Environmental Engineering — Engineering Reference

1. At a glance

Environmental engineering is the applied discipline that protects human health and the natural environment by designing the engineered systems that treat, transport, contain, monitor, and remediate the four classical environmental media — water, wastewater, air, and solid/hazardous waste — together with their cross-cutting layers of stormwater management, contaminated-site remediation, sustainability/LCA, and greenhouse-gas accounting. It is the civil-engineering sub-discipline most tightly coupled to public-health policy, regulatory law, and ecosystem science.

The hardware spans seven orders of magnitude:

A 4-bedroom septic system (drain field 30 m², 2 m³ tank) serving a single household.
A package WWTP (10–500 m³/d) for a rural school, subdivision, or industrial facility.
A 50 000 m³/d municipal drinking-water plant — coagulation/flocculation/sedimentation/filtration/disinfection train serving a city of 150 000.
A 500 000 m³/d coastal seawater reverse-osmosis (SWRO) plant — Sorek B (Israel), Carlsbad (California), Ras Al-Khair (Saudi Arabia).
A 4 000 MW coal-fired power station’s flue-gas treatment train — ESP + SCR + FGD scrubber + ACI mercury control, handling ~12 million m³/h of flue gas.
A regional sanitary landfill (1–5 million m³ capacity) with composite liner, leachate collection, and 4 MW gas-to-energy plant.

Where environmental engineering sits in the discipline stack: it sits on top of chemical-process-fundamentals (every treatment train is a sequence of unit operations on a mass-balanced flowsheet), fluid-mechanics (open-channel + pipe + porous-media flow), heat-transfer (incineration, sludge drying), pumps-turbomachinery (every lift station and aerator), and the supporting earth-science disciplines (hydrology, hydrogeology, atmospheric science). It is governed by US EPA statutes (SDWA, CWA, CAA, RCRA, CERCLA, TSCA), state implementation plans, the EU Water Framework Directive (2000/60/EC) and its CAFE/IED air counterparts, WHO Drinking-Water Guidelines, and trade-association standards from AWWA, WEF, A&WMA, and SWANA.

As of 2026 the field is being reshaped by four converging pressures: PFAS (“forever chemicals”) regulation forcing GAC/IX/RO retrofits at thousands of utilities; microplastics monitoring and removal; mandatory GHG / Scope-1-2-3 accounting under EU CSRD and US SEC climate rules; and net-zero water/wastewater plant design coupling on-site solar, anaerobic-digester biogas, and heat recovery.

2. Why it matters

Clean piped water and sewered sanitation are credited (WHO, 2024) with adding more years to global life expectancy than antibiotics and vaccines combined. Outdoor and indoor air pollution is estimated to cause 7 million premature deaths per year worldwide (WHO 2024 Air Quality Database). Climate change layers compounding water, flood, drought, wildfire, and ecosystem stresses on top. Global environmental-compliance spend exceeds US $1 trillion per year. The economic, public-health, ecological, and legal stakes are simultaneous and non-negotiable.

Specific consequences of bad environmental engineering:

Walkerton, Ontario 2000 — E. coli O157:H7 in a municipal well, inadequate chlorination, 7 deaths, 2 300 illnesses. Triggered Ontario’s Clean Water Act.
Flint, Michigan 2014–2019 — switch to Flint River source without corrosion-control orthophosphate dosing, lead leached from service lines, ≥ 12 deaths from associated Legionellosis, mass childhood lead exposure. Triggered the Lead and Copper Rule Revisions (LCRR 2021) and proposed LCRI (2024).
Bhopal 1984 (cross-listed with chemical-process: MIC release) — drove EPCRA and the Risk Management Program.
Love Canal 1978 — 21 000 t of chemical waste under a housing development, dioxins + chlorinated solvents, declared federal emergency; triggered CERCLA (1980 — “Superfund”).
Camp Lejeune 1953–1987 — TCE/PCE/benzene/vinyl chloride in base drinking water, decades of cancer/birth-defect litigation.
GenX / PFOA discharges, Cape Fear NC 2017 — Chemours PFAS to drinking-water source for 200 000+ people, catalyzed the 2024 EPA MCL of 4 ppt for PFOA/PFOS.

3. First principles

Environmental engineering rests on a thin layer of conservation laws plus heavy use of partition + kinetic models.

3.1 Mass balance

The framework is identical to chemical-process-fundamentals §3:

Input  −  Output  +  Generation  −  Consumption  =  Accumulation

Applied to a reactor, settling basin, river reach, atmospheric box, or aquifer cell. Steady-state ⇒ Accumulation = 0.

3.2 Reactor kinetics

Most natural-system contaminant decay is well-modeled as first order:

dC/dt = −k·C          ⇒   C(t) = C₀ · exp(−k·t)

with half-life t₁/₂ = ln 2 / k. Examples: BOD exertion (k ≈ 0.23 d⁻¹ base e at 20 °C, Streeter–Phelps 1925); chlorine decay in distribution; UV photolysis. Biological reactions often follow Michaelis–Menten / Monod:

r = r_max · S / (K_s + S)

where K_s is the half-saturation constant. Second-order kinetics (r = k·C·B) describes ozonation of micropollutants, where B is the oxidant.

