Abstract:For the stable operation of simultaneous nitrification-endogenous denitrification and phosphorus removal (SNEDPR) system under domestic sewage, sequencing batch reactor (SBR) was selected to inoculate SNEDPR granular sludge under artificial water distribution. The actual domestic sewage and sludge were pretreated by mixed hydrolysis acidification and taken as the influent matrix, and the stable operation of the system was realized by optimizing the pretreatment unit hydraulic retention time (HRT) combined with SBR sludge discharge modes. Results showed that when the HRT of the pretreatment unit decreased from 12 h to 4 h, the content of acetic acid, propionic acid, and the proportion of volatile fatty acids (VFA) provided by the hydrolytic acidification solution increased, the anaerobic poly-hydroxyalkanoate (PHA) storage in aerobic granular sludge (AGS) increased, the simultaneous nitrification and denitrification (SND) efficiency increased to 58.1%, and the effluent TN decreased to about 7.8 mg/L. At the same time, under bottom sludge discharge, the proportion of small particles in SBR increased, LB-EPS increased, and the settlement performance of the system became worse. The proportion of mature and aged large particles under top sludge discharge increased, and the moisture content and strength of particles decreased. Under selective sludge discharge, the particle size in SBR was concentrated in 0.5-0.9 mm, with dense particle structure and high strength. The layered distribution of bacteria in the particles enhanced the SND effect. Batch experiments showed that the activities of nitrite-oxidizing bacteria (NOB) and glycogen accumulating organisms (GAOs) were reduced, the proportion of denitrifying polyphosphate accumulating organisms (DPAOs) with NO-₂ as electron receptor in the system was increased to 40.5%, and the efficiency of simultaneous nitrification and endogenous denitrification phosphorus removal was improved. The short HRT combined with selective sludge discharge of the pretreatment unit had stable performance in the treatment of domestic sewage particles. The effluent TN and TP met the first-level A standard specified in GB 18918—2002 Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant. The system can achieve efficient and stable removal of nitrogen and phosphorus.

Abstract:Short-cut nitrification and denitrification phosphorus removal has the advantages of low oxygen consumption, low carbon source demand, and low sludge yield, but aerobic granular sludge (AGS) is prone to disintegration and instability due to large particle size. In order to solve the problem of instability and ensure sufficient anoxic zone for denitrifying phosphate accumulating organisms (DPAOs), long-term cultivated granular sludge with artificial water distribution was used as seed sludge, and the particle size distribution was regulated by optimizing hydraulic shear strength, so as to realize the stable operation of domestic sewage treatment and explore the influence of hydraulic shear strength on particle structure. Results showed that the particle size range with optimal simultaneous nitrogen and phosphorus removal performance and dense particle structure was 800-1 400 μm. By adjusting the hydraulic shear strength to 1 435.2, the proportion of particles in this size range was increased to 53.39%. When the granulation reached stable, the effluent COD concentration was maintained below 50 mg/L, the TN removal rate was about 90%, the effluent TN concentration was 4.28 mg/L, the average removal rate of TP was 93.45%, and the effluent TP concentration was below 0.5 mg/L. In addition, the analysis of extracellular polymeric substances (EPS) by excitation-emission matrix (EEM) combined with parallel factor (PARAFAC) showed that increasing hydraulic shear strength could reduce the content of humic acid and increase the proportion of protein, which was conducive to optimizing particle sedimentation performance and improving compactness. By optimizing the particle size range, the larger anoxic zone of granules could be used to enrich DPAOs and gradually eliminate nitrite oxidizing bacteria, avoiding the expansion of filamentous bacteria, and realizing stable short-cut nitrification, denitrification, nitrogen, and phosphorus removal of domestic sewage.

Abstract:Pharmaceuticals and personal care products (PPCPs) are a new pollutant of widespread concern at home and abroad, and their presence in the water environment of the Arctic region has attracted great attention of governments and environmental organizations. The source, concentration, distribution, and fate of PPCPs in the Arctic water environments were reviewed in this paper. The river transportation from pan-Arctic regions, long-distance transportation by the atmosphere or the ocean, direct discharge of waste liquids, and residues from sewage treatment plants were found as the main pathways for PPCPs entering into the Arctic water environments. The antiepileptic drugs, non-steroidal anti-inflammatory drugs (NSAIDs), antidepressants, stimulants, and plasticizers were typical PPCPs in the Arctic water environments, and they had "pseudo-persistence", which were similar to the environmental behavior of PPCPs in the water environments of the non-Arctic regions. At present, researches on the pollution control technology of PPCPs in the Arctic region, the pollution level of PPCPs in the Arctic freshwater environment, the physical and chemical properties of PPCPs in the Arctic water environments and their environmental behaviors need to be improved.

