Which of the following combinations most strongly suggests that a wastewater is difficult to treat by conventional biological process alone?
- AModerate BOD, moderate COD, near-neutral pH
- BLow COD, high DO, low turbidity
- CHigh COD, low BOD/COD ratio, possible extreme pHCorrect
- DModerate alkalinity, low colour, low suspended solids
Solution
Concept
Biological wastewater treatability and inhibitory conditions
Formula
BOD/COD < 0.2 indicates refractory or toxic wastewater
Step-by-step solution
1. Microorganisms require biodegradable substrate (indicated by BOD) and a hospitable pH environment (6.5 to 8.5). 2. High COD coupled with low BOD indicates that most contaminants are non-biodegradable chemicals. 3. Extreme pH inhibits or inactivates microbial enzyme systems. 4. This combination makes conventional biological treatment alone ineffective without advanced chemical pretreatment. Answer: C
Explanation
Conventional biological treatment depends on living microorganisms consuming biodegradable organic matter at near-neutral pH (6.5–8.5). A wastewater with high COD, a low BOD/COD ratio (<0.2), and extreme pH contains large fractions of refractory (non-biodegradable) or toxic organic compounds that cannot be degraded biologically and will inhibit bacterial growth.
Why the other options are wrong
A) Moderate BOD and COD with neutral pH represents readily treatable municipal sewage. B) Low COD and high DO characterizes clean or polished water. D) Moderate alkalinity and low color are easily managed in biological units.
Shortcut
High COD + Low BOD/COD + Extreme pH = Toxic/Non-biodegradable = Biological failure.
Common mistake
Choosing option A by confusing easy treatability with difficult treatability.
Correct answer: C) High COD, low BOD/COD ratio, possible extreme pH