Chinese
The rubber belt filter is a highly efficient solid–liquid separation device based on the principles of chemical flocculation contact filtration and mechanical squeezing. Because of its advantages such as simple process flow, high degree of automation, continuous operation, convenient control, adjustable operating conditions, and the elimination of sludge thickening tanks—which helps reduce construction costs—it has been increasingly widely used in many industries.
After flocculation, the sludge first enters the gravity dewatering zone, where most of the free water is removed through the filter belt under the action of gravity. As the belt moves forward, the sludge enters the wedge zone formed by two filter belts. In this area, the two belts apply gradual pressure to the sludge, causing it to thicken and lose fluidity before entering the pressing zone. In the pressing zone, the sludge is subjected to increasing squeezing pressure and shear forces generated by the alternating upper and lower positions of the two belts. Most of the remaining free water and pore water in the sludge is removed, and the sludge becomes a sheet-like filter cake with relatively low moisture content. The upper and lower filter belts separate at the discharge roller, and due to the curvature change of the belt and the action of a scraper, the filter cake is removed. The belts are then washed and reused for the next filtration cycle.
1. The main technical and economic indicators involved in practical engineering applications of rubber belt filters include:
① Processing capacity
② Moisture content of the sludge cake
③ Dosage of chemical reagents
④ Power consumption
⑤ Washing water consumption
⑥ Belt tension
⑦ Effective belt width
⑧ Filter belt operating speed
⑨ Air source pressure
Among these indicators, processing capacity is the most important parameter for evaluating the overall performance of a rubber belt filter. Many factors influence the processing capacity, mainly including the gravity dewatering zone, the pressing zone, filter belt running speed, belt tension, roller diameter (size, wrap angle, and center distance), filter belt selection (air permeability), and chemical conditioning effects. These factors also reflect the overall quality of structural design and manufacturing of the equipment. Therefore, understanding the calculation method of the processing capacity is helpful for optimizing the design of rubber belt filters, selecting appropriate operating parameters, and determining reasonable chemical dosage.
2. Capacity Calculation
Taking the thickness of the produced wet sludge cake as the main calculation parameter, the wet sludge cake output is calculated first, and then the feed amount (i.e., processing capacity) is derived. The calculation formula is as follows:
Qwet cake = B • ξ • δ • v • s • γ • β
Where:
Qwet cake — Wet sludge cake output (t/h)
B — Filter belt width (m)
ξ — Belt width utilization coefficient, generally 0.85–0.9
δ — Thickness of wet sludge cake (m), usually 6–10 mm (0.006–0.01 m)
v — Actual operating speed of the filter belt (m/min), generally 3–6 m/min
s — Unit time, 60 min/h
γ — Specific gravity of wet sludge cake (t/m³), usually about 1.03 t/m³
β — Solid recovery rate, usually ≥95%
Feed capacity calculation:
Qfeed = (solid content of wet cake / solid content of feed sludge) × Qwet cake (t/h)
From the formula above, it can be seen that this calculation method mainly uses the thickness of the wet sludge cake as the key parameter. The formation of sludge cake thickness is closely related to the operating parameters of the rubber belt filter, such as belt speed and filtration pressure. At the same time, it is also strongly affected by sludge characteristics, including solid concentration, viscosity, and specific resistance after chemical conditioning. In addition, the thickness of the wet sludge cake also depends on structural design factors of the filter press, such as the length and capacity of the thickening section, filtration time, filtration cycle, and air permeability of the filter belt. In the formula, Qwet cake has a linear relationship with sludge cake thickness δ. The thickness generally ranges from 3–10 mm, and in practical operation the cake thickness may not be uniform across the belt width.
Therefore, this calculation method does not fully integrate the key parameters of the thickening section, pressing section, and sludge properties. It also does not fully reflect the influence of sludge conditioning, filter press structural design, or operational parameter changes on the processing capacity. As a result, the calculated value of Qwet cake often has a wide range and is mainly suitable for equipment selection during design rather than for detailed structural optimization or operational guidance.
In municipal wastewater treatment plants, after chemical conditioning, the specific resistance of sludge r is generally controlled within (1–4) × 10¹² m/kg. In laboratory tests, the r value is used to determine the most economical chemical dosage. For rubber belt filters, the conditioned sludge typically has a resistance range of (1–3) × 10¹² m/kg, while centrifuge dewatering sludge may have r values of (2–4) × 10¹² m/kg. At an environmental temperature of 20°C, the kinematic viscosity of sludge μ is approximately 0.001 Pa·s.
In the pressing section, the pressure P can be calculated from belt tension and the contact area between pressing rollers and the belt. The concentration of sludge entering the pressing section (C₀) is the sludge concentration leaving the thickening section (8–10%), while Ck represents the final cake concentration (20–25%). The filtration time t = m / T, where m = ts / T (i.e., t = ts / T²). Here, ts is the actual filtration time, determined by the contact length between the belt and pressing rollers and the belt running speed, while T is the filtration cycle time.
The total processing capacity of the rubber belt filter equals the combined processing capacities of the thickening section and the pressing section. Because the calculation does not include the resistance of the filter medium (Rf), and Rf depends on the filter belt material and air permeability—generally selected within 8000–10000 m³/(h·m²)—the final calculated capacity should be multiplied by a correction coefficient K (typically 0.9–0.95). This corrected value represents the actual processing capacity of the rubber belt filter.
3. Conclusion
Although the second calculation method is more complex than the initial one, it incorporates structural design parameters of the rubber belt filter, sludge property parameters, and operating conditions. Therefore, it provides more practical guidance for optimizing structural design, determining appropriate chemical dosing, selecting operating parameters, and improving overall processing capacity. Practical calculations show that the capacity of the thickening section plays a major role in determining the overall processing capacity of the rubber belt filter.
