Environmental Science and Engineering involves the application of scientific and engineering principles to protect and improve the environment (air, water, and land resources). It aims to provide healthy water, air, and land for human habitation and other organisms, and to remediate polluted sites. It bridges multiple disciplines, relying heavily on biology, chemistry, and physics to understand complex natural systems and human impacts on them. Engineers in this field design systems for water treatment, waste management, air quality control, and sustainable development.

The field is increasingly focused on the transition to a circular economy, minimizing waste by designing products and systems where resources are continuously reused or recycled. Understanding the foundational science of ecosystems is critical for engineers because human infrastructure—from dams to urban sprawl—profoundly alters these natural cycles.

An ecosystem is a community of living organisms (biotic) in conjunction with the nonliving components of their environment (abiotic), interacting as a system.

Biotic Components:

• Producers (Autotrophs): Organisms like plants and algae that produce their own food through photosynthesis.
• Consumers (Heterotrophs): Animals and humans that rely on other organisms for energy.
• Decomposers (Saprotrophs): Fungi and bacteria that break down dead organic matter, recycling nutrients.

Abiotic Components:

• Physical Factors: Sunlight, temperature, water, and wind which dictate where organisms can survive.
• Chemical Factors: Essential nutrients (Carbon, Nitrogen, Phosphorus), pH levels, and salinity.

The modern environmental movement was catalyzed in the mid-20th century by growing awareness of the ecological impacts of industrialization. A seminal moment was the publication of Rachel Carson's Silent Spring (1962), which documented the detrimental effects of indiscriminate pesticide use (specifically DDT) on bird populations and ecosystems. This, along with highly visible environmental disasters like the Cuyahoga River fire in 1969, led to immense public pressure.

In the United States, this culminated in the establishment of the Environmental Protection Agency (EPA) in 1970 and the passage of landmark legislation such as the Clean Air Act and Clean Water Act. These events shifted engineering practices from merely managing waste (e.g., building sewers) to proactively protecting and restoring ecosystems and public health.

Energy flows through an ecosystem in a one-way stream, originating from the sun, moving to primary producers, and then to various consumers. Unlike energy, matter is conserved and continuously recycled within and between ecosystems through biogeochemical cycles. Ecosystems rely on a delicate balance, and disruptions—such as pollution or habitat destruction—can have cascading effects across multiple trophic levels. The study of thermodynamics is highly relevant here, particularly the Second Law of Thermodynamics, which dictates that energy transfers are never 100% efficient, explaining the shape of ecological pyramids.

• Hydrologic (Water) Cycle: The continuous movement of water on, above, and below the surface of the Earth. Key processes include evaporation, transpiration, condensation, precipitation, and runoff. This cycle drives almost all other biogeochemical cycles and is foundational to water resources and environmental engineering.
• Carbon Cycle: Essential for all organic life. It involves photosynthesis (uptake of $CO_2$ by plants), respiration (release of $CO_2$ by organisms), and combustion of fossil fuels, which significantly impacts global climate.
• Nitrogen Cycle: Atmospheric nitrogen ($N_2$) must be "fixed" into usable forms (ammonia, nitrates) by specialized bacteria or industrial processes (Haber-Bosch). Excess nitrogen from agriculture leads to water pollution.
• Phosphorus Cycle: A slow cycle, mainly found in rock formations and ocean sediments. It does not have a gaseous phase. Like nitrogen, excess phosphorus from runoff can cause severe eutrophication in aquatic ecosystems.

Graphical representations of the trophic structure of an ecosystem.

• Pyramid of Energy: Shows the rate of energy flow through each trophic level over time. Always upright, reflecting the 10% rule where significant energy is lost as heat.
• Pyramid of Biomass: Shows the total mass of organisms at each trophic level. Usually upright in terrestrial ecosystems but can be inverted in aquatic ecosystems (e.g., phytoplankton have lower biomass than zooplankton but reproduce rapidly).
• Pyramid of Numbers: Shows the number of individual organisms at each level. Can be upright, inverted (e.g., one large tree supporting many insects), or spindle-shaped.

• Provisioning Services: Products obtained from ecosystems, such as food, fresh water, wood, fiber, genetic resources, and medicines.
• Regulating Services: Benefits obtained from the regulation of ecosystem processes, including climate regulation, natural hazard regulation, water purification, and waste management.
• Cultural Services: Non-material benefits people obtain from ecosystems through spiritual enrichment, cognitive development, reflection, recreation, and aesthetic experiences.
• Supporting Services: Services necessary for the production of all other ecosystem services, such as soil formation, photosynthesis, and nutrient cycling.

