Fluoride Dosing

Background

Water fluoridation is the process of adjusting the fluoride content in drinking water. Fluoridation is conducted at the majority of water treatment plants to achieve levels of 0.7 milligrams per Liter (mg/L) Fluoride is added to drinking water supplies at the last stages of the water treatment process. A chemical dosing system is utilized to carefully control and monitor the fluoride addition and ensure the rate is appropriate for the current water throughput in the plant. One of the common chemicals that is used in fluoride dosing is hydrofluorosilicic acid (H2SiF6). Hydrofluorosilicic acid is supplied to the treatment plant in liquid form and stored in special tanks. Acid is drawn from the vessel and fed through plastic piping to a chemical dosing system and then added to the water supply.

Challenges

Hydrofluorosilicic acid is a dangerous and highly corrosive chemical. Since the required dosing rates are low, small-bore flowmeters are required. Chemical dosing systems often do not utilize flowmeters with the correct wetted parts. That leads to premature failure and high replacement costs. Since the chemical will also crystalize if water is present, flowmeters with diagnostics to detect adhesion provide added benefits.
 

Challenges	Risk
Dangerous	Hydrofluorosilicic acid is dangerous to plant personnel who require special protective gear.
Highly Corrosive	Hydrofluorosilicic acid is highly corrosive and requires special materials for chemical compatibility.
Crystallization	Presence of water (from flushing, etc.) leads to crystallization, coating pipes and flow meters electrodes and earthing electrodes.
Low Dosing rates	Very small flows are required even at larger plants to meet regulated fluoride dosing levels.
Minimizing Costs	Poor flow control leads to excessive chemical use.
Compliance	Dosing rates must be tightly controlled to ensure fluoride dosing meets government-regulated levels.
Sustainable use of Chemicals	Lost opportunity.

Solution

Yokogawa ADMAG AXG Total Insight magnetic flowmeters offer many features that are well-suited to the application:

Highly Accurate

• No risk of overdosing
• Reduction in chemical use and costs

Small Bore Available

• Flowmeters of 2.5mm and 5mm bore are available for very low flow measurement.

Robust and Reliable Dosing

• Dual frequency excitation provides stable flow measurement, even at low and pulsating flows.
• The metal punch plate reinforced PFA liner provides superior reliability.

Corrosion Resistance

• Platinum-Iridium electrodes and earthing are available for superior corrosion resistance and long service life
• PFA liner provides superior chemical compatibility

Adhesion Diagnostics

• The AXG meter offers adhesion diagnostics as standard to provide a pre-warning that crystals are forming and maintenance is required.

Built-in Verification

• Minimize and simplify maintenance with built-in meter verification for confidence in flow measurements.