Wednesday, August 07, 2013

More recognition to grey water recycling!

Grey water (1, 2, 3) recycling has always been a less glamorous cousin to rainwater harvesting. After all, rainwater is clean water for drinking, bathing, cooking... all important processes for our health and well-being, while grey water is wastewater best kept out of sight and mind... as long as someone else is taking care of its disposal.

News update: Water is water is water (no typo here) by whatever name. The only difference lies in the "impurities" in the water, hence giving rise to black water (toilet bowl), grey water (sink, laundry, bath), yellow water (urine only), dark grey water (kitchen sink, diapers laundry). All water is intricately connected in the water cycle - both natural and human. In other words, rainwater or any other "clean" water can come from grey water. Therefore, it pays to conserve (read recycle) and handle your grey water properly.

It is therefore great news when I see that PUB has added a section on the guidelines of grey water recycling in Singapore.

1. Alternate sources of water
On this page, rainwater and grey water are officially recognised as alternative sources of water, not just at the national level or at the residential/individual level. I have written about rainwater harvesting in an earlier post.

2. The document itself

Some highlights
  1. It goes without saying - no potable use. But irrigation and general washing are also forbidden. Why then do you recycle grey water???

    Well.... it can only be used for toilet flushing and as cooling tower make-up water.

    Personally, depending on your treatment process, grey water can even be made potable. Even with simple treatment, its water quality can be made adequate for general washing and irrigation.
  2. As for rainwater harvesting, grey water recycling must follow PUB's standards for fittings and code of practice for water services, as well as NEA's standards for prevention of mosquito breeding.
  3. Raw grey water should not be stored for more than 24 hours. This IS reasonable as any time longer will lead to anaerobic conditions and foul odours.
  4. Treated grey water should not be retained for more than 72 hours. This DEPENDS on the type of treatment. If the final water quality is that of drinking water, I am sure the duration can be stretched.
  5. Incidentally, there is no mention of WHAT type of treatment process you should use... Instead, whatever treatment must produce a water conforming to the water quality guidelines (listing the various water quality parameter and their limits) given in this document. This is immensely useful as previously, there were no such guidelines.
  6. And also very useful is the schedule for water sampling and monitoring e.g. testing for E. coli and coliform should be done monthly.

 Figure: PUB guidelines for treated grey water quality

Figure: PUB sampling regime for treated grey water

Source of the above 2 figures: PUB document (

In conclusion, I believe we are on the right track on publishing such standards for grey water in terms of its water quality and testing schedule. I hope more developers and individuals will take up the challenge to implement grey water recycling on a residential or building scale.

Natually, there is much more to be done. E.g. I am impressed when habitats use plants not only to clean up grey water but also to provide edibles for their inhabitants. Bananas can be clearly seen in my previous posts (12) illustrating this concept in an Earthship.

Figure: Bananas, anyone? (Source:

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