Wastewater treatment using MFCs was studied to remove and recover pollutants, such as heavy metals, organic carbon and ammonia, using biodegradation of organic matter to generate
e- e-
CO2 H+
H2
Cathode
Anode
Exoelectrogens H+
CxHyOz
e- M
+
M Power source
FIG. 15.12 Schematic of the MEC system with single chamber. CxHyOz, organic compounds; M+, cation.
392 Current Developments in Biotechnology and Bioengineering
electricity [294]. MFCs are able to oxidize organic carbons into CO2 and allow the biochemi- cal reduction to transport electrons [295]. This electron transport is similar to batteries, where the substrates on the anode side are the fuel required for the biochemical conversion [296].
Through the degradation of substrates on the anode side, MFCs generate protons and electrons which are transported through a resistor and a membrane to the cathode where they react with oxygen (O2) and generate electricity [297,298]. The main advantages of MFC technology include high efficiency of substrate conversion into energy, reduced sludge volume, and the recovery of high-value products [299].
However, MFCs still has bottlenecks, such as high electrode cost, low power output and catalyst cost [300]. Therefore, studies have focused on the improvement of MFCs, such as the electrode behavior at micro levels [301], the use of electromotive force and internal resistance for digital projection [302], the application of MFCs in MBR for wastewater treatment [303], as well as the application of multichamber MFC [296,303,304]. Multichamber MFCs were studied to improve the efficiency of pollutant removal and energy generation (Fig. 15.13) [305–307].
Multiple units can be connected in parallel or in series, increasing the current output but this technology requires electrical connections and multiple fluid flows (Fig. 15.13(A)) [305]. Wu et al. (2016) reported that the COD removal efficiency of stacked MFCs was 97% and power den- sity production amounted to 50.9 W/m3 [308]. Moreover, in the case of a high current rate, the feed substrate cannot support this rate, resulting in charge reversal and reverse polarity [301].
Peng et al. (2018) reported that with the 1000 L stacked MFCs, the COD removal efficiency can reach 90% and the maximum power density is 125 W/m3 [309]. Fig. 15.13(B) shows a MFC with multianaerobic anode and a single air cathode [306]. Kim et al. (2017) reported that it could achieve a maximum power density of 24 W/m3. Other configurations used cassette electrodes to improve the interaction between the anode and the wastewater (Fig. 15.13(C)) [310]. Miyahara et al. (2013) reported this configuration with power density of 150 W/m2 and coulombic effi- ciency of 20%.
Together with the generation of energy from MFCs, recovering other valuable compounds present in wastewater such as heavy metals and nutrients, can improve the economic viabil- ity of MFCs. This is especially the case for high-strength wastewaters from animal, food, and agriculture sources, as they contain high concentrations of nutrients [311]. Recently, studies have examined different types of settings to recover soluble nutrients through different sizes and architectures of MFCs [312]. Fig. 15.14 illustrates the configurations of MFCs, including the single-chamber/air-cathode MFC (Fig. 15.14(A)), dual-chamber MFC (Fig. 15.14(B)) and three- camber MFCs (Fig. 15.14(C)), and the main mechanisms involved in their resource recovery systems [313,314]. The presence of selective membranes or porous separators reduces current densities and increases the internal resistance, resulting in a lack of cations supply for cathodic reaction [313]. This imbalance between the hydrogen ions passing through the membrane and cations consuming by cathodic determines the increase in the pH of the cathodic solution [313,314]. These mechanisms made it possible to recover valuable elements from wastewater by salts precipitation at high pH, such as carbonates and phosphates at pH 8.5 [315]. P and N in struvite salts could also be formed in MFCs [316]. Zhang et al. (2012) reported that 95% of phosphate could be recovered from MFCs in the presence of level ratios between P, N and Mg
MFC-1 MFC-2 MFC-3 MFC-4 MFC-5
R
MFC-1 MFC-2 MFC-3 MFC-4 MFC-5 Ianode
Icathode
Ianode
Icathode
Ianode
Anode Cathode Anode Cathode Anode Cathode
Titanium
meshes CEM
PEM Pt-carbon cloth
Anode-left Anode-right
Vent
From right recirculation pump R To right anode media pump
Air Cathode
From left recirculation pump To left anode media pump
Anode Influent
Effluent (1) (2) (3) (4) (x)
Cathode (A)
(B)
(C)
FIG. 15.13 Different MFC configurations with multiple chambers. (A) stacked MFCs connected in series with parallel collection and independent collections; (B) triple-chamber MFCs with a single air cathode; (C) MFCs with cassette electrodes.
394 Current Developments in Biotechnology and Bioengineering
M+ Organics
HCO3-
O2
OH-
(1) M
(3)
(4) NH3 + -
e- e-
Wastewater with
organics and nutrients Treated effluent
(2)
M+ Organics
HCO3-
(1)
NH3
+ -
e-
Wastewater with
organics and nutrients Treated effluent
M+ OX/O2
SO42-
NO2-
NO3-
(7) (6) (2) (5) (5) (4)
Red/OH- (2)
N- N-
e- e-
Organics
HCO3- (7)
+ -
Wastewater with organics and nutrients Ion-rich
water a N-
Treated effluent
OX/O2
Red/OH-
(1) (2) (2)
(8) M+ M+
N-
(A)
(B)
(C)
e-
FIG. 15.14 The nutrient recovery mechanism of (A) single-chamber/air-cathode MFC, (B) dual-chamber MFC, and (C) three-camber MFC. (1) cation (M+) migration; (2) cathodic reductive precipitation of M+; (3) inorganic salts/hydroxides precipitation; (4) NH3 stripping; (5) bioaccumulation and biomass deposition; (6) microbial oxidations (e.g., nitrification and sulfuration); (7) anion (N−) migration; and (8) anode oxidative precipitation of N+.
Chapter 15 • Biological nutrient recovery from wastewater for circular economy 395
(1:1:1) [317]. However, MFCs allow a limited amount of recovered ammonia. The removal of ammonia in MFCs is mainly done by nitrification, followed by denitrification that occurs either in the presence of organic molecules or by autotrophic denitrifying bacteria that accept elec- trons from the cathode [318,319].
A new generation of METs called microbial recycling cells (MRCs) is emerging, based on biocompatible and biogenic materials with the recovered organic carbon and nutrients, to pro- duce soil conditioners and fertilizers. The proposed MRCs are based on fully recyclable materi- als (e.g., biomass-derived char coal, paper, terracotta, clay, and lignocellulosic) that can be used as biological electrodes, structural frames and separators [314,320]. In the traditional METs architectures (based on technological materials, such as plastic panels, carbon cloths, bind- ers, and membranes), the sedimentation and adsorption of inorganic salts and the biofouling caused by the deposition of organic-matter are considered to be the main problems of block- age and other hindrances [320]. In MRCs, these mechanisms could be maximized, in which the materials can be fully recycled as a basis for organic-mineral fertilizers or as agricultural soil conditioners when the systems are saturated [305,320].