3.3 Phase partitioning

Equilibrium distribution coefficients tell where a contaminant ends up:

K_d [L/kg] — soil-water partition: C_soil / C_water. Drives sorption + retardation in groundwater (R = 1 + ρ_b·K_d/n).
K_OC — organic-carbon-normalized K_d; K_d = f_OC · K_OC. Tabulated per compound.
K_H — Henry’s law constant, dimensionless (C_air / C_water) or atm·m³/mol. Drives air-stripping, volatilization, indoor-air vapor intrusion.
K_OW — octanol-water; log K_OW > 3 ⇒ bioaccumulative; ≥ 5 ⇒ persistent in food web.

3.4 Adsorption isotherms

For GAC, ion exchange, and soil sorption:

Langmuir:    q = q_max · K·C / (1 + K·C)
Freundlich:  q = K_F · C^(1/n)

q [mg/g sorbent], C [mg/L]. Freundlich (1909) is the workhorse; Langmuir (1918) is mechanistic for monolayer sorption.

3.5 Dissolved oxygen + BOD

Aerobic biology requires DO; freshwater saturation at 20 °C ≈ 9.1 mg/L (decreases with T and salinity). BOD₅ is the 5-day, 20 °C oxygen demand exerted by biodegradable organics in a sample. Streeter–Phelps (1925) river DO sag model:

D(t) = (k_d·L₀ / (k_r − k_d)) · (e^(−k_d·t) − e^(−k_r·t)) + D₀·e^(−k_r·t)

where D = DO deficit, L₀ = ultimate BOD, k_d = deoxygenation rate, k_r = reaeration rate.

3.6 Treatment stoichiometry — nitrification + denitrification

The classical nitrogen-cycle stoichiometry (per Metcalf & Eddy):

Nitrification:     NH₄⁺ + 1.83 O₂ + 1.98 HCO₃⁻ → 0.021 C₅H₇NO₂ + 0.98 NO₃⁻
                   + 1.04 H₂O + 1.88 H₂CO₃              ≈ 4.57 g O₂ / g NH₄-N
Denitrification:   NO₃⁻ + 1.08 CH₃OH + H⁺ → 0.065 C₅H₇NO₂ + 0.47 N₂
                   + 0.76 CO₂ + 2.44 H₂O                ≈ 2.86 g BOD / g NO₃-N

These two numbers — 4.57 g O₂/g N nitrified and 2.86 g BOD/g N denitrified — drive aeration energy and external-carbon dosing on every BNR (biological nutrient removal) plant.

3.7 Hydrology

The basic surface-water tools:

Rational method (Mulvaney 1851, Kuichling 1889): Q_peak = C · i · A, with C the runoff coefficient (0.05–0.95), i the design rainfall intensity [mm/h or in/h], A the catchment area. SI form: Q [m³/s] = C · i_mm/h · A_ha / 360.
SCS Curve Number method (USDA SCS 1954, now NRCS TR-55): Q_runoff = (P − I_a)² / (P − I_a + S), with S = 25 400/CN − 254 mm.
Unit hydrograph (Sherman 1932) — convolution kernel for runoff response.
HEC-HMS / HEC-RAS — USACE reference models.

3.8 Open-channel hydraulics

Manning’s equation (Manning 1889) for uniform flow in a prismatic channel:

V = (1/n) · R^(2/3) · S^(1/2)            [m/s, SI]
Q = (1/n) · A · R^(2/3) · S^(1/2)        [m³/s]

R = A/P is hydraulic radius, S the slope, n the Manning roughness (smooth concrete 0.012, natural earth 0.025, vegetated 0.035–0.05). Froude number Fr = V/√(g·D_h) classifies flow (Fr < 1 subcritical; Fr > 1 supercritical); hydraulic jumps occur in transition.

3.9 Atmospheric dispersion

The Gaussian plume model (Sutton 1947, Pasquill 1961, Gifford 1961) is the regulatory workhorse:

C(x,y,z) = Q/(2π·u·σ_y·σ_z)
           · exp(−y²/(2σ_y²))
           · [exp(−(z−H)²/(2σ_z²)) + exp(−(z+H)²/(2σ_z²))]

Q [g/s] emission rate, u [m/s] mean wind, H effective stack height (physical + Holland or Briggs plume rise), σ_y, σ_z dispersion coefficients tabulated by Pasquill stability class A–F and downwind distance.

4. Water treatment — drinking water

4.1 Sources

Source	Typical TDS [mg/L]	Treatment burden
Surface (river/lake)	100–500	Turbidity, microbial, NOM/DBP precursors
Groundwater	200–1000	Iron, manganese, hardness, As, nitrate
Brackish (estuarine, deep aquifer)	1000–10 000	RO desalting
Seawater	35 000	SWRO, energy-intensive (3–4 kWh/m³)
Reclaimed wastewater (IPR/DPR)	varies	Multi-barrier: MF/UF + RO + UV/AOP
Atmospheric (fog/dew/dehumidification)	< 50	Emerging niche

4.2 Conventional treatment train

Stage	Unit operation	Typical chemistry/parameters
1	Screening + pre-oxidation	Trash rack; KMnO₄ or ClO₂ for Fe/Mn/taste/odor
2	Coagulation	Alum Al₂(SO₄)₃·14H₂O 20–80 mg/L; FeCl₃ 10–40 mg/L; cationic polymer 0.1–2 mg/L; rapid mix G = 600–1000 s⁻¹
3	Flocculation	Tapered slow mix G = 20–70 s⁻¹, Gt = 30 000–150 000
4	Sedimentation	Overflow rate 30–80 m³/(m²·d); plate/tube settlers 2–3× boost
5	Filtration	Rapid sand 5–15 m/h; multimedia (anthracite + sand + garnet); GAC; or membrane
6	Disinfection	Free Cl₂ ≥ 0.2 mg/L residual; CT per SWTR for 4-log Giardia + 6-log virus; UV 40 mJ/cm² for 4-log Cryptosporidium
7	Stabilization	pH 7.5–8.5, Langelier or CCPP for corrosion; orthophosphate 1–3 mg/L for Pb

Stokes’ law settling velocity for a discrete particle (Stokes 1851):

V_s = g·(ρ_p − ρ_w)·d² / (18·μ)

A 50-µm floc with ρ_p ≈ 1100 kg/m³ in 20 °C water (μ = 1.0 × 10⁻³ Pa·s): V_s ≈ 1.36 × 10⁻⁴ m/s ≈ 11.7 m/d — comfortably below a 40 m/d overflow rate.

4.3 Advanced + membrane

Process	MWCO / pore	Pressure [bar]	Use
Microfiltration (MF)	0.1 µm	0.2–1	Turbidity, Cryptosporidium, primary for membrane bioreactor
Ultrafiltration (UF)	10 nm / 10–100 kDa	1–3	Virus, colloids; pretreatment to RO
Nanofiltration (NF)	1 nm / 200–1000 Da	5–15	Softening, DBP precursor, divalent ion removal
Reverse osmosis (RO)	< 0.5 nm	8–80	Desalting; SWRO 55–70 bar, 35–50 % recovery
GAC adsorption	(sorbent)	—	TOC, DBPs, taste/odor, PFAS (with limits)
Ion exchange (IX)	(resin)	—	Nitrate, As, PFAS (SBA resins), softening
Advanced Oxidation (AOP)	—	—	UV/H₂O₂ or O₃/H₂O₂ — micropollutants, 1,4-dioxane, NDMA

4.4 Contaminants of concern

Microbial: bacteria (coliforms, E. coli, Legionella), viruses (norovirus, enterovirus), protozoa (Giardia, Cryptosporidium — chlorine-resistant cysts, UV-susceptible).
Inorganic: arsenic (MCL 10 µg/L), lead (LCRR action level 15 µg/L → LCRI proposed 10 µg/L), copper, nitrate (10 mg-N/L), fluoride (4 mg/L MCL, 0.7 mg/L optimal).
Disinfection by-products (DBPs): TTHM (80 µg/L), HAA5 (60 µg/L), bromate (10 µg/L), chlorite (1 mg/L).
Organic: VOCs (TCE, PCE, benzene), pesticides (atrazine 3 µg/L), 1,4-dioxane.
Emerging: PFAS — final 2024 EPA NPDWR sets MCLs PFOA 4 ppt, PFOS 4 ppt, PFHxS / PFNA / HFPO-DA 10 ppt, plus a Hazard Index for mixtures; microplastics (CA SB 1422 monitoring framework 2024); PPCPs (estrogens, sulfamethoxazole, carbamazepine).

5. Wastewater treatment

5.1 Collection + pretreatment

Sanitary or combined sewers, gravity-flow at slope sufficient for 0.6–3 m/s self-cleansing velocity. Manholes typically 100 m spacing on straight runs. Lift stations where gravity fails.

Headworks: bar screen (25 mm coarse + 6 mm fine), grit chamber (V ≈ 0.3 m/s), primary clarifier (overflow 30–50 m³/(m²·d), 30–50 % TSS / 25–40 % BOD removal).

5.2 Secondary biological

Process	Mode	MLSS [mg/L]	SRT [d]	Notes
Conventional activated sludge	Suspended, aerobic	1500–3000	5–10	Aeration tank + secondary clarifier
Extended aeration	Suspended, aerobic	3000–5000	20–40	Small communities, MBR-like waste production
Trickling filter	Fixed-film	n/a	n/a	Plastic or rock media, recirculation
RBC (rotating biological contactor)	Fixed-film	—	—	Disc stacks 40 % submerged
MBR (membrane bioreactor)	Suspended + UF	8000–12 000	15–30	No secondary clarifier; permeate quality
MBBR / IFAS	Hybrid	2000–4000 + carriers	8–15	Plastic carriers retrofit
SBR (sequencing batch reactor)	Suspended, time-cycled	2000–4000	5–15	Fill/react/settle/decant in one tank

5.3 Tertiary / advanced

Nitrogen removal — BNR configurations: pre-anoxic MLE (Modified Ludzack–Ettinger), 4-stage Bardenpho, A²/O, MUCT (Modified University of Cape Town), step-feed.
Phosphorus: chemical precipitation (alum, ferric, lime) or EBPR (enhanced biological P removal) using PAOs (phosphate-accumulating organisms) in an anaerobic selector.
Effluent polishing: cloth-disk filter, deep-bed sand, MF/UF, GAC.
Disinfection: UV (dose 30–40 mJ/cm²) is now standard for wastewater (no chlorine residual issue for receiving waters); chlorination + dechlorination (SO₂ or NaHSO₃) is legacy; peracetic acid (PAA) is gaining for CSO/wet-weather.

5.4 Sludge / biosolids

Thickening: gravity thickener, DAF (dissolved air flotation), gravity belt thickener (GBT) — 1–4 % → 4–8 % solids.
Stabilization: anaerobic digestion (mesophilic 35 °C or thermophilic 55 °C, 15–25 d SRT, ~50 % VS destruction, biogas 0.8–1.1 m³/kg VS destroyed, ~60–65 % CH₄). Aerobic digestion for small plants.
Dewatering: belt filter press → 15–25 % solids; centrifuge → 20–30 %; screw press → 18–25 %; thermal dryer → 90 %+.
End-use: Class A biosolids (PFRP — process to further reduce pathogens) land-applied unrestricted; Class B (PSRP — to significantly reduce pathogens) restricted-use. Landfill disposal, monofills, incineration. Regulated by 40 CFR Part 503 (“the 503 rule,” 1993).

5.5 Energy

Aeration accounts for 45–60 % of WWTP electricity. Diffused fine-bubble + DO-control trim and variable-speed blowers cut 20–30 %. Biogas from AD typically returns 30–50 % of plant electricity via CHP. Net-zero-energy WWTP — Strass (Austria), Sheboygan (WI), Ejby Mølle (DK) — combine high-rate primary capture (CEPT or A-stage), AD, CHP, and on-site solar.

6. Stormwater + low-impact development

6.1 Hydrology

Runoff coefficient C ranges 0.05 (forest, sand) to 0.95 (asphalt, concrete). Design storms by return period — 2-yr for minor drainage, 10-yr for storm sewers, 25–100-yr for detention sizing, 500-yr for floodplain.

NOAA Atlas 14 (and the in-progress Atlas 15, incorporating climate non-stationarity, 2024–2026) gives precipitation-frequency for the US.

6.2 Green infrastructure (GI) / LID

Practice	Surface footprint	Function
Bioretention / rain garden	5–10 % of contributing	Filtration + ET + infiltration
Permeable pavement (PICP, porous concrete/asphalt)	100 %	Infiltration in place
Green roof	Roof area	Detention + ET + insulation
Bioswale	Linear	Conveyance + treatment
Cistern + reuse	Above/below ground	Volume control + non-potable supply
Tree well / urban forest	1–3 %	Canopy interception, root infiltration
Constructed wetland	1–3 %	Polishing for nutrients + metals

6.3 Detention vs retention

Detention — temporarily store + release at controlled rate (peak shaving). Pre-development peak normally the release target.
Retention — permanent pool; volume capture + treatment by sedimentation + biological uptake.
Subsurface infiltration galleries (StormTech, Cultec) where surface space is unavailable.

6.4 Regulatory

NPDES MS4 permits (40 CFR 122.26) require municipalities to control discharges from separate storm sewers via SWPPP, illicit-discharge detection, construction-site controls, post-construction stormwater management, and public education (“six minimum measures”).
CSO Long-Term Control Plans for combined-sewer cities (consent-decree-driven; tens of billions of US infrastructure spend through 2030s).
Construction General Permit (CGP) for sites ≥ 1 acre; SWPPP required.

7. Air quality engineering

7.1 Criteria pollutants — NAAQS (40 CFR Part 50)

Pollutant	Primary NAAQS	Averaging
O₃	70 ppb (0.070 ppm)	8 h
PM₂.₅	9 µg/m³ (annual, 2024 revision) / 35 µg/m³ (24 h)	annual / 24 h
PM₁₀	150 µg/m³	24 h
NO₂	100 ppb (1 h) / 53 ppb (annual)	1 h / annual
SO₂	75 ppb	1 h
CO	9 ppm (8 h) / 35 ppm (1 h)	8 h / 1 h
Pb	0.15 µg/m³	3-mo rolling

7.2 Source categories

Mobile: light-duty (Tier 3 + LEV III), heavy-duty (HD-FE Phase 3, 2027), nonroad, marine, aircraft.
Stationary point sources: EGUs (power plants), refineries, cement, steel, pulp/paper, chemical.
Area sources: residential heating, dry cleaners, gas stations, agricultural burning.
Biogenic + wildfire — dominant PM₂.₅ source in many western US summers.

