Ask the Expert
Excessive fluoride and rotting teeth
Question: I belong to a nonprofit charity organization that is now focusing its efforts on East Asia. In Vietnam, currently, there is a village composed of people whose teeth have literally all rotted away. The reason for this is an enormous amount of excess fluoride in their drinking water. We’ve thought about buying tubs to collect rain, but then the people would only have water when it rained. So now I’m wondering if you could help me. What is the most effective and most inexpensive way of neutralizing (getting rid of) the fluoride in said drinking water?
Answer: The fluoride ion is a real big problem in many areas around the world, even in the United States. The coastal areas are particularly at risk and you see this in the Carolinas on the Atlantic Coast. Treatment technologies include RO, ion exchange and activated alumina. Typically, the most effective, competitive method is activated alumina (AA) if other contaminants are not a problem. It’s regenerated with sodium hydroxide (NaOH) and hydrochloric acid (HCl). These chemicals in the regeneration effluent need to be neutralized before discharge and care must be taken when handling concentrated chemicals and the regeneration effluent. It sounds as if the fluoride (F–) content is extremely high. This can be knocked down to the 8-to-10 parts per million (ppm) range by pre-treating with lime (CaO). It forms an insoluble CaF2, which precipitates out. They would then have to clarify (filter) it and then use the AA, which is available in filters with automatic regeneration capabilities. A 10 gallon per minute (gpm) plant would cost about US$35,000 and would treat 15,000 gallons per day (gpd). Smaller plants could be built as well as manually regenerated ones for less money. Bone char is another common treatment for this. It also has to be regenerated (similar to AA). In the United States, AA may be the most common method. Reverse osmosis (RO) also works but can be more expensive, in some cases, depending on pretreatment requirements; however, it will need less operator attention and maintenance once properly installed. In this case I would ask for a complete water analysis and assume you’ll find high TDS or chlorides also above World Health Organization (WHO) or U.S. Environmental Protection Agency (USEPA) safe drinking water levels. Then RO would be more than justified.
Cleaning water in NYC
Question: I’m a high school student enrolled at School of the Future in New York City. I have been recently given a chemistry assignment by my chemistry instructor and, frankly, it’s quite taxing. Rancid and foul water was composed for my chemistry class in order for us to make the water pure once more. This foul water is composed of oil, coffee, garlic and various other ingredients (which are non-significant in the composition of the solution) that I have yet to determine. I have a simple request to ask of this website (www.wcp.net)—after all, it does seem insightful and knowledgeable about water purification. I would like for someone to explain the best possible way to separate the water from everything else mentioned earlier. I’ve even come close to using filters that one can buy at the grocery store, but they were restricted by my teacher as well as any other water purification mechanisms that can be bought. Any information would be graciously accepted and seriously considered.
New York, N.Y.
Answer: The question isn’t as simple as it seems. In order to design a process that would be commercially economical and efficient would require the use of several different technologies. As the student indicates, off-the-shelf processes are not acceptable; therefore, go with the basics. Distillation will provide good quality water from this specialized composite polluted water source. Boil the water and condense the vapor. If you’d like to run the finished product through a small amount of carbon—such as that you can find at a fish store for aquarium filters—to remove any remaining tastes or odors. I don’t think the teacher cares that the apparatus will foul and by itself isn’t a commercial option due to the cleaning requirements. However, it will give potable water in a pinch!
Iron and Saudi Arabia
Question: I’m a civil engineer working in the technical division of the local water authority. We face a problem in well water that produces a high dissolved iron concentration (2-2.5 ppm). We need to locate a suitable plant or system for removing dissolved iron to health limit (<0.3 ppm). Our parameters are:
Also, we need to ask about:
- Is an aeration tank enough for removing iron?
- What is the best catalyst for filtration?
- How can I calculate the backwash flow rate? And how can I recycle it again?
- Could you please give us some technical information for iron filters?
Your early response is highly appreciated.
Buraidah, Saudi Arabia
Answer: There are several factors that influence treatment choice. In the 900 TDS range, how many other oxidizable ions are present? How many cation exchangeable ions are present? Is water conservation a high priority?
Ion exchange is one approach. You can get relatively high flow rates through resin beds—they require low backwash rates, and are simple to operate. However, if you have high hardness, the hardness will compete with the iron for exchange sites causing frequent regeneration and some iron leakage to service.
Oxidation doesn’t remove iron but simply changes its state from dissolved to ferric. In oxidation by aeration, chlorination or ozonation, the application is normally followed by a 2-to-3 minutes detention time and filtration to remove ferric hydroxide—Fe(OH)3—precipitate. Filtration can range from conventional sand filters to oxidation media like birm or manganese greensand, a catalytic filter media that requires 8-to-10 gallons per minute per square foot (gpm/ft2) of filter area for backwash. Use of oxidative media provide a safeguard for under aeration or Cl2/O3 addition normally due to variable feed water quality. This filter media is often regenerated with a potassium permanganate feed. Service flow rates are 3-to-5 gpm/ft2 of bed (surface) area. A high quality backwashable sand filter should be sufficient, and I would use a flow rate of about 5 gpm/ft2 of bed area. Backwash rates vary based on filter media used and water temperature. At your assumed high water temperature (30+°C), 12-to-15 gpm per square foot is typical. Backwash water can be collected, allowed to settle and then refiltered with a bag-type filter unit to reclaim some of this valuable resource