Effects of sewage sludge application methods on the transport of heavy metals with runoff and their mechanisms

Woodland utilization is a promising disposal method for sewage sludge (SS). However, the potential risk of heavy metals (HMs) transport with runoff must be considered. Among the various factors influencing HMs loss, SS application methods (Holing application, HA; Broadcasting and mixing application, BM; Broadcasting application, BA) are likely to cause significant effects by altering soil erosion and soil aggregates. This study aimed to determine how SS application methods affect HMs loss, soil aggregates erosion, and how they are related. Accordingly, the losses of HMs in surface runoff, interflow, and sediment were quantified during six simulated rainfalls. The results demonstrated that all methods reduced surface runoff, but BA was the most effective. Additionally, BA significantly reduced the total sediment yield and the total proportion of the <0.05 mm fraction aggregates. Moreover, BA had the smallest cumulative losses of Pb and Cd through surface runoff and Cu, Pb, and Cd through sediment. Sediment was the most important pathway for HMs loss, through which over 76.56 % of HMs were lost. In BA, the <0.05 mm fraction aggregates had the lowest HMs load, whereas in other treatments had the highest (54.33 %-80.33 %). The potential ecological risk coefficient of Cd was beyond "moderate" in all the pathways of BM and "high" in the interflow of each SS treatment. Nonetheless, when the multi-elements were evaluated collectively, the potential ecological risk index for each SS treatment was categorized as "low". Overall, BA not only reduced soil erosion but also posed no risk of HMs pollution. It should be noted that the loss of Cd in the interflow had a great impact, while the <0.05 mm fraction aggregates played a significant role in the HMs load. Thus, the current study not only provides an effective approach for the environmentally safe disposal of SS but also proposes a scientific method for the application of SS in woodlands.

Study of experimental and numerical simulation on the influence of gravel on the interflow of slope land

The formation of interflow is of great significance for the stability of mountain slopes and soil erosion. Hillsides are often covered with a certain amount of gravel, and research on interflow of slope land with different gravel ratios needs to be carried out. This article is based on indoor experiments and numerical models to study the formation law of interflow in hillside soil under different gravel coverage ratios. It was found that the interflow in the soil rapidly increased in the early stage and began to decrease after briefly reaching equilibrium. The formation of interflow is a complex process that is related to slope, soil characteristics, and rainfall intensity, but the correlation is not high when viewed separately. The lattice Boltzmann model can effectively simulate such problems and achieve high simulation accuracy. The [Formula: see text] of the simulated data and measured data ranged from 0.5217 to 0.7403, and [Formula: see text] of the simulated data and measured data ranged from 0.4051 to 0.5711.

Surface and subsurface runoff generation processes and their influencing factors on a hillslope in northern China

Runoff processes are essential to the hydrological cycle in mountainous areas. However, many aspects of surface and subsurface runoff generation mechanisms and their influencing factors remain to be fully understood. In this study, rainfall simulation experiments were conducted in micro runoff plots in different slope positions on a typical hillslope to explore runoff processes and their influencing factors in the Taihang Mountain region in northern China. The surface and subsurface runoff and soil water content (SWC) variation processes were analyzed. Moreover, the impact of the soil properties, such as soil saturated hydraulic conductivity (Ks), bulk density (BD), capillary porosity (CP), non-capillary porosity (NCP), and soil organic matter (SOM), on these processes were investigated. The results revealed that the response of the SWC to rainfall was significantly different in different soil layers and slope positions. The response time was slower and the period was longer on the lower slope. However, the middle and upper slopes had a faster response time and shorter period. The surface runoff was the dominant type in the lower slope (67.26 % of the total runoff), while the subsurface runoff was the dominant type in the middle (78.83 %) and upper (83.67 %) slopes. The subsurface runoff was mainly generated in the 40 cm layer on the lower slope, 20 and 30 cm layers on the middle slope, and 30 and 40 cm layers on the upper slope. These layers exhibited good correspondence with the Ks' vertical distribution, but were inconsistent with the other soil properties. These results indicate that the Ks was the most critical factor influencing the runoff generation process. The ratio of the upper layer's horizontal Ks to the lower layer's vertical Ks controlled the subsurface runoff generation process in the hillslope. These findings provide useful information for understanding the hydrological processes in mountainous areas.

Nitrogen removal from low COD/N interflow using a hybrid activated sludge membrane-aerated biofilm reactor (H-MABR)

A hybrid activated sludge membrane-aerated biofilm reactor based on a two-stage simultaneous nitrification-denitrification (SND) process was built, and its utility for treating interflow with low chemical oxygen demand (COD)/total nitrogen (TN) (COD/N) was explored. The operating performance, functional microbial communities, and functional genes for nitrogen metabolism were evaluated at low COD/N (4-1.3). The reactor could achieve stable operation at COD/N = 4-1.5, and the removal efficiency of COD, TN, and ammonia nitrogen was stable at 90.30 ± 2.36 %, 85.69 ± 2.22 %, and 89.52 ± 6.06 %, respectively. The SND rates were 70.89 % and 50.75 % when influent COD/N was 2.0 and 1.7, respectively, indicating that SND makes an important contribution to nitrogen removal under these two COD/N conditions. Microbial analysis revealed that the sampling sites with a high abundance of denitrification genes in the outer ring experienced aerobic conditions, inferring that aerobic denitrification also plays an important role in denitrification.

