4.1.3. Outputs

In particular, intermediate fuels provided by methane from wastewater can be used as energy products for heat, electricity generation, and even in propulsion vehicles. Currently available systems are much less effective and can transform only 25 to 35 percent of thermal energy into electrical energy, resulting in tremendous energy losses [8,37]. For higher recovery and better efficiencies, therefore, combined heat and power application is recommended. Therefore, technologies that convert inputs into intermediates by harnessing carbon-bound energy into biogas, bio-ethanol, and biodiesel, and then converting these intermediates into outputs via combustion/gasification into gaseous fuels, or converting inputs directly into outputs via heat recovery through heat pumps, and microbial fuel cells for electricity generation [38–40]. To maximize the potential for wastewater energy recovery, each of these technologies must first be understood in terms of physical, chemical, and biological concepts and constraints, and in terms of maturity and current penetration level.

#### *4.2. Valuable Energy Products from Wastewater*

Controlled treatment of wastewater can produce a variety of value-added items. Biological wastewater systems are commonly used to obtain valuable materials from wastewater. There may be either aerobic and anaerobic digestion, or effluent co-digestion. Wastewater treatment produces a large number of energy products that can be used in the form of biofuels, which are described in Table 2 as well as in the section below:






