4.5 Environmental Engineering (Water & Wastewater)

Water-Quality Parameters

Environmental items test the indicators that define water quality. Biochemical Oxygen Demand (BOD) is the oxygen microbes consume to break down biodegradable organics; the standard BOD₅ is measured over 5 days at 20°C. First-order kinetics give BOD_t = L₀(1 − e^(−kt)), where L₀ is the ultimate BOD and k is the rate constant (day⁻¹). Chemical Oxygen Demand (COD) oxidizes both biodegradable and non-biodegradable matter chemically, so COD ≥ BOD always.

Other key parameters: Dissolved Oxygen (DO) sustains aquatic life (a healthy stream holds several mg/L; saturation DO falls as temperature rises, so warm water holds less oxygen — thermal pollution is harmful). High BOD discharged to a stream depletes DO downstream, creating the classic DO sag curve (modeled by the Streeter-Phelps equation, balancing deoxygenation against reaeration). Turbidity (cloudiness, in Nephelometric Turbidity Units, NTU) indicates suspended particles and interferes with disinfection; drinking water is typically held below ~1 NTU after filtration.

pH (0–14, neutral 7) measures acidity/alkalinity and affects coagulation, disinfection, and corrosion. Coliforms (especially E. coli) are fecal-contamination indicator organisms — easy to test for and signaling possible pathogens. S. Safe Drinking Water Act sets a Maximum Contaminant Level Goal (MCLG) of zero for total coliforms. Other regulated items include total suspended solids (TSS), total dissolved solids (TDS), nitrogen and phosphorus (nutrients that cause eutrophication), and alkalinity/hardness (mostly Ca²⁺ and Mg²⁺, reported as mg/L of CaCO₃).

Drinking-Water Treatment Train

Conventional surface-water treatment follows a sequence; know the order and the purpose of each step:

1. Coagulation — add a coagulant (e.g., alum, Al₂(SO₄)₃) and rapid-mix to destabilize colloidal particles by neutralizing their charge.
2. Flocculation — gentle, slow mixing lets destabilized particles collide and grow into settleable floc.
3. Sedimentation — floc settles out by gravity in a clarifier; design uses the overflow rate (surface loading) = Q/A_surface, which equals the critical settling velocity captured.
4. Filtration — typically rapid sand or dual-media filters remove remaining fine particles.
5. Disinfection — chlorine, chloramine, ozone, or ultraviolet (UV) light inactivates pathogens; chlorine also leaves a measurable residual to protect the distribution system, which UV and ozone do not provide. Disinfection effectiveness follows the CT concept (disinfectant concentration × contact time): a higher CT achieves greater log-removal of organisms.

Softening (lime-soda or ion exchange) may be added to remove hardness, and fluoridation for dental health. Sedimentation-basin sizing hinges on the overflow rate v_o = Q/A_surface: a particle settles out only if its settling velocity exceeds v_o, so a larger surface area captures finer particles — note that detention time alone does not govern removal, the surface loading does.

Wastewater Treatment & the Activated-Sludge Process

Municipal wastewater treatment removes organics, solids, and nutrients in stages:

• Preliminary — bar screens and grit removal protect downstream equipment.
• Primary — quiescent sedimentation removes ~50–60% of suspended solids and ~30–35% of BOD by settling.
• Secondary (biological) — microorganisms metabolize dissolved/colloidal organics. In the activated-sludge process, an aeration basin grows a biomass (measured as Mixed Liquor Suspended Solids, MLSS) that is settled in a secondary clarifier and partly recycled (Return Activated Sludge). Secondary treatment removes ~85–95% of BOD. Trickling filters are an attached-growth alternative.
• Tertiary/advanced — nutrient removal (nitrogen, phosphorus), filtration, polishing.
• Disinfection — chlorination/dechlorination or UV before discharge.

A frequent exam concept is removal efficiency E = (C_in − C_out)/C_in × 100%, and combining stages (overall removal compounds through the train).

Mass Balances, Detention Time & a Worked Example

The workhorse tool is the mass balance on a control volume: accumulation = mass in − mass out ± reaction. At steady state, accumulation = 0, so in = out ± reaction. For a completely-mixed flow reactor (CMFR) with first-order decay, the effluent concentration satisfies C_out = C_in/(1 + kθ).

The hydraulic detention (retention) time is θ = V/Q, the average time a parcel of water spends in a tank of volume V passing flow Q. It sizes sedimentation basins, aeration tanks, and disinfection contact chambers. Watch your units — flow in MGD or m³/min must be converted so V comes out in the desired unit; carrying minutes versus hours is a common slip.

For a simple flow-blending mass balance with no reaction, the combined concentration of two merging streams is C = (Q₁C₁ + Q₂C₂)/(Q₁ + Q₂) — a flow-weighted average. The same in/out bookkeeping handles chemical dosing (mass added = Q × dose) and removal across a unit process. When a reaction consumes the constituent (e.g., first-order decay of BOD or a disinfectant), include the reaction term: rate = −kC·V for first-order kinetics. These mass-balance setups, combined with detention time, answer most FE environmental design questions, so always write 'in − out ± reaction = accumulation' first and then simplify.

Worked Detention-Time Example

A sedimentation basin must hold wastewater for θ = 2 hours at a flow of Q = 5 million gallons per day (MGD). Find the required volume.

• Convert Q: 5 MGD = 5,000,000 gal/day ÷ 24 h = 208,333 gal/h.
• V = Q·θ = 208,333 gal/h × 2 h = 416,667 gallons.
• In cubic feet: 416,667 gal ÷ 7.48 gal/ft³ = 55,700 ft³.

Solid waste and air (high level): municipal solid waste (MSW) is managed by reduction, recycling, composting, waste-to-energy, and engineered landfills with liners and leachate/gas collection. Air topics include criteria pollutants (PM, SO₂, NOₓ, CO, O₃, Pb) and basic dispersion concepts — these appear lightly on FE Civil.