Suppression of arbuscular mycorrhizal fungi increased lead uptake in maize leaves and loss via surface runoff and interflow from polluted farmland

Arbuscular mycorrhizal fungi (AMF) are ubiquitous in farmland. But the knowledge on AMF impact on lead (Pb) migration in farmland is limited. A field experiment was conducted in the rainy season (May-October) for two years in a Pb-polluted farmland. Benomyl was used to specifically suppress the native AMF growth in the farmland. The effect of benomyl-induced AMF suppression on the Pb uptake in maize, and Pb loss via surface runoff and interflows (20 cm and 40 cm depth) from the farmland was investigated. The benomyl significantly inhibited the AMF growth, resulting in decreases in the colonization rate, spore number, and contents of total and easily extractable glomalin-related soil protein (GRSP); and promoted the Pb migration into maize shoots and mainly enriched in leaves. The particulate Pb accounted for 83.2%-90.6% of Pb loss via surface runoff, while the proportion of particulate Pb loss via interflow was decreased and the proportion of dissolved Pb loss increased with the increase of soil depth. The AMF suppression led to a decrease in dissolved Pb concentration and loss, but an increase in particulate Pb concentration and loss, and enhanced the total Pb loss via surface runoff and interflows. Moreover, significant or very significant negative correlations were observed between the AMF colonization rate in roots with the Pb uptake in leaves, and the content of easily extractable GRSP with the particulate Pb loss. These results indicated the native AMF contributed to immobilizing Pb in soil and inhibited its migration to crops and the surrounding environment.

Pollution in the interflow from a simple landfill in a mountainous and hilly area in Southwest China

Simple landfills lack pollution prevention measures and therefore continuously release pollutants into the surrounding environment. There are a large number of simple landfills in the mountainous and hilly areas in China, and the interflow accounts for a large proportion of runoff. However, the pollution in the interflow stemming from the simple landfill has not been extensively studied. Here, the pollution of the interflow caused by the simple landfill near the Yunxi Town Landfill in the mountainous and hilly region in Southwest China was studied. The composition and pollution release potential of aged refuse in the landfill were determined, and the water quality of interflow around the landfill was monitored for five months. Seasonal changes in water quality of the interflow were observed, and the concentration of pollutants in the interflow around the simple landfill greatly exceeded the water quality standard for the local water function zoning throughout the sampling period. Specifically, the chemical oxygen demand concentrations of the interflow were 247.90 ± 81.57 mg/L, and more than 50 types of refractory organics were detected, with as many as 10 types of polycyclic aromatic hydrocarbons and Environmental Protection Agency priority pollutants. The total nitrogen concentration of the interflow was 132.45 ± 108.68 mg/L; organic nitrogen (53.27%) was the main component, followed by nitrate nitrogen (32.28%) and nitrous nitrogen and ammonia nitrogen (14.45%). The results highlight the need for the remediation of interflow around simple landfills in mountainous and hilly areas. Generally, the basic data could be used to aid the development of remediation technology.

Effects of rainfall intensity and slope gradient on runoff and sediment yield from hillslopes with weathered granite

The method of indoor artificial rainfall simulations was applied to compare the characteristics of runoff and sediment yield under different slope gradients (5°, 8°, 15°, and 25°) and rainfall intensities (30, 60, 90, 120, and 150 mm/h) for two kinds of different hillslopes with weathered granite and with exposed soils respectively from the laterite layer (L-soil) and sand layer (S-soil). The results show that the distribution of runoff yield significantly varied with soil types as the surface flow was predominant for L-soil while interflow was the main runoff form for S-soil. Both surface flow and sediment yield of L-soil was more than that of S-soil, and the changing trends for L-soil were more regular. The relationships between surface flow, sediment yield, and rainfall intensity can be expressed by power functions (R² > 0.68). Interflow was positively related to slope gradient and displayed a single peak curve with the prolongation of runoff time. For S-soil, the surface flow increased with increasing slope gradient under light rainfall intensities but showed a decreasing trend under heavy rainfall intensities. Surface flow for L-soil showed a decreasing trend with increasing slope gradient under all rainfall intensities. The combined effects of slope gradient and rainfall intensity on runoff and sediment yield could be accurately described by linear correlation equations (R² > 0.59). The impact of rainfall intensity on surface flow and sediment yield was much greater than that of slope gradient. Slope gradient presented a more significant effect on interflow. The eroded sediment consisted of a relatively higher content of clay, silt, and fine sand, which was approximately 1.26 times greater than the original soils. There was a grading limit of particle size (0.25 mm) for sediment transport. These results not only demonstrate the effects of rainfall intensity and slope gradient on sloping runoff and sediment yield but also provide valuable information for loss prediction and conservation of soil and water.