MF and UF can be operated under positive pressure or negative pressure (i.e., vacuum pressure). Positive pressure systems typically operate between 3 and 40 psi, whereas, vacuum systems operate between -3 to -12 psi. There is no significant difference between the range of pressures at which MF and UF systems operate. The pressure differential is used to pass the feed water through the membrane to and produce a filtrate stream. Particulate contaminants are retained by the membrane and removed during backwashing of the membrane filter much the same way particulates are removed from a rapid sand filter.