Mg2+ distribution in activated sludge and its effects on the nitrifying activity and the characteristics of extracellular polymeric substances and sludge flocs
Graphical abstract
Introduction
Activated sludge, which is composed of microbial populations, extracellular polymeric substances (EPS) secreted by bacterial metabolism and inorganic matters, plays a predominant role in the biological wastewater treatment processes since it can mineralize organic compounds due to microbial metabolism. There is a considerable interest in understanding the bacterial metabolites and the characteristics of the sludge flocs because nitrification and bioflocculation are major bottlenecks in improving the biological nitrogen removal capacity. Once the nitrifiers are washed out due to poor-flocculated sludge, recovery of the nitrification capacity is time-consuming due to the slow growth rates of the nitrifiers. It is important to maintain optimal bioflocculation of sludge for the nitrification process.
The presence of divalent cations was reported to have positive effects on bioflocculation as they could change the metabolites, settleability and filterability of the sludge flocs [1]. EPS mainly arises from the bacterial metabolism and its major constituents include proteins, polysaccharides, humic compounds, nucleic acids and lipids [2,3]. It has great effects on the properties of microbial aggregates based on the adsorption, adhesion, surface charge and biodegradation characteristics. The analysis of EPS characteristics is conducive to better understand the mechanisms of bioflocculation by the divalent cations.
Mg2+ is ubiquitous in the wastewater and it is also used as a conditioner during the wastewater treatment. Mg2+ enhanced the sludge microbial aggregation process through three ways: (1) Mg2+ could neutralize the negative charge on bacterial surface and promote the sludge aggregation [4]; (2) Mg2+ acted as a bridge and then formed EPS-Mg2+-EPS cross-linkage by binding to the functional groups of EPS [5]; (3) Mg2+ could interact with alkalinity and then form carbonate and phosphate precipitates [6]. Meanwhile, Mg2+ was a necessary inorganic salt for microbial growth as it could stimulate the enzyme reactions associated with the synthesis of cell materials [7], especially for microorganisms with low specific growth rates such as ammonia oxidizers and nitrite oxidizers. However, it was unclear how Mg2+ affected nitrification process and enhanced the enzyme activity of nitrifiers. Therefore, the impact research of Mg2+ on nitrification activity, EPS and flocculation characteristics is of great significance for the improvement of biological wastewater treatment performance.
In the present study, a lab-scale sequencing batch reactor (SBR) was used to study the corresponding distribution of Mg2+ in the activated sludge. The performances of nitrogen and COD removal and the nitrification activity of sludge were explored under different Mg2+ concentration. The effect of Mg2+ on EPS was investigated by employing three-dimensional excitation-emission matrix (EEM) fluorescence spectra and X-ray photoelectron spectroscopy (XPS). In addition, the influence mechanisms of Mg2+ on the bioflocculation were discussed based on the above information. The results of this study are expected to provide a theoretical basis for improving the stability and nitrification efficiency of wastewater biological treatment systems.
Section snippets
Experimental set-up and operation conditions
The experiment was carried out using a lab-scale cylinder SBR with a working volume of 4.0 L (Fig. 1). The SBR was operated two cycles each day. Each cycle included five steps: fill (15 min), aeration (240 min), anoxic reaction (120 min), settle (30 min) and draw (15 min). The replacement ratio was 80% and the hydraulic retention time (HRT) was 7 h. Fine air bubbles were supplied through a diffuser at the bottom of the bioreactor during the aeration. Temperature was kept at 20–23 °C. Dissolved
Effect of Mg2+ on the nitrification activity
Fig. 2 showed the changes of removal efficiencies of NH4+-N, TN and COD under different Mg2+ concentrations. The pollutants removal were positively affected by Mg2+ concentrations. The average removal efficiency of COD was stable at 90.7%. The average removal efficiencies of NH4+-N and TN increased from 92.3% and 65.3% to 98.1% and 74.7%, respectively. It demonstrated that microorganisms could adapt to lower divalent ion concentrations by regulating their own metabolic systems. Low
Conclusions
The effects of Mg2+ concentration on the nitrifying activity, EPS and floc characteristics of the activated sludge were investigated and the conclusions were as follows:
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The nitrification activity of the activated sludge promoted under low concentrations of Mg2+, while reduced under higher concentrations of Mg2+. When Mg2+ concentration was 1.1 mmol/L, the reactor had the optimal pollutant removal capacity and nitrification activity. Meanwhile, the sludge had good bioflocculation and
Acknowledgements
The authors thank to the National Natural Science Foundation of China (grant numbers 51678119 and 51808254), the Science and Technology Development Program of Jilin Province (grant numbers 20180201016SF, 20180101079JC and 20180101309JC) and the Science and Technology Research Project from Jilin Provincial Department of Education (grant numbers JJKH20180453KJ and JJKH20180454KJ) for their financial supports.
