Effects of shear on two microalgae species. Contribution of pumps and valves in tangential flow filtration systems

The circulation of microorganisms in tangential flow filtration systems ind uces perturbations and then the damage of brittle cells. This work is focus ed on the shearing of two marine microalgae species (Skeletonema costatum a nd Haslea ostrearia), both largely cultivated in western France (Region des Pays de la Loire). We have studied the effects of the circulation of these cells in pumps and valves. For the pumps, it is shown that shear stress is due to the type of pump, but that mechanical shear can have different effe cts even if the pumps and the number or frequency of loops are the same. He nce other intrinsic parameters of the pump must be taken into account: rota ting velocity (omega), capacity (Cyl = output flow/rotating velocity) or in ternal leakage (K = inner volume/capacity). In throttling valves, the aim i s to correlate the effect of shear to a parameter related to the inner geom etry of the valve and to operating conditions. An overall parameter is then evaluated: the pressure drop coefficient K-v which integrates both the typ e of valve (ball valve or globe valve type) and its opening degree. K-v is derived from the relationship Delta H= K(v)u(2)/2g. The modelling of the sh ear effects is now conceivable: basic descriptive data used so far (type of pump, geometry or opening degree of the valve, etc.) being completed and p artially substituted by quantitative parameters (rotating velocity, capacit y, or internal leakage for the pumps, K-v coefficient for the valves). (C) 1999 John Wiley & Sons, Inc.