| According to the data, the annual wet swage sludge production has exceeded 0.3 billion ton in China. The implementation of the new environmental law and the release of the water pollution control plan of action stressed the urgency of sludge treatment and disposal in China. Composting is commonly used for sludge stabilization. While sludge composting is accompanied with large amounts of greenhouse gases(green house gas, GHG), not only causes nitrogen loss and reduces compost fertilizer, but also could contribute to Greenhouse Effect. However, we do lack the primary data in this area and a great uncertainty of the effect and GHG emission characteristics of sludge composting process also exists, especially under different conditions. This study is aiming to investigate the emission characters of GHG from composting by applying two different bulking agents, adding rice straw biochar, different biochars of varied proportions(2.5%, 5% and 10%) into dewatered urban sludge. By solid and gas sampling and analysis, we get the result of how bulking agents, adding biochar and different biochars in various proportions affect the emission of GHG and nitrogen reduction. Therefore, the total GHG emission factors and Greenhouse effect of different treatments are caculated and environmental indexes are analized. The main results are as follows:(1) The results of basic physical and chemical properties during waste sludge composting showed that the varation trend of different bulking agents were roughly the same. Temperature rose sharply as composting started and maintained for a week’s high temperature then fell to environment level. Taking straw as bulking agent was benificial to composting process; the water remove rate of straw treatment and sawdust treatment were 20.85% and 19.43%, respectively; the pH value increased gradually with composting going on; the TOC remove rate were 10.91% and 9.12%, respectively.Treatments of applying biochar and without biochar both kept a range of high temperature for more than a week, while the one with biochar was more slowly; compared to treatment without biochar, by applying biochar to the pile, the varation of pH value was more gentle and with a higher data when composting ended. The application of biochar increased the evaporation rate and water content of the pile, TOC content showed a trend of slow decrease.Compared to the bamboo charcoal, adding rice-straw biochar was more conductive to the rotten process, and the higher proportions of biochar maintained a longer time of high temperature. Adding 10% of this two kinds of biochar the temperature were significantly higher than that of other concentrations, and temperature droped in a lower speed. For the TOC content, the higher ratio of biochar application remained the higher TOC content, of which the loss were mainly during the first 2 weeks, the average loss rates were 55.45% and 74.68%, respectively.(2) The total GHG emission varied with composting environment, materials applied, pile volume, operation process and so on. The accumulative CH4 emissions of straw dust and cornstalk were 0.13 and 0.14 kg·t-1 dry sludge, respectively; of which more than 75% were during the first 2 weeks; total N2 O of straw dust and cornstalk were 0.099 and 0.073 kg·t-1 dry sludge, of which more than 90% of it produced during the rotten stage. The GHG emission equivalence of constalk was lower than that of wood dust(34.46 and 26.69 kg·t-1 dry sludge, respectively). On the whole, because of the related short duration of high temperature as well as the low temperature during mature stage, the process came to an especially low emission of CH4 but a relatively high discharge of N2 O.The tatal GHG emission(CO2 equivalent to dry sludge meter) of applying biochar and without biochar were 61.20 kg·t-1and 70.38 kg·t-1, respectively. Adding straw biomass carbon was conducive to the reduction of GHG. The CH4 and N2 O cumulative emissions(dry sludge meter) of adding rice straw biochar were lower than that without adding biochar: CH4 was 0.952 kg·t-1、0.911 kg·t-1, respectively; N2 O emissions(dry sludge meter) is 0.138 kg·t-1and 0.112 kg·t-1, respectively.Compared with bamboo biochar, rice-straw biochar is benificial to GHG reduction, adding a proportion of 5% rice-straw biochar caused the lowest equivalence CO2 emission compared with other proportions( 53.17 kg·t-1 dry sludge), while for bamboo biochar, a proportion of 10% conducts the lowest GHG emission( 80.83 kg·t-1 dry sludge).(3) The results of nitrogen transformation during sludge composting indicated that the TN content of straw and sawdust showed a downward trend, composting sites were 21.15% and 17.35%. Therefore, sawdust treatments is superior to nitrogen preservation. The addition of biochar can improve the efficiency of composting process and reduce total nitrogen loss. The TN remove rate of adding rice straw biochar is lower than that without adding biochar, 9.57% and 12.89%, respectively. The application of rice-straw biochar is more efficient than bamboo biocoal in nitrogen preservation on the whole, while a proportion of 5% is much more benificial to reduce nitrogen loss compared with other proportions.(4) The GHG emission from sewage sludge composting is affected by the factors like pile temperature, moisture content, pH value and so on. According to the results, most of the CH4 emission flux were significantly positively correlated with pile temperature(P<0.05), and the significantly or extremely significant correlated with TOC, while the correlation with other factors was not significant. Most of the treatments showed a significant positive correlation(P<0.05) between N2 O emissions and water content, while a significant negative correlation with pile temperatur, namely high temperature inhibited N2 O emission by inhibiting the activity of nitrifying bacteria. Partial treatment of this study showed a significantly negatively correlation between N2 O emission and NO3--N content, while a significantly positively correlation with NH4+-N. TN content had no significant correlation with pile temperature, moisture content and pH value. On the whole, the NO3--N change was significantly negative correlated with moisture content and pH value, while NH4+-N content had significant or extremely significant positive correlation with moisture content and pH value, and negatively correlated with pile temperature. |
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