Clogging in intermittently dosed sand filters used for wastewater treatment

Clogging in intermittent sand filter (ISF) systems was analyzed using an unsaturated flow model coupled with a reactive transport model. Based on the results of a model sensitivity analysis, several variables were determined to be important in the clogging phenomena observed in ISFs, including hydraulic loading rate, influent chemical oxygen demand (COD) concentration, filter dosing frequency, and time of operation. Several modes of operation were identified that minimize the growth of bacteria at the filter surface. Following the sensitivity analysis, several case studies where ISF clogging was documented were simulated using the model. The results from the case study model simulations were found to be correlated with the total suspended solids loading rate (TSSLR) at the point of clogging. A model was developed that relates biomass development at the surface of ISFs with the TSSLR that can be sustained without clogging. The engineering significance of the model is presented in terms of operational and design considerations.     

潮白河河道回灌入渗试验研究

通过对潮白河河道进行现场单双环试验和坑灌试验,查明包气带水文地质条件。用包气带水流模拟软件HYDRUS建立受水区地下水流模拟模型,预测潮白河河道包气带回灌能力,为南水北调余水合理回灌利用提供科学依据。预测结果表明:在每年的回灌期290 d内,保证12 km回灌河道有水的情况下,通过包气带底部边界流出的水量最小值为76.56亿m3。随着回灌水头的增加,通过包气带底部边界流出的水量相应增加,二者呈正相关。     

Fate and transport of particle matter during GEOTextile tube dewatering of a dioxin and furan (PCDD/F) contaminated sediment

Geotextile tube dewatering is a pre-treatment method utilized in the remediation of high-water content materials (i.e. sediments and slurries). However, given the association some contaminants (e.g. dioxins and furans, (i.e. PCDD/F)) have with particulate matter in these contaminated sediments, understanding the fate and transport of this particle matter in the dewatered effluent is essential. In this paper, pressure filtration tests (PFTs) were conducted to investigate the effect of pressure and filter cake formation on both geotextile filtration efficiency and effluent quality. Transport tests were then performed to evaluate the particle transport through the developed filter cake, as well as the hydraulic characteristics of the medium during dewatering. HYDRUS, a one-dimensional model contaminant transport model was then employed to simulate the experimental particle transport test results. Three different mechanisms of particle transport (i.e. attachment only, detachment only, both attachment and detachment) through saturated porous media were examined to identify the possible mechanisms of observed effluent particle concentration. HYDRUS modeling outputs suggest that both attachment and detachment mechanisms are involved in a given particle's fate and transport during geotextile dewatering. In addition, parameters such as confining pressure and filter cake properties affect the particle concentration in the effluent.     

A shift in pathway of iron-mediated perchloroethylene reduction in the presence of sorbed surfactant--a column study

Surface modification of zero-valent iron (ZVI) to enhance its reduction rates for chlorinated ethanes and ethenes has recently attracted great attention. In this research, the enhancement of perchloroethylene (PCE) reduction by ZVI in the presence of sorbed micelles of the cationic surfactant hexadecyltrimethylammonium (HDTMA) was examined in a series of laboratory column tests with varying flow rates and input PCE concentrations. Model simulations using HYDRUS-1D showed that the overall pseudo first-order rate constants for PCE reduction by ZVI increased by a factor of four in the presence of sorbed HDTMA admicelles. The increase in reduction rate was attributed to a higher distribution coefficient (K_d) for contaminant sorption on surfactant-modified ZVI (SM-ZVI) compared to untreated ZVI. Modeling results also showed that in the presence of HDTMA admicelles 58–100% of PCE reduction occurred via hydrogenolysis. In contrast, only 12–25% PCE underwent hydrogenolysis when HDTMA was absent. The significant increase in TCE production during PCE reduction by SM-ZVI verified a shift in reaction pathway previously observed in batch studies, most likely from β-elimination to hydrogenolysis. Although this shift in reaction pathway resulted in a higher accumulation of TCE, the combined concentrations of chlorinated hydrocarbons in the effluent were 1.5–5 times lower when SM-ZVI rather than unmodified ZVI was used.     

A simplified dynamic method for field capacity estimation and its parameter analysis

This paper presents a simplified dynamic method based on the definition of field capacity. Two soil hydraulic characteristics models, the Brooks-Corey （BC） model and the van Genuchten （vG） model, and four soil data groups were used in this study. The relative drainage rate, which is a unique parameter and independent of the soil type in the simplified dynamic method, was analyzed using the pressure-based method with a matric potential of -1/3 bar and the flux-based method with a drainage flux of 0.005 cm/d. As a result, the relative drainage rate of the simplified dynamic method was determined to be 3% per day. This was verified by the similar field capacity results estimated with the three methods for most soils suitable for cultivating plants. In addition, the drainage time calculated with the simplified dynamic method was two to three days, which agrees with the classical definition of field capacity. We recommend the simplified dynamic method with a relative drainage rate of 3% per day due to its simple application and clearly physically-based concept.     

