Modelling of coliform removal in 186 facultative and maturation ponds around the world

The paper presents a very extensive evaluation of the coliform decay in facultative and maturation ponds, based on data from 186 different ponds in the world. The ponds encompass a very wide diversity in terms of physical and operating conditions, covering most situations encountered in practice. The median values for the coliform removal efficiencies were 1.8 log units (98% removal) for primary facultative ponds, 1.0 log units for secondary facultative ponds (90% removal) and 1.2 log units (94% removal) for each maturation pond in the series. Two equations to be used for design purposes were derived for estimating the die-off coefficient K(b) (dispersed flow, 20 degrees C) in facultative and maturation ponds. The first equation led to a slightly better fitting with the observed logarithm of the effluent coliform concentrations (R2 = 0.874), and related K(b) with the pond detention time t and depth H (K(b) = 0.682 H(-1.286) t(-0.103)). The other equation also led to a satisfactory fitting (R2 = 0.845), but was slightly simpler, depending only on the pond depth (K(b) = 0.549 H(-1.456)).