Robustness of UASB Reactors Treating Sewage Under Tropical Conditions

This PhD thesis presents results and discussions to elucidate the matters of performance and robustness of the Upflow Anaerobic Sludge Blanket (UASB) reactors for the treatment of municipal wastewater in tropical countries. The research focuses on the main operational parameters (hydraulic retention time -HRT, influent COD concentration - COD Inf , organic loading rate - OLR, and sludge retention time - SRT) that affect the UASB loading potentials and its performance in "steady state" conditions, and on the response of the system when submitted to transient conditions. The experimental investigation was performed using 11 pilot-scale UASB reactors (120 L) which were organised into three sets: Set 1, five reactors were operated with the same hydraulic retention time (HRT = 6 h) and different COD Inf , ranging from 92 to 816 mg/L. Set 2, four reactors were operated with approximately the same COD Inf (~800 mg/L), but with different HRTs, ranging from 1 to 6 hours. Set 3, the HRTs were identical to the second phase but the COD Inf was adapted to have approximately the same OLR in the four reactors (~3.3 kgCOD/m 3 .day). In the first experimental part of the research, data was collected in order to evaluate the "steady state" performance and robustness of UASB reactors on the basis of COD removal efficiency, effluent variability, and pH stability. After the "steady state" condition was achieved, the sludge of each reactor was tested in terms of Specific Methanogenic Activity (SMA), biodegradability, settleability and, expansibility. In the second part of the experimental research, the robustness and stability of the system were evaluated under hydraulic and organic shock loads. Four indicators were defined for that purpose: COD removal efficiency, effluent variability, pH stability, and recovery time. Under "steady state" conditions, UASB reactors can treat sewage with COD as low as 200 mg/L, and HRT as low as 2 h, but the maximum efficiency is achieved with an HRT longer than 4h, and COD Inf higher than 300 mg/L. Effluent variability is highly dependent on the influent variability, showing that the reactors do not attenuate the daily fluctuation of the COD Inf . UASB reactors treating sewage in tropical countries are extremely stable with regards to pH and buffer capacity. With regards to the biological properties of the sludge, the reactors operated with a short HRT produce sludge with a high SMA. Moreover, sludge of reactors operated with a long HRT and with a low COD Inf resulted in low biodegradability. With respect to the hydrodynamic characteristics of the sludge, reactors operated with a high COD Inf and/or a short HRT produced sludges with high settleability and low expansibility. Results show that it is useless to design a UASB reactor with a longer HRT to cope with organic or hydraulic shock loads. Under shock load conditions the reactors resulted in COD removal efficiencies in the same range as during "steady state" conditions. The effluent COD fluctuates in the same range of the COD Inf variation, showing that the reactors are unable to attenuate strong variation in the OLR. The recovery time from a shock load is always very short, as the reactors needed less than 18 hours after the shock ceased to resume performance. The reactors showed signals that they would acidify if the organic shock load continued. However, hydraulic shock loads barely affected the pH stability of the reactors. Finally, the results obtained during this study showed that this kind of reactor is very robust with regards to the COD removal efficiency, as it keeps its maximum performance, in either "steady state" or transient conditions. Regarding to the pH stability, the UASB reactors also demonstrated extraordinary robustness, even when they are operated under extreme operational situations, which rarely occur in municipal wastewater treatment plants. When a shock load is imposed they need a very short time to recover. However, the UASB reactors are not robust with regard to effluent variability. Regarding this matter, the robustness of the reactors can be improved with the implementation of a secondary settler.