Using a tubular photosynthetic microbial fuel cell to treat anaerobically digested effluent from kitchen waste: Mechanisms of organics and ammonium removal
Introduction
Kitchen waste constitutes a large component (30–50%) of municipal solid wastes (Shin et al., 2015, Levis and Barlaz, 2011). Anaerobic digestion has been proved to be an effective technology to manage kitchen waste and also produce abundant energy resources like methane (Huang et al., 2015). Nonetheless, anaerobically digested effluent from kitchen waste (ADE-KW) is still of severe environmental concern for its high content of nutrients (Shin et al., 2015). Microbial fuel cells (MFCs) are bioelectrochemical systems that have been recommended to complement anaerobic digestion for ADE-KW treatment (Li et al., 2013a, Li et al., 2013b). However, some disadvantages limit the practical application of MFC, such as incapable of ammonium removal and energy-intensive (e.g. aeration).
A tubular photosynthetic microbial fuel cell (PMFC) system, using an algae-assisted cathode, is a promising technology that can not only convert chemical energy stored in organic matter to electric energy but also recover nutrients using microalgal technology. The oxygen produced by the photosynthesis of algae will save the additional energy that would otherwise be needed for aeration. Previous studies demonstrated that it is feasible to cultivate algae in wastewater, such as diluted ADE-KW, to produce biodiesel (Shin et al., 2015, Vasconcelos Fernandes et al., 2015, Pei et al., 2017). Thus, using diluted ADE-KW as algal culture medium, which also functions as the catholyte, can simultaneously achieve organics removal, nutrient removal and bioenergy production. Additionally, the tubular PMFC that a MFC was installed in a photosynthetic bioreactor (PBR) allows the algae to grow in the “U” shape sector so that the light is more accessible (Xiao et al., 2012, Ma et al., 2017). Furthermore, the large effective area of cation exchange membrane (CEM) can reduce the internal resistance of the PMFC markedly, which was beneficial for power generation (You et al., 2006).
ADE-KW was characterized with high nutrients (ammonium) and salinity (Pei et al., 2017). Thus, recovery of nitrogen from ammonium-rich ADE-KW was a goal of this study. It is notable that the ammonium could transfer from the anodic chamber to the cathodic chamber through the CEM, especially in the tubular PMFC, in which the CEM has a large specific surface area of about 23 m2 CEM m−3 anolyte. The effect of current generation is a driving force for ammonium transportation from an anodic chamber to a cathodic chamber. Diffusion is another reason for ammonium transportation, especially in ammonium-rich anolyte (Haddadi et al., 2013). Thus, it is expected that vast amounts of ammonium would transfer into the catholyte which could function as the nutrient for algal growth. The ammonium in the catholyte can also be oxidized and then removed as electron acceptors on the cathode. To the best of our knowledge, the study was the first attempt to study the ammonium migration effect in an algae-assisted bioelectrochemical system.
It is estimated that the N/P (molar) ratio of ADE-KW was more than 100 which much higher than the ideal N/P ratio for algal growth (Chisti, 2007). Phosphorus (P) is the second most frequent macronutrient that limits algal growth after nitrogen (Solovchenko et al., 2016) Therefore, it is necessary to introduce additional phosphorus to the diluted ADE-KW to regulate the N/P ratio of the culture. In this study, anaerobically digested effluent from kitchen waste (ADE-KW) was used as the substrate for bioelectricity generation in the anodic chamber of a tubular PMFC. Diluted ADE-KW was used as the microalgal culture, and also functioned as the catholyte. The objectives of this study were to: (1) optimize the performance of PMFC by varying phosphorus concentration; (2) reveal the mechanisms of organics removal in the PMFC with a substrate of ADE-KW; (3) study the ammonium removal processes in an algae-assisted bioelectrochemical system.
Section snippets
Experimental set-up
The tubular Plexiglas anode chamber of the PMFC had a working volume of 350 mL (height 200 mm, diameter 60 mm) and ten rectangular holes (height 160 mm, width 5 mm) on the anode chamber for cation exchange between anolyte and catholyte. The cathode chamber (working volume 1500 mL) consisted of a cylindrical section (height 200 mm, diameter 13 mm) joined to a conical section, and also functioned as an algal bioreactor (Fig. 1). A cation exchange membrane (CEM) was wrapped around the tubular
Algal biomass
The results shown in Fig. 2a indicated that the algal growth was limited under the control conditions, i.e. without extra P addition. After increasing P concentration by 1 and 3 mg/L, the algal biomass reached 0.88 ± 0.06 and 0.94 ± 0.03 g/L, respectively, whereas only about 0.71 ± 0.02 g/L algae was obtained without extra P addition. In the systems with 6 and 9 mg/L extra P, the algal growth rates was lower than the systems with small dose of extra P and eventually yielded biomass
Conclusion
An extra P concentration of 3 mg/L was optimal for the tubular PMFC in this study. Soluble microbial byproduct-like material and aromatic proteins were the dominant organics in the ADE-KW, which were also readily degradable in PMFC system. Ammonium was removed mainly as electricity acceptors at the cathode after being oxidized by oxygen, whereas algal assimilation only accounted for about 14.6% of the overall nitrogen. Further study could be conducted to improve the performance of the tubular
Acknowledgement
This research was funded by: National Science Fund for Excellent Young Scholars (51322811); Science and Technology Development Planning of Shandong Province (2012GGE27027); Program for New Century Excellent Talents in University of Ministry of Education of China (Grant NO. NCET-12-0341); and Foundation for Outstanding Young Scientists in Shandong Province (ZR2016EEB26).
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