monteiro1997.pdf

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Pergamon

PH: 50273-1223(97)00670-7

Wal

Sci.

T.ch. Vol. 36,

No.

II ,

pp. 61-67,

1997.

C 1997 IAWQ. Published by Elsevier Science Lid

Prinled in Great Britain.

0273-1223/97 S

l7-llO

+0'00

THE

INFLUENCE OF THE ANAEROBIC

DIGESTION PROCESS ON

THE

SEWAGE

SLUDGES RHEOLOGICAL

BEHAVIOUR

Paulo Santos Monteiro

IHRH

lnstituto

de Hidrliulica

e Recursos Hidricos

Faculdade de Engenharia da

Universidade do

Porto

Rua

des

Bragas

4099

Porto

Codex

Portugal

ABSTRACT

During the anaerobic digestion process, a significant part of the organic mailer in sewage sludge is

decomposed to form other organic and inorganic compounds in dissolved form. This biological

transformation of a substantial part of the organic solids has, cenainly, a strong influence on the rheological

characteristics of the sludge. In this paper a test racility was set up to simulale sewage sludge digestion and

periodic observalions on the evolution of the sludge characteristics were carried oul. Results of this study

show that imponant changes on the sludge rheological behaviour occur during anaerobic digestion and that

the evolution

of

those changes is related to the degree

of

digestion. Moreover. it is shown that the verified

high degree of physical changes can not be explained only by the total solids concentration variation and two

hypotheses are proposed to explain those changes.

@

1997

IA

WQ. Published by Elsevier Science Ltd

KEYWORDS

Sludge; anaerobic digestion; rheology

of

sludges.

INTRODUCTION

About 3

of

the wastewater that reaches any urban treatment plant is transformed into a final residue called

raw sludge. Raw sludge is an unstable solids suspension that must be subjected to a specific and complex

treatment before an environmentally acceptable product is obtained for final disposal.

In a conventional activated sludge wastewater treatment plant. raw sludge is normally a mixture of the

primary sludge and the excess biological sludge. In general. raw sludge has about 3-5 (by weight) total

solids, among which about 70-80 is organic matter.

Due to its high organic solids content, raw sludge must be stabilized by digestion processes. in order to

obtain a stable product which is easier to handle and to dispose of. During the anaerobic digestion.

heterotrophic bacteria reduce about 40-50 of the organic compounds, especially those less complex and

readily biodegradable. These compounds are normally in soluble and colloidal form and may

be

classified as

a complex mixture

of

nutrients, proteins, carbonhydrates and organic acids - small organic particles linked

to other solids

of

greater dimension or suspended in the liquid phase.

The biological assimilation of these solids reduces slightly the total solids concentration of the sludge but

certainly plays a major role in the well known great rheological differences between raw and digested

.NSl1,.\I.C

61


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62

P. S. MONTEIRO

sludges, as concluded in studies carried out in the Water Research Centre (Frost, 1982, 1983; Johnson,

1981),

by

Mulbarger et al. (1981) and by Hendo and Kanari (1980).

In the above-mentioned studies important results are obtained on the sludge rheological characteristics

relating to the influence of two specific parameters - solids concentration, and sludge nature. However, the

application of the proposed methods to raw and digested sludges with an average solids concentration of,

respectively, 3-4 and 2-3 , gives very different results. According to the Water Research Centre studies

these sludges are Herschell-Bulkley fluids, in Mulbarger et al. studies they are Bingham fluids and, finally,

for Hendo and Kanari they are pseudo-plastic fluids.

The aim of this study is to provide more information about the rheological characteristics of sewage sludges

with a particular focus on the influence

of

the anaerobic digestion process in order to minimize the

uncertainty faced by the designers of sludge transport systems.

METHODS

Figure I shows the experimental set-up used to simulate the sewage sludge digestion. The installation

includes a heated anaerobic reactor with a volume

of

0.7 m

3

, equipped with a mechanical mixer in order to

simulate the anaerobic digestion process of sludges at a constant temperature of 32-34OC.

Figure

I.

Experimental sct-up.

The biological activity was permanently followed

by

the measurement

of

the volume of biogas produced and

by periodic measurements of the total and volatile solids concentration. When necessary small amounts

of

lime were added in order to maintain the pH

of

the digesting sludge

in

the range of 6.0 to 8.0.

Weekly a rheological test was performed in a Physica Rheolab MClOO rheometer using a rotational

viscometer (MS Z2 DIN model

of

the concentric cylinders geometry).The bob and the cup radius

of

this

geometry are, respectively, 0.0225 m and 0.0244 m.

The rheological measurements were performed in the controlled shear rate mode. The range of the

measurements was carefully studied to avoid any interference at high values

of

shear rate, caused by the

secondary cellular motions that develop in the flow, the so-called Taylor instabilities. On the other hand the

low shear rate limit was also made an object of special study in order to minimize the influence

of

wall

depletion on the viscometric results since this effect is most likely to occur in this kind

of

suspension.

RESULTS AND DISCUSSION

Two experiments were performed using raw sewage sludge from a domestic wastewater treatment plant.

Figure 2 shows the evolution

of

solids concentration during the anaerobic digestion process.


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Anaerobic digestion process

63

. .

. -.,

I

•

; u .......... . . : ~ : : - - - - - - - -

•.

:=-:;;::

'.t------..--•• ••----..

Figure 2. Tolal solids concentration

durin,

the anaerobic digestion process.

