Nitrification and denitrification using biofilters packed with sulfur and limestone at a pilot-scale municipal wastewater treatment plant

A pilot-scale municipal wastewater treatment plant composed of a fixed film activated sludge (IFAS) system with sulfur-limestone autotrophic denitrification (SLAD) was operated for a year and the influence of different operational factors was investigated. Nitrification efficiency was found to be above 91% at temperatures above 25°C even at short hydraulic residence times (HRTs), but declined to 51 ± 2% when the temperature dropped to 22 ± 3°C. The minimum HRT (HRT_min) to achieve nitrification efficiency > 90% was found to be 12 h at temperatures above 25°C. Denitrification efficiencies were found to be 89% and 79% at a nitrate loading of 0.36 kg NO^-₃ -Nm^-3 d^-1 and at 0.18kg NO^-₃-Nm^-3 d^-1, respectively. The minimum empty bed residence time (EBRT) to achieve denitrification efficiency above 80% without methanol addition was 3h at a nitrate loading rate of 0.27-0.38kg NO^-₃-Nm^-3 d^-1. The amount of nitrate removed as a function of the sulfate formed was found to be 0.188g NO^-₃-N/g SO^2-₄. The nitrate load removed by the biofilter as a function of the alkalinity consumed was found to be very close to the theoretical stoichiometric value. The application of the pilot plant was proven to be feasible and the performance of the SLAD system, especially with respect to the minimum EBRT to achieve denitrification efficiency above 80%, the maximum denitrification rate and performance at temperatures below 10°C. To achieve a nitrification efficiency above 90% in the IFAS system, temperature changes and the minimum HRT were found to be the most influential operational parameters.