Experimental Study on Fly Ash for Conditioning of Specific Resistance of Sludge Water

doi:10.4236/gep.2013.12005

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Journal of Geoscience and Environment Protection

2013. Vol.1, No.2, 22-25

Published Online October 201 3 in SciRes (http://www.scirp.org/journal/gep) http://dx.doi.org/10.4236/gep.2013.12005

Experimental Study on Fly Ash for Conditioning of Specific

Resistance of Sludge Water

Bingtao Liu, Pengju Ding

School of Environmental and Municipal Engineering, North China University of Water Resources and Electric

Power, Zhengzhou, C hina

Email: [email protected]

Received August 2013

Discharge of sludge from water works into waters will result in the increased deterioration of water envi-

ronment. Therefore, an experimental study was conducted on the addition of conditioning agent to im-

prove sludge dewatering performance. The result shows that coagulation is inadaptable to the treatment of

sludge from water works and use of powdered coal ash from power plant as conditioning agent can great-

ly reduce the specific resistance of sludge and the dewatering performance can be improved. The optimal

dosage of fine powdered coal is 20 g/100 mL and coarse powdered coal is 30 g/100 mL. Powdered coal is

mixed with sludge to form filter cake, which is blended with coal in certain proportion to make into fuel.

So that incineration of sludge from water works can be realized.

Keywords: Fly Ash; Sludge Water; Sludge; Specif i c Resi s t ance

Introduction

The sludge water of waterworks includes sludge water of set-

tling tank and backwash wastewater of filter. After the simple

sedimentation and separation, the supernatant can be reused,

sediment forms so-called sludge of sludge water. At present,

most of the existing waterworks directly discharge sludge of

sludge water into natural water in China which will cause pol-

lution to water (Liu Hui & Xu Jianhua, 2001; Lu jiajuan, 2005).

In the 1970s, the United States, Japan and other developed

countries began to pay attention to the treatment and disposal of

sludge of waterworks (John T. Novak, 1999; J Kopp & N Dichtl,

2001). The treatment method is similar to sludge treatment me-

thod of municipal sewage plant (Xu Qiang , 2003). However, the

sludge from waterworks and sewage plant has different charac-

teristics, even if the sludge is all from waterworks. There are

significant differences in properties because of the different raw

water quality and the different production technology. We have

an experimental research on methods of treatment and disposal

of sludge from inclined tube sedimentation tank of water treat-

ment plant in Zhengzhou.

Experiment

Main Reagents and Mud Sample

PAC, light yellow powder, alumina content 31%, basicity

76%, confect concentration of 10 g∙L−1 when use i t.

Sludge samples, Activated sludge from Zhengzhou city se-

wage sludge treatment plant emissions, take underlayer preci-

pitation after standing.

Two kinds of fly ash that contains fly ash of coarse particle

and fine particle in the experiment are from Zhengzhou thermal

power plant. Its quality is shown in Table 1.

Experiment Method

Test is divided into two steps. First of all is test of coagula-

Table 1.

Quality index of powdered fly ash (%).

Items Finenes

(Remai ni ng amount of 45 µm screen)

Burning

loss

Fly ash of coarse particle 55.2 4.41

Fly ash of fine particle 12.9 3.13

tion, sedimentation and pre-concentration of waterworks sludge

is in progress and we can obtain settlement curve. Then specific

resistance of mud sample is measured (an important index of

sludge dewatering performance) and make optimization of the

sludge conditioner of specific resistance in order to achieve the

sludge dehydration and drying.

The purpose of adding 1 g PAC and 5 mg PAM coagulant to

per liter of sludge water is to destabilize colloids that inorganic

coagulant is the first dosing and PAM is dosing after 5 S. Oper-

ating parameters: in the mixing process, n = 300 r/min, T = 30

S; in the flocculation process, n1 = 150 r/min, T1 = 3 min, n2 =

30 r/min, T2 = 10 min. Static precipitation continues for 2 h

after coagulation, a decline height of muddy liquid level along

with time is recorded and subsidence curves is depicted.

Specific resistance has a special measuring device shown in

Figure 1. Including Büchner funnel, filter media, smoke filters,

cylinders, vacuum table and vacuum pump.

