Inclined plate settlers to treat stormwater solids

It has been well-documented that, in stormwater runoff, many of the problem pollutants are associated with the particulate fraction. Past characterization of urban runoff and source contributions has shown that correlations were observed between TSS and particulate runoff concentrations of chromium, copper, and zinc, indicating that solids removal may reduce total metals concentrations. The first concern when investigating innovative treatment methods is determining the needed level of stormwater control. Specific treatment goals usually require 80% reduction in suspended solids concentrations. In most runoff, this would require the removal of most, if not all, particulates greater than about 10 μm in diameter (about 1% of the 1mm size that must be removed to prevent sewerage deposition problems). The ability of inclined cells (inclined plates/tube settlers) to provide excellent treatment of stormwater for a variety of pollutants was demonstrated by Pitt et al. (1999) in the report on the multi-chambered treatment train (MCTT) at the University of Alabama at Birmingham. The current project is adding to that body of knowledge by investigating the potential of inclined plate settlers to treat stormwater runoff both in the field (at the City of Harrisburg Public Works Yard) and through a full-scale "laboratory" demonstration. Inclined plate settlers can be designed in one of two ways - through the use of Stokes' Law and through the use of the Hjulstrom diagram, which accounts for scour and re-suspension. The test device in this research was sized using the Hjulstrom diagram. The results showed that the Hjulstrom diagram may be a very effective tool to predict the performance of inclined plate sedimentation devices based on the particle size for which 100% control is desired. Once the runoff's particle size distribution is known, estimating the average percent removal for the system would be trivial, and could be done using Stokes' Law. Particle size analysis of the influent and effluent samples collected from the inclined plate settlers evaluated in this researcher demonstrated that the inclined plates were capable of removing particles in >250-μm size range, even when these particles were a substantial part of the mass load to the system. An analysis of the TSS and SSC data demonstrates that inclined plate settlers are able to achieve the high removals desired for particles with a specific gravity similar to that of sand. As with most stormwater sedimentation devices, the plate settlers have poorer removals for small and/or lighter particles compared to large sand particles. One item of note is that removal efficiency was not dependent on continuous operation of the system. Interevent drying of the plates did not affect performance.