Comparative Study of Sludge Dew Ate Ring Units

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JOURNAL OF ENVIRONMENTAL SCIENCE AND HEALTH

Part A—Toxic/Hazardous Substances & Environmental Engineering

Vol. A39, No. 2, pp. 473–482, 2004

A Comparative Study of Sludge Dewatering Units

for Sludge Management

Saleh Al-Muzaini*

Environment Sciences Department, Kuwait Institute for

Scientific Research, Safat, Kuwait

ABSTRACT

A comparative study of three different types of municipal sludge dewatering unit

was performed including sludge dewatering, centrifuge, and sand drying bed. Thepurpose of that was to examine the capability of each dewatering unit for

dewatering municipal sludge at the Jahra treatment plant. The testing program

was performed to select the most feasible and effective unit under local

conditions. From the experiment approach, it was clear that the application

results of various dewatering units were different and depends entirely on the

basic properties of each unit. It has been also found that mechanical dewatering

units can operate and are the most feasible and effective to reduce the large

quantities of sludge as a treatment byproduct.

Key Words: Sludge dewatering; Belt filter; Centrifuge; Mechanical dewatering;

Sand dry bed.

*Correspondence: Saleh Al-Muzaini, Environment Sciences Department, Kuwait Institute

for Scientific Research, P.O. Box 24885, Safat 13104, Kuwait; E-mail: smuzaini@safat.

kisr.edu.kw.

473

DOI: 10.1081/ESE-120027538 1093-4529 (Print); 1532-4117 (Online)

Copyright & 2004 by Marcel Dekker, Inc. www.dekker.com



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INTRODUCTION

The Jahra treatment plant was built in 1981 and serves the western and eastern

side of the city of Jahra. The Jahra city is located about 15 km to the west of Kuwait

City. The Jahra plant was designed to treat 70,000 m3/d of sewage. Current studies

show that the plant is receiving about 55,000 m3/d of sewage (MPW, 2000). The

sludge produced by the plant is sent after treatment directly to drying beds. There are

30 drying beds and they are able to dry about 100 m3 of sludge. The drying beds in

the plant are being fully used and it is apparent that the plant will require 3–4 times

the existing drying capacity area when it becomes fully operational. Lack of

sufficient area in the vicinity of the plant restrict the expansion of existing sand-

drying beds which lead to the generation of odors especially during summer causes

public annoyance and complains of headaches. Generally, municipal wastewater in

Kuwait is of high strength and is often septic because of low flows, long retention,

and high temperature.[1–3]Sand-drying beds are less complex, easier to operate, and require less operational

energy than mechanical dewatering systems. They easily produce a sludge cake with

25 to 46% solids and can exceed 60% solids with additional drying times.[4–6]

Although the sand drying beds currently use and produce a satisfactory

dewatered sludge, pilot tests on two mechanical systems were conducted to compare

their efficiency with the existed sand dry units. Equipment tested included belt filter

unit and centrifuge unit. Reynold[7] showed that the dewatered sludge content by

mechanical dewatering depends primarily upon the nature of the sludge, its original

solid content, and whether or not a polymer is used.

Steel and McGhee,[8] Al-Layla et al.,[9] TCBFP,[10] and Innocenti[11] have all

shown that municipal sludge has liquid, organic, and inorganic solids content.

Mechanical dewatering units, such as the belt filter and centrifuge are used to removethe liquid and to increase solids contents. Novak et al. [12] reported that use of the

polymer was found to be important in the mechanical dewatering processes.

The purpose of the this article is to compare results of a testing program in

term of performance capabilities of a belt filter, centrifuge, and sand dry unit for

dewatering municipal sludge at the Jahra treatment plant. The following presents a

summarized comparison of the pilot-testing program undertaken at the Jahra

facility.

Sand Dry Bed

Sand-drying bed processes found early applications in wastewater to dewatersludge but their use declined with wide-scale use of mechanical units. Sand beds are

usually used for small industrial or community waste treatment plants. The sludge

can be dewatered on open or covered sand beds. The method is very simple and

requires minimum operator attention. They also require considerable amounts of

land and are subject to such uncontrolled variables as rainfall, temperature,

humidity, and drainage rate.

