The filter belt and bearings used are all high quality parts. As a supporting equipment for sewage treatment, it can press and dewater the suspended matter and sediment after air flotation treatment as well as the sludge produced in each biological section of sewage treatment, and press it into mud cake to achieve the purpose of preventing secondary pollution, which is a good solid-liquid separation equipment; the machine can also be used for slurry concentration, extraction of black liquor and applicable to various solid-liquid separation processes.

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- low electricity consumption and large processing capacity.
- high dewatering efficiency and high solid content of mud cake.
- Easy operation and management, easy maintenance.
- high degree of automation, continuous production.
- Low noise and long service life.
- Less accessory equipment, economic and reliable, wide application range.

Preface

The belt type thickening and filtering sludge dewatering machine is an efficient solid-liquid separation equipment based on the principles of chemical flocculation contact filtration and mechanical squeezing, which has a series of advantages such as simple process flow, high degree of automation, continuous operation, easy control and operation and adjustable working process, and saves the sludge thickening tank and construction funds to a certain extent, and is being more and more widely used.

The flocculated sludge first enters the gravity dewatering zone, where most of the free water is filtered out through the filter belt under the action of gravity; with the operation of the filter belt, the sludge enters a wedge-shaped zone consisting of two filter belts, which slowly pressurise the sludge, which gradually thickens and reduces its liquidity, and transitions to the press zone; in the press zone, the sludge is subjected to increasing squeezing pressure and the shear force generated by the alternating changes in position on and off the two filter belts. In the press area, the sludge is subjected to increasing squeezing pressure and the shearing force generated by the alternating change of position on and off the two filter belts.

The main technical and economic indicators involved in the actual engineering application of the belt filter press are.

①Processing capacity.

②Water content of the sludge cake.

③ Chemical dosage.

④Power consumption.

⑤ Flushing water consumption.

⑥Belt tension.

⑦Effective bandwidth.

⑧Running speed of the filter belt.

⑨ Main indicators such as air source pressure.

Among them, the processing capacity is the primary indicator to evaluate the comprehensive performance of the belt filter press. There are many factors affecting the processing capacity of the belt filter press, but they are mainly reflected in the gravity dewatering area, the pressing area and its belt running speed, belt tension, roller diameter (size, wrap angle and centre distance), belt (air permeability) selection, dosing conditioning effect, etc. It is also a comprehensive reflection of the quality of the belt filter press structure design and manufacturing. Therefore, understanding the calculation method of the processing capacity of the belt filter press has certain guiding significance for the optimal design of the belt filter press, the selection of operating parameters, and the selection of reasonable dosing of drugs.

Calculation of processing capacity

2.1 The first algorithm

Using the thickness of the wet mud cake output of the belt filter press as the main calculation parameter, according to the calculated wet mud cake output, and then calculate the feed volume (i.e. treatment capacity), the calculation formula is as follows.

Q wet mud cake = B – ξ – δ – v – s – γ – β

Where: Q wet mud cake – wet mud cake output t/h

B – the width of the filter belt m

ξ – filter belt width utilization coefficient, generally take 0.85 ~ 0.9

δ – wet mud cake thickness m, generally take 6 ~ 10mm (0.006 ~ 0.01m)

v – the actual working speed of the filter belt m/min, generally take 3-6m/min

s – unit time 60min/h

γ – specific gravity of wet mud cake t/m3, generally taken as 1.03 t/m3

β – solid phase recovery rate, generally taken ≥ 95%

Q feed = (wet mud cake solid rate / feed solid rate) × Q wet mud cake (t/h)

From the above formula, it can be seen that the calculation method is based on the thickness of the wet mud cake output of the belt filter press as the main calculation parameter, while the formation of the wet mud cake thickness on the one hand has a great relationship with the operating parameters of the belt filter press such as the running speed of the filter belt and the filtration pressure; on the other hand, it also has a great relationship with the nature of the sludge such as solids concentration, viscosity and the specific resistance of the sludge after dosing and conditioning; the formation of the wet mud cake thickness also depends on the structural design of the filter press. The formation of the wet mud cake thickness also depends on the structural design of the filter press, such as the length of the thickening section, the capacity of the thickening section, the filtering time and the filtering cycle, and the choice of the air permeability of the filter belt. In the calculation formula, Q wet mud cake is linearly related to the thickness of wet mud cake δ. The thickness of wet mud cake can be selected from 3 to 10 mm, and the thickness of wet mud cake formed in the actual operation of many belt filter presses is not uniform within the width of the filter belt.

Therefore, this calculation method is not combined with the main technical parameters of the thickening section, the press section and the main nature parameters of the sludge, and does not reflect the effect of the sludge dosing and conditioning effect, the design of the structure parameters of the filter press, the changes in the operating parameters and other factors on the processing capacity of the belt filter press, and the calculated value range of Q wet mud cake is large, which is generally applicable to the design and selection of the belt filter press, and has little significance for the optimal structure design and guidance of the operation of the belt filter press.

