Performance of geosynthetic filters in treatment of urban storm water runoff

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Current urban infrastructure is highly reliant on impervious surfaces, including roadways, parking lots, and building rooftops. Rainfall that strikes these surfaces cannot infiltrate into the soil and subsurface and rapidly becomes surface runoff. They mobilize and transport particulate matter and other pollutants. Storm water control measures (SCMs) currently in use for storm water treatment are retention ponds, detention basins, wetland ponds, and grass swales, which requires large land area.

Sand filters are common subsurface storm water runoff treatment systems used in urban areas. Sand filters clog, all or a portion of the sand must be replaced to ensure adequate drainage through the treatment system. A geotextile was hypothesized to be as effective as a sand filter at capturing suspended solids in storm water runoff while maintaining adequate drainage during solids accumulation.

Geotextiles are any permeable, synthetic, textile material used with foundation, soil, rock, earth, or any other geotechnical engineering related as an integral part of a man made project , structure or system. The purpose of geotextiles are in separation, drainage, reinforcement and filtration. They generally fall in to two categories,

Woven and Non woven. Woven geotextiles consist of fibers or yarns of a polymer that are oriented in two perpendicular directions, one over other. Non woven geotextiles consist of discrete fibers which may be oriented or randomly distributed. Geotextiles are mainly differentiated by the Polymer type, Fiber type and Manufacturing process.

Polymer type

Geotextiles are generally made from synthetic fibers rather  than natural fibers. The synthetic materials or polymers are made in chemical processing plants from the polymerization of thermoplastics. Geotextiles are commonly made from the polymers of Polypropylene (PP), Polyester (PET), Polyamide (nylon) and Polyethylene(PE).

Fiber type

There are 4 main fiber types which are used to manufacture woven geotextiles are Monofilament fibers,  Multifilament fibers, Silt film fibers and Fibrillated fibers. Fiber types used to manufacture nonwoven geotextiles are Continuous filament fibers and Staple fibers.

Manufacturing process

Woven geotextiles - The weaving technique is performed in four steps are Shedding, Picking, Battening and taking up, Letting off.

Nonwoven geotextiles - Are generally made from a spun bonding process. There are 4 major steps in the spun bonding process. That are fiber preparation, web formation, web bonding and winding into rolls. The fibers may be bounded together by one of three techniques - Mechanical bonding, Thermal bonding and Chemical bonding.

Mechanical Bonding - Needle punching is the mechanical process used to bond nonwoven geotextiles. In this process the web is passed under a needle board, which is made up of thousands of barbed needles. The needle design, punch density and depth of punch are the variables which may affect the pore size distribution of a geotextile.

Thermal Bonding - It is a bonding process which melts the web together  at fiber cross over points. The web is passed through a source of heat, such as pressurized steam or hot air which causes fusion at fiber cross over points. Strong, flexible bonds may be formed at cross-over points.

Chemical Bonding - A chemical binder, such as an acrylic resin, may be applied by total immersion or by spraying. After the binder is applied, the web is passed through an oven or hot rollers to cure the chemical binder. Another chemical bonding technique uses hydrogen chloride gas.

Filtering Mechanism

A filter should prevent excessive migration of soil particles, while at the same time allowing liquid to flow freely through the filter layer. Filtration is therefore summarized by two seemingly conflicting requirements. A filter must retain soil, implying that the filter pore spaces or openings should be smaller than a specified maximum value. The filter must be permeable enough to allow a relatively free flow through it, implying that the size of filter pore spaces and number of openings should be larger than a specified minimum value. For better performance of filter, it should satisfy some criteria, that are Retention : Ensures that the geotextile openings are small enough to prevent excessive migration of soil particles. Anti clogging : Ensures that the geotextile has adequate openings, preventing trapped soil from clogging openings and affecting permeability. Permeability - Ensures that the geotextile is permeable enough to allow liquids to pass through without causing significant upstream pressure buildup. Survivability - Ensures that the geotextile is strong enough to resist damage during installation due to stress applied on it. Durability - Ensures that geotextile is resilient to adverse chemical, biological, and ultraviolet (UV) light exposure for the design life of the project.

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