Wiki Page, Community Fluoridation: Module 4 - Water Fluoridation Equipment Part 3

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2017-06-27T12:46:38-06:00

Acid Feed Systems

*Note: HFS Acid and FSA are interchangeable*

Water Fluoridation 3D from Enos C Inniss on Vimeo.

HFS Acid is added with a metering pump. These systems are most cost effective for very large systems. When HFS is the additive of choice, the process can be quite simple. If the facility is sufficiently large, then delivery can be by tanker truck and the HFS storage tank, with a 3-month supply, is filled by the tanker truck. A transfer pump would refill a day tank with a maximum 3-day storage volume. The final step is a solution metering pump to deliver a precise quantity to the water flow. Each step along the way would have a backflow preventer and anti-siphon device to minimize accidents. Unless a leak occurs, there is very little operator exposure. The solution pump is the control point for a HFS system.

From the CDC:

Fluorosilicic Acid Systems

1. To reduce the hazard to the water plant operator, fluorosilicic acid (hydrofluosilicic acid) must not be diluted. Small metering pumps are available that will permit the use of fluorosilicic acid for water plants of any size.

2. No more than a 7-day supply of fluorosilicic acid should be connected at any time to the suction side of the chemical feed pump. All bulk storage tanks with more than a 7-day supply must have a day tank. A day tank should only contain a small amount of acid, usually a 1- or 2-day supply.

3. Day tanks or direct acid-feed carboys/drums should be located on scales; daily weights should be measured and recorded. Volumetric measurements, such as marking the side of the day tank, are not adequate for monitoring acid feed systems.

4. Carboys, day tanks, or inside bulk storage tanks containing fluorosilicic acid must be completely sealed and vented to the outside.

5. Fluorosilicic acid should be stored in bulk, if economically feasible.

6. Bulk storage tanks must be provided with secondary containment (i.e., berms) in accordance with state/local codes or ordinances.

Verify with manufacturers on the compatibility of the materials. Cross-linked polyethylene tanks are the best choice for FSA, but polypropylene, rubber-lined steel, or Kynar-lined steel tanks are acceptable. Rubber-lined tanks require inspection and repair of the rubber lining periodically. Fiberglass can be used with qualification in that the tanks need periodic inspection for integrity due to glass reinforcement being subject to attack.

Steel tanks should be avoided. The nickel in austenitic stainless, such as grade 316, will form a partial protective nickel-fluoride surface, but the resistant film will not be continuous, and welded surfaces will be attacked. Fiberglass tanks can be used, but should only be used at larger facilities that have the resources and knowledge for inspection and repair of damage to the lining as the hydrogen fluoride can attack the embedded glass fibers. If concrete is used as a base for the tank or as a chemical containment barrier, then it should be coated to resist minor exposure using an epoxy undercoat with a urethane topcoat. Alternately, a manhole rehabilitation spray-on polyurethane coating could be satisfactory.

Rubber-lined neoprene tanks were the original material of choice. Gutta percha and paraffin bottles were the original materials used for containment of FSA. Gutta percha, made from latex-bearing sap of Palaquium Gutta or Isonandra Gutta trees native to the Malayan archipelago, was one of the first natural plastics to be exploited by man. Paraffin bottles were used in laboratories for small quantities, since it’s molecular structure of carbon chains with 20 atoms is similar to the long-chained carbon-based Polyethylene (PE).

PVC is commonly used for piping and has good chemical compatibility.

Use fluoride-compatible materials such as polyvinyl chloride (PVC), chlorinated PVC (CPVC), and HDPE (High Density Polyethylene) in pipe manifolds. Be sure to include numerous shut-off valves and unions at key locations to facilitate pipe repairs. Use a color-coding system to clearly mark that the supply is a fluoride solution. A recommended painting scheme is light blue with red bands, with an arrow indicating the direction of flow and the word “Fluoride” on the pipe.

Day Tanks and Pumps

Now, it's time for a quiz! Click next at the bottom for Module 4 - Quiz 3.