Lesson 18:

Dry Feeders


Up to this point, we have been concerned only with solution feeders.  However, dry feeders are commonly used to feed solid fluoride chemicals in large plants.  The most common fluoride chemical fed using a dry feeder is sodium fluorosilicate. 

Like solution feeders, dry feeders are used for many purposes besides fluoridation in the water treatment plant.  In all cases, the dry feeder measures a dry chemical and mixes it with water in a solution tank.  Then the resulting solution is delivered into the main flow of water using a pump or injector. 

There are two types of dry feeders - volumetric feeders and gravimetric feeders.  We will discuss each type below.



Volumetric Feeders

Volumetric feeders feed a measured volume of dry chemical within a given time interval.  The picture below shows one type of volumetric feeder, in which the chemical begins in the hopper and is measured by a rotating screw.  The dry chemical is then dropped into a solution tank where it is mixed with water to produce a fluoride solution.  The solution can then be pumped to the water supply.  

volumetric feeder.

Volumetric feeders can also measure chemicals using rotating disks, oscillating pans, vibratory pans, rotating rollers, or star wheels. 



Gravimetric Feeders

Gravimetric feeders measure a certain weight (rather than volume) of chemical within a given time period.  These feeders are more expensive, more complicated, and require more space than volumetric feeders, but they also feed larger amounts of chemicals and are more accurate.  A typical gravimetric feeder is shown below.

Gravimetric feeder.



As mentioned previously, excess fluoride concentrations in water can cause illness.  The operator should monitor the fluoride concentration at least once per day to prevent overfeeding.  Underfeeding is less of a problem than overfeeding and fluoridation equipment can be shut down for cleaning and maintenance for short time periods without shutting down the entire water treatment plant. 

In addition to monitoring the fluoride concentration manually, a type of flow meter called a pacing meter can be be used to match the feed rate of fluoride to the flow rate of the water being treated.  Pacing meters measure the total water flow rate and then produce a signal which allows for automatic adjustment of the fluoride feed rate.  This type of meter is only required in systems in which the flow rate is very variable.  Even in systems containing pacing meters, the fluoride concentration in the water should be tested daily by the operator. 

A second type of flow meter is also used in fluoride systems.  This is a simpler water meter used to merely measure the flow rate of water within the plant.  This type of meter is not connected to the chemical feeder and is used by the operator to determine what flow rates the feeder must accommodate. 




If daily monitoring shows a change in the fluoride concentration of treated water, the operator will need to be able to determine and correct the problem.  A variety of factors can influence fluoride concentrations, and some of the most frequent problems will be discussed below. 

The most common cause of low or erratic fluoride concentration is improper equipment maintenance or operation.  The following factors may cause low fluoride readings in treated water:

  • Undersized solution tanks, detention time of less than five minutes, inadequate solution water, and inadequate mixing in dry feeders.
  • Inadequate chemical depth in saturators.
  • Fluoride feed ahead of the filters.
  • Unfluoridated water mixing with treated water in the distribution system.
  • Low chemical purity. 

In contrast, high fluoride readings are often the result of improper testing procedures.  If the high fluoride readings are accurate, they may result from not taking the initial fluoride content of the water into account when calculating optimal fluoride dosage.

The water quality can also influence fluoridation to some extent.  When water with a hardness greater than 10 ppm is fluoridated using a saturator, scaling can form on the saturator walls.  The scaling consists of low solubility calcium and magnesium fluoride compounds.  Scaling can be prevented by water softening upstream. 




Fluoride is added to drinking water to prevent dental decay.  Several chemicals can be used in fluoridation, but the most common are hydrofluorosilicic acid, sodium silicofluoride, and sodium fluoride.  The optimal concentration of fluoride in drinking water is approximately 1 ppm, with the exact amount depending on a region's average daily temperature.  Fluoride concentration in water must be monitored daily.  When handling fluoridation chemicals, operators must follow appropriate safety precautions.



New Formulas Used






Complete the assignment for this lesson. You must be logged into Canvas to submit this assignment. Make sure you choose the appropriate semester.




Read through the Fluoride lab to see the steps involved for testing concentration.





Answer the questions in the lesson quiz. You must be logged into Canvas to take this quiz. You may take the quiz up to three times; an average will be taken for final grade calculation. Make sure you choose the appropriate semester.