As discussed in an earlier section, there are two types of toxicity
assays: the batch-based direct measurement of toxic effects on
the native biological community; and, the indirect measurement of
toxic effects on bioluminescent indicator organisms or biosensors.
The two types of toxicity assays have their own application niches:
the batch-based direct measurement is more applicable when a
new chemical or a new wastewater has to be evaluated for basic
toxicity. The indirect measurement is more suitable as a front-line
screening device to protect an operating WWTP. A comparison of
the above-mentioned toxicity assays is listed in Table 2.
Recently, both the direct and indirect toxicity assays have
been used in full-scale biological WWTPs. A wastewater treatment
facility receiving more than 85% of wastewater from industrial
sources in Virginia, USA, had experienced a number of random toxic
upsets [169]. Thus, an early warning system (EWS) based on ATP
luminescence assay developed by LuminUltra (Fredericton, New
Brunswick, Canada) was implemented, which identified a significant
toxic upset and facilitated troubleshooting after the upset. In
another case, an EWS based on a full performance reactor-based
biosensor – BioCoilTM, which consisted of a small aeration tank
and an unaerated plug-flow reactor, was successfully used over
the past six years [170]. The pH, temperature, suspended solids,
and dissolved oxygen were sensed in the EWS, and the OUR was
determined continuously. The EWS successfully warned of a chemical
spill incident, and helped to identify the source of the incident.
These examples clearly demonstrate that anEWSbased on an effective
toxicity assay can protect WWTPs from toxic shocks. However,
research is still needed in the following areas: