All Stormwater Particles Are Not The Same. Part 1: Particle Size And Composition

The EPA selected a removal standard of 80% total suspended solids (TSS) removal as the target pollutant of concern due to high TSS concentrations impact on water quality and degradation to aquatic habitat. Many other pollutants of concern are particle-bound, and TSS is thereby a surrogate for other pollutants. Testing methodologies for stormwater control measures (SCMs) in respects to TSS can vary greatly.

There are many sediment characteristics that should be considered when evaluating a SCM for TSS removal performance to ensure apples and apples are being compared among removal efficiencies for SCMs. Therefore, design engineers and regulatory authorities should be aware that not all SCM claims are based on the same type of sediment, and a better understanding of the sediment tested will allow for SCM selection appropriate for the application.

In part one of this two part series, we’ll look at particle size and particle composition and their effect on TSS removal.

Particle Size

Suspended sediment particle size distribution (PSD) characterizes the particle size fractions of a given sediment, and is an important evaluation tool as size fractions will vary from site to site based on land use. It is also known that different pollutants will adsorb to different size fractions. Particle size and surface area are inversely proportional. Smaller particles have larger surface areas, and will often exhibit more sorbed dissolved pollutants per mass of sediment due to the increased surface area. TSS is typically defined as particles greater than 0.45 microns with the upper bound defined by the regulatory authority; generally it is those not remaining in suspension, which can vary dependent upon the flow rate during sampling.

A SCM may demonstrate meeting an 80% removal TSS standard while treating a medium-density residential neighborhood, but the same SCM TSS removal may decrease significantly when used to treat a right-of-way near a lumber mill with high-density truck traffic. This change in TSS removal is often due to the shift in PSD contributed by fine, organic dust. It is imperative that SCMs be selected based on the applicable land use. For example, pretreatment SCMs are designed to capture larger particles while filters are often used as stand-alone practices or as polishers to target the smaller sized particles. New Jersey Department of Environmental Protection (NJDEP) distinguishes the differences in removal mechanisms for hydrodynamic separators (HDS) and filter SCMs by developing separate laboratory sediment testing protocols with different sediment removal rate requirements (50% for HDS and 80% for filters) to address these different treatment processes.

Commonly used ground silica sand gradations used in laboratory testing protocols for SCMs have included OK-110, F-95, and Sil-Co-Sil 106. The average particle size (d50) for these various product gradations can vary significantly. 80% removal by one SCM using OK-110 (d50 = 110) as a test sediment is not the same as 80% removal by another SCM using Sil-co-sil 106 (d50 = 19 microns). While both may achieve 80% TSS removal, the SCM having used test sediment Sil-co-Sil 106 has greater capacity for removing finer sediment, and would be a better choice for land uses with small-sized silts and clays, or for use as a polisher in a treatment train design.

The Washington State Department of Ecology (Ecology) requires the use of Sil-Co-Sil 106 ground silica sand to represent a typical PSD for laboratory testing of Basic Treatment SCMs for Conditional Use Designation. Ecology, through implementation of the Technical Assessment Protocol – Ecology (TAPE) Program, recognizes that water quality samples containing different PSDs with the same TSS concentration can result in significant differences in SCM removal efficiency. The TAPE program attempts to alleviate the issue of varying PSD data among SCMs by requiring the field influent PSD contain mostly silt-sized particles (i.e., 3.9 to 62.5 microns) to represent Pacific Northwest stormwater.

Particle Composition

Laboratory testing allows SCMs to be tested under the same protocols using the same sediment characteristics (i.e. specific gravity, inorganic ground silica) with precisely defined PSDs for apples and apples comparison when evaluating sediment performance among different SCMs. Testing is performed in a controlled environment under an approved testing standard requiring a specific test sediment PSD, allowing for repeatable results. As previously mentioned, inert, ground silica sand is often utilized as a surrogate test sediment to representative field sediment in stormwater runoff. However, it should be noted that ground silica sand is not typically representative of field performance and therefore, should only be used as supplementary data for evaluating performance among different SCMs. Field sediment is often made up of both organic and inorganic particles including sands, silts, clays and organic matter. Field sediment removal performance and hydraulic capacity will vary significantly in comparison to a sediment consisting of ground silica sand alone.

Field sediment will cause most SCMs to occlude more quickly than ground silica sand due to the caking effects of organic material. Organic material is also subject to biological degradation which further impacts performance by breaking down sediment into smaller size fractions. SCM field installs are subject to biofilm development and increased loading rates, ultimately restricting hydraulic capacity. Field monitoring of SCMs is more reflective of real world operational performance by providing longevity and storage capacity data. This data can be used as an indicator for predictive maintenance frequency, and when coupled with laboratory sediment testing, provides more equal comparison across SCMs.

Field sediment particles will have different reactivity with other pollutants depending on the composition of the sediment. Reactive filtration can occur through surface adsorption via complexation, ion exchange, precipitation, electrostatic or electrokinetic forces and biofilm development. For example, organic particles and clays have a strong affinity for metals and hydrocarbons, and can cause smaller particles to flocculate to form larger particles, which can increase TSS removal efficiency.

In part two, we’ll continue our look at stormwater sediment and discuss particle shape and density and their effect on TSS removal.