Selected BMPs: Portable Sediment Tank

Definition: A portable sediment tank is a compartmentalizedcompartmentallizedcompartmented container through which sediment-laden water is pumped so sediment can be trapped and retained.

Purpose: Portable sediment tanks are used during construction to retain sand, silt, and fines in water thatduring construction to trap and retain sediment from water has been pumped from trench excavations, construction sumps, and other small projects.

before the water is pumped to drainageways, adjoining properties, and rights-of-way that are below the sediment tank site.

Application: SA sediment tanks areis most frequently to be used to treat sediment -laden water produced from small construction projects, including trenching operations and foundation excavations. They can also be used in lieu of sediment traps to protect small areas of disturbed land in urban settings. In this application, a construction sump is excavated and pumps used to transfer the collected runoff to a sediment tank beforeprior to discharging to a nearby storm sewer. Sediment tanks can also be used to manage slurry recovered from construction activities associated with water bodies (see SILT CURTAIN).. Before discharging water from a sediment tank to a natural water waterway, it should be filtered using a SILT FENCE, STRAW BALE BARRIER, FILTER STRIP, or similar measure.on sites where excavations are deep and space is limited, such as urban construction, where direct discharge of sediment-laden water to stream and storm drainage systems is to be avoided. A sediment tank also is used where an excavation extends below the seasonal high water table causing a sump pump to be used. Sediment tanks are generally not suitable where runoff from areas larger than one1  acre must be treated.

Recommended Design Criteria:

Requirements for Regulatory Compliance

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Performance-Based Guidelines

The Illinois Natural Resources Conservation Service recommends athat portable sediment tanks be constructed with three equal-sized, baffled compartments. The inlet and outlet pipe should be a minimum diameter of 3  inches. Illinois recommends providing storage for 4 hours of pumping (the volume in cubic feet can be estimated by multiplying the pump discharge rate, in gallons per minute (gpm),) by 32). The volume can be modified to meet specific site conditions.

Ideally, sediment tank dimensions should be tailored to site -specific conditions of sediment size and flow rate. Hydraulic loading rates (HLR), computed as the discharge rate divided by the vertical cross-section areas of a device, should be based on the settling rates for sediment particles. The settling velocity of sediment particles depends on the specific gravity and the diameter of the particles. The velocity, assuming particles are spherical, can be approximated by using Stokes Law. The time (in minutes) required for a particle to settle 1 foot under quiescent conditions at 68°F is:

t_s = 0.00935 / ((sg - 1) x D²)

where: sg = specific gravity of sediment

D = diameter of particle (in mm)

Operation and Maintenance: A portable sediment tank should be cleaned out when one-third of the storage volume is filled with sediment. All sediment collected in the tank must be disposed of in an approved sediment-trapping device, such as described in BMP descriptions for TEMPORARY SEDIMENT TRAP or SEDIMENT BASIN, or disposed of on the construction site or at an approved disposal facility. s approved by the engineer inspector.

Considerations: The sediment tank should be located where it will be easy to clean out and dispose of the trapped sediment and where interference with construction activities and pedestrian traffic is minimal.

Portable sediment tanks should be used only when other sediment trapping practices cannot be installed for reasons such as lack of space. Portable sediment tanks are not as efficient as other types of BMPstanks for removing sediment and the portable sediment tanks require a great deal of manual labor to operate and maintainremove the sediment.

References:

State of Illinois, Soil Conservation Service. Conservation Practice Standards. Portable Sediment Tank, Code 895.U.S. Department of Agriculture, Natural Resources (formerly Soil) Conservation Service. National Handbook of Conservation Practices (latest edition).

Specifications and Methodology: Plan drawings are not necessary for portable sediment tanks. Specifications for installing portable sediment tanks should show the required storage volume of the tank. Other container designs can be used if the storage volume is adequate and approval is obtained from the local regulatory agency.

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