7.3 Control technologies

Pollutant	Control	Mechanism	Typical efficiency
PM (coarse)	Cyclone	Inertial separation	70–90 % for > 10 µm
PM (fine)	ESP (electrostatic precipitator)	Corona charging + plate collection	99–99.9 %
PM (fine)	Fabric filter / baghouse	Surface + depth filtration	99.9 %+
PM (fine + acid gas)	Wet scrubber (venturi, spray tower)	Impaction + absorption	80–99 % PM, 90 %+ SO₂
NOₓ	SCR (selective catalytic reduction, NH₃ over V₂O₅-WO₃/TiO₂)	Catalytic reduction at 300–400 °C	80–95 %
NOₓ	SNCR (selective non-catalytic)	Urea/NH₃ at 850–1100 °C	30–60 %
NOₓ	Low-NOx burner	Combustion staging, FGR	30–60 %
SO₂	Wet FGD (lime/limestone)	Absorption into slurry	90–98 %
SO₂	Dry sorbent injection	Trona / hydrated lime	50–80 %
VOC	Thermal oxidizer / RTO	Combustion at 760–870 °C	95–99 %
VOC	GAC adsorption	Physisorption + regeneration	90–99 %
VOC	Condensation	Cool below dew	50–90 %
Hg	ACI (activated carbon injection)	Sorption upstream of PM device	80–95 %
Acid gas (HCl)	Dry/wet scrubber	Neutralization	95–99 %

7.4 Dispersion modeling

AERMOD (US EPA preferred steady-state Gaussian plume, replaced ISCST3 in 2005) for permits ≤ 50 km. CALPUFF (puff/Lagrangian) for long-range transport + complex terrain. WRF-CMAQ (CMAS Center) regional photochemical for ozone + PM SIPs. ADMS (CERC, UK) is the European AERMOD analogue.

7.5 Regulatory architecture

NAAQS (CAA §109): set primary (health) and secondary (welfare) standards.
SIPs (State Implementation Plans, §110): state-by-state path to attainment.
NSPS (§111): per-source-category technology floors for new sources.
NESHAP / MACT (§112): hazardous air pollutants (HAPs, 188 listed), Maximum Achievable Control Technology.
PSD / NSR (§§160–169, 171–179): preconstruction permitting for major sources.
Title V (§§501–507): operating permits.
GHG endangerment finding (2009, reaffirmed 2022) → §111(d) rules for power plants (ACE 2019, replaced by 2024 Clean Power Plan 2.0).

8. Solid + hazardous waste

8.1 Waste hierarchy

Reduce  →  Reuse  →  Recycle  →  Recover energy  →  Treat  →  Dispose

EU Waste Framework Directive 2008/98/EC formalizes the same five-step hierarchy.

8.2 MSW (municipal solid waste)

US generation (2018 EPA MSW Facts): 292 Mt/yr, 32 % recycled/composted, 12 % combusted with energy recovery (WTE), 50 % landfilled. Composition by mass: paper 23 %, food 21 %, plastics 12 %, yard 12 %, metals 9 %, wood 6 %, glass 4 %, textiles 6 %, other 7 %.

Sanitary landfill — RCRA Subtitle D: composite liner (60 mil HDPE + GCL + 0.6 m compacted clay, k ≤ 1 × 10⁻⁹ m/s); leachate collection (300 mm gravel + perforated HDPE pipe); gas collection (vertical wells on ~60 m grid); daily cover; final cap (40-mil flexible membrane + drainage + 0.6 m vegetative cover). Methane yield 100–200 m³/t MSW over 20–40 yr; collected gas → flare or LFGTE (landfill-gas-to-energy) at 30–50 % collection efficiency.
WTE / mass-burn incineration: combustion at 850–1100 °C, > 2 s residence per EU Industrial Emissions Directive; followed by SNCR/SCR + spray-dryer absorber + ACI + baghouse. Modern fleet: Covanta (US), Veolia (EU), Hitachi-Zosen Inova (CH).
MRF (Materials Recovery Facility): single-stream (commingled recyclables, US/CA dominant) or dual-stream; sorted by trommel + disk screen + magnetic + eddy-current + optical (NIR) + manual QC.
Composting: aerobic, C:N ≈ 25–30, target 55 °C × 3 d for pathogen reduction (EPA PFRP); windrow, ASP (aerated static pile), or in-vessel.
Anaerobic digestion of food waste: separate from sewage AD, gaining ground (NYC, San Francisco mandates).

8.3 Hazardous waste — RCRA Subtitle C

Definition (40 CFR 261): a solid waste that exhibits a characteristic (ignitability, corrosivity, reactivity, toxicity by TCLP) or is listed (F-list non-specific source, K-list specific source, P-list acutely toxic discarded commercial chemical products, U-list toxic discarded commercial chemical products).

Cradle-to-grave manifest (EPA Form 8700-22, now e-Manifest): generator → transporter → TSDF (Treatment, Storage, Disposal Facility). LDR (Land Disposal Restrictions) require BDAT treatment before land disposal.

8.4 Site remediation — CERCLA / RCRA Corrective Action

In-situ technologies: pump-and-treat (legacy); in-situ chemical oxidation (ISCO — permanganate, persulfate, Fenton’s reagent); in-situ chemical reduction (ISCR — ZVI, ZVI emulsions); bioremediation (aerobic + anaerobic, biostimulation, bioaugmentation with KB-1 etc.); thermal (electrical resistance heating, steam-enhanced extraction); soil-vapor extraction (SVE); air sparging; permeable reactive barriers (PRBs).
Ex-situ: excavation + landfill (presumptive remedy for shallow contamination); soil washing; thermal desorption (low/high temp); biopiles.
Brownfield redevelopment: state VCPs (Voluntary Cleanup Programs) with covenants-not-to-sue.
PFAS destruction (emerging 2024–2026): supercritical water oxidation (SCWO — 374 °C, 22 MPa), electrochemical oxidation (BDD anodes), plasma (Aquagga), hydrothermal alkaline treatment (HALT). GAC + IX still dominant for separation (concentration), but the spent media + concentrate require destruction.