References (40)
- et al.
The relationship between cation ions and polysaccharide on the floc formation of synthetic and activated sludge
Desalination
(2008) - et al.
Bioflocculation in activated sludge: an analytic approach
Water Res.
(1993) - et al.
Effects of K+ salinity on the sludge activity and the microbial community structure of an A2O process
Chemosphere
(2019) - et al.
Quantification of the exchangeable calcium in activated sludge flocs and its implication to sludge settleability
Sep. Purif. Technol.
(2011) - et al.
Impact of calcium on the membrane fouling in membrane bioreactors
J. Membr. Sci.
(2008) - et al.
The dynamic effects of potassium limitation on biological phosphorus removal
Water Res.
(1996) - et al.
The influence of key chemical constituents in activated sludge on surface and flocculating properties
Water Res.
(2003) - et al.
Influence of loosely bound extracellular polymeric substances (EPS) on the flocculation, sedimentation and dewaterability of activated sludge
Water Res.
(2007) - et al.
Component analysis of extracellular polymeric substances (EPS) during aerobic sludge granulation using FTIR and 3D-EEM technologies
Bioresour. Technol.
(2012) - et al.
Effects of magnesium on thermophilic acetate-degrading granules in upflow anaerobic sludge blanket (UASB) reactors
Enzyme Microb. Technol.
(1993)
Effect of heavy metals on nitrification performance in different activated sludge processes
J. Hazard. Mater.
Adsorption of Cu2+ and Zn2+ by extracellular polymeric substances (EPS) in different sludges: effect of EPS fractional polarity on binding mechanism
J. Hazard. Mater.
Kinetics of inhibition in the biodegradation of monoaromatic hydrocarbons in presence of heavy metals
Bioresour. Technol.
Composition and functional group characterization of extracellular polymeric substances (EPS) in activated sludge: the impacts of polymerization degree of proteinaceous substrates
Water Res.
Characteristics of extracellular polymeric substances (EPS) fractions from excess sludges and their effects on bioflocculability
Bioresour. Technol.
The influence of additives (Ca2+, Al3+, and Fe3+) on the interaction energy and loosely bound extracellular polymeric substances (EPS) of activated sludge and their flocculation mechanisms
Bioresour. Technol.
Factors influencing the production of extracellular polymeric substances by Rhodopseudomonas acidophila
Int. Biodeterior. Biodegradation
New insights into the rapid formation of initial membrane fouling after in-situ cleaning in a membrane bioreactor
Process. Biochem.
Fate of NaClO and membrane foulants during in-situ cleaning of membrane bioreactors: combined effect on thermodynamic properties of sludge
Biochem. Eng. J.
Influences of extracellular polymeric substances (EPS) on the characteristics of activated sludge under non-steady-state conditions
Process. Biochem.
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2022, ChemosphereCitation Excerpt :Hence, the effects of sludge floc surface properties on lysozyme transfer also need to be studied. Divalent cations such as calsium ions (Ca2+) and magnesium ions (Mg2+) are commonly found in wastewater and WAS and are essential to the metabolism of microorganisms (Zhang et al., 2020; Zulkeflee et al., 2012). Moreover, the divalent cation bridging (DCB) theory states that divalent cations play a vital role in constructing the EPS matrix by binding negative function groups of EPS while monovalent cations like Na+ have no such effect (Kara et al., 2008).
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Lanhe Zhang and Qiannan Zhao are co-first authors.