Test run at room temperature. Measure out 100 ml of sludge

in the beaker, stir in a certain pH by adding flocculents, stir in

medium-speed about 5 minutes and pour into the Büchner fun-

nel, the vacuum filtration dehydrat ion until no drop. Then record

at different times (t) and the corresponding filtrate volume (v),

with t/v as the vertical coordinate, v the horizontal mapping, t/v

and v is linear, the slope b, constant pressure filtration

2PA b

γµ

Where

B. T. LIU, P. J. DING

Figure 1.

Experimental set of specific resistance measurement.

P: filtration pressure (for the cake pressure difference be-

tween top and bottom surfaces), N∙m−2

A: filter media area, m2

b: filtration time/filtrate volume and filtrate volume formed

by the slope, s∙m−6

µ: filtrate dynamic viscosity, (N.s)∙m−2

C: filter biscuits dry quality by the filtrate, kg∙m−3, according

to the weight of dry solid and the filtrate volume by filtrate to

calculate.

We can calculate the specific resistance of sludge and meas-

ure its filtrate turbidity, filtration time and filtrate volume, and

needed time to get same filtrate. Sludge dewatering perfor-

mance reflect by the specific resistance, the greater the value

the worse the dehydration properties. By contrary, dehydration

performance is better.

At present, in order to improve dewatering performance of

the sludge of waterworks, sludge usually be added the right

amount of polyacrylamide (PAM) to reduce specific resistance

of sludge before its mechanical dehydration. But PAM exists

many problems in practice. Firstly the price is expensive. It

cannot reach to the effect of reducing specific resistance when

dosage is insufficient and dosage is more and it will make the

operation cost become too high. Secondly PAM is a kind of

viscous slurry. It is easy to stick on the cloth filter and choke

filtration pore which affects the dehydration efficiency. So the

material such as fly ash of power plant is chosen as a condi-

tioner. Optimization experiment is carried out in order to im-

prove the separation efficiency and reduce the production cost.

Results and Discu ssion

Curve of Natural Sedimentation and Coagulation

Sedimentation

Settlement curve can be seen by Figure 2, Whether the nat-

ural sedimentation of original sludge water or static precipita-

tion under optimum amount of coagulant, the drop height of

muddy liquid are not very satisfactory, which decrease by only

3.2%, 4.0% and 8.1%, respectively, thus we need not set up the

coagulation process when we design the process of sludge wa-

ter treatment for waterworks, only set up an intermediate regu-

Figure 2.

Settlement curve of sludge water.

lating pond, and preliminary sinking separation pool is not ne-

cessary.

Specific Resist ance C onditioning Curve of Sludge

Measurement results of specific resistance of original sludge

are shown in Table 2. Literature Novak J indicates that, gener-

ally speaking, when the specific resistance < 1011 m/kg, sludge

is easy to dewater, and specific resistance > 1013 m/kg, sludge

dewatering becomes difficult. It is obvious that original sludge

dewatering is difficult, suction filtration time is long, and it

needs to be conditioning.

Figure 3 shows that specific resistance of sludge decrease

quickly with the increase of dosage of fly ash from the pre-

treatment magnitude of 1013 to 1010. When the dosage of fly ash

increased to 30 g per 100 ml of mud sample, the curve tend to

gentle. In the case of low dosage, that the effect of improving

sludge dewatering performance for fine fly ash is more obvious

than coarse fly ash, with the increase of dosage of fly ash, the

ability of improving specific resistance for the coarse and fine

fly ash tend to be the same. In the measurement experiment of

specific resistance, the linear slope (b) formed by filtering time/

filtrate volume and filtrate volume can indirectly reflect the

speed of filtration rate, the smaller of the value of b, the greater

of the speed of filtration rate. The results show that, the value of

b drops rapidly with the increase of the dosage of fly ash (the

filtration rate increases rapidly), when the dosage of coarse fly

ash is 30 g or the dosage of fine fly ash is 20 g, the speed of

filtration rate is the largest, after that, the filtration rate is de-

creased with the increase of the dosage of fly ash.

