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The Prediction of Filter Belt Press Dewatering Efficiency for Activated Sludge By Experimentation on Filtration Compression Cells Article  in  Environmental Technology · January 2005 DOI: 10.1080/09593332508618474 · Source: PubMed

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THE PREDICTION OF FILTER BELT PRESS DEWATERING EFFICIENCY FOR ACTIVATED SLUDGE BY EXPERIMENTATION ON FILTRATION COMPRESSION CELLS J. OLIVIER1,2, J. VAXELAIRE1,*

1- Laboratoire de Thermique, Energétique et Procédés, ENSGTI Rue Jules Ferry, BP 7511, 64075 Pau cedex, France (*tel : 33 (0)5 59 40 78 11, fax : 33 (0)5 59 40 78 01,  : [email protected])

2- E.M.O. s.a. 40 rue du Bignon, Immeuble le "Ponant ", B.P. 17, 35574 Chantepie Cedex France

ABSTRACT The filter belt press is commonly used to dewater activated sludge. However, little research has been done on this process and the prediction of its efficiency. Experimentation have been carried out in a filtration compression cell (FCC) and in a pilot scale filter belt press. It offers a way of determining filter belt press efficiency thanks to simple laboratory research. The pressure distribution around the pressing roller was measured inside the pilot scale filter belt press. It showed progressive increase (up to a certain maximum value : plateau), which was followed by a rapid decrease. The impact of the progressive increase of applied pressure onto the dry solid sludge content was observed in FCC. Similar dry solid contents were obtained from both the above laboratory devices when the application of the pressure is comparable (in time and increasing rate).

Keywords: Wastewater treatment, filter belt press, dewatering, sewage sludge.

INTRODUCTION

The enforcement of current European regulation on wastewater treatment has led to a significant increase in the amount of municipal sludge being produced. Consequently, management and treatment of these sludges have became more important. Currently, two

major methods are available for sludge disposal; incineration and landspreading. For both, a preliminary step of dewatering is usually required. Due to their relatively low energy cost, mechanical devices, such as filtration or centrifugation, are currently used to achieve this dewatering. Amongst the common devices, filter belt presses are widely used in municipal wastewater treatment plants. In spite of its widespread usage, the design of this type of press remains essentially empirical and the operating parameters are generally fixed according to field testing [1,2]. However tests on field industrial systems are not always easy to set up and some alternative methods of characterisation may be preferred. Only a few published studies specifically deal with the prediction of the efficiency of the belt filter press. Some authors [3] have established, from a study carried out on a laboratory scale pilot belt press (containing 8 rollers), different equations for predicting the efficiency of the belt filter press. These equations allow one to estimate the solid content of the final cake, the total filtrate flow rate, the cake’s width and the solids’ recovery efficiency according to the input sludge’s flow rate, the initial solid content of sludge and belt speed (Table 1). Two years later, from a compilation of performance data obtained from a survey of over 100 American installations, some general equations to predict dry solid content in pressed cakes, as compared to dry solid content when entering the device, were proposed [4] (Table2).

These different equations do not correlate well with the majority of the experimental data from current literature. Consequently the interest of such global relationships remains very limited. To establish more practical equations, filter belt press efficiency must be evaluated with regard to some measurable physicochemical characteristics of sludges. In this way, a study [5] tried to define the major factors (among 30) that affect the dewatering characteristics of sewage sludge in belt filter presses. They showed that only the viscosity of raw sludge could be correlated to the moisture content of dewatered cake for mixed and anaerobically digested sludges. The viscosity was affected by the composition in terms of colloids, fibres, volatile suspended solids and ash.