Abstract:To resolve the problems such as the relatively low utilization value of hydrothermal products and excess processing liquid, this study proposed a multiple recycling process scheme with hydrothermal processing liquid as the HTC medium. By analyzing the transformation and distribution of nitrogen (N), phosphorus (P), and organic matters in hydrothermal solid and liquid phases, the feasibility and advantage of the hydrothermal recycling approach for improving the utilization value of sludge by-products and saving water resources were investigated. It was found that under the HTC condition of 250 ℃, N-containing components in hydrothermal liquid increased with recycling. After three cycles, the concentration of NH+₄-N and P in liquid reached 17 000 mg/L and 200 mg/L respectively (being 4 and 5 times the primary hydrothermal processing liquid), significantly improving the efficiency of hydrothermal resource recovery. The conversion of organic components to solid phase promoted carbon formation, with the utilization value of hydrochar products increased. In conclusion, regulating the intensity of hydrothermal reactions and increasing the recycling cycles of processing liquid tend to maximize the utilization value of hydrothermal products and improve the efficiency of water use. The results provide a fresh insight for popularizing the hydrothermal technique to achieve sludge reduction and resource utilization.

Abstract:In order to study the characteristics of heat transfer in subcooled flow boiling of circulating cooling system, the unsteady numerical calculation of the subcooled flow boiling process of a horizontal pipe was carried out based on the VOF multiphase model and the Lee phase change model. The correlation of boiling onset proposed by Bergles was introduced for correcting the Lee model, taking into account the influence of the boiling onset. The heat transfer properties of the subcooled flow boiling process in the horizontal pipe and their variation rules were examined from two aspects: the thermal boundary layer development and the boiling development. The distribution regularity of relevant parameters under different heat flux conditions was summarized, as well as the distribution rules of heat transfer coefficient along the flow direction. Results showed that the development of the thermal boundary layer and boiling would enhance the instability of the flow field, and the fluctuation amplitude of the heat transfer coefficient in the rear was twice as large as in the front. Moreover, the increase of the heat flux accelerated the variation of the flow and heat transfer parameters along the flow direction. In the 250 kW/m² condition, the area affected by the thermal boundary development was about 60% that of the 150 kW/m² condition. The heat transfer coefficient in the front of the heating section showed a descending trend with the development of the thermal boundary layer. Therefore, when the heating area is in high flux condition, we can shorten the length of the heat exchanger to increase the heat exchange efficiency and weaken the fluctuation of the heat transfer coefficient caused by flow boiling.

Abstract:In order to study the dynamic change mechanism of droplet dynamic contact behavior under electrowetting conditions, the dynamic contact angle theory is used to establish a numerical model of droplet under electrowetting and the droplet transient behaviour is analysed. In particular, the effects of volume and wall conditions on the dynamic behaviour of droplet are systematically investigated. The results show that the maximum amplitude of the droplet contact radius increases from 0.16 mm to 0.23 mm when the droplet volume increases from 2 μL to 6 μL, and the amplitude increases from 0.13 mm to 0.18 mm when the initial droplet angle increases from 100° to 115°. At the same time, with the slip length from 0.5 μm increased to 2 μm, the maximum amplitude increases from 0.21 mm to 0.29 mm. The larger the droplet volume, the smaller the wall resistance, the stronger the hydrophobicity, and the higher the kinetic energy and the greater the oscillation amplitude of the droplet. By revealing the dynamic contact behavior mechanism of droplets in the process of electrowetting, a theoretical basis is provided for the study of improving the heat transfer characteristics of microchannels through the electrowetting effect.

