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Biological treatment of gold mine sewage
Sulfate can seriously affect the use of many water sources. When it is discharged in large quantities, it is also the direct cause of mineralization of the receiving water body, and it also indirectly causes some problems due to the high salt content, including the corrosion related to salinity, causing the smell of drinking water to make the boiler and a heat exchanger fouling, and the effect as the use of corrosion-related problems with the biological substrate microorganism, in many mines, often derived from bacterial oxidation sulfate pyrite, sewage and industrial waste and often contains some sulfuric acid. Sulfate can be removed by well-known methods such as reverse osmosis and electrodialysis, but these methods are expensive, so other treatment methods need to be studied. A promising new method is the reduction of sulfate to sulfide by Desulfovibriodesulfuricans. This sulfide is then converted to elemental sulfur.
In the past, many scholars have studied the bioreduction method of sulfate. This article is to introduce the South African Scientific and Industrial Research Council, National Institute of Water to develop a complete biological methods from the gold mine effluent, the results of removing the sulphate process. While studying the removal of these final products, the performance of the packed bed and activated sludge layer reactors was also compared, and the problem of simultaneous removal of heavy metal cyanide complexes was also investigated.
First, continuous test device
Figure 1 shows a first stage anaerobic, aerobic and second stage anaerobic reactor and stripper. The bottom of the reaction column is shrunk (ф25) to aid in liquefaction of the medium when flushing excess sludge with dead bacteria and heavy metal deposits, usually by increasing the circulation at an upward flow rate of 50 m/h. Speed ​​achieved. In order to remove these sludges, a reactor volume of liquid is required. The samples were backflushed at different time intervals and analyzed for the amount of solid suspended solids and volatile solid suspended solids to determine the relationship between sludge accumulation time and unit biological strains.
Figure 1 Sulfate biological reduction experimental device
1—liquid tank containing mine sewage and 3mL/L syrup; B—first stage anaerobic treatment reactor anaerobic treatment; C—stripper; D—inflating tank (required gas treatment stage); E - Settling tank (same D); F - second stage anaerobic treatment reactor (second stage anaerobic treatment; ф - pump medium B medium volume = 750 mL residence time in the reactor; 10 h, reactor Volume of D = 750 mL, residence time in reactor D = 24 h; medium vessel in reactor F = 750 mL, residence time in empty reactor = 10 h); G - overflow chamber (same as F)
The liquid was circulated between the anaerobic reactor and the stripper at a rate of 20 L/d (compared to a feed rate of 1.8 L/d). Hydrogen sulphide is separated in the stripper by a carrier gas, a mixture of nitrogen and CO 2 . An overflow port is provided on the stripper for use as a feed for the aerobic treatment process. Cycle with a compression pump and feed with a NIWR Piston pump.
The 2/3 volume of the anaerobic reactor contains dolomite pebbles with a diameter of 2 to 3 mm. The porosity of the medium was 50%, including the process water (3 ml syrup, 6 mg cyanide, 2 mg cobalt and 2 mg nickel per liter) and stored at 4 °C. A layer of bioactive membrane is produced on the pebble medium by injecting activated sludge into the reactor.
This anaerobic sludge layer reactor is very similar to an upflow packed bed reactor, but uses suspended sludge instead of dolomite pebbles. The inoculum consisted of sludge that was discarded when the packed bed reactor was backwashed.
The aerobic treatment process includes an aeration tank (fully mixed reactor) and a settling tank. Like anaerobic reactors, bioactive sludge is also initially inoculated. The mud age in the aeration tank was 5d, and the oxygen content was maintained at 4 mg/L by continuous inflation. By adding to the liquid flowing into the aerobic treatment step of adding an appropriate amount of ammonia and phosphate, that the ratio is maintained at 50:5:1 C:N:P.
The effluent from the aerobic treatment process is pumped to the second stage anaerobic treatment process. The process includes an upflow packed bed reactor (similar to the one used in the first stage anaerobic treatment process) and an overflow chamber. In this case, the reactor is filled with activated carbon (Filtrabsorb F300 type), which is mainly used for decolorization, but also supports the growth of bacteria. The reactor is filled with the sludge obtained from the activated sludge plant as described above.
Second, the test results
(1) In the first anaerobic treatment process, the influent containing 2480 mg/L sulphate is converted into 180 mg/L sulphate (as SO 4 ) and 220 mg/L sulphide (based on SO 4 ) . ), a solution of elemental sulfur and metal sulfides. At this stage, many heavy metals have been effectively removed, while the calcium carbonate content has reached the level of supersaturation.
(b) In the anaerobic treated water, the final product (ie, soluble organic carbon, residual hydrogen sulfide, calcium carbonate, etc.) can be effectively removed in the next aerobic treatment stage. At this stage, the chemical oxygen demand is reduced to 300 mg / L, hydrogen sulfide is reduced to 20 mg / L, and 1070 mg / L calcium carbonate is crystallized. The content of complex cyanide decreased from 6.5 to less than 1.0 mg/L (by CN). It was found that the ratio of C:N:P=50:5:1 makes the aerobic bacteria function effectively.
(C) The second stage of anaerobic treatment can further purify the acid produced by the decomposition of the syrup and reduce the content of certain heavy metals below the lower limit of detection.
(d) When treating water containing 2480 mg/L of sulfate, the syrup cost required to completely remove these sulfates is 27 cents/m 3 (8 cents/kg syrup price). If you use a waste carbon source such as sludge, you can greatly reduce the cost.
The experiment confirmed that the effluent after three treatments has reached a good water quality standard.