8.5 E-waste, batteries, plastics

E-waste 62 Mt/yr globally (UN Global E-Waste Monitor 2024); lithium-ion battery recycling (Redwood Materials, Li-Cycle) recovers Li, Co, Ni, Cu; lead-acid battery recycling is the most successful circular product worldwide (> 99 % recovered in US/EU). EU Battery Regulation 2023/1542 mandates recycled-content thresholds 2027+.

9. Worked examples

Example A — Activated-sludge aeration tank sizing

Given: Q = 10 000 m³/d, raw BOD = 250 mg/L, target effluent BOD = 10 mg/L. Operate at SRT (θ_c) = 8 d, MLVSS X = 3000 mg/L. Yield Y = 0.6 g VSS/g BOD removed, endogenous decay k_d = 0.06 d⁻¹.

Aeration-tank volume (CMAS with recycle, neglecting influent VSS):

V = (Q · θ_c · Y · (S₀ − S)) / (X · (1 + k_d · θ_c))
  = (10 000 · 8 · 0.6 · 240) / (3000 · (1 + 0.06·8))
  = (10 000 · 8 · 0.6 · 240) / (3000 · 1.48)
  = 11 520 000 / 4 440
  ≈ 2595 m³

HRT = V/Q = 0.26 d ≈ 6.2 h ✓ (5–8 h typical).

Oxygen demand:

O₂ ≈ 1.5 · (BOD removed)·Q − 1.42·P_x
   = 1.5 · (240 g/m³ · 10 000 m³/d) · 10⁻³ kg/g
   = 3600 kg O₂/d

(P_x term ~10 % refinement, omitted for brevity.) At typical SOTR/AOTR ≈ 0.5 and 1.5 kg O₂/kWh wire for fine-bubble diffused aeration, electric power ≈ 3600 / 1.5 / 24 ≈ 100 kW continuous, plus ~30 % blower turn-up margin → 130 kW installed.

Example B — Stormwater detention pond sizing (rational + modified rational)

Given: catchment A = 5 ha; weighted post-development C = 0.60 (60 % impervious); pre-development C = 0.20. Design storm 25-yr 1-h intensity i = 100 mm/h. Assume rectangular hyetograph t_storm = 30 min, t_c = 15 min.

Peak flows:

Q_pre  = C·i·A / 360   = 0.20 · 100 · 5 / 360 = 0.278 m³/s = 278 L/s
Q_post = 0.60 · 100 · 5 / 360                 = 0.833 m³/s = 833 L/s

(SI rational with i in mm/h and A in ha gives Q in m³/s if divided by 360.)

Modified-rational detention volume (release at Q_pre):

V_det ≈ (Q_post − Q_pre) · t_storm
      = (833 − 278) L/s · 1800 s
      ≈ 1.0 × 10⁶ L  ≈  1000 m³

Outlet structure: low-flow orifice sized for ≤ Q_pre at design WSE; overflow weir for 100-yr storm. Always check water-quality volume (typically the first 12–25 mm of runoff) separately per state stormwater manual.

Example C — Gaussian plume ground-level concentration

Given: SO₂ stack emission Q = 100 g/s, stack physical height h_s = 60 m, exit T_s = 420 K, exit velocity 15 m/s, stack diameter 3 m. Wind u = 5 m/s at stack height, neutral atmosphere (Pasquill class D), T_amb = 290 K.

Plume rise (Holland 1953) for neutral, simplified:

ΔH ≈ (v_s · d / u) · (1.5 + 2.68 × 10⁻³ · P · (T_s − T_a)/T_s · d)
   ≈ (15 · 3 / 5) · (1.5 + 2.68e-3 · 1000 · 130/420 · 3)
   ≈ 9 · (1.5 + 2.49)
   ≈ 36 m

Effective stack height H = 60 + 36 = 96 m (round to ~90 m for textbook tables).

At x = 1000 m downwind, Pasquill D: from Briggs (1973) rural coefficients, σ_y ≈ 75 m, σ_z ≈ 40 m.

Centerline ground-level concentration (y = 0, z = 0, image term included via doubling at ground):

C = Q / (π · u · σ_y · σ_z) · exp(−H² / (2·σ_z²))
  = 100 / (π · 5 · 75 · 40) · exp(−96² / (2 · 40²))
  = 100 / 47 124 · exp(−2.88)
  = 2.12 × 10⁻³ · 0.0561
  ≈ 1.19 × 10⁻⁴ g/m³
  = 119 µg/m³

This exceeds the 1-h SO₂ NAAQS of 196 µg/m³ at the 99th-percentile metric only marginally, but a permit demonstration would need to account for stack-tip downwash, building wakes, and meteorology over a full year — exactly what AERMOD does. Hand-Gaussian gets you to first-order yes/no; AERMOD finalizes it.

