Assessment – Design – Solution
One of the principal contaminants that must be treated in landfill leachate, and in waste water in general, is ammonia. Ammonia-N in normally present in waste water or in leachate in several forms. Treatment is therefore essential.
Ammonia is a compound of nitrogen and hydrogen with the formula NH3. In its simplest form ammonia is a colourless gas with a characteristic 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 create anoxic conditions, it can be deadly to aquatic lifeforms.
Ammonia in raw domestic sewage is found in concentrations of about 30 mg/litre. In leachate from landfill sites and other organic wastewater streams it may rise to several thousands of milligrams per litre. As ammonia is biologically oxidised 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 has developed a thermally-driven ammonia stripper that requires no chemical additions apart from minor additions of anti-foam agent. The single important input is waste heat from, for example – the exhaust from on-site engines, which drives the chemical reactions.
Conventional ammonia removal 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 waste-water is not adequate to drive the necessary conversions to nitrogen gas. When a waste-water stream has very high ammonia levels (circa <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 waste-water 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 waste-water, 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.
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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