Air/Ammonia Stripping Technology
Ammonia Stripping (also known as Air Stripping) removes the volatile contaminants such as ammoniacal nitrogen and Volatile Organic Compounds (VOCs) from leachate from liquid to the gaseous phase.
This is achieved by dosing the leachate to raise the pH to between 10.5 and 11.5, followed by air contact by aeration, or by percolation of the leachate downward through a column of aerated media fed by an upward air flow. The media over which the leachate flows is filled with a large surface area material, or may cascade through perforated trays.
Treated leachate is collected in a sump at the base of the tower, and recirculation of a proportion of the flow may be required. The off-gases produced during ammonia stripping are released from the tower.
The limitations of this technology are substantial, and include:-
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- the need for multiple passes to reduce the ammoniacal nitrogen concentration to levels which are suitable for discharge to sewers and watercourses;
- high operating costs due to the need to continuously add alkali and then reduce pH with acid dosing before discharge takes place;
- fouling (calcification) of the packed stripping tower and pipework, due to the formation of a combination of calcium carbonate scale, and oxides of iron (unless these are removed separately);
- a need to apply acid to the tower for cleaning, or remove and physically clean the trays) with the associated Health & Safety risks arising from handling and storage of these dangerous chemicals;
- downtime associated with cleaning;
- release of ammonia in the off-gases to the atmosphere with potential environmental and health effects as well, if people work or reside within the ammonia “fall-out zone”. (Plants of this type have been known to produce an off-gas of sufficient strength under some circumstances, to strip the paint from nearby parked cars.);
- potential for odour generation.
This type of plant technology has at times been utilised after a primary settlement stage, during which a flocculant (eg polyelectrolyte) is added. Again, this requires expensive tank and dosing infrastructure to achieve and odour generation from such tanks is substantial where average and strong leachates are present.
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