|Fig. 1: Schematic of two-chamber MFCs.|
Wastewater contains various pathogens or hazardous chemicals. To remove these contaminants from wastewater before discharging it into natural environment, multiple processes are required, making the wastewater treatment energy-intensive. About 2 × 106 joules electrical energy is consumed for treating per m3 wastewater using the traditional aerobic activated sludge treatment and anaerobic sludge digestion techniques.  In the US, approximately 3% of the total electricity consumption goes to wastewater treatment.  On the other hand, there is certain amount of chemical energy stored in wastewater, mainly in the form of reducing matters such as carbohydrates and ammonia, providing the opportunity of extracting energy from wastewater.
According to the guideline of designing wastewater treatment plants, the wastewater generation rate in the US is 100 gallon per capita per day.  Taking account that the population of the US is about 307 million, 1.16 × 108 m3 wastewater is generated every day. Although this number is a little bit lower than that reported in the literature, which is 1.26 × 108 m3/day, it is reasonable to estimate that the total wastewater generation rate in the US is about 1.2 × 108 m3/day, 4.4 × 1010 m3/year. 
The concentration of reducing matters in wastewater is usually expressed as the COD (chemical oxygen demand) value, which indicates how much oxygen is required for oxidizing the reducing matters. A typical wastewater has a COD value of ∼0.5 kg/m3.  Based upon a theoretical 1.47 × 107 joules energy production per kg COD oxidized to CO2 and H2O, the energy density of wastewater is 0.74 × 107 J/m3. [4,5] In another paper published recently in Environmental Science and Technology, Heidrich et al. directly apply experimental methods to determine the internal chemical energy of wastewater. It is estimated to be 1.68 × 107 J/m3 for a wastewater mixed with domestic wastewater and industrial wastewater, and 0.76 × 107 J/m3 for a pure domestic wastewater.  Therefore, a fair estimation of the theoretical energy density in wastewater is in the order of 107 J/m3, which is 5 times as much as the energy consumed to treat the wastewater. Considering the wastewater generation rate in the US estimated above, the total potential energy in wastewater is 1.2 × 1015 J/day, 4.4 × 1017 J/year.
Currently, anaerobic wastewater treatment is the most widely used technology to recover energy from wastewater. The energy is harvested as methane production. Taking off the energy lost due to the heat dissipation during energy transfer from various reducing matters to methane, the energy consumption for maintaining the microbial activity and, the residual reducing matters in the wastewater after treatment, 80% of the chemical energy contained in the original reducing matters can be transferred into methane. Considering that only ∼35% of the chemical energy of methane can be converted into electricity through combustion process, the overall energy recovery efficiency is ∼28%. This number can potentially increase to 40% when more effective CH4-driven chemical fuel cells are developed. 
Microbial fuel cell (MFC) is a novel technology, which can directly convert the chemical energy in wastewater into electrical energy. A schematic of classic two-chamber MFCs is shown in Fig. 1. The basic mechanism of MFCs is similar to that of chemical fuel cells: oxidation of reducing matters at an anode releases electrons that travel through an external circuit to a cathode where oxygen is reduced.  However, different to chemical fuel cells that usually employ a pure fuel, such as hydrogen and alcohol, MFCs apply wastewater in the anodes as the fuel and the oxidation of the COD in wastewater is mediated by microorganisms instead of precious metal catalysts.  The MFC technologies are mainly under bench scale study and only a few pilot scale MFCs have been built.  It is promising that the MFCs can harvest electricity directly from wastewater, but their performance is still too low (less than 1%) for practical application.  Even though the energy recovery efficiency of MFCs can be close to that of anaerobic treatment potentially, the capital cost is about 800 times higher.  Therefore, several issues need to be addressed before the real application of MFCs in the wastewater treatment plants for energy recovery.
Wastewater contains certain amount of energy that may be recovered. Extracting energy from wastewater cannot affect the overall energy consumption structure in the US, since it only contributes a very small portion of the total energy consumption in the US even with 100% energy recovery efficiency. However, it is possible to make the wastewater treatment plant self-sustainable for energy demand by applying anaerobic wastewater treatment or MFC technologies. Such sustainability is of significance, especially in developing countries, to prevent the deterioration of aqueous environment accompanying with the economic growth.
© Xing Xie. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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