The removal of site-specific pollutants can be maximized with the variety of filtration media available. Understanding the hydraulics of the media you choose is a key, albeit misunderstood, factor in determining the effectiveness of the filtration system. Here are five things you should consider.
1) Evaluate the specific flow rate (q) through the media.
The specific flow rate, or flux, is in units of flow per unit area or gallons per minute per square foot (gpm/ft2). Given the specific flow rate times the surface area (A) of the filter, the total flow rate (Q) can be calculated (Q = qA). A good reference point is rapid sand filtration with rates of about 4 gpm/ft2. In general, the higher the rate, the higher the head loss. Finer media are typically more efficient for total suspended solids (TSS) removal, as well as other pollutants, but have high head loss characteristics. Coarser media can handle higher flow rates but are less efficient in TSS removal. Claims of high filtration rates with high pollutant removal capabilities and low head loss are highly suspect.
2) Compare the design specific flow rate to specific flow rates in lab and field studies.
Does the specific flow rate of the proposed design match the specific flow rate associated with performance data? For example, does performance data have a specified rate of 2.0 gpm/ft2 while the design rate is 20.0 gpm/ft2? For proper filter design, this is an absolute factor. Also evaluate different model sizes of the proposed BMP. Given the same driving head, the specific flow rate should be the same. If the specific flow rates vary with model size, this should raise questions.
3) Consider the thickness and head loss of the media.
Darcy’s Law (q = ?hKA/L) states that flow rate (q) increases with increased head (h), surface area (A), or driving head (K) and decreases with an increased bed length (L). The thickness of the media coupled with the specific flow rate determines the amount of contact time the water has to be treated. The longer the contact time, the more effective pollutant removal will be, particularly when soluble pollutants are being removed through reactive processes. Thicker media will have higher removal rates but will increase head loss. Thicker media will also increase media costs and maintenance costs.
For porous media such as perlite, TSS removal efficiency increases with thickness because there is more opportunity for particles to be captured as water follows a tortuous path though interstitial pores. For fine media such as sand, the majority of the TSS capture is at the surface, and media thickness has less influence on TSS removal. Observations and studies of sand filters show that the majority of fine solids removal occurs within the first 2 to 3 inches of bed thickness.
In general, Darcy’s law applies to flow through porous media. However, for many filtration systems with high conductivity and a relatively short bed, Darcy’s law does not behave with the same accuracy as in a groundwater application. Another confounding factor of stormwater filters is that the conductivity is a variable. As the media loads with solids, K will slowly decline. Toward the end of the filter life, K approaches zero asymptotically.
4) Calculate the contact time.
Compare the calculated contact time with the contact time used in lab or field studies. For example, if the flow rate is doubled and the thickness is reduced by half, the contact time is one fourth. This will have a direct impact on pollutant removal effectiveness. It is important to check the contact time of the design and the test data presented.
One should also look at the flow paths through the filter. Is there a uniform pressure distribution across the media? A non-uniform pressure distribution results in differential loading of the filter and non-uniform contact time.
5) Consider fouling and occlusion of the media.
This will ultimately control the specific flow rate through the media. One critical aspect is the surface of the filter. In general, finer media can remove finer particles but have a much higher clogging factor. Sand can retain about 1.1 pounds of dry sediment per cubic foot before it rapidly clogs and fails, whereas a coarser perlite media can retain about 6 pounds of sediment per cubic foot. This is a common tradeoff, having the benefit of finer media achieving higher performance while exhibiting a much higher clogging factor.
Filtration media that is effective in trapping fine solids will accumulate a thin layer of solids on its surface (smutzdecke) that begins to occlude the filter and reduce the flow rate. An accumulated layer as thin as 1 mm will control the filtration rate and reduce the specific flow rate to very small amounts (< 0.1 gpm/ft2). For example, a horizontal sand filter that builds up a smutzdecke demonstrates this. Without cleaning mechanisms to prevent surface clogging, this problem becomes endemic to all filtration systems.
Following are two important aspects to consider regarding the filter surface:
1) Know and examine the percent open area of the outer wall of the filter vessel, which includes the filter body and any internal mesh. The lower the percent open area, the more susceptible the housing is to surface clogging and failure. If the surface of the filter is exposed to light, algal growth can rapidly clog the filter surface.
2) Understand the capability of the technology to prevent surface fouling by sediments. It is a fact that filters will eventually clog with sediments. The question is how long does it take. Clearly, if a filter has no active mechanism to remove accumulated sediment from a filter surface, its life will be less than a filter that does. Pretreatment by settling will help, but research indicates that the majority of the fouling is by fine sediments, organic matter, and bacterial growth, all of which are difficult to remove by pretreatment through rapid settling. In fact, research at Monash University in Melbourne, Australia, indicates that it is particles of 20 µm or smaller that cause clogging of sand filters.