不同目标函数对土壤水力参数反演效果评价

土壤水力参数是表征土壤水力性质的重要参量,是准确预测土壤水分及溶质运移等过程的基础。基于GML非线性参数优化算法,构造了三个不同的目标函数,同时反演van Genuchten-Mualem模型中三个土壤水力参数(进气吸力倒数α、孔隙体积大小分布指数n和饱和导水率Ks),并分析了同一目标函数下不同降雨条件对反演结果的影响。结果表明,选取不同的目标函数对反演结果有显著影响,以累积入渗量作为目标函数能有效反演参数Ks,以压力水头和累积入渗量作为多目标函数则能同时有效反演α、n、Ks三个参数;此外,降雨条件对反演结果也有一定影响,相对于均匀降雨,在非均匀降雨条件下以压力水头作为目标函数对反演结果的精确度和准确度有较大的提升;以累积入渗量作为目标函数对反演结果的精确度略有提升但降低了反演结果的准确度。     

低影响开发单项设施模拟模型研究与应用进展

随着我国城市化进程的加快,城市内涝、雨水径流污染及水质恶化等问题日益严峻。低影响开发(LID)作为目前国际上城市水环境保护和可持续发展的雨洪管理新策略,在削减城市雨洪和滞留污染物等方面效果显著。为更好的模拟LID措施对环境带来的影响,不同尺度低影响开发模拟模型应运而生。本文总结了SWC、HYDRUS、DRAINMOD、RECARGA等典型小尺度单项设施模拟模型的特点、结构、适用条件和优缺点,分析了现有典型单项设施模拟模型的不足之处,指出模型参数选择、模型适应性检验以及水质水量耦合模型研发为今后的研究方向。     

Implementation of quadratic upstream interpolation schemes for solute transport into HYDRUS-1D

Numerical solution of the advection–dispersion equation, used to evaluate transport of solutes in porous media, requires discretization schemes for space and time stepping. We examine use of quadratic upstream interpolation schemes QUICK, QUICKEST, and the total variation diminution scheme ULTIMATE, and compare these with UPSTREAM and CENTRAL schemes in the HYDRUS-1D model. Results for purely convective transport show that quadratic schemes can reduce the oscillations compared to the CENTRAL scheme and numerical dispersion compared to the UPSTREAM scheme. When dispersion is introduced all schemes give similar results for Peclet number Pe < 2. All schemes show similar behavior for non-uniform grids that become finer in the direction of flow. When grids become coarser in the direction of flow, some schemes produce considerable oscillations, with all schemes showing significant clipping of the peak, but quadratic schemes extending the range of stability tenfold to Pe < 20. Similar results were also obtained for transport of a non-linear retarded solute transport (except the QUICK scheme) and for reactive transport (except the UPSTREAM scheme). Analysis of transient solute transport show that all schemes produce similar results for the position of the infiltration front for Pe = 2. When Pe = 10, the CENTRAL scheme produced significant oscillations near the infiltration front, compared to only minor oscillations for QUICKEST and no oscillations for the ULTIMATE scheme. These comparisons show that quadratic schemes have promise for extending the range of stability in numerical solutions of solute transport in porous media and allowing coarser grids.     

新疆膜下滴灌棉田暗管排盐的数值模拟与分析Ⅱ:模型应用

为了降低新疆地区盐碱棉田根区土壤盐分含量,尤其是排走长期使用膜下滴灌技术造成的下部根系层(40~60 cm)累积盐分,针对当地实际情况设计了2种改进排盐模式,分别是淋洗防渗排盐模式(情景1)和暗管局部冲洗排盐模式(情景2),利用经过校验的模型和参数对不同模式下暗管排水、排盐动态过程进行模拟。情景1在暗管下方铺设一定宽度(L_f,分别设定为20、50、100、250和500 cm)的防渗材料以增加汇流面积,提高排水、排盐量;情景2先通过暗管直接供水湿润周围土壤,达到设定时间(T_i,分别设定为0.25~10 d的9种情形)后停止供水,然后再通过暗管进行排水、排盐,以期利用较小的冲洗定额达到排盐的目的。对情景1进行模拟时,以0~40 cm和40~60 cm土壤含盐量分别低于3 g/kg和6 g/kg作为结束淋洗的标准进行对比分析,结果表明:当暗管处于非饱和区域、下方无防渗处理(传统暗管排盐模式)时,其排盐率仅为9.8%,单方水的排盐效率约1.86 kg/m~3;在暗管下方进行防渗处理有利于增加暗管排盐量,排盐率可达11.9%~32.1%,排盐效率可提高至2.27~3.15 kg/m~3;然而,随着Lf增加,施工难度和成本均会大幅上升,尤其是当Lf100 cm,单方水排盐效率的提高程度却较为有限,因此建议Lf在100 cm以内。而采用暗管供水进行局部冲洗(情景2)时,即使在暗管下方进行防渗处理,其单方水的排盐效率最高也不超过0.6 kg/m~3,整体表现并不理想,在新疆地区使用不太现实。