Second Law Analysis of Resource Recovery from Wastewater

The energy characteristics of water reclamation plants are compared using the first and second law of thermodynamics. Example analyses are presented to compare different water reclamation plants in terms of both first and second law thermodynamic parameters. Levels of wastewater treatment including chemically enhanced primary, conventional activated sludge, and different levels of biological nutrient removal are compared. A plant without supplemental organics is compared to a plant with codigestion. The beneficial use of different energy products are also compared including biogas fueled boilers, CHP system, biomethane, and effluent heat recovery systems using heat pumps. Results from the example analyses are presented to demonstrate a methodology for analyzing the energy supply and demand from resource recovery systems from wastewater. The intents is to develop a methodology to better describe the energy characteristics of a water reclamation plant. Results from the first and second law analysis are presented in thermodynamic parameters of energy balance, availability, first and second law efficiencies. An energy quality parameter is defined as the availability divided by the energy source. Different energy forms can easily be compared for quality. For example, hot water @ 330 K has an energy quality parameter of 0.05 compared to steam @ 950 kPa with a parameter of 0.30. Electricity is assumed to have an energy quality parameter of 1.0. The first law efficiencies of a biogas fueled boiler system and a CHP system are shown to be about the same at 75 percent, but their second law efficiencies varied greatly at 19 and 40 percent, respectively. The first law efficiency of a heat pump system for extracting heat from plant effluent had a 99 percent first law efficiency but only a 35 percent second law efficiency. A second law analysis provides a comprehensive comparison of different wastewater plants with different energy profiles and levels of resource recovery. An energy system with higher second law efficiency (production of more high quality energy) is an indication that it will be more adaptable to future energy demands. Both the first and second law of thermodynamics should be considered when comparing energy systems for recovery resources from wastewater.Biogas utilization systems for water reclamation plants are analyzed using the first and second laws of thermodynamics. Biogas produced from the anaerobic digestion of sludge is assumed to be composed of 68 percent methane, CH4, and 32 percent carbon dioxide, C02. Four systems for the utilization of biogas are compared on a first and second law basis. These four systems are identified by their major equipment: (i) steam boilers, (ii) internal combustion engines, (iii) gas turbines, and (iv) gas scrubbers. Second law parameters that indicate efficiency, energy quality, and utility are shown to be beneficial in selecting a system. The steam boiler system has a high first law efficiency but the lowest second law efficiencies. The internal combustion engine and gas turbine systems both have comparable and relatively high first and second law efficiencies but higher quality heat is recoverable from the gas turbine system. The gas scrubber system has the highest first and second law efficiencies, but the biomethane produced has not been utilized yet. It is concluded that a second law analysis should be performed when analyzing the energy characteristics of a water reclamation plant and choosing a biogas utilization system.