Thursday, February 23, 2017

Revisiting rainwater harvesting in Singapore

Hi there,


I came across your blog when I was doing some research on rainwater harvesting systems (we are in the process of renovating an old inter-terrace house that that we just bought, and thought that this might be worth exploring).


I know that this is a question that you fielded a number of years ago (hopefully things have changed since then, as your answer then was that there wasn't much demand for this), but would you know what is available in the market in Singapore in terms of rainwater harvesting systems for domestic households (we are looking for a fairly straightforward above-ground system) and are there any that you recommend? 


Thank you,



Dear Felix,
Indeed, the rainwater harvesting (RWH) market appears to have developed in the past few years. I believe RWH is becoming more widely known and accepted in SG. That comes with a rise in demand for RWH and naturally, businesses will be built around servicing this demand.

I am actually happy to see this happening as it shows that people are becoming green not just in words but in action too. However, we also have to bear in mind that the current RWH solutions in the market are more geared towards a private house rather than a HDB flat. (On the other hand, RWH can be done at community level by e.g. HDB at Treelodge@Punggol in which the whole HDB estate is designed and built for RWH.)

Over the past years, I have come into contact with some of these businesses. A quick online search should yield the same results.
  1. Green Eco Harvesting
    A quick glance shows that it is offering tanks and filters for RWH
  2. Netatech
    It advertises construction of RWH system.
  3. Hydro
    It features tanks and filters for RWH.
As for your question, no, I have no preference for the above or any other vendors. But if you decide to go ahead to integrate a RWH system into your house, a few points to keep in mind...
  1. Does the vendor only sell you the tank & filter or does it include installation into your existing house structure?
  2. What kind of maintenance is needed for your RWH system? Does the vendor offer such a service or are you on your own after its installation? Obviously, the cost and frequency of maintenance should be taken in consideration when you make your decision to get a RWH system.
  3. Is your setup legal??? Legally speaking, the plans for your RWH setup have to be approved by PUB and NEA (if gutters are involved in RW collection). These plans are typically submitted by a QP/PE/architect to the authorities. Is the vendor doing these for you or are you expected to engage your own QP/PE/architect to do so? Finally, legal means additional costs so factor that in too!
Good luck!

Figure: using a zinc roof to collect rainwater (Water for Life, Hesperian Foundation)
Who says RWH has to be complicated? It has been and still is done simply and cheaply in developing countries. Mosquito breeding is a concern but not insurmountable with care and effort.

Monday, February 13, 2017

More questions on silver in water filters: Is it really safe?

Dear Chen Ko


Thank you for your reply and I just saw your timely blog post - thank you so much! And it is so comprehensive and helpful, as always.


I must say the facts on silver were not terribly assuring. I have a few quick follow up questions if you could help:


  1. I saw the info below from They mentioned that the silver content is 0.07%, but I don't know how that number sits with the 0.1ppm (parts per million) you mentioned. Pls advise?

Why do the ceramic elements contain silver?  Are they safe?

In a multilayer candle, the protective effect is even more critical because carbon is a natural growth medium for bacteria.  In carbon-only filter elements, which are commonly found, the carbon filter often ends up as being a bacteria repositary if not changed regularly.
The silver in the Doulton® ceramic elements is a specially formulated self-sterilizing (bacteriostatic) agent.  The result is that bacterial growth is prevented from occurring within the Sterasyl® ceramic element (which is possible with the other ceramic filter elements).  More importantly, bacterial "grow-through" is eliminated.  
The silver content is about 0.07%, which is well within the allowable levels recommended by the World Health Organization and the EPA.  Because of the silver, Sterasyl® filter elements do not require sterilization after cleaning.  Oligodynamic silver in Sterasyl® is permanently locked into the pore structure of the ceramic.

  1. Puzzled - why would the amount of leaching from silver in a filter increase over time? And how would changing the filter regularly help, since the silver in the new filter will also leach? 

3. I stumbled on the "perfect" solution for this silver issue. Doulton has an EWC pre-filter cartridge which is supposedly based on USA KDF system. I did a quick read on KDF and one highlight is that it inhibits bacterial growth without silver...! Having said that, I realised it is a pre-filter, meaning to say it will still go through the main ceramic filter with the silver. 😕 It sounded promising for awhile though. Any thoughts on this KDF business? Only downside is, I doubt it removes fluoride.


4. Using the Doulton filter (Sterasyl ceramic) as an example, how much water approximately would one need to drink to consume e.g. 10g of soluble silver (or maybe not even drink, if say you wash your hands with it)? 

5. Does silver pass out of the human body...? I'm sensing that it doesn't. 😐

Dear Paloma,

  1. The value of 0.1ppm as given in the previous post refers to concentration of dissolved silver in drinking water. I suspect the 0.07% quoted in the Arkwater webpage refers to the amount of silver in the filter element i.e. if the filter element weighs 100g, silver will take up 0.07g. Therefore, we are not comparing apples to apples and it becomes meaningless to compare 0.1ppm to 0.07%.
  2. Actually, I made the statement based on my experience in many applications, including outside the topic of water. Any material will wear and tear over time, both mechanically and chemically, the latter possibly due to attack by other chemicals e.g. chlorine which is a pretty aggressive compound.

    As the bonds within the material break down, anything held within e.g. silver will be released. As a material ages, more bonds will be broken, releasing more stuff.

    Yes, you are right, a new filter will leach silver but an old filter will leach even more.
  3. KDF (also known as redox) filters are designed to remove lead, chlorine, hydrogen sulphide and guess what, fluoride too. When combined with other filters e.g. activated carbon, they seem quite effective at removing a good variety of pollutants. I personally have not done any experiments on them but they look promising.
  4. Assuming the 0.07% does not apply here, if you base on a concentration of 0.1ppm silver in your drinking water, you have to drink for 70 years (2 litres per day) exclusively from that silver impregnated filter before you drop from argyria or silver poisoning.
  5. The WHO guidelines on drinking water listed silver retention in humans and animals to be 0-10% which is pretty low.
All the best in choosing your filter!