Nanofiltration: membrane water softeners

membrane water softeners: consistent and cost effective water softening


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Membrane water softeners are based on Nanofiltration (NF) membranes, which reduce operating costs and provides optimum hardness rejection.

1. Nanofiltration (NF): the key to membrane water softeners

Nanofiltration membranes are designed to remove divalent ions, which cause water hardness (Calcium and Magnesium). Therefore, they are also referred to as softening membranes. 

Typically, NF membrane have higher permeability than brackish-water membranes and comparable rejection of bivalent ions (i.e., Ca, Mg, Mn, Fe, SO4), large organic molecules and of pathogens (bacteria, protozoa, and viruses). 

However, Nanofiltration membranes have a lower rejection of monovalent ions (Sodium, Chloride, Boron etc) compared with brackish-water membranes or seawater membranes. Their looser membrane technology reduces  the capacity to reject most monovalent salts but are capable of delivering a higher filtered water, hence nanofiltration systems operate at lower feed pressure than reverse osmosys sistems. 

Modern Nanofiltration (NF) membranes offer a high hardness rejection and membrane water softeners can be designed to deliver any desired softening level. Nanofiltration membranes also provide excellent removal of organic matter what eliminates the risk of disinfection by-products.
Membrane water softening by means of Nanofiltration membranes is an excellent option when a certain level of salt rejection is beneficial, in particular salinity related to bivalent ions such as Calcium and Magnesium, typically responsible for water hardness.
Nanofiltration membranes remove 99% of organic matter, responsible for color, taste and odor in water as well as 99.999% of bacteria, viruses and contaminants such as sulfates, colloidal matter and
heavy metals. 
For this reason membrane water softening with Nanofiltration membranes is commonly described as partial desalination.

2. Membrane water softeners: not aggressive water and easy concentrate disposal

Although Nanofiltration membranes offer a high rejection to bivalent ions such as Calcium and Magnesium, there is still a low-level of Calcium hardness remaining in the NF permeate, which provides a slight scaling tendency necessary to prevent corrosion of water lines.
Concentrate disposal in membrane water softeners based on NF membranes is simpler that disposal of
concentrate from reverse osmosis (RO) plants.  
RO concentrate contains very high levels of dissolved salts due to the high ion concentration in the water feed and the high salt rejection capability.
On the other hand, Nanofiltration is usually applied to low-salinity water feeds and NF membranes offer a lower salt rejection capacity than RO membranes, hence NF concentrate will typically containing less than 4000 mg/l of dissolved solids. This way, the concentrate from membrane water softeners can be used directly for irrigation purposes.

3. Membrane water softeners vs lime softening

Nanofiltration is a verycost-effective option compared to lime softening. Initial costs of a membrane water softener system or a lime softening plant are about the same, however the operating costs of membrane water softeners are more affordable than for lime softening plants: chemical dosing required for PH adjustment is reduced and sludge is non-existent.
As lime softening capacity must be installed in large increments, excess capacity is often installed to prevent lack of potential growth in projected increased demand. Membrane water softeners offer a high scalability and capacity can be increased and paid for as it is needed.
Another factor in favor of membrane water softeners is the reduced footprint. Membrane water softeners require approximately one-third of the space of lime softening plants, what can be a major project saving.
Last but not least dust, visual impact, odors, sludge disposal and overall plant operations can make lime softening plants unacceptable in many communities.

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