| In recent years, with the improvement of people’s living standard, urban sewage emissions are increasing. How to treat fast and dispose efficiently has become a pressing issue in today’s society. The traditional methods of sludge treatment and disposal include incineration, landfill, etc. They not only consume large amounts of energy, but also are likely to cause secondary pollution. While the anaerobic fermentation with the feature of environmental protection and energy saving has been seen as a green and sustainable sludge treatment method. However, the difficulties of breaking sludge cell walls and the low ratio of carbon to nitrogen result in the low efficiency of sludge anaerobic digestion. Research shows that pretreatment helps to breaking cell walls. Besides, carbon investment can improve ratio of carbon to nitrogen. Therefore, pretreatment and carbon investment can improve anaerobic fermentation efficiency. In this study, corn straws were added into sludge as induce carbon source to promote the ratio of carbon to nitrogen. Then the straw and sludge mixture was thermal and alkali pretreated, respectively, and co-fermented to further promote breaking cell walls and increase the production of VFAs (Volatile fatty acids).Firstly, in order to determine the optimal co-fermentation temperature under condition of adding corn straws as induce carbon, a set of sequencing batch experiment was conducted. The results showed that high SCOD (soluble chemical oxygen demand) value could be obtained under 55 ℃. The maximum daily SCOD reached 4149 mgCOD/L. Under that condition, there were more butyrate acid and valerate transformed to acetic acid and formic acid, which could effectively utilize sludge resource and increase the production of VFAs. On the 4th day, under 55 ℃, the VFAs value of WAS (wasted activated sludge)+straw reached the maximum value of 4017 mgCOD/L, which was approximately 34-fold greater than that of raw sludge and 32-fold greater than that of non-pretreated sludge. Although the SCOD value of 4271 mgCOD/L at 90℃ was more than that at 55 ℃, the concentration difference was not obvious. Considering economic factors, the optimal pretreatment temperature was 55 ℃.Based on the optimal thermal pretreatment from sequencing batch experiment, co-fermentation with sludge and corn straws was conducted under thermal and alkali pretreatment respectively. It suggested that the groups adding corn straw to increase C/N indicated better performance of breaking cell walls:the soluble protein concentration of group WAS+straw reached 1443 mgCOD/L, which was 26-folded greater than that of non-adding corn straw. Besides, the performance of alkali pretreatment was better than thermal pretreatment whether adding corn straw or not:the soluble carbohydrate of group WAS+straw+pH10 reached 588 mgCOD/L which was approximately 1.43-fold greater than group WAS+straw+55 ℃ and 1.61 folded greater than group WAS+straw. The VFAs of each experiment group first increased then decreased. Acetic acid accounted for most of proportion in group that added straw. The maximum value of acetic acid of WAS+straw+pH10 could reached 6867 mgCOD/L. While propionic acid was the most component, accounting for 50%-60%, of VFAs in group that non-added straw. Seeing that acetic acid was the reactants of anaerobic methane-producing, adding corn straw played an important part in anaerobic fermentation process. In addition, using alkali pretreatment could obtain more acetic acid than thermal pretreament. Taking groups of adding straw as examples, the maximum daily acetic acid yield of WAS+straw+pH10 was up to 6869 mgCOD/L, which was 1.59 fold greater than WAS+straw+55℃. Thus, in terms of VFAs yield and cell walls breaking, the performance of alkali pretreatment was better than thermal pretreatment. |
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