Encyclopedia
Please note this is a comparison between Version 1 by Lei Huang and Version 3 by Jessie Wu.
Extracellular polymeric substances (EPS)EPS are complex extracellular polymers with high molecular weight, which are metabolic products of microorganisms and result from effluent organic matter and microbial lysis or hydrolysis. EPS were proved to have the ability to absorb and biodegrade certain substances. The main components in EPS were found to influence the properties of microbial floccules, such as transfer, surface hydrophilicity/hydrophobicity, and aggregate stability.
- wastewater
- extracellular polymeric substances
- nitrogen removal
1. Fundamentals of Extracellular Polymeric Substances
1.1. Extracellular Polymeric Substances Structure and Distribution
1.1. EPS Structure and Distribution
Extracellular polymeric substances (EPS) have a three-dimensional structure with a gel-like and highly hydrated matrix; the microorganisms are embedded and more or less immobilized in EPS [1][14]. Owing to the distinct structure, EPS possess a vast surface area and carry numerous functional groups (e.g., carboxyl, phosphoric, amine, and hydroxyl) [2][15], significantly affecting the physico-chemical characteristics of sludge flocs, such as adhesion, hydrophobicity, settling, and dewatering. Two forms of EPS can be subdivided outside of microorganisms cells, including bound EPS (sheaths, capsular polymers, condensed gels, loosely bound polymers, and attached organic materials) and soluble EPS (soluble macromolecules, colloids, and slimes) [3][16]. Although the interaction between soluble EPS and cells is weak, the soluble EPS are found to have impacts on microbial activity and sludge properties [4][17]. However, the research on soluble EPS is limited, and the EPS mentioned in this resviearch w without being specified are bound EPS. Bound EPS surrounding bacteria or in sludge flocs likely possess a dynamic double-layered structure where tightly bound EPS (TB-EPS) forms an inner layer, and loosely bound EPS (LB-EPS) diffuses in the outer layer [5][18]. LB-EPS are highly hydrated and tend to form a dispersible and loose slime layer without a significant edge (dispersible part). Sludge settleability and dewaterability were found to be more strongly correlated with LB-EPS than TB-EPS [6][19], while TB-EPS were found to have effective flocculation activity compared to LB-EPS [7][20].
1.2. Methods of Extracting Extracellular Polymeric Substances
1.2. Methods of Extracting EPS
Several EPS extraction methods can be employed from various wastewater sludge (biofilm and aerobic or anaerobic activated sludge). Centrifugation is the most common extraction method for soluble EPS. Notably, lots of extraction methods for bound EPS have been generated, and new approaches are being developed [8][21]. These extraction methods can be classified as physical methods and chemical methods, listed in Table 1. The physical extraction methods usually adopt external forces, which are created by ultrasonic, centrifugation, or heating, to encourage the EPS to detach from cells and then dissolve in solution. The chemical extraction methods involve adding chemical compounds to disrupt the binding interactions between the EPS and the cells to accelerate the dissolution of the EPS. In general, the extraction efficiencies of the physical extraction methods are lower than those of the chemical extraction methods. As LB-EPS loosely bind with cells, mild methods, such as high-speed centrifugation, heating at low temperatures, or high-rate shear, should be utilized to prevent TB-EPS release. In addition to various physical methods, chemical methods, and enzyme treatments, the extraction of TB-EPS usually adopts more harsh combination methods, including CER + sonication, alkaline + heating, and formaldehyde + heating.
Table 1. Different Tightly Bound-Extracellular Polymeric Substances Extraction methods.
Different TB-EPS extraction methods.
| Methods | Mechanism | Features | Disadvantages | References | |
|---|---|---|---|---|---|
| Physical | Centrifugation | EPS are separated from cell surface and then dissolve to solution under the centrifugal force. |
|
|
[9][10][11][12][13][8,11,22,23,24] |
| Heating | EPS dissolution will be accelerated by enhancing molecular movement with heating. |
|
|
[10][11][14][15][11,22,25,26] | |
| Sonication | EPS of biofilm matrix are extracted under different impulsive pressures created by sonication. |
|
|
[10][11][16][17][18][11,22,27,28,29] | |
| Sonication/centrifugation | EPS will dissolve into solution under the impulsive pressure created by the sonication and centrifugal force. |
|
|
[11][22[19],30] | |
| Chemical | Acidic treatment | EPS are fallen away from the cell surface, as the interaction between EPS and cells is disrupted by the repulsive force. |
|
|
[9][10][11][8,11,22] |
| Alkaline treatment | Alkaline treatment with NaOH addition causes the groups to be ionized, resulting in a strong repulsion between EPS and cells. |
|
|
[9][10][8[11],11,22] | |
| CER | CER removes the divalent cations resulting in EPS falling apart. |
|
|
[9][10][11][20][8,11,22,31] | |
| EDTA | EPS matrix will fall apart, because divalent cations for the cross-linking of charged compounds are removed using EDTA. |
|
|
[9][10][11][21][8,11,22,32] | |
| Enzymatic extraction | The carbohydrate and protein-hydrolyzing enzymes are used to disrupt the structure of sludge and dissolve EPS. |
|
|
[10][11][22][11,22,33] | |
| HCHO/NaOH | HCHO reduces the cell lysis caused by NaOH addition. |
|
|
[10][18][23][11,29,34] | |
2. Biological Characteristics of Extracellular Polymeric Substances
2.1. Stability
In biological treatment systems, the stability of microbial floc is important to solid/liquid separation [24][35]. EPS exist in the structure of microbial floccules and interact with cells by polymer entanglement, electrostatic interaction, and ion bridging, as well as hydrogen bonds and van deer Waal’s force. These intermolecular interactions contribute to the stability of microbial aggregates. This means that higher EPS contents lead to higher stability in sludge [25][36]. Sludge has been reported to have a multiple-layer structure with two independent parts. The outer layer is dispersible and could be extracted readily, and the internal part is stable, where the non-readily extractable EPS glue the residual sludge cells. Thus, closer relationships with sludge stability exist among the readily extractable EPS [26][37].
2.2. Adhesion Ability
The formation of biofilm in a liquid environment is related to micro-surface characteristics. EPS present on cell surfaces could enhance microbial deposition [27][38]. Some functional groups in EPS contribute to microbial adhesion onto surfaces [28][29][39,40]. In the previous study, the number of adherent bacteria on the sludge surface was found to be reduced after EPS removal [30][41], and EPS-rich strains have stronger adhesion to bacteria than the EPS-deficient strains under similar surface charge conditions [31][42]. Thus, the adhesion of cells created by EPS can help to generate bioflocculation [32][43].
2.3. Biodegradability
Carbon and energy from EPS can be used by activated sludge [33][44]. Proteins and polysaccharides are the main substances in EPS, and the degradation enzymes for these polymers are adequate in biological wastewater treatment reactors. The excreted EPS were found to be utilized by bacteria for metabolic activity when there was a nutrient shortage [34][35][45,46]. However, some studies showed that some EPS could not be degraded by microbes [3][16]. EPS located in the inner layer of aerobic granular sludge were reported to be biodegradable, but the situation was on the contrary in the outer layer [36][47]. Although some EPS can be used as carbon resources, the degradation can cause deflocculation in the activated sludge system. Moreover, the non-degradable EPS can be discharged along with the effluent and bring negative effects on the effluent quality [37][48].
