Saturday, August 17, 2013

Help! My rainwater harvesting system (RWHS) is not working (well)!

Further to my earlier posts on rainwater harvesting (RWH), I thought I should also present some possible problems faced by the rainwater harvester lest the reader thinks that rainwater harvesting is a big bed of roses. Nevertheless, I stand by my proposition that RWH is the best source of domestic water in rural/developing countries as long as you are living in a sub-Saharan climate.

These pointers are based by my experiences gained from my RWH projects so they are by no means exhaustive. But then again, RWH is not rocket science. As long as you exercise some common sense, do some basic research and you are on your way. Teething problems? Of course you will have them but nothing even close to life-and-death or that cannot be solved with hard work and ingenuity. "Improvise, adapt, overcome" If this motto is good enough for the US Marines, it is good enough for us.

Back to my pointers.

  1. A first flush diverter (FFD), roof washer or whatever-you-call-it is mandatory. It is basically a device to divert, filter or remove the first batch of rainwater from your roof - also the dirtiest part of the rain. Some people advise the first 5/10/15/(put your favourite number here) minutes of rain to be removed. Anyway, we tested the water quality from our FFD vs. the rainwater collected throughout the entire duration of the rain and no surprise, the FFD turned up significantly dirtier for almost all the water quality (WQ) parameters.

    Moral of the story: don't skim on your FFD/roof washer. Remember the old adage - penny wise, pound foolish.
  2. We tested out about 14 WQ parameters e.g. pH, conductivity, turbidity, nitrate, chloride, coliform, E. coli, hardness, alkalinity on rainwater itself, FFD water, final treated rainwater and compare them against PUB WQ from their water treatment works (no longer available on PUB's website but you can refer to their drinking WQ report for 2012) The parameter for final treated water that half the time did not meet PUB's guidelines is pH. Acceptable range is 6.5 - 9.5 while we got an average of 6.6 with a few data points below 6.5.

    No big deal actually for drinking (no, we didn't drink the rainwater though) but a small consideration for washing of metal parts e.g. car. Corrosion could be enhanced by acidic waters.
  3. Like the FFD, a vital piece of equipment is the strainer at the top of the downpipe (joining the gutter to the tank). If you place your RWHS in a well vegetated area like we did (in a garden), you will get lots of leaves, twigs and unidentified stuff trapped there in rainy Singapore. Clean it regularly or you will find your system clogged up or worse, a hot bed for mosquito breeding.
  4. Treat your rainwater treatment train (filters, UV plus whatever purification device) like your car - check them, clean them and change them when necessary.

    Case in point. UV disinfection is simple to install and use. No chemicals are needed. Just plug into a power source and presto. Right? No really... you  have to check the UV lamp regularly for fouling and clean it if fouled. A fouled lamp does not disinfect well. Ultimately, the lamp will gradually lose its intensity and you have to change it... remember pennies and pounds.
Good luck, folks!

Figure: The PVC portion jutting down from the tee forms the FFD

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