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免滤膜秸秆排水体滤层渗透特性及孔隙特征研究
Permeability and Pore Characteristics of Filter Layer in Non-filter Membrane Straw Drainage Bodies
在真空预压处理疏浚淤泥时,免滤膜秸秆排水体(NSD)具有无需滤膜和防淤堵的优势,应用前景良好。然而,目前秸秆滤层的渗透特性及其孔隙特征尚不清楚,影响了其推广运用。通过室内渗透试验,探究了免滤膜秸秆排水体滤层的渗透系数随真空预压时间、滤层厚度及疏浚淤泥初始含水率变化的规律,并结合CT扫描进一步探讨了不同处理条件下,免滤膜秸秆排水体孔隙结构演化对其渗透特性的影响。结果表明:①初始NSD内部存在纵横连通的缝隙排水通道,这些通道通过连接孔隙构成连续的渗流网络,使NSD具有优越的渗透性能。其中空隙占NSD总体积的31.42%,缝隙结构体积是孔隙结构体积的14倍。②秸秆滤层在渗透性能上优于传统土工织物滤膜。自反滤层-免滤膜秸秆排水体(NSD-Reverse,NSD-R)反滤体系的渗透系数随真空预压时间增长先快速下降后趋于稳定,在30 min后稳定在10-5 cm/s这一数量级,相较于滤膜过滤黏土的反滤体系渗透系数10-6 cm/s高一个数量级。滤层厚度的增加会导致反滤体系渗透系数减小,疏浚淤泥初始含水率较高的NSD-R反滤体系渗透系数较大。③真空预压时间增长使NSD-R反滤体系中的秸秆滤层空隙结构发生变化。随着真空预压时间的增长,NSD滤层空隙率迅速下降,厘米级缝隙结构占比明显减少,有效阻断细颗粒连续迁移,随后毫米级孔隙结构增多,既提升了保土性能,又确保了透水效率,从而实现了“保土-透水”功能的平衡。
[Objective] The non-filter membrane straw drainage body (NSD) offers significant advantages in vacuum preloading treatment of dredged sludge, such as eliminating the need for filter membranes and preventing clogging, making it a promising solution for practical applications. However, the permeability characteristics and pore structure of the straw filter layer are not yet well understood, which limits its widespread adoption. [Methods] Laboratory permeability tests were conducted to investigate the variation in the permeability coefficient of the non-filter membrane straw drainage body with vacuum preloading time, filter layer thickness, and the initial water content of the dredged sludge. CT scanning was also used to further explore the influence of pore structure evolution under different treatment conditions on the permeability characteristics. [Results] (1) Initially, the NSD contained vertically and horizontally interconnected fissure drainage channels, and these channels formed a continuous seepage network through the connection of pores, which endowed the NSD with superior permeability. The voids occupied 31.42% of the total volume of the NSD, and the volume of fissure structures was 14 times greater than that of the pore structures. (2) The permeability performance of the straw filter layer was superior to that of conventional geotextile filter membranes. The permeability coefficient of the NSD-Reverse (NSD-R) filtration system decreased rapidly and then stabilized with increasing vacuum preloading time, ultimately reaching a stable value on the order of 10-5 cm/s after 30 minutes, which was one order of magnitude higher than that of the reverse filtration system using geotextile filter membranes for clay (around 10-6 cm/s). An increase in the filter layer thickness led to a decrease in the permeability coefficient of the filtration system, while a higher initial water content of the dredged sludge corresponded to a larger permeability coefficient of the NSD-R filtration system. (3) As vacuum preloading time increased, the pore structure of the straw filter layer in the NSD-R filtration system evolved. With the increase in preloading time, the porosity of the NSD filter layer decreased rapidly, with the proportion of centimeter-scale fissure structures significantly reduced, effectively blocking the continuous migration of fine particles. Subsequently, the proportion of millimeter-scale pore structures increased, enhancing soil retention while ensuring water permeability, thereby achieving a balance between soil retention and water permeability. [Conclusion] These findings provide theoretical support for the engineering application of NSD in environmentally sustainable stabilization of dredged sludge.
免滤膜秸秆排水体 / 真空预压 / 渗透特性 / 疏浚淤泥 / 自反滤层
non-filter membrane straw drainage body / vacuum preloading / permeability characteristics / dredged sludge / self-filtration layer
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[Objective] To address the engineering challenge of drainage efficiency reduction caused by filter membrane clogging in traditional plastic drainage bodies during vacuum preloading of dredged sludge, this paper innovatively proposes a fully biodegradable, non-filter membrane straw drainage body (NSD) technology. Model tests were conducted to verify the engineering applicability of the NSD, reveal its drainage consolidation mechanism, and provide sustainable solutions for the green treatment of dredged sludge. [Methods] A PVC cylinder with a height of 50 cm and a diameter of 30 cm was used as the test tank, filled with dredged sludge at a water content of 147.5% (approximately 2.5 ωL). The sludge was collected from a disposal site in Wuhe County, Anhui Province, with a liquid limit of 59% and clay content of 21.7%. The control group used traditional plastic drainage bodies, consisting of rigid tubes wrapped with filter gauze and fabric. Two setups were tested: one with constant vacuum loading and another with staged vacuum loading. The experimental group employed NSDs, made of rigid tubes wrapped with straw ropes. Four setups were tested: (1) constant vacuum loading, (2) staged vacuum loading, (3) constant vacuum loading after installing a 2-5 mm self-filtering soil layer, and (4) staged vacuum loading with the pre-installed 2-5 mm self-filtering layer. During the testing period, the effluent discharge volume was recorded every 24 hours, and the solids content of the extracted tailwater was measured during each cycle. Upon completion of the vacuum preloading, the soil’s moisture content and particle gradation were determined. [Results] Drainage efficiency significantly improved, with the experimental group’s cumulative effluent volume 7.9%-22.1% higher than the control group, indicating that the three-dimensional pore structure of straw effectively alleviates the impact of clogging on drainage. Soil reinforcement was enhanced, with the experimental group’s average water content after vacuum preloading reduced by 6.8%-15.3% compared to the control group. Particle size distribution analysis revealed that when the self-filtering soil layer was pre-installed, the clay content (d<0.005 mm) increased by 5%-11.1%, confirming that the NSD, when combined with the pre-installed self-filtering layer, not only achieved effective soil filtration but also enhanced soil stabilization performance. [Conclusion] Technical innovation: The NSD achieves membrane-free drainage through its crisscrossing internal channels, overcoming the clogging bottlenecks of traditional plastic drains while increasing drainage efficiency by more than 15%. Mechanism breakthrough: A synergistic mechanism combining the NSD with the self-filtering soil layer has been proposed, demonstrating significantly enhanced drainage performance without compromising consolidation effectiveness. Application value: An efficient and eco-friendly dredged sludge treatment technology has been developed, providing a novel approach to vacuum preloading treatment of dredged sludge. |
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