The dissolved oxygen sag downstream of an organic wastewater discharge is primarily caused by:
Correct answer: C. Microbial oxidation of biodegradable organic matter
- A. Chemical precipitation of suspended solids
- B. Rapid evaporation of oxygen from the water
- C. Microbial oxidation of biodegradable organic matter
- D. Increased oxygen production by aquatic plants
Explanation
Microorganisms use dissolved oxygen while decomposing biodegradable organic matter, so oxygen levels can fall downstream from a discharge. Natural reaeration and photosynthesis may later restore oxygen, producing the characteristic sag and recovery pattern.
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Environmental engineering examines water supply, water quality, treatment processes, wastewater collection and treatment, solid waste management, air pollution and public health protection. It covers sedimentation, coagulation, filtration, disinfection, biological treatment and disposal methods, while distinguishing drinking-water treatment from sewage treatment and BOD from COD.
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