Intertidal wetland sediment as a novel inoculation source for developing aerobic granular sludge in membrane bioreactor treating high-salinity antibiotic manufacturing wastewater
Graphical abstract
Introduction
High-salinity wastewater was mostly generated from industrial processes, e.g. mariculture, seafood processing, petroleum, pharmaceuticals, etc. (Corsino et al., 2015, Song et al., 2018, Wang et al., 2017). Antibiotic wastewater is a typical industrial saline wastewater, characterized by problematic recalcitrant/toxic organics and high salinity (Shi et al., 2018). Treatment of antibiotic wastewater has induced a great of concern recently, because discharge of such wastewater without proper treatment could pose high risk to the ecosystem and public health. Currently, biological process with advantages of low-cost and high-effectiveness is still widely recognized as the most promising option for industrial saline wastewater treatment (Ou et al., 2018a). However, a challenge faced by biological process is the inhibitory effect of high salinity to microorganisms. The huge osmotic pressure imposed by high salinity could cause severe cellular plasmolysis and reduction of microbial activities involved in biodegradation and nutrients conversion (Ou et al., 2018b). Thus, enhanced biological processes (e.g. halophilic bacteria, biofilm system, etc. (Song et al., 2020)) with high tolerance to salinity stress were required to enable efficient treatment performance.
Recently, aerobic granular sludge (AGS) has attracted increasing interest as one of the most promising technologies for industrial saline wastewater treatment due to its distinctive compact and layered structure (Liu et al., 2011). On one hand, the outer layer comprised of dense discrete microbial cells and extracellular polymeric substances (EPS) matrix (Ou et al., 2018b) acts as buffer layer for external stress, minimizing the concentration of salinity or toxicants in inner cells, thereby improves the biological vitality under extreme conditions; On the other hand, the layered structure of AGS enabled the formation of multiple redox micro-environments (e.g. aerobic, anoxic and anaerobic) (Nancharaiah et al., 2018), where a great diversity of functional microorganisms can be enriched and performed excellent biodegradation and nutrients conversion abilities (simultaneous nitrification–denitrification) (Corsino et al., 2016a, Corsino et al., 2016b).
Integrating AGS with membrane bioreactor (AGS-MBR) has been demonstrated to be a further enhanced biological process for wastewater treatment due to the combination of advantages of both MBR and AGS (Li et al. 2005). On one aspect, AGS-MBR normally showed better pollutants removal performance than separate AGS or MBR system. The concentrated biomass and rich biodiversity in AGS enabled excellent organics degradation and simultaneous nutrients removal (Iorhemen et al., 2018, Li et al., 2012), moreover the high retention rate of membrane guaranteed steady good effluent quality. On the other aspect, spherical granular sludge with large size and compact structure exhibited substantially lower fouling rate compared with flocs sludge in conventional MBRs (Tu et al., 2010, Xuan et al., 2010). However, most of the reported AGS-MBR were operated using pre-cultured AGS through a separate sequencing batch reactor (SBR). In the perspective of practical application, direct AGS cultivation in MBR is more advantageous than a pre-culturing and seeding mode in terms of long-term stability and operation convenience. As of now, only a very recent study reported direct cultivation of AGS in a continuous-flow MBR with internal circulation (Chen et al., 2017), while no direct AGS cultivation in MBR under high salinity condition has been reported.
The cultivation of AGS was governed by a series of factors, i.e. seed sludge, substrate composition, feast-famine regime, reactor design (e.g. height to diameter H/D), settling time, hydrodynamic shear force, EPS, multivalent cations and particles (act as nuclei for granulation), etc. (Winkler et al., 2018). With regards to treating saline wastewater, the effect of salinity to AGS needs to be considered. Some previous studies found that the granule disintegration was observed when salinity was increased to a specific level (Jeison et al., 2008, Pronk et al., 2014). Contrarily, more recent researches demonstrated successful application of AGS in high salinity condition (Corsino et al., 2015, Corsino et al., 2017, Ni et al., 2020, Qi et al., 2020, Corsino et al., 2016a, Corsino et al., 2016b, Zhao et al., 2019). Li et al. (2017) reported that AGS was cultivated with 100% seawater (approximate salinity of 34 g/L), and the salinity not only facilitated granulation but also generated stronger granular structure; Abundant Ca2+ and Mg2+ in seawater played key roles in AGS formation (i.e. forming ALE cross-linkages and precipitates to initiate granulation). Ou et al. (2018b) also successfully developed a robust salt-tolerant AGS system by a stepwise increase of salinity, where the enrichment of high salt-tolerant bacteria was the main reason for AGS stability. Although the effect of salinity to AGS is still controversial, it is believed to be feasible to apply AGS for saline wastewater treatment with adequate cultivation and operation strategy.
