Anaerobic treatment, such as the Upflow Anaerobic Sludge Blanket - UASB - has many advantages: a compact system, with practically no equipment in the anaerobic vessel, low operational costs, very low energy consumption, and low excess sludge produced.

However, taking into account its poor effluent quality, and the legal water quality standards, post treatment is a must. Brazil is experiencing the scheme UASB plus aerobic secondary treatment, aiming reduction in investment and mainly in operational costs, with excellent results.

Three cases are discussed in this paper: two small plants, Barreto (0,14 m3/s) and Itaipu (0.07 m3/s, 1.6 MGD); and the Rio Preto plant (1.34 m3/s), the first two already operational. All adopt the UASB plus activated sludge process, the last two with denitrification.

Several other important plants are in the stage of design or construction in Brazil, with flows as high as 3.35 m3/s, UASB plus activated sludge with nitrogen and phosphorus removal and UASB plus trickling filters.

Design criteria for the anaerobic reactor and for the different secondary treatment processes, and available operational data as well, are discussed in the paper. Particular attention is given to special restraints with activated sludge as post treatment, such as the higher sludge age required for nitrification, and the difficulty in denitrifying the anaerobic effluent.

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