Question 18 of 19: Strategies to Reduce Fish Toxicity from WWTP Effluents

Question 18: Strategies to Reduce Fish Toxicity from WWTP Effluents (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.

Whole-effluent toxicity to fish from a treatment plant is most often driven by residual chlorine, un-ionized ammonia, and residual metals or organic toxicants that pass through secondary treatment, so effective strategies target these directly. Dechlorination (sulfur dioxide or sodium bisulfite addition, or switching disinfection to UV) removes residual chlorine and its toxic disinfection by-products (chloramines) before discharge — chlorine residual is one of the single most common causes of acute effluent toxicity. Enhanced nitrification/ammonia control reduces un-ionized ammonia, which is acutely toxic to fish at concentrations far below what affects other measured parameters, by ensuring the biological process reliably nitrifies (adequate SRT, dissolved oxygen and alkalinity) rather than passing ammonia through untreated. Source control and industrial pretreatment stops metals, cyanide and toxic organic compounds from ever reaching the plant at treatable-but-still-toxic residual concentrations, addressing the problem upstream of the outfall rather than trying to remove every toxicant at the plant itself; this is typically implemented and verified through routine whole-effluent toxicity (WET) testing on the final effluent to confirm the combined strategy is working.