. Also, the addition of biochar improved the efficiency of acidogenesis and acetogenesis. However, adverse effects were observed with the biomethane production decreased by 33.98% and the lag phase extended by 35 h in T5 (3.86 g/gTS sludge). Especially, the results showed that the adsorption of biochar played important roles in digestion. In addition, acetoclastic Methanosaeta which considered to be involved in interspecific electron transfer (IET) was enriched after biochar added and the highest diversity of acetogens was obtained in T1. Oppositely, microbial networks analysis showed that the excessive biochar may destroy the diversity of microorganism drastically.

Fate of Antibiotic Resistance Genes and Virulence Genes in Enterococci During Anaerobic Digestion Process of Thermal Hydrolyzed Sludge

Municipal sludge contains large amounts of enterococci, which can harbor antibiotic resistance genes (ARGs) and virulence genes (VGs). ARG- and VG-containing enterococci therefore present potential resistance and virulence and, as a consequence, represent a significant health risk to humans. Therefore, the resistance phenotype of enterococci and the prevalence of ARGs and VGs in the enterococci isolated from the mesophilic (40℃) and thermophilic (55℃) anaerobic digestion of thermal hydrolyzed sludge was investigated. Results showed that the enterococci isolated from thermal hydrolyzed sludge showed significantly higher resistance to azithromycin than that to spiramycin and tetracycline. Thermophilic anaerobic digestion resulted in a greater reduction of enterococci abundance (by two orders of magnitude and one order of magnitude, respectively), and a greater reduction in the antibiotic resistance rates of the enterococci. However, thermophilic digestion can promote the expression of tetracycline resistance genes in the enterococci. Furthermore, both mesophilic and thermophilic digestion can facilitate horizontal genes transfer (HGT) between enterococci, which might result in an increase in the occurrence of double- or multiple-resistance. Mesophilic digestion reduced the prevalence of co-occurring ARGs and VGs in enterococci, while thermophilic digestion had the opposite effect. This research improves understanding of the occurrence and fate of ARGs and VGs in potential pathogens during the treatment of municipal sludge.

Cadmium Addition Effects on Anaerobic Digestion with Elevated Temperatures

Anaerobic fermentation with biogas as an energy source is influenced by the presence of heavy metals. However, the availability of the heavy metals is dependent on the digestion temperature. In this study, the impacts of Cd on the characteristics of biogas, substrate biodegradation, and enzyme activity during anaerobic co-digestion were investigated under varying digestion temperatures. The results showed that 1 mg/L initial Cd concentration improved cumulative biogas yields by 404.96%, 16.93%, and 5.56% at 55 °C, 45 °C, and 35 °C, respectively. In contrast, at low temperatures (25 °C), the yield decreased by 0.77%. In the 55 °C group, Cd addition improved the activity of cellulase (p < 0.05) and coenzyme F420 (p < 0.01). The total chemical oxygen demand (COD) during the peak period and the transformation of hydrolytic organic components into volatile fatty acids (VFAs) influenced the CH4 and biogas yields. There were no significant differences in cellulase, dehydrogenase, and coenzyme F420 activities with or without Cd addition when the digestion temperature was 45 °C, 35 °C, and 25 °C. Therefore, thermophilic digestion is recommended for the efficient degradation of Cd-contaminated biowaste. Moreover, the impact of metals on the performance of anaerobic digestion should be considered together with temperature conditions in future research and practice.

Rapid release of internal carbon source from excess sludge with synergistic treatment via thermophilic microaerobic digestion and microcurrent

Synergistic treatment via thermophilic microaerobic digestion and microcurrent was adopted to release the internal carbon source from excess sludge, and the effects of the microcurrent on sludge digestion and carbon source release were investigated. Compared with the control group where no current was applied, the digestion supernatant in the tested R1-45 °C reactor maintained a relatively lower phosphate concentration. The optimal technical parameters included a digestion temperature of 45 °C, a current intensity of 175 mA, and a digestion time of 96 h; the concentrations of soluble chemical oxidation demand (SCOD), total nitrogen (TN), and total phosphate (TP) for the test group were 12 189, 903, and 105 mg L−1, respectively, and the ratios of SCOD/TN and SCOD/TP reached 13.5 and 116.1. The sludge flocs were disaggregated into smaller particles upon thermophilic treatment coupled with microcurrent application, and abundant organic matters were released into the digestion supernatant, resulting in a rapid increase in the SCOD. The hydroxyl radicals produced by the microcurrent could directly oxidize fatty acids into end molecules, and it also facilitated regeneration of NAD+ (nicotinamide adenine dinucleotide) through the oxidation of NADH; the orthophosphate in digestion supernatant could be continuously absorbed to synthesize ATP, accompanied by accumulation of acetic and propionic acids. In addition, synergistic treatment accelerated the production of humic acid-like substances and played an important role in the transformation of soluble microbial products.

