Thermodynamics and Conservation of Mass and Energy

Ecological engineering principles are constrained by the laws of conservation of mass and energy and the laws of thermodynamics, just as chemical, mechanical, or electrical engineering principles are constrained by these laws. Ecosystems are open systems that require a continual input of energy to maintain their structure and function. Two of the most inviolable principles of ecological science can be described as energy flow and material recycling.

Energy flow

Energy inputs, driven by solar radiation, are required to maintain structure and function in the face of the physical tendency toward disorder (the increase of entropy). Traditionally engineered systems use human and hydrocarbon-based energy to maintain order (keep the system intact and functioning). Ecosystems use photosynthesis, driven by solar energy, as their energy source. Biological energy flow can be measured by rates of production (biomass accumulation) and respiration (energy used for production). Physical energy flow can be measured by the mobilization, transport, and deposition of organic and inorganic materials by the kinetic and potential energy of fluids or solids such as water, wind, and sediment. Both biological energy and physical energy are constrained by conservation of mass and energy laws. Ecology as the interaction of biotic and abiotic processes looks at the interactions of both types of energy. Some energy is lost at each transformation so while total entropy increases in accordance with the second law of thermodynamics, order is locally increased. This has been described as the self-organization feature of ecosystems or exergy.

Emergy, an accounting system developed by H. T. Odum, can be used to put all natural and human production into common units based on solar radiation. Emergy measures the inputs to make a product or service. It is a measure of energy used in the past and thus is different from a measure of current energy use. This provides a way to evaluate the costs of ecological goods and services in the same units as the costs of human production of goods and services. Ecological engineering designs seek to maximize the use of renewable energy (e.g., solar radiation) and minimize the use of nonrenewable energy.

Material recycling

Nutrient and material (re)cycling is another major ecological principle. Material is conserved by the continual reuse of materials and the transfer of those materials between organic and inorganic states through biogeo-chemical cycles. Organic and inorganic materials cycle through the system appearing in different locations and forms through time. Waste disposal is seldom an issue in a functioning ecosystem as the output from one system is used as input to another. Natural biogeochemical cycles mobilize, transport, and store material in the atmosphere, biosphere, hydrosphere, and lithosphere. Producers, consumers, and decomposers transfer organic matter and nutrients among themselves and the storage compartments. Many traditional human engineering designs lead to the accumulation of waste materials that cannot be reused by the original process and can contaminate other processes. Ecological engineering designs seek to minimize waste production and to utilize wastes (material not related to the primary function of the design) as inputs for other processes. One example of this is using ecological processes to clean up waste products such as using wetlands to treat wastewater or phytoremediation to clean up soil contamination.

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