Study of an aerobic concentrated culture reactor coupled to separation by crossflow micro- or ultra-filtration through inorganic membranes. Initial approach to a depollution application
Urban or industrial waste water organic pollution is must often treated by aerobic biological systems. In this case, commonly used activated sludge plants need large tanks. Membrane bioreactors would give compactness and very high and constant quality effluent. Up to now, organic membranes have been mainly utilized for polishing effluents from conventional secondary treatment. MEMCOR process, using hollow fibers membranes, allows permeate fluxes of 100 l/h.m2 for several months under a 1 bar pressure. Chaize (1990), using inorganic membranes for urban waste water treatment, reached high organic and nitrogen removal. Biomass was about 10 to 20 g/l and permeate fluxes of 20 and 28 l/h.m2 at 1.1 and 1.4 bar transmembrane pressure with residuals less than 20 ppm COD and SSM and 5 ppm TKN. Materials and methods : The pilot studied here combined in the same module a compact bioreactor and a filtration membrane. A synthetic waste water has been treated by crossflow micro or ultrafiltration with aerobic microorganisms and pure oxygen injection. This effluent had a DCOIDBO ratio of 1.5 with very low suspended solid matters (38 mg/l). a-Alumina (pore size : 0.2 μm) or Zirconium oxide (pore size : 500 Å) inorganic membranes have been used. As membrane fouling is the main weakness of this processes, two treatments have been experimented with both membranes : 1. total biomass recycling without back flush system ; 2. excess biomass draining with back-flush system operating. Experiments lasted continuously for 3 to 7 days. Crossflow velocity was about 3.8 m/s, pH was regulated at 7.0 by concentrated sodium hydroxide addition. Heat generated by pumps needed heat exchange with tap water in order to maintain temperature around 30 °C, wich was considered optimal for biological reactions. Permeate COD and nitrogen, biomass Suspended Volatile Matters (SVM) and Suspended Solid Matters (SSM) have been monitored. Results and discussion : 1) Effluent quality was constant and better than for conventional extended aeration. The two membranes retained the whole of SSM and bacteria, permeate COD was less than 30 mg/l, even under high load conditions (5 to 20 kg COD/kg SVM.d), elimination rates reached 98 % BOD and 97 % COD. Biomass production and oxygen consumption were lower than for high load activated sludge process, tact reported to pure oxygen utilization and unfloculated bacteria predominance. Nitrification remained very low due to short sludge age (< 90 h). 2) Membranes cleaning state at the beginning of the tests didn't modify stabilized permeate fluxes. Particulate fouling predominated with 0,2 μm membrane, problem solved using back-flush system, but fast slime fouling developed in the first 24 h. Electron microscopy membrane surface examination shown heavy fouling by a biofilm which real composition remains unknown, and by numerous small particles (site range from 0.04 to 0.3 mm). With total biomass recycling, permeate fluxes were very low for both types of membranes : 15 l/h.m2 for 500 Å membrane and 2 l/h.m2 for 0.2 μm membrane. Treatment had to be stopped alter only 99 h. Biomass concentration increased up to 32 g/l in 3 days. When using back flush system and regularly draining excess biomass, becter stabilized permeate fluxes could be observed : 35 l/h.m2 for 500 Å membrane and 24 l/h.m2 for 0.2 μm membrane. Mean biomass concentration was about 15 g/l. Enzymatic digestion of slimes on fouled membranes pointed out the role of bacteria colt walls and proteins. These compounds were thought to be produced mainly under substrate limitation conditions. Fouling could thus be avoided by regularly draining excess biomass. Technology of inorganic membranes is still recent and involves relatively high costs. However, as it may ha interesting for soma industrial effluents, tests are carried on to confirm chose results.