Characterization of the extracellular polymeric substances and microbial community of aerobic granulation sludge exposed to cefalexin

Highlights

• The presence of CLX decreased granular stabilities.

• EPS contents were significant changed.

• The species richness for bacteria was increased over the adding of the CLX.

• The abundance of Bacteroidetes and Chloroflexi underwent a highly significant increase.

Abstract

This study characterizes the extracellular polymeric substances and bacterial community composition of aerobic granules exposed to cefalexin (CLX). The presence of CLX potentially decreases granular stabilities, resulting in a lowered granule diameter. Chemical oxygen demand and ${{\text{NH}}_4}^{+}$–N removal efficiencies were slightly decreased and the denitrification process was inhibited with CLX addition. Extracellular polymeric substance contents were significantly increased in aerobic granules exposed to CLX. The shifts of fluorescence intensities and peak locations in 3D-EEM fluorescence spectra indicated changes of EPS components. High-throughput sequencing analysis showed aerobic granules with CLX addition in synthetic wastewater had superior diversity of microbial species, and this was the reason that the level and components of EPS changed. The species richness for bacteria was increased from 341 to 352, which was revealed by Chao1. The Shannon index of diversity rose slightly from 3.59 to 3.73 with CLX addition. The abundance of Proteobacteria significantly decreased, while the abundance of Bacteroidetes and Chloroflexi underwent a highly significant increase in aerobic granules exposed to CLX.

Introduction

Pharmaceuticals are used extensively to treat diseases in humans and animals. The growing occurrence of residues of human and veterinary pharmaceuticals, such as antibiotics, in the environment is causing increasing concern (Sarmah et al., 2006). The major sources of antibiotic residues in the environment include wastewater from hospitals, households, livestock farming, and pharmaceutical factories. Wastewater can contain extremely complex mixtures of various antibiotics and other drugs, such as cefalexin (CLX), chloramphenicol (CHL), carbamazepine (CAR), ibuprofen (IBU), and naproxen (NAP) (Sui et al., 2010, Krkošek et al., 2014). These kinds of pharmaceuticals can reach wastewater treatment plants (WWTPs) from different routes and are usually detected at levels ranging from ng L⁻¹ up to mg L⁻¹ not only in domestic and hospital effluents but also in effluents of pharmaceutical manufacturing facilities, which can present higher levels, reaching concentrations up to the mg L⁻¹ range (Lin et al., 2009, Yang et al., 2014).

Cefalexin is one of the most prescribed antibiotics and is produced in great quantities, as indicated by its frequent occurrence and high concentrations in municipal sewage treatment plant influents (Evgenidou et al., 2015). Common wastewater treatments such as conventional activated sludge systems are unable to remove pharmaceuticals and personal care products efficiently (Zhou et al., 2009). Although relatively higher removal efficiencies of CLX were observed after treatment in an activated sludge system, the presence of CLX in effluents is a clear indication that CLX cannot be removed completely in current sewage treatment plant processes (Liu et al., 2011, Liu et al., 2012). Previous studies have already shown that aerobic granular sludge is extremely promising for the treatment of effluents containing toxic compounds (Miao et al., 2014). Compared with conventional activated sludge, the aerobic granulation sludge is less sensitive to fluctuation than activated sludge, due to a higher tolerance to toxicity as well as a higher biomass concentration. To date, treatment of wastewater containing CLX has been reported using physical–chemical processes such as the electro-Fenton oxidation process and sorption processes (Ledezma Estrada et al., 2012). However, there is limited information available on the interaction of CLX and aerobic granulation sludge.

The limited literature available seems to indicate that when aerobic granulation sludge is used for removal of antibiotics, sorption is an important mechanism due to the porosity and high amount of surface area; initial adsorption onto the surface is followed by intra-particle diffusion (Shi et al., 2011). The production of EPS is a response to the stress situation. Extracellular polymeric substances (EPS) secreted by cells could contribute to the adhesion, promoting microbial aggregation and the formation of matrix structure, and enhancing the communication between cells and the stability of granules (Kong et al., 2014). Characterization of EPS contents and components of aerobic granules exposed to CLX has not been carried out previously. In addition, microbes responsible for degradation and the toxicity of CLX on those microbes should be investigated, as should the qualitative and quantitative changes to the microbial community structure when using aerobic granulation technology to remove CLX and their effect on the granules.

The aim of this study was to evaluate the removal of CLX using aerobic granulation technology. The effect of CLX on COD and nitrogen removal was investigated. Additionally, changes in EPS and the characterization of microbes in aerobic granular sludge exposed to CLX were also evaluated with three-dimensional excitation–emission matrix (3D-EEM) and high-throughput sequencing techniques, respectively.