Nutrients removal and recovery in bioelectrochemical systems: A review

Highlights

• Nutrients are key contaminants to be removed in bioelectrochemical systems (BES).

• Nitrogen is removed by biological and bioelectrochemical reactions.

• Nitrogen is recovered through ammonia migration and volatilization.

• Phosphorus is removed and recovered in precipitates due to high electrolyte pH.

• Effective and efficient nutrients removal/recovery will make BES more competitive.

Abstract

Nutrient removal and recovery has received less attention during the development of bioelectrochemical systems (BES) for energy efficient wastewater treatment, but it is a critical issue for sustainable wastewater treatment. Both nitrogen and phosphorus can be removed and/or recovered in a BES through involving biological processes such as nitrification and bioelectrochemical denitrification, the ${\text{NH}}_4^{+}/{\text{NH}}_3$ couple affected by the electrolyte pH, or precipitating phosphorus compounds in the high-pH zone adjacent a cathode electrode. This paper has reviewed the nutrients removal and recovery in various BES including microbial fuel cells and microbial electrolysis cells, discussed the influence factors and potential problems, and identified the key challenges for nitrogen and phosphorus removal/recovery in a BES. It expects to give an informative overview of the current development, and to encourage more thinking and investigation towards further development of efficient processes for nutrient removal and recovery in a BES.

Introduction

In a bioelectrochemical system (BES), organic compounds are oxidized by microorganisms, and the electrons generated from this oxidizing process can be used to produce energy and other value-added compounds (Wang and Ren, 2013). Direct conversion of chemical energy into electric energy in a BES holds potential advantages over the existing technologies in terms of energy recovery from organic compounds, and the intensive studies of BES configuration/operation, microbiology, electrochemistry, and application have occurred in the past decade. The representative BES includes microbial fuel cells (MFCs), microbial electrolysis cells (MECs), and microbial desalination cells (MDCs). A BES can be potentially applied to treat wastewater, to power remote sensors, to act as a platform for studying fundamental microbial interaction with a solid electron acceptor/donor (e.g., in a micro-MFC), or to produce value-added compounds through electrochemical or electrosynthetic processes.

The use of the low-grade substrates such as wastewater as an electron source is attractive because of the increasing demand for sustainable water/wastewater treatment with a low carbon footprint. Various substrates including pure organics and domestic/industrial wastewaters have been examined in the BES for electricity generation (Pant et al., 2010), the BES size has been enlarged from milli-liter to liter-scale or even larger at a pilot scale, and its long-term performance outside the laboratory has been reported (Zhang et al., 2013a). However, at this stage the energy recovery in a BES is still too low to make it practically competitive, and a benchmark power density of 1000 W m⁻³ (Arends and Verstraete, 2012) was realized only in very small-scale reactors. The low energy recovery, as well as the low energy consumption (due to the reduced use of aeration) in a BES, indicates that its primary function, if designed for energy recovery from wastewater treatment, may be contaminant removal, rather than energy recovery that would be a beneficial plus to offset energy use by the treatment process, thereby furthering energy benefits by using BES (He, 2013). In addition, because of a low conversion efficiency (from organic to electric energy), a BES will be more applicable to the low-strength wastewater, such as domestic wastewater.

The main goal of contaminant removal in a domestic wastewater treatment process is to reduce the concentrations of organic pollutants and nutrients (mainly nitrogen and phosphorus). BES can efficiently remove organic compounds within a reasonable time; however, the anaerobic condition in the anode of a BES does not effectively facilitate nutrient removal, which may require aerobic conditions (e.g., nitrification, and enhanced biological phosphorus removal). Therefore, nutrient removal has become a key challenge to develop BES for efficient wastewater treatment. Nitrogen and phosphorus are the key contaminants and also the important elements for improving agricultural production; due to the stricter discharge regulation and the depleting reserve, there is an increasing trend of research and development of wastewater treatment technologies to remove and/or recover nutrients from wastes (Rittmann et al., 2011). A BES capable of removing or recovering nutrients will certainly make it promising for future deployment. The objectives of this review paper are to examine the past research on nutrient removal/recovery in BES (with a focus on wastewater treatment), introduce developed technologies, analyze removal efficiencies, and discuss the challenges for future development of BES for effective and efficient nutrient removal and/or recovery. The studies of nitrogen removal in biofilm-electrode reactors (BERs) are excluded because the denitrification in a BER relies on in situ produced hydrogen gas as an electron donor (Ghafari et al., 2008), which is different from a BES described here.