Introduction:
The worldwide increase in the production of organic wastes has increased the need for treating and using them as manures, fertilizers, nutritive feeds or energy substrates. Integrated multitrophic aquaculture (IMTA) systems along with wastewater treatment techniques is a novel area of research which in future can be established as a cutting-edge technology in the field of agriculture and aquaculture. The sediment microbial fuel cell (SMFC) assisted wastewater treatment system has the potential of simultaneous agriculture/livestock/synthetic wastewater and aquacultural wastewater treatment along with promising power production for powering different low-energy up-taking sensors such as water quality or environmental sensors. Activated aquacultural biochar catalysts can be prepared from aquacultural wastes, which can potentially improve the performance of SMFC acting as cathode catalyst. Biochar-based catalysts are made of naturally available sources, mainly wastes which makes them a better sustainable alternative to metallic catalysts making them more cost-effective for large-scale applications. Waste minimization, wastewater treatment, water utilization and renewable energy production are the primary advantages of SMFC-assisted aquacultural systems.
Materials and methods:
A sediment microbial fuel cell (SMFC) reactor with carbon felt as the electrode was designed and fabricated for wastewater treatment and power generation studies (Fig. 1). After filling up the pond sediment and aquacultural wastewater, an anode electrode (carbon felt of dimension 18cm ×3cm ×0.5 cm) was placed at the bottom attached to the pond sediment and a cathode electrode (three carbon felt each of 6cm ×3cm × 0.5cm) which was stacked parallelly, was placed in the overlying water. The water quality parameters of the aquacultural pond water sample were analyzed and synthetic wastewater was prepared considering the maximum COD range that can occur in an integrated poultry-fish system for more accurate studies . The SMFC reactor was operated and monitored for more than 50 days (10 experimental cycles).
A novel activated aquacultural biochar catalyst was synthesized from aquacultural wastes and used as a cathode catalyst for improving the performance of SMFC. Material and electrochemical characterization studies were carried out in order to analyze the property of aquacultural biochar catalyst. Polarization studies were carried out by varying external resistance from 10 kΩ to 10 Ω to compare the performance of SMFC reactor with and without activated aquacultural biochar-based cathode catalyst in treating aquacultural and synthetic wastewater (poultry-fish based). A detailed design of an intensive poultry fish culture system assisted with SMFC-based wastewater treatment was proposed after validating the data from the experimental SMFC reactor’s performance.
Results:
SMFC started producing an open circuit voltage (OCV) of 772 mV and an operating circuit current of 0.4 mA on the first day after its acclimatization phase. The stable operating voltage was generated until the COD value reached the optimal range and hydraulic retention time (HRT) was determined to be 4 days. An average COD removal efficiency of 86.31% was obtained over the 10 experimental cycles . SMFC generated a maximum operating voltage of 0.422 V when connected to an external resistance of 975 Ω on the 21st day of operation. The characterization studies of the synthesized aquacultural biochar catalyst such as cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) exhibit the synthesized aquacultural biochar catalyst is an active electrocatalyst which can help in faster oxygen reduction reaction (ORR) rate which ultimately leads to higher power output and overall efficiency of the cell. Raman spectroscopy and X-ray diffraction (XRD) analysis suggest that the biochar shows good electrochemical activity and catalytic properties.
The SMFC with aquacultural waste-based biochar cathode catalyst treating the synthetic wastewater showed the highest maximum power density (MPD) with a range of 101.63 mW m-2 (1693.83 mW m-3 ) and lowest internal resistance (49.34 Ω) which depicts the improved performance of the SMFC by the use of cathode catalyst. The results validate the reliability of using SMFC in actual aquacultural systems, thus proposing a system design of SMFC reactor-assisted small-scale integrated poultry fish culture system. SMFC used for the design study was proposed for the aquacultural fish tank of inner dimensions of 1.75 m ×2.1 m with a tank depth of 1.2 m. The design specifications for the integrated poultry fish culture are proposed in such a way that the poultry cage set-up should be installed on the top of the fish tank to easily pass the poultry droppings into the fish tank (Fig. 2). The poultry chickens will be kept inside the cages and the fish will be reared in the same fish tank where SMFC has been developed. SMFC equipped inside the fish tank will treat the wastewater and maintain the aquaculture water quality with simultaneous power production.
Conclusion:
This study represents the experimentation, analysis and validation of incorporating wastewater treatment techniques such as SMFC into IMTAs including integrated poultry-fish culture. The operating parameters such as pH, DO, temperature, SMFC reactor design, electrode material, and stocking density of fish and chicken are to be considered before incorporating SMFC into an integrated poultry-fish culture. SMFCs can be integrated into aquacultural systems, such to provide a complementary source of energy and improve water quality reducing the risk of disease outbreaks in aquacultural systems. SMFCs have the potential to act as power generators for different water quality and environmental sensors which in fact provides multiple advantages of assisting this reactor with the IMTA system. SMFC using aquacultural waste-based biochar catalysts can effectively be incorporated into IMTA systems resulting in a potentially valuable technology for sustainable IMTA systems. SMFC-based IMTA systems improve water quality, generate renewable energy, improve fish growth and survival and maintain sustainability.