Investigation and Experience with Post Treatment for UASB Reactors in Brazil

The use of anaerobic treatment for domestic sewage is under current expansion in Brazil. Anaerobic treatment is already a well known technology in Brazil, the main units designed and constructed being the Upflow Anaerobic Sludge Blanket reactors – UASB.They present many advantages, such as being a compact system, with practically no equipment in the anaerobic vessel, low construction and operational costs, very low energy consumption, low excess sludge produced. However it presents an important disadvantage, which is the limited efficiency for BOD, COD and TSS removal: around 45 to 75% for COD removal, 55 to 75% for BOD removal; BOD effluent concentration always above 60 mg/L, up to 120 mg/L; TSS effluent concentration between 40 and 80 mg/L; less than 1 logarithmic order for FC removal; practically no efficiency for N and P removal.In order to avoid poor effluent quality, and meet legal water quality standards, post treatment has been investigated and practiced, with excellent results, and great reduction in investment costs.Several new plants are designed or constructed in Brazil, according to such investigations, some of them with a flow as high as 3.0 and 1.5 m3/s (68 and 34 mgd). Such secondary wastewater treatment plants preceded by a UASB reactor have proven several advantages and benefits: total construction and operational costs comes out to be lower than that of the conventional primary plus secondary plant; the plant can be implemented in stages, according to available funds, with effluent quality in the UASB first stage better than in the conventional primary treatment phase.Seen as pretreatment for a secondary plant, the UASB can double the BOD or COD removal rates of conventional primary treatment, while at the same time producing lower volume of sludge. Total construction and operational cost for such type of plant comes out to be lower than that of the conventional primary plus secondary plant. Also, according to available funds, the plant may be constructed in stages: first the UASB reactor, with effluent quality better than conventional primary treatment; afterwards, as the second phase of the project, an aerobic polishing treatment.The paper describes the main features of the investigations and presents the new projects undergoing design or construction, concluding that benefits from the use of this treatment train are clear. It may be accepted as very feasible, particularly in warm weather developing countries, where construction costs are always a limiting factor for sewage treatment expansion.