Water Environment Federation (WEF)

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 the following: (a) soil disturbance increased the TSS and turbidity in the runoff; and (b) 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 specify about 80% reductions in suspended solids concentrations. In most stormwater, this would require the removal of most, if not all, particulates greater than about 10 μm in diameter, which is about 1% of the 1-mm 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. This 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.

A sieve analysis of the influent and effluent, using a 250-μm sieve, 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 density similar to that of sand. As with most stormwater sedimentation devices, the plate settlers operate as designed and removals for small and/or lighter particles are not as high as they are for large sand particles. Removal efficiency also was not dependent on continuous operation of the system. Interevent drying of the plates did not affect performance.

The preferred method for treating solids in stormwater runoff has been through sedimentation in detention facilities. Using the terminology of the water and wastewater treatment industry, three types of sedimentation may occur in stormwater detention basins: Type I (discrete particle settling), Type II (floc settling) and Type III (zone/blanket settling). Type III settling is the settling that would occur near the bottom of the detention pond as the solids interact sufficiently to create a “blanket” which moves downward with the characteristics of a large, thin layer settling through water. In a wet detention basin or other wet sedimentation device (any device where the trapped water is not completely discharged), zone/blanket settling is likely to occur below the outlet structure and therefore, would have little impact on the solids removal efficiency of the device. Discrete particle and floc settling (Types I and II) are the settling methods of concern in design stormwater sedimentation facilities.

Design has been based on determining the desired surface overflow rate (which is equal to the flow through the basin divided by the basin’s surface area). This is the flow rate through the outlet structure when the basin is “full,” i.e., before activation of any freeboard area or overflow/bypass channel. If it is assumed that the particle settling in the sedimentation basin occurs by Type II settling (floc settling) where the interactions of particles affect the settling rate (usually by creating a larger, less dense particle), then the sedimentation characteristics of the floc must be determined in the laboratory, typically through the use of a settling column. The floc’s settling characteristics, usually measured as total suspended solids (TSS) at specific depths in the water column over time, are graphed. Then the desired effluent TSS or removal efficiency and water depth are used to select the retention time in the basin. To size the basin (volume), the effluent flow rate at design flow is multiplied by the retention time required to achieve the desired removal efficiency.

Most stormwater sedimentation facilities are designed assuming Type I, or “discrete particle,” settling. In Type I settling, it is assumed that the particle settles without interacting with other particles and that the settling occurs in laminar flow conditions (small Reynolds’ numbers). Therefore, the settling rate is determined by the balance of the gravitational forces on the particle and the buoyancy of the water. The primary equation used to determine settling rate is Stokes’ Law, which calculates the settling velocity of a particle, given its size, diameter and density.

Complete sedimentation is assumed for all particles whose settling velocity is greater than the surface overflow rate. For particles whose settling velocities are less than the surface overflow rate, partial sedimentation occurs if the influent water column is well mixed.

As noted above, one of the primary controlling factors for sedimentation is the surface area available for settling. Various methods – hydrodynamic separators, chemically-enhanced coagulation followed by detention – have been used to enhance, and reduce the surface area required for, sedimentation. Another method, common in the water treatment industry for enhancing sedimentation efficiency, is the use of inclined plate settlers. Inclined tubes (or plate settlers) increase solid removal by reducing the distance particles travel to the chamber floor and by reducing scour potential (Davis, et al.1989). The main settling chamber operates much like a settling tank, but with the tube settlers increasing the effective surface area of the tank. The increase in performance is based on inclined cells that overlap each other. Each cell forms the ceiling of the next cell, etc. The projected area of each base forms the settling surface of each cell. However, the horizontal distance between each plate is a fraction of the horizontal projection of the cell base. Hence, the efficiency in settling surface is obtained by this cellpacking arrangement. If the plates are relatively flat and close together, the increase in performance is greater than if the plates are steeper and wider apart. The effective increase is usually about 3 to 5 fold, and in the drinking water industry where this technology has been studied and optimized, the increase has been about 10 fold. The drinking water industry uses these inclined plates or tubes to save on plant space, increase settling of the floc from the treated water, and ultimately to increase the filter run life.

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