| Kitchen wastewater characterized with high contents of organic matter, would pose environmental problems by its acidification and decomposition. Anaerobic digestion is supposed as an effective approach for kitchen wastewater treatment due to its high efficiency for both organic materials removal and energy recovery. However, high contents of grease in kitchen wastewater might lead to serious inhibition to anaerobic methanogenesis, and reduce the efficiency of wastewater anaerobic digestion and stability of the system.Rhizopus chinensis lipase was applied for hydrolysis of kitchen grease in this paper. Firstly, properties of Rhizopus chinensis lipase and its optimum hydrolysis conditions were investigated. Secondly, anaerobic digestion performance of kitchen wastewater with grease by lipase pretreatment was carried out with batch experiments under different conditions. Finally, the effect of enzymatic pretreatment on kitchen wastewater anaerobic digestion in a continuous anaerobic membrane bioreactors(An MBR) was investigated. The main results were as follows.(1) Kitchen grease hydrolysis by Rhizopus chinensis lipase were investigated under different conditions of enzyme dosage, ratio of water to oil, p H and temperature. Results showed that the appropriate ranges of kitchen grease hydrolysis by lipase were enzyme dosage 0.30%~2.00%, ratio of water to oil 0.30~1.50, p H 7.0~9.0 and temperature 35.0~50.0 oC. Further optimization of kitchen grease hydrolysis conditions with response surface methodology showed that the optimized conditions were determined as enzyme dosage 1.15%, ratio of water to oil 0.9, p H 8.0, temperature 42.5 oC. Under the above-mentioned conditions, the hydrolysis rate was predicted at 86.6% and the repeated experimental result was 88.1±2.2%, which revealed that the response surface model could well predict the influence of 4 factors on the hydrolysis reaction.(2) Anaerobic digestion performance of kitchen wastewater under different conditions such as lipase concentration, F/M and pretreatment time were investigated. Results showed that the cumulative production of methane from kitchen wastewater anaerobic digestion by lipase pretreatment were higher than the control group, and the second peak of daily methane production showed relative delay with increase of lipase addition. The cumulative methane production revealed an increase of 124.88%, 97.27%, 75.06%, 66.11% and 57.62% with the lipase addition of 0.10%, 0.50%, 1.00%, 5.00% and 10.00% respectively, which indicated that appropriate enzyme pretreatment could improve methane production during kitchen wastewater anaerobic digestion. Moreover, VFAs, NH4+-N and lipase activity were gradually increased with increasing of lipase addition. For F/M experiments, results showed that both low F/M(0.25) and high F/M(4.00) might exert negative effects on methanogenesis. Cumulative methane production increased from 672.19 m L/g·COD to 844.46 m L/g·COD, when F/M was risen from 1.00 to 2.00. p H and VFA were observed to be stable in the normal range during processes. Although NH4+-N concentrations were found to be accumulated to 1716.26 mg/L, 1787.78 mg/L and 1852.79 mg/L(higher than half inhibition concentration 1500 mg/L) respectively in 3 batch experiments, daily methane production showed high level during processes. Although grease hydrolysis efficiency was influenced by the pretreatment time, second peak of daily methane production and cumulative methane production revealed less correlation. Shorter pretreatment time showed a significant increase in lipase activity at the early stage of anaerobic digestion.(3) According to the results of batch experiments under different conditions, the appropriate condition ranges of kitchen wastewater anaerobic digestion were determined as lipase addition 0.10%~5.00%, F/M 1.00~2.00 and pretreatment time 0~48 h, respectively. Further investigation was carried out to optimize the above-mentioned 3 factors by BBD during kitchen wastewater anaerobic fermentation, with accumulative methane production as the response value. Results showed that optimized conditions were determined as lipase addition 0.10%, F/M 1.50, and pretreatment time 48 h, respectively. Under these conditions, the accumulative methane production was predicted at 700.29 m L/g·COD, which was closed to the repeated experimental result(723.89±46.16 m L/g·COD) and within the 95% confidence interval. This indicated that the response surface model could well predict the influence of 3 factors on kitchen wastewater methanation.(4) Effect of enzymatic pretreatment on fermentation of kitchen wastewater was continuous investigated in 2 An MBRs. The reactors start-up and water volume loading enhancement was according to the ratio of VFAs/alkalinity, and operation was ensured with 95% total COD removal efficiency. Results showed that biogas production in the pretreated group was 11.11% higher than that of the control group, and average methane content of biogas from the pretreated group was 9.09% higher than that of the control group, under the same condition of volume load 18 kg COD/(m3·d). This revealed that enzymatic pretreatment could improve biogas production and methane content by enhancing methane conversion efficiency. During processes, p H of the system were observed to be stable in the range of 7.3-7.8, indicating good stability of the An MBR. Furthermore, fluctuation of VFAs in the pretreated group was less than that of control group during operating process, which indicated a much less inhibition by grease and a higher microbial activity was beneficial to maintain the stability of VFAs in anaerobic system with enzymatic pretreatment. Lower membrane cleaning frequency and integral membrane pollution cycle(3 times) and TMP average rising rate(6.29 k Pa/d) of the pretreated group indicated that less grease in the anaerobic tank after enzymatic pretreatment had a lower adhesion on the membrane surface, which was benificial to alleviate membrane fouling. |
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