The new European Guideline on municipal wastewater discharges will require nitrogen removal in all sensitive areas below 15 mg TN/l. To achieve this limit within reasonable time and budget, infrastructure cost has to be minimised. To construct new sewage treatment plants close to urban neighborhoods or to upgrade existing facilites within current plant boundaries, high-rate reactors for nitrogen were investigated. The retained option was an upflow aerated filter on a floating granular media.

The lightweight media facilitates backwashing, and an anoxic zone at the filter bottom achieves denitrification. After extensive pilot tests to quantify the performance of the reactor, a demonstration plant of one full-scale filter cell was constructed and operated for two years to verify technological as well as process options. The dynamic behaviour of the reactor could be assessed by continuous reading of ammonia and carbon pollution. This paper describes the results obtained and the observed system limits.

The reactor was fed with primary settled wastewater containing about 450 mg COD/l and 65 mg TKN/l. Filtration velocity was varied between 1 and 2 m/h, and 100 to 300 % of the feed flow was recirculated. The removal rates at different temperatures were measured, under an applied load of 5 kg COD/m3 d, nitrification and denitrification capacities in the respective zones were around 1 kg N/m3 d at 15 °C. Up to 50 mg TN/l could be removed and the required effluent quality of the EC guideline could thus be achieved with an empty bed contact time of 2 hours. An additional on-site pilot test showed that within this same detention time, total nitrogen residuals down to 5 mg/l could be achieved at 10 °C.

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