Abstract
Abstract The water volume used for blowdown of a mesh filter is one of the important indicators to evaluate the economic benefit of a mesh filter. The amount of water used for drainage of the mesh filter can be controlled by setting the start-up time of the blowdown residue discharge of the mesh filter. Through the indoor prototype test, the sediment of the Manas riverbed in Xinjiang, China, was used as the filter medium to test the mesh filter and analyze the water usage volume at each stage. The test focused on the analysis of the trend of the pressure difference between the inlet and outlet of the mesh filter over the filtration time. The results showed that as the filtration time was prolonged, the inlet and outlet pressure difference of the mesh filter showed an S-shaped curve with time. The pressure difference between the inlet and outlet can be divided into four stages, i.e., the start of filtering to the first inflection point of the abrupt change in the pressure difference, the first inflection point of the abrupt pressure difference change to the median point of filtering, the median point of filtering to the second inflection point of the abrupt change in the pressure difference, and the second inflection point of the abrupt pressure difference change to the end of filtering. At the same time, the water usage volume for blowdown was calculated under various working conditions. This paper analyzed the start-up time of blowdown at three different time points, i.e., after the completion of the entire filtration cycle of the traditional mesh filter, at the first inflection point of pressure difference, and at the second inflection point of pressure difference. The tests were performed under the condition that the filtration cycle of the mesh filter was ten days. The results have shown that compared with the traditional screen filter that starts the blowdown at the end of the entire filtration cycle, starting the blowdown at the first inflection point of pressure difference can save 200–300 m3 of water resources for a single mesh filter, and starting the blowdown at the second inflection point of pressure difference can save 30–90 m3 of water resources for a single mesh filter. Meanwhile, the working conditions in the analyses in this paper were consistent with the actual engineering operating conditions. It is recommended that the mesh filter in the micro-irrigation system use the first inflection point of pressure difference to control the start-up time of the blowdown residue discharge, which can not only save water resources, but also ensure that the mesh filter runs at the optimal filtering conditions.
Highlights
Kai Shi Zhenji Liu Yan Xie Man Li College of Water Conservancy and Civil
The pressure difference between the inlet and outlet of the mesh filter is mainly caused by the clogging of the pores and the inner surface of the filter
In the actual projects and in many studies, the starting point of the blowdown of the filters was set at the second inflection point of the pressure difference
Summary
Where A1 is the initial value, A2 is the final value, x0 is the center, and dx is a constant. The mathematical expression of the filter pressure difference of the mesh filter with the filter time can be shown in Equation (2): THEORETICAL ANALYSES OF PRESSURE DIFFERENCE AND WATER USAGE FOR BLOWDOWN. The change of the pressure difference of the mesh filter with time was mainly affected by the filtration flow rate. When the clogging on the surface of the mesh reached a certain thickness, the pressure difference between the inlet and outlet of the filter reached the peak value. Where M1 is the amount of blocked sand in the filter screen during the clogging stage (kg), S1 is the influent sand content (g/L), Q1 is the influent filtration flow rate (m3/h), and t1 is the time to reach an inflection point (s), and γ is the percentage of impurities greater than the pore size of the filter. Where β is the number of blowdowns in a filtration cycle
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