Changes Of PH In The Aerobic Fermentation Process Of Organic Solid Waste And Effects Of Raw Material On Product PH

With the rapid development of agriculture and industry and the improvement of urban living standards in China,the production of organic solid wastes(OSWs)has increased in recent years.OSWs vary in composition as well as properties,because of a wide range of sources.The improper disposal of OSWs can not only lead to extensive occupation of land resources and waste of organic resources,but contaminate soil and water.Aerobic composting is a widely used method of sludge disposal in the world,which is an effective way to treat biodegradable organic solid waste.Aerobic composting process is a complex process of biological fermentation.In the process of transformation of different organic compounds,the type and the amount of intermediate products are different,due to different material composition and C/N ratio.pH has a great influence on the microbial flora composition and the transformation efficiency of OSWs in the fermentation process.Appropriate pH can be achieved by varying composition of feedstocks,which could then affect fermentation and pH of products.To date,little work has been documented in this aspect.In the thesis,we investigated the change of pH during the aerobic fermentation process of organic solid waste at three different scales.(1)Laboratory incubation experiment to determine aerobic mineralization of three organic materials(wheat straw,soybean leaves,cow manure)with different C/N ratios.Three organic materials were well mixed with quartz sand at mass ratio of 1:20,respectively.Subsequently,same volume of soil extract was added,and soil moisture was adjusted to 50%of maximum water holding capacity.The mixture was incubated at 30?for 30 days,and pH,total nitrogen,ammonium nitrogen,nitrate nitrogen and release rate of CO2 was analyzed during the incubation.(2)Aerobic fermentation of small piles by mixing sludge with organic materials(wood dust,rice straw and corn stalk)at ratio of 6:1 on wet weight basis.The temperature,pH,total organic carbon(TOC)and dissolved organic carbon(DOC),total nitrogen(TN),ammonium nitrogen and nitrate nitrogen were monitored for 60 days during the fermentation process.(3)Fermentation experiment in a pilot plant of dreg and sludge at mass ratios of 0.5:1,1:1,1.5:1 and 2:1.In the period of 60 days,temperature,pH,TOC,DOC,total nitrogen,ammonium nitrogen and nitrate nitrogen were monitored during the fermentation process.Through three different sizes of experiments,the following conclusions were obtained:(1)During the laboratory aerobic cultivation in decomposition of organic carbon is related to TOC and DOC of the organic material.High levels of TOC and DOC could increase the amount of organic carbon released from the material.The release of nitrogen from organic materials is related to nitrogen content and C/N ratio.Higher nitrogen content and lower C/N ratio could promote nitrogen release.The released amount of nitrogen was highest and the release rate was the fastest at the beginning of cultivation.(2)Lab aerobic cultivation trial showed that concentrations of ammonium and nitrate nitrogen changed in reversed trends for mixture of cow manure and soybean leaves with low C/N ratio.This indicated that ammonium nitrogen could be transformed into nitrate nitrogen.However,there was no significant correlation between ammonium nitrogen and nitrate nitrogen of wheat straw with a high C/N ratio.At the initial stage of culture,amination could lead to a pH increase,and conversion of ammonium nitrogen to nitrate could lead to a pH decrease.During the culture,.the greatest pH increased was observed for soybean leaves with the lowest C/N ratio.(3)Decarboxylation of organic acids in the mineralization can consuming H+,contributingto increase of pH.Therefore,the contents of DOC and organic acids in organic materials have great impact on pH of products.The degree of pH increase was:soybean leaves>wheat straw>cow dung,consistent with the content of soluble organic carbon and organic acid.(4)Sludge composting of small piles showed that the final pH of sludge composting was lower than that of the initial compost.This was because nitrogen released from nitrogen enriched sludge can be converted to nitrate ions,concurrent with release of proton.(5)Sludge composting of small piles also showed that pH of the compost decreased with the decrease of ammonium nitrogen as well as increase of nitrate nitrogen.The final pH of the compost was:corn stalk treatment>rice straw treatment>wood dust treatment,which was consistent with order of their organic acids.This was consistent with the results of lab incubation experiment confirming that the higher content of soluble organic acid leaded to higher pH of the product because of decarboxylation process.(6)The results of pilot plant revealed that pH changed during the aerobic composting of organic materials was related to the generation and transformation of ammonium and nitrate nitrogen.When the ammoniation was dominant process,ammonium nitrogen increased and pH increased.As the nitrification dominates,pH decreased when ammonium nitrogen was converted to nitrate nitrogen.Thus,pH of raw materials at the initial stage of composting could affect the final product pH,but the composting process generally narrowed the difference between them.