Applying halophilic biomass as inoculant is a prospective approach of developing AGS for saline wastewater treatment. Intertidal wetlands, especially temperate-zone salt marshes, tropical mangroves, and mudflats, are environments with primary productivity that provide diverse essential ecosystem services (e.g. biodegradation, nitrogen conversion etc.) (McGenity, 2014). Owing to the rich biodiversity and high-salinity environment, intertidal wetland sediment (IWS) was observed to contain considerable amounts of halophilic/tolerant microorganisms that are able to survive under high salinity stress and perform pollutants removal (Shi et al., 2015). Besides this, the fine inorganic particles (mainly as CaCO3 from shells) in IWS can also work as nucleus for initiating AGS development. Given these, IWS could be a promising inoculant for developing AGS and treating saline wastewater. However, to date, the application of IWS for developing AGS under high salinity condition has yet to be documented.
Therefore, present study employed natural IWS as novel inoculant to directly cultivate AGS in a continuous flow membrane bioreactor (MBR) for treating saline antibiotics wastewater. To reveal the advantages of IWS over conventional activated sludge for AGS development under saline wastewater environment, two lab-scale MBRs were setup in parallel and operated under identical conditions. The sludge properties, pollutants removal performances, membrane fouling behavior and key microbial populations of the two MBRs were comprehensively analyzed and compared.
Section snippets
Saline antibiotic wastewater characterization
The wastewater for this study was collected from the equalization tank of a pharmaceutical company located in Singapore, which produces antibiotic of amoxicillin. The wastewater was collected and stored at 4 ◦C prior to feeding to the MBR systems with pH adjustment to about 7 using 1 M HCl. Amount of phosphorus was supplemented in form of KH2PO4 to ensure a COD: P ratio of 100: 1. Table 1 summarizes the characteristics of the wastewater used in this study. It comprised an average chemical
Granule development during system start-up
To reveal the granule formation process, the mean particle size and SVI30 of sludge were monitored for both IWS-MBR and Co-MBR systems throughout the whole start-up period of 30 d and the results are shown in Fig. 2a. Initially, similar mean particle sizes were observed in both IWS-MBR and Co-MBR (0.085 and 0.061 mm, respectively); While after a short acclimation of approximate 7 days, a number of small bio-aggregates (mean particle size of 0.32 mm) appeared in IWS-MBR indicating the initiation
Conclusions
IWS was used as natural microbial inoculant to cultivate AGS in MBR treating high salinity wastewater. The increased sludge particle size (3.1–3.3 mm) and improved sludge settling property (SVI of 23.8 ml/g) demonstrated that AGS were successfully cultivated in IWS-MBR. The abundant inorganic particulates and distinctive microbial structure in IWS played key roles in granulation. The IWS-MBR exhibited superior TOC (90.3 ± 6.1%) and TN reduction (31.2 ± 5.0%) efficiencies as well as longer
CRediT authorship contribution statement
Weilong Song: Conceptualization, Methodology, Formal analysis, Data curation, Writing - original draft. Dong Xu: Methodology, Formal analysis, Data curation. Xuejun Bi: Supervision, Conceptualization, Methodology, Writing - review & editing. How Yong Ng: Supervision, Conceptualization, Methodology, Writing - review & editing. Xueqing Shi: Funding acquisition, Supervision, Conceptualization, Methodology, Data curation, Writing - original draft.
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.
Acknowledgement
The research is supported by the Taishan Scholars Program of Shandong Province, China (No. tsqn201812091).
References (48)
- et al.
Aerobic granular sludge: recent advances
Biotechnol. Adv.
(2008) - et al.
The role of extracellular polymeric substances on aerobic granulation with stepwise increase of salinity
Sep. Purif. Technol.
(2018) - et al.
Cultivating granular sludge directly in a continuous-flow membrane bioreactor with internal circulation
Chem. Eng. J.
(2017) - et al.
Study of aerobic granular sludge stability in a continuous-flow membrane bioreactor
Bioresour. Technol.
(2016) - et al.
Cultivation of granular sludge with hypersaline oily wastewater
Int. Biodeterior. Biodegrad.
(2015) - et al.
Simultaneous nitritation-denitritation for the treatment of high-strength nitrogen in hypersaline wastewater by aerobic granular sludge
Water Res.
(2016) - et al.
Physical properties and Extracellular Polymeric Substances pattern of aerobic granular sludge treating hypersaline wastewater
Bioresour. Technol.
(2017) - et al.
A comprehensive study into fouling properties of extracellular polymeric substance (EPS) extracted from bulk sludge and cake sludge in a mesophilic anaerobic membrane bioreactor
Bioresour. Technol.
(2015) - et al.
Stability of aerobic granules during long-term bioreactor operation
Biotechnol. Adv.
(2018) - et al.
Simultaneous organics and nutrients removal in side-stream aerobic granular sludge membrane bioreactor (AGMBR)
J. Water Process Eng.
(2018)