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FP5

AGROIWATECH Sintesi della relazione

Project ID: ICA2-CT-2002-10010
Finanziato nell'ambito di: FP5-INCO 2
Paese: Netherlands

Report on feasibility of anaerobic MBR systems for the treatment of specific agro-industrial waste waters (extreme conditions)

The feasibility of ananerobic membrane bioreactors (AnMBR) for the treatment of wastewaters from agroindustrial proceses was assessed. Experiments were performed at mesophilic and thermophilic conditions using three 3.7 L useful volume Anaerobic Membrane Bioreactors (AnMBR) equipped with submerged polysulphone micro-filtration membranes (Triqua, The Netherlands).

An emphasis was put into membranes performance, since this represents one of the most important cost in membrane bioreactors for wastewater treatment. In a first stage, the effect of flux, gas sparging, solids concentration and temperature on the critical flux was evaluated. Critical flux concept is used to define flux at which noticeable cake layer formation begins to take place. Cake formation rates over critical flux and its reversibility were also studied. In a second stage, long term operation of MBR reactors was studied, both under mesophilic and thermophilic conditions. Finally, the presence of non-acidified organic matter was studied under thermophilic conditions.

Biomass concentration showed to be the most important factor for cake formation in mesophilic MBRs. An increase in solids concentration reduce the critical flux and increases cake layer formation rate due to an increase of convective flow of particles towards the membrane. Gas flow rate has the opposite effect, increasing critical flux and decreasing cake formation. Thermophilic operation reduced drastically the effect of operational conditions, in comparison with mesophilic operation. Long term operation showed an important decrease in critical flux, probably due to a reduction in particle size distribution.

This may have been the result of cell lysis products that accumulate in the reactor. Cake formation showed to be reversible under mesophilic and thermophilic conditions, when studied in short term experiments. Nevertheless, on long term operation cake consolidation may occur, especially under mesophilic conditions. This cake layer cannot be removed by back-flush cycles, and a physical cleaning is required.

Presence of non-acidified organic matter induced the development of a high amount suspended acidogenic bacteria. This produced an increase in suspension viscosity and a decrease in particle size, which a consequent decrease in critical flux.
Anaerobic submerged MBRs economical feasibility is restricted by low critical fluxes attained in this study. Due to fast cake formation rates over critical flux, operational flux is likely to be restricted to a range close to critical value. Low levels of irreversible fouling were observed, with the exception of cake consolidation, as already mentioned. This means that the increase in shear forces over membrane surface should be efficient in increasing levels of critical flux. In an external membrane configuration, hydraulics can be more easily controlled. Further research in that direction would be of interest. Anyhow attention has to be set to the eventual negative effect of extreme shear forces over bacteria viability.

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