## 1 Introduction  
Anaerobic digestion (AD) is a widely applied and mature technology that is used to treat the organic fraction of municipal wastewater and produces methane for biofuel, which is considered as an effective solution to the energy crisis and global pollution threat [1]. Among the different types of AD, primary sludge digestors (PSDs) and anaerobic membrane bioreactors (AnMBRs) receive the most attention for the biological treatment of wastewater. PSDs are mostly utilized in AD for wastewater treatment and are regarded as cheap and low energy input processes, with an overall positive energy output [2]. PSDs degrade the majority of the organic content in wastewater sludge, greatly reducing the total amount of solids produced. PSDs stabilize sludge to control odors while also reducing pathogens [3]. In addition to PSDs, AnMBRs have been widely incorporated into the anaerobic biological wastewater treatment process because of their solid-liquid separation and high effluent quality [4]. Due to their high operation stability, AnMBRs are suitable for treating wastewater with a high content of COD and suspended solids, high salinity, and large variations of flow and composition, even in the presence of fat, oil, and grease or inhibitory compounds [5; 6]. The complex microbial communities in AD play a significant role in maintaining stable operation and efficient biogas production [7]. To enhance biogas production in AD of organic wastes, it is vital to comprehensively understand the microbial communities involved in terms of their taxonomic composition; the interaction networks; and the relationships among operational conditions, biodiversity, and system functions [8]. AD degrades organic compounds and generates biogas and involves four sequential stages: hydrolysis, fermentation (acidogenesis), acetogenesis, and methanogenesis [9; 10]. AD is reliant on the tightly coupled synergistic activities of complex microbial consortia, which are delicately balanced between their two major domains: _Bacteria_ and _Archaea_. In general, bacteria are responsible for hydrolysis, acidogenesis, and acetogenesis, whereas methanogens in AD bioreactors are affiliated with _Methanobacteriales_, _Methanomicrobiales_, and _Methanosarcinales_[11; 12]. The composition and diversity of fermenting bacteria, syntrophic bacteria, and methanogens are very related to temperature, fed substrates, and process conditions [13; 14; 15]. The microbiome in PSD and AnMBR in AD remains only partially elucidated because these communities are rich in uncharacterized taxa, and they are difficult to explore with traditional cultivation-based methods [16]. Metagenomics was first applied to AD in 2008 with the analysis of a German full-scale biogas plant treating farm waste [17]. Since then, several studies have examined AD metagenomes, mainly focusing on taxonomy and gene-centric functional analyses [12; 18; 19; 20]. Recently, metagenomic analysis has shifted toward reconstructing the important metabolic pathways and genomes present in AD systems [21]. Of the AD metagenomes analyzed to date, samples have been obtained from full-scale biogas plants treating farm waste [17], industrial [19] and municipal [12; 20] sludge digesters, and lab-scale reactors [18; 21]. DWTS combined the advantage of PSD and AnMBR in wastewater treatment, particularly for low strength (COD \(<\) 1000 mg kg\({}^{-1}\)) municipal wastewater at relatively low temperatures (\(<\)18 \({}^{\circ}\)C). However, to date, no effort has been dedicated to analyzing the microbial community structure in DWTS, as well as the differences of microbial and functional divergence between PSD and AnMBR. Furthermore, the microbial interactions of the syntrophic consortium is pivotal in understanding the kinetics of bio-mediated element-cycling and methane generation in DWTS.