Secondary effluent purification towards reclaimed water production through the hybrid post-coagulation and membrane distillation technology: A preliminary test

https://doi.org/10.1016/j.jclepro.2020.121797Get rights and content

Highlights

  • Secondary effluent purification towards production of high-grade reclaimed water .

  • Post-coagulation and membrane distillation hybrid procedure is proposed.

  • Emerging polytitanium coagulation is included with superior performance.

  • Clear reclaimed water, enhanced permeate flux and mitigated fouling are achieved.

Abstract

Shortage of water resource makes the production of high-grade reclaimed water become an important topic. This study proposed the post-coagulation and membrane distillation hybrid technology for further treatment of the secondary effluent to produce the reclaimed water with high quality. The emerging high-efficient polytitanium coagulant was utilized in post-coagulation unit, with the results suggesting its superior coagulation performance than the conventional polyaluminum chloride in terms of both particulates and organic matter removal. The titanium coagulated effluent then flowed to the subsequent direct contact membrane distillation unit for further purification. Permeated flux of the direct contact membrane distillation reached stable value of around 5.1 L/m2·h, which was accompanied by the production of the filtrate with high quality (turbidity < 0.4 NTU and dissolved organic carbon < 1.0 mg/L). Post-coagulation could not only withdraw the foulants from raw secondary effluent, but also improve the subsequent permeate flux and mitigate membrane fouling during membrane distillation process. The foulants attached on membrane surface was the main cause of membrane fouling during membrane distillation procedure. Both economical and practical prospect are included in this study, with the results demonstrated that post-coagulation of the secondary effluent followed by the subsequent membrane distillation procedure was a feasible and high-efficient strategy towards reclaimed water production.

Introduction

Municipal wastewater is mainly treated by activated sludge (AS) process including adsorption-biodegration (AB) process, sequencing batch reactor (SBR) activated sludge process and anaerobic-anoxic-oxic (A2/O) process (Daverey et al., 2019; Huang et al., 2019). Vast majority of the treated municipal wastewater is directly discharged into the receiving water bodies, which is actually a waste of valuable freshwater resource (Qiao et al., 2018; Gu et al., 2019). The secondary effluent of the conventional biological process is used as a significant source of the high-grade reclaimed water in Singapore, which is named NEWater (PUB, 2019). The treated-used water is further purified by the advanced membrane technology and ultra-violet disinfection to produce NEWater. The reclamation of secondary effluent from municipal wastewater treatment plants has received much more attention owing to its large reserve and relatively easy to reach the standard (Jin et al., 2013; Si et al., 2019).

The reclaimed water can serve as an alternative source of water for multifarious applications, such as agriculture, urban, environmental and industrial uses (Gikas and Tchobanoglous, 2009; Zalacain et al., 2019). Moreover, reclaimed water is a stable and comparatively untapped source of water, less affected by climatic and seasonal changes (Racar et al., 2019). Previous studies reported the production of reclaimed water from municipal wastewater through pressure-driven microfiltration (MF), ultrafiltration (UF) and reverse osmosis (RO) (Ghayeni et al., 1996; Ravazzini et al., 2005; Gu et al., 2019). Whilst, high energy consumption and serious membrane fouling restricted the application of pressure-driven membrane filtration technology.

Different to the pressure-driven membrane separation processes (MF, UF, RO, etc.), membrane distillation (MD) is a thermal-driven membrane separation method (Khumalo et al., 2019). Membrane fouling in MD procedure is less problematic because MD utilizes a microporous hydrophobic membrane whose pores are relatively large and not easy to be clogged (Lawson and Lloyd, 1997). Moreover, MD procedure requires low operating temperature, and the unit could be combined with alternative energy sources, such as industrial waste heat, solar and geothermal energy (Hejazi et al., 2019). Recently, application of MD technology for water purification has received increasing attention in water and wastewater treatment (Li et al., 2016).

