Wastewater from institutions are considered with low strength and they may have a low C/N ratio and pH values higher than 7.0 units. In developing countries, anaerobic treatment for low strength wastewater is widely applied. However, COD removals and biogas production can be low due to inhibitions in the process. To improve the effluent quality, aerobic posttreatment has been used. The aim of this study was accessing the influence of pH and C/N ratio on methane yield of institutional wastewater, taking into account the aerobic sludge recirculation towards the anaerobic reactor. A factorial design 32 was used, evaluating different pH values: 6.9±0.1, 7.5±0.05 and 9.0±0.5; and C/N ratio: 4.9±0.2, 8.2±0.18 and 14.2±0.17. Biochemical methane (BMP) tests were performed using as inoculum a sludge with the hydrolytic, acidogenic and specific methanogenic activity of 2.79 gCOD/gVSS-d, 2.80 gCOD/gVSS-d and 0.14 gCODCH4/gVSS-d, respectively. Results indicating that with high C/N ratio, the methane yield increased. The maximum BMP was 318 L CH4/kgVSS with pH 7.5 and C/N ratio of 8.2±0.18; however, with the C/N ratio of 14.2±017 a major COD removal and methane production rate were observed.
An anaerobic membrane reactor (AnMBR) treating municipal wastewater was evaluated. The experiments were performed using a pilot-scale up-flow anaerobic sludge blanket reactor with a submerged tubular ultrafiltration membrane at a hydraulic retention time of 8 hours. The system worked at an intermittent filtration mode (4 min on/1 min off) with and without nitrogen gas bubbling during the relaxation time (IF4NP and IF4P, respectively). The chemical oxygen demand (COD) removal achieved by the AnMBR was 68.6% and 87.9% for IF4P and IF4NP. Nitrogen bubbling also improved the filtration performance, as the elapsed time to reach 40 kPa for IF4NP and IF4P were 443 and 108 hours, respectively. Results show that intermittent filtration combined with nitrogen bubbling during the period of relaxation was an effective operation strategy in order to minimize membrane fouling and to increase COD removal.
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