Where do chlorides in water come from and what to do with them?

This indicator is almost always present in detailed water analyses but its description in various sources differs. Today we will try to understand what the water analysis for chlorides say, how to interpret them correctly and what measures should be taken in case of an increase in this indicator.

What are chlorides in water?

In chemical terms chlorides are salts or anions of hydrochloric acid (HCl). They are a complex of a cation (metallic or organic) and anion Cl-. The most typical chloride is table salt NaCl.

It is important not to confuse anion in salts with chlorine for water disinfection. The first is quite inert and practically does not participate in redox reactions in an aqueous environment. The second actively oxidizes various organic impurities, microorganisms and in the process forms new substances, including chlorides.

The norm of chlorides in water

According to international sanitary standards, the maximum chloride content in drinking water should not be more than:

• 250 mg/l for tap and bottled water;
• 350 mg/l for water from wells and catchment areas.

For mineral water this indicator can be higher due to the high content of minerals. But the salty taste of water can be felt already when 200 mg/l of sodium and potassium chloride is reached.

Where do chlorides in water come from?

Chloride ions are a typical mineral component of natural water. They are found both in surface and underground waters.

The main source of mineral salts (particularly chlorides) entering water is the dissolution of natural mineral salts. Sodium, potassium and calcium chlorides are typical and are leached from a variety of minerals such as sylvinite.

The concentration of chlorides in fresh natural water is usually kept within the norm but anthropogenic factors (environmental factors caused by accidental or intentional human activity) can change it for the worse. The exception is groundwater in coastal sea zones which are characterized by high salinity. In this case the water is usually so salty that it cannot be used for drinking.

A number of actions can be attributed to anthropogenic factors:

• Use of salt for sprinkling roads and sidewalks. In winter dozens of tons of salt are used on the roads which, in the process of melting snow and stormwater, enter the rivers. It can cause an increase in the level of chlorides in the winter and spring period.
• The use of inorganic fertilizers, for example, potassium chloride, which are washed from the fields by rain into the groundwater and cause an increase in the indicator in wells and in rivers.
• Municipal wastewater.
• Industrial waters, especially mine waters, have very high salinity due to active interaction with mineral rocks. An increased content of chlorides in surface waters is observed almost always after the discharge of industrial wastes.
• The filtrate of solid waste landfills.

The least negative impact is by effluents from private septic tanks, animal feed and drainage of irrigation systems.

Another source of chloride are water chlorination processes where active chlorine is reduced to anion and its purification with the help of coagulants (iron and aluminum chlorides).

Impact on the environment

If we look at chlorides through the lens of impact on water bodies and soils, the impact becomes more obvious. Salinity leads to changes in species diversity, primarily among plants which lead to the disappearance or appearance of certain types of fish. Increasing mineral levels in low salinity waters is critical for biological balance.

Chlorides in water: pros and cons

This ion is not a toxic substance like heavy metals or organochlorines. The indicator is used for a comprehensive assessment of water. However, a high content of chlorides indicates a high content of sodium, potassium and calcium which already have an effect on the human body.  In particular, excessive sodium consumption is associated with hypertension which leads to heart attacks and strokes. The relationship between high sodium content and the development of osteoporosis has also been proven.

The daily norm of chlorides for an adult recommended by the World Health Organization is 9 mg/kg. This is approximately 0.5g at a weight of 60 kg. If the maximum daily rate of salt for an adult is 5 g, it is almost 2 g of sodium and 3 g of chlorine in the form of Cl-.

The required amount of water per day for an adult is approximately 2 liters. Consuming 2 liters of water with a maximum allowable concentration of chlorides (250 mg/l), you will be able to get approximately 500 mg or 0.5g of chlorides from it, which corresponds to the full daily rate of chloride consumption and 1/10 of salt consumption.

Another important aspect is the electrochemical properties of this ion. Since this ion increases the electrical conductivity of water, it increases its corrosion activity. When water is supplied through metal pipes, chlorides and other anions can react with metals and form soluble salts which pollute the drinking water.

Water purification from chlorides

As we explained above, chlorides are not toxic and dangerous by themselves and their significant excess can indicate a high content of inorganic salts. They can lead to health problems and malfunctions of household appliances.

The most common method of reducing chloride concentration is reverse osmosis.

Reverse osmosis

This is a universal technology for both domestic conditions and industry. The household reverse osmosis system is able to retain 99.9% of water impurities, including chlorides and sulfates of sodium, potassium, calcium, magnesium and heavy metals. Reverse osmosis is also effective against large organic molecules. In domestic systems preliminary cleaning from mechanical particles as well as chlorine and organics is provided which reduces the load on the membrane element and also increases its efficiency and service life.

Other methods are mainly used in industry, such as distillation and electrodeionization.

Read also: Manganese , Iron , Sulfates .