Effect Of Sewage Sludge Particle Size On Vermicomposting

With the continuous increase of sewage sludge production and the improvement of treatment and disposal requirements in China,it is imminent to develop diversified treatment and disposal methods to face the dilemma.Vermicomposting is expected to provide a new outlet for current sludge treatment because it is environmentally friendly,safe and sustainable.Vermicomposting still belongs to aerobic composting,and sludge re-oxygenation capacity is an important factor restricting its composting efficiency.The sewage sludge with different particle sizes have different specific surface areas,so that the areas contacting with oxygen are different,resulting in different degradation efficiency.In this study,the sewage sludge was made into 5 mm and 14.5 mm pellets.After adding earthworms,the following investigations during vermicomposting process with two particle sizes were made: 1)Changes of eight physicochemical indexes(Organic matter(OM),moisture content,conductivity(EC),p H,NH4+,NO3-,dissolved organic carbon(DOC)and microbial biomass carbon(MBC))and two spectral properties(fourier transform infrared spectra(FTIR)and three-dimensional fluorescence excitation-emission matrix spectra(3D-EEM));2)Changes of seven kind of enzyme(Dehydrogenases(DHA),FDA enzyme,invertase,cellulase,lipase,urease,protease)activities;3)Changes of 16s rDNA and 18s rDNA microbial communities;4)Effect of different sludge re-oxygenation capacity on the relationship among substances,enzyme activities and microbial communities.The main conclusions are as follows:(1)The particle size of sewage sludge has a significant effect on the physicochemical indexes of vermicomposting.Principal component analysis shows that the reduced particle size can accelerate organic carbon mineralization in the early stage of vermicomposting,and can accelerate organic nitrogen mineralization in the middle and later stages by accelerating ammoniation and advancing nitrification process.Furthermore,it can also accelerate the decomposition of difficult-to-use organic matter in the middle period.FTIR results show that the reduction of sludge particle size contributes to the degradation of cellulose,hemicellulose,etc.At the same time,the process of vermicomposting may not be based on the aromatization of organic matter as a fundamental means of stabilization.The results of 3D-EEM show that the mineralization of proteinaceous organic nitrogen occurs in the late stage of vermicomposting.The composting process does not stabilize the sludge by humification of the organic matter,but further decomposing the relatively refractory organics,and reducing the sludge particle size helps to strengthen this process.(2)Enzyme activities show that reducing the particle size of sludge has a limited role in facilitating the use of organic carbon sources such as sucrose and lipids by microorganisms,but it has a significant role in promoting the use of organic carbon sources such as cellulose.Reducing the particle size to promote the mineralization of organic nitrogen in sludge is not achieved by proteases,but by promoting deamination and promoting urea decomposition through urease.The principal component analysis also shows that the reduction of particle size accelerates the utilization of readily available organic carbon in the 0 to 10 days and difficult-to-use organic carbon such as cellulose in 10 to 30 days,and accelerates the deamination and nitrification after 30 days.(3)The 16s rDNA phylum which always maintains a dominant position during vermicomposting process is Proteobacteria,Bacteroidetes,Actinobacteria,Chlorobi,and Chloroflexi.Decreasing the particle size can decrease the type of 16s rDNA dominant phylum at the end of the experiment.In addition,reducing the sludge particle size can increase the proportion of 16 S r DNA bacteria that have the ability to decompose cellulose from 87.39% to 92.54%,and it is also beneficial to reduce the proportion of anaerobic bacteria in sludge.During the vermicomposting process,the 18s rDNA phylum that always maintains a dominant position is only Ascomycota,but at the beginning and the end of vermicomposting,Basidiomycota and Hyphochytriomycetes are also the dominant phylum.The Zygomycota soon replaces the Ciliophora as the most dominant phylum in the composting process.There are predatory relationships among earthworms,the two kinds of dominant protozoa(Ciliophora and Choanoflagellida),as well as Zygomycota.Reducing the particle size creates a condition for the proliferation of Zygomycota by accelerating the predation of protozoa.In addition,due to the reduction in the readily available organic carbon,at the end of composting,the sum of the three species of Ascomycota,Basidiomycota,and Zygomycota that could make use of carbon sources that are difficult to use in two treatment groups are both significantly higher than in the early days of composting which the increases are more than 60%,and there is not much difference between the two treatment groups.(4)The correlation between the changes of substances and enzyme activities during vermicomposting process shows that the degradation of cellulose in the small particle size treatment group contributed more to the degradation of organic matter than to the big particle size treatment group.During the vermicomposting process,the process from the macromolecules organic nitrogen like protein to the small molecule organic nitrogen such as amino acids and then to NH4+ does not occur at the same period.However,the process from urea to NH4+ is carried out from the beginning of composting.Nitrification occurs when the organic matter has already degraded to a certain degree.Reducing the particle size of the sludge helps to accelerate the nitrification process.The correlation of microbial communities and enzyme activities in the composting process shows that the phylum related to the secretion of invertase during the composting process is Saccharibacteria,and the phylum related to the secretion of cellulase are Choanoflagellida,Cryptomycota,and the phylum related to the secretion of lipase are Saccharibacteria,Proteobacteria,Firmicutes,Choanoflagellida,Ciliophora,and the phylum related to urease secretion include Saccharibacteria,Firmicutes,Actinobacteria,and Choanoflagellida,and the phylum related to the secretion of proteases is Zygomycota,and all of the correlations are above 0.5.The specific functions of the remaining dominant species in vermicomposting need further study.Before the 30 th days of vermicomposting,lipase,invertase,urease,and protease play an important role in the rapid degradation of easy-to-use substances and the accumulation of NH4+ and amino acids in the system.After the 30 th days,the deamination and nitrification dominates.The small particle size treatment group accelerates the use of organic carbon in the first 30 days,and accelerates the decomposition of urea into NH4+ in the first 20 days,providing conditions for the start of the nitrification process after 20 days.The acceleration of the deamination after the 30 th days provides large amount of nitrification substrate,NH4+,so that the nitrification intensity is significantly higher than that of the big particle size treatment group.