Question 15 of 28: When to Select a Physical-Chemical Treatment Train

Question 15: When to Select a Physical-Chemical Treatment Train (3 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

1. The waste is non-biodegradable or has a low BOD5/COD ratio (< ≈0.3) — refractory organics, heavy metals or inorganics that a biological process cannot metabolize.
2. The waste is toxic or bio-inhibitory to a biological process at the concentrations present, so biological treatment would fail or need extensive dilution/pretreatment first.
3. The flow is small, highly variable, or intermittent (batch) — a biological process needs a sustained, relatively steady food supply to maintain a healthy microbial population; a small or batch flow suits physical-chemical unit operations (equalization, precipitation, adsorption) better.
4. Very stringent or rapid start-up/shutdown requirements exist — physical-chemical processes respond immediately to changed conditions, unlike a biological process that needs an acclimation period.
5. Space is limited and detention times must be short — physical-chemical unit operations (coagulation, sedimentation, filtration) generally need much shorter residence times than biological treatment.