Improvement of vacuum pressure on pumping performance of vertical wells in municipal solid waste landfills.

The pumping performance of the traditional vertical well is often poor in municipal solid waste (MSW) landfills due to the blocking effect of landfill gas on leachate migration. To improve the pumping performance, a vacuum vertical well was designed and then installed at the Tianziling landfill. When the leachate was drawn out through submersible pump, the landfill gas was simultaneously extracted through vacuum pump to form vacuum pressure in the well. The vacuum pressure could increase the hydraulic gradient of leachate flow as well as the relative liquid permeability of MSW. Pumping tests were carried out to explore the effectiveness of the vacuum pressure on improving the pumping performance of vertical well. When the vacuum pressure increased from 0 kPa to - 30 kPa, the steady leachate pumping rate increased from 1.58 to 2.34 m/h, and the steady leachate level drawdown increased from 5.9 to 10.3 m at the distance of 5 m. The vacuum pressure mainly affected the leachate level drawdown within the distance of 15-20 m. When the vacuum pressure in the pumping well was - 30 kPa, it attenuated to - 14.7 kPa and - 6.6 kPa at the distance of 5 m and 10 m, respectively. The influence radius of vacuum pressure was about 15 m. Numerical modeling indicates that the leachate pumping rate and drawdown will decrease with the increase in decreasing rate of hydraulic conductivity with depth, degree of heterogeneity, and anisotropy of hydraulic conductivity of waste. The experimental and numerical results demonstrate the effectiveness of vacuum pressure and provide working parameters for the application of the vacuum wells in MSW landfills.