Parameter optimization in anaerobic co-digestion of NaOH pretreated sugarcane leaves and pig manure-dairy manure

Abstract: Tropical agriculture in south of China produces a large amount of tropical agricultural wastes every year including sugarcane leaves (SL). According to statistics, the annual output of SL in China is as high as 36 million tons, while the utilization rate is less than 20%. Lots of SL is discarded or even incinerated, causing environmental pollution. At the same time, South China is traditional dominant area for pig and herbivore due to rich and cheap feed. A great amount of animal manure (AM) is produced every year, which causes ecological and environmental pressure. Anaerobic digestion (AD) technology is considered to be an effective way to deal with wastes from planting and breeding. However, the low efficiency and biogas production resulting from large amount of lignocellulose limits large-scale application of SL. The biodegradability of SL can be effectively improved by pretreatment, and the anaerobic co-digestion can improve the biogas production performance of SL and AM. In this paper, the anaerobic co-digestion performance of AM and SL pretreated by NaOH was studied. The AD performance of pretreated SL with pig manure (dairy manure) at different mixing ratios (2∶1, 1∶1, 1∶2, 1∶0, 0∶1) was investigated. After that, the effects of temperature (30, 35, 40 ℃) and C/N (20∶1, 25∶1, 30∶1) of modified SL, pig manures and dairy manures were determined with Central Composite design method. The results of the tests indicated that the cumulative gas production of the mixed materials was significantly higher than that with single material and the best mixing ratio was 1:1, with which the values reached 6 075.57 and 5 903.61 mL respectively and were 8.13%-15.01% higher than that without pretreated SL or AM. The modified Gompertz model could simulate the kinetics of anaerobic co-digestion of SL and AM, and the correlation coefficient was more than 0.998. The synergistic effect was found in anaerobic co-digestion and resulted in nutrients balance and promotion of organic matter. Regression model analysis showed that the temperature and C/N had significant effects on AD. The optimal process conditions were determined by response surface methodology, that the temperature was 36.5 and C/N was 27. The measured values of biogas production of the mixture of SL and AM was 337.5 mL/g under the optimized conditions above, which was very close to the predicted value (331.92 mL/g). The model verification results showed that RSM could be used to optimize the AD process parameters of mixture.