Due to the current problem of anticipating the behaviour of such an elusive material to study, it seems useful to develop some laboratory tests capable of easily predicting the efficiency of the industrial devices. Several techniques, classically used to characterise solid-liquid separation, have been tested; such as gravity drainage methods [6,7], capillary suction time measurement

(CST) [6,8,9], measurement of the specific resistance of filtration (SRF) [6,9,10] in filtration compression cells (FCC) or on funnel-type vacuum units [8], laboratory centrifugation [5,11], or the measurement of infinite cake solid content [12]. The inability of all these tests to accurately predict the efficiency of industrial belt filters are due to several aspects, such as differences in dewatering times [6,13] and in the value of applied pressures. To progress on these aspects other methods of investigation were proposed. A classical FCC was modified with a rotary piston to take into account the shearing observed in belt filters [10]. Nevertheless, the results obtained from this modified device cannot be directly applied to the design of industrial devices. Other authors [14, 15] have used a permeable piston and consequently modified the FCC to a double-sided piston filter press. This modification enables better prediction of industrial effectiveness, but does not consider the pressure distribution around the roller. Some specific laboratory devices dedicated to the study of the belt filter press have also been developed. A device constituted of a belt (length = 1.8 m, width = 0.2 m) tensed by two rollers (diameter = 0.25 m) was used [16]. The belt speed (1-17 m.min-1) was set by an electric motor and a pneumatic jacket allowed to fix the belt tension. For experimentation, a sample of drained sludge was laid on the belt and trapped with a piece of filter cloth (length = 0.6 m) which was on the sludge and clipped at the ends. This stage of installation of the sludge is delicate and can have some effects on the experimental results, which could explain why this apparatus is used rarely. Another laboratory device called the "Crown Press " was also developed [17]. It is a PVC pipe cut in half lengthwise (the crown) with slots to allow filtrate drainage [18-21]. A curved surface was used to better simulate the pressure distribution around pressing rollers. A piece of filter cloth was fixed on the crown and a sample of sludge was laid on it. Then, another piece of cloth was fixed on the crown and put over the sludge to squeeze it. The bottom of this second piece of cloth was tensed by a hook attached to a pinion system. This specific device has given some interesting results but it is not often used. Equipment suppliers generally prefer simpler and more standard tests such as FCC.

The aim of this paper is to propose a procedure to carry out FCC tests according to belt filter specificity. This work should improve and facilitate the design of industrial belt filter presses for biological sludge dewatering.

MATERIALS AND METHODS Experiments were carried out on activated sludge with two different devices: a pilot belt filter press and a filtration compression cell (FCC). The laboratory scale belt filter press The pilot device was a small scale belt press (0.5 m * 2 m * 1.5 m). This is where the sludge dewatering was studied, only around one pressing roller, the roller 3 (Fig 1). Different diameters of the pressing roller can be adapted to this device, the one chosen for the present work was of 0.27 m. All the other rollers were necessary to ensure the position, the tension and movement of the two belts. More specifically, the rollers 1 and 11 are fixed to pneumatic jacks to set the belt tension. An electric motor equipped with a regulator allowed us to set the belt speed. The roller 9 is covered by a rubber liner and transmits the movement. The direction of the moving belt can be reversed to study dewatering under several series of pressing cycles in order to come closer to industrial devices (which operate with a series of pressing rollers). The belt was a 16-6 herringbone twilled wave belt manufactured by Rai-Tillères. The filtrate was collected and weighed for every pressing cycle around the designated roller. The dewatered cake was removed and weighed at the end of the experiment. The dry solid content in the final cake was measured after being dried at 105°C. It was also calculated after each cycle (across the pressing roller), from a mass balance based on the mass of collected filtrate. Four replicated tests were carried out to evaluate the reliability of the experiments in terms of dry solid content in the final cake. The relative error ( Cmax Cmin ) was estimated to be under 3%. Caverage Moreover, some tests were carried out to compare dewatering efficiency between the laboratory device and an industrial belt filter press operating in the Pau-Lescar municipal wastewater treatment plant (France). These experiments were performed on the same sludge in similar operating conditions, in terms of belt tension, belt speed, sludge loading, and the number of cycles (across pressing rollers). The results of this comparison have shown accurate agreement between the laboratory device and the industrial one [22].

