BACKGROUNDWater purification is challenged by the co-presence of contaminants, some of which are too toxic for bacteria or too opaque for photocatalytic processes. This challenge may be overcome by utilizing two types of reactors (for example biological and photocatalytic) operating in sequence. Realization of this approach should be based on a steady state model for calculating the optimal design under nominal conditions, a non-steady state responding to fluctuating conditions and fast measurement of toxicity and turbidity. RESULTSA steady-state model, examining the effect of coexisting toxicity and turbidity on the optimal sequence, on the relative retention times in the two reactors and on the effect of recycling, under an estimated cost function constraint is presented. It was found that the optimal sequence depends on the relative importance of toxicity and turbidity, that a single branch operation is likely to be more cost effective than dual branch (Biophotocatalytic in parallel with photocatalyticBio) system. CONCLUSIONIt is recommended to add the possibility for altering flow direction to account for operation under non-nominal conditions. To decrease instability during flow-direction switching, a recycling option should be added. Recycling may also be beneficial in particular when the activated biological medium has sluggish kinetics. (c) 2017 Society of Chemical Industry