Environmental engineering and sustainability

Environmental engineers protect people from the environment and the environment from people. They design the water treatment plants that make river water safe to drink, the sewers and treatment plants that return used water to rivers without harming them, the landfills and recycling systems that handle solid waste, and the controls that keep smokestacks and tailpipes from fouling the air. They also predict what a new highway, mine or factory will do to the air, water, soil and living things around it, and help decide whether and how it should be built. This course introduces the whole field: the chemistry and mass balances that every calculation rests on, the quality of water and air and how it is measured and regulated, the treatment of drinking water and wastewater, the management of solid and hazardous waste, and the environmental assessment of projects.

The examples follow Willowbend Environmental, an invented consulting firm in Sioux Falls, South Dakota, on the Big Sioux River. Nadia Okoro is an engineer intern who joined the firm after graduating in civil engineering. She works with Martin Lefebvre, a senior environmental engineer, and Joy Kwan, who runs the firm's small laboratory. This term Willowbend has five jobs: an upgrade of the city's wastewater treatment plant and its discharge to the river, a nitrate problem in the wells of a nearby farming town, leachate from a closed municipal landfill, an air permit for a new asphalt plant, and the environmental review of a new raw water intake and reservoir. Each lesson uses one or more of these jobs.

Where the field came from

Environmental engineering grew out of sanitary engineering, the nineteenth-century work of supplying clean water and removing sewage from crowded cities. In 1854 the physician John Snow traced a London cholera outbreak to a single contaminated pump on Broad Street, one of the founding stories of epidemiology. Cities soon learned that filtering and later chlorinating drinking water cut deaths from typhoid and cholera dramatically. Through the twentieth century, industrial growth added new problems: rivers so polluted they caught fire, smog that killed thousands in London in 1952, acid rain falling on lakes far from the smokestacks, and toxic waste buried in places such as Love Canal in New York. Each crisis brought new laws and new engineering, and the field widened from water and sewage to air, waste, soil, groundwater and whole ecosystems.

American experience shaped the field too. The Cuyahoga River fire in Cleveland in 1969 became a symbol of industrial pollution and helped lead to the Clean Water Act of 1972. In Flint, Michigan, a 2014 switch of water source without proper corrosion control released lead from old service lines into the drinking water of a city of about 100,000 people. Flint showed that treatment, distribution, monitoring and public trust are one system, and it pushed new federal rules on lead service lines.

What environmental engineers do

The work falls into a few broad areas, and most engineers move between several of them during a career:

Environmental engineers work for consulting firms like Willowbend, for municipalities and utilities that own treatment plants, for industry, and for the governments that write and enforce the rules. Many are licensed professional engineers, because the public relies on their designs and judgments for health and safety. The work is intensely practical: the answer to most questions is a number with units, such as a concentration in milligrams per liter, a load in kilograms per day or a removal efficiency in percent, and that number must be compared with a guideline, standard or permit limit.

Sustainability

The most widely quoted definition of sustainable development comes from the 1987 report of the World Commission on Environment and Development, often called the Brundtland report: development that meets the needs of the present without compromising the ability of future generations to meet their own needs. Engineers usually think about sustainability in three linked parts, sometimes called the triple bottom line: environmental (resources, emissions, ecosystems), social (health, equity, community) and economic (cost over the whole life of the project). A design that is cheap to build but costly to run, or that solves a water problem by creating an air problem, is not sustainable.

The United Nations Sustainable Development Goals, adopted in 2015, set targets that environmental engineers work toward directly, such as clean water and sanitation (goal 6), affordable and clean energy (goal 7), sustainable cities (goal 11) and climate action (goal 13). Professional engineering codes of ethics in both Canada and the United States now ask engineers to consider the environment and sustainability alongside public safety.

Life-cycle thinking

Every product and structure has a life: raw materials are extracted, processed and transported; the product is manufactured, built, used and maintained; and at the end it is reused, recycled or disposed of. Life-cycle assessment (LCA) adds up the inputs (energy, water, materials) and outputs (emissions, wastes) over all these stages and translates them into impacts such as climate change, smog formation, acidification or water use. The international standards ISO 14040 and ISO 14044 set out the four phases of an LCA: goal and scope definition, inventory analysis, impact assessment and interpretation.

Life-cycle thinking often changes the answer. A wastewater plant that uses a lot of electricity for aeration may have a larger carbon footprint than one that recovers biogas, even if both meet the same effluent limits. A water main made of one material may be cheaper to install but need replacement sooner than another. The functional unit, the quantity of service being compared, such as one cubic meter of treated water or one metric ton of waste managed, makes comparisons fair.

