Concrete Washout Pits & You

Such a common part of many construction sites that the inclusion of a concrete washout pit (CWOP) on a Stormwater Pollution Prevention Plan (SWP3) is second nature. Concrete washout is non-sediment pollutant that must be prevented from discharging off-site. It has an extremely high pH and contains other admixture chemicals. It is the responsibility of the permittee to ensure that proper washout is occuring. The design engineer can get things started by thinking about the most appropriate placement.

Concrete wash water is considered a process water and is not authorized for discharge under the General Storm Water Permit associated with Construction Activities. Waste water must be collected and properly disposed of at an approved disposal facility. Listed below are some key items related to concrete washout pits.

  • pH - Concrete washout water is a slurry containing toxic metals, is caustic and corrosive, and has a pH near 12. The safe pH ranges for aquatic life habitats are 6.5 – 9 for freshwater and 6.5 – 8.5 for saltwater.
  • Hydrologic Flow Line - When concrete washout enters storm water infrastructure, it may disturb or inhibit water flow. This may cause the storm water infrastructure to not work as designed.
  • Placement of Washout Pits - Even though the structure is included in the SWP3, thus meeting a requirement; well thought out placement is often neglected. Here are some tips to consider:
    • Is the CWOP in an easily accessible area?
    • Is the CWOP in an area with enough space for a delivery driver to back-up?
    • Can the delivery driver swing the trucks chute out over the CWOP?
    • Is the CWOP out of the way of other site traffic or vehicles entering the site?
    • Is the CWOP away from storm drain catch basins or other water courses?
    • Is the CWOP greater than 50 feet of storm drain inlets, open ditches or water bodies.
  • Inspecting Washout Facilities - Washout facilites should be inspected daily and after heavy rain events to check for leaks and damage to the lining.
  • Wash water treatment - Wash water can either be recycled or treated (see below). Coarse aggregate, sands, and fines should be properly filtered. Remaining water should be reduced in pH. After pretreatment is completed on site or by a delivery service, it can be delivered to a publicly owned treatment works for further treatment.
  • Concrete Washout Container - A portable washout container is a portable, reusable way to remove excess concrete from trucks or construction equipment. A vinyl pit is easier to set up compared to a hay bale washout pit and comes with a biodegradable filter bag to easily remove solids. Cardboard washout containers are easy to set up and come in a range of sizes.
  • After solids are removed, wash water can be allowed to evaporate and the container can be reused. Alternatively for quicker removal, wash water can be removed using a separate filter bag and a gelling agent. The gelled wash water and filter bag can be removed and disposed of together after about five minutes of the gelling agent application. More information on concrete washout pits can be found here.

Blog Authors: Brian White, Urban Conservationist and Adam Delaney, Natural Resource Intern

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