The filtration compression cell (FCC)

The filtration compression cell was built according to French standards (the AFNOR T 97-001). It is a 0.15 m deep cylindrical stainless steel chamber with an internal diameter of 0.07 m. A perforated disk was located at the bottom of the cylinder in order to support the filter medium. The filter cloth was similar to the belt used on the pilot device. The pressure on the piston was applied and controlled by pressurised air. The pressure range was 0.5 to 15 bar. The mass of collected filtrate was recorded over time on a personal computer using a software program developed in the laboratory. The reliability of the experiment was evaluated on four replicated tests. The curves describing the kinetic of dewatering were accurate and the relative error of the dry solid content in the dewatered cake was estimated to be under 6% [22].

The sludge The activated sludge extracted from the thickener at the Pau-Idron municipal wastewater treatment plant (France). It was conditioned by a high molecular weight and high charge density (80%) cationic polymer (SNF Floerger, France, ref. EM 840 TRM). The conditioner dose was fixed from preliminary experiments to 7 g kgDS-1. The polymer solution (2.15 g l-1) was added with a syringe while the sludge was gently shaken in a stirred vessel for 30 s. In order to simulate industrial practices the sludge was drained during five minutes on a gravity drainage system before the dewatering experiments began (on the pilot device and in FCC).

RESULTS AND DISCUSSION Pressure measurements To evaluate belt filter press effectiveness it is important to know the operating parameters, especially the pressure applied around the pressing rollers. Usually this pressure is determined by the following equation [19,23] (not including rolling friction and belt elasticity) :

Proller 

2Tbelt (i) Drollerlbelt

According to this equation, the pressure is constant around the roller. However, it was shown (from direct measurements of pressure in a belt filter press) that equation (i) considerably under-predicts the true applied pressure [24]. Besides, the pressure was not constant around the roller. To clarify this aspect, we have set up some direct pressure measurements in the pilot filter belt press. A miniature pressure transducer (Kyowa, Japan, ref. PS-5KA) was placed between the two belts and carried around the pressing roller while in rotation. These measurements have shown, similar to the results of the literature [24], that the pressure was not constant, with a progressive increase, a plateau and steep decrease (Fig 2). The shape of the pressure distribution around the roller is not really affected by the tension applied on the belts (Fig. 2a) but depends on the roller diameter (Fig. 2b). Moreover, the maximal pressure reached (Pmax) was much higher than the one calculated from equation (i). In this study (0.3 m belt width and a 0.27 m roller diameter) it can be calculated from the following empirical equation : Proller  1.06 10 3 Tbelt  0.6186 (ii)

with Tbelt in N and Proller in bar.

It is this "true" pressure which should be used for tests in FCC. The impact of the applied pressure Another important aspect to specify, was the influence of the applied pressure on sludge dewatering by compression. On this particular point, the data reported in current literature generally concerns initial liquid sludge (not drained) and leads to quite divergent results. Indeed, for undrained sludge, few authors [25,26] have observed during the filtration step, the significant influence of the applied pressure, whereas others [27,28] have shown that for a highly compressible material such as activated sludge, pressure has no effect on filtrate flow rate. During the compression phase (a phase particularly important in the belt filter press process), it was shown [12,29] that a variation of pressure (after several minutes of compression) had no effect, either on the filtrate flow rate, or on the dry solid content in the final cake. Due to this lack of agreement and of data on drained sludge, some experiments were carried out on FCC at different pressures. The results, obtained when the whole pressure was applied instantaneously, do not show a significant impact of the pressure on the dry solid content of the sludge (the observed variations are within the limits of accurate reliability). However, when the pressure was increased progressively over the first two minutes of the experiment, a significant evolution in the flow rate of the filtrate is observed (Fig 3). The dry solid content in the

dewatered cake is also very different under these operating conditions. When the increase of pressure was delayed after a time lapse of two minutes, no impact was observed in the dewatering. This is similar to the results reported in literature [12,29]. The impact of the pressure in the initial phase of compression is an important aspect which should be considered to analyse belt filter presses because in these devices the sludge is progressively pressurised when it reaches the first roller. Laboratory procedure to predict industrial efficiency To predict filter belt press dewatering efficiency, it seems interesting, according to the previous results of this work, to carry out some tests in FCC with a progressive pressurisation. Thus, comparative experiments were performed simultaneously in both devices: the pilot belt filter press and the FCC, on identical sludge.

