Types of Discharge at Construction Site

Thomas Sim PEA2066

Q.
List down the different types of discharge from a construction site, and how to prevent them from polluting the environment.

A.
Purpose of drainage in a construction site is posted earlier here.

There are 10 types of discharge from a construction site listed below:

  1. Surface Run-Off
  2. Groundwater
  3. Boring and drilling water
  4. Wastewater from concrete batching and Precast concrete casting
  5. Wheel washing water
  6. Bentonite slurries
  7. Water for testing and sterilization of water retaining structures and water pipes
  8. Waste water from building construction
  9. Acid cleaning, etching and pickling waste water
  10. Waste water from site facilities

Steps to prevent pollution are:

1. Surface Run-Off

  • Well designed sand/silt removal facilities, sand traps/silt traps and sediment basins. Channels, Earth Bunds and Sand Bags barriers should guide the run-off to proper storm drains with filters/removal facilities. These facilities should be maintained and clear off blocks and avoid being clogged.
  • Reduce soil excavation work during rainy seasons/landas seasons.
  • Earth work final surfaces should be compacted to prevent erosion.
  • Minimize ingress of water into trenches, and avoid accumulation of water into puddles.
  • Stockpiles of construction materials should be covered from rain.
  • Manholes should be covered to prevent debris and construction materials being washed down into it.

2. Groundwater

  • Groundwater pumped out of wells, for the lowering of groundwater level in basement or foundation construction and groundwater seepage pumped out of tunnels or caverns during construction should be drained off into storm drain with sand/silt removal facilities.

3. Boring and drilling water

  • Water used in site drilling should be recirculated after sedimentation. It should later be discharged into the storm drain with sand/silt removal facilities.

4. Waste water from concrete batching and precast concrete casting

  • Waste water from washing of mixer trucks and drum mixers should best be recycled. Discharge waste water should be kept to a minimum.
  • As this waste water should not be mixed with the surface run-off because it is contaminated with concrete batching and casting. Hence, it should be treated to remove sand and silt, restoring its pH to 6-10 by a pH adjustment facility before being discharged to storm drain.

5. Wheel washing water

  • All vehicle and plant should be cleaned before leaving the construction site to prevent carrying the mud and debris into the public road.
  • The waste water should be collected from the washing bay as the vehicle is leaving the site. This waste water should be filtered from sand and silt before being discharged into the storm drain.

6. Bentonite slurries

  • Bentonite slurries used in diaphragm wall and bore-pile construction should be reconditioned and reused whenever possible.
  • If disposal is inevitable, it should be disposed of at a marine spoil grounds subject to proper licensing being issued.

7. Water used in testing or sterilization of water retaining structure and water pipes

  • Water used in the testing of leakages, or sterilization (added chlorine) should be reused. Surplus unpolluted water can be drain off to storm drains. Treatment for anti-chlorination should be referred to the relevant authorities.

8. Waste water from building construction

  • Before construction, all sewer and drainage should be sealed to prevent building debris, soil, sand etc. from entering public sewer and drainage.
  • Waste water generated from building activities, e.g. concreting, plastering, internal decoration, cleaning of works, or similar works should NOT be discharged into the storm drain.
  • If the waste water were to be discharged into the foul water system (foul sewers), proper treatment of removal of settleable solids, silt and sediments and pH restoration is necessary.

9. Acid cleaning, etching and pickling waste water

  • Acidic waste water should undergo pH adjustment to 6-10 before discharging into foul sewers.
  • If there is no foul sewer drainage system nearby, it should be tankered off site to the nearby sewer system for disposal.

10. Waste water from site facilities

  • Sewage from site toilet, kitchen and similar facilities should be discharged into a foul sewer/drainage system. If there is no sewer nearby, a septic tank or soakaway system be used. If the foul discharge is in large amount, a separate sewage treatment plant should be built for such considerations.
  • Oil spillage or leakage (e.g. from vehicles/plant) should be contained and cleaned up immediately. Waste oil should be collected for reuse, or disposed in accordance to the local regulations.

Ref:
Construction Site Drainage, Environmental protection department, UK HK, 1994. http://www.epd.gov.hk/epd/english/resources_pub/publications/files/pn94_1.pdf