Question 15 of 18: Handling a Bio-Inhibitory Compound Before Biological Treatment

Question 15: Handling a Bio-Inhibitory Compound Before Biological Treatment (6 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.

The challenge is that the inhibitory compound would otherwise disqualify an economically preferred biological process, so the strategy is to remove or reduce the inhibitor to below its toxic threshold rather than abandon biological treatment altogether. Several complementary approaches are available. Dilution/equalization can bring the inhibitor concentration below the threshold that harms the biomass, though this only works if the equalized concentration is still low enough and does not simply postpone the problem downstream. Physical or chemical pretreatment ahead of the biological stage — air stripping (for a volatile inhibitor), activated-carbon adsorption, or chemical oxidation/precipitation — can remove or destroy the inhibitor before it ever reaches the biological process, which is generally the most reliable approach if the inhibitor's chemistry allows it. Biomass acclimation is a third option: gradually increasing the inhibitor's concentration in the feed over an extended start-up period can develop a tolerant microbial population capable of treating a waste that would immediately upset an unacclimated culture, though this takes time and the resulting tolerance is not unlimited. If none of these is economically viable, the inhibitor-bearing sub-stream can be segregated and treated separately (physically/chemically) while the remaining, non-inhibitory waste continues to a conventional biological process, avoiding the need to protect the whole flow.