Water softening, Deionization (DI), and Reverse Osmosis (RO) are three treatment methods used to remove hardness in water, but they work differently and have different use cases.
How do they work in removing hardness in water
Water softening: This process uses ion exchange resin to remove hardness ions (primarily calcium and magnesium) from the water and replace them with sodium ions.
Deionization (DI): Deionization is a process that removes all ions from water, leaving it almost completely pure (H2O). TDS of DI treated water is usually zero. It typically uses ion exchange resin to remove positive and negative ions from the water.
Reverse Osmosis (RO): Reverse osmosis is a process where water is forced through a semi-permeable membrane, leaving impurities behind. RO removes a wide range of contaminants including hardness ions, salts, and organic materials.
The flow rate of water through the softening, DI, or RO system can impact the effectiveness of the treatment process. If the flow rate is too high, the system may not have sufficient time to remove the hardness or other impurities, resulting in reduced effectiveness. If the flow rate is too low, the system may not be able to keep up with the demand for treated water.
Water softening: Depending on the volume of the resin bed, flow rate has to be limited to get to the enough contact time for ion exchange. For the size of the whole house point of entry water softener system, usually the flow rate can be as high as 8 GPM, and the flow reduction is not significant.
Deionization (DI): DI resin is more expensive than the resin in water softener, as such DI system usually have a much smaller size of the resin bed. Because of the smaller size, water flow usually have to be regulated to allow sufficient contact time to completely remove TDS in water. Depending on the resin bed, the flow rate can range from less than 0.1 GPM for a DI post filter of a RO system to 2 to 3 GPM for a DI car rinse system.
Reverse Osmosis (RO): RO systems usually have the lowest flow rates. RO membrane is usually rated for gallon per day as its rate flow rate. The higher it goes, more expensive the membrane will be. To achieve higher flow rate, multiple membranes are put together to treat water simultaneously.
In general, RO systems have the lowest flow rates, followed by DI and then water softening.
The capacity of a water softening, DI, or RO system refers to the amount of water it can treat.
Water softening: The resin bed of water softener can be regenerated after the ion exchange capacity is exhausted. The replacement of resin bed only becomes necessary after many years of service. In other words, the capacity of a water softener can be extended with each regeneration until after many years.
Deionization (DI): The regeneration of the resin bed of a DI system usually requires sophisticated system and process. As such it is not usually regenerated. The anion resin and cation resin have different capacities. The capacity is based on the volume and quality of the resin. Generally speaking, the capacity of DI resin is quite limited.
Reverse Osmosis (RO): RO membrane produces RO filtered water, but also generate drain water with higher brine content. The membrane is usually flushed by the drain water, or the system can run source water over the membrane once in a while. Such flushing helps to remove sediments and contaminants that are blocked by RO membrane which usually clog membrane pore. Eventually though membrane will be clogged (fouled) and has to be replaced. Depending of the qualify of source water, time for replacement can range from several months to a few years.
In general, water softener has the largest capacity, followed by RO systems. DI system has the smallest capacity.
Cost is composed of the one time cost of system purchase, the cost of running the system, and the cost of replacement filters and resin.
Water softening: The one time purchase cost of a whole house water softener is usually more expensive than a DI system due to the volume of resin bed, but less expense than a RO system. Because the resin can be regenerated with inexpensive salt, the entire cost of water softening is arguably the lowest among the three.
Deionization (DI): The one time purchase cost of a DI system is usually the lowest. But as DI resin can not be regenerated, new resin has to be purchased when the limited ion exchange capacity is reached, the overall cost of ownership of a DI system is usually more expensive than water softening.
Reverse Osmosis (RO): RO system is expensive. The limitation of a RO system is more about the flow rate than its capacity. If higher flow rate needs to be achieved, it can be quite expensive, especially for the upfront system purchase.
Non-industrial use case
All the characteristics of each treatment methods in terms of how it works, flow rate, capacity and cost, set the stage of its use case.
Water softening: As a water softener replaces calcium and magnesium ions with sodium ions, the total TDS of the water is not changed. If sodium presents issue for a use case like in agriculture or car rinse (sodium ion also leaves stain), then water softeners can't be used. A water softener is good for whole house water treatment because of its cost advantage.
Deionization (DI): DI systems produce pure water. It is used where ions in water may present issues like for aquarium. It is commonly chosen for car rinse system due to its advantage in flow rate and system purchase cost over RO systems.
Reverse Osmosis (RO): RO system is commonly used for household drinking water as flow rate is not much a concern and the health concern from water is more than hardness. RO system is also used in specific agricultural applications like hydroponics due to its operating cost being lower than a DI system, even though such system is usually quite expensive itself.