Physical/chemical elimination of ammonium from digester supernatant with magnesium-ammonium-phosphate (MAP) precipitation and with air stripping is investigated in pilot scale and compared with separate intermittent denitrification, denitrification in wastewater treatment and in tertiary filtration. MAP-precipitation is feasible but most expensive due to required chemicals as well as dewatering and drying of the precipitate. Air stripping is slightly cheaper, but still more expensive than biological treatment due to the complex process, cost of chemicals and reconditioning of the ammonium sulfate solution to a fertilizer product. In nutrient removal plants the additional nitrogen from the digester supernatant can be eliminated by increasing the anoxic volume or using a carbon source. Denitrification with methanol will be the cheapest solution with today's methanol price. If digester supernatant inhibits nitrification substantially or if the anoxic zone is too small for organic carbon addition a separate intermittent denitrification with addition of a carbon source, as used in some treatment plants in Scandinavia, might be a good solution.

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