Effect Of Carbon Regulation On Nitrogen Losses And Its Mechanism During Sewage Sludge Composting

The improvement of wastewater treatment ability has brought about an increase in the generation of sewage and environmental pollution. Because of lower cost and simple processing condition, composting is becoming the mainstream technology of the present domestic and foreign sewage sludge treatment and disposal. However, an unfortunate occurrence during composting is the loss of nitrogen (16%-74%of initial nitrogen), which reduces the agricultural value of the product and contributes to air pollution, such as the formation of atmospheric particles and odor. According to statistics, approximately46.8%-77.4%nitrogen loss is caused by ammonia emission. Thus, controlling ammonia emission is an effective way for reducing nitrogen loss.It is well known that the emission of ammonia is greater if the C/N ratio is <15. Sewage sludge normally has a high moisture content and low C/N ratio, which are favorable conditions for ammonia emission. To resolve above problem, different types of carbon compounds have been used to increase the C/N ratio, such as rice straw, sawdust and branch. However, when different materials have been added, their effectiveness in reducing ammonia emissions has not been evident. There is little information available in literature regarding the mechanism of reducing ammonia emissions by adding carbon source. It is difficult to explain why different types of carbon sources show different effects on reducing ammonia emissions.In this studythe effects of carbon source type, adding ratio and adding time on the normal physicochemical parameters and nitrogen loss during sewage sludge composting process were investigated. On the basis of composting process successful complete, the proper adding condition of carbon source for reducing ammonia emissions could be obtained. The biodegradation mechanism and structural characterizations of organic matter were investigated. Moreover, dissolve organic carbon, respiration index and emission rate of CO2were measured to evaluate of bioavailability of organic carbon. The interactions between carbon bioavailability and nitrogen loss were presented. Both conditional plate culture and culture-independent technology were used to investigate the dynamic changes of microbial population and microbial community structure succession during the composting process. It can be concluded from this study that ammonia assimilation play a crucial role in reducing nitrogen loss by adding carbon source.The type of carbon source had a significant effect on nitrogen loss, and the glucose and sucrose performed better than straw. According to the results of ammonia emission and nitrogen loss, addition of the mixture of sucrose and straw obtained the best effect on reducing nitrogen loss. The cumulative ammonia emission decreased by37%, and nitrogen loss reduced by47.8%compared with control group. With the optimal adding ratio of straw and sucrose (3/7), the nitrogen loss reduced by47.4%. At last, the best condition for carbon regulation has been obtained. Addition of the mixture of straw and sucrose with the ratio of3/7during thermophilic phase of composting could significatantly decrease ammonia emission by55%and the nitrogen loss was only10.8%.A relationship between biodegradable of organic mater and nitrogen loss could be found in this study. However, it is difficult to explain this relationship only by qualitative analysis in the change of organic carbon. According to the evolution of different bio-fractions of organic matter, organic matter with different composition showed different degradation characteristics during composting process. Moreover, the effect of nitrogen loss was associated with the degradation capacity of organic carbon by microrganisms. Assesment of carbon availability by emission rate of CO2(dCO2/dt) could well illustrate the interaction between carbon availability and ammonia emission. Under the optimal condition of carbon regulation, the maximum value of dCO2/dt was105.1that occurred after144h composting. This maximum value of dCO2/dt was simultaneous with the maximum value of ammonia emission. Moreover, it was much higher than the control group (60.4). Thus, a higher dCO2/dt always goes with lower ammonia emission during the intensive stage of ammonia emission.Accordding to the changes of microbial population and community structure, the mechanism of reducing nitrogen loss by carbon regulation is based on microbial ammonia assimilation, but not on nitrification. It should be noted that ammonia assimilating microorganisms play a crucial role in reducing ammonia emission. A large amount of Pseudomonas sp. which can assimilate ammonia was detected in the glucose treatment during the thermophilic stage of composting. The population of ammonia assimilating bacteria in surose composting increased to91×106which was much more than straw adding composting (16×105). The ammonium can be converted to organic nitrogen by ammonia assimilationwhich reduces nitrogen loss.-Ketoglutarate plays a crucial role in ammonia assimilation, which can be obtained by the tricarboxylic acid cycle with the formation of carbon dioxide. Thus, the change of carbon dioxide emission can reflect the intensity of ammonia assimilation by microorganisms and the effect of carbon regulation on reducing nitrogen loss. The relationship between dCO2/dt and ammonia emission could be well verified by ammonia assimilating pathway.