Comparative Analysis of Bacterial and Archaeal Community Structure in Microwave Pretreated Thermophilic and Mesophilic Anaerobic Digesters Utilizing Mixed Sludge under Organic Overloading

Gokce Kor-Bicakci,Cigdem Eskicioglu,Emine Ubay-Cokgor

doi:10.3390/w12030887

Gokce Kor-Bicakci, Cigdem Eskicioglu + Show 1 more

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https://doi.org/10.3390/w12030887

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Abstract

The effects of microwave (MW) pretreatment were investigated by six anaerobic digesters operated under thermophilic and mesophilic conditions at high organic loading rates (4.9–5.7 g volatile solids/L/d). The experiments and analyses were mainly designed to reveal the impact of MW pretreatment and digester temperatures on the process stability and microbial community structure by correlating the composition of microbial populations with volatile fatty acid (VFA) concentrations. A slight shift from biogas production (with a reasonable methane content) to VFA accumulation was observed in the thermophilic digesters, especially in the MW-irradiated reactors. Microbial population structure was assessed using a high-throughput sequencing of 16S rRNA gene on the MiSeq platform. Microbial community structure was slightly affected by different MW pretreatment conditions, while substantially affected by the digester temperature. The phylum Bacteroidetes proliferated in the MW-irradiated mesophilic digesters by resisting high-temperature MW (at 160 °C). Hydrogenotrophic methanogenesis (mostly the genus of Methanothermobacter) was found to be a key route of methane production in the thermophilic digesters, whereas aceticlastic methanogenesis (mostly the genus of Methanosaeta) was the main pathway in the mesophilic digesters.

Highlights

There is a significant effort to develop sustainable approaches and technologies for the minimization of excess sludge production all over the world
After the pretreatment condition of MW2—160 ◦ C was applied, the soluble phase concentrations of chemical oxygen demand (COD), protein, humic acids, and sugar in the mixed sludge increased to 4028 ± 688, 199 ± 1.9, 276 ± 9.6, and 38.4 ± 1.3 mg/L normalized by % volatile solids (VS) by weight, whereas these ratios were found as 2107 ± 444, 30 ± 0.41, 61 ± 1.3, and 4.6 ± 0.2 mg/L/%VS by weight before the pretreatment
This means that the disintegration of the complex sludge structure was achieved by MW pretreatment and extracellular and intracellular biopolymers that were released into the soluble phase, resulting in faster hydrolysis and, higher anaerobic digestion (AD) improvement compared to conventional digestion

Summary

Introduction

There is a significant effort to develop sustainable approaches and technologies for the minimization of excess sludge production all over the world. Microwave (MW) irradiation is an effective thermal pretreatment method that has been proven to disrupt the floc structures and microbial cell membranes. Hereby, it improves sludge disintegration and the AD performance, while destroying pathogens and enhancing sludge dewaterability [7]. Many studies were conducted to investigate the impacts of different MW pretreatment conditions (e.g., temperature, sludge retention time (SRT), or heating ramp rate) on sludge disintegration and the performance of anaerobic digesters in batch and continuous flow mode [8,9,10,11,12]. The effects of MW temperature (80, 120, and 160 ◦ C) and heating ramp rate (3, 6, and 11 ◦ C/min) on sludge solubilization and performance of thermophilic batch AD were evaluated in the study of Hosseini Koupaie and Eskicioglu [8]

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