Using a tubular photosynthetic microbial fuel cell to treat anaerobically digested effluent from kitchen waste: Mechanisms of organics and ammonium removal

Highlights

• Adding 3 mg/L P in catholyte improved the bioenergy output of the PMFC to 4.13 kWh/m³.

• Aromatic proteins and microbial byproducts were degradable in the PMFC.

• The majority of NH₄⁺-N in the anolyte migrated to catholyte through CEM.

• NH₄⁺-N was removed at the cathode after being oxidized by oxygen produced by algae.

Abstract

Anaerobically digested effluent from kitchen waste (ADE-KW) was used herein as the substrate of a tubular photosynthetic microbial fuel cell (PMFC) for power production, and also, after being diluted, as a medium for cultivation of algae in the cathodic chamber. Adding 3 mg/L phosphorus to the catholyte could efficiently enhance the algal growth and the PMFC performance. About 0.94 g/L algal biomass and 0.57 kWh/m³-ADE-KW bioelectricity were obtained from the PMFC. Soluble microbial byproduct-like material and aromatic proteins were the dominant organics in the ADE-KW, which were readily degradable in the system. About 79% of the 1550 mg/L ammonium in the anolyte transferred to the catholyte through the cation exchange membrane. The ammonium was removed mainly as electron acceptors at the cathode after being oxidized by oxygen, whereas algal assimilation only account for about 14.6% of the overall nitrogen.

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.