Experience with biofilters, acquired over many years, has made it possible to control their operating parameters. The goal of this article is to present an assessment of operational biofilters, in particular their air consumptions, their sludge productions, their energy balances as well as the duration of their filtration cycles. Some tests, carried out on medium to large full scale units, are described at the same time as the process results. These experiences in the design and operation of biofilters led to their optimization and make it possible to build very large scale installations. Examples of such plants, now being built or started up, are also presented, treating flows in excess of 50 000 m3/d even for low nitrogen residuals.

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.

Biological filters with fixed submerged beds are normally used for secondary or tertiary treatment and are recognized for their good performance. These processes are designed to meet, simultaneously, strict requirements of quality and operation. The achievement of the discharge consent requires the use of fine media which features a large support surface for the growth of biomass. On the other hand, the achievement of long filter run times is intrinsically related to the use of coarse media. Nowadays, all biological filters have mono‐media, which offers a compromise between these two conflicting criteria. The B2A process is a newly developed compact filter which ensures high filtration performance and long filter run times, even when used for crude sewage. The technology has been in use since 1992 for nitrification/ denitrification using an applied load of 0.7 kg/m3. d to obtain an effluent quality of 20 mg/l total nitrogen. Further tests have proved its efficiency in the removal of carbonaceous pollution. The B2A process is the first compact filter capable of treating screened crude sewage yet producing effluent qualities which are equal to that from conventional biological filters receiving settled sewage.