AN INNOVATIVE PROCESS FOR WASTE-WATER TREATMENT - THE CIRCULATING FLOATING BED REACTOR

Increasing volumes of wastewater combined with limited space availabil ity and progressively tightening European standards promote the develo pment of new intensive biotechnologies for water treatment. Fixed biom ass processes offer several advantages compared with conventional biol ogical treatments: higher volumetric load, increased process stability and compactness of the reactors. The purpose of this paper is to pres ent a new concept of gas-lift mobile bed, the circulating floating bed reactor (CFBR). The reactor design is simple and does not require any complex technical devices (easier effluent and air-flow distribution, no primary settling, no back-washing). This new process is studied an d developed in an industrial-scale prototype. The optimum hydrodynamic characteristics of the CFBR (liquid circulation velocity 0.3-0.4 m s( -1), k(L)a 50-300 h(-1), average mixing time 85 s) were not deteriorat ed by the high solid hold-ups (up to 40% v/v) of the floating media. O n the contrary, three-phase operating improves the local gas hold-up i n the downcomer. Improved hydrodynamics in the CFBR guarantee high nit rification rates and operation stability either in tertiary (up to 2 k gN m(-3) d(-1)) or secondary (up to 0.6 kgN m(-3) d(-1)) nitrification . The results show that nitrification is the limiting step in simultan eous C+N treatment. The negative effect of the increasing C/N ratio is more pronounced than stepwise decreasing of the temperature. The stud y of the biofilm composition and activity shows an effective control o f the attached biomass growth by the high liquid circulation velocity. It is concluded that this new three-phase bioreactor ensures not only an enhanced process stability and biological reaction rate through an effective biofilm control but also guarantees an excellent synergy be tween hydrodynamic and biological performances. These advantages are h ighlighted by the simplicity of the reactor design. Thus, this innovat ive technology will be an attractive solution for intensive wastewater treatment for nitrogen and carbon removal. Copyright (C) 1996 IAWQ. P ublished by Elsevier Science Ltd.