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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. |