Abstract:To investigate the effect of surface groove method on the interfacial shear behavior of CFRP-concrete, a series of simple shear tests on CFRP-concrete specimens with surface groove method were carried out, focusing on the effects of groove width-depth ratio (0.5,1.0 and 1.5) and groove shape (rectangle, normal trapezoid and inverted trapezoid) on the interfacial shear behavior of CFRP-concrete, especially the failure modes, fracture energy, and average shear stress. The bond interfacial shear design method of surface groove was proposed. The results show that the failure mode of surface groove method is changed from classⅡdebonding failure to class Ⅵ fracture failure, with the interfacial fracture energy increased by 136.4%, giving a full play to the tensile strength and utilization ratio of CFRP sheets. Furthermore, the groove width-depth ratio is positively correlated with the average shear stress, presenting an optimum volume, and with the average shear stress of normal trapezoidal groove improved, 10.1% and 11.8% higher than that of the inverted trapezoidal groove and rectangular groove respectively. As the surface groove method performs better shear behavior, the proposed method can be used to guide the shear design of bonding interface.

Abstract:Stable initial groundwater exists in most shallow slopes in mountainous and hilly areas of Zhejiang province, which leads to complex distribution of soil water content and difficulty of rainfall infiltration process and slope stability analysis. Considering the characteristics of unsaturated soil, the exponential distribution model of water content of slope under initial groundwater was determined. The rainfall condition was divided into weak and heavy rainfall according to the relationship between rainfall intensity and soil saturated permeability coefficient. On the basis of the Green-Ampt infiltration model and water balance principle, the analytical solution of governing equations for rainfall infiltration of shallow slope with groundwater was derived for the two conditions. Combined with the shallow infinite slope model, the stability of shallow slope under rainfall was analyzed considering the influence of initial groundwater. The modified model could be approximately degenerated into the traditional model when the initial water content was uniformly distributed. When the initial water content was in exponential distribution, the effectiveness of the modified model was verified through numerical simulation. Results showed that shallow slope failure induced by rainfall occurred near the bedrock surface when groundwater existed in the soil layer over the bedrock in shallow slope, and the failure time was prior to that predicted by the traditional model. Under weak rainfall conditions, the failure time was nearly exponentially shortened with the increase in rainfall intensity. Under heavy rainfall conditions, the failure time tended to be stable when the rainfall intensity was increased, on the condition that the promoting effect of slope surface runoff on infiltration was not considered.

Abstract:To describe the evolution law of tunnel plastic zone and the principle of support design under in-situ stress field, this paper presented equations of plastic zone radius at horizontal and vertical axes of a circular tunnel under non-hydrostatic pressure with the total load invariant method. The two equations were based on the Mohr-Coulomb criterion and the elastic-brittle-plastic model, and a stress perturbation solution in the elastic zone was introduced. Then, the equations were extended to other azimuth angles according to the geometric similarity principle. The obtained equations of plastic zone radius were compared with the results of the total load invariant method (based on the Kirsch stress formulation), Kastner method, complex variable function method, and measured data. An analytical solution of plastic zone displacement was derived using the non-associated flow law. Finally, parametric studies were performed to investigate the effects of lateral pressure coefficient and brittle softening on the plastic boundary, distribution of plastic zone displacement, and ground response curve. Results showed that the second-order perturbation solution taken as the stress equation in the elastic zone of a non-hydrostatic tunnel was more reasonable than the total load invariant method and the Kastner method, and it was verified against the complex variable function method. Lateral pressure coefficient had an obvious influence on the shape and range of tunnel plastic boundary, so support type and size should be determined for specific azimuth angles to control support pressure and stable rock deformation at the intersection point of convergence-confinement analysis. Plastic zone radius and tunnel wall displacement increased significantly with the decrease in rock post-peak strength, so the elastic-brittle-plastic model was suggested to calculate ground response curve.

