Heavy metals removal from anaerobically digested sludge

152 Pages·2003·1.52 MB·English

by  Marina Maya Marchioretto

Heavy metals removal from
anaerobically digested sludge

Marina Maya Marchioretto
Promotor
Prof. Dr. Ir. W. H. Rulkens
Hoogleraar in de Milieutechnologie, Wageningen Universiteit

Co-promotor
Dr. Ir. H. Bruning
Universitair docent, Sectie Milieutechnologie, Wageningen Universiteit

Samenstelling promotiecommissie
Prof. Dr. M. A. P. Reali (São Paulo Universiteit, EESC-USP, Brazilië)
Prof. Dr. W. H. van Riemsdijk (Wageningen Universiteit, Nederland)
Prof. Dr. Ir. C. J. N. Buisman (Wageningen Universiteit, Nederland)
Dr. Ir. A. H. M. Veeken (Wageningen Universiteit, Nederland)
Marina Maya Marchioretto

Heavy metals removal from
anaerobically digested sludge

Proefschrift
ter verkrijging van de graad van doctor
op gezag van de Rector Magnificus
van Wageningen Universiteit,
Prof. Dr. Ir. L. Speelman,
in het openbaar te verdedigen
op maandag 3 november 2003
des namiddags te 13:30 in de Aula.
Author: Marchioretto, M. M.
Title: Heavy metals removal from anaerobically digested sludge
Publication year: 2003
Thesis Wageningen University, Wageningen, the Netherlands - with summary in
Dutch and Portuguese
Keywords: anaerobically digested sludge, bioleaching, chemical leaching, heavy
metals, hydroxide precipitation, sulfide precipitation
ISBN: 90-5808-908-8
To my parents - my first instructors

To my brother - my first friend
ABSTRACT

Marchioretto, M. M. (2003). “Heavy metals removal from anaerobically
digested sludge”. Doctoral thesis, Wageningen University, the Netherlands,
142 pages.
Although studies in the field of heavy metals removal from sewage sludges
are numerous, knowledge gaps still exist, which are the reason for the low
attention paid to the implementation of techniques for this purpose. Such
techniques still remain in laboratory and pilot plant scale. This work aims to
contribute to the further applicability of technologies that might be used to
remove heavy metals from anaerobically digested sludge.
A physical classification scheme based on wet-sieving was applied and
showed that heavy metals prevailed in the fractions < 0.063 mm,
constituting 70 % of the total sludge mass and 78-85 % of the heavy metals
present in the sludge. In all the separated fractions, however, most of the
metals concentrations exceeded the Dutch standard for sludge application
on land. To determine the chemical forms of heavy metals in the sludge,
three sequential chemical extraction (SCE) schemes were tested: Tessier
(1979), Veeken (1998), and Sims & Kline (1991). Modified versions of the
original schemes were also applied as a tentative to investigate the
differences among them. Despite uncertainties, the SCE method gives useful
information concerning metals availability. The results demonstrated that Cr
(65-67 %) and Cu (65-87 %) were released in an oxidizing environment. Zinc
(78-86 %) was extracted by a reducing agent, whereas Pb was solubilized at
strong acidic conditions and high temperature. A chelating agent released
considerable amounts of Cr (56-57 %), Pb (57-78 %) and Zn (50-62 %).
About 70 % of Cu was also extracted at a pH of 12.6. The modified schemes
were valuable to interpret the differences in the results of the original
schemes.
To assess the heavy metals solubilization, several organic and inorganic
acids were tested at different conditions of pH, reaction time, and redox
potential. Oxidation with aeration or H O was also investigated. The best
2 2
results achieved with the most effective acid (HCl) were: Cr: 85 % with H O ,
2 2
Cu: 100 % with H O , Pb and Zn: 100 % with aeration and H O . Moreover,
2 2 2 2
Cu solubilization was highly influenced by oxidation. The effect of
bioleaching on heavy metals extraction, with application of elemental sulfur
and ferrous iron, was studied. The results were compared to those obtained
by chemical leaching with H SO and HCl. With addition of ferrous iron, the
2 4
maximum extraction achieved for Zn was 80.8 % with a pH of 2.7. For Cu, it
was 65.5 % with a pH of 2.5. Chromium was solubilized only when the pH
was 2.5 and less than 1 % of Pb was extracted by bioleaching. HCl is more
effective than H SO to solubilize all the metals studied.
2 4
After metals solubilization, the next step was the separation of the sludge
solids from the metal-rich acidic liquid (leachate) by centrifugation and
filtration. The filtered leachate was submitted to hydroxide precipitation with
NaOH and sulfide precipitation with Na S, both separately and in
2
combination. Each precipitation step was followed by filtration. The results
showed that, when Fe and Al are present in the sludge (as it was the case),
adsorption and/or coprecipitation of the heavy metals to ferric or aluminium
hydroxide precipitate might occur. This was especially observed when
hydroxide precipitation was solely applied. The use of NaOH at a pH of 4-5
followed by filtration and further addition of Na S to the filtered liquid at a
2
pH of 7-8 considerably decreased the dosage of the second precipitant, when
this was exclusively applied. The best removal efficiencies achieved were: Pb:
100 %, Cr: 99.9 %, Cu: 99.7 %, and Zn: 99.9 %.
Two conceptual designs of a treatment process to be applied in practice for
heavy metals removal from sewage sludge were discussed. One referred to a
physical-chemical treatment and the other to a biological-physical-chemical
treatment. In the first concept, heavy metals solubilization is achieved with
HCl and an oxidative pre-treatment with aeration or H O . In the second
2 2
concept, the approach is the use of the bioleaching to solubilize heavy
metals. This system consists of an aerobic bioreactor fed with So. A
biological sulfate-reducing reactor, where sulfate is converted into sulfur, is
included and a closed sulfur cycle is achieved. Based on a brief technical,
environmental and economical evaluation, the biological-physical-chemical
process seems to be more attractive than the physical-chemical process. In
further development of the process to a practical scale, attention has to be
paid to the reuse possibilities of heavy metals recovered from the sludge and
how to deal with the remaining liquid residues.

CONTENTS

Chapter 1 - General introduction 1
Chapter 2 - Sludge characterization 17
Chapter 3 - Chemical leaching of heavy metals 37
Chapter 4 - Bioleaching of heavy metals 63
Chapter 5 - Heavy metals removal from the leachate 81
Chapter 6 - General discussion 97
Chapter 7 - Summary 112
- Samenvatting 117
- Sumário 122
References 129
Acknowledgements 136
Curriculum vitae 139