Ammonium recovery by an MFCBioelectrochemical systems (BES)
Bioelectrochemical systems (ie Microbial Fuel Cells) are engineered systems in which the electronic transfer chain associated with microbial respiration is short-circuited. Electrons that would naturally flow from the substrate toward oxygen or another electron acceptor are collected at an electrode, on which the micro-organisms form a biofilm. Like in all fuel cells, the anodic reaction is coupled to an electron consuming reaction at a cathode. If the oxidation of organic matter at a bioanode is coupled to the reduction of oxygen at the cathode, the positive cell potential and the flow of electrons results in electricity production. The valorisation of organic wastes by electricity production is the usually foreseen application of Microbial Fuel Cells. The use of bioelectrochemical systems for ammonium recovery is a completely novel approach.
Microbial fuel cells (MFCs) are an emerging technology with a wide range of applications (Hamelers et al., 2010). In MFCs, bacteria catalyze the oxidation of organic substrate (i.e., acetate) and produce electrons at the anode according to:
CH3COO- + 4H2O --> 2HCO3- + 8e- + 9H+ (1)
The electrons are used to reduce an electron acceptor (i.e., O2) at the cathode. Given the neutral to alkaline environment (pH > 7) of the cathode, the reduction of oxygen results in the
production of hydroxyl ions according to:
O2 + 4e- + 2H2O --> 4OH- (2)
Anode and cathode are often separated by an ion exchange membrane (Logan et al., 2006). Ion exchange membrane and the cathode-electrode can be combined to form so called membrane electrode assembly (MEA) (Larminie and Dicks, 2003; Pham et al., 2005; Prakash et al., 2010). In an MEA the membrane separates the anode from the cathode and serves as an ion conductor, while the electrode can be directly exposed to the gas phase (air-cathode).
The principle of ammonium recovery by an MFC was reported in literature (Kuntke et al., 2011) using a sacrificial K3Fe(CN)6- cathode and synthetic wastewater.
At the anode, electrons are produced (Eq. (1)) and transported via an external load (resistor) to the cathode, where oxygen is reduced (Eq. (2)). The electron transport induces a charge transport (i.e., anion or cation transport) across the membrane to maintain the charge neutrality of the system. In
case of the applied Cation Exchange Membrane (CEM), cation transport (i.e., H3O+, Na+, K+, Mg2+, Ca2+, NH4+ ) occurs from the anode chamber through the CEM to the cathode chamber (migrational flux) and leads with time to a concentration gradient between cathode and anode chamber of the MFC. The pH in the cathode chamber increases during operation, due to the production of hydroxyl ions (OH-) according to Eq. (2) and a migrational transport of cations other than H3O+ and
During continuous MFC operation an equilibriumwill be reached, where forward (anode to cathode) migrational flux and backward (cathode to anode) diffusion flux of cations will be equal and amaximum concentration of cations and OH- in the cathode chamber is reached. At this point the cathode pH remains stable, because the constant production of OH- leads to a diffusion flux of OH- from cathode to anode (Rozendal et al., 2006; Sleutels et al., 2009a). The high pH in the cathode chamber results in to a formation of volatile NH3. NH3 is stripped (Cord-Ruwisch et al., 2011) from the liquidegas boundary at the MEA by the air stream supplied to air-cathode. Subsequently, NH3
can be recovered from the gas stream leaving the cathode by adsorption in an acid as NH4+. Therefore, this innovative concept couples energy production from urine by an MFC with NH3 stripping.