Model evaluation of different mechanisms driving freeze–thaw N2O emissions

N₂O emissions from soil contribute significantly to global warming. Pulse emissions of N₂O from soils during freeze-thawing were recently recognized as important atmospheric sources. In this modelling study we explore three different hypotheses for explaining freeze–thaw related N₂O emissions: (1) soil frost or snow cover may reduce gas diffusion and create anaerobic conditions that stimulate N₂O production via denitrification, (2) microbes that die of frost deliver easy decomposable organic carbon and nitrogen to the soil, which stimulates microbial growth and vigorous N₂O production during freeze–thaw, and (3) the enzyme nitrous oxide reductase, which is responsible for the reduction of N₂O to N₂ during denitrification, is more sensitive to low temperatures than other enzymes, so that N₂O becomes the dominating end-product of denitrification at low temperatures. These hypotheses were tested with a biogeochemical model that combines hydrology and physics calculations with a newly developed, parameter-poor biochemistry module. The model was first calibrated with field datasets on soil–atmosphere fluxes of N₂O, NO and CO₂ and soil NO₃ and NH₄ concentrations that were measured in a spruce forest in Southeast Germany in the years 1994–1997. Subsequently, additional model mechanisms were implemented that allow the model to describe the outlined mechanisms potentially driving freeze–thaw N₂O fluxes. After each implementation the model was recalibrated. We were able to mimic dimension and timing of high N₂O emissions when either one of the first two hypotheses were assumed, but found no confirmation for the third. The best model fit was achieved by combining hypothesis one and two, indicating that freeze–thaw N₂O emissions are not mono-causal.     

臭氧对膜法水处理中膜污染的影响

膜过滤是一种高效水处理技术,包括污水处理中的膜-生物反应器（membrane bioreactor,MBR）和针对二级生物处理出水（生产再生水）、地表水（生产饮用水）的膜过滤工艺等,其中膜污染是制约膜工艺应用的一个主要问题.臭氧具有强氧化性,在水处理上得到了广泛的应用.近十几年来,针对臭氧-膜过滤组合工艺的研究正变得越来越多,因此本文对这些研究进行了系统综述与分析.在MBR组合臭氧控制膜污染方面的研究中,目前结论相对较少.可以通过对MBR进水预臭氧化或者投加适量臭氧到MBR混合液这2种方式来减轻MBR膜污染.在针对微污染水的臭氧-膜过滤组合工艺中,根据臭氧的功能和结构形式主要分为3种,分别是污染膜臭氧清洗、分离式臭氧-膜工艺和一体式臭氧-膜工艺.绝大部分报道认为臭氧可以有效地控制膜污染的发展,但也有少数投加臭氧后膜污染加剧的情况.目前,臭氧化影响膜污染机制的研究主要集中在有机物方面,根据原水水质和工艺结构参数等的不同,颗粒物、微生物和无机物质也在这个过程中起着相应的作用.另外,在迄今为止的研究中,还缺乏对于臭氧投加方式和分散方式的统一衡量和比较标准.作者建议应该加强对一体式臭氧-膜组合工艺的研究,并且要注意臭氧投加的工艺经济性.     

Fouling of nanofiltration membrane by e uent organic matter: Characterization using di erent organic fractions in wastewater

The UF membrane with molecular weight cuto (MWCO) ranging from 2 to 100 kDa and XAD-8 resin were employed to identify the characteristic of molecular weight (MW) distribution of wastewater e uent organic matter (EfOM) in terms of TOC and UV254, as well as the amounts of the hydrophilic/hydrophobic organic fractions in di erent MW ranges. Then, the nanofiltration (NF) membrane fouling experiments were carried out using the above fractionated water to investigate the e ect of MW distribution and hydrophilic/hydrophobic characteristics of EfOM on the membrane flux decline using the fractionated water samples. The experimental results have shown that 45.61% of the total organics belongs to the low MW one, among which the percentage of the hydrophilic organics with low MW (less than 2 kDa) was up to 28.07%, while that of the hydrophobic organics was 17.54%. In particular, the hydrophilic fraction was found to be the most abundant fraction in the e uents. MW distribution has a significant e ect on the membrane fouling. When the MWwas less than 30 kDa, the lower the MW, the larger was the specific flux decline, while in the case of MW higher than 30 kDa, the higher the MW, the larger was the specific flux decline, and the decline degree of low MW organics was larger than the high MW one. With the same MW distribution range, specific flux decline of the hydrophilic organic was considerably slower than that of the hydrophobic organic, which indicated that the hydrophobic organic fractions dominantly contribute to the flux decline.     

