During the past decade, a number of different media have been used for stormwater filtration and more recently for bioretention. Media such as sand, peat, and compost have been used successfully. Uses of perlite, zeolite, carbon, and other "exotic" media have expanded the choices for targeting specific pollutants. Media are now being used to target TSS, Petroleum Hydrocarbons, Dissolved Phosphorus, dissolved metals and even bacteria. More recent research in biofiltration adds elements of biological uptake of nutrients and metals by plants, conversion of Nitrogen into ammonia in anaerobic zones.
There are some fundamental media properties that the designer or researcher must consider before selecting a media for use in filtration processes.
Consider the physical properties of the media used for sediment removal. Most media remove solids by mechanical processes. The gradation of the media, irregularity of shape, porosity, and surface roughness characteristics all influence 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. Biofiltration tends to operate at very slow rates at about 5 inches per hour. This enables fine media, which operates as a cake filter, where solids are retained on the surface. Other rapid filtration technologies, operating from 50 to 100 inches per hour, tend to operate as depth filters where solids are removed progressively through the entire media bed. Hence, solids loading is a critical aspect of media performance and longevity.
Understand the chemical properties and mechanisms used to remove stormwater pollutants. Many types of pollutants such as nutrients, metals, and oil and grease are insoluble 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. Ideally one wants to balance the life of the media reactivity with the solids loading to even out the maintenance frequency.
The reviewer should also consider if the media can add constituents to runoff. For example, organic media can elevate ortho-phosphorus, which leaches from the media. In some watersheds, this is not desirable, and an alternate should be selected. Other media can raise or lower pH or can add unintended constituents to the effluent. It should be a standard process to evaluate media by toxicity testing. In addition, claims made about a media killing bacteria can trigger USEPA FIFRA regulations and requirements to be registered as a biocide.
Evaluate whether the properties of the media will 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. 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.
Check media availability and cost. Many times, media are available in small production quantities, or it is difficult to find media that meet all the specifications. Some 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. Proper media gradation, content, and configuration are all critical to filtration performance, longevity, cost and maintainability.
Volumes have been and will be written on this subject. Media filtration, biofiltration, and bioretention all utilize filtration processes. As these solutions become the mainstream approach for managing water quality and volume, media selection will become a key element for land use specific management approaches.