Numerical investigation and optimal design of capillary barrier cover with passive gas collection pipes on the performance at limiting landfill gas emissions.

Relying solely on soil properties may not fully ensure the performance of capillary barrier covers at limiting landfill gas (LFG) emissions. This study proposed to install passive gas collection pipes in the coarse-grained soil layers of capillary barrier covers to enhance their performance at limiting LFG emissions. First, the LFG generation rate of municipal solid waste and its influencing factors were analyzed based on empirical formulas. This information provided necessary bottom boundary conditions for the analyses of LFG transport through capillary barrier covers with passive gas collection pipes (CBCPPs). Then, numerical simulations were conducted to investigate the LFG transport properties through CBCPPs and reveal relevant influencing factors. Finally, practical suggestions were proposed to optimize the design of CBCPPs. The results indicated that the maximum whole-site LFG generation rate occurred at the end of landfilling operation. The gas collection efficiency (E) of CBCPPs was mainly controlled by the ratio of the intrinsic permeability between the coarse- and fine-grained soil (K2/K1) and the laying spacing between gas collection pipes (D). E increased as K2/K1 increased but decreased as D increased. An empirical expression for estimating E based on K2/K1 and D was proposed. In practice, CBCPPs were supposed to be constructed once the landfilling operation finished. It is best to select the fine- and coarse-grained soils with K2/K1 exceeding 10,000 to construct CBCPPs.

Influence of pH on the leaching behavior of a solidified arsenic contaminated soil.

Stabilization/solidification is widely used for treatment of arsenic (As)-contaminated soils. The stability of the soil may deteriorate significantly when exposed to acid or alkaline leachate. In this study, semi-dynamic leaching tests under different pH were carried out to investigate the leaching behavior of As from the solidified soils. Spectroscopic and microscopic analyses were performed to reveal the related mechanisms. The results showed that the leaching of As was closely correlated with the pH of the leachate, because the encapsulation effect of the cementitious matrix and the chemical speciation and valence of As were all significantly influenced by pH. In the strongly acidic leachant (pH 3.0), the leached As concentration increased by an order of magnitude, and the effective diffusion coefficient of As reached 3.71 × 10-13 m2/s. This is because that pores and cracks increased owing to the acidic corrosion of CSH, such that the physical encapsulation effect was reduced and the mobility of As increased. The leachability index showed that the solidified soil was unsuitable for 'controlled utilization' under strongly acidic conditions. The leached As concentration was the lowest in the weakly alkaline leachant (pH 9.0) because under weakly alkaline conditions the hydration process of the cement was facilitated, and more CSH gels were attached to the surface of the soil particles, forming a tighter structure for As encapsulation. However, as pH increased from 9.0-11.0 the leached As concentration increased due to an increased content of As(III)-O in the solidified soil.

Study on the migration mechanisms of water-soluble agents in high-pressure rotary jetting remediation.

High-pressure rotary jetting (HPRJ) remediation is a recent-applied technology for in situ remediation of contaminated soils. The effectiveness of remediation depends upon the migration and distribution of the injected agents in the soil. However, the corresponding migration mechanisms have received little attention. In this study, laboratory HPRJ tests and numerical simulations were performed using chlorine (Cl-) as a tracer to investigate the transport during HPRJ and the subsequent advection and diffusion. The test results showed that the HPRJ transported Cl- into the mixing zone by eroding the sand, and the radius of the mixing zone could be reasonably predicted by the erosion model. The Cl- concentration decreased linearly along the radial direction in the mixing zone. In addition, the Cl- transport distance increased with the increase in nozzle diameter, jetting times, especially injection pressure, and decreased with an increasing rotation speed. The Cl- concentration and radial uniformity were correlated positively with rotation speed, particularly nozzle diameter and jetting times. Numerical simulation showed that part of Cl- migrated from the mixing zone to diffusion zone by advection-diffusion after rotary jetting, which contributed positively to the agent distribution distance and uniformity. The Cl- migration was dominated by advection in the initial stage (30 days), while diffusion became more important thereafter.

Performance of a compacted loess/gravel cover as a capillary barrier and landfill gas emissions controller in Northwest China.

Loess is widely distributed in Northwest China where the rainy season coincides with the warm and vegetation growth period. The use of loess as a capillary barrier cover (CBC) material is promising. However, how the loess/gravel CBC perform as a capillary barrier and landfill gas emissions controller remains elusive. In this study, the performance of a designed CBC comprised 1.3 m-thick compacted loess underlain by 0.3 m-thick gravel in extremely wet and dry years of Xi'an city from 1950 to 2000 was analyzed using numerical modeling. An instrumented CBC test section comprised 0.9 m-thick compacted loess underlain by 0.3 m-thick gravel was constructed to show the hydraulic responses in real conditions from January 2015 to January 2017. The numerical results indicated that the designed CBC performed well as a capillary barrier as no percolation occurred during the extremely wet periods. Despite adopting a CBC of 0.4 m thinner than the designed one, the test section produced only 16.16 mm percolation during the two-year monitoring period, and that can meet the recommended limit of 30 mm/yr. The effect of the capillary break on increasing the water storage within the CBC was observed at the test section in fall. The increased water storage can significantly decrease the gas permeability, and thus improve the performance of the CBC as a LFG emissions controller. Furthermore, the LFG emissions can be controlled to meet the limit set by the Australian guideline by decreasing the bottom gas pressure and artificial watering. Finally, a procedure was proposed to enhance the performance of CBCs.

Clinical research on comprehensive treatment of senile vascular dementia.

OBJECTIVE: To observe the curative effect of combined Chinese medicine, acupuncture and rehabilitation therapy on vascular dementia (VD), and to compare the Western medicine piracetam. METHODS: Total 134 VD patients screened in reference to the standard for VD diagnosis in DSM-IV were randomly divided into a Chinese medicine plus rehabilitation group (32 cases), a Chinese medicine plus acupuncture group (33 cases), a Chinese medicine and acupuncture plus rehabilitation group (37 cases) and a Western medicine piracetam group (32 cases). Mini Mental State Examination (MMSE) and Bless Behavior Scale (BBS) were used to assess changes in intelligence disorder before treatment and after 12-week treatment. RESULTS: BBS score, living ability and daily habit were enhanced after treatment in all groups. Cognitive function and behavioral ability were improved with similar total curative effects in all the 4 groups. Directional ability and short-term memory ability were significantly enhanced after treatment (P < 0.05) in both the Chinese medicine plus rehabilitation group and the Chinese medicine plus acupuncture group groups. Living ability was significantly improved after treatment in the Chinese medicine and acupuncture plus rehabilitation group (P < 0.01). However, no obvious difference was shown before and after treatment in the piracetam group. CONCLUSION: The improvement of living ability in the comprehensive treatment group is better than that in the other groups. Chinese medicine and acupuncture plus rehabilitation treatment can improve intelligence and living ability of senile VD patients.