10. Sustainability + ESG + GHG accounting

GHG Protocol (WRI/WBCSD, Corporate Standard rev 2013; Scope 2 Guidance 2015; Scope 3 Standard 2011): Scope 1 direct (combustion, fugitive refrigerants, process); Scope 2 purchased electricity/steam/heat (market vs. location method); Scope 3 15 categories, value-chain (purchased goods, transport, use of sold products, etc.). ISO 14064-1:2018 is the parallel ISO standard.
Life-cycle assessment (LCA) — ISO 14040:2006 / 14044:2006: goal + scope, LCI (inventory), LCIA (impact assessment via ReCiPe / TRACI / CML / Impact World+ / EF 3.1), interpretation. Cradle-to-gate, cradle-to-grave, cradle-to-cradle. Software: SimaPro, GaBi/Sphera LCA FE, openLCA (free), One Click LCA (construction), Ecochain Mobius. Datasets: Ecoinvent v3.10 (2024), GaBi databases, NREL US-LCI, EF reference.
EPD (Environmental Product Declaration) — EN 15804+A2:2019 for construction products, ISO 14025 / 21930 framework; third-party verified.
Decarbonization levers: electrification of heat, renewable PPA, on-site solar/wind, refrigerant transition (HFO/A2L), CCUS (carbon capture, utilization, storage — amine scrubbing, calcium looping, oxy-fuel, direct air capture).
Corporate disclosure: CDP (formerly Carbon Disclosure Project), SASB (sector-specific, now ISSB IFRS S1/S2), GRI (broader sustainability), TCFD (climate-financial, now incorporated into ISSB).
Regulation: EU CSRD + ESRS mandatory 2024/2025/2026 phase-in; EU Taxonomy activity-level screening; US SEC climate-disclosure rule (2024, currently stayed); California SB 253 / SB 261 scope-1-2-3 + climate-financial-risk disclosure 2026+.
Net-zero by 2050 alignment via SBTi (Science Based Targets initiative) sector pathways; near-term targets typically 42–50 % reduction by 2030.

11. Tools / software

Water-treatment process design: Stoat, AQUASIM, BioWin (EnviroSim, the BNR plant-design standard), GPS-X (Hydromantis), WEST (DHI), SUMO (Dynamita), WaterCAD / WaterGEMS (Bentley) for distribution. EPANET 2.2 (US EPA, open-source) — the reference for pressurized distribution hydraulics + water-quality.

Wastewater + collection: InfoWorks ICM (Innovyze/Autodesk), SWMM 5.2 (US EPA, open-source — the urban-drainage and CSO modelling reference), PCSWMM (CHI commercial wrapper), MIKE+ / MIKE URBAN (DHI), XPSWMM / 12d.

Hydrology + hydraulics: HEC-HMS (USACE, free, watershed runoff), HEC-RAS (USACE, free, 1D/2D river hydraulics + sediment + water quality), MIKE 11 / MIKE 21 (DHI, river/2D), TUFLOW (BMT, 1D/2D flood), MODFLOW 6 (USGS, free, groundwater), MT3D-USGS / MODPATH (transport + particle tracking).

Air dispersion: AERMOD (US EPA, regulatory, free), CALPUFF (regulatory long-range, free), ADMS 6 (CERC, UK), WRF-CMAQ (CMAS, free) regional photochemistry, CALMET meteorological preprocessor, AERMET + AERSURFACE, CAMx (Ramboll).

LCA / GHG: SimaPro (PRé), GaBi / Sphera LCA FE, openLCA (free, GreenDelta), One Click LCA, EC3 (Embodied Carbon in Construction Calculator — Building Transparency, free), Athena Impact Estimator, Ecochain Mobius, Persefoni, Watershed, Sweep, Greenstone.

GIS / spatial: ArcGIS Pro (ESRI), QGIS (open-source), ESRI Spatial Analyst, Whitebox Tools (open-source).

Reference datasets: Ecoinvent v3.10 (LCA), ICE Inventory of Carbon & Energy (Hammond & Jones, U. Bath, free), NREL US-LCI, NEI (US EPA National Emissions Inventory), TRI (Toxics Release Inventory).

12. Cross-references

chemical-process-fundamentals — mass + energy balances, reactor kinetics, separations underpinning every treatment train.
fluid-mechanics — open-channel flow, pipe friction, porous-media (Darcy) flow.
heat-transfer — sludge drying, incineration heat balance, thermal desorption.
pumps-turbomachinery — WWTP influent/effluent pumps, blowers, lift stations.
electric-motors — aeration blower drives, every pump motor.
hvac-fundamentals — IAQ + ventilation (ASHRAE 62.1), shared air-quality engineering vocabulary.
transportation-engineering — road runoff, MS4 + post-construction stormwater.
digital-control — SCADA, DO trim loops, BNR aeration control.
microcontrollers — Modbus-over-RS-485 instrumentation typical for WWTP I/O.
materials-polymers — HDPE liners, PVC/PE membranes, fiber-reinforced tanks.
scientific — EPA SWMM .inp grammar (planned).
scientific — AERMOD pathway/keyword input grammar (planned).
scientific — EPANET input file format (planned).

