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The Importance of Accurate and Reliable Dosing of Zinc Orthophosphate


Zinc Orthophosphate (also known as “ZOP”) is a commonly used corrosion inhibitor in municipal and well water treatment applications. This corrosion inhibitor is particularly important because it aids in prevention of lead leaching from the pipes into the water distribution network.

There are many different types and formulations of ZOP, depending on the ratios of Zinc to Phosphate. While many different formulations exist to suit various application needs, all ZOPs generally work in the same way.

Zinc Orthophosphate is composed of Zinc Sulfate, Phosphoric Acid, and other orthophosphate compounds.

ZOP should be paired with materials of construction such as PVC, Polypropylene, PVDF, Acrylic, or PTFE should be used and the user should avoid ceramics and natural rubbers (e.g. EPDM / BUNA).

While this acid is stable, it should avoid contact with strong alkalines and avoid materials such as aluminum, zinc, steel, and any strong caustics. Chemical compatibility is always important when selecting a dosing pump.

In other pipe materials, such as cement or concrete, a zinc carbonate film forms to protect the cement pipes from degrading.


For Zinc Orthophosphate to be most effective, many factors need to be accounted for when determining the dose rate to optimize corrosion inhibition. For example, alum (leftover from coagulation) can greatly influence the effectiveness of the orthophosphate. Additionally, varying pH levels can alter the effectiveness so is important to periodically reevaluate the dose rate. According to the Ontario Ministry of Environment, Orthophosphate treatment tends to increase over time. As many water distribution systems have varying characteristics, it is important to properly size your chemical dosing pump. Selecting a product with a high turndown ratio, such that there is room to increase dosage rate when exposed to real world application conditions.

Many municipalities operate with Zinc Orthophosphate (or any chemical) stored in bulk storage and then transferred to a day tank in the chemical feed room during water production. Due to the nature of this chemical and process; Air Operated Double Diaphragm (AODD) pumps are used regularly for transfer of this chemical from the storage tank to the day tank. AODDs are a good choice for this application because of their high reliability and low operating costs for best in class total cost of ownership.

Both diaphragm metering pumps and air operated double diaphragm pumps have a seal-less design and high levels of reliability to minimize leaks. In conjunction with these designs and long times between preventative maintenance cycles, these types of pumps reduce risk of chemical exposure to operators and maintenance technicians.

Zinc Orthophosphate has recommended limits as defined in NSF 60/61 and the EPA to provide guidance and ensure safety of populations. Zinc Orthophosphate is a chemical that is needed to be injected in precise amounts:

  • Too little ZOP may not offer enough protection on the distribution system
  • Too much ZOP may be harmful to the consumer

Due to the nature of balancing this control limit, selecting a reliable, accurate, and repeatable dosing pump is key to controlling this process properly.


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