Asediment control is a practice or device designed to keeperodedsoil on aconstruction site, so that it does not wash off and causewater pollution to a nearby stream, river, lake, or sea. Sediment controls are usually employed together witherosion controls, which are designed to prevent or minimize erosion and thus reduce the need for sediment controls. Sediment controls are generally designed to be temporary measures, however, some can be used for stormwater management purposes.[1]
Treatment of silt impacted water using equipment and chemical addition, commonly called anactive treatment system, is a relatively new form of sediment control for the construction industry. These systems are designed to reduceTotal Suspended Solids (TSS) from entering nearby water bodies where silt pollution can be of environmental concern. Sediment-laden stormwater is collected and or pumped, and a chemicalflocculant is added to aide inclarification. Types of flocculant include;
Extreme caution should be observed when usingcationic flocculants likechitosan or positively chargedpolyacrylamide orpolyDADMAC which causehypoxia in fish. The use of anionic,negatively charged, flocculants is best practice on open loop treatment systems to ensure the protection aquatic habitat, fish andinvertebrates.
The water is then either filtered (sand or cartridge filter,) or settled (lamella clarifier orweir tank) prior to discharge. Chemical sediment control is currently used on some construction sites around theUnited States and Europe, typically larger sites where there is a high potential for damage to nearby streams.[3] Another active treatment system design useselectrocoagulation to flocculate suspended particles in the stormwater, followed by a filtration stage.[4] Active treatment systems require technical expertise to operate effectively as multiple types of equipment are utilized.
Chemical treatment of water to remove sediment may also be accomplished passively.Passive treatment systems use the energy of water flowing by gravity through ditches, canals, culverts or other constructed conveyances to effect treatment. Self dosing products, such asGel Flocculants, are placed in the flowing water where sediment particles,colloids and flow energy combine to release the required dosage, thereby creating heavy flocs which can then be easily filtered or settled. Natural woven fibers like jute are often used in ditch bottoms to act as filtration media. Silt retention mats can also be placed insitu to capture floccules. Sedimentation ponds are often utilized as a deposition area to clarify the water and concentrate the material. Mining, heavy construction and other industries have used passive systems for more than twenty years. These types of systems are low carbon as no external power source is needed, they require little skill to operate, minimal maintenance and are effective at reducingTotal Suspended Solids, someheavy metals and the nutrientphosphorus.
Stormwater treatment can also be achieved passively. Stormwater management facilities (SWMF's) are generally designedStokes' law to remove particulate matter larger than 40 micron in size, or to detain water to reduce downstream flooding. However, regulation on theeffluent from SWMF's is becoming more stringent, as the detrimental impact from nutrients likePhosphorus either dissolved from (fertilizers), or bound to sediment particles from construction or agriculture runoff, cause algae and toxiccyanobacteria (akaBlue-green algae) blooms in receiving lakes.Cyanotoxin is of particular concern as many drinking water treatment plants can not effectively remove this toxin. In a recent municipal stormwater treatment study,[5] an advanced sedimentation technology was used passively in large diameter stormwater mains upstream of SWMF's to remove an average of 90% of TSS and phosphorus during a near 50 year rain event.
Allstates in the U.S. have laws requiring installation of erosion and sediment controls (ESCs) on construction sites of a specified size. Federal regulations require ESCs on sites 1 acre (0.40 ha) and larger. Smaller sites which are part of a common plan of development (e.g. a residentialsubdivision) are also required to have ESCs.[6] In some states, non-contiguous sites under 1-acre (4,000 m2) are also required to have ESCs. For example, the State ofMaryland requires ESCs on sites of 5,000 sq ft (460 m2) or more.[7] The sediment controls must be installed before the beginning of land disturbance (i.e. land clearing, grubbing andgrading) and must be maintained during the entire disturbance phase of construction. Approval for use of any chemical flocculant must be obtained prior to its deployment.