2.2 An alternative algorithm.

Sludge dewatering systems for municipal wastewater and industrial wastewater require dosing and conditioning of the sludge prior to sludge dewatering. The purpose of dosing and conditioning is to improve the dewatering performance of the sludge, reduce the affinity of the water in the filter press wastewater treatment sludge and reduce the filtration specific impedance value (i.e. the resistance of the filter cake) r and capillary absorption time CST of the sludge.

At the beginning of the filtration press, the filtrate must overcome the resistance of the filter medium (filter belt), and when the filter cake is gradually formed, it must also overcome the resistance of the cake itself, which is the basic form of filter cake filtration. The basic equation for filtration (i.e. processing capacity) can be calculated using the basic equation for Carman filtration, which is derived from the basic principle of liquid flow through the filter residue layer.

According to the basic equation of Carman filtration.

(1)

where: V – volume of filtrate m3

t – filtration time s

P – filtration pressure Pa

A – filtration area m2

μ – dynamic viscosity of the filtrate Pa-s

ω – weight of dry solids retained on the filtration medium per unit volume of filtrate filtered kg/m3

r – specific resistance m/kg, i.e. the resistance of the filter cake, defined as the resistance per unit dry weight of the filter cake per unit filter area

Rf – impedance of the filter medium 1/m2

From the definition of ω the following equation can be written.

(2)

Where: Q0 – incoming sludge volume (treatment volume) m3

Qf – volume of filtrate m3

Ck – concentration of solids in the filter cake kg/ m3

According to the liquid-phase equilibrium relationship: Q0= Qf + Qk

According to the solid phase equilibrium relationship: Q0C0= QfCf+ QkCk

gives

(3)

Where: C0 – solids concentration in the incoming raw sludge g/L

Cf – concentration of solids in the filtrate g/L, sludge dewatering system generally requires solids recovery rate ≥ 95%, Cf value is very small, the actual calculation can be taken Cf = 0

Qk – the volume of filter cake L

According to the definition of filtration yield: the dry weight of the filter cake produced on the unit filtration area per unit of time kg/(m2-s) or kg/(m2-h).

For the convenience of calculation, temporarily set the impedance of the filter medium Rf = 0, then the basic equation of Carmen filtration (1) equation becomes

that is

(4)

Set the dry weight of the filter cake is W, then W = ωV, V = W/ω into (4) finishing: that is: the belt filter press processing capacity L (in terms of the amount of absolute dry sludge, that is, the dry weight of the filter cake) is: (kg/m2.s)

(5) (5) is another formula for calculating the processing capacity of the belt filter press.

For urban wastewater plant sludge dewatering systems, the sludge is conditioned by dosing, and the specific resistance r of the sludge is controlled at (1 to 4) x 1012 m/kg (the laboratory is to determine the more economical dosing amount by measuring the r value), where for the belt filter press, the laboratory generally conditions the sludge by dosing with the specific resistance r at (1 to 3) x 1012 m/kg (centrifugal dewatering r = (2 to 4) x 1012m/kg). The kinematic viscosity of sludge μ = 0.001 Pa-s at an ambient temperature of 20°C.

When using the formula (5), the calculation should be divided into two parts for the belt filter press: the thickening section and the pressing section. The thickening section is a gravity filtration dewatering, the filtration pressure P is calculated according to the height of the sludge entering the sludge tank in the thickening section (average thickness, 1mm = 9.5 Pa), the concentration C0 is the incoming sludge concentration, Ck out of the thickening section sludge concentration, which can generally be calculated as 8-10% (the concentration when the sludge basically loses its liquidity after gravity dewatering through the thickening section). The filtration time t is obtained according to the length of the thickening section and the walking speed of the filter belt.

For the press section, the pressure P is calculated by the tension of the filter belt, the contact area between the press roll and the filter belt, the concentration C0 entering the press section is the sludge concentration out of the thickening section (8-10%), Ck is the final filter cake concentration (25%-20%), the filtering time t=m/T, m=ts/T (i.e. t=ts/T2), ts is the actual filtering time (obtained from the contact length between the filter belt and the press roll in the press section and the running speed of the filter belt in the press section), and T is the filtering cycle (running time of the press section).

The sum of the processing capacity of the thickening section and the pressing section is the processing capacity of the belt filter press. Since the impedance of the filter belt is not taken into account in the calculation, and the impedance of the filter belt is related to the material of the belt and the ventilation volume [generally 8000-10000m3/(h-m2)], the final treatment volume should be the above calculated treatment volume multiplied by a factor K (K is generally taken as 0.9-0.95), which is the treatment volume of the belt filter press.

Conclusion

The second algorithm above is more complicated than the first algorithm, but the calculation method contains the influence of the structural design parameters of the belt filter press, sludge nature parameters and operating parameters on the treatment capacity, which has certain guiding significance for optimising the structural design of the belt filter press, reasonable dosing and selection of operating parameters to improve the treatment capacity of the belt filter press. It is also calculated by example that the processing capacity of the thickening section plays a major role in the processing capacity of the belt filter press.