###### Abstract  
Biological treatment of table olive processing wastewater (TOPW) may be problematic due to its high organic and polyphenic compound content. Biomass acclimatization is a necessary, yet sensitive, stage for efficient TOPW biological treatment. Next-generation sequencing technologies can provide valuable insights into this critical process step. An aerobic membrane bioreactor (MBR) system, initially inoculated with municipal activated sludge, was acclimatized to treat TOPW. Operational stability and bioremediation efficiency were monitored for approx. three months, whereas microbial community dynamics and metabolic adaptation were assessed through metagenomic and metatranscriptomic analysis. A swift change was identified in both the prokaryotic and eukaryotic bio-community after introduction of TOPW in the MBR, and a new diverse bio-community was established. _Thauera_ and _Paracoccus_ spp. are dominant contributors to the metabolic activity of the stable bio-community, which resulted in over 90% and 85% removal efficiency of total organic carbon and total polyphenols, respectively. This is the first study assessing the microbial community dynamics in a well-defined MBR process treating TOPW, offering guidance in the start-up of large-scale applications. Keywords:next-generation sequencing; membrane bioreactor; start-up; table olive processing wastewater; microbial community dynamics; biomass acclimatization +
Footnote †: journal: Applied Soft  
Biological wastewater treatment is likely the most widely used wastewater treatment technology [5]; nonetheless, the specific physicochemical characteristics of TOPW render its treatment uncertain through common biological processes [6; 7]. Membrane bioreactor (MBR) technology, which combines the biological degradation process of activated sludge with a direct solid-liquid separation through membrane filtration [8], has inherent advantages, including the complete retention of suspended biomass, and the growth of a widely diversified bio-community that enhances the breakdown, and eventual assimilation and mineralization, of difficult to biodegradable organic components (xenobiotics, phenolic compounds, etc.) [8]. [PERSON] et al. [9] demonstrated in long-term experiments (six months), that MBR technology, after appropriate active biomass acclimatization, could successfully be employed for TOPW treatment, exhibiting stable performance with high percentages of total organic carbon (TOC) and total polyphenols (TPh) removal. The inoculation and acclimatization of the suspended biomass during the start-up period of a MBR is an aspect that critically influences its performance, as in all such biological-based wastewater treatment processes [10; 11]. Considering the specific characteristics of TOPW, a prolonged acclimatization period for the biomass community may be required to achieve satisfactory biological treatment [6]. TOPW may either introduce new microorganism species in the MBR or can exert a selective pressure on the bio-community leading to the supremacy of those species able to grow under the specific physicochemical conditions. Specific microorganism species have been identified as capable to metabolize the toxic polyphenic compounds under such harsh conditions [12]. Studies on the acclimatization period of MBR systems have recognized the significance of advanced experimental techniques to gain useful insights into the phenomena involved [10] by analyzing the bio-community dynamics. In recent years, several studies have exploited next-generation, high-throughput, sequencing technologies (NGS) for phylogenetic and functional analysis of microbial bio-communities in environmental samples. Mainly, three NGS approaches are used to characterize microbial communities: (i) meta-barcoding or targeted amplicon sequencing, (ii) whole metagenome DNA sequencing, and (iii) whole metatranscriptome sequencing [13]. Activated sludge has been used as a specimen for developing novel analytical methods and data-processing techniques in metagenomic of complex samples [14; 15; 16].