求翻译：由此可见，当磁粉投加量在4g/L左右时，微生物的生长最好，因此初步确定磁粉浓度为4g/L是微生物生长的最适浓度。是什么意思？

Thus, when the amount of magnetic powder dosing when at around 4g/L, best microbial growth, preliminary determines magnetic particle concentration is so 4g/L is the optimal concentrations of microbial growth.