Rainwater harvesting: Water quality

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The important water quality parameters for harvested rainwater differ from other types of LID. This is due to the potential for direct human contact, rather than environmental discharge. As of July 2017, the CSA and ICC are finalizing a standard[1] which specifies different water quality treatments according to the source (roof runoff versus stormwater runoff) and the intended use.

Tier 1 end uses are most readily achievable, requiring only that larger particles are filtered out of the water. Removing the bulk of the solid particles reduces the nutrient concentration in the water and prevents clogging of the water distribution system. Toilet and urinal flushing are the next most popular use of harvested rainwater. If flushing or other higher tier end uses are desired, disinfection of some type is required and consideration may be given to colour and odour of the water. Technologies for achieving higher standards of water quality include:

  • Ultraviolet (UV) disinfection requires additional filtration to remove particles so that the light can penetrate the water and destroy the viruses and bacteria,
  • Chlorine disinfection also requires additional filtration to remove larger particles,
  • Micro- or Ultra- filtration uses such fine membranes that the vast majority of harmful viruses, bacteria etc. are excluded from the water directly.

All three water purification technologies require specialist design and consultation with a subject matter expert is recommended for higher tiers of water use.

Water quality treatment required for multi-residential and commercial applications (Abridged from CSA B805 draft)
Application Roof runoff pathogen reduction Stormwater pathogen reduction
End Use Tier Example uses Viruses Bacteria Protazoa Viruses Bacteria Protazoa
  • Sub-surface irrigation (drip/bubbler)
  • Spray irrigation (restricted access)
  • Fire suppression
  • Ice rinks
0 0 0 0 0 0
  • Toilet and urinal flushing
  • Clothes Washing
  • Rooftop cooling
0 99% 99% 99.99% 99.99% 99.9%
HVAC systems In accordance with ASHRAE 188
  • Hose bibbs
  • Pressure washing
  • Vehicle washing
  • Spray irrigation (non-restricted)
  • Decorative fountains
99.9% 99.9% 99.9% 99.99% 99.99% 99.99%
  • Human consumption
  • Oral care
  • Food preparation
  • Dish-washing
  • Bathing, showering, and hand washing
  • Pool/hot tubs/spas/splash pads
  • Misting stations
  • Swamp coolers
0 99.999% 99.999% Outside of the scope of the CSA standard

Source Water Quality[edit]

A study of many types of roof surfaces in Texas found:

  • 'Cool' membrane, concrete tile, and metal roofs all produced water of similar good quality for non-potable use,
  • The runoff water from asphalt shingle and green roofs contained significantly more dissolved organic carbon (DOC). The DOC can add a yellow colour to the water. If the water is treated with chlorine, for drinking purposes, DOC can produce toxic compounds[1].

Research in Hamilton, ON assessed the water quality of rain collected from three highly reflective 'cool roof' membranes[2]. Key findings:

  • The water was free from significant contamination with by-products of plastic manufacture and did not show elevated levels of the five metals tested.
  • Increased microbiological contamination was found in runoff from roof areas where ponding occurred.

Note: Increased microbiological contamination in roof runoff is also associated with warmer weather [3].

  1. Mendez CB, Klenzendorf JB, Afshar BR, et al. The effect of roofing material on the quality of harvested rainwater. Water Res. 2011;45(5):2049-2059. doi:10.1016/j.watres.2010.12.015.
  2. Cupido, A., B. Baetz, Y. Guo, and A. Robertson. 2012. An evaluation of rainwater runoff quality from selected white roof membranes. doi: 10.2166/wqrjc.2012.011.
  3. Vialle C, Sablayrolles C, Lovera M, Jacob S, Huau MC, Montrejaud-Vignoles M. Monitoring of water quality from roof runoff: Interpretation using multivariate analysis. Water Res. 2011;45(12):3765-3775. doi:10.1016/j.watres.2011.04.029.