Units and concentrations

Environmental work uses a small set of units constantly, and many mistakes come from mixing them up. In water, concentrations are usually given as mass per volume, in milligrams per liter (mg/L) or micrograms per liter (µg/L). Because a liter of dilute water has a mass of almost exactly one kilogram, 1 mg/L is very nearly one part per million by mass (ppm), and 1 µg/L is very nearly one part per billion (ppb). In air, the same terms mean something different: ppm and ppb in air are parts by volume, which for gases equals parts by moles, and they must be converted to micrograms per cubic meter with the gas laws (lesson 9). In soil and sediment, concentrations are mass per dry mass, in milligrams per kilogram.

The second key idea is the load, or mass flow rate: the mass of a substance passing a point per unit time. A treatment plant permit often limits both the concentration in the effluent and the load to the river, because a small concentration in a very large flow can still deliver a large mass. The load is simply the flow times the concentration, with care over units:

The factor of 10⁻³ appears because 1 mg/L is the same as 1 g/m³, and there are 1,000 g in a kilogram. If the flow is in cubic meters per second, multiply by 86,400 seconds per day as well, giving the convenient factor of 86.4: a flow of 1 m³/s carrying 1 mg/L delivers 86.4 kg/d.

Nadia uses a short program for loads, because the plant has several outfalls and the flows change by season. The same calculation works for any substance once the units are consistent.

# Daily loads (kg/d) from flow and concentration
streams = [
    ("Plant effluent",   30000, 2.5),     # name, flow m3/d, total phosphorus mg/L
    ("Storm outfall A",   4200, 0.40),
    ("Upstream river", 1.21e6, 0.08),
]
for name, q, c in streams:
    load = q * c / 1000            # mg/L = g/m3, so q*c is g/d
    print(f"{name:16s} Q = {q:>9,.0f} m3/d  C = {c:4.2f} mg/L  load = {load:6.1f} kg/d")
total = sum(q * c / 1000 for _, q, c in streams)
print(f"Total phosphorus load: {total:.1f} kg/d")

Plant effluent   Q =    30,000 m3/d  C = 2.50 mg/L  load =   75.0 kg/d
Storm outfall A  Q =     4,200 m3/d  C = 0.40 mg/L  load =    1.7 kg/d
Upstream river   Q = 1,210,000 m3/d  C = 0.08 mg/L  load =   96.8 kg/d
Total phosphorus load: 173.5 kg/d

The result shows something that surprises many students: the upstream river, with the lowest concentration, carries the largest load, simply because its flow is so large. Managing a river's phosphorus means looking at every source in the watershed, including farm runoff, not only the treatment plant at the end of the pipe.

Laws, guidelines and standards

In the United States, Congress passed most of the major environmental statutes in the 1970s and 1980s: the National Environmental Policy Act (1970), the Clean Air Act (major amendments 1970, 1977 and 1990), the Clean Water Act (1972), the Safe Drinking Water Act (1974), the Resource Conservation and Recovery Act (1976) and the Comprehensive Environmental Response, Compensation, and Liability Act, or Superfund (1980). The Environmental Protection Agency (EPA) writes regulations under these laws, and most states run the permit programs themselves under delegated authority, often with rules at least as strict as the federal ones. In South Dakota, Willowbend's permits come from the state environmental agency acting under EPA-approved programs.

Three words are used carefully. A guideline or objective is a recommended value that is not enforceable by itself. A standard is a value set by a government body, which may or may not be legally binding depending on the law behind it. A permit limit or license condition is the enforceable number written for a particular facility, often stricter than the general standard because of local conditions such as a small receiving river. Engineers design to the permit, check against the standards and keep an eye on guidelines that are likely to become standards later.

Risk and the precautionary principle

Many environmental decisions are about risk: the probability that harm will occur and how severe it would be. Health-based drinking water and air quality values are usually derived from studies of the dose that causes harm, divided by safety factors to allow for uncertainty and sensitive people. For substances believed to cause cancer at any dose, regulators often set a level that corresponds to a small lifetime risk, such as one additional case in 100,000 or a million people. Where the science is uncertain but the possible harm is serious or irreversible, the precautionary principle says that lack of full certainty should not be a reason to postpone cost-effective measures. Engineers do not usually set these values, but they need to understand where they come from and how much margin they contain.