The secondary effluent from municipal wastewater treatment plant contains a large amount of dissolved and refractory organic matter, inorganic substances and pathogens (Gao et al., 2019; Ibn Abdul Hamid et al., 2019). Pretreatment of the secondary effluent before MD filtration procedure is quite necessary. Coagulation, which can remove colloidal particles, suspended solids (SS) and natural organic matter (NOM), is one of the most commonly-used and economical wastewater treatment process (Zhu et al., 2016; Zhao et al., 2017). Previous studies reported wide application of primary coagulation with conventional Al-based coagulants before MF (Jin et al., 2015), nanofiltration (NF) (Yu et al., 2016) and UF-RO processes (Shang et al., 2019), with the results demonstrated the enhanced removal of foulants and reduced membrane fouling. Similar conclusions could be achieved during coagulation-MD hybrid process as previously reported (Wang et al., 2008; Li et al., 2016; Choudhury et al., 2019). Different from the biologically treated coking wastewater and recirculating cooling water as they reported, secondary effluent from municipal wastewater treatment plants has larger reserve and is relatively easier to reach the reuse standard. Since MD is a recently developed membrane filtration technology, few studies reported the performance of the hybrid coagulation and membrane distillation (C-MD) procedure for further purification of secondary effluent, especially using the emerging titanium coagulants for coagulation.

Compared with the conventional Al- and Fe-based coagulants, the emerging Ti-based coagulants are becoming increasingly attractive due to i) its strong coagulation capability at least comparable to that of Al- and Fe-based coagulants (Wu et al., 2011; Zhao et al., 2013, 2015), and ii) the titanium-coagulated sludge recycling to produce functional nano-TiO2 particles (Huang et al., 2015; Chi et al., 2019). This study investigated both coagulation performance of polytitanium chloride (PTC) for the treatment of secondary effluent (PAC coagulation as reference), and the subsequent MD filtration performance in terms of both permeation flux and membrane fouling. Influence of operation conditions (with/without coagulation, temperature and flow arte, etc.) on the performance of the hybrid C-MD process were included. Mechanisms of the PTC-coagulation and the membrane fouling during MD filtration were also studied.

Section snippets

Raw water and coagulants

Raw water was the secondary effluent of the A2/O process from the secondary sedimentation tank of Everbright Water Limited in Jinan, Shandong Province, China. General properties of the test water included: residual turbidity = 1.07 ± 0.14 NTU, zeta potential = −9.28 ± 3.48 mV, ultra-violet absorbance at 254 nm (UV254) = 0.097 ± 0.009 cm−1, pH = 7.91 ± 0.53 and dissolved organic carbon (DOC) = 16.42 ± 0.07 mg/L.

Two typical coagulants, polyaluminum chloride (PAC) and polytitanium chloride (PTC),

Performance of post-coagulation

Performance of post-coagulation was investigated in terms of both post-coagulation efficiency and post-coagulation mechanism. The emerging PTC coagulant was expected to produce the effluent with higher quality than conventional PAC coagulant. High-quality effluent indicated the superior performance of the subsequent DCMD procedure in terms of both enhanced permeation flux and reduced membrane fouling. Investigation of coagulation mechanisms would be helpful for the follow-on analysis of the

Conclusions

This study proved that coagulation-MD hybrid treatment process was feasible to purify secondary effluent towards reclaimed water production. Two coagulants, PAC and PTC, were utilized for secondary effluent pretreatment, followed by DCMD to further purify coagulated effluent for improving effluent quality and mitigate membrane fouling. The PTC coagulated effluent achieved lower residual turbidity and higher UV254 and DOC removal efficiencies, and the flocs formed by PTC had bigger size and

CRediT authorship contribution statement

Xiao Liu: Methodology, Software, Validation, Investigation, Formal analysis, Data curation, Writing - original draft. Chang Tian: Software, Resources, Supervision. Wenhao Sun: Investigation. Yanxia Zhao: Conceptualization, Methodology, Resources, Writing - review & editing, Project administration, Funding acquisition. Kaimin Shih: Supervision, Writing - review & editing.

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.

Acknowledgments

This work was supported by the grants from the National Natural Science Foundation of China (No. 51978311) and Natural Science Foundation of Shandong Province, China (No. ZR2019BEE044).

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