Physical Nitrogen Removal
Nitrogen can be removed from wastewater through physical means. The most common method is ammonia stripping. Ammonia (NH3) exists as a gas in solution. Ammonium (NH4) is a dissolved solid. Depending on the pH, nitrogen will exist as ammonia or ammonium.
Ammonia, because it is a gas, will come out of solution readily and exit into the atmosphere (strip). Ammonium, as a dissolved solid, will not. Therefore, to effectively strip ammonia from wastewater, the pH must be raised to a point that most of the compound will exist as ammonia. At pH 11 and a temperature of 25º C, the percentage of the compound that exists as ammonia is 98%. At this pH, agitation of the wastewater or spraying the wastewater into fine droplets will result in a high level of ammonia removal through stripping. Caustic ammonia stripping usually employs a packed tower with a counter current of forced air. High levels of ammonia removal can be reliably achieved, however the cost is generally prohibitive.
To a lesser extent, this phenomenon is responsible for much of the nitrogen removal that is achieved by lagoon systems. Because the pH of lagoons is often elevated over 8.5 during periods of high algae activity, ammonia stripping can be responsible for removal of much of the nitrogen in the wastewater. Although this method offers an inexpensive and simple means for nitrogen removal using lagoons, please note that the effectiveness is limited and effluents with less than 10 mg/L total nitrogen are hard to obtain.
Chemical Nitrogen Removal
There are two common chemical methods for removing nitrogen from wastewater; breakpoint chlorination and ion exchange. Neither is used on a widespread basis as a means of discharge permit compliance.
Ammonia/ ammonium nitrogen can be oxidized to nitrogen gas with chlorine. Breakpoint chlorination is the term used to describe the process. To reach the chlorine breakpoint, enough chlorine must be added to satisfy all of the demand in the wastewater. Any ammonia in the wastewater is oxidized to nitrogen gas, and all other pollutants are oxidized as well. When the addition of more chlorine results in a comparable increase in the free residual, the breakpoint has been achieved.
Breakpoint chlorination requires relatively high volumes of chlorine compared with the amount of ammonia being oxidized. In fact, the chlorine to ammonia ratio is around 10:1. This means that it takes 10 pounds of chlorine to oxidize one pound of ammonia into nitrogen gas. The high demand makes breakpoint chlorination impractical for any use other than for polishing an effluent following another nitrogen removal process. In this practice, the bulk of the influent ammonia is removed through biological processes and then the remaining 1 –2 mg/L of ammonia is removed through breakpoint chlorination.
The ion exchange process can be used to remove a variety of pollutants from wastewater, including ammonia. Ion exchange involves passing ammonia-laden wastewater through a column that contains natural or synthetic ion exchange resins. A naturally occurring resin (or zeolite) known as clinoptilolite is commonly used. The columns are generally 4 – 5 feet in depth packed with 20 X 50 mesh particles. As wastewater passes through the column, ammonium ions in the wastewater are absorbed by the clinoptilolite. When the absorptive capacity of the resin is used up, the column is regenerated through a caustic wash, which releases the absorbed ammonium from the resin by converting it into ammonia, which is then removed through gas stripping. Clinoptilolite resin can also be regenerated using brine solution. When the brine is passed through the column, the sodium in the brine replaces the absorbed ammonium at the exchange sites of the resin. The brine can then be discarded or the ammonium can be removed and the brine can be reused to regenerate the column.
Ion exchange is generally considered an expensive method of ammonia removal. However, it is also very effective when properly employed. Because of plugging of the ion exchange resin by bio-slimes, this method is generally only applied to high quality secondary effluents that have been filtered and disinfected.