By: Joel Garbon – Regulatory Manager
Oil-grit separators (OGS) have a decades-long history of use in Canada as primary treatment devices for reducing pollutant loads in stormwater runoff. OGS devices are typically utilized to capture gross pollutants, particulates (TSS), and oil. The dominant treatment mechanisms within OGS devices are sedimentation and floatation, with each functioning on the principle of gravity-based separation. Pollutants with specific gravity higher than that of water (such as road sand and degraded asphalt) settle to the bottom of the device as sediment, while pollutants with low specific gravity (such as oil and most plastics) rise to the top of the treatment chamber.
The settling velocity of particulates is largely a function of particle size and specific gravity. Relatively large particles (> 100 microns in size) with high specific gravity, such as coarse sand, settle very rapidly. Fine particles (< 50 microns) and particles with low specific gravity, such as brake dust and vegetative material, settle much more slowly. In order to achieve substantial pollutant load reduction, an OGS device must be sized to provide adequate detention time for capture of the range of particle sizes and specific gravities and for the range of runoff flow rates generated by the site for the majority of storm events. Undersized OGS devices do not provide adequate detention time for substantial load reduction, particularly of the fine particles on which much of the toxic metals and nutrient loads are concentrated. Additionally, undersized devices generally have inadequate sediment storage capacity and are much more prone to resuspension and washout of previously captured pollutants during a high intensity storm event.
As stormwater quality regulations continue to evolve toward greater focus on treatment of both particulate-bound pollutants of concern (metals, nutrients, hydrocarbons, bacteria) and dissolved pollutants, emphasis is placed on a high degree of capture of the fine particle fractions (silts and fine sands). Stormwater filtration devices are the proper choice to do this job, and ensure 80% TSS removal consistently. Whereas particulate load reduction with OGS devices are sensitive to influent flow rate and detention time (i.e. removal efficiency decreases as influent flow rate increases), the pollutant removal performance of filter devices is much less sensitive to changes in runoff flow rate.
The primary fine particle removal mechanism within a filter device is entrapment of particles within the filter media matrix. The pore size of the filtering media is a primary factor in determining the particles sizes captured. For filters that use granular media, such as sand filters, the gradation of the granular media establishes the average pore size. For a membrane-based filter, the membrane itself establishes the pore size. Membrane-based filters typically have a much higher hydraulic capacity and sediment capacity and much smaller footprint compared to granular media filters, due to provision of exceptionally high filtering surface area within the compact “tube bundle” membrane cartridge design. The membrane cartridges also have significant service life and maintenance advantages in that they can be rinsed and reused compared to granular media filters which require routine removal and disposal of spent media.
As stormwater quality regulations have become increasingly more stringent, use of filtration devices has become commonplace across North America, based on capability to consistently remove very high percentages of the particle size fractions and particulate-bound pollutants typically found in urban stormwater runoff. OGS devices will continue to play an important function as pretreatment measures targeting coarser particle fractions and oil capture, and applied within a treatment train approach.