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Process Description

The sludge drying bed unit has dimension of 25 m by 15 m. It consists of a 40 cm

thick layer of sand over 20 cm of graded gravel. The sand has a uniformity coefficient

of not more than 40 and an effective grain size of 0.3 to 0.75 mm. A network of

buried pipes collects the filtrate that percolates through the sand and gravel. The

buried pipe includes 10 cm lateral subdrains that connect to a single 15 cm main

subdrain for each cell. The subdrain pipes are 50 cm long sections of asbestos cement

pipe with open joints. A piece of tar paper is placed on the top of each open joint to

prevent material from falling into the pipe. The filtrate from the subdrains flows via

a 20 and 25 cm drainage sewer to a manhole upstream of the treatment plant, where

it repeats the treatment process. Sludge that remains on the top of the sand bed is

solidified by the percolation of water downward into the sand and also from

evaporation from the surface of the sludge. The sludge can be dried in nine days in

the summer and within 15 days in the winter, and produces a cake of up to 40%solids. The schematic layout of a sand dry bed is shown in Fig. 1.

Performance

The study at the Jahra Plant was conducted for approximately a year. Samples

were collected from a representative dry bed. The collected samples were analyzed

for moisture content, oil and grease, sand content, and alkalinity. Samples were also

analyzed for heavy metals and bacteriological indicators.

The lead, nickel, copper, cadmium, and chromium levels in the dried sludge were

determined using the graphite furnace of a Shimadzu (model 680) atomic adsorption

spectrophotometer.

Parameters such as total coliform, fecal coliform, and salmonella were selectedto monitor the level of pollution in the dry sludge in the Jahra plant. The total

suspended solids and volatile suspended solids were measured. Total oil and grease

content were determined by Horbia Ocma—200 (Japan).

Sample preparation and analyses were conducted according to the standard

methods for the examination of water and wastewater.[13]

Dry Sludge Characteristics

Monthly samples were collected and analyzed to determine the physical and

chemical characteristics of dry sludge produced at the Jahra plant. The results are

presented in Tables 1 and 2. Oil, grease, and sand content in the sludge are above thetypical values, which are 15 and 70 mg/L respectively. It is well-established that the

high sand content in the sludge is due to sandy storms during summer. The sludge in

Jahra is of high-strength oil and grease because the highest organic content flows

into domestic sewage.

The amounts of total suspended solids and volatile suspended solids are high.

This suggests that the Jahra plant always receives high concentrations of suspended

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F i g u r e 1 .

T h e s c h e m a t i c l a y o u t o f a s a n d d r y b e d .

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solids, which may cause septic conditions in sewers. The presence of high

concentrations of solids in the sludge indicates low dewaterability and high specific

resistance to filtration. Reduction of volatile solids is recommended to help reduce

odor problems and pathogenic exposure.

Table 2. Chemical characteristics of sludge at the Jahra treatment plant

compared with typical values.

Pollutant

Concentration

(monthly average)

(mg/g)

Pollutant concentration

allowable

(mg/kg)

Ceiling

concentrations

(mg/kg)

Arsenic NM 41 75

Cadmium 0.6 39 85

Chromium 20.0 1,200 3,000

Copper 110.0 1,500 4,300

Lead 70.0 300 840

Mercury NM 17 57

Molybdenum NM N/A 75

Nickel 45.0 420 420

Selenium NM 36 100

Zinc NM 2,800 7,500

TKN 1.9 N/A N/A

NHþ

4 -N NM N/A N/A

NO3-N NM N/A N/A

N/A—not available, NM—not measured.

Table 1. Physical characteristics of sludge produced at

the Jahra plant.

Parameter Concentration

Solids in wet sludge (%) 3.6

Moisture (%) 15.6

Oil and grease 510.0a

Sand content 20.0a

Alkalinity (CaCO3) 4.0a

Total suspended solids (TSS) 7,599b

Volatile suspended soild (VSS) 4,933b

Dissolved oxygen (DO) 1.10b

Chlorides (Cl) 400.0a

Phosphate (PO4) 0.80a

Total coliform 160.0c

Fecal coliform 300.0c

Salmonella 150.0c

aAs mg/g dry.bAs mg/L.cAs colonies/g dry basis.


