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.

Stringent effluent quality programs to limit wastewater discharges into receiving waters require extensive upgrading of conventional wastewater treatment plants. Large facilities built some decades ago are now often located in densely urbanised areas where land is unavailable. Since nitrogen and phophorus removal often require additional unit processes, innovative solutions have to be found to upgrade existing plants for nutrient removal. This paper shows large scale examples of compact technology and the additional upgrading flexibility provided. New facilities are implemented in sensitive neighborhoods by creative siting under sports stadiums, parks or buildings. In covered plants, air emission control becomes of primary importance. To reduce visual impacts and facilitate odour control, more and more underground treatment plants are constructed, allowing multiple use of plant surfaces. Several plants are illustrated in inner-city locations, avoiding infrastructure cost to pump sewage to remote sites. Most of the presented plants incorporate spacesaving settling facilities and high rate biological reactors to reduce the ‘footprints' of the installations and thus favour coverage. Parallel plates in primary setllers reduce the surface to about one tenth of conventional systems. Biocarbone aerated filters combine biodegradation with very high removal rates and retention of particles in one reactor, without additional clarification or filtration. Air treatment for large plant is mostly performed by chemical scrubbing, completely eliminating environmental nuisances. Performance results of both air and water treatment technology are given. Examples include recent sewage treatment plants on the French Mediterranean Coast. A physico-chemical treatment plant for 1 Million p.e. has operated since 1987 under a stadium in Marseille. In Monaco, the sewage treatment plant for 100 000 p.e.is located in the city center underneath a building of 3000 m2. Primary lamella settlers are followed by biological treatment on Biocarbone aerated filters and air is chemically deodourised. Similar technology is used in Antibes' 200 000 p.e. plant, integrated underneath a park close to the beach.

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