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Finding a cheap way to disinfect water for social project

  1. Mar 4, 2016 #1
    Hello there,
    A couple of us have been tasked to come up with a way to produce drinkable water from a source which is turbid and I'm trying to figure out a way to do this. It has to be cheap and work well without needing too much knowledge to operate.

    My initial idea is to use a coagulant (polyferric sulfate), flocculant (polyaluminium chloride) and chlorine dioxide tablets together to reduce turbidity and disinfect. Is this a good combination and how effective will it be? Are there better coagulants and flocculants than the ones I chose?

    As for the amount of each, I looking at using 0.5mg/l of chlorine dioxide but I'm not sure about how much to use for the other two. The biggest issue with this is chlorate production but I was reading that iron based coagulants can reduce this and at the concentration of 0.5mg/l I was thinking that it won't be too much of an issue.

    I would also be really interested in any alternative ideas that you guys might have (I've ruled out using the sun but I am open to something that uses electricity), the above is an initial idea that seems to be quite cheap to implement and could be used to filter 1000 liters of water for less than $2.

    Advice will be very much appreciated.
    Thanks for reading.

    Wasn't really sure if I should post this thread in the Chemistry or Biology section, so I hope this is ok.
  2. jcsd
  3. Mar 4, 2016 #2
    How much water do you need? Does it need to be continuous for years, or just a few hours? What sort of budget?

    Also, I think civil engineers do this, so maybe engineering forums?
  4. Mar 5, 2016 #3
    Thanks for getting back to me.

    This will be done at a household level with a daily drinking water need of ~10 liters (depending on the amount of occupants) for the foreseeable future. As for the budget, the cheaper the better.

    I posted here because I wanted to get the chemistry right.
  5. Mar 5, 2016 #4
    I would think about starting with a sand filter. I would also consider a backwash system of some sort.

    If tubidity is your only concern that may do it.

    I seem to recall CaCO3 from my Environmental engineering course. First they drove the pH up, then down. This was supposed to clean out most chemical contaminant. I don't know if you need that.

    If you are in Flint, MI (or areas of the Former Soviet sphere) buy bottled water from out of the area. Lead is a problem today, but they've had heavy industry for decades. PCBs, etc. wouldn't surprise me. Not all of the possible bad stuff would show up in the standard tests. Lacking that a rain water system might be a solution.

    BTW, a reputable water purification system is likely cheaper than anything you can build yourself. They buy and manufacture in bulk, so unless you have access to cheap plumbing supplies they will come in with a competitive bid.
  6. Mar 5, 2016 #5
    This is for households in a small Indian village (Andhra Pradesh).

    I didn't really consider sand filters due to their maintenance and contamination issues.
  7. Mar 5, 2016 #6
    Andhra Pradesh is a big area with diverse geography. Is this a surface source?

    Sand filters need to be backwashed. That means periodically forcing the water through them backwards to dump the out the collected dirt. Other than that they work pretty well. Picking the right sand for them is difficult since size and density are critical, with the fine sand being the most dense so it settles to the bottom after backwash.

    The sand slowly builds a biofilter matrix that does a really good job cleaning water. As it gets clogged it does a better job, but filters more slowly (creating hydrostatic head). When the pressure builds too high, the system needs backwashing.

    So you will need two deep sand tanks so one can wash while the other flows (or live with some down time). You will need a high tank with clean backwash water. You will need a drain above the sand slurry for draining the backwash.

    Typically one starts with a settling pond. Then some sort of bio-reactor to eat biochemicals. Then a second pond with flocculant. Then a sand bed. Then chlorination. Finally the pipes.

    You need to be aware of how dangerous this can be. Contaminated potable water sources can kill thousands of people. Normally I would say to hire a reputable engineering firm, but if your alternate choice is to do nothing and watch people die, all I can say is to study hard and try really hard to not make mistakes.