For these experiments the quantity of sludge

introduced into the FCC was adjusted to obtain a thickness of the final cake close to 1 cm. This value was approximately the thickness of the dewatered cake on the belt filter press. The pressure was progressively increased in the FCC according to pressure measurements carried out around pressing rollers, and finally fixed to a constant value which corresponded to P max (Fig 4). The time of pressurisation into the FCC was set according to the number of pressing cycles and the belt speed in the filter. This procedure correlates with our previous results which have shown that after an initial period of a few minutes, the increase of pressure did not affect the compression anymore. However, the time pressurisation modified significantly the dry solid content of the final cake. To take into account this aspect, the duration of pressurisation in the FCC was equal to the real time of pressing in the belt filter press. This time was calculated by :

tn

Lc s

(iii)

One example of these comparative tests is reported on figure 5. It shows good agreement between both experiments with a maximal deviation of 1%. The result of one complementary experiment, carried out in FCC at a constant pressure (Pmax), indicates the importance of the procedure of progressive pressurisation (Fig 5). The data presented on figure 5 also indicates that the number of pressing rollers seems to have an impact only on the pressing time. Other similar tests

performed under different operating conditions led to the same results and

confirmed the interest of measurements in FCC to predict filter belt press dewatering efficiency.

CONCLUSION This work has contributed to the application of a classical laboratory test to the design of filter belt presses. It has presented a procedure to predict, from experiments carried out in the filtration compression cell (FCC), the effectiveness of industrial devices. This study has shown the necessity to be very aware of the distribution of pressure around the pressing rollers, especially the first one. The addition of other pressing rollers essentially contributes to increase the time of pressing. When these two aspects (progressive pressurisation and time of pressing) are correctly assessed for experiments in FCC, the dry solid content of the sludge, dewatered by the filter belt press, can be accurately estimated.

ACKNOWLEDGEMENT This study was carried out on behalf of E.M.O. Ltd. NOTATION C

: Final dry solid content of filter belt press (gDS gcake-1)

Cmax : Maximal dry solid content measured during experiments of reliability (gDS gcake-1) Cmin

: Minimal dry solid content measured during experiments of reliability (gDS gcake-1)

Caverage : Average dry solid content obtained during experiments of reliability (gDS gcake-1) C0

: Initial dry solid content of sludge (gDS gcake-1)

C’0

: Initial dry solid concentration of sludge (kgDS m-3)

Droller : Diameter of belt filter press roller (m) E

: Solid recovery efficiency (-)

l

: Cake width (m)

lbelt:

: Belt width (m)

Lc

: Length of the pressing zone around a given roller (m)

n

: Number of pressing rollers

Proller : Pressure drop around the cake around belt filter press rollers (Pa) Q

: Filtrate flow rate (m3 h-1)

Q0

: Input sludge flow rate (m3 h-1)

s

: Belt speed (m s-1)

Tbelt

: Belt tension (N)

X

: Position in the pressing zone around a roller (m)

REFERENCES 1.

Haworth P.J. and Roberts K., Development and application of filterbelt presses. Filt. Sep., 12, 572-576 (1975).

2.

Johnson G., Buchanan G.G. and Newkirk D.D., Optimizing belt filter press dewatering at the Skiner filtration plant. J. Am. Water Works Association, 84, 47-52 (1992).

3.

Lotito V., Spinoza L. and Santori M., Analysis of sewage sludge dewatering operation by beltpress. In: Proc. of the 4th World Filtr. Congress, Ostende, Belgium, organised by the Royal Flemish Society of Engineers, 4.43-4.47 (1986).

4.

ASCE (Task Committee on Belt Filter Presses), Belt filter press dewatering of wastewater sludge. J. Environ. Eng., 114, 991-1006 (1988).

