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Leachate Treatment

Ammonia Air Stripping

Ammoniacal-N removal by Air Stripping

Ammonia can be removed from leachates as a gas, using air stripping, as an alternative to biological nitrification. Ammonia dissolves in water to form the ammonium ion in the following manner: NH> +H 2 O= NH 4 + +OH - Ammonia gas ammonium The relative proportions of dissolved ammonia gas, and of ammonium ions, depend on the pH-value and the temperature of the water. Only the ammonia form is removed (as ammonia gas) by air stripping, and at normal temperatures and neutral pH-values in leachates or other waters, only a small proportion (<2 percent) of the total ammoniacal-N will be in the gaseous ammonia form.


At raised pH-values or temperatures, concentrations of dissolved ammonia gas adjust to an equilibrium between liquid and gaseous phases, and ammonia can be stripped from the liquid within the gas stream (usually air). The efficiency of the process is increased significantly by increasing values of pH or temperature, and with increasing efficiency the quantity of air required will decrease, and the concentration of ammonia gas in the exhaust air increases. Typically, either pH values in excess of 10.0, or temperatures in the order of 60-70C, are needed to achieve greater than 80 percent of ammoniacal-N in the gaseous ammonia phase, to provide an efficient removal process.


Unlike many other treatment processes, the required air volume removes a constant percentage of the incoming ammonia, regardless of influent concentrations in leachate, the progressive removal of ammoniacal-N therefore operating in a half-life manner. This has two consequences first, at very high concentrations of ammoniacal-N, the stripping process is increasingly cost-effective; and second, it becomes difficult or costly to achieve low effluent concentrations of ammoniacal-N, such as below 50 or 100 mg/l. On this basis, ammonia stripping will generally only prove to be cost-effective, where partial pre-treatment is required, for example, prior to discharge into the public sewer, or before further removal of ammoniacal-N in a subsequent stage of biological treatment.

In achieving relatively low effluent values of ammoniacal-N (e.g. <50 mg/l), very large volumes of air will be required and this generally makes air stripping uncompetitive in cost terms for such applications.

Process Type:

Ammonia stripping can be carried out in tanks or lagoons, packed towers, or in counter-current, multi-stage reactors. A consequence of optimisation of the process, to achieve reduced aeration requirements, is that air containing high
concentrations of ammonia gas (to tens of grammes per cubic metre, equivalent to 10 percent by volume), can be released. This is likely to cause unacceptable health hazards at most sites, and must therefore be controlled. One option
would be absorption of the ammonia in sulphuric acid, to produce ammonium sulphate, which may have potential for use as an agricultural fertiliser.

Another possible solution is thermal destruction of the ammonia to nitrogen gas, ideally within a high efficiency landfill gas flare.

A few based on alkali dosing have failed, or rapidly been abandoned, as a result of environmental impact, operational difficulties, or excessive cost of reagents. At least one plant in the UK plant uses leachate heating to enhance the stripping of ammonia.

In recent years this technology has become established as a pre-treatment step for leachates in Hong Kong, from some of the largest landfill sites in the World (e.g. see Eden, 2001).

At three initial sites where such systems were installed in Hong Kong, leachate flows were typically in the range 720-1800 m3/d, and concentrations of ammoniacal-N of 6700 mg/l in leachate were used for design purposes. The plants could efficiently remove these high concentrations of ammoniacal-N down to below 100 mg/l, before subsequent biological treatment of effluent in sequencing batch reactor (SBR) plants. Landfill gas was used to raise leachate temperatures to 70C before passage to the stripping tower, and effective thermal destruction of ammonia gas >99.99 percent) has been achieved within the landfill gas flare.

Advantages:

None known to the author when compared with other available technologies, including biological treatment.

Disadvantages:

According to the EA (UK) Guidance for the Treatment of Landfill Leachate Page 45 of 187 Final DRAFT - Sector Guidance Note IPPC S5.03, the treatment of 250 m3/d will involve a plant costing in excess of £2.5M (GBP) (UK 2005 prices).

Very large quantities of acid and other chemicals are involved, for example, about 1 percent by volume of 32 percent w/v hydrochloric acid (i.e. 10 litres of acid per m3 of leachate treated). This is not only expensive, but if leachate is concentrated by a factor of 20 times, will in itself result in concentrations of chloride in excess of 60,000 mg/l of chloride in the concentrate/sludge.

Typical chloride levels of 2,000 mg/l in leachate would raise total concentrations of chloride in sludge to greater than 10 percent by weight.

Operation is relatively labour-intensive, estimated at 2 hours per day for a skilled operator.

A further and by no means least issue is the control of air emissions from the process.

Where best used:

None known to the author when compared with other available technologies, including biological treatment.

Costs comments:

No significant cost advantages when compared with other testament techniques.

Sustainability comments:

Major sustainability disadvantages due to the disposal requirements for the chemicals used.

Energy usage comments:

Not particularly high energy use.

Chemical usage/by-product production:

High. Highest of all the leachate treatment processes and by-product production is equally so.

 


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