Anaerobic co-digestion of commercial laundry wastewater and domestic sewage in a pilot-scale EGSB reactor: The influence of surfactant concentration on microbial diversity
Graphical abstract
Introduction
Surfactants are substances that have both polar (hydrophilic) and apolar (hydrophobic) regions in their chemical structures. This characteristic has led to their use in a wide variety of products, mainly in the field of detergents (soaps, soap powders, detergents). In this context, an anionic surfactant, namely, linear alkylbenzene sulfonate (LAS), has received attention. With a worldwide production of millions of tons per year, LAS is the most widely used anionic surfactant in the world (García et al., 2005). Commercially, LAS is sold as a mixture of homologs (depending on the size of the alkyl chain) and position isomers (depending on the position of the aromatic ring). LAS is structurally composed of an alkyl chain with different numbers of carbon atoms (from 10 to 14), while the hydrophilic part corresponds to the sulfonate group. The sulfonate group may be attached to any carbon atom except for the terminal carbons of the alkyl chain.
Inadequate treatment wastewater containing LAS can lead to the formation of foams, which inhibit the natural self-purification processes of rivers and lagoons as well as the aerobic and/or anaerobic processes in sewage treatment plants. Approximately 0.02–1.0 mg LAS L−1 in aquatic environments may cause gill damage and excessive mucus secretion in fish, decreased respiration in Pomatoschistus microps, and damaged movement in mussels (Venhuis and Mehrvar, 2004); while 40–60 mg LAS g−1 in soil interferes with the reproduction and growth of invertebrates (Holmstrup et al., 2001).
Due to the high consumption of surfactants, especially LAS, and due to the effects of the inadequate treatment of surfactants, there is a need to develop technologies to treat this macro-pollutant. In this context, the biological removal of LAS has been sought through the use of different configurations of anaerobic reactors. Commercial laundry wastewater is characterized by the presence of high concentrations of LAS (12–1024 mg LAS L−1) and organic matter (620–4800 mg DQO L−1; Braga and Varesche, 2014). Depending on the products used during washing (stabilizers and neutralizers), the sulfate concentration in commercial laundry wastewater can be as high as 372 ± 223 mg L−1 (Delforno et al., 2014). In the literature, few studies deal with the biological treatment of wastewater from commercial laundry; physicochemical treatments, such as advanced oxidative process and reverse osmosis, are the most commonly used treatment methods for commercial laundry wastewater.
Microbiology is a valuable tool for the elucidation of the microbial consortium that participates in the degradation of the LAS molecule. According to Almendariz et al. (2001), acidogenic bacteria are responsible for the degradation of this complex molecule, while methanogenic archaea are more sensitive to LAS and do not participate directly in the degradation process. In this regard, populations belonging to the Bacteria domain are significantly affected by LAS because they play a central role in the degradation of this compound. Higher surfactant concentrations and longer exposure times have a toxic effect on Bacteria, inhibiting or decreasing bacterial activity. On the other hand, low LAS concentrations (<3 mg L−1) can have a positive effect on bacterial populations (Mosche and Meyer, 2002).
The degradation of LAS is initiated by Ω-oxidation, which oxidizes the methyl end group, thereby enabling further β-oxidation. The second stage of LAS degradation begins by desulfonation of the sulfate group, followed by the opening of the aromatic ring, yielding carbonic gas, water, sulfate and biomass (Scott and Jones, 2000, Kuntze et al., 2008, Mungray and Kumar, 2009, Lara-Martin et al., 2010). Kuntze et al. (2008) proposed the use of the bamA gene as a molecular marker for the identification of microorganisms that have the coenzyme 6-oxocyclohex-1-ene-1-carbonyl, which is related to the last step of aromatic ring cleavage.
Therefore, the present study aimed to evaluate the influence of LAS concentration on the composition and dynamics of microbial communities during the anaerobic co-digestion of commercial laundry wastewater and domestic sewage in an expanded granular sludge bed (EGSB) pilot-scale reactor by using a combination of DGGE fingerprinting, 16S rRNA gene sequencing and bamA marker-gene amplification.
Section snippets
EGSB reactor and experimental setup
The EGSB pilot-scale reactor consisted of an acrylic and polyvinyl chloride (PVC) apparatus with a volume of 62 L, a height of 4.1 m and a diameter of 0.150 m (Fig. 1). At the top end of the reactor, there was a device to separate the solid, liquid and gas phases, and at the base of the reactor, there was a flow distributor. Twelve sampling points were installed in the reactor. The reactor was operated for 232 days at room temperature with a hydraulic retention time (HRT) of 40 h and an up-flow
Sequencing statistics, coverage and alpha/beta diversity
Overall, after quality control, the Illumina MiSeq 2 × 300 bp sequencing yielded 128,839, 111,059 and 220,538 sequences from samples 16S_0, 16S_4 and 16S_16, respectively. In the normalization process, using QIIME tools, the number of sequences retained for each sample was 111,000, with lengths of 420–470 bp. A total of 2997 OTUs were defined with 1,095, 1109 and 793 OTUs from samples 16S_0, 16S_4, and 16S_16, respectively. Coverage analysis (using the Good equation) showed that above 99% of
Conclusions
Low values of diversity and richness were obtained at the highest concentration of surfactant. An integrated analysis of the results revealed that the Pseudomonas genus may be associated with all steps of LAS degradation and is the only genus responsible for the Ω-oxidation; greater richness (>6 different genera) was observed in the other steps, indicating the need for complex microbial diversity for complete LAS mineralization. The detection of taxonomic groups with similar physiological and
Acknowledgements
The authors are grateful to the São Paulo Research Foundation – FAPESP, Processes no. 2015/08219-7, 2014/16426-0, 2016/11948-3 and 2015/06246-7 and the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Process no. 131391/2016-0 for their financial support.
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