Enhanced nutrients enrichment and removal from eutrophic water using a self-sustaining in situ photomicrobial nutrients recovery cell (PNRC)
Graphical abstract
Introduction
In recent years, the presence of elevated levels of inorganic nutrients (particularly nitrogen and phosphorus) in surface water, due to excessive agricultural/industrial production and uncontrolled wastewater discharges, has become an utmost concern of populations, as it leads to eutrophication of aquatic systems even serious threats to human health (Bricker et al., 2018; Sinha et al., 2017; Wilson et al., 2018). Conventional biological treatments such as activated sludge systems and anaerobic/anoxic/oxic processes, which are known to be efficient in removing nitrogen and phosphorus from various wastewater (Hou et al., 2018; Xiao et al., 2014), exhibit poor performance for surface water with low nutrient levels and not feasible for treating widely distributed surface waters. In addition, traditional ex-situ treatment techniques usually suffer from high energy or chemical consumption, lacking practicality and generating secondary pollution. In-situ treatment technologies have garnered particular attention for environmental compatibility, low energy consumption, and high operational flexibility. This gives priority to the exploitation of an advanced in situ nutrients removal technology for surface water in an environmentally benign and sustainable way.
Bioelectrochemical systems (BESs), which can accomplish the removal of organics while simultaneously recovering energy in form of electricity, hydrogen, or methane, has drawn extensive attention and offered new information to an entirely renewable wastewater treatment concept (Li et al., 2014; Liu et al., 2014; Logan and Rabaey, 2012). One of the BES - microbial desalination cell (MDC), exhibits great promise for desalination through deriving biochemical energy from oxidizing organics in wastewater (Ping et al., 2016; Yuan et al., 2016). The desalination function was achieved by introducing an additional chamber between the anode and cathode chamber (separated by a cation exchange membrane and an anion exchange membrane) and utilizing difference of the electrode redox potential to deploy the migration of anions (i.e. chlorine) and cations (i.e. sodium) towards the anode and cathode chamber, respectively (Chen et al., 2016). During the past few years, MDCs have demonstrated with versatile applications to remove or recover ionic substances from various impaired waters, among which MDC for nutrients recovery has drawn wide attention (Chen et al., 2017; Hou et al., 2017; Qin et al., 2018). A novel MDC namely microbial nutrient recovery cell (MNRC) was recently developed to recover nutrients in domestic water (Chen et al., 2015). In-situ treatment for nitrate-containing groundwater was achieved by a submerged microbial desalination-denitrification cell (Zhang and Angelidaki, 2013). Therefore, MDCs are good candidates for nutrients recovery from eutrophic water.
Microalgae are photosynthetic microorganisms, universal in various marine and freshwater environments (Bahadar and Bilal Khan, 2013; Safi et al., 2014). Microalgae-based technologies exhibit great promise for wastewater treatment, as microalgae can not only efficiently uptake nitrogen and phosphorus from wastewaters, but also recycle these nutrients into microalgae biomass which can be further converted to biofuels or other valuable co-products (Beuckels et al., 2015; Mennaa et al., 2015; Mujtaba and Lee, 2017). Besides, microalgae-based wastewater treatment provides a cost-effective and safe alternative to mechanical aeration, as microalgae could release oxygen through photosynthesis (Arbib et al., 2014; Su et al., 2011; Zhao et al., 2018). These advantages enable microalgae to be a promising technology for nutrient recovery (Whitton et al., 2016). In view of these merits of microalgae, substantial attempts have been made to explore the potential of cooperating algae with BESs (Saba et al., 2017; Xiao et al., 2012). An air-lift-type microbial carbon capture cell (ALMCC) was constructed for CO2 fixation and municipal wastewater treatment (Hu et al., 2015). The introduction of stainless steel mesh with Chlorella biofilm as the cathode material of photosynthetic microbial fuel cell (PMFC) has been proposed to produce concentrated algae biomass and purify wastewater (Ma et al., 2017).
In light of the synergetic advantages of BES and microalgae, we expect to establish a symbiotic system for the simultaneous nutrients removal and electricity production without energy input. Herein, a self-sustaining photomicrobial nutrients recovery cell (PNRC) capable for in-situ separation and enrichment of nutrient ions and subsequent recovery as biomass from eutrophic water was proposed. The PNRC system took advantage of electricity generated from organics oxidation to drive charged nutrient ions across ion exchange membranes from eutrophic water, and achieve highly enrichment in the PNRC chambers utilizing the unique configuration. These enriched nutrients were recovered based on the synergistic interaction between Chlorella vulgaris (C. vulgaris) and various bacteria in the system. The superiority of the PNRC system was effective in-situ separation and enrichment of nutrient ions in an energy-efficient manner, which could reduce the treatment volume and facilitate the final recovery. Meanwhile, microalgae cultivated in the cathode would help in the in-situ production of oxygen as an electron acceptor for electricity generation, which avoids mechanical aeration. This newly proposed strategy, with advantages of high flexibility and easy scale-up, held huge potential for practical nutrient recovery and sustainable treatment of eutrophic water.
In this study, a self-sustaining in situ photomicrobial nutrients recovery cell, was developed for enhanced nutrients enrichment and removal from eutrophic water. The feasibility of the PNRC was evaluated by varying operational parameters such as external resistance and initial nutrients concentration in terms of nutrients enrichment and removal efficiency. The identification of the microbial community structure of the anode and cathode was performed to explore the possible mechanisms involved in the organics and nutrients removal. This work suggested a new approach to in-situ efficient nutrients removal and offers the potential for cost-effective eutrophic waters treatment and bioenergy production.
Section snippets
Reactor construction
The PNRC reactor was established by cubic-shaped nonconductive polycarbonate blocks, consisting of an anode chamber and a cathode chamber (inside dimensions 3 cm in diameter, 3.5 cm and 3 cm in length respectively). An anion exchange membrane (AEM, JAM-II, Ting Run, Beijing), a perforated plate and a cation exchange membrane (CEM, JCM-II, Ting Run, Beijing) were placed between the anode and cathode chambers. The perforated plate (5 cm × 5 cm × 1 cm), with a cylindrical chamber (3 cm in
Electricity and power generation of the system
The current output performance of the system under different conditions was monitored. As shown in Fig. 2A, the current density of the PNRC reactor reached the maximum level (2.0 A m−2), which was 43% higher than that with air aeration (1.4 A m−2). The discrepancy between these two operations might demonstrate the superiority of aeration by microalgae than that by mechanical aeration. Obviously, almost no electricity was produced without the addition of C. vulgaris in the cathode compartment
Conclusion
In this study, in-situ nutrient enrichment and removal from eutrophic water has been accomplished by a self-sustaining photomicrobial nutrients recovery cell (PNRC). Rather than introducing eutrophic water directly to the anode/cathode chamber, the PNRC system made it possible for nutrient ions to be highly concentrated in the internal compartment (NO3−-N and PO43--P in the anode chamber, NH4+-N in the cathode chamber) driven by the self-generated electric field and recovered as microalgae
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 National Key R&D Program of China (Grant No. 2016YFC0401101) and National Natural Science Foundation of China of China (Grant No. 51408156). Authors also acknowledged the Open Project of State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (No. QA201935).
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