Experimental Study On Degrading Lignocellulosic Materials Contained Inexcess Sludge By White-Rot Fungi

The pursuit of “carbon netrual” in wastewater treatment plants’(WWTPs) operation revives the common theme of energy conversion of excess sludge. Among the energy conversion technologies, anaerobic digestion is commonly recognized as the preferred process. However, due to the weak policy caused by insufficient recognition, the application of anaerobic digestion in our country remains less common. In addition, the poor energy conversion efficiency and very low methane content in the biogas are two significant bottlenecks in application. Previous studies show that the high levels of lignocellulose in the excess sludge(~ 32% of sludge dry weight) and the refractory nature of lignocellulose greatly limit the anaerobic digestion efficiency. Although sludge pretreatment methods can effectively improve the biodegradability, the high cost limits their practical application.White rot fungi are the most effective microorganisms in nature to degrade lignin. This feature provides us with a potentially cost-effective method of biological pretreatment of lignocellulose. Based on this, this study tried to enrich white rot fungi to degrade lignocellulose in excess sludge, thereby promoting conversion efficiency of the refractory organics. This study investigated the effectiveness of a variety of white-rot fungi in degrading lignocellulose in excess sludge. Meanwhile, the ability of white rot fungus in enriching heavy metals was explored as well.In the first phase of the experimental study, edible mushroom(white rot fungus) was tested to investigate its effectiveness in treating dewatered sludge. Excess sludge after dewatering was used to cultivate oyster mushroom in laboratory conditions. After 50 days of cultivation, fruiting bodies grew out successfully, but the number and height of the fruiting bodies were less than that of the oyster mushrooms cultivated in regular conditions. Lignocellulose in the upper layer of the sludge was degraded to a certain extent. Lignin, cellulose and hemicellulose were degraded by 35%, 20% and 20.8%, respectively. Subsequently, the cultivation was carried out in a mushroom house. Oyster mushroom and shiitake mushroom were tested. A variety of cultivation schemes have failed to grow out of fruiting bodies, indicating that using excess sludge as the sole substrate is not likely to be successful in mushroom cultivation.In the second phase of the experiment, the model species of white rot fungi- P. chrysosporium strains were used in the pretreatment of the sludge. The strain was inoculated to the sludge, and secreted lignin-degrading enzymes to degrade lignocellulose during its growth. Multiple sets of comparative experiments showed that inoculation of Phanerochaete chrysosporium suspension into sterilized sludge achieved the best results, in which the degradation of lignin, cellulose and hemicellulose reached 58.9%, 25% and 25.8% respectively. When this pretreated sludge was applied as feedstock to anaerobic digester, the biogas production was increased by 33.6% and the methane production was increased by 40.3%. Meanwhile, in sludge samples inoculated with mycelium balls of Phanerochaete chrysosporium, content changes of four kinds of heavy metal including Cu, Zn, Cd and Pb were analyzed. The results showed that P. chrysosporium had a high Pb-enriching capability, with an enrichment of 92%.