New Drinking Water Treatment Plant

Port LaBelle’s new water treatment plant is a “membrane softening” system that uses a process called Nanofiltration.  Nanofiltration is a membrane treatment process that falls between reverse osmosis and ultrafiltration.  Membrane softening (membrane filters) is a name often applied to nanofiltration (NF) because it will selectively remove double-charged hardness ions of calcium and magnesium better than the single charged ions of sodium, potassium and chloride. 

The nanofiltration process can pass more water at lower pressure operations than reverse osmosis, can remove particles in the 300 to 1,000 molecular weight range such as humic acid and organic color bodies present in water, and can reject selected (typically polyvalent) salts. Nanofiltration may be used for selective removal of hardness ions in a process known as membrane softening. 

Water Purification System

Micropore size, nanometers

(10-9 meters)

Cross-flow Microfiltration

100 to 10,000

Ultrafiltration

1 to 100

Nanofiltration

(Membrane Softening)

0 to 1

Reverse Osmosis

Pure Water

Nanofiltration (NF) is a membrane technology on the rise, primarily because it's more cost-effective than reverse osmosis (RO) in some applications. In those situations, it can provide a specified degree of water quality at lower operating costs. Nanofiltration is a membrane filtration process which removes substances with an effective diameter of around a billionth of a meter (a nanometer) or greater. As a reference, a human hair is roughly 100,000 nanometers in diameter. A typical bacterial cell is around 1,000 nanometers in diameter, and a helium atom has a diameter of around 0.1 nanometer. 

The largest users of NF technology are municipal drinking water plants. Florida has, by far, the largest number of these plants. It's estimated that more than 100 million gallons per day (mgd) of NF permeate will soon be offered by these plants, some of which process more than 10 mgd (6,900 gpm). In Florida, new plants invariably use membrane softening or RO rather than conventional lime softening. By processing feedwater with NF and removing most of the THM precursors, water can usually be chlorinated without exceeding the regulated limit for THM concentration. 

Cross-flow microfiltration 

This technique may be defined as a solid-liquid separation process that uses membranes with pore diameters of 0.1 to 10 µm. It can retain suspended particles, bacteria and indirectly colloids and some ions after adsorption by larger particles through precipitation or flocculation. The theoretical difference between ultrafiltration and microfiltration is very clear, since ultrafiltration operates in a uniform liquid phase while microfiltration enables solid-liquid separation. In terms of technology, however, the processes can intersect. As a result, ultrafiltration membranes may be used to perform microfiltration, to minimize clogging and prevent solid particles from passing through the pores. Similarly, a microfiltration membrane can be used for ultrafiltration or even reverse osmosis once a coating of very-fine pore gel (dynamic membrane) is formed during the process.  

Ultrafiltration 

This technique uses membranes with micropores of 1 to 100 nm in diameter. The membranes let through small molecules (water, salts) and retain high-molecular-mass molecules (polymers, proteins, colloids). They are used in many applications, such as the concentration of macromolecular solutions (proteins, polysaccharides, various polymers) and the elimination of macrosolutes in effluent or in water for industrial or medical applications.   

Nanofiltration 

This new membrane separation technique lies between reverse osmosis and ultrafiltration. The membranes have pores with a diameter of around one nanometer, which is why the technique is called nanofiltration. The membranes can retain multivalent ions (e.g., calcium, magnesium, aluminum, sulfates) and non-ionized organic compounds with a molar mass exceeding about 300 g/mol. However, most monovalent ions and organic compounds with a molar mass lower than 300 g/mol pass through. Potential applications include selective demineralization (for water softening) and concentration of organic compounds with low molar mass.  

Reverse osmosis 

The reverse osmosis technique uses dense membranes that allow solvents, water in most cases, to pass through while retaining any salts. It is used to demineralize water (for seawater or brine desalting, or production of ultrapure water), or to concentrate solutions such as fruit juices.