5.8.1 Nitrate removal
Nitrate in water has become a significant problem and the EU Directive sets a maximum admissible concentration of 50 g m−3 measured as NO3−. This is equivalent to 11.3 g m−3 as N. High nitrate levels can cause cyanosis or methaemoglobinaemia in babies. Legislation allows the designation of nitrate-vulnerable zones and these help to prevent nitrate levels in natural waters increasing in affected areas.
Ion exchange is used in some treatment plants to remove nitrates from drinking water. In this process the water is passed through an ion exchange resin which removes the undesired ions and replaces them with ions which do not affect the water quality. This technology for nitrate removal was developed from water softening systems, which were used to remove the hardness-conferring ions Ca2+ and Mg2+. At first, ion exchange was carried out with zeolites, which are naturally occurring insoluble sodium aluminosilicates. Zeolites were able to exchange sodium ions for other ions such as Ca2+ and Mg2+. Artificial zeolites such as permutit are now produced. If the cation exchange sodium resin is represented by Na2R, where R is the complex resin base, then the reaction for water softening is
The treated water then becomes richer in sodium and, unless the water was particularly hard, this is less of a problem.
When all the sodium ions in the exchange resin have been replaced, the resin can be regenerated by passing a strong solution of sodium chloride through it:
For removal of nitrate ions, the exchange is with R*Cl where R* is another complex resin base:
The ion exchange vessels are taken out of service sequentially for regeneration using a brine solution which displaces the captured nitrate ions. The nitrate-rich brine product has to be disposed of. Recently, a process has been developed whereby this brine is electrolysed to convert the NO3 to N2 gas, allowing reuse of the brine.
As mentioned earlier, ion exchange is also used to reduce the hardness of a water (for example, the small units available for the home) by removing calcium and magnesium ions from water. It can also be used as a desalination system to reduce the salt content of a water. Small-scale ion-exchange units are commonly used in laboratories to produce pure water called deionized water, an alternative to distilled water. Deionized water requires the use of both cationic and anionic exchangers.
Reverse osmosis (explained in the next paragraph) has become popular in removing pollutants such as trace organics and salts and it is worth considering for nitrate removal.
When a solution of a salt is separated from pure water by a semi-permeable membrane that permits the passage of pure water but prevents that of the salt, water will tend to diffuse through the membrane into the salt solution, continuously diluting it. This phenomenon is called osmosis. If the salt solution is in an enclosed vessel, a pressure will be developed. This pressure in a particular solution is known as the osmotic pressure of that solution. Reverse osmosis is a process in which water is separated from dissolved salts in a solution by filtering through a semi-permeable membrane at a pressure greater than the osmotic pressure caused by the dissolved salts in the water. The pressure required increases in direct proportion to the concentration of salts. The salts could be in any form including nitrates. Removal rates in excess of 93% for nitrate have been reported for reverse osmosis systems. Operating costs and space requirements are said to be less than for equivalent ion exchange plants.
The basic components of a reverse osmosis unit are the membrane, a membrane support structure, a containing vessel, and a high pressure pump. Cellulose acetate and nylon are the most commonly used membrane materials.
The water to be treated is pumped at high pressure through the membrane module, and clean water is collected as permeate (Figure 31), with the unwanted material remaining in the retained liquid (retentate).
The concentration of retained material in the feed builds up with time, and the membrane can get clogged. To prevent this, periodic backwashing with either water or gas under pressure is undertaken.
Continuous filtration with the feed flowing over the membrane surface is preferred over a batch process, as the flow promotes self-cleaning and enables longer runs between backwashing or replacement.
To prevent clogging of the membrane, prior filtration of the feed water is necessary. To decrease scaling potential, iron and manganese removal may also be necessary. The pH of the feed should be adjusted to a range of 4.5–7.5 to inhibit scale formation. Figure 32 shows a schematic of a reverse osmosis system.
Other options for nitrate removal include electrodialysis (Section 3.14.3) and biological denitrification.