Reduction in organic effluent static acute toxicity to fathead minnows by various aeration techniques

This study compared results of no aeration, intermittent aeration, and constant aeration strategies in determining the static acute (48-h) toxicity of phenolic-based effluents to adult fathead minnows (Pimephales promelas). Toxicity was greatest in no aeration tests followed by intermittent aeration and constant aeration. Two factors were considered responsible for the observed patterns of toxicity. First, in side-by-side tests of no versus intermittent aeration and intermittent versus constant aeration, toxicity reductions were directly attributed to maintenance of dissolved oxygen above 5.0 mg litre(-1) in aerated containers. Secondly, toxicity was reduced when treatment system temperatures were warmest, probably due to increased microbial activity and volatilisation during late spring to early autumn (temperatures > 16 degrees C). Effluent was slightly more toxic on- than off-site, presumably due to degradation of phenolic compounds during transport and set-up at the off-site laboratory (approximately 4.5 h). Gill tissue ultrastructure and histopathology were used to determine the extent of effluent-induced damage and the recovery of minnows to short (6-h) effluent exposures. After a 48-h exposure to the approximate LC(50) level, gill tissue lamellae were characteristically desquamated with epithelium lifting from the basement membrane. Gill tissue was similarly damaged after a 6-h exposure to 100% effluent and had recovered to pre-exposure conditions after 42 h in clean water. Aeration strategies in these studies demonstrated potential air-stripping of volatile compounds, although stress to test organisms from low dissolved oxygen was relieved.