Methods to model particulate matter clarification of unit operations subject to unsteady loadings.

Stormwater, and also wastewater unit operations (UOs) to a much lower extent, are subject to unsteady hydrodynamic and particulate matter (PM) fluxes. Simulating fully transient clarification of hetero-disperse PM requires much greater computational expense compared to steady simulations. An alternative to fully unsteady methods are stepwise steady (SS) methods which use stepwise steady flow transport and fate to approximate unsteady PM clarification of a UO during transient hydraulic loadings such as rainfall-runoff. The rationale is reduced computational effort for computational fluid dynamics (CFD) compared to simulating continuous unsteadiness of such events. An implicit solution stepwise steady (IS) method is one approach which builds upon previous SS methods. The IS method computes steady flows that are representative of unsteady PM transport throughout an unsteady loading. This method departs from some previous SS methods that assume PM fate can be simulated with an instantaneous clarifier (basin) influent flowrate coupled with a PM input. In this study, various SS methods were tested for basins of varying size and residence time to examine PM fate. Differences between SS methods were a function of turnover fraction indicating the role of unsteady flowrates on PM transport for larger basins of longer residence times. The breakpoint turnover fraction was between two and three. The IS method best approximated unsteady behavior of larger basins. These methods identified limitations when utilizing standard event-based loading analysis for larger basins. For basins with a turnover fraction less than two, the majority of effluent PM did not originate from the event-based flow; originating from previous event loadings or existing storage. Inter- and multiple event processes and interactions, that are dependent on this inflow turnover fraction, are not accounted for by single event-based inflow models. Results suggest the use of long-term continuous modeling combined with the IS method for hydraulic, PM and chemical loadings to a UO when the turnover fraction is less than three.