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 induces perturbations and then the damage of brittle cells. This work is focused on the shearing of two marine microalgae species (Skeletonema costatum and Haslea ostrearia), both largely cultivated in western France (Région 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 effects even if the pumps and the number or frequency of loops are the same. Hence other intrinsic parameters of the pump must be taken into account: rotating velocity (omega), capacity (Cyl = output flow/rotating velocity) or internal leakage (K = inner volume/capacity). In throttling valves, the aim is to correlate the effect of shear to a parameter related to the inner geometry of the valve and to operating conditions. An overall parameter is then evaluated: the pressure drop coefficient Kv which integrates both the type of valve (ball valve or globe valve type) and its opening degree. Kv is derived from the relationship DeltaH = Kvu2/2g. The modelling of the shear effects is now conceivable: basic descriptive data used so far (type of pump, geometry or opening degree of the valve, etc.) being completed and partially substituted by quantitative parameters (rotating velocity, capacity, or internal leakage for the pumps, Kv coefficient for the valves).