Anaerobic treatment of domestic wastewater in subtropical regions

In this thesis, the application of upflow anaerobic sludge bed (UASB) reactors for the treatment of low-strength domestic sewage was studied for the city ofSalta, in northwesternArgentina. The climate in this region can be defined as subtropical with a dry season. Mean ambient temperature in the city is 16.5ºC. Mean sewage temperature during the experiments was 23.0°C (monthly minimum: 17.2°C; daily minimum: 12.6°C). A literature review on the use of upflow reactors for sewage treatment was performed, and a brief description of laboratory, pilot-scale, and full-scale applications from all over the world is presented.

Experiments were performed in two pilot plants especially built for this thesis. The first pilot plant was a UASB reactor installed after a conventional full-scale sedimentation tank (settler). The second pilot plant was a two-stage UASB system with posttreatment in five waste stabilization ponds (WSP) in series. In the first pilot plant, chemical oxygen demand (COD) removal efficiencies up to 84% in total COD and 92% in suspended COD were achieved at a mean hydraulic retention time (HRT) of 2 h in the settler and 5.6 h in the reactor, equivalent to an upflow velocity (V _up ) of 0.71 m/h. A granular sludge developed in the UASB reactor probably due to the low concentration of suspended solids (SS) and COD in the influent and an adequate combination of HRT and V _up . Some of the granules were surprisingly big (up to 5 cm in diameter). The system studied was highly robust andefficient,it consistently delivered a final effluent in compliance with discharge standards for COD and SS, and produced a small amount of well-stabilized sludge. In the two-step UASB system of the second pilot plant, COD removal efficiencies up to 89% were obtained at mean HRTs of 6.4 + 5.6 h (V _up = 0.62 + 0.70 m/h), with 83 and 36% removal in the first and second steps, respectively. The effluent concentration was similar to that obtained in the first pilot plant. The performance of the two-step system was not affected during the coldest period of the year, which usually lasts about three months. The anaerobic sludge showed good stability, especially in summer time, and could be directly disposed of without further treatment. After the posttreatment, the effluent also complied with discharge standards for pathogenic microorganisms. It was concluded that a single-stage UASB reactor followed by a series of WSP could be a very efficient, reliable, compact, and simple system for the treatment of raw sewage in subtropical regions likeSalta.

Finally, a comparative assessment of the sustainability of three technological options for sewage treatment was performed, in terms of a series of technical, environmental, social, and economic criteria and indicators. In this preliminary assessment it was found that, under local conditions, a system UASB + WSP was more sustainable than (a) an aerobic high-rate treatment system based on trickling filters, and (b) a system of conventional WSP. The assessment method used (a multi-criteria weighted-scale matrix) was simple to perform and sensitive enough to detect differences in sustainability between the options compared. A representative panel of local stakeholders must perform the actual assessment in a transparent and participatory way before any concrete policy decision is taken. The final plead of this thesis is that sustainable development will only be achieved through a fully democratic way of decision-making that can go beyond political and economic motivations and that may be able to solve environmental problems and social injustices.