5.

Hashimoto M. and Hiraoka M., Characteristics of sewage sludge affecting dewatering by belt press filter. Water Sci. Tech., 22,143-152 (1990).

6.

Baskerville R.C., Bruce A.M. and Day M.C., Laboratory techniques for predicting and evaluating the performance of a filterbelt press. Filt. Sep., 15, 445-454 (1978).

7.

Poduska R.A. and Collins B.H., A simple laboratory technique for determining belt-filter press operation. Environ. Tech. Letters, 1, 547-556 (1980).

8.

Karr P.R. and Keinath T.M., Influence of particle size on sludge dewatering. J. Ferm. Bioeng., 50, 1911-1930 (1978).

9.

Pan J.R., Huang C., Cherng M., Li K-C. and Lin C-F., Correlation between dewatering index and dewatering efficiency of three mechanical dewatering devices. Adv. in Environ. Res., 7, 599-602 (2003).

10.

Halde R.E., Filterbelt pressing of sludge – a laboratory simulation. J. Water Pollut. Control Fed., 52, 310-316 (1980).

11.

Bullard C.M. and Barber J., A factor in belt filter press performance. Water Environ. Tech., 8, 67-70 (1996).

12.

Gazbar S., Evaluation et amélioration des performances des procédés de déshydratation mécanique des boues résiduaires. Thèse, Institut Polytechnique de Lorraine, Nancy, France (1993).

13.

Rhemat T. Branion R., Duff S. and Groves M., A laboratory sludge press for characterizing sludge dewatering. Water Sci. Tech., 35, 189-196 (1997).

14.

Novak J., Knocke W., Burgos W. and Schuler P., Predicting the dewatering performance of belt filter presses. Water Sci. Tech., 28, 11-19 (1993).

15.

Miller S.A., Development of industrially relevant test methods for belt-press filtration. In: Proc. of the 9th World Filtr. Congress, New Orleans, Louisiana, organised by the American Filtration and Separation Society, 224-238 (2004).

16.

Reitz D.D., Municipal sludge dewatering using a belt filter press. M.Sc. Thesis, Virginia Polytechnic Institute and State University, Blacksburg, Department of Environmental Engineering (1988).

17.

Severin B.F. and Collins B.H., Advances in predicting belt press performance from lab data. In: Proc. of the Water Environ. Fed., 65th Annual Conf. and Expo., New Orleans, Louisiana, (1992).

18.

Galla C.A., Freedman D.L., Severin B.F. and Kim B.J., Laboratory prediction of belt filter press dewatering dynamics. In: Proc. of the Water Environ. Fed., 69th Annual Conf. and Expo., Dallas, Texas, (1996).

19.

Emery B.P., Predicting belt filter press performance using laboratory techniques. M.Sc. Thesis, University of Illinois, Urbana, Department of Civil/ Environmental Engineering (1994).

20.

Galla C.A., Laboratory prediction of belt filter press dewatering dynamics. M.Sc. Thesis, University of Illinois, Urbana, Department of Civil/ Environmental Engineering (1996).

21.

Galla C.A., Freedman D.L., Severin B.F. and Kim B.J., Pressing solids. Water Environ. Lab. Solut., 4, 8-10 (1997).

22.

Olivier J., Etudes des filtres à bandes pour la déshydratation mécanique des boues résiduaires urbaines. PhD Thesis, Université de Pau et des Pays de l’Adour, France (2003).

23.

Tokunaga K., Fujinami S., Ishimi T., Okahashi H. and Nakano I., High pressure filtration and/or squeezing of sewage sludge. Filt. Sep., 20, 450-456 (1983).

24.

Badgujar M.N. and Chiang S.-H., An analysis of belt filter press dewatering mechanism. Filt. Sep, 26, 364-367 (1989).

25.

La Heij E.J., An analysis of sludge filtration and expression. PhDThesis, Technische Universiteit Eindhoven, Netherland (1994).

26.