The Amplification Test under the Optimal Do sage

According to the front of the test results, it shows that the

optimum dosage of coarse fly ash and fine fly ash is 30 g/100

ml and 20 g/100 ml of sludge, respectively. The optimal dosage

of sludge dewatering performance and the nature of the filtrate

are shown in Table 2.

From Table 3, we can know that the specific resistance of

sludge decreases rapidly and the speed of filtration rate in-

creases significantly after the conditioning of fly ash, the suc-

tion filtration can be completed in about 3 minutes, and the

B. T. LIU, P. J. DING

Table 2.

Specific resistance measurement of original sludge.

Items Suction filtration time/S Filtrate volume/ml Moisture content of filter cake/% specific resistance/1013∙m∙kg−1

Numeric value 930 94.5 92.25 1.92

Table 3.

Dewatering performance and filtrate quality at optimum dosage.

Conditioning dose Dosage (g) Specific resistance

(1011 m/kg) Filtering

time (s) Filtrate

volume (ml ) Moisture content

of filter cake (%) Filtrate turbidity

(NTU) Chromaticity

of filtrate (NCU) CODMn

of filtrate (mg/L)

Coarse fly ash 30 .50 160 70 41.12 4 32 16.00

Fine fly ash 20 .70 200 75 47.80 4 23 15.04

Amplification of 1 1500 .45 140 73 40.04 4 30 20.77

Amplification of 2 1000 .80 220 80 47.96 4 28 17.45

Figure 3.

Effect of Sludge specific resistance.

filter cake is dry, it is easy to strip from the filter paper, the

value of filtrate turbidity, chromaticity and CODMn is not high,

it can be used as municipal miscellaneous water.

Ignition loss is due to the unburned carbon particles burning

again in the laboratory. The ignition loss in the filter cake

formed by the fine fly ash after dehydration is 6.47%, and igni-

tion loss of fine fly ash is only 3.13%, so it can be inferred that

the increase combustible comes from combustible components

of the sludge. It can be taken use of, for example, it can be

made of blend coal cake through mixing with proportion coal,

which is available for civilian or power boiler burning, so as to

achieve the final incineration disposal of sludge.

Analysis Results of Scanning Elec tron Microscop e

The section of filter cake of original mud and conditioning is

sprayed gold, then scanning electron microscope is carried out.

The results are shown in figure. The sludge particle size of

sludge water is very small, about the quality of 76% of particle

size is less than 4 microns, so the channel of filter cake formed

is small (Fei Xiali & Cui Fu yi, 2004). In addition, there is a layer

of hydration shell in granular bed of filter cake whose flowabil-

ity is poor. It can be seen from Figure 4. The filter cake gap of

original sludge is very small and it will make filtration resis-

tance increases and the fly ash is a variety of mechanical mix-

Figure 4.

SEM appearance made from original sludge.

ing of powder particles. Generally the particle size is more than

30 microns due to the unique structural characteristics of fly ash.

The mud sample of conditioning can be used as a skeleton

which make the sludge flocs form the structure of persistence

and strong. The channel of filter cake is larger, which can form

the solidified layer of loosened structure and increase the void

fraction as is shown in Figures 5 and 6. At the same time, the

carbon that is not burning enough and porous vitreous content

of fly ash is less, the content of glass beads is more, it is not

easy to store water for caves and gap of sphere but coarse fly

ash is instead, so in the case of low additive amount of fly ash,

fine fly ash is better to the improvement of the sludge dewater-

ing effect .

Conclusion

When the sludge of sludge water of waterworks carries on

the treatment of dewatering and drying, it is better to choose fly

ash as conditioners (filter aid). In the process design of treat-

ment of sludge water of waterworks, we need not set up the

coagulation process, and only set up an intermediate regulating

pond. It is easy to dewater for the sludge of waterworks that

have been conditioning by fly ash, mud cake that comes from

the process of filter-press and suction filtration is mixed with

coal to make coal cake, which can realize the final incineration

disposal.

B. T. LIU, P. J. DING

Figure 5.

SEM appearance by coarse fly ash.

Figure 6.

SEM appearance by fine fly ash.

Acknowledgements

We thank the financial support by North China University of

Water Resources and Hydroelectric Power.

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