The current gap in global access to safe and reliable drinking water, especially in rural communities, calls for a reevaluation of the design bases of treatment technologies. Robust and cost-effective design requires optimization of the various unit processes in a water treatment train. The mechanism of primary particle removal in floc blanket clarifiers was characterized via size-based performance curves obtained from light blocking particle counters that measured the effluent of a laboratory-scale floc blanket clarifier treating synthetic raw water. Particle capture was first order with respect to depth up to a depth of 30 cm, after which diminishing removal of non-settleable particles was observed as depth increased. Observed particle removal is best described by finite particle capture via internal flow through flocs, as set by floc properties such as porosity. This mechanism is consistent with the observed changes in performance with floc blanket depth and time.