It

was also

observed that the

biogas

product ion during both experiments, measured at the gasmeter, was

about 0.7 m

3

/kg

of

the

volatile solids destroyed.

Rheological

data

was in the first place analysed in a shear rate versus viscosity coordinate system in order

to

remove

any

interference ofwall depletion or

Taylor

instabilities in the results.

Recent studies by Escudier

et al.

(1995), revealed that the development ofTaylor vortices

on

non-Newtonian

shear-thinning fluids is

marked

by a progressive

and

gradual increase in the instabilit ies, in contrast

to the

abrupt way it takes place

in

Newtonian fluids. Moreover, experimental studies by Coelho

et al.

(1996) with

polymers confirmed the results of Escudier and showed that the flow instabili ty was reached only when Ta •

3100.

I·,..,

.........

IIi.. 1

· -

_

-:s=::

FiglR 3. Viscometric results with raw sludge (2nd clIperirnellt).

J.,... .....__

Z

'-IIIIII

.::

* \ ; ; ..

Fiaure 4. Viscornetric results willi diacslina siudac (2nd

uperimcnt).


4/7

64

P. S. MONTEIRO

- -.-,--- -, ;;

..

,. .....,•

D U ....

TI ·I.

Figure

S

Viscometric ~ u t with digested sludge (2nd experimenll.

Figures 3. 4 and 5 show, using a shear rale versus viscosily coordinate system. three examples

of

lhe

viscometric results obtained in the rheological tests perfonned during the second experiment.

Due

to the

great reduction

in

the viscosity of the sludges during digestion and in order to avoid any interference of the

Taylor instabilities. the shear rate upper limit of the experiments was fixed at 700 s-I for raw sludges and

300

s·1

for digested sludges. With a shear rate lower limit of 10,2

s,1 it

was not possible to

find

any

interference due

to

wall depletion.

Figure 6. Rheological results with raw sludge (2nd experiment).

, ruT

-\,/Il l ~ I U J Q G

i .

..•,M J .....

I

y

i

1.,

•• .........,.,

.. I .

PCMa

.1

1

Figure 7. Rheological results with digested sludae (2nd experiment).

The

rheological measurements showed that the best statistical approximations to the measured values are

obtained with

the

rheological models that include the yield stress parameter, namely the Herschell-Bulkley

model and

the

Bingham model. The correlation coefficient obtained for raw sludge is 0.99 for the

Henchell-Bulkley model and 0.95 for the Bingham model. On the other hand. for digested sludge these


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coefficient is 0.99 for both models. Figures 6 and 7 show two examples of the results obtained in the

rheological tests performed in the first and last runs

of

the second experiment with the best fit lines for

the

Herschell-Bulkley and Binghammodels.

. . .

.

. . _ . ~ _ . . . ~ .

Figure 8. Evolution of the yield stress (Herschell-Bulkley model).

.. 1lOD. .T

~

· ·

utI;--

..

~

Figure 9. Evolution of the consistency index (Herschell-Bulkley model).

·.1·...-....

...

. _ ~ --- . --- . -- .

--

Fi,lun: 10. Evolution of the power

index

(Herschell-Bulkley

model).

Finally. Figures 8, 9 and 10 show the evolution

of the

parameters

t

c

•

K and n, applying the Herschcl l•

Bulkley model to the experimental data, and Figures II and 12 show the evolution of the t

c

(yield stress)

and 1B (plastic viscosity)

of

the Bingham model during the anaerobic digestion

of

the sludges. The time

evolution

of

the rheological parameters clearly shows that the digestion process has a very strong influence

on the rheological behaviour

of

the sludges.


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66

P. S. MONTEIRO

· · ~

....

-

....

- - - -


7/7

REFERENCES


67

Coelho, P. M., Pereira,

A.

S. and Pinho. F. T. (1996).

Rheology of Tylose. CMC

and

Xantham Gum Aqueous Solutions.

Internal

Report. Departamento de Engenharia Meclinica e Gestao Industrial. Faculdade de Engenharia da Universidade

do

Porto.

Portugal.

Escudier, M. P

.•

Gouldson,

I.

W. and Jones, D. M. (1995). Taylor vortices in Newtonian and shear-thinning liquids.

Proc.

R

Soc.

Lond

449, 155-176.

Frost, R. C. (1982). Prediction

of

Friction Losses for the Flow

of

Sewage Sludges in Straight Pipes. Technical Report TR 175,

Water Research Centre.

Frost, R. C. (1983).

How to Design Sewage Sludge Pumping Systems.

Technical Report TR 185. Water Research Centre.

Johnson, M. (1981).

First Reporton the WRC Sewage Sludge Pumping Project.

Technical Report

TR

162, Water Research Centre.

Levine, A. D., Tchobanoglous, G. and Asano, T. (1985). Charactenzation of the size distribution of contaminants in wastewater:

treatment and reuse implications.

WPCF 57(7), 805-816.

Monteiro, P. and Valente,

J.

T. (1996).

Flow Characteristics

of nauobic

Digesting Siudgu

Slurry Handling and Pipeline

Transport - HYDROTRANSPORT 13. BHR Group Conference Series, Publication n° 20, 3-19.

Mulbarger, M. C., Copas. S. R., Kordic, 1.

R.

and Cash. F. M. (1981). Pipeline friction losses for wastewater sludges.

WPCF

51(8).

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