Elsevier

Bioresource Technology

Volume 245, Part A, December 2017, Pages 502-510
Bioresource Technology

Impact of temperatures on microbial community structures of sewage sludge biological hydrolysis

https://doi.org/10.1016/j.biortech.2017.08.143Get rights and content

Highlights

  • VFA and biogas production increased with increase in biological hydrolysis (BH) temperatures.

  • BH microbial community structures varied with BH temperatures.

  • Different groups of fermenting bacteria were determined with BH at 35 °C, 42 °C, and 55 °C.

Abstract

This study investigated the biological hydrolysis performance at 35 °C (BH35), 42 °C (BH42), and 55 °C (BH55) and the effect of temperatures on microbial communities of the hydrolyzed sludge. The results showed that the suspended solid reduction, volatile fatty acids (VFA) production, and biogas production increased with the BH temperatures. VFAs produced in the sludge BH included acetic acid, propionic acid, isobutyric acid, butyric acid, and isovaleric acid with the fractions of acetic acid increased with BH temperatures. The Illumina MiSeq sequencing analysis showed that the microbial taxonomic structures of the BH systems varied with BH temperatures. It was found that Acidaminobacter at 35 °C, Proteiniphilum and Lutispor at 42 °C, and Gelria at 55 °C were the main protein fermenting bacteria genera, while the carbohydrate fermenting bacteria might belong to the genera of Macellibacteroides and Paludibacter at 35 °C, Fronticella at 42 °C, and Tepidimicrobium at 55 °C.

Introduction

Anaerobic digestion (AD) of activated sludge has gained increasing interests due to its essential role in reducing carbon footprints of wastewater treatment plants via energy recovery and waste reduction. Biological hydrolysis/acidification (BH) is a widely used pre-treatment process to increase the methanogenic degradation of complex organic compounds for methane production (Ding et al., 2017, Gao et al., 2011, Ucisik and Henze, 2008, Wang et al., 2014, Yang et al., 2015a). Temperature is one of the key BH process parameters that can exert a profound impact on the hydrolysis performance (Chu et al., 2008, Duarte et al., 2015, Liu et al., 2013). Although the effect of BH conditions on the biomass hydrolysis performance have been assessed by a number of studies (Chu et al., 2008, Zhang et al., 2009), knowledge of microbial communities structures in sludge BH systems is far from adequate for understanding sludge BH mechanisms.

The anaerobic conversion of organics into methane gas involves the hydrolysis, acidogenesis, acetogenesis, and methanogenesis steps (Lee et al., 2014). Hydrolysis and acidogenesis bacteria, or the so-called primary fermenters, catalyze the extracellular hydrolytic degradation of polymers into oligo- or monomers and intracellular conversion of sugars, amino acids, fatty acids and other monomers into fatty acids, lactate, alcohols, etc. Acetogenesis bacteria, or the secondary fermenters, degrade products of the primary fermentation into acetate, H2, and CO2. The methanogenesis converts acetogenesis products into methane gas via two main pathways: acetoclastic and hydrogenotrophic CH4 production (Da Silva et al., 2015). The complete conversion of complex organics to methane relies on syntrophic interactions of primary fermenters, secondary fermenters, and methanogens because mutual metabolic dependencies between different groups of anaerobic microorganisms eliminate the accumulation of intermediate hydrolysis products, making the conversion of complex organics to methane thermodynamically favorable (Schink and Stams, 2013). The syntrophic interactions in the sludge BH systems will depend on the microbial community structures and compositions. Thus, the determination of bacterial and archaeal community structures of BH systems would be of great importance for the understanding of microbial syntrophic interactions and process mechanisms of the sludge biological hydrolysis.