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The analysis of heavy metals showed that the levels of Cd (0.001 mg/kg), Cr

(0.02 mg/kg), Cu (0.110 mg/kg), Pd (0.070 mg/kg), Ni (0.04 mg/kg), Cl (400 mg/g

dry), and phosphate (0.80 g/g dry). No sludge samples showed any significant

concentrations of heavy metals, and the detected levels in most cases were below

the recommended levels for controlled sludge disposal. The dry sludge was also

analyzed for TKN (0.002 mg/kg); however, the levels of TKN were below

international levels.

Comment

Operation of the sand dry bed unit is satisfactory. However, there are several

areas which, may need improvements such as shortening the exposure time and

adding chemicals as a conditioner. As can be seen by slow drying, a vacuum-assisteddry bed system for dewatering should be installed as an appropriate technology for

sludge dewatering.

Belt Filter Press

The Belt Filter Press unit was developed in the 1960 s and receives considerable

attention by municipal authorities. The description of process unit which has been

used for this study is shown in Fig. 2 (P.E. Trading, SDN and BHP, Malaysia).

Figure 2. Schematic diagram of the belt filter unit.

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Process Description

The unit consists of two belts forming a closed loop around a series of metal

rollers. These rollers can be adjusted in a manner that brings the belts closer and

closer. The belts are driven by a variable speed motor, which allows the belt speed to

be adjusted. Both the belts and rollers are fixed on a stainless steel frame. The sludge

enters the unit and flows between the belts, which forces the liquid out of the sludge

by compression. The dried sludge is discharged and the belts pass through a series of

sprays for cleaning. In addition, a polymer-mixing tank is mounted on a stainless

steel frame. Polymer No. 7633-1 and sludge feed pumps and necessary piping are

provided.

Performance of Belt Filter Unit

Conditioning of wet sludge was necessary to achieve satisfactory yield from the

belt filter unit. Conditioning of the sludge was allowed to drain the water freely. In

addition, the belt speed and roller pressures were adjusted to obtain the desired cake

solid content. For a small plant, such as Jahra Plant, the dewatering unit was run 3

wk to one month.

The performance of the belt filter can be measured in terms of the yield of solid

on a dry weight basis expressed as percent. The quality of the produced cake was

measured by its moisture content on a wet weight basis expressed as in percent. In

this experiment, the unit was operated at highest performance so that it could

produce a cake that could be separated easily from the filter with high solid content.

Table 3 presents the performance testing program of the belt filter. A design rate of

13.10 content kg/m2/h is estimated for this type of filter and is based on experimental

output.

Comment

It is relatively simple to operate and it operates in a continuous mode. The

results of tests were representative of what could be expected. If the unit put on a full

scale, it will behave quite good without any problems in operation.

Table 3. Performance of belt filter.

ExperimentNo.

Before

dewateringTSS (mg/L)

After

dewateringTSS (mg/L)

TVS(mg/L)

Dewatered

sudge% Solids

Yield(kg/m2/h)

1 0.004 10,000 6,000 19–23 12.98

2 0.005 10,850 6,200 19–24 13.70

3 0.006 12,000 6,400 20–25 13.10

4 0.005 11,000 7,200 18–25 13.25


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Centrifuge

Various types of centrifuges are used for solid–liquid separation in municipal

fields for over 50 years. Improvements in the centrifuge equipments have resulted

excellent results for dewatering of the municipal sludges. Recently, centrifuges are

becoming more known and have been utilized widely for sludge dewatering.

Description of Unit

The most widely used centrifuge for wastewater sludge treatment is the solid

bowl or decanter type with electric motor drive. The machine consists of a

cylindroconical rotor that rotates between two bearing blocks supported by a base

frame and includes a decantation bowl, screw conveyor, and a speed reducer. The

unit is driven by horizontal axle double electric motors. The screw conveyor drivenby the speed reducer turns at slightly higher speed than the bowl.

Polymer No. 7633-1 was added to the sludge and mixed thoroughly before the

treatment. In general, mixed sludge and polymer is fed into a rotating mechanism

that separates it into a dense cake containing the most solid discharge stream and a

dilute central stream containing the remaining fine density solids. Figure 3 presents a

diagram of the centrifuge unit (Scientific Supply House Company, Germany).

