One of the principal contaminants that must be treated in landfill leachate, and in waste water in general, is ammonia. Ammonia-N is normally present in wastewater or in leachate in several forms. Treatment is therefore essential.
Composition of Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula NH3. In its simplest form ammonia is a colorless gas with a characteristically pungent smell.
Environmental Impact of Ammonia
Nitrogen is a major plant nutrient, but too much nitrogen in the form of nitrates in the effluent can cause explosive plant growth and algal blooms in watercourses, lakes and ponds. As the process also creates anoxic conditions, it can be deadly to aquatic lifeforms.
Ammonia in raw domestic sewage is found in concentrations of about 30 to 60 mg/liter. In leachate from landfill sites and other organic wastewater streams it may rise to several thousands of milligrams per liter. As ammonia is biologically oxidized to nitrate it exerts an oxygen demand on the receiving water. This can reduce dissolved oxygen in the water to a point where aquatic lifeforms cannot survive. Lethal concentrations range from 2.5 to 25 mg/l. Ammonia can also act as a fertilizer. Uncontrolled release of untreated water can cause a profuse growth of stringy bacteria and/or fungi.
Organics USA has developed a thermally-driven ammonia stripping process to remove ammonia fro wastewater or leachate that requires no chemical additions apart from minor additions of an anti-foam agent. The single important input is waste heat from, for example – the exhaust from on-site engines, which drives the chemical reactions.
Ammonia Removal Solutions
Conventional ammonia removal from wastewater comes in two forms:
The first, and most commonly employed method, is the use of bacteria in aerated or anoxic reactors. For waste-water streams with relatively low ammonia and comparatively high carbon loading, it has proven to be a robust, viable and long-term solution. Difficulties only arise when the carbon available in the wastewater is not adequate to drive the necessary conversions to nitrogen gas. When a wastewater stream has very high ammonia levels (greater than 3000 mg/l) and carbon is inadequate, the cost of operation for an extended duty-of-care period can become excessive.
The second process often employed is that of pH-driven ammonia stripping. The wastewater stream is dosed with lime or caustic soda to raise the pH to above 11. This process converts ammonium ions to ammonia gas. Air is then passed through the wastewater, typically in a packed column or plate tower at a ratio of approximately 3,000:1. The effluent may be finally dosed with acid to reduce the pH to acceptable discharge levels. The big issue here is the cost of chemical additions to drive the pH adjustment. Unless a large and in-house source of lime is available the cost can be prohibitive.
Organics Ammonia Removal Solution
Thermally-driven ammonia stripping was developed by Organics to offer an alternative route to that of pH adjustment for continuous-process ammonia stripping. With a thermally-driven stripper no chemical additions are required, apart from the minor addition of anti-foam agent.
The single important input is waste-heat with which to drive the chemical reactions. If waste-heat is not available, the fuel cost can be as prohibitive as that of chemical additions or carbon source requirements. Where waste-heat is available, from an engine-exhaust or unwanted biogas, the long-term operating cost can be confined to plant Operation and Maintenance, as well as electricity costs.
The preferred option within the Organics process is to use the ammonia-laden air as combustion air in the heat-raising process. By this means ammonia gas is destroyed as a part of the process of thermally powering the system.
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