Elsevier

Water Research

Volume 205, 15 October 2021, 117706
Water Research

Recovery of extracellular biopolymers from conventional activated sludge: Potential, characteristics and limitation

https://doi.org/10.1016/j.watres.2021.117706Get rights and content

Highlights

  • Biopolymers/ALE extraction from CAS ranged from 90 to 190 mg/g VSS.

  • Good performances of ALE from CAS mean a potential application as commercial alginates.

  • Compositional content and property of ALE from CAS were dependent.

  • Influent substrate and nutrient content affected the ALE formation.

  • Microbial community was a decisive factor affecting the ALE formation.

Abstract

Extracellular polymeric substances (EPS) are biopolymers that can be recovered from excess sludge, which could contribute to a more sustainable wastewater treatment plant (WWTP) operation. An example is alginate like extracellular polymers (ALE) contained in the biopolymers could be a potential resource with a highly-added value. EPS extraction for ALE from aerobic granules sludge (AGS) has already been well studied and applied in the Netherlands. On the other hand, there is little attention to the recovery of biopolymers from conventional activated sludge (CAS). In this study, flocculent sludge from eight CAS-WWTPs in China was collected and their EPS/biopolymers were extracted to investigate their recovery potential, chemical & physical properties and limiting factors. The results revealed that the biopolymers extracted and purified from CAS ranged from 90 to 190 mg/g VSS. The compositional characteristics of the biopolymers were observed by FT-IR, 3D-EEM and UV-Visible spectra, demonstrating some differences in the composition and property of the biopolymers from the different WWTPs. The biopolymers had a similarity of about 60% to a commercial alginate with respect to chemical functional groups and the alginate equivalent was >400 mg/g biopolymers. Moreover, the biopolymers consisted of poly (guluronic acid) blocks (20%-30%) and poly (guluronic acid-mannuronic acid) blocks (8%-28%), and the ionic hydrogel formation tests indicated that condensed beads were immediately formed once the drops of the biopolymers came in contact with CaCl2 solution. These results demonstrated that the biopolymers extracted had a relatively high gel-forming capacity and might also have a potential application as commercial biopolymers. Furthermore, the factors influencing the biopolymers’ formation such as influent substrate, nutrient content and microbial community and the related mechanisms were investigated. Among them, increasing soluble organics (SCOD) content and low nutrient content (C/N/P) in the influent could promote the biopolymers’ formation. Also, different bacteria in BNR processes might have positive or negative effects on the biopolymers’ formation. In conclusion, the diversity and abundance of bacteria were identified to be a crucial and decisive factor controlling biopolymers’ extraction and composition.

Introduction

Resource recovery from wastewater is currently emphasized to develop a sustainable wastewater treatment system. In this context, excess sludge can be considered a potential resource for recovering highly-added value products such as phosphate, cellulose, bioplastics, biopolymers, etc. (van Loosdrecht and Brdjanovic, 2014; Li et al., 2015). Activated sludge flocs are actually a matrix of bacterial cells embedded in extracellular polymeric substance (EPS). The EPS accounts for about 10%−40% of the total dry weight of activated sludge (Basuvara et al., 2015). EPS mainly consists of polysaccharides, proteins, lipids, nucleic acids and humic substances (Felz et al., 2016; Boltz et al., 2017). The EPS substances embed the microcolonies and form a matrix in which these microcolonies and single cells are embedded by flocculation. Various physical and biochemical interactions such as electrostatic force, hydrogen bonding or ionic bonds form the mechanisms for the floc formation and stability (Basuvaraj et al., 2015; Mahendran et al., 2012). EPS is considered as the protective matrix for environmental pressures, mechanical stability, external heavy metals and toxic compounds and also as the storage of carbon or water (Sheng et al., 2010; Schambeck et al., 2020). The biopolymers in the matrix form a unique structure that can be recovered for potential highly valuable biomaterials (Sheng et al., 2010; More et al., 2014).

Much attention on recovering the biopolymers has focussed on alginate like extracellular polymers (ALE) from aerobic granular sludge (AGS) (Lin et al., 2015, 2018; Xue et al., 2019). ALE is a biopolymer that can be used in the food, paper, textile, medical and construction industry and also in agriculture and horticulture (Rehm 2010; Lin et al., 2015, 2018; Xue et al., 2019; Felz et al., 2020; Kim et al., 2020). Lin et al. (2015) have evaluated the performance of ALE extracted and purified from AGS as non-flammable surface coating and Kim et al. (2020) showed that ALE could comply with the aviation requirements for aircraft interiors by the Federal Aviation Regulations (FAR). Currently, the extraction of the EPS/biopolymers from AGS has been scaled-up to demonstration scale in an installation in Zutphen (the Netherlands). Because of its unique properties, the biopolymer/ALE has been branded Kaumera®.

