Question 10 of 20: When to Use Equalization Basins

Question 10: When to Use Equalization Basins (5 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.

An equalization basin dampens variability in flow and/or concentration ahead of downstream treatment, and is worth considering under five circumstances. (1) Highly variable flow — batch discharges, shift changes or intermittent production create hydraulic surges that would otherwise overload downstream unit processes sized on an average flow. (2) Highly variable concentration/strength — a process with widely fluctuating BOD/COD or pH between batches can shock a biological system that needs a relatively steady feed to maintain a stable microbial population. (3) Periodic slug loads of a specific pollutant (e.g. a tank dump or cleaning cycle) that would otherwise pass through treatment largely untreated in the short time it takes to transit the plant. (4) A need to blend incompatible or sequential waste streams from different parts of the plant so the combined feed presented to treatment is more uniform than any single stream alone. (5) A downstream biological process that is sensitive to shock loading and needs a controlled, steady feed rate to avoid upset (equalization is frequently paired specifically with activated-sludge or other biological treatment for this reason). In all five cases the underlying justification is the same: smoothing flow and load variability protects downstream capital-intensive treatment processes from being either overwhelmed at peak or under-loaded (and therefore inefficient) at trough. The trade-off is that an equalization basin adds its own capital and footprint cost, so it should be sized against the actual measured coefficient of variation of the stream (from the industrial waste survey in Question 9), not added reflexively to every design.