内压式中空纤维超滤膜污染模型研究

以流体力学和过滤理论为基础,建立了一个描述内压式中空纤维超滤膜污染的数学模型。该模型将膜的性质、原水的性质、清洗条件等多种因素对超滤过程的影响,用一个参数——膜堵塞系数表示。实验以造纸黒液稀释水为原水,对模型参数进行了拟合、验证,模拟计算结果与实验实测数据比较吻合,说明该模型能较好的模拟内压式中空纤维超滤膜过滤过程。     

一体式膜生物反应器处理生活污水的研究

研究了一体式膜生物反应器对生活污水的处理效果。实验结果表明,膜生物反应器对COD、TOC、NH4+-N、TN、TP的平均去除率为93.4%、96.2%、97.2%、63.8%、60.6%,并且污泥活性随着运行时间的延长而有所降低。     

基于能值分析的城市物质代谢研究

采用能值分析方法,构建了基于城市物质代谢通量指标、代谢存量指标和代谢效率指标的城市物质代谢指标体系,并以北京、天津、上海和重庆四个直辖市为例计算了2005年四城市物质代谢状况。分析结果表明:北京市代谢效率最低;上海市代谢通量和代谢存量最大,但是代谢效率不容乐观;重庆市代谢通量和代谢存量在四个城市中居于末位,但是代谢效率最高;天津市在四个城市中代谢呈中等水平。最后根据分析结果对城市今后的可持续发展提出了意见和建议。     

冷冻取芯过程煤样温度变化特性研究

为探究冷冻取芯过程煤样温度变化特性,基于模拟试验的相似性,自主设计冷冻取芯模拟测试试验装置,选取榆家梁矿、首山一矿、六龙煤矿、九里山矿4个矿区不同变质程度煤样,用以测定瓦斯吸附平衡压力为0、1.0、2.0、3.0 MPa下冷冻取芯过程煤样的温度实时变化数据,并统计分析各条件下的数据。研究结果表明:冷冻取芯技术的冷冻效果与煤变质程度及瓦斯吸附平衡压力呈正相关关系;冷冻取芯对含瓦斯煤冷冻效果优于不含瓦斯煤;冷冻取芯对高变质煤冷冻效果优于低变质煤;冷冻取芯过程中,煤样温度随时间变化曲线符合指数函数关系。     

Thermal effects of a sonic jet fire impingement on a pipe

Although the effects of jet fires are often limited to rather short distances, if their flames impinge on a pipe or a vessel collapse can occur in very short times. In such cases, the heat flux on the affected equipment is very high and wall temperature can increase very rapidly. This can happen in parallel pipelines, if a release occurs and impinges on another one. Nevertheless, jet fire impingement has been scarcely studied. In this communication the results obtained from an experimental set-up are presented. Sonic jet fires impinged on a pipe containing stagnant air or water. The temperatures of the flames impinging on it were measured for the worst case (flame front-bright zone), as well as the evolution with time of the pipe wall temperature at different locations. Initial temperature increases up to around twenty °C/s were registered for the air inside, with maximum values of up to 600 °C reached in 2.5 min, and 800 °C in approximately 9 min. In the case of pipe containing water, in the zone of the wall in contact with the liquid the heating rates were much lower, the maximum temperature reached being up to approximately 150 °C. From the temperatures of the jet flames and of the pipe, the heat fluxes reaching the pipe and the corresponding heat transfer coefficients were obtained. The results obtained emphasized that safe distances are essential in pipelines, together with fire proofing and other safety measures.     

煤田火区温室气体通量测量方法研究进展

为提高煤田火区温室气体通量测量的准确性,科学制定温室气区减排措施,笔者对国内外煤田火区温室气体测量方法进行梳理,对比分析各种测量方法的原理、优缺点和适用范围等,提出测量方法存在的问题及以后的研究方向。研究表明:目前所用的测量方法存在测量精度低、连续性差、假设理想化等缺陷,检测设备的改进应朝着能适应当地地表高温环境并能实现连续监测的方向发展,测量方法应更注重和新技术的结合。     

平板膜生物反应器临界通量测定方法研究

临界通量在膜污染控制中是一个非常重要的概念.本文采用通量阶梯式递增法测定平板膜生物反应器的临界通量,在此基础上考察了不同测定参数对临界通量的影响.结果表明,测定临界通量过程中,当初始启动通量小于临界通量时,其对临界通量的测定值影响不大;当通量阶梯递增量增大时,临界通量的测定结果变小;当测定间隔时间段变长时,临界通量的测定结果相应变小.在测定临界通量时,推荐以下测定参数:通量阶梯递增量不大于4 L/(m2·h),测定间隔时间段取15～30 min;初始启动通量的选择要适宜,太大可能导致试验结果的误差.