Iwa international Specialist Conference

- With addition of ferrous iron (Fe, Fe/Acid, Fe/Culture, Fe/Acid/Culture):

Eight 1-L glass bottles were filled with 400 ml of sludge and amended with ferrous iron substrate in form of FeSO₄.7H₂O. The pH of four bottles was adjusted to 5 by sulfuric acid and the remaining bottles were preserