Aeration control for simultaneous nitrification-denitrification in a biological aerated filter using internal model approach.

Experience has shown that simultaneous nitrification/denitrification in a biological aerated filter is possible. However these systems react very sensitively to the aeration control strategies applied. Poorly adapted control strategies induce a strong decrease in treatment efficiency. A new control strategy for simultaneous N/DN is developed. The strategy proposed makes use of ammonia measurements and the inlet and outlet: a Feedback/Feedforward block. Passing by a calculation of the load to be eliminated, an estimation of the air flow velocity to be applied is carried out dynamically. A retroactive loop corrects this prediction in order to reach exactly the desired set point. This control approach has been implemented and tested at pilot plant scale for a period of 18 months. The pilot plant consists of two coupled BAF cells, reflecting closely industrial scale situations. Comparative studies reveal clearly the improved performance of the developed FF/FB control strategy compared to classical controllers. The benefits include 5% increase in nitrogen removal performance and a reduction of 15 to 20% in air requirement, offering a rapid return of investment costs.

Experience with an up-flow biological aerated filter (BAF) for tertiary treatment: from pilot trials to full scale implementation.

Biofilters can be added to existing non-nitrifying activated sludge plants for tertiary ammonia removal and effluent polishing. It is a convenient, efficient, and cost effective way of meeting more stringent consents. In order to prove this technology, a biofilter pilot plant was installed in a large activated sludge plant with challenging conditions, since almost half of the load is industrial effluent and the water temperature can be as low as 7 degrees C. Trials were conducted over a two year period, providing the following information: low water temperature does not affect the process; the optimum rise rate at nominal flow is 8.5 m/h for this wastewater; the consent can be achieved for ammonia loads up to 0.94 kgNH4N/M3 media/day; about 50% of the post-secondary TSS and BOD can be removed in the BAF; a large proportion of industrial effluent has not had any noticeable effect on the beads after two years. A full scale plant with a nominal flow of 8257 l/s was then built and commissioned in summer 1998. Seeding was completed within three weeks of starting the filters. Two months later, a 28 day takeover was started. Results obtained on full scale plant during and after this test confirm the results obtained on the pilot.

Simultaneous nitrification and denitrification in biofilters with real time aeration control.

The aim of this article is to present a new biological aerated filter (BAF) for nitrogen removal based on simultaneous nitrification and denitrification. Contrary to the systems which integrate both an aerated and a non-aerated zone to allow complete nitrogen removal in one compact or two different units (pre-denitrification and nitrification), this upflow BAF system is based on the principle of simultaneous nitrification and denitrification since the filter is completely aerated. The denitrification process is possible due to the diffusion effect which dominates biofilm processes. The real time aeration control allows us to maintain a low dissolved oxygen value (0.5 to 3 mg O2/l). In this case, the biofilm will not be fully (or less) penetrated with oxygen and denitrification will be carried out in a large part of the biofilm. Therefore, nitrification and denitrification is running simultaneously in different depths of the biofilm. By using 50% less air this BAF gave the same results (less than 20 mg TN/l) on pilot plant as a classical nitrification and denitrification BAF (Toettrup et al., 1994). Less recirculation was necessary to achieve the same denitrification.