National Pollutant Discharge Elimination System/State Disposal System permit guidance on dewatering and basin draining
This document is meant to provide guidance on dewatering and basin draining practices of construction sites that are compliant with the Minnesota National Pollutant Discharge Elimination System/State Disposal System (NPDES/SDS) construction stormwater permit (Permit).
Dewatering is the removal of surface or ground water to dry and/or solidify a construction site to enable construction activity. Examples of dewatering include pumped discharges, trenches, and ditch cuts that are used for drainage.
The Minnesota NPDES/SDS Permit regulates the discharge of stormwater. The Permit identifies certain requirements for dewatering and basin draining to effectively prevent and/or control erosion and sediment problems on a construction site. Turbid, or sediment-laden, water that is removed during construction must be released to an onsite sedimentation basin or to a system constructed with appropriate Best Management Practices (BMPs) before the water may be discharged to surface waters. The water must be properly treated or filtered to remove sediment and other contaminants prior to discharge. Dewatering activities must not result in adverse impacts such as nuisance conditions, erosion or scour, flooding, or inundation of wetlands and other surface waters. Energy dissipation BMPs, such as riprap, plastic sheeting, or sand bags, must be installed at the discharge points of the dewatering system for adequate protection from erosion and scouring. Dewatering and basin draining practices must be monitored while in operation, and must not cause nuisance conditions.
Sedimentation treatment chemicals such as flocculants and polymers may be used to treat water if the following requirements are met:
For filters with backwash systems, use these methods to manage backwash:
If the removed water contains oil or grease, an oil-water separator or suitable filtration device must be used to treat the water before its release. If other contaminants are present, they must be treated appropriately.
Often the individual who developed a construction site plan will have determined if the need to dewater the site is necessary before construction begins. However, site conditions discovered while excavating may present the need to implement a dewatering and basin draining practice, as well. Here are some things to consider should the need to dewater arise:
Knowing the extended forecast and what to expect given the time of year is very important when planning to conduct a dewatering and basin draining practice. Here are some things to consider:
The placement of a dewatering and basin draining practice on a construction site is very important in order to perform the practice in a compliant manor. Here are some things to consider when deciding where to construct an onsite temporary or permanent sedimentation basin or a system constructed with appropriate BMPs:
The following are suggestions for possible dewatering practices to utilize on the site:
A sediment trap is a temporary containment area that allows sediment in collected stormwater to settle out during infiltration or before the runoff is discharged through a stabilized spillway. Sediment traps are formed by excavation or construction of an embankment to detain sediment-laden runoff and allow sediment to settle out before discharging.
A wet sedimentation basin is a temporary or permanent basin with a controlled release structure formed by excavation or construction of an embankment to detain sediment-laden runoff and allow sediment to settle out before discharging. A wet sedimentation basin should always have a permanent pool of water. The removal efficiency of this practice can be improved if the outlet structure is designed to withdraw water from the surface with a floating head skimmer (floating weir).
In a dewatering tank, flow enters the BMP through the top of the tank and is discharged from the tank after adequate settling time. Weirs can be added to enhance settling.
A filter bag, also referred to as a dewatering bag, is a square or rectangular bag made of non-woven geotextile fabric that collects sand, silt, and fines. Water to be treated is pumped into one side of the bag and seeps through the bottom and sides of the bag. It is beneficial to direct the effluent to a pond or vegetated area for additional treatment. Alternatively, a secondary barrier, such as a rock filter bed or straw bale barrier, is placed beneath and beyond the edges of the bag to capture sediments that escape the bag.
Generally, sand filters are used as a polishing step or final treatment. They are often used after a significant amount of sediment and other pollutants are removed with other treatment technologies. Sand filters can be used as a standalone treatment or in conjunction with a filter bag or other BMP.
An Active Treatment System (ATS) uses a coagulant or flocculent for the treatment of water with a sedimentation basin for turbidity reduction. In addition, pH adjustment or bag or sand filters may be included. The Construction General Permit does not require the use of an ATS; however, for waters and sites where the reliability of the stormwater is of concern, these systems are recommended. In general, these systems are most useful for construction sites with: 1) severely impaired receiving waters, 2) sediment that is difficult to remove from suspension, and 3) sites with small disturbed areas. In each of these cases an ATS provides an extra degree of control over the quality of the stormwater leaving the construction site.