The failure of an upstream filtration membrane, or the presence of a foulant in a fluid is detected using a membrane-based detector. The fluid or an effluent from the filtration membrane is directed to permeate through a first permeable membrane, and from the first membrane to permeate through a second permeable membrane. A ratio between (P.sub.1-P.sub.2) and (P.sub.2-P.sub.3) is determined, where P.sub.1 is a first pressure at a feed side of the first membrane, P.sub.2 is a second pressure between the first and second membranes, and P.sub.3 is a third pressure at a permeate side of the second membrane. The ratio is correlated with the failure of the filtration membrane, or with the presence of the foulant.