Performance of Centrifuge Unit

During the testing program, the testing operating parameters of the centrifuge

unit were varied systematically achieve the driest percentage of dry cake. Table 4

presents data on centrifuge performance. The performance of the unit was

Figure 3. Centrifuge unit.

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satisfactory during the test period. The unit produced approximately 19–20% solids

from a feed line containing 0.005 mg/L solids or 1.2% solids.

Comments

Operation of the centrifuges is satisfactory. It was noted that the unit was able to

increase thesolids content of sludgewhich will reduce thehandlingcosts. Theproduced

sludge was quite dry, and it will lend itself more ready to applications for reuse.

SUMMARY AND CONCLUSIONS

Comparative pilot plant testing for municipal sludge dewatered was conducted

including a belt filter, centrifuge, and sand drying bed unit. The main objective of the

testing program was to examine the capability of each dewatering equipment for use

in Kuwait. The results of comparative study of municipal sludge dewatering

equipment can be summarized as follows.Belt filter unit was operated at a loading of 13.10 kg/m2h, producing an average

cake solid of 20–25%. Centrifuge unit was operated at loading rate of 4.97 kg/min,

producing average cake solid of 20%. Polymer no. 7633-1 was selected and showed

high effectiveness for sludge produced at the Jahra plant. The suitable dosage was

estimated to be 3.5 mg/L. Sand drying beds was operated and produced about 4 m3

of sludge per cell, with 25–40% solids and can exceed 60% solids with additional dry

time. From the achieved results showed that sand drying beds are inadequate to

handle the projected sludge production. The centrifuge produces a cake that appears

very dry compared with belt filter. The polymer type and quantity for both belt filter

and centrifuge almost identical. From the data collected, more extensive testing is

required to determine the best unit that could meet the Kuwait public works

requirements for sludge dewatering.

REFERENCES

1. Samhan, O.; Ghobrial, F. Trace metals and chlorinated hydrocarbons in sewage

sludge of Kuwait. Wat. Air Soil Pollut. 1987, 36, 239–246.

Table 4. Centrifuge performance.

Description Unit Value

Loading kg/min 4.97

Dewatering sludge % solids 19–20

Concentrate solids mg/L 11,000

Polymer dosage mg/L 3–4


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2. Al-Muzaini, S.; Smhan, O.; Hamoda, M.F. Sewage related impact on Kuwait’s

marine environment: a case study. Wat. Sci. Tech. 1991, 23, 181–189.

3. Tomar, M.; Abdullah, T.; Abdullah, J. Effect of aeration on generation and

emission of hydrogen sulfide in wet wells lifting stations. Wat. Air. Soil. Pollut.

1995, 81, 385–399.

4. Hammer, M.J. Water and Wastewater Technology; John Wiley and Sons Inc.:

New York, USA, 1975.

5. Eckenfelder, W.W., Jr.; Santhanam, C.J. Sludge Treatment. Pollution

Engineering and Technology; Series No. 14, Marcel Dekker, Inc.: New York,

1981.

6. EPA. Process Design Manual for Dewatering Municipal Wastewater Sludges,

Report No. EPA-625/1-82-014; US Environmental Protection Agency, 1982.

7. Reynold, T.D. Unit Operation and Processes in Environmental Engineering;

California Brooks, Cole Engineering Division: New York, U.S.A., 1982.

8. Steel, E.; McGhee, T. Water Supply and Sewage; McGraw Hill: New York,1979.

9. Al-Layla, M.; Ahmed, S.; Middlebrooks, E. Handbook of Wastewater Collected

and Treatment; Garland STPM: London, 1980.

10. TCBFP. Belt filter press dewatering of wastewater sludge. Task committee on

belt filter presses. Journal of Environmental Engineering 1988, 114 (5),

991–1007.

11. Innocenti, P. Techniques for handling water treatment sludge. American Water

Works Association 1988, 14 (2), 1–4.

12. Novak, J.T.; Prendeville, J.F.; Sherrard, J.H. Mixing intensity and polymer

performance in sludge dewatering. Journal of Environmental Engineering 1988,

114 (1), 190–198.

13. APHA. Standard Methods for the Examination of Water and Wastewater, 19th

Ed.; American Public Health Assoc.: New York, 1995.

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Comparative Study of Sludge Dew Ate Ring Units