Combining hydrodynamic and enzymatic treatments to improve multi-species thick biofilm removal

The high water recycling ratio used in some industries can cause nutrients to accumulate in the system, leading to thick, rapidly growing biofilms inducing detrimental effects. Nowadays, chemical treatments are widely used but the increasingly restrictive environmental regulations induce the necessity to take the environmental aspect into account in the choice of a suitable treatment. With this in mind, this study analyses a method for controlling thick, dense biofilm development by applying synergistic actions having low environmental impact, i.e., an enzymatic and a mechanical (shear stress) treatment well suited to industrial water networks. For that purpose, biofilms were grown on plastic plates set in a Couette–Taylor reactor (CTR) that was inoculated using a “white water” sampled from a paper industry network. Development of the biofilms was controlled by applying a constant continuous feeding with a high COD/N ratio of 20, a well-defined shear stress and an external aeration, those conditions leading to a well-controlled G-value and a γS/O2 value close to 1, and consequently, to the development of thick mainly aerobic heterotrophic biofilms representative of that encountered in high loaded industrial water networks. Optimal operating conditions for biofilm treatments were determined considering the penetration time for the enzymatic treatment taking into account both internal and external mass transport (using Biot number calculation). In addition, an optimal shear stress increment of 2.5Pa was selected in order to maximise biofilm detachment while avoiding compression phenomena due to the mechanical treatment. The combined treatment led to an increase of 80% in biofilm mass removal (COD) compared to the enzymatic treatment alone and removed a large part of the basal layer of the biofilm, 80% reduction being observed in the support coverage. ► Biofilms were grown under well defined conditions. ► In response to hydrodynamic treatments, both biofilm detachment and compaction occur. ► Enzymatic treatments applied alone do not induce detachment of biofilm. ► Combined treatments greatly improve biofilm removal.