When the dosage of powdered activated carbon is below 20 mg/L, the effluent tends to have a stronger odor. This occurs because organic matter in the water occupies the adsorption sites of the carbon, preventing the removal of odor-causing substances. Additionally, the negative charge of the powdered activated carbon can interfere with coagulation, especially at lower dosages. As a result, turbidity increases, and the coagulation process becomes less effective, leading to higher odor levels in the treated water.
As the dosage increases from 20 mg/L to 50 mg/L, more adsorption sites become available, allowing the remaining odor compounds to be effectively removed, which gradually lowers the odor value of the effluent. However, once the adsorption capacity reaches saturation, further increases in the dosage do not significantly reduce the odor level.
When potassium permanganate is used in combination with powdered activated carbon, the results are even more promising. At a fixed dose of 0.5 mg/L of potassium permanganate, increasing the powdered activated carbon dosage from 20 mg/L to 40 mg/L reduces the effluent odor from 25 to 5, with no detectable odor. This method outperforms both potassium permanganate pre-oxidation and powdered activated carbon alone in terms of deodorization efficiency. Moreover, using this combined approach allows for a reduction of about 20% in the amount of powdered activated carbon needed while still maintaining an effluent odor threshold below 10.
In cases where the raw water has a high odor threshold (greater than 90), the dosage of powdered activated carbon must be increased to 40 mg/L to ensure that the treated water has an odor threshold of 7 or lower, meeting national drinking water quality standards. In real-world applications, factors such as the ability of filters to remove residual odors and cost considerations often lead to a reduced dosage of powdered activated carbon.
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