The choice between plug-flow and complete-mix hydraulic regimes for a biological reactor comes down to how each handles the incoming substrate concentration gradient: plug-flow exposes biomass to a high substrate concentration at the inlet and a low one at the outlet, favouring treatment efficiency and floc formation for a fairly consistent waste; complete-mix instantly dilutes any incoming waste throughout the entire tank volume, favouring resistance to shock or toxic loading at some cost in effluent quality.
14.1 — Plug-flow example. A dairy or food-processing plant with a fairly steady, consistent, moderate-strength wastewater and a requirement for a low effluent soluble BOD5 would be a good candidate: the declining substrate gradient along a plug-flow tank drives the reaction closer to completion, and the resulting biomass typically has better settling characteristics (lower filamentous bulking tendency) than a complete-mix system treating the same waste.
14.2 — Complete-mix example. A metal-finishing or batch chemical-manufacturing plant, where the wastewater strength and composition vary widely from batch to batch and an occasional slug of a moderately toxic or high-strength discharge is a real operating risk, would be a good candidate: because the entire tank volume mixes instantaneously, an incoming shock load is diluted across the whole biomass inventory immediately, rather than exposing biomass at the inlet to the full undiluted concentration, which protects the microbial population from being killed off by a slug that a plug-flow reactor would deliver at full strength.
