If landfill leachate is simultaneously poor in biodegradable organic carbon (BOD) and rich in ammonium nitrogen , i.e. BOD/N < 1 more or less, biological autotrophic nitrogen removal through partial nitritation and anammox (anaerobic ammonia oxidation) was proved to be an effective solution. Please refer, for instance, to Van Hulle et al.'s 2010 review 'Engineering aspects and practical application of autotrophic nitrogen removal from nitrogen rich streams', Chemical Engineering Journal 162 (2010), 1-20.

Physic-chemical treatment of the liquid by means of air stripping: adjustment of pH (pH>10, tests shall be performed using different types of available chemicals), solids removal (formation of insoluble compounds) from the liquid, stripping unit (packed column) with air, air flow cleaning-ammonia recovery with an acid scrubber (usually sulphuric acid, producing ammonia sulphate). Air can be recirculated. It is not a cheap system to operate, but it works.

Leachate from sanitary landfills is a strong wastewater in terms of organic matter and ammonia. Organic matter can be reduced by anaerobic plus aerobic treatment; however, ammonia reduction by nitrification often poses problems due to inhibition. Aerobic sequencing batch reactors (SBR): an activated sludge SBR (ASBR) and a granular sludge SBR (GSBR) showed good treatment performance of old landfill leachate.

It is an impressive question about landfill leachate. A high level of ammonium (>200 mg/L) can inhibit the activity of microorganisms found in biological treatment methods. However, Electrochemical oxidation (EO) processes are able to degrade simultaneously both the organic matters, especially biorefractory content in old landfill leachate and ammonium through production the free radicals such as OH and Cl.

For further information, you can refer to the manuscript addressed below:

Article Review on landfill leachate treatment by electrochemical oxi...

Vikalp Saxena