| Waste activated sludge (WAS) originated from the wastewater treatment plants (WWTPs) in China contains many hardly degradable organic matters and nutrients of nitrogen and phosphours. It will be harmful to environmental security and human health if WAS is disposed only with reduction process. At present, anaerobic acidification is one of methods that can realize WAS harmlessness and resources. During the process of anaerobic acidification, many organic matters have been converted into biodegradable substances, such as volatile fatty acids (VFAs) and ethanol, which can be applied to the wastewater treatment for solving the problem of the shortage of carbon sources and improving nutrient removal efficiency. However, the reliability of VFAs generation is often not adequate for denitrification bacteria in fact, so a proper control of conditions could be essential to obtain the maximal VFAs and ethanol. Moreover, VFAs composition varies for different conditions, which also has some effects on denitrification rate and nitrate removal rate. Thus, based on the exited studies of anaerobic acidification at home and abroad, this study aimed to investigate the influences of several factors on WAS acidification liquid using single factor method and response surface methodology (RSM), and then investigated wastewater denitrification using different VFAs composition as carbon source in an upflow anaerobic sludge blanket (UASB) reactor.Firstly, initial pH, oxidation reduction potential (ORP) and shaking rate were controlled independently to study soluble chemical oxygen demand (SCOD), VFAs and its composition in WAS anaerobic acidification, and then determine the optimal value of each factor. The results indicated that when the initial pH was 6.0 or 8.0, most degradable substances in sludge were decomposed rapidly, VFAs maximal production was 340.7 mg/L and 325.8 mg/L respectively, and acidification rate of WAS rose to 65.9% and 63.6%. The initial ORP had a little influence on VFAs production, and its composition almost remained constant during fermentation process, but the lower initial ORP could increase the contents of acetic and valeric acids so that the optimal initial ORP was 200 mV. The rates of hydrolysis and acidification were speeded up with proper shaking rate which was also related to fermentation time. When the condition was 60 r/min at 4 h or 150 r/min at 12 h, VFAs concentration was maximal to 318.0 mg/L and 278.2 mg/L respectively, and acidification rate was 65.3% and 59.3%. In addition, it was beneficial to the accumulation of acetic acid at higher shaking rate.Considering interaction of several factors, sludge suspended solids (SS), initial ORP and shaking rate were chosen to make up different conditions using RSM in anaerobic acidification of pretreated sludge with solubilization by thermophilic enzyme (S-TE). The optimum conditions were determined by analyzing response surface contour plots and the model equations. The results indicated that compared with SS, VFAs yield enhanced at lower initial ORP and shaking rate which might be caused that the lower initial ORP could make the ORP of fermentation process suitable for the growth and acitivities of hydrolysis and acidification bacteria, and the lower shaking rate could increase contacts of organic matters and microbes. The maximal VFAs was 1400.8 mg/L which was 29 times than the raw sludge when the condition was 8.0 g/L、0 and 60 r/min. Similarly, SCOD content was little effected by SS and its predicted maximum was 8331.7 mg/L with the optimal condition of 8.0 g/L, 143.7 mV and 60.0 r/min. In addition, the SS was closely related to shaking rate on accumulation of acetic acid and ethanol. Initial ORP could change the fermentative type which influenced productions of propionic and butyric acid. The interaction of SS and initial ORP had more influence on valeric acid and protein. However, the role of three factors in generating carbohydrate was not clear because of its little content, which the reason might be that carbohydrate degradation occurs prior to other substances in the fermentation of anaerobic bacteria.The application of VFAs on denitrification process was studied, and the results showed that:(1) When acetic and propionic acids with different proportion were used as carbon source, the denitrification rate and nitrate removal efficiency performed well. Their maximum appeared when acetic/propionic was 1:2, which was seconded by 1:1. In addition, the nitrate removal could be enhanced at more propionic acid inVFAs which proved that propionic acid was more suitable for carbon source in detrification than acetic acid; (2) Different proportions of acids-acetic, propionic, butyric and valeric-had significant influences on wastewater denitrification. The optimal proportion was 30%:60%:5%:5%, and the corresponding concentration of each acid was 482.5 mg/L,205.7 mg/L,52.3 mg/L and 26.9 mg/L. Although the denitrification rate was not fastest in the above proportion, nitrate content in effluent declined continually with reaction time and reduced to below 2.0 mg/L. The removal efficiency of nitrate increased to 97.5% which indicated that the depth of sewage denitrification could realize in the proper ratio of VFAs composition. Moreover, when acetic and propornic acids were main components of VFAs, adding butyric and valeric acids contents could also promote the rapid degradation of nitrate. |
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