Secondary disinfection:

Secondary disinfection is a process during which an essential residue is provided to prevent the regrowth of microorganisms in the distribution network. Chlorine, chloramines and chlorine dioxide are usual secondary disinfectants among which the use of chloramines as secondary disinfectants is more common.

Chloramines have no virucidal power and are only slightly effective in killing Giardia cysts. However, low reactivity, high stability, non-formation of THMs, and easy conversion of residual free chlorine into residual chloramine by adding ammonia are among their most important properties.

Water fluoridation:

Fluorine is found in nature and due to the dissolution of fluorine-containing sediments or ornamental rocks, it can enter raw water sources. 

It rarely occurs in sufficient quantities in surface water and is concentrated only in the underground water of some geographical areas. Its low concentration is beneficial, but its high concentration is toxic for humans and other animals. Drinking water containing fluorine with a concentration of approximately 1 mg/L prevents tooth decay in children.

During the formation of permanent teeth, as a result of a chemical combination process of fluoride with tooth enamel, it provides more hardness and resistance to teeth, making them resistant to decay. The result is a reduction in the rate of tooth decay, tooth extraction and filling. However, high concentrations of fluoride may stain teeth and excessive amounts of it (more than 5mg/L in drinking water) can cause brittle of teeth, Fluorosis or skeletal abnormalities.

Fluoridation is used to keep the fluoride concentration at an optimal level in many water treatment facilities. Assuming that people consume more water in warmer climates, ambient temperature is usually the basis for adjusting water fluoride levels.

Fluorination of chemicals and their storage:

Currently, several compounds are used for fluoridation of drinking water, which are calcium fluoride, sodium fluoride, sodium silicofluoride, ammonium silicofluoride and hydrofluorosilicic acid.

Fluorination of dry chemicals:

To protect the dry chemicals used in fluorination from moisture, they are kept in covered places. Since sodium fluoride powder is corrosive, dust collection equipment should also be included if conventional exhaust systems are used in the system design.

Proper ventilation is one of the requirements of the storage place, and its capacity should be 1.2 to 1.5 times the amount of stored chemical. In larger units, steel silos should be used. To transfer the chemical from the storage silo to the injection equipment a transfer system is required.

Fluorination of liquid chemicals:

Barrels containing hydrofluorosilicic acid should be stored in a well-ventilated area. For chemicals in larger quantities, a large storage tank should be used. For this purpose, it is necessary to use steel tanks covered with rubber materials (neoprene) or a suitable thermoplastic (such as polyethylene). The capacity of the tank should be enough to consume at least one to two months. In the design of storage tanks important considerations must be observed, which are as follows:

1. For cargo unloading necessary preparation should be made.
2. Access to a faucet is necessary to wash away leaks and chemical spills.
3. In case of tank or pipe breaks, large above-ground tanks should be equipped with a storage structure in order to keep the entire volume of chemicals.
4. It is necessary that the concrete storage structure has a suitable protective cover
5. A leak detection system is necessary in ground storage tanks.
6. All pipes and fittings must be made of suitable thermoplastic material

System of Fluoride injection:

The chemicals used in fluorination are added dry or dissolved to water. Fluoride solution can be prepared from dry chemicals before consumption. Difficulty in dissolving dry chemicals in water depends on their solubility.

Among the dry compounds used in silico fluorination, ammonium fluoride and sodium fluoride are the most soluble, therefore they easily dissolve in water.

Injectors are of the type of dry injector or solution injector, although a tank is also used in dry injectors to prepare the solution before the injection point.

Dry injection:

A dry injection system usually includes:

1. Chemical transfer equipment
2. Dry injector
3. A solution tank with a one-day consumption capacity equipped with a stirrer
4. A system of dust collection

There are two types of dry injectors: volume and weight injectors. Both volume and weight injectors deliver a specific volume and weight of the chemical in a certain time. All components of dry injectors are made of iron or steel.

• Volume injectors are used to deliver smaller amounts of chemicals.
• The range of accuracy of volume injectors is 3 to 5% and the accuracy of weight injectors is 1%.
• Volume injectors have simpler and cheaper installation.
• Automatic control of weighing injectors is more easier.
• Weight injectors are usually used in large water treatment plants.

Solution injection:

A system of solution injection usually has the following components:

1. Large storage tank and transfer pump
2. Storage tank with one day capacity
3. The injecting material of the daily storage tank solution must be made of resistant thermoplastic and have the capacity for one-day consumption of fluoride solution.

A saturated form or in any desired concentration of the solution can be prepared. A stirrer that is resistant to corrosion is also required. Solution injectors include devices that are used to measure and deliver a specific volume of solution over a specified period of time.

Diaphragm pumps, piston pumps and rotary displacement pumps are different types of solution injectors, among which diaphragm pumps are the most widely used type.