Figures (9)  Tables (1)

    • Figure 1. 

      Wet sludge generation and disposal in China, 2011–2022.

    • Figure 2. 

      Percentage of sludge disposal methods in major developed countries.

    • Figure 3. 

      Development history of the sludge co-combustion technology.

    • Figure 4. 

      CFB boiler sludge co-combustion process flowchart.

    • Figure 5. 

      Application of sludge.

    • Figure 6. 

      Process path diagram for sludge combustion.

    • Figure 7. 

      Effect of sludge co-combustion on boiler efficiency at different loads. The figure was redrawn and redesigned based on data from Li et al.[63].

    • Figure 8. 

      Ignition and burnout temperatures for sludge co-combustion. The figure was redrawn and redesigned based on data from Tong et al.[68].

    • Figure 9. 

      Effect of sludge co-combustion on heavy metal content in bottom slag and fly ash. (a) Fly ash. (b) Bottom ash[74].

    • Method Effectiveness (in SCOD) Advantages Disadvantages
      Physical method Microwave pretreatment SCOD/TCOD = 0.069% ± 0.009% Rapid thermal transition
      (> 7.5 °C/min), easy to control      		 Expensive equipment, few engineering applications (high-power microwave)[46]
      Ultrasonic pretreatment SCOD = 954.33 mg/L Easy to control, shortening the processing time (4–8 d) High energy consumption, complex equipment (dedicated ultrasound equipment)[47]
      Thermal pretreatment SCOD = 1,180.4 mg/L Easy operation Maillard reaction
      Ball milling pretreatment SCOD = 8,256 mg/L Simple, easy to use Noise and abrasion, high energy consumption (the crushing process lasts 3–7 h)[48]
      Chemical method Acid-base pretreatment SCOD = 2,934.9 mg/L Excellent results 3× compared
      with ultrasonic      		 Corrosion of equipment
      Oxidation pretreatment SCOD = 709.0 mg/L Deep Dehydration Difficult to control the amount of reagents
      and by-products
      Salt pretreatment SCOD = (3,330 ± 196.5) mg/L High solubilization, approx.
      3.3 × 103 mg/L, higher than
      acid/base      		 High dosage and high cost (50 mg of
      high-iodate must be added per gram of dry solids in the sludge)[49]
      Surfactant pretreatment SCOD = (24,306.67 ± 2,143.49) mg/L Efficient, easy to use Expensive, high equipment requirement
      Biological method Enzyme pretreatment SCOD/TCOD = 61.63% Gentle reaction conditions, no secondary pollution Enzymes are expensive and time-consuming

      Table 1. 

      Comparison of the effectiveness of common sludge pretreatment methods