So just what is hydrodynamic separation? The terminology has proliferated, yet no one seems to have provided a definition. Our use of the descriptor initially referred to vortex separators. To this group has recently been added small manufactured wet vaults and even, by some, generic oil/water separators.
Common to the early evolution of new fields of technical application is the multiplicity of duplicate and overlapping terms. Perhaps this occurred with automobiles in the early 20th century and computers in the late 20th century. In the meantime, confusion and misperceptions can hold sway, but more importantly the inconsistent application of design criteria. In our field we have several names for essentially the same treatment system, different systems with the same name, words without apparent definition, and the misapplication of words. Does not hydrodynamic or vortex convey the impression that the labeled treatment system has some extra-special characteristic that improves its performance? These two words are applied to systems with relatively small footprints, implying they are capable of performance equivalent to systems with much larger footprints.
What about swales? We have biofilters, grassy swales, vegetated swales, grass channels, landscaped swales, wetland swales, bioretention swales, dry and wet swales, and enhanced dry and wet swales. No wonder we have confusion over “what is best.” The first six terminologies usually refer to the flow-through swale–that is, to areas where treatment is received as the stormwater passes through the swale, discharged to a roadside pipe or ditch. This swale is sized to the peak flow of the design event. Infiltration may or may not occur. If it occurs, it is commonly not credited in the performance if based on concentration change, but is if based on loading reduction. In contrast, dry and wet swales are sized based on volume, equal to the volume of the design event. Common east of the Rockies, these swales are, in effect, wet extended detention basins and filters, respectively. The former is used where groundwater is near the surface; hence, the stormwater can neither infiltrate nor filter. The latter system commonly has underdrains and native soil—ergo, a filter. Presumably it need not have underdrains if we have good infiltrating soil. Some call this system bioinfiltration—i.e., a grass-covered infiltrating swale with no surface outlet excepting flooding of adjacent areas during the extreme event.
And often we have what appears to be the same system, but with different names. Consider the organic filter, presented in some manuals, and a bioretention system with underdrains—i.e., a filter. The bioretention medium is a mixture of 50% sand and 50% organic media, sometimes all loam soil, sometimes a mix of loam soil and mature compost, with a depth of 2 to 4 feet. The organic filter medium is a mixture of 50% sand and 50% organic media, usually peat. This comes as an 18-inch layer placed above an 18-inch layer of sand. One may have grass growing on the surface. The bioretention filter has grass plus shrubs and possibly trees. One system is landscaped; the other may or may not be. Although the two systems are essentially identical, we have two different sizing procedures. The organic filter is sized as a filter, using Darcy’s Law, a specified drawdown time, and a specified hydraulic conductivity. Surface area is calculated from these inputs. The bioretention filter is sized as an infiltration system, because this was its original form. Manuals commonly specify a maximum water depth, an infiltration rate, and that the storage volume above the unit be equal to the volume of the design event. The surface area is calculated by dividing this volume by the specified water depth. The infiltration rate does not play a direct role in sizing the unit but is presumed to be sufficient. It should be noted that infiltration rate and hydraulic conductivity are two different parameters, although they have the same units.
As for name-ology, how does bioretention differ from bioinfiltration? The differences do not seem significant. By those who use the term, bioinfiltration is a surface infiltration system covered usually with grass, but possibly trees or shrubs. Bioretention uses a specified medium, imported to the site. Bioinfiltration uses the native soil present at the site. Are these differences sufficient to warrant two different names? To leave the native soil in place is a specification, and this soil is a filter medium. The incremental benefit of trees and shrubs over grass has been conjectured but not demonstrated. One manual uses the words porous landscape detention instead of bioretention.
Is not a dryswale a bioretention system? They both are vegetated, although perhaps only with grass in the dry swale. Or must the system have shrubs and trees to be considered bioretention? Should we have different names based solely on the form of plants? Yes, with bioretention a specified soil mix is used, whereas it is not with the dry swale. But soil is a filter medium. One manual refers to bioretention cells and bioretention swales.
So what is hydrodynamic separation? The terminology is used by chemical and mining engineers, in medicine, and to describe the expansion of the fireball of a nuclear weapon. Excluding the latter use, one common definition of hydrodynamic separation is the removal of particles by gravity from a moving fluid. The fluid can be air or water. The particles may be more or less dense than the fluid. Chemical engineers also narrowly define hydrodynamic separation as removal of a particle from a fluid that experiences a very abrupt change in its direction of flow. If we apply the first definition to stormwater treatment, which seems most appropriate, wet ponds and constructed wetlands are hydrodynamic separators, as are extended detention basins. Hydrodynamic separation also applies to flow-through grass swales. In effect, the term is meaningless.
My recommendation is to simplify and clarify. Let’s not use terms that imply a process or performance expectation that does not exist, or whose definition applies so broadly as to be meaningless. Such is the case with hydrodynamic separation. Its use should be discontinued. We need to establish single names for what are essentially the same systems, with variants in the design criteria to reflect different conditions with respect to soil, site topography, pollutant removal objective, etc.
We need a formal, logical, and simplified classification system. In the March/April 2006 issue of Stormwater, I will present a proposed framework, including the concepts of unit processes and unit operations.
