Abstract
Hydrodynamic separation is a preliminary unit operation frequently utilized in wastewater and more recently in storm water for separation of coarse particulate matter (PM) and gross solids. In order to examine the behavior and separation mechanisms of a screened hydrodynamic separator (HS) not influenced by scour, this study examined the event-based performance of an empty-bed (clean sump and volute) HS for PM fractions transported in eight runoff events from a 1,088 m(2) paved source area urban watershed. Influent particle-size distributions (PSDs) {d(50) m from 270 to 2,202 mu m} and HS particle separation efficiency (PSE) (from 38 to 70% of mass) exhibited variations influenced by hydrology and previous loadings. When examined as PM size fractions, results demonstrate separation of the sediment fraction (>75 mu m) ranging from 76 to 94% while for settleable and suspended (1-25 mu m) fractions, the PSE was variable and significantly lower; from 3 to 57% and 2 to 43%, respectively. Results demonstrate a correlation between higher influent PM concentration and coarser PSDs, illustrating why higher PM concentrations promote higher PSEs; and why HS performance must be specified at a PM concentration, PSD, and flow rate. Results demonstrate that HS behavior is influenced by influent PSDs coupled with flow rate. Hydrodynamic separation is effective for high-rate gross solids control. However, current HS designs require incorporation of hydrologic control, methods of frequent sludge zone management before scour, and stored runoff management to control interevent redox conditions.