Dealkalisation Plant – Frequetly Asked Questions (FAQs)
What is a Dealkalisation Plant?
A dealkalisation plant is used to lower the alkalinity (temporary bicarbonate hardness) in a water stream. This is achieved by an ion exchange process using a resin which is specific for alkalinity removal, known as a weak acid cation resin.
The three main components of the dealkalisation system are:
- A pressure vessel containing the weak acid cation resin (in the hydrogen form)
- A degasser tower
- A softening plant (see water softener section) containing a strong sodium form exchanger
Other ancillary components are break tanks, transfer pumps, pH correction equipment, control panel and often a magnadol effluent neutraliser.
The size of each component varies according to the flow rate, the alkalinity in the water and the permanent hardness.
How does the Dealkalisation Plant work?
A standard dealkalisation plant works by allowing the water containing bicarbonate ions (also known as temporary hardness) to pass through the weak hydrogen cation exchange resin in the dealkalisation column. This resin selectively removes the calcium and magnesium salts associated with the alkaline hardness and, in the outlet water, the bicarbonate ion then exists as a solution of carbon dioxide and water. This carbon dioxide is then removed by gas stripping in the degasser column.
Remaining in the water is the permanent hardness which then passes through the simple softening process where the hardness is converted to sodium salts.
When the resin in the dealkalisation column becomes fully used up, i.e. exhausted, then the unit regenerates with hydrochloric acid to replace the hydrogen sites on the resin, ready for the next service run. Similarly, when the softening plant resin becomes exhausted, it is regenerated by brine (sodium chloride) which replaces the sodium ions back onto the resin, ready for the next service run.
What are the typical regenerants for a Dealkalisation Plant?
28% hydrochloric acid is typically the acid used to regenerate the acid for the dealkalisation column. Derwent Water Systems also design plants to operate on sulphuric acid.
Brine, in the form of a saturated salt solution, is used for the softening plant regeneration.
What is the quality of the treated water from a Dealkalisation Plant?
For most waters, the process will produce the following water:
Bicarbonate alkalinity – less than 5% of the input total alkalinity
Total dissolved solids reduction – approximately equal to the total hardness
Total hardness to service – commercial zero or less than 4 ppm
How do you regenerate a Dealkalisation Plant?
The Derwent range of dealkalisation plants regenerate automatically according to pH. In some instances, a water meter can be used but pH is normally the preferred option.
When the pH of the outlet water rises to 5.6, the unit automatically educts acid from the acid measure tank into the dealkalisation column. After regeneration of the resin, the outlet pH is 3.8. The service cycle then carries on until the pH reaches 5.6 again and the regeneration process is repeated.
Why do I need pH correction on a Dealkalisation Plant?
The water leaving the dealkalisation column will be between pH 3.8 to 5.6 and will be saturated with carbonic acid (water and carbon dioxide). After degassing, the pH will rise but still the pH will be acidic and, for most normal use and to prevent down stream components from corrosion, the pH will need to be raised to an alkaline condition.
To correct the pH, a 15% caustic soda solution is injected after the degasser column to raise the pH to around 7.5 to 8.5.
What is the Degasser Tower and what is its purpose?
The degasser tower is filled with packing of polypropylene rings (or similar). The packing is to ensure maximum surface area between the water and the air which is blown upwards through the packing from an air blower at the degasser base. The water containing carbon dioxide then passes down through the tower and the carbon dioxide is blown off through the top of the degasser.
Is a Dealkalisation Plant suitable for all waters?
The dealkalisation process is normally used on high hardness and high alkalinity waters, e.g. borehole waters. With these high hardness waters, the process gives maximum benefit by removing the high levels of bicarbonate alkalinity which would give serious costly condensate corrosion if the water was fed direct to a boiler plant.
The process works best when the total hardness is greater than the total alkalinity.
How do I size a Dealkalisation Plant?
The sizing of the dealkalisation column depends on the input alkalinity and the capacity required between regeneration and the flow loading on the resin. Typically, the flow loading on the resin should be in the range of 8 to 36 bed volumes per hour.
For help on sizing, please contact the Derwent Water Systems technical support team.
Can Derwent Water Systems service Dealkalisation Plants?
Yes. We offer single-day services or service contracts on all water treatment equipment. All of our water treatment service engineers are fully qualified and experienced on the full spectrum of water treatment equipment available. We have defined service specification sheets (available on request) which define the exact service schedule proposed for a dealkalisation plant.