Here I'll provide more details on the experimental design.
One of the factors in the DoE was the protein concentration (in g/L) in the input solution. It was intended to filter the same volume of input solution for each experiment (we measure this). Therefore, the protein content filtered in each experiment would be different. The protein content is reported as g/m2 filter and is considered as the nuisance factor (perhaps a better term is required). In some experiments the filters blocked and not all of the solution could be filtered (most likely due to high protein concentrations). Therefore, we proposed to determine the protein content filtered in each experiment and to see if this had an impact on the responses (most interesting would be the effect on flux decay).
To answer your question about curvature, we in fact detected curvature from the center points and therefore added star points to the design. Also, we are not trying to optimise our process but establish ranges that ensure process robustness.