The effects of clay a m e n d m e n t and composting on metal speciation in digested sludge liang

~) Pergamon

PlI: S0043-1354(96)00290-4

Wat. Res. Vol. 31, No. 5, pp. 951-964, 1997

© 1997 Elsevier Science Ltd. All rights reserved

Printed in Great Britain

0043-1354/97 $17.00 + 0.00

THE EFFECTS OF CLAY AMENDMENT AND

COMPOSTING ON METAL SPECIATION IN DIGESTED

SLUDGE

LIANG QIAO @ and GOEN HO *@

Institute for Environmental Science, Murdoch University, Murdoch 6150, Western Australia, Australia

(First received October 1995; accepted in revised form September 1996)

Abstract--Sewage sludge usually contains significant heavy metals that may limit its land application.

Heavy metals in municipal solid waste have been shown to be less mobile by amendment with bauxite

refining residue (red mud) prior to the composting process. In the present research a sequential step

extraction was employed to investigate metal speciation (into exchangeable, bound to carbonate, to Mn

and Fe oxides, to organic matter and in residue phase) and the effect of red mud on metal speciation in

compost of sewage sludge for Cr, Cu, Ni, Pb and Zn. The effect of red mud addition and composting

process on metal distribution in sewage sludge compost is significant. Red mud addition generally reduces

metal leachability and therefore the potential hazard of releasing metals from sludge compost through

adsorption and complexation of the metals on to inorganic components to different extents for the

different metals. Red mud cannot desorb, however, metals bound to organic matter in the sludge. The

composting process breaks down organic matter in the sludge and may release the heavy metals. The

addition of red mud prior to the sludge composting binds the released metals on to the red mud for those

not strongly readsorbed by the remaining organics (Cu, Ni and Zn). © 199 "7 Elsevier Science Ltd

Key words--bauxite refining residue (red mud), composting, heavy metals, metal mobility, metal

speciation, :~ewage sludge

INTRODUCTION

In a previous experiment (Qiao et al., 1993) the effect

of bauxite refining residue (red mud) on the metal

speciation in sewage sludge, without composting the

red mud sludge mixture, was investigated. The effect

of red mud is delx,ndent on the metal distribution in

the sludge. In general, the exchangeable fraction of

metals can be effectively reduced and converted into

more stable forms, by precipitation and adsorption

into oxides of the red mud, either directly or

indirectly through complexation. The ratio of

exchangeable meta.l to total metal content in sludge

is therefore an important factor in assessing the role

of clay addition in controlling the mobility and plant

availability of metals.

The metals in sludge are generally in very stable

insoluble forms and most of the metals are bound to

the organic fraction that cannot be desorbed by red

mud or extracted by DTPA. The latter is usually used

as a measure of ple, nt available metals. The action of

microorganisms in composting may make the metals

more available due to metal release from the

decomposed organic matter in the sludge. Red mud

amendment significantly reduced the mobility and

plant availability of metals in municipal solid waste

*Author to whom correspondence should be addressed

[Fax: (61) 9 310 z[997].

(MSW) compost when the red mud was added at the

beginning of the composting process (Hofstede,

1994).

The addition of a clay material, such as red mud,

to sewage sludge and composting the mixture is

expected to reduce the mobility of heavy metals along

similar lines to MSW compost (Hofstede and Ho,

1992; Hofstede, 1994). Differences exist between

sewage sludge and MSW because heavy metals in

sewage sludge have been in contact with the organic

matter in the sludge for a longer period of time. With

digested sludge, anaerobic digestion of the sludge

means bacterial processes may have transformed

readily mobile metals into more stable complexes. It

is desirable therefore to investigate the effect of red

mud addition on heavy metal mobility during sewage

sludge composting by determining the metal specia￾tion during the composting process. The results of the

study are reported in this paper. The red mud

addition has been shown to improve the composting

process of digested sludge (Qiao and Ho, 1997).

MATERIALS AND METHODS

Samples of sludge compost

Sludge compost samples were taken from the sludge

composting experimental mixtures in which 0, 10 and 20%

red mud were added to the raw materials before the

composting process (Qiao and Ho, 1997). To obtain

successful composting and drying 0.5 kg of sugar was added

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