Thermal Inactivation of Escherichia coli and Salmonella Typhimurium in Poultry Carcass and Litter at Thermophilic Temperatures

SUMMARY The disposal of by-products, such as poultry litter, and carcasses is a serious issue because of risks associated with microbial pathogens, and controlling the pathogen risks requires identifying improved pre-treatment methods capable of inactivating pathogens. As poultry litter and carcasses are known to be major reservoirs of pathogenic microorganisms such as Salmonella and Escherichia coli (E. coli), improvement in existing understanding of the inactivation of these pathogens in poultry litter and carcasses is needed to determine the effective treatment time and temperature. Here we conducted a study to assess the thermal inactivation of 2 common bacteria in poultry productive systems: Escherichia coli (E. coli) and Salmonella enterica (Salmonella). The inactivation study was conducted at 50°C and 60°C using 3 different feedstocks: (1) poultry carcasses, (2) poultry litter, and (3) mixture of poultry litter and carcasses. Each feedstock was inoculated with known concentrations of E. coli and Salmonella prior to thermophilic digestion experiments at 50°C and 60°C. Regardless of feedstock types, E. coli survival was extended beyond 3 d at 50°C. In contrast, Salmonella was no longer detectable within 3 d at 50°C. At 60°C, both E. coli and Salmonella were undetected within an hour. There was no significant difference (at P

Long-term biogas desulfurization under different microaerobic conditions in full-scale thermophilic digesters co-digesting high-solid sewage sludge

Long-term (81 weeks) microaerobic biogas desulfurization was investigated in three full-scale thermophilic digesters in series co-digesting sewage sludge with co-products of source segregated domestic food waste. Residual oxygen (RO) concentrations in the digester headspaces were maintained at fixed values from 2.0% to 0.2% (v/v). Complete H2S removal was obtained at all tested RO concentrations under organic loading rate (OLR) and hydraulic retention time (HRT) varying between 1.5 and 2.3 kg VS/m3 d and between 44 and 71 d, respectively. Specific oxygen and air consumptions of 0.88 kg O2/ton VSin and 0.73 %ton air/ton biogas, respectively, were obtained by linear regressions with the RO values maintained in the biogas. Mass balance and stoichiometric evaluations revealed that from 13% to 59% of the consumed O2 was used for processes other than partial H2S oxidation to elemental sulfur (S0). The relatively high ammonium concentrations (3.5–3.8 g N–NH4+/kg digestate) in the liquid phase of the three digesters presumably limited the biological H2S production and S0 accumulation in the headspace. The results of this study can be used for designing the aeration systems of full-scale thermophilic digesters.

Thermophilic anaerobic conversion of raw microalgae: Microbial community diversity in high solids retention systems

The potential of microbial communities for efficient anaerobic conversion of raw microalgae was evaluated in this work. A long-term operated thermophilic digester was fed with three different Organic Loading Rates (OLR) (0.2, 0.3 and 0.4 g·L−1·d−1) reaching 32–41% biodegradability values. The microbial community analysis revealed a remarkable presence of microorganisms that exhibit high hydrolytic capabilities such as Thermotogae (~44.5%), Firmicutes (~17.6%) and Dictyoglomi, Aminicenantes, Atribacteria and Planctomycetes (below ~5.5%) phyla. The suggested metabolic role of these phyla highlights the importance of protein hydrolysis and fermentation when only degrading microalgae. The ecological analysis of the reactor suggests the implication of the novel group EM3 in fermentation and beta-oxidation pathways during microalgae conversion into methane. Scenedesmus spp. substrate and free ammonia concentration strongly shaped thermophilic reactor microbial structure. Partial Least Square Discriminant Analysis (PLS-DA) remarked the resilient role of minor groups related to Thermogutta, Armatimonadetes and Ruminococcaceae against a potential inhibitor like free ammonia. Towards low-cost biogas production from microalgae, this study reveals valuable information about thermophilic microorganisms that can strongly disrupt microalgae and remain in high solids retention anaerobic digesters.