Treatment of extremely high turbid water by chitosan and aluminium salts
Date Issued
2011
Date
2011
Author(s)
Chen, Fu-Ling
Abstract
Because a rapidly changing climate, special topography and over-exploitation of catchment areas, the frequently high turbidity raw water is caused by heavy rains and increase year by year. Therefore, for water supply units with surface water as their major water source, a supply of sufficient safe drinking water in compliance with provisions has become a severe challenge during storms. The utilization of aluminum salt coagulant currently results in increased sludge output, leading to excessive loads for water purification units and reducing or even cutting-off water supplies. Increased aluminum salt coagulant will increase the aluminum content in water and thus bring more health risks. Hence, it is important to improve the treatment efficiency of coagulants in high turbidity raw water and reduce the aluminum content in water.
In this study, 10000 Nephelometric Turbidity Unit (NTU) of artificial raw water was prepared in a laboratory and 5000NTU of raw water was sampled from the water intake of Zhitan Water Treatment Plant during Typhoon Maggie, respectively. The aluminum salt (Polyaluminum Chloride, aluminum sulfate) and natural polymer chitosan were used as the coagulants. Experiments with varying operational parametersof pH, residual turbidity, sludge volume and dissolved aluminum analysis were evaluated for the overall effectiveness of different coagulation models.Results from the test with 10000 NTU artificial raw water showed that addition of aluminum salt coagulant would increase the sludge volume to 60-70ml/L and cause the residual dissolved aluminum content in water to exceed 300μg/L. The Polyaluminum Chloride coagulant revealed better turbidity removal performance than the aluminum sulfate coagulant. Sludge volume generated by chitosan could be maintained at about 40 ml/L, and dissolved aluminum in the water could be reduced to less than 50μg/L. Turbidity removal efficiency of chitosan was similar to that of aluminum chlorohydrate. But none of the three could reduce residual water turbidity to less than 10 NTU with increased dosage.When chitosan was combined with fixed-concentration (0.2mg/L, 0.5mg/L) aluminum salt to treat 10000 NTU of artificial raw water, the residual turbidity and residual dissolved turbidity reduced to less than 10 NTU and 25μg/L, respectively, at low additive dosage, while sludge volume unchanged. When aluminum salt was combined with chitosan with a fixed concentration of 1mg/L, the turbidity removal efficiency was similar to that of chitosan alone. However, it only required a relatively low dosage. In addition, sludge volume and residual dissolved aluminum would increase with increased dosage while the content of dissolved aluminum in water was 200 μg/L. This result was lower than that of aluminum salt alone. Test results of the high turbidity raw water coagulation model during a typhoon were agreement with those of artificial high turbidity raw water. Nevertheless, the turbidity removal efficiency did not show significant difference because erosion of heavy rainfall produced particles more likely to settle, resulting in the trend of generated sludge volume and residual dissolved aluminum being the same as that of 10000 NTU artificial raw water.
From the above results, it can be concluded that chitosan combined with a small amount of aluminum salt coagulant has good turbidity removal effect on high turbidity raw water. It can produce low sludge output which remains unchanged, thereby reducing the load for water purification and treatment units during storms. Also, increasing water supply efficiency, reducing the aluminum content in raw water and avoiding increased risk of illness caused by excessive aluminum content can be achieved by combination of chitosan and aluminum salt coagulant.
Subjects
chitosan
coagulant
dissolved aluminum
high turbidity raw water
sludge
Type
thesis
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