Towards on-demand photocatalysis: Controlling the operation of a photocatalytic reactor based on real-time, automatic monitoring of toxicity towards the working bacteria of a proceeding bioreactor

Wastewater streams contain a large number of contaminants, part of which can be quite toxic for the working bacteria in bioreactors, thus hampering its efficiency. In such cases, pretreatment by an Advanced Oxidation Process (AOP) might be needed. Since, in general, biotreatment is more economic than AOP, it is sensible to design the integrated system in a manner that would reduce the load on the AOP as much as possible. This is a very challenging task, since the type and the concentration of the toxic compounds constantly varies over time. The problem is aggravated by lack of inexpensive and fast technologies that are able to evaluate the toxicity under conditions in which the nature of the toxic compounds is unknown. Here we report on the developing of a unit that automatically measures the extent by which polluted water might put at risk a biological treatment unit. The sensing unit monitors in real time the viability of the reporting bacteria Bacillus Subtilis, using resazurin. The toxic compounds were modelled by three antibiotics (chloramphenicol, tetracycline and ciprofloxacin). The potential of embedding the sensing unit in a multi-technology system (AOP-biological) was demonstrated by connecting the sensing unit to a photocatalytic reactor and controlling the number of operating lamps in the photocatalytic reactor autonomously according to predetermined toxicity setpoint. The approach can be easily applied to almost any AOP-biotreatment tandem system, while altering any controllable parameter (residence time, light, etc.) of the AOP unit according to needs.