Effect Of Oil Palm Waste Pretreatment On The Humification Of Co-composting With Paper Mill Sludge And Its Mechanism

Composting is one of the effective ways to recycle the increasingly widespread oil palm waste（OPW）and paper mill sludge（PMS）,which can promote the resource recycling and sustainable development of the oil palm industry and papermaking industry.However,the poor bioavailability of lignocellulose hinders the humification process of oil palm waste and paper sludge composting.Pretreatment technology can destroy the stable structure of lignocellulose,so as to improve its bioavailability.At present,pretreatment technology has been widely used in the resource disposal process of lignocellulosic waste,such as anaerobic fermentation and aerobic composting,but its influence on the humification process of composting is still unclear.This study took co-composting of OPW and PMS as the research object,and focused on the effects of acid-thermal pretreatment（ATP）and hydrothermal pretreatment（HTP）on the physicochemical properties,organic matter transformation characteristics and microbial communities of co-composting of OPW and PMS,with a view to provide technical and theoretical guidance for improving the humification efficiency of lignocellulose composting.The specific research content and results are as follows:（1）The influence of different pretreatment methods on the composition and structure of OPW:The orthogonal experiment of OPW pretreatment was carried out with temperature,time and acid concentration as factors.Based on the dissolved organic carbon content of OPW,120℃,1 h,6%（H₂SO₄,w/w）and 120℃,1 h were selected as the optimized ATP and HTP conditions,respectively.The effects of ATP and HTP on OPW fiber structure were studied by sugar concent determination,scanning electron microscope（SEM）and Fourier transform infrared spectroscopy（FTIR）.Results showed that both ATP and HTP could effectively destroy the lignocellulose structure of OPW.The results of sugar content showed that after ATP and HTP,reducing sugar content of OPW increased to 61.96 mg/g and 34.07 mg/g,respectively,and polysaccharide content of OPW decreased to 17.90 mg/g and 77.47 mg/g,respectively.SEM showed that after ATP,OPW fiber structure was broken,and a lot of folds and depressions appeared on the surface.HTP can also cause a lot of folds on the surface of OPW fiber,but it had little effect on the overall fiber morphology.FTIR analysis showed that the hemicellulose and cellulose of OPW were degraded duiring ATP and HTP,and the degradation degree was greater during ATP.（2）Effect of pretreatment on physicochemical properties of OPW and PMS co-composting:Acid-heat pretreated composting（ATPC）and hydrothermal pretreated composting（HTPC）were prepared by PMS with OPW after ATP and HTP respectively,the traditional composting（TC）without pretreatment was taken as control.The differences in temperature,organic matter content,C/N and other physicochemical properties of ATPC,HTPC and TC were compared and analyzed.It was found that the fermentation effect of ATPC and HTPC was obviously better than that of TC.ATP and HTP significantly increased the fermentation temperature and extended the thermophilic period to 14 and 16 days,respectively.At the end of composting,the organic matter content in ATPC and HTPC decreased to 54.93%and 55.2%,respectively,and the total nitrogen content decreased to 1.71%and 1.68%,respectively,while the organic matter content and total nitrogen content in TC were 56.54 and 1.42%,respectively.The C/N in ATPC and HTPC tended to be stable after day 30,while the C/N in TC still kept a decreasing trend at day 40.The C/N of ATPC and HTPC products were 14.85 and 14.46,respectively,which were significantly lower than TC（15.43）.These results show that compared with TC,ATPC and HTPC could effectively improve the fermentation temperature,accelerate the maturity of composting,and have obvious advantages in degradation of organic matter and nitrogen fixation.（3）Effect of pretreatment on organic matter transformation during OPW and PMS co-composting:The characteristics of organic matter transformation in ATPC,HTPC and TC processes were analyzed and characterized by UV-Visible（UV-Vis）spectroscopy and 3dimension-excitation-emission matrix（3D-EEM）spectroscopy combined with fluorescence region integral（FRI）and parallel factor（PARAFAC）analysis.With the progress of composting,SUVA₂₅₄ in HTPC increased from 0.06 to 0.11,while SUVA₂₅₄ in TC had no obvious change.At the end of composting,E₂₅₀/E₃₆₅ in ATPC decreased to 4.50,while E₂₅₀/E₃₆₅ in TC decreased to 5.46,indicating that the organic matter in ATPC and HTPC had higher humification degree and aromaticity.3D-EEM combined with FRI analysis showed that the formation rate of humic acid-like substances and the degradation rate of protein-like substances in ATPC and HTPC were significantly faster than that of TC.PARAFAC analysis suggested that the maximum fluorescence intensity(F_max)of humic acid-like substances in ATPC and HTPC products were59.19 and 56.04,respectively,and the F_max of protein-like substances were 40.80 and 43.95,respectively.The F_max of humic acid-like substances and protein-like substances in TC products were 48.16 and 51.83,respectively.These results proved that ATP and HTP could promote the humification process of composting and improve the quality of composting products.（4）Microbial mechanism of the influence of pretreatment on the humification process of OPW and PMS co-composting:The evolution of microbial communities during ATPC,HTPC and TC processes was explored using 16S r RNA high throughput sequencing technology,and the internal relationships among physicochemical properties,humification parameters,microbial communities and maturity degree during composting were analyzed by redundancy analysis（RDA）and least partial square path model（PLS-PM）analysis.16S r RNA sequencing results showed that Firmicutes was the dominant phyla during composting,and its relative abundance during ATPC and HTPC was significantly higher than that during TC.At the family level,ATP and HTP increased the relative abundance of Bacillaceae,Sphingobacteriaceae,Streptosporangiaceae,and Thermonosporaceae during composting.RDA and PLS-PM analysis indicated that Bacillaceae were the key bacterium for the rapid degradation of organic matter during ATPC and HTPC processes and Sphingobacteriaceae,Streptosporangiaceae and Thermonosporaceae were closely related to the formation of humus.These four bacteria had obvious decay promoting effect,which were the key reason to accelerate the humification process of ATPC and HTPC.