The discipline in philosophy that studies the moral relationship of human beings to, and also the value and moral status of, the environment and its non-human contents.

• Anthropocentrism: The belief that human beings are the central or most significant entities in the world.
• Biocentrism: The ethical perspective that all life deserves equal moral consideration or has equal moral standing.
• Ecocentrism: A nature-centered, as opposed to human-centered, system of values that places inherent value on all living organisms and their natural environment.

The survival and growth of organisms within an ecosystem are governed by critical limits and tolerances to environmental factors.

The gradual process by which ecosystems change and develop over time, following a disturbance or the initial colonization of new habitat.

• Primary Succession: Occurs in essentially lifeless areas—regions in which the soil is incapable of sustaining life as a result of such factors as lava flows, newly formed sand dunes, or rocks left from a retreating glacier. "Pioneer species" like lichens are the first to colonize.
• Secondary Succession: Occurs in areas where a biological community has already existed but some or all of that community has been removed by small-scale disturbances that did not eliminate all life and nutrients from the environment (e.g., after a forest fire or agricultural clearing).
• Climax Community: The theoretical endpoint of succession, resulting in a stable, mature, and self-sustaining community (e.g., an old-growth forest) that remains in dynamic equilibrium until another major disturbance occurs.

The Mass Balance Principle is the fundamental tool used by environmental engineers to track pollutants in any system (a lake, a reactor, a city). Based on the Law of Conservation of Mass, it states that matter cannot be created or destroyed, only transformed. By defining a specific "control volume" (boundary), engineers account for all materials entering, leaving, and reacting within that volume.

Environmental chemistry dictates how pollutants dissolve, react, and transport through air, water, and soil.

Many environmental systems are "buffered", meaning they resist changes in pH when acids or bases are added. Alkalinity is the measure of water's capacity to neutralize acids. The carbonate system ($CO_2$, $HCO_3^-$, $CO_3^{2-}$) is the primary buffer in natural waters and is essential for maintaining a stable environment for aquatic life and wastewater treatment microbes.

• Precipitation and Dissolution: Governs whether a contaminant is dissolved in water (mobile and bioavailable) or forms a solid precipitate (immobile). This is heavily dependent on pH. For instance, heavy metals like Lead or Copper can be removed from industrial wastewater by raising the pH (adding lime), causing them to form insoluble hydroxide precipitates that can be physically settled out.
• Oxidation-Reduction (Redox) Reactions: Reactions involving the transfer of electrons between species. Crucial in biological processes and contaminant degradation.
  • Oxidation: The loss of electrons (e.g., converting toxic ammonia to less toxic nitrate).
  • Reduction: The gain of electrons (e.g., converting toxic hexavalent chromium to less toxic trivalent chromium).

Microorganisms (bacteria, archaea, fungi, protozoa) are the primary engines of biogeochemical cycles and are intentionally cultivated in wastewater treatment to consume organic pollutants.

Environmental engineers quantify these organic pollutants using two key metrics:

• Biochemical Oxygen Demand (BOD): The amount of dissolved oxygen needed by aerobic biological organisms to break down organic material present in a given water sample at certain temperature over a specific time period.
• Chemical Oxygen Demand (COD): The total measurement of all chemicals (organics & in-organics) in the water/waste water that can be oxidized; it represents the oxygen equivalent of the organic matter that can be oxidized chemically using dichromate in an acid solution. COD is always higher than BOD.

How microbes obtain energy and carbon.

Energy and Carbon Sources:

• Heterotrophs: Obtain carbon from organic compounds (like the pollutants measured by BOD in wastewater).
• Autotrophs: Obtain carbon from inorganic $CO_2$.
• Phototrophs: Obtain energy from light.
• Chemotrophs: Obtain energy from chemical reactions (oxidation of organic or inorganic compounds). Most bacteria in biological wastewater treatment are chemoheterotrophs.

Oxygen Requirements:

• Aerobes: Require oxygen as the final electron acceptor in metabolism. Yield the most energy and grow fastest.
• Anaerobes: Cannot use oxygen (it may be toxic to them). They use other electron acceptors (like sulfate or carbon dioxide) and grow much slower, often producing methane or hydrogen sulfide.
• Facultative Anaerobes: Can grow with or without oxygen, preferring oxygen if available.

Understanding how populations grow is crucial for managing resources, predicting ecological changes, and understanding human environmental impact. Populations change due to births, deaths, immigration, and emigration. These models are essential for estimating future infrastructure needs, such as water demand and wastewater treatment capacities for growing cities.