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Today, the issue is not only protecting water sources, but also finding & permitting them.  Schreuder, Inc. provides cost-effective, creative & customized solutions to water use/reuse & environmental issues facing you.

Tailing Sand Reduction of Surface Water Turbidity by Filtration

     Schreuder, Inc. performed an investigation of the feasibility of using tailing sand as a filtration medium to reduce the high level of turbidity in the surface waters at a phosphate mine. The cause of the high turbidity readings appeared to be colloidal material.  The widespread availability of tailing sand led to the hypothesis that filtration of the surface water through a tailing sand filter bed could reduce the turbidity concentrations.

     To determine the boundaries of the problem, SI set-up a bench scale test site at their office. Initially SI ran filtration tests through a 36-inch deep (short) test filtration system, measuring percolation capacity and turbidity data.  Based on the results, to measure the effectiveness of a longer flow path a 96-inch deep (tall) filter system was added.  The same tailing sand was placed in the tall and short filtration system.  At regular time intervals the SI hydrologic technician would measure the discharge from each of the filter pipes and record the number. After the rate of flow had been diminishing significantly, he disturbed the surface of the sand/accumulated sediment. This would result in an instantaneous increase in outflow. These incidents were recorded in an attempt to predict the frequency of “clogging”.

     The raw turbidity values varied from 2120 - 1118 NTU. The mean value was 1575 and the median 1528.  The NTU concentrations in the filtrate from the tall column ranged from 259 - 77.2 NTU with a mean of 108.2 NTU and a median of 100.7 NTU.  For the short column the values ranged 445 - 111 NTU, with a mean of 198.0 NTU and a median of 206.5 NTU.  On a median basis the short column reduced the concentrations of the turbidity by 86.5 %, while the tall column reduced turbidity by 93.4 %.

     At the advice of Dr. Harold Bentley, the president of and senior scientist with Hydro Geo Chem, Inc., SI set-up a small bench test to investigate the use of electric current (electro-coagulation) to improve the settlement of colloidal materials.

1)     Filtration by tailing sand will remove turbidity by up to 93%. The bench tests show that the longer the flow path through the sand the greater the reduction in the turbidity. 

2)     The suspended solids, in particular the clay fractions, will significantly reduce the rate of percolation through the sand/water interface.  Even a slight disturbance of the surface will restore the percolation capacity.

3)     Based on a low percap value of 5 ft/day obtained from these bench tests, it may take a filter basin with a surface area of 60 acres to filter 30,000 gpm through a 10 ft thick filter bed under a 5 ft gradient. It will require surface maintenance.

4)  Bench tests indicate electro-coagulation appears to reduce turbidity almost entirely.  The use of stainless steel electrodes and a higher voltage load is preferable than a lower voltage load. The treated water would require a stilling basin to allow the coagulated sediments to settle.

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