Bio-assay test (bioassay test)

The bio-assay test is a biological toxicity test that checks whether treated effluent is acutely harmful to aquatic life, using live fish as the test organism. The Indian standard requires that 90% of the test fish survive after 96 hours of exposure to 100% effluent (the undiluted treated discharge). It is a mandatory general condition for effluent discharge and is the regulatory backstop against toxicity that the chemical parameters might miss.

Its value is that it is an integrative, whole-effluent test. The numerical standards check individual parameters — BOD, COD, specific metals, cyanide and so on — but an effluent could pass all of those and still be toxic because of an unmeasured substance, a synergistic combination, or an interaction the individual tests do not capture. The fish do not care about the chemistry list; if the water kills them, it fails, regardless of whether every measured parameter was within limits.

For recyclers, the bio-assay test is the catch-all that makes genuine treatment, not just parameter-chasing, necessary. It is especially relevant where effluent carries complex or variable toxics: hydrometallurgical metal recovery (metal mixtures and residual reagents), chemical recycling and pyrolysis wastewaters (dissolved organics and additives), and any stream with cyanide, phenols or biocides. An effluent that meets the metal limits individually can still fail the bio-assay if the combined toxicity is too high.

The practical implication is that an ETP must be designed and run to produce genuinely non-toxic effluent, not merely effluent that ticks the parameter boxes. Passing the 96-hour fish bio-assay is the proof that the treatment is real and the combined toxicity is controlled. For metal and chemical recyclers, this often means adequate tertiary polishing and careful control of residual reagents, since their effluents are the most likely to pass individual chemical limits yet fail the whole-effluent toxicity test.