Abstract:The effect of beaded caverns on vertical bearing characteristics of pile foundation was investigated. The load-settlement characteristics, variation rule of vertical ultimate bearing capacity, distribution rule of pile axial force, side resistance, and partial load of piles under different roof thickness-pile diameter ratios were studied by centrifugal model test, when the pile passed through two layers of caves and there was an underlying cave. A reasonable method for calculating roof thickness-pile diameter ratio was proposed by comparing with the test results of the case without underlying karst cave. Results show that when the pile passed through the cavern, the roof thickness to pile diameter ratio of the underlying caverns had a great influence on the vertical ultimate bearing capacity of the pile. When the roof thickness-pile diameter ratio increased from 0.5 to 3.0, the influence degree of vertical ultimate bearing capacity of the pile decreased from 57.4% to 4.0% compared with the case without karst cave. When the roof thickness-pile diameter ratio was greater than 2.5, the influence degree of vertical ultimate bearing capacity of the pile was less than 5.0%. When the roof thickness-pile diameter ratio increased, the axial force of the pile was almost not attenuated in the cavern. The decay rate of the axial force of the pile in the upper and middle cave roof was slow, and the load transmitted to the cave roof was large. The side resistance of the pile barely had influence in the cave that the pile passed through. With the increase in the roof thickness-pile diameter ratio, the side resistance of the pile decreased in the upper and middle cave roof. As the roof thickness-pile diameter ratio increased, the proportion of pile side resistance and pile tip resistance gradually decreased and increased, and the pile gradually transformed to frictional end bearing pile. When the roof thickness-pile diameter ratio was greater than 2, the side resistance of the pile decreased less than 10% compared with the condition without underlying karst cave. In beaded karst cave area, when the pile passes through two-layer karst caves and is above an underlying karst cave, it is recommended that the influence of the underlying karst cave on the vertical bearing characteristics of the pile can be ignored if the roof thickness-pile diameter ratio is great than 2.5, and a reasonable roof thickness can be determined according to the bearing requirements of the pile foundation.

Abstract:To study the dynamic mechanical properties and rupture mechanism of laminated composite rock mass under the action of impact loading, we carried out loading tests of laminated soft and hard coal rock composites consisting of coal monomer and white sandstone under different rates and impact directions by using the split Hopkinson pressure bar (SHPB) device and LS-DYNA finite element analysis software combined with Holmquist-Johnson-Cook (HJC) constitutive model. Results show that the strength of the laminated coal-rock composite did not change with the loading direction under static loading. The peak stress and dynamic increase factor (DIF) of the laminated coal-rock composite under dynamic loading increased linearly with the impact velocity. The mechanical properties of HS (H and S represent white sandstone and coal monomer respectively) composite with better wave impedance matching were always better than those of SH composite, and this phenomenon gradually decreased with the increase in the impact velocity. The dissipation energy density and incident energy density of the laminated coal-rock composite showed a quadratic growth relationship. The fractal dimension increased with the velocity, and the effect obtained when the stress wave transferred from hard into soft was better than that from soft into hard. The degree of fragmentation of the laminated coal-rock composite became more and more intense with the increase in the impact velocity, the degree of destruction of HS composite was greater than that of SH composite under the same conditions, the white sandstone presented more shear-like fragmentation, and the coal monomer presented more crushed conical destruction. The strength at the interface of the laminated coal-rock composite was not consistent with that in other regions, resulting in a change in the order of destruction of the composite. The overall strength law of the composite from small to large was: coal monomer non-interface region, coal monomer interface region, white sandstone interface region, and white sandstone non-interface region.

Abstract:To investigate the dynamic damage mechanical behaviors of bedded rock-like material based on 3D printing, dynamic compression tests on five groups of printed samples with different dip angles were carried out using a split-Hopkinson pressure bar. Then, according to the obtained stress-strain data and Zhu-Wang-Tang constitutive model, a viscoelastic constitutive model was constructed by connecting the linear spring body, the Weibull distribution damage body, and the Maxwell body in parallel, and the damage correction coefficient was introduced in combination with the residual strength properties of the samples. Finally, the model was extended to explore the deformation patterns of black shale to verify its applicability. Results showed that under impact loading, the peak stress of samples displayed a "V" shaped trend with the increase in dip angle, which was consistent with the variation pattern of natural layered rock. The damage constitutive model accurately characterized the shape of the stress-strain curve and the mechanical behaviors of the 3D printed bedded rock-like material. After considering the damage correction, it also better depicted the stress-strain change characteristics and residual strength of the samples at the post-peak stage. This study is useful for understanding the deformation pattern of bedded rock subjected to dynamic loading.

Abstract:Biopolymers can effectively improve the mechanical properties of soil. In order to evaluate the application prospect of biopolymer-modified sludge in practical engineering, the effects of xanthan gum (XG), sodium alginate (SA), and cationic guar gum (GG) on the compressibility and permeability of high water-content sludge were studied. On the basis of one-dimensional osmotic consolidation test, the effects of different types and dosages of biopolymers on the yield stress, compression index, and coefficient of permeability of soil were investigated. Meanwhile, the microscopic mechanism of the modification of high water-content sludge by biopolymer was analyzed by scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) tests. Results show that the compressibility of modified sludge was reduced because of the formation of clay-polymer network structure. With increasing dosage, the compressibility of modified sludge decreased, while the structural yield stress increased gradually. When the vertical effective stress was greater than the structural yield stress, the C_c value of the modified sludge increased significantly, and it further increased with the increase in dosage. The permeability of the modified sludge was significantly reduced. Among the polymers used in the study, XG had the most significant effect, achieved two orders of magnitude when the dosage was 1.5%.

Abstract:Under long-term loading from aircraft, the cumulative deformation of airport subgrade filled with saline silt in northwest China can be a key factor that affects the service performance of the airport. Taking the silt in a typical area of northern China as example, the dynamic triaxial test was carried out to investigate the influence of salt content (mass fraction) on the dynamic characteristics of silt under cyclic loading. A cumulative plastic strain of 4% was selected as the failure strain based on the test results, and a prediction model of dynamic strength of salinized silt was proposed. Results show that with the increase in dynamic stress amplitude, the plastic deformation of salinized silt gradually changed from plastic stability to incremental failure. The salt content largely affected the cumulative axial strain, dynamic modulus, and critical dynamic stress of silt. At the same dynamic stress amplitude, as the salt content increased from 0 to 5%, the cumulative axial strain decreased firstly and then increased at the salt content of 1%. At low dynamic stress amplitude, the dynamic modulus decreased with the increase in salt content. The strength of salinized silt experienced a short growth at 1% salt content and a gradual attenuation at high salt content, which might be explained from a microscopic view that the strength of salinized silt sample was closely related to the ion concentration in the soil pores and the arrangement of soil particles. According to the dynamic strength prediction model, the critical dynamic stress for the silt with 1% salt content was about 2.2 times that of silt with 5% salt content. The conclusions of the study can provide reference for the airport runway design and construction in saline silt area.

Abstract:To investigate the influence of polymer material and contact surface roughness on the shear mechanical properties of soil-rock contact surface, we designed precast concrete samples as similar materials for rock surface and conducted a series of improved direct shear tests. The influence of organic polymer material and contact surface roughness was discussed. The improvement mechanism of polymer material was revealed by scanning electron microscope (SEM). Test results show that the polymer material could greatly improve the shear mechanical properties of the base soil and the contact surface by increasing the cohesive force, and the cohesive force of the substrate sample with 2% doping reached 41.76 kPa, which was increased by about 3 times. The cohesive force of different contact surface samples with 2% doping increased by about 2-7 times. The increase in the contact surface roughness could enhance the shear properties of contact surface by increasing the cohesive force, and the cohesive force of different contact surfaces with roughness of 6.5 mm increased from 12.27 kPa to 23.77 kPa, which was increased by 0.4-3.9 times. For flat contact surface and rough contact surface, the shear properties of polymer materials were strengthened in two different modes. The polymer material and the contact surface roughness had a synergistic strengthening effect on the shear properties of contact surface.

Abstract:The deformation characteristics of frozen soil are important parameters for testing the stability of foundation deformation in foundation design of cold region, changing with the change of stress direction. However, the influence of principal stress axes direction on the deformation characteristics of frozen soil is rarely considered in existing studies. This paper carries out a series of principal stress unidirectional rotation tests on frozen clay by using frozen soil hollow cylinder apparatus. The influence of rotation rate and rotation direction of principal stress angle α on deformation characteristics of frozen clay is explored. It is found that plastic deformation can be generated in the frozen clay without changing the stress amplitude in pure principal stress axes direction rotation tests. The strain peak of frozen clay lags behind the stress peak in the deformation process. The α-angle rotation rate can affect the axial strain and shear strain of frozen clay, and there is a critical α-angle rotation rate to make the frozen clay play better carrying capacity. Different α-angle rotation direction changes the stress process of frozen clay, and the larger the shape variable of frozen clay, the more obvious the influence of α-angle rotation direction on its deformation development. Through the analysis of the hysteresis curve, it is found that α-angle rotation rate will have a greater impact on the strength of the frozen clay when the α-angle rotation rate is small. Nevertheless, its impact on the strength of frozen clay will be reduced when the α-angle rotation rate is large. The influence of α-angle rotation direction on the strength of frozen clay is closely related to the selection of stress path parameters.