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