Two Letters Re: Building a Backyard Water Treatment Plant, by J.S.M.

Dear Sir:

Regarding “Building a Backyard Water Treatment Plant, by J.S.M.“, I wanted to comment on a couple of errors I saw in the last section on treating surface water with alum. I once worked for a manufacturer of aluminum sulfate and was involved in several trials at municipal water systems.

  1. First thing, I would not bother waiting a day for an initial settlement. Once the leaves, twigs, and tadpoles have been removed, you should go ahead and treat the water. It seems paradoxical, but the cleaner the water is when you start, the harder it is to get good flocculation and settlement.
  2. I double checked the tuna can measure and was surprised to find that a 5 oz. tuna can does contain about 5 oz. of dry alum. This reference shows that the average density of dry alum ranges from a low of 38 – 45 lb/ft3 for powdered alum to 63 – 71 lb/ft3 for ground. Five ounce tuna cans are about 3 ¼” in diameter and 1 ¼” tall, so the volume of one is 3.14 ÷ 4 * ( 3.25in )2 * 1.25 in ÷ ( 1728 in3/ft3 ) = 0.005998 or about 0.006 ft3. If you know whether you have powdered or ground you should use the correct value. Using an average of both of gives you about 55 lb/ ft3. Using this value and multiplying by the tuna can volume gives 55 lb/ ft3 * 0.006 ft3 * ( 16 oz./ lb ) = 5.28 or about 5 oz.
  3. The fourth step should be amended to say that the alum is mixed with clean water and is shaken until all of the alum is dissolved. This is your working solution.
  4. The fifth step in the procedure contains several errors.
    1. The original post says nothing about mixing the 5 gallon bucket. For best results, the bucket should be mixed vigorously for about a minute after adding the alum and mixed gently after that, if it is mixed at all. Vigorous mixing helps contact of the alum with the suspended solids in the water. Very gentle mixing can help the small floc particles (pin floc) that are initially formed bump into one another and grow into larger particles. These larger floc particles settle faster.
    2. Adding all of the 5 oz. of alum to the 5 ½ gallons of water will grossly overdose the solution. Here’s the calculation: 5 oz. alum ÷ ( 16 oz/lb ) ÷ ( 5 gallons water to be treated + 0.5 gallons of water the was alum mixed in ) ÷ 62.4 lb of water/gallon * 1,000,000 ppm/ lb/lb = 911 ppm. I remember that 10 – 20 ppm is a more typical dose. About 1 tablespoon of the working solution would be a dosage of 8 ppm. That calculation is 5 oz. alum ÷ ( 16 oz./lb ) ÷ ( 128 tablespoons per ½ gallon of water ) ÷ ( 5 gallons water to be treated and ignore the water the alum mixed in because it’s small ) ÷ 62.4 lb of water/gallon * 1,000,000 ppm/ lb/lb = 7.83 or about 8 ppm. All surface water is different. If I was doing this myself, I’d add a tablespoon at a time, mix, and then inspect for pin floc. If I didn’t see any, I’d repeat.
    3. Using alum to flocculate water works best when the pH of the water after adding the alum is maintained at 6 – 7. Adding all 5 oz. of alum will drive the pH too far below the optimum pH. Even adding normal amounts can decrease the efficiency of the alum. For best results, pH should be adjusted with lime after addition of alum.
    4. It shouldn’t take a day to flocculate the dissolved solids in the water. You should be ready to move to the next step in a couple of hours.
    5. Don’t wash out the container with the alum solution. That’s your working solution and will be enough alum to treat up to 500 gallons of water.
  5. To transfer the clean water after flocculation and sedimentation, I’d recommend siphoning into another bucket instead of pouring. Pouring is ok if you have to, but the bucket will slosh, and you could get some of that sediment into your clean bucket.

– S.M.