Environmental and Sanitary Engineering
Environmental and sanitary engineering questions test water and wastewater treatment fundamentals, effluent standards and the Egyptian regulatory framework. The exam expects mass balances, BOD/COD reasoning, treatment-process selection and awareness of Law 4/1994, its amendments, and Law 48/1982 for Nile protection.
Core rules and concepts
- BOD and COD: BOD5 measures biodegradable organic matter over five days; COD measures chemically oxidisable matter. COD is normally higher than BOD, and the BOD/COD ratio indicates biodegradability.
- Treatment train: preliminary (screens, grit), primary (sedimentation), secondary (activated sludge, trickling filters, lagoons) and tertiary (filtration, disinfection). Sludge treatment and disposal are part of the design.
- Activated sludge: key controls are F/M ratio, mixed-liquor suspended solids, sludge age, aeration and return sludge. Nutrient removal may be required for discharge to sensitive water bodies.
- Disinfection: chlorine dose and contact time (CT) determine inactivation; excess chlorine creates disinfection by-products and must be quenched where required.
- Regulation: discharge to the Nile or its branches is controlled under Law 48/1982; general environmental impact assessment and discharge controls fall under Law 4/1994 as amended. Industrial pretreatment is required before sewer discharge.
Worked example
A wastewater plant receives Q = 10,000 m³/d with influent BOD5 = 250 mg/L and effluent BOD5 = 20 mg/L. The influent BOD load is 10,000 × 250/1000 = 2,500 kg/d. The effluent load is 10,000 × 20/1000 = 200 kg/d. Removal efficiency is (250 − 20)/250 = 92%. If the activated-sludge F/M ratio is 0.25 d⁻¹ and MLSS is 3,000 mg/L = 3 kg/m³, the required aeration-tank volume is V = BOD load/(F/M × MLSS) = 2,500/(0.25 × 3) = 3,333 m³, giving a hydraulic retention time of about 8 hours. Final design must also check oxygen demand, sludge production and clarifier loading.
Traps that cost marks
- Treating BOD and COD as interchangeable.
- Using influent load instead of effluent load to check compliance.
- Ignoring sludge return, waste sludge and nutrient loads.
- Designing chlorination without contact time or residual control.
- Overlooking Law 48/1982 restrictions when discharging near the Nile.
- Forgetting that industrial waste may need pretreatment before municipal sewers.