Brackish water is water which is more saline than fresh water. It is not quite as saline as seawater. It may result from the mixing of seawater with fresh water, as in surface waters such as estuaries or other watery ecosystems, or it may occur underground in brackish fossil aquifers.

A water softener is an appliance that uses sodium chloride, also known as salt, to treat hard water.
Hard water contains an excess of minerals such as calcium, magnesium, manganese, and iron that can be an expensive nuisance for a home. These minerals are taken up in the underground water supply and, as the water is heated in the home, they crystallize and stick to household surfaces. Sodium chloride, the effective component of water softener, works to replace these unwanted minerals

Function
Water softener systems use a process called ion exchange to remove calcium and magnesium, which are the trademarks of hard water. Overly hard water can make household cleaning difficult, as it often leaves scum on kitchen and bathroom fixtures, dulls clothing, and leaves spots on dishes.
Water softening must be used with care, however, because it exchanges sodium for the minerals that give water hard characteristics, and it may be harmful for people on low-sodium diets. Sodium produced by softening water can also be harmful to plants. The different types of water softener systems all utilize ion exchange, but they vary in the amount of input they require from the operator.
Water softeners are fairly easy to operate and maintain. All that needs to be done after installation is to periodically add sodium chloride, and the appliance does the rest.

Twin System Water Softener 

Hard water measurement

originates from groundwater sources, such as aquifers, because calcium, magnesium, and other minerals dissolve into the water from the surrounding rocks. The calcium and magnesium cause the characteristics that are referred to as hard. Hardness is measured in grains per gallon (GPG), with one grain of hardness equal in weight to one kernel of wheat. Any water with greater than 1 GPG of calcium and magnesium is considered hard, with greater than 10.5 GPG being classified as very hard.

How to tell hard water ?

One of the easiest ways to tell if family use hard water just by bathing. If the water is hard, soap will not lather well, and baths and sinks tend to have a build up of soap scum that is difficult to remove. The soap scum is actually the mineral deposits left by hard water. It may also build up on dishes, coffee makers and in dishwashers.

Though they are great indicators of hard water, soap scum and the inability for soap to lather properly are not the sole reasons a person might need a water softener. 

These minerals can cause severe problems with plumbing and appliances that use water, such as water heaters and dishwashers. Calcium, magnesium, and the other minerals in hard water build up in pipes and will begin to interfere with pipe drainage and water pressure. Eventually, this build up can lead to completely blocked pipes.