Kaumera recovery from AGS is moving towards a commercial market introduction. However, according to the existing studies, the content of EPS from conventional activated sludge (CAS) is only 7%−19% VSS (Lin et al., 2010, 2013; Yang et al., 2014; Schambeck et al., 2020), which is lower than that of AGS (35.1 ± 1.9% SS) (Lin et al., 2010; Kim et al., 2020). On the other hand, the total amount of the excess sludge in conventional WWTPs is huge. For example, there was more than 60 million t/a (80% moisture content) of excess sludge from CAS in China in 2019. Moreover, handling excess sludge accounts for nearly a half of the total investment and operation costs of WWTPs (Kroiss, 2004). If the EPS recovery could be applied as high value biopolymers like ALE from AGS, it could offset some the costs of handling sludge and also make WWTPs more sustainable towards the circular economy (van Loosdrecht and Brdjanovic, 2014; Li et al., 2015; Hao et al., 2019). Conversely, there have been relatively few studies on recovering extracellular biopolymers from CAS, while the existing studies on EPS from CAS mostly focused on identifying and evaluating their roles in biological nutrient removal and sludge disposal processes (Sheng et al., 2010; Seviour et al., 2019). The economic value of EPS from CAS seems largely ignored. The EPS recovery from CAS could give a large potential for extraction and application.

For this reason, a study was initiated to investigate EPS/biopolymers recovery from CAS, towards ALE. Based on the excess sludge from eight CAS-WWTPs from different regions of China, EPS/biopolymers were extracted and purified by the previous methods (Felz et al., 2016, 2019), to obtain the yield and the purity of ALE. Then, the amounts and compositions of ALE were investigated and evaluated by comparing with Kaumera (ALE) from AGS. Next, the physical and chemical properties of the biopolymers/ALE were analysed to clarify their performance gaps relative to commercial alginate. Finally, the influences and related mechanisms of such factors as substrate and nutrient content in influent were summarized to provide some possible technical strategies for increasing the EPS/biopolymers production in CAS.

Section snippets

Sludge characteristics

Flocculent excess sludge was collected from eight WWTPs (WWTP 1–8, the conventional biological nutrient removal processes for municipal wastewater) in different regions of China. The locations and flocculent excess sludge characteristics were listed in Table S1. The average influent quality and other detailed information are listed in Table 1. The collected flocculent sludge was concentrated by a 0.15-mm filter, sieved, and then stored in a refrigerator (4 °C) for use.

Extraction protocols

Extracellular biopolymers

Extraction potentials

The amounts of extracellular biopolymers extracted from different flocculent sludge (WWTP 1–8) are shown in Fig. 1. WWTP 3 had the lowest yield of the biopolymers (92.9 ± 3.3 mg/g VSS) about half as compared to the highest yield from WWTP 1 (187.9 ± 12.3 mg/g VSS). The amounts of the biopolymers ranged from 90 to 190 mg/g VSS (9%−19% VSS). Interestingly, the biopolymer yields are comparable to those analyzed in the previous studies with the same extraction methods, e.g., 187±94 mg ALE/g VSS

Factors limiting biopolymers formation

Obviously, extracting biopolymers from the flocs of CAS is attractive as a potential substitute for commercial biopolymers. However, there was still the difference on extracted biopolymers amount and its physical and/or chemical properties from CAS in different WWTPs and also comparing with AGS, as indicated above. Thus, it is necessary to further analyze the factors causing the difference and limitation, including influent substrates, nutrient content and microbial community, to figure out the

Conclusions

The yield and the purity of the extracellular biopolymers extracted from the flocs of conventional activated sludge (CAS) were analyzed and evaluated, and the physical and chemical properties of the biopolymers were also elucidated. Moreover, the influencing factors and related mechanisms of the biopolymers formation were identified. Based on the results, main points can be addressed as follows:

  • Ø

    The biopolymers/ALE extraction from the flocs of CAS (eight wastewater treatment plants, WWTPs)

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The study was financially supported by the National Natural Science Foundation of China (51878022), Beijing Advanced Innovation centre of Future Urban Design (2021).

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