During the past few years, a number of different filter media have been used for stormwater filtration. Media such as sand, peat, and compost have been used successfully. Recent uses of perlite, zeolite, carbon, and other “exotic” media have expanded the choices for targeting specific pollutants. Here are some questions you should ask when selecting the right media for your application.
1. What are the physical properties of the media?
Most media remove solids by mechanical processes. The gradation of the media, irregularity of shape, porosity, and surface roughness characteristics all influence solids (TSS) removal characteristics. Finer media are more effective at removing TSS than coarse media but create higher head loss and have higher clogging factors. This tradeoff is a fundamental consideration. Media gradation is critical to performance, the finer the media the higher the performance but the slower the flow rate with the same amount of driving head.
2. What are the chemical properties and mechanisms for removing pollutants?
Many types of pollutants such as nutrients, metals, and oil and grease are in soluble or free form and can be removed through chemical and/or biological processes. Common processes are cation exchange, precipitation, chelation, and adsorption. When claims are made for soluble pollutants, there needs to be a documented process by which these reactions take place.
In addition, these reactions have limits in terms of sorption capacity and reaction kinetics. For example, media may have a sorption capacity of “X” mg/kg of media. Given the mass of the media, the total mass of pollutant that can be removed can be calculated and then compared with what is generated from the site. Reaction kinetics also cause a slowing of pollutant removal rates as media saturation increases and/or pollutant concentration decreases.
3. Can the media add constituents to runoff?
An example is organic media can elevate ortho-phosphorus, which leaches from the media. In some watersheds this is not a desirable media and an alternate should be selected. Other media can raise or lower pH.
4. Will the properties of the media change over time?
Stormwater is a complex mixture of sediments, nutrients, organic matter, bacteria, and other pollutants. Many times, media may perform well in the short-term, but in the long-term may be compromised by biological decomposition, bacterial slimes, or simple decomposition by continuous saturation in water. For example, does the media decompose or dissolve when exposed to stormwater? Cellulose-based media such as treated pulp, corn cobs, or rice hulls will decompose when exposed to these elements.
5. Does the media swell or shrink on wetting and drying cycles?
Check to see if the media is free draining or submerged. Permanently submerged media can lead to anoxic conditions, causing anaerobic decomposition and release of many of the trapped pollutants.
6. Is the media readily available? How much does it cost?
Many times, media are available in small production quantities or it is difficult to find media that meet all the specifications. Systems can also facilitate multiple-media options. Multiple-media systems have the versatility to fine tune media to site-specific pollutants, as well as adapt to future improvements in media effectiveness.