Bacterial community structure in treated sewage sludge with mesophilic and thermophilic anaerobic digestion

Abstract

Stabilized sewage sludge is applied to agricultural fields and farmland due to its high organic matter content. The aim of this study was to investigate the effects of two types of sludge stabilization, mesophilic anaerobic digestion (MAD) and thermophilic anaerobic digestion (TAD), on bacterial communities in sludge, including the presence of pathogenic microorganisms. Bacterial community structure and phylogenetic diversity were analyzed in four sewage sludge samples from the Czech Republic. Analysis of 16S ribosomal RNA (rRNA) genes showed that investigated sludge samples harbor diverse bacterial populations with only a few taxa present across all samples. Bacterial diversity was higher in sludge samples after MAD versus TAD treatment, and communities in MAD-treated sludge shared the highest genetic similarities. In all samples, the bacterial community was dominated by reads affiliated with Proteobacteria. The sludge after TAD treatment had considerably higher number of reads of thermotolerant/thermophilic taxa, such as the phyla Deinococcus-Thermus and Thermotogae or the genus Coprothermobacter. Only one operational taxonomic unit (OTU), which clustered with Rhodanobacter, was detected in all communities at a relative abundance >1 %. All of the communities were screened for the presence of 16S rRNA gene sequences of pathogenic bacteria using a database of 122 pathogenic species and ≥98 % identity threshold. The abundance of such sequences ranged between 0.23 and 1.57 % of the total community, with lower numbers present after the TAD treatment, indicating its higher hygienization efficiency. Sequences clustering with nontuberculous mycobacteria were present in all samples. Other detected sequences of pathogenic bacteria included Streptomyces somaliensis, Acinetobacter calcoaceticus, Alcaligenes faecalis, Gordonia spp., Legionella anisa, Bordetella bronchiseptica, Enterobacter aerogenes, Brucella melitensis, and Staphylococcus aureus.

Is anaerobic digestion a reliable barrier for deactivation of pathogens in biosludge?

“…Actinomycetales as typical member) were found to be the dominant phyla in raw sludge, while their abundances declined during the digestion, along with the enrichment of Bacteroidetes. It was also found that the microbial diversity was further lowered when AD was turned from mesophilic to thermophilic operation with notable shifts towards the higher number of thermotolerant taxa and lower abundances of Bacteroidetes and Proteobacteria (Sun et al, 2016;Stiborova et al, 2015). It is worth to note that Bacteroidetes and Proteobacteria had been considered as potential pathogens that harbored ARGs, while Actinomycetales, Bacilli Corynebacterium were known as sul and integron carrier (Sun et al, 2017).…”

“…Among the detected pathogens, Escherichia coli, Salmonella, Yersinia enterocolitica, Enterococcus were vulnerable to AD, while Campylobacter jejuni, and Streptomyces, Collinsella aerofaciens, Streptococcus salivarius and Gordonia bronchialis etc. were reported to be much more resistant to and hardly removed by AD (Ju et al, 2016;Kearney et al, 1993b;Stiborova et al, 2015). It should also be noted that some pathogens can be enriched, and even some become emerging and reemerging after AD (Li et al, 2015).…”

The presence of contaminations in sewage sludge – The current situation

“…Sewage sludge processing, mainly hygenization with the use of high temperature, anaerobic digestion, trickling filters (TF) or autothermal thermophilic aerobic digestion (ATAD) may control the presence of pathogens as E. coli or Salmonella (De los Cobos-Vasconcelos et al, 2015;Fu et al, 2014;Marin et al, 2015) and modify bacterial community structure. Stiborova et al (2015) described that sludge after TAD treatment had considerably higher number of thermotolerant/thermophilic taxa, such as the phyla Deinococcus-Thermus and Thermotogae or the genus Coprothermobacter. However, the occurrence of fecal contamination indicators is frequently not correlated with the presence of other pathogenic microorganisms that may inhibit sewage sludge and survive the treatment processes.…”

Dynamics of brominated flame retardants removal in contaminated wastewater sewage sludge under anaerobic conditions

“…The most common methods of sludge use and disposal are agriculture, compost, landfilling and incineration. Before land application, sludge is treated to reduce odour and pathogens (Stiborova et al, 2015b). Furthermore, the land-applied sludge needs to meet the regulatory criteria for the amount of heavy metals.…”

“…As the molecular methods have further advanced, not only have been methanogens described to be highly abundant during anaerobic sludge digestion but also in various terrestrial and aquatic environments (Brablcova et al, 2015;Praeg et al, 2014). In the anaerobically stabilized sludge from WWTPs Brno and Hradec Kralove methanogenic populations were detected previously (Stiborova et al, 2015b).…”