Illumina MiSeq Sequencing is an effective method for the characterization of microbial community structures. Compared to the conventional cultivation and biological molecular methods, Illumina MiSeq Sequencing can generate an enormous number of sequences, providing an excellent platform for the analysis of microorganism communities in wastewater treatment systems (Lin et al., 2016, Sheng et al., 2017, Xie et al., 2016, Zamanzadeh et al., 2016). With the use of Illumina MiSeq sequencing platform, Xie et al. (2016) revealed that the phyla of Bacteroidetes, Proteobacteria, and Firmicutes were dominant in the hydrolysis acidification reactors treating dyeing wastewater. Lin et al. (2016), who investigated the effect of temperature on microbial communities in the anaerobic digestion of organic food waste, revealed that the phyla of Firmicutes, Chloroflexi, Bacteoridetes, and Actinobacteria were dominant under mesophilic conditions while the phyla of Firmicutes, Thermotogo, Synergistales dominated under thermophilic conditions.

The objective of this study was to investigate the effect of temperature on microorganism communities in the sludge BH systems. Temperature is one of the most critical process parameters for the sludge BH treatment. It can exert a critical impact on the types of microorganisms, fermentation pathways, and end-products of sludge BH. In this study bench-scale hydrolysis and biochemical methane potential (BMP) tests were carried out to assess the performance of sludge BH at 35 °C, 42 °C, and 55 °C. The high throughput Illumina MiSeq sequencing platform was used to determine the microbial community structures at the BH temperatures tested. Based on the MiSeq sequencing results, the characteristics of microbial community structures, syntrophic interactions and main functional fermenting bacteria under mesophilic and thermophilic BH conditions were discussed.

Section snippets

Sludge source and hydrolysis experiments

The raw sludge (RS) and seed sludge used in the BMP tests were taken, respectively, from the secondary clarifier and the anaerobic digester of Guelph Wastewater Treatment Plant, Ontario, Canada, and were transported to the laboratory within 2 h.

For the BH experiments, the RS was filled into a 1.5 L plastic bottle without addition of seed sludge, flushed with N2 for 1 min, and then sealed for hydrolysis reactions. The hydrolysis experiments at 35 °C, 42 °C, and 55 °C were carried out in parallel by

Effect of temperatures on the solid solubilization and biogas production

Experiments demonstrated that BH could result in a considerable suspended solid solubilization and VFA production. Fig. 1a–c show the TSS and VSS reductions, VFA concentrations, and VFA fractions at the end of 6-day hydrolysis at 35 °C, 42 °C, and 55 °C. The VSS reductions of 35.4%, 37.3%, and 42.4% and TSS reductions of 28.9%, 31.0%, and 33.5% were achieved in the 6-day hydrolysis at 35 °C, 42 °C, and 55 °C, respectively (Fig. 1a). Accordingly, the VFA concentrations in sludge were increased from the

Impact of biological hydrolysis temperature on microbial structures

The experimental data obtained in this study showed the impact of temperatures on the BH of sewage sludge. The sludge BH at 55 °C enhanced the reduction of suspended solid (TSS and VSS) and production of VFA, in agreement with the previous studies reported by Zhang et al. (2009), who showed that the VSS reduction by thermophilic fermentation was greater than that by mesophilic fermentation. These notable processing changes of hydrolyzed sludge at different BH temperatures were further linked to

Conclusions

The results showed that the volatile suspended solids (VSS) reduction, volatile fatty acids (VFA) production, and biogas production increased with the increase in the BH temperature. The Illumina MiSeq sequencing analysis revealed: Bacteroidetes (46.8%), Firmicutes (19.8%), Proteobacteria (14.1%), and Spirochaetes (13.3%) dominated in the BH35 sludge; Bacteroidetes (37.9%), Firmicutes (30.9%), and Proteobacteria (25.2%) dominated in the BH42 sludge, and Proteobacteria (47.6%) and Firmicutes

Acknowledgements

The authors wish to thank Ontario Centres of Excellence (OCE), The Natural Sciences and Engineering Research Council of Canada (NSERC), and GE Water & Process Technologies for support. Special thanks to Guelph Wastewater Treatment Plant for providing sludge samples.

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