Experimental Study On Improving Degradation Of Sludge Lignocelluloses By Pretreatments

Under the dual pressures of the energy crisis and carbon emission reduction, the energy conversion of excess sludge via anaerobic digestion has regained intensive recognition nowadays. However, one major problem is that the energy conversion efficiency of excess sludge is still low, which is mainly attributed to the fact that there is often a large non-biodegradable fraction-lignocelluloses existing in the sludge.In this study, several pretreatment methods were tested to breach the structure of lignocelluloses in excess sludge, which could obviously facilitate the rate-limiting process-hydrolysis in anaerobic digestion where the stable organic macromolecules are broken into small molecule products, and improved methane production and overall energy conversion efficiency could then be expected.This study started with the analysis of the content and origin of lignocelluloses in the excess sludge. And the biodegradability of lignocellulose was elaborated with its structural characteristics. By comparing the similarities of sludge pretreatment and wood cellulose pretreatment in principle and technical aspects, proper pretreatment methods, which were effective in deconstruction of both microbial cells and lignocelluloses, were expected to be available.There were four pretreatment methods tested in the experiments:acid, alkaline, pyrolysis and ultrasonic. The results showed that higher extent of pretreatment resulted in better effect. In these4pretreatment experiments, the highest sludge COD dissolution rate and lignocelluloses reduction rate were as follows respectively:1) pyrolysis:50.9%and11.9%;2) ultrasonic:44.4%and5.1%;3) alkaline hydrolysis:33.9%and7.6%;4) acid hydrolysis:25.8%and5.6%. Although the direct lignocelluloses degradation after the pretreatments were still low, its structural stability had been destroyed which was conducive to the further biotransformation in the anaerobic digestionLaboratory scale anaerobic digestion testes were then carried out with the sludge pretreated by different methods mentioned above under SRT of20days. And higher methane production rates were obtained in all these tests with methane yield of81.4ml CH4/g VSS (pyrolysis),74.2ml CH4/g VSS (ultrasonic),67.2ml CH4/g VSS (alkaline hydrolysis) and65.7ml CH4/g VSS (acid hydrolysis) respectively. Compared with unpretreated sludge, the methane production rates were increased by53.7%,40.0%,25.7%and22.8%respectively. Meanwhile, high lignocelluloses degradation rates after anaerobic digestion were obtained as52.6%,43.5%,38.9%and39.1%for pretreatment of pyrolysis, ultrasonic, alkaline hydrolysis and acid hydrolysis respectively. In particular, cellulose and hemicellulose were almost degraded completely.In conclusion, the experiments indicate that applying appropriate pretreatment methods could break both microbial cells and lignocelluloses simultaneously, and thus facilitate the overall energy conversion efficiency of excess sludge. If these pretreatments are coupled with enriched lignocelluloses degrading bacteria, the energy conversion rate of excess sludge can be further improved.