1. Background of the case
A certain textile printing and dyeing factory experienced blackening in the wastewater treatment system during the production process. After preliminary investigation, it was found that the main reason for the blackening of wastewater is the reaction between ferrous sulfate and sulfide dyes to produce ferrous sulfide, as well as the production of iron oxides during aeration and biochemical processes. In addition, insufficient nutrition and aeration also exacerbate the blackening of wastewater.
2. On site issues
a. Blackening of wastewater color: The wastewater color is significantly darker, which affects the appearance of the effluent and may indicate internal problems in the treatment system.
b. Decreased treatment effect: Due to the blackening of wastewater, the biochemical system is affected, resulting in a decrease in treatment efficiency and difficulty in meeting effluent standards.
c. Increased energy consumption: In order to address the issue of blackening, the aeration rate and stirring intensity have been increased, resulting in an increase in energy consumption.
3. Data representation
Measurement |
Normal range |
Current Data |
Wastewater color |
Yellowish to colorless |
Black color |
CoD(Chemical Oxygen Demand) |
≤100mg/L |
200mg/L |
BOD(Biological Oxygen Demand) |
≤30mg/L |
50mg/L |
Sulfide content |
≤1mg/L |
5mg/L |
Aeration volume |
200m³/h |
300m³/h |
4. Handling measures
a. Increase the dosage of ferrous sulfate: Depending on the nature of the wastewater, increase the dosage of ferrous sulfate appropriately to improve the coagulation and adsorption effect. After adjustment, the dosage of ferrous sulfate was increased from 0.5% to 0.8%, and the color of the wastewater was improved.
b. Adding coagulants: Adding coagulants (such as lime, polyacrylamide, etc.) to wastewater can accelerate the bonding speed of ferrous sulfide and promote its rapid aggregation into large chunks of alum precipitate. After experimental verification, the addition of 0.2% lime and 0.05% polyacrylamide effectively removes ferrous sulfide from wastewater.
c. Optimize the aeration system: Optimize the aeration system to ensure sufficient aeration volume and uniform distribution. Meanwhile, adjust the aeration time and intensity according to the nature of the wastewater. After adjustment, the aeration rate was reduced from 300m ³/h to 250m ³/h, while ensuring sufficient oxidation of the wastewater.
d. Increasing nutrient intake: To address the issue of nutrient deficiency, it is appropriate to increase the dosage of nutrients (such as urea, phosphate, etc.) to meet the nutritional needs of the biochemical system.
5. Processing effect
After the implementation of the above measures, the blackening phenomenon of wastewater has been effectively controlled, and the treatment effect has been significantly adjusted. The following is the processed data:
Measurement |
Post-treatment data |
Wastewater color |
Yellowish |
CoD(Chemical Oxygen Demand) |
80mg/L |
BOD(Biological Oxygen Demand) |
25mg/L |
Sulfide content |
0.5mg/L |
Aeration volume |
250m³/h |
By comparing the data before and after processing, it can be seen that the color of the wastewater has changed from black to light yellow, and the COD, BOD, and sulfide contents have all decreased. The aeration rate has also decreased, and the treatment measures have achieved good results