Environmental pollution by wastewater occurs when contaminants are introduced to an ecosystem at a specific location. As a result, the regulations of discharges into water sources, water reservoirs and sea are an important aspect of the preservation of overall water quality and environmental ecosystem. One of the most common pollutants which is also toxic to aquatic life is ammonia. Current methods for ammonia removal from wastewater are mainly biological and are based on bacteria which are sensitive to environmental conditions and are difficult to control and maintain.
Ammonium (NH4+) is an important nutrient in primary production of many industrial plants and is present in the wastewater of many industries. It is one of the main constituents in all types of human and animal wastewater. Ammonium ions are the primary form of widespread nitrogen pollution in the hydrosphere and present a major challenge for municipal and industrial wastewater treatment. Ammonia (NH3) in its liquid and gas forms is toxic to the human body as well as the environment, fish and other forms of aquatic life in very low concentration (~0.2 mg/L). High ammonium loads can cause eutrophication of natural waterways, contributing to undesirable changes in water quality and ecosystem. While ammonium pollution comes from diffuse agricultural sources, making control difficult, industrial or municipal point sources also contribute significantly to overall ammonium pollution. These latter sources can be targeted more readily to strictly control ammonium release into water systems.
Common ammonia removal treatment is mostly based on biological technologies which are less than ideal. The treatment processes are themselves sensitive to environmental conditions such as toxic composites, low or high temperatures, pH, ammonium concentration levels, changes in wastewater compositions and more.
Biological methods also require reactors, mixers, diffusers, blowers, clarifiers, sludge treatment (e.g. thickening, digestion, dewatering), and sludge removal and disposal. Biological processes are not flexible in their production capacity and are not suitable for cases where wastewater composition and ammonium concentration are not stable neither fully predictable. A polishing physico-chemical system might be added in such cases to ensure effluent quality at all time. Biological systems tend to fail from time to time, and then, start-up (and regrown) of the bacteria population can take much time starting from a few days and up to months. A back-up solution might be required for such cases. Regulation limits present a challenge for the traditional ammonia treatment techniques. Polishing is a good way to ensure compliance, but biological processes are not well suited for polishing and do not ensure the required levels defined by the regulations.
Air stripping is a well-known process for ammonia removal from wastewater which is not biological. The removal is performed by means of a stripping tower in a traditional process (less used today), in which the wastewater pH is first elevated and then the wastewater is brought into contact with gas (usually air) so that ammonia (and some VOC) is released to a second circuit. More steps are needed in order to reduce the pH before discharge of the wastewater, and for the treatment of the now-polluted air. The main disadvantages of this technique are:
- the need to add a strong base for attaining a pH range of 10.8-11.5, at which NH3 is the dominant specie, and then reducing the pH before discharge (which translates to complexity and material costs);
- the outflowing ammonia gas (second circuit) needs to be treated separately (this is in fact transfer process rather than ammonia treatment process); and
- Scaling in the stripping towers requires periodical maintenance.
Air stripping can be economical where the ammonia concentration is at least few hundred mg/L and the resulting effluent ammonia concentration will not meet most regulation regimes.