Physicochemical Removal of Nitrogen

As described previously, biological nitrogen removal is not the only technology available for nitrogen compound removal from wastewaters. Some nonbiological processes are able to recover nitrogen compounds in their dissolved forms for potential uses as fertilizers and are viable alternatives under some circumstances. On the whole, however, physicochemical processes for removing nitrogen from wastewater are not practically popular. The reasons for the unpopularity are often cited as cost, inconsistent performance, and operating and maintenance problems. The principal processes employed for nitrogen removal from wastewaters are air stripping, breakpoint chlorination, and selective ion exchange.

Air stripping is used to remove ammonia from wastewater because ammonia in water can be easily volatilized and carried away by the flowing air stream, particularly in high pH values. The gaseous ammonia in air can later be captured. The process is temperature-sensitive, and fogging and icing occur in cold temperature. This is an expensive operation that requires lime for pH control, and it can be justified only in some special cases, such as the need for a high pH for other reasons at the time.

Breakpoint chlorination is a process that involves the addition of chlorine to wastewater to oxidize the ammonia in the wastewater to nitrogen gas and other possible stable compounds. It can, with proper control, theoretically remove all ammonia in wastewater and therefore it is used to polish the effluents from other nitrogen removal processes. The downside of the breakpoint chlorination process is that the process is sensitive to pH, thus requiring proper control of pH with skillful operators during the application of chlorine. The process is expensive because of chlorine and skilled labor requirements, and its residual may be toxic to the aquatic life.

Ion exchange is a separation technology (see Chapter 3 for the description of ion exchange systems) that utilizes ion-selective resins (mostly synthetic, but also natural, such as zeolites) to remove certain ions from wastewater. In ion exchange operations, the ions in wastewater displace the ions on the resins, thus separating from the rest of the components in the wastewater streams. The removed ions in the resins can be washed out with appropriate regenerants. For nitrogen removal with a zeolite ion exchange system, lime can be used to regenerate zeolites. Because of the presence of organic materials in wastewater, restorants such as sodium hy droxide, hydrochloric acid, methanol and bentonite are employed to remove organic materials from resins.

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