Question 8 of 19: When to Select a Physical-Chemical Treatment Train

Question 8: 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.

A physical-chemical train is favoured over biological treatment when biological treatment cannot reliably do the job, rather than as a default choice, because it is typically more chemical- and energy-intensive on a per-unit-removed basis. The circumstances that point toward physical-chemical treatment include: the waste is toxic, non-biodegradable, or has a high refractory-COD/biodegradable-BOD ratio (low BOD₅/COD ratio); the waste contains constituents that are bio-inhibitory even after the mitigation measures discussed in Question 6; the flow is highly variable, intermittent or batch in nature, which biological systems (needing a stable, continuously-fed biomass) tolerate poorly; the plant footprint is too small for the retention time a biological process would need; specific pollutants must be targeted that biological treatment does not remove well, such as heavy metals, oils and greases, colour, or very low residual suspended solids requiring polishing; and regulatory effluent limits are stringent enough that physical-chemical polishing is needed even after (or instead of) biological treatment.