Léonard A., Etude du séchage convectif des boues de stations d’épurations – suivi de la texture par micrographie à rayon X. PhD Thesis, Université de Liège, Belgique (2002).

27.

Novak J.T., Agerbaek M.L., Sorensen B.L. and Hansen J.Aa., Conditioning, filtering and expressing waste activated sludge. Water Res., 34, 1-20 (1999).

28.

Lee D.J., Ju S.P., Kwon J.H. and Tiller F.M., Filtration of highly compactible filter cake: variable internal flow rate. AIChE J., 46, 110-118 (2000).

29.

Baudez J-C., Rhéologie et physico-chimie des boues résiduaires pâteuses pour l’étude du stockage et de l’épandage. PhD Thesis, Ecole Nationale du Génie Rural, des Eaux et Fôrets, Centre de Paris, France (2001).

Table1. Estimation of belt filter press efficiency on municipal sludges by Lotito et al. [3]. (for 1 < Q0 < 3 m3 h-1, 3 < sb < 12 m min-1 and 13 < C’0 >34 kg m-3) Parameter

Equation

Correlation coefficient (R)

Cake width

l lbelt

Filtrate flowrate

 88.945Q0

Q  0.007Q0

Solid content

C  9.241Q0

Solid recovery efficiency

E  788.61Q0

s

1.135 0.108

0.093

s

C' 0

0.975

0.646

 s 0.006  C' 0

0.0443

0.912

0.706 0.381

0.279

 s 0.298  C' 0

0.414

0.880 0.915

Table 2. Cake solid content versus solid content at the filter entrance (ASCE [4]).

Primary

Secondary

sludge

sludge (%)

Equation

Number

Correlation

of points coefficients (R)

Standard error

(%)

0 - 10

10 – 40

40 - 60

60 – 80

80 - 100

90 – 100

60 - 90

40 – 60

20 - 40

0 - 20

C

C0 0.044  0.0426 C0

30

0.89

4

C

C0 0.0297  0.0402 C0

7

0.90

3

C

C0 0.059  0.0307 C0

35

0.84

3

C

C0 0.062  0.0306 C0

17

0.82

3

C

C0 0.071  0.0266 C0

12

0.87

3

Figure 1. Laboratory filter belt press. Pneumatic jack

Roller driven by the electric

11

9

motor Belt

10

Gravity drainage zone

8

12

Wedge zone 1

4

2

Filtrate tank

5

Belt

D

High pressure zone (pressing roller)

3

R A I

7

N

Filtrate tanks

A G E Z O N E

6

Figure 2. Applied pressure versus a dimensionless length: ratio of any length over the whole length (in the roller pressing zone). 2a. Effect of belt tension on pressure distribution around a roller. ( : 206 N,

: 382 N,

: 971 N,

:1383 N,

: 2354 N,

: 2943 N,

: 3531 N)

2b. Effect of roller diameter on pressure distribution around a roller ( : Droller= 0.27 m, : Droller= 0.17 m). Belt tension: 197 N.

4.5

1.6

a

4.0

Pressure ( bar)

3.5

Pressure (bar)

b

1.4

3.0 2.5 2.0 1.5

1.2 1.0 0.8 0.6

1.0

0.4

0.5

0.2

0.0 0.0

0.0 0.0

0.2

0.4

0.6

X / Lc

0.8

1.0

0.2

0.4

0.6

X / Lc

0.8

1.0

Figure 3. Effect of the pressurisation procedure (

: 1.5 bar applied instantaneously,

1.5 bar applied progressively over the first two minutes)

:

Figure 4. Pressure distribution: in FCC ( ______ ), and in the pilot filter belt press for a experiment carried out with a series of seven pressing rollers (diameter 0.27 m) and a belt speed of 1.5 m min-1 (------).

Figure 5. Comparison of FCC (

) and the pilot filter belt press (

solid content.

15

Dry solid content (%)

14 13 12 11 10 9 8 7 6 0

20

40

60

Time (s)

View publication stats

80

100

) results in terms of cake