13. Citations

Davis, M. L. Introduction to Environmental Engineering, 6th ed. McGraw-Hill, 2020. ISBN 978-1260441260. The canonical undergraduate text.
Metcalf & Eddy / Tchobanoglous, G.; Stensel, H. D.; Tsuchihashi, R.; Burton, F. L.; Abu-Orf, M.; Bowden, G.; Pfrang, W. Wastewater Engineering: Treatment and Resource Recovery, 5th ed. McGraw-Hill, 2014. ISBN 978-0073401188. The wastewater bible.
Crittenden, J. C.; Trussell, R. R.; Hand, D. W.; Howe, K. J.; Tchobanoglous, G. MWH’s Water Treatment: Principles and Design, 3rd ed. Wiley, 2012. ISBN 978-0470405390. The water-treatment bible.
Sawyer, C. N.; McCarty, P. L.; Parkin, G. F. Chemistry for Environmental Engineering and Science, 5th ed. McGraw-Hill, 2003. ISBN 978-0072480665.
Cooper, C. D.; Alley, F. C. Air Pollution Control: A Design Approach, 5th ed. Waveland Press, 2018. ISBN 978-1478623533.
Vesilind, P. A.; Worrell, W. A. Solid Waste Engineering: A Global Perspective, 3rd ed. Cengage, 2011 (3rd ed. 2017). ISBN 978-1305635203.
Masters, G. M.; Ela, W. P. Introduction to Environmental Engineering and Science, 3rd ed. Pearson, 2007. ISBN 978-0131481930.
Davis, M. L.; Cornwell, D. A. Introduction to Environmental Engineering, 5th ed. McGraw-Hill, 2012.
WHO Guidelines for Drinking-Water Quality, 4th ed., incorporating the first and second addenda. World Health Organization, Geneva, 2022.
US EPA National Primary Drinking Water Regulations — 40 CFR Part 141; PFAS NPDWR Final Rule, 89 Fed. Reg. 32532, April 26 2024.
US EPA Long Term 2 Enhanced Surface Water Treatment Rule (LT2 — Cryptosporidium); Stage 2 D/DBPR.
US EPA Lead and Copper Rule Revisions (LCRR, 2021); Lead and Copper Rule Improvements (LCRI, proposed 2023).
AWWA Manuals of Practice (M-series) — M11 Steel Pipe, M19 Emergency Planning, M22 Sizing Water Service Lines and Meters, M51 Air-Release/Vacuum Valves, M60 Drinking Water Distribution Systems.
WEF Manuals of Practice (MOP) — MOP 8 Design of Municipal Wastewater Treatment Plants (6th ed., 2018), MOP 11 Operation of WWTP, MOP FD-3 Preliminary Treatment, MOP 25 Solids Process Design and Management.
ASHRAE 62.1-2022 Ventilation and Acceptable Indoor Air Quality — shared cross-reference for IAQ.
Manning, R. On the Flow of Water in Open Channels and Pipes. Transactions of the Institution of Civil Engineers of Ireland, vol. 20, 1889, pp. 161–207.
Pasquill, F. The estimation of the dispersion of windborne material. Meteorological Magazine 90 (1063), 1961, pp. 33–49.
Gifford, F. A. Use of routine meteorological observations for estimating atmospheric dispersion. Nuclear Safety 2(4), 1961, pp. 47–51.
Holland, J. Z. A meteorological survey of the Oak Ridge area. USAEC Report ORO-99, 1953.
Briggs, G. A. Diffusion estimation for small emissions. ATDL Contribution File No. 79, NOAA, 1973.
Streeter, H. W.; Phelps, E. B. A study of the pollution and natural purification of the Ohio River III. US Public Health Service Bulletin 146, 1925.
Stokes, G. G. On the effect of internal friction of fluids on the motion of pendulums. Trans. Cambridge Phil. Soc. 9, 1851.
Langmuir, I. The adsorption of gases on plane surfaces of glass, mica and platinum. J. Am. Chem. Soc. 40(9), 1918, pp. 1361–1403.
Freundlich, H. Über die Adsorption in Lösungen. Z. Phys. Chem. 57, 1907, pp. 385–470.
GHG Protocol Corporate Accounting and Reporting Standard, Revised Edition. WRI / WBCSD, 2004 (rev. 2013) — plus Scope 2 Guidance (2015) and Scope 3 Standard (2011).
ISO 14001:2015 Environmental management systems — Requirements with guidance for use.
ISO 14040:2006 / 14044:2006 Life cycle assessment — Principles and framework / Requirements and guidelines.
EN 15804:2012+A2:2019 Sustainability of construction works — Environmental product declarations — Core rules for the product category of construction products.
US Code — Clean Water Act (33 USC §§1251 et seq., 1972 + amendments); Safe Drinking Water Act (42 USC §§300f et seq., 1974); Clean Air Act (42 USC §§7401 et seq., 1970 + 1990 amendments); RCRA (42 USC §§6901 et seq., 1976); CERCLA (42 USC §§9601 et seq., 1980).
EU Directive 2000/60/EC Water Framework Directive; 2008/98/EC Waste Framework Directive; 2010/75/EU Industrial Emissions Directive.

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Environmental Engineering (Water, Wastewater, Air Quality, Solid Waste) — Engineering Reference