The Importance to both Industry and Agriculture of Ammonia Recovery Systems
Two forms of ammonia are encountered in wastewater from agroindustrial processes, the ionic form (NH4+) and the gaseous form (NH3). The relationship between ammonia gas and the ammonium ion is:
NH4+ + OH– ↔ NH3 + H2O
Dissociated ammonium (NH4+) is converted to undissociated, or free, ammonia gas (NH3) by the addition of a base, such as sodium hydroxide. The balance of this equation is a function of pH and temperature, with low pH and low temperature pushing the balance toward NH4+. Conversely, by increasing the temperature, the pH required to maintain the ratio of ammonia in the gas phase of total ammoniacal nitrogen decreases. The consequence is that by increasing temperature alone, the requirement to add a chemical base is obviated.
Thermally driven ammonia recovery and removal systems have been successfully deployed on several sites in Hong Kong, with plants having been installed for the treatment of both landfill leachate and for the highly ammoniated wastewater produced during the Anaerobic digestion of food waste. Waste heat for these ammonia recovery systems is provided either by using waste landfill gas or, more efficiently, by using waste heat from Biogas engines.
The implications for employing ammonia stripping systemsin the treatment of wastewater are clear in that ammonia and ammonia by-products are not only pollutants they also have many commercial applications. For example, compression of ammonia converts it into a liquid with relative ease so that when converted back into a gas it absorbs heat, hence its application in refrigeration engineering. The textile industry uses ammonia in the tanning of leather and the dyeing of wool, cotton, and silk, as well as in the production of nylon. Household floor cleaners and detergents commonly use ammonia. Chemical process industries facilities use ammonia for pH control and to manage NOx which, when combined with ammonia under the correct conditions, produces nitrogen and water. Ammonia is also an important ingredient in fermentation processes as it serves as a source of nitrogen to encourage microorganism growth and pH adjustment.
The cost for the control, recovery, and disposal of ammonia from leachate and wastewater can run into hundreds, if not thousands, of dollars per cubic metre treated but as indicated, ammonia is also a valuable commodity. The possibility of converting what many see as a liability into an asset is obviously an attractive commercial opportunity that would not only result in the removal of a significant environmental threat but would result in a substantial reduction of operating costs in the management of wastewater treatment plants.