Enhancement of nutrient removal in SBR systems through acid fermentation of raw wastewater

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Danesh, Shahnaz
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Acid fermentation of degritted raw wastewater was investigated using a sequencing batch mode of operation. The conditions of the experiments included sludge retention times of 4, 8 and 13 days, influent pH ranges of 7.0-7.6 and 6.1-6.4, mixing periods of 6 h/cycle and 0.25 h/cycle, and hydraulic retention time of 12 h, 9 h, and 6 h. The impact of prefermentation of raw wastewater on enhancement of nutrient removal in SBR systems was explored in conjunction with the fermentation process. This was done by developing a two-stage anaerobic-aerobic SBR system (PAF-SBR) in which the wastewater was first fermented in a sequencing batch reactor named PAF (Primary Acid Fermenter). The effluent from the fermenter was conducted to another SBR for carbon, nitrogen and phosphorus removal. The performance of this system was evaluated against a parallel conventional-SBR. All experiments were carried out at room temperature (20 $\pm$ 2$\sp\circ$C). It was found that acid fermentation of degritted raw wastewater is a stablereliable process. Acetic acid was the main component (78-96%) of volatile fatty acids (VFA) produced under the various investigated conditions. Sludge retention time (SRT) had a pronounced impact on the solubilization of organic matter and production of VFA. Both processes improved as SRT increased from 4 d to 13 d. Overall efficiency of VFA production diminished as the influent pH range was reduced from its natural range of 7.0-7.6 to the range of 6.1-6.4. Influent pH also affected VFA distribution. Acetic acid production was reduced at the pH range of 6.1-6.4, while production of butyric, aleric and propionic acids was enhanced. Acid fermentation was affected by the mixing period of each operational cycle. Specific rates of solubilization and VFA production were lower at the mixing period of 0.25 h/cycle. Volatile fatty acids composition was also influenced by the duration of the mixing period in each operational cycle. Reduction of HRT from 12 h to 9 h did not affect the fermentation process significantly. However the decrease from 9 h to 6 h was associated with a reduction in net solubilization and VFA production. Prefermentation of raw wastewater had a remarkable impact on the improvement of biological phosphorus removal. Regardless of the operational conditions, soluble phosphorus of the effluent from the PAF-SBR was always below 1 mg/L, while in the parallel conventional-SBR, the concentration ranged from less than 1 mg/L to over 4 mg/L with an average range of 1.5-2.6 mg/L. An operational cycle of 8 h with a long initial anoxic/anaerobic period of 2-3 h was found to deteriorate effluent phosphorus quality. Best results with respect to nitrogen and phosphorus removal were obtained in 6 h operational cycles employing intermittent aeration schemes. Phosphorus removal up to 90-94% was achieved in the PAF-SBR system leading to an average effluent soluble phosphoros content of 0.3 mg/L. Nitrogen removal efficiencies were 69% in the PAF-SBR system and 65% in the parallel conventional-SBR. Step feeding did not enhance nitrogen removal. The average effluent SS concentration was less than 20 mg/L. Bulking and deterioration of effluent quality were often problems in the conventional-SBR. The PAF-SBR system produced about 12% less sludge than the conventional-SBR system. (Abstract shortened by UMI.)