    Perhaps you can visit a nearby water plant and talk to the operators to see what they are doing? Clean water has saved more lives than all the doctors in history, so it is worth doing. But doing it wrong will cost some of the lives you were trying to save.
  8. Mar 6, 2016 #7
    Yes, some of them get the water from a surface source but there are others who get their water from a borehole hand pump but the water isn't clear from either source and we haven't run any tests for bacteria and viruses.

    The issue that someone who has worked with this village and a number of others mentioned to us was that if a method needed you to maintain or clean a certain aspect of the system, they end up not bothering to do it and simply go back to doing what they're used to doing.

    You mention about the danger of contaminated water. Is there anything dangerous about the method in my fist post? As long as the people are told the amount they should use and the time they should give for the water to disinfect everything would be under safe levels right?
  9. Mar 6, 2016 #8
    I would use a solar/wind powered pump from the well into a cistern. Have the cistern overflow in a way that's easy for the villagers to collect, then drain back into the ground avoiding wasted water. Well water is usually pretty safe, and turbidity is not usually dangerous. You probably won't need much flow, just a trickle.

    Adding chemicals is a problem. It's easy to screw up the dosage, so yes it's dangerous. (John adds a dose, but Mary doesn't realize he did, so adds a dose too, children play with chemicals, etc.)

    Besides, $2/1000 liters sound expensive. You should be able to set up the solar pump for well under $1000 (mostly the cistern cost) and have it last years. Spend a hundred dollars more for a larger solar panel and there will be electricity to charge cell phones and perhaps run a small cooler for drugs the pharmacist might need for drugs.

    Do consider getting the well water tested though. Mostly the earth cleans the water as it passes, but sometimes not so much.

    Untreated surface sources are a bigger problem and getting people off them is a good thing.
  10. Mar 7, 2016 #9
    I don't know where OP got $2 per 1000 liters (maybe locally sourced product) but if you can import them in bulk you could potentially get it down to $0.50 per 1000 liters.

    As for double dosing ClO2 what I would do is use an effective dose that's on the lower end like 0.25-0.30mg per liter and then even if someone doses twice or they put a 10 liter dose into 7 liters of water it won't be a huge issue. As for children playing with chemicals the family should me mentioned about the danger of ingesting the chemical. The same can be said for other things that can be found around a house.

    As for the question about mixing the floc, coagulant and ClO2 together in one go. I'm not really sure about this but I would think that the initial two will create a sediment at the bottom while the latter will turn into gas and clean the water.
  11. Mar 7, 2016 #10


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    Arsenic might be a problem as well.
  12. Mar 7, 2016 #11
    If the choice is a surface source with the chemicals or not, go with the chemicals. But if the choice is a proved clean well, I would skip the chemicals. Turbidity is not dangerous by itself. Besides, a large, flat cistern will provide some settling by itself. (Settling is proportional to the surface area, not the volume, BTW.)

    Each year about 300,000 American children are poisoned (~900 deaths). Cleaning products are the number two cause (after cosmetics). So it's a small risk compared to drinking contaminated water, but it's still not worth keeping the chemicals around in a village with otherwise clean water, IMO.

    It might be worth it if the chemicals were locked up, but that means buying somewhere safe to keep them and developing strong procedures to keep them safe.
  13. Mar 7, 2016 #12
    Let me add that spending the money for professional level clean water is a good investment. The community will be healthier. Thus they will earn more income and spend less on doctors.
  14. Mar 8, 2016 #13


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  15. Mar 8, 2016 #14
    One of the reasons that I chose polyferric sulfate was because there is research to show that it removes arsenic as well as other metals such as lead.

    I'm not sure if it'll remove fluoride.
  16. Mar 8, 2016 #15
    Fluoride is a water additive in much of the U.S. Unless present in large quantities I wouldn't worry about it.

    Arsenic and lead are serious problems. But they are only problems if they are in the water. They mostly are not. Still, that's one reason to get the water tested before deciding on a plan.

    BTW, if you do have lead or arsenic, I wouldn't rely on iron(III) oxide to remove it. At that point professional help is a must.

    There are online mail in water test kits, even in India. Find one and use it.
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