Elsevier

Process Biochemistry

Volume 88, January 2020, Pages 120-128
Process Biochemistry

Mg2+ distribution in activated sludge and its effects on the nitrifying activity and the characteristics of extracellular polymeric substances and sludge flocs

https://doi.org/10.1016/j.procbio.2019.10.002Get rights and content

Highlights

  • Low concentration of Mg2+ (< 3 mmol/L) promotes nitrification activity of sludge.

  • The distribution of Mg2+ in the activated sludge floc was as follows: pellet > supernatant > LB-EPS > TB-EPS.

  • The number of groups in EPS increased with the Mg2+ concentration.

  • Aromatic PN-like substances and humic acid-like substances were identified in LB-EPS and TB-EPS.

Abstract

Divalent cations act as bridges among extracellular polymeric substances (EPS) and form cross-linkage for the self-immobilization of microbial biomass. However, their effects on the nitrification performance during the biological nitrogen removal are still unclear. In the present study, the effects of Mg2+ on the nitrifying activity, EPS and floc characteristics were investigated using a lab-scale sequencing batch reactor. The distribution of Mg2+ was quantified at different level of sludge floc. The results indicated that the nitrification activity was significantly improved when influent Mg2+ was below 1.1 mmol/L, but suppressed at 3 mmol/L. The overall performance characterized by COD, NH4+-N and TN, the particle size and sludge flocculation ability rapidly increased with the increase of Mg2+ concentration. Mg2+ was mainly distributed in the pellet and changed slightly in supernatant, LB-EPS and TB-EPS. The four fluorescence peaks detected by three-dimensional excitation-emission matrix spectra were attributed to PN-like substances and humic acid-like substances in the LB-EPS and TB-EPS. The results of XPS analysis demonstrated that LB-EPS and TB-EPS comprised similar elements. Therefore, the types of EPS functional groups was unchanged under varied Mg2+ concentrations, while their proportions changed and LB-EPS/EPS was key factor for the changes of bioflocculation.

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:

  • (1)

    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.

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