Measures for Nitrogen Removal in Environmental Water in Developing Countries
Excess nitrogen in waters all over the world is a problem, and nitrogen is a factor that induces eutrophication in water. In drinking water, health problems particularly from nitrate-nitrogen and nitrite-nitrogen are concerning.
Nitrogen in water can exist in states of organic nitrogen, ammonium-nitrogen, nitrite-nitrogen, nitrate-nitrogen and nitrogen gas. Each relation is roughly as follows.
Organic nitrogen is oxidized to ammonium-nitrogen and then to nitrite-nitrogen. Furthermore, it is oxidized to nitrate-nitrogen. Nitrite-nitrogen and nitrate-nitrogen are finally transformed into nitrogen gas. Oxidizing agents and organics are involved in these reactions.
Removal of nitrogen existing in the groundwater is often conducted in order to ensure safe drinking water. The nitrogen removal process in water is roughly divided into two processes: one is a technique to purify water after water-intake and the other is to purify water in soil or the groundwater – in situ.
Technically, they are divided into a physicochemical process and a biological process. For nitrogen removal after water intake, chlorine, reverse osmosis membrane, ion-exchange membrane and ion-exchange fibers are generally used. For in-situ removal, anaerobic ammonia oxidation, a so-called anaerobic ammonia oxidation (ANAMMOX) other than the nitrification denitrification process using anaerobic denitrifying bacteria, which has known for a long time as biological nitrogen removal process, has been applied recently. Most of these techniques are applied to water containing comparatively highly-concentrated nitrogen compounds such as municipal wastewater, industrial wastewater and so on. On the other hand, for environmental water, particularly in a case where the application is drinking water, it is clear that applicable nitrogen removal techniques are strictly limited. Furthermore, if the application is provided in developing countries, the conditions for construction and administrative costs become even tougher.
With comparatively low concentration of nitrogen in mind, we review a few typical nitrogen removal techniques from an above mentioned viewpoint. Since an oxidizing agent like sodium hypochlorite is essential for breakpoint chlorination that has many past records for water supply, there are some by-products that cause damage to human health in addition to cost and safety management. Therefore, the technique is not recommended. Reverse osmosis technology for a water purification that has recently been prevailing in various fields is good for effective separation cleaning regardless of nitrogen form. However, the technology involves high operating costs because the membranes used are expensive and need to be replaced after use. In addition, it should be remembered that the technique has high disposal costs because concentrated liquid containing highly-concentrated nitrogen is produced. The biological nitrogen removal process has a slight cost advantage because of technology using the function of organisms. Coexistence with organisms for denitrification reaction, however, is essential. Given the difficulties in control and necessity of organism, unfortunately it is not a good technology for water supply.
Therefore, one recommendation we offer is ANAMMOX. Since autotrophic bacteria are used for ANAMMOX, the application of organic matter is not required. In addition, operating cost for ANAMMOX is kept down. It is good for water supply as a whole because there are few disposables which need to be replaced. This technique has very few study cases in addition to past applications to water containing low-concentrated nitrogen. However, Japan and Vietnam jointly are now promoting research and development of ANAMMOX in Hanoi, Vietnam; further results are expected.
※References：”JOUNAL of ENVIRONMENTAL CONSERVATION ENGINEERING”,Vol42,No.12, 'Pollutant source of nitrogen in environmental water and its treatment and water-resource management.' Special Edition