Fluoridation - Solution Feeders

Fluoridation - Solution Feeders

Feed Equipment

Equipment used to feed fluoride into water comes in two categories - solution feeders and dry feeders. In both cases, the chemicals are added to water in a liquid form. The distinction is whether the chemical is measured as a liquid (in solution feeders) or as a solid (in dry feeders.) In general, solution feeders are more expensive than dry feeders and are used in smaller systems. Dry feeders are used in large treatment plants.

Acid Feed System

Despite the name, solution feeders can be used for either liquid or dry chemicals. If the fluoride source is a liquid, then the fluoride can be fed directly into the water main using an acid feed system. If the fluoride source is a solid, a saturated solution can be produced using a saturator and the solution can then be fed into the water main. In either case, positive displacement pumps are used to pump the appropriate quantity of the fluoride solution into the water being treated.

In this section, we will be concerned with acid feed systems, which are usually used for pumping hydrofluosilicic acid into water. The setup is known as an acid feed system since hydrofluosilicic acid is pumped into the water in the same state in which it was delivered.

The hydrofluosilicic acid begins in the shipping container, which is placed on a scales to measure the remaining liquid in the container (just as was done for chlorine cylinders.) A pump pulls acid from the shipping container into a physical break box, the purpose of which is to prevent a major overfeed of acid into the main flow of water. From the break box, the fluoride flows to the other side of the pump and is pumped to the water line.

Positive Displacement Pumps

In the acid feed setup shown in the previous section, the amount of fluoride solution added to water is measured by a pump. Positive displacement pumps are used in solution feeders to feed a measured volume of a liquid chemical during a specific time period. They have several advantages over other types of pumps, being accurate and capable of feeding a solution against pressure into a pipe or tank of water as well as into open tanks of water.

All pumps move liquids, thus creating a flow from one side of the pump to the other. Positive displacement pumps work by delivering a constant volume of solution with each stroke or pulse, so volumes of the solution are discharged at intervals. This differs from non-positive displacement pumps which create constant flows of solution which are more difficult to measure.

Positive displacement pumps are used for many purposes in the water treatment plant in addition to fluoridation. The only specific requirement for fluoride pumps is that the pumps deliver an accurate quantity of the fluoride at a constant rate.

The operation of a positive displacement pump is shown below. First, the solution enters a chamber in the pump. Since the chamber has a specific volume, it measures a set volume of solution.

Once the chamber has been filled, the inlet valve is closed and the outlet valve is opened. The solution is pushed out of the chamber by a piston, shown in pink in the picture below.

The volume of solution fed by the pump is determined by the piston stroke length and by the stroke frequency. As you can see in the pictures above, the length of the piston stroke determines the volume of solution which is fed with each pulse. If the piston is drawn back a shorter distance, the chamber is smaller and fills with less solution. If the piston is drawn back further, then the chamber is larger and fills with more solution. The stroke frequency also influences the amount of solution fed since faster strokes will feed more solution.

Some types of positive displacement pumps may use diaphragms, gears, or screws instead of pistons. Volume of solution fed can be monitored in these types of pumps in much the same way as for a piston pump.

Break Boxes

Feeding a vastly excess amount of fluoride into water can cause illness in the customers. Break boxes are often used to prevent overfeeding. A diagram of a break box is shown below:

During normal flow conditions, the fluoride solution flows from the shipping container to the break box, entering through the inlet at the top left on the diagram above. The solution then flows out of the container through the outlet and to the water being treated.

If there is a malfunction in the pump which results in an excess amount of fluoride solution being fed, then the break box operates as shown below.

Some solution continues to flow out to the water being treated. However, once the solution level in the break box rises above the normal full level, the solution can enter the overflow line. The excess solution flows out of the break box through the overflow line and back to the shipping container.

Saturators

As mentioned above, solution feeders can be used to feed dry fluoride chemicals if a saturator is part of the setup. A saturator is a device used in fluoride feed systems which produces a fluoride solution. The saturator dissolves dry fluoride chemicals so that they may be fed into the water line using a solution feeder.

Saturators can either be upflow, with water flowing from the bottom to the top of the saturator, or downflow, in which water flows down through the saturator. The upflow saturator is the preferred type because it is easier to clean and maintain, so we will consider an upflow saturator here.

The diagram above shows a typical upflow saturator. Some form of dry fluoride is placed in the bottom of the saturator. The dry fluoride is most typically sodium fluoride, but can also be sodium silicofluoride.

Once the fluoride is in place, water is pumped down to the bottom of the saturator and is then spread throughout the saturator bottom using a distributor. The water flows up through the sodium fluoride, dissolving some of the fluoride and producing a fluoride solution.

A pump intake floats at the top of the fluoride solution. The pump pulls the solution out of the saturator and pumps it to the water main just as it would in an acid feed system.

Saturators are often used to dissolve dry fluoride chemicals because no measuring is required. Whole bags of sodium fluoride can be dropped into the saturator. The saturator always produces a 4% solution of sodium fluoride (or a 0.76% solution of sodium silicofluoride) due to the solubility of the dry fluoride chemical. The saturator completely saturates the incoming water with the fluoride chemical.

Backflow

Backflow is the reverse flow of water in a system. Backflow problems can occur at several locations in fluoridation systems, especially around saturators, so control devices are added to prevent the reverse flow. You can read about some of the techniques used to prevent backflow in ENV 110 Lesson 10. The simplest way to prevent backflow is by using an air gap, which is a vertical separation between the inlet line and the outlet line. However, in some cases vacuum breakers are required.