The role of urban wastewater on the rate of cohesive sediment erosion in water transfer channels

One of the important influence factors on the critical shear stress of erosion is consolidation. Fluid mud consolidates gradually as a result of self-weight. Many factors affect the consolidation process, including initial concentration, water temperature, hydrodynamic conditions, particle characteristics, and salinity. The critical shear stress for the erosion of cohesive sediment is found to increase with consolidation pressure as well. Bed age can influence the transport characteristics of the deposited sediment by altering the critical shear stress for erosion through compaction. A major number of investigators used circular flume for studying the transport of cohesive sediments. These flumes essentially consists of two main components: a circular channel and an annular cover plate that fits inside the channel. The goal of this study is to better understand the wastewater effects on erosion and self-weight consolidation of cohesive sediment. Also, some experiments were carried out in annular flume using a mixture of cohesive sediment and water with a combination of three concentration of wastewater and four times of consolidation for evaluating their effects on the erosion of cohesive sediments.

In this work, three sets of experiments with three different levels of wastewater (0%, 30%, and 60%) were conducted in an annular flume. The optimal speed ratio between the ring and the flume to be 1.1 for a water depth of 0.2 m. For the plane bed experiment, the water depth above the bed was maintained at 0.2 m. The sediment was sampled from Pribalut dam reservoir, located in the northern Karun basin, Iran. These fine sediments contain 76% silt and 24% clay. Wastewater used in the experiments was taken from Shahrekord wastewater treatment plant outlet, which is located east of Shahrekord city, Iran. In order to test self-weight consolidation of the cohesive sediment, the mixture of three levels of wastewater (0%, 30%, and 60%) with three different initial concentrations of cohesive sediment (200, 300 and 400 g/l) were poured into nine settling columns. By using the linear interpolation method, in order to have 10 cm height of deposited sediment in the annular flume, the initial concentration of the used mixture of fluid and sediment to fill the flume for consolidation times of 1, 3, 14 and 28 days were found 279, 283, 289 and 291 g/l, respectively. To determine the appropriate ratio of the rotational speed of the ring to the flume, velocity and shear stress profiles were measured in the rotating flume, using an Acoustic Doppler Velocimeter (ADV). The flume was operated at a high speed, corresponding to a bed shear stress of 11.2Pa, for 30 minutes to ensure a homogenous suspended sediment mixture. This routine was followed for four consolidation periods of 1, 3, 14 and 28 days in which the bed was left undisturbed. For each erosion experiment in the presence of wastewater, the flume was started from rest and the rotational speed was increased in time increments of 60 min from the first shear stress to forth shear stress and was considered 240 minutes for the last shear stress. Consequently, all experiments were done for a period of 480 min; meanwhile the test samples were collected in a sampling interval of 15 min during the first hour and 30 min thereafter. The samples were taken from depths 15.2, 18.3, 22.2 and 26.8 cm from the bottom.

During the one-day consolidation period, the process of changes in the erosion rates is quite different. In this period, consolidation of the results showed that the presence of wastewater increased the resistance of bed sediment and reduced erosion rate. In this case, due to the results of the sediment consolidation process, the incompleteness of the consolidation process and biota affecting resulted from to the presence of wastewater can be regarded as the main reasons for this difference. During the one-day period of the third shear stress (0.41 N/m2), the erosion rate increased and the highest erosion rate occurred in the fluid containing 60% wastewater and in the fifth stress. It can be inferred that the presence of wastewater in addition to impact on sediment consolidation caused more cohesion of the sediments that withstood the turbulence of the flow until the fourth shear stress and then experienced abrupt erosion at the last stress. This event occurred for pure water and 30% wastewater at the third shear stress. At consolidation periods of 3, 14 and 28 days, the results show that at different concentrations of wastewater, the erosion rate is increased with increase of the shear stress, such that the highest erosion rate occurred at the largest shear stress. ESP also increased with increasing wastewater percentage in the one-day consolidation period. The presence of wastewater also increased the shear stress of the erosion threshold, which is a trend that is unlike the consolidation periods of 3 to 28 days.

The results of this study show that the presence of wastewater has a dual role in the sediment consolidation process such that the rate of consolidation for different time periods in the fluid, it is 30% lower in pure water and 60% in fluids containing wastewater. Compared to pure water and 60% wastewater it was observed that sediment consolidation in 60% fluids is more than pure water. Given the similarity of the conditions of the consolidation experiments (time and type of sediments), the dual role of the wastewater in the consolidation process can be attributed to the difference in fluid quality parameters. In calculating the amount of erosion threshold shear stress, the dual impact of the wastewater is also evident, such that at different time periods, the lowest critical shear stress is for sediments in the fluid containing 30% of wastewater and the highest is for fluids containing 60% of wastewater. Accordingly, the ESP values showed a different trend for the 30% wastewater in all consolidation periods except for one day. For this purpose, the mean value of ESP index and relative height at different levels of wastewater showed that the trend of h/h0 changes for different wastewater percentages was the same as the ESP index changes. Accordingly, ESP values indicate that in fluids containing 30% of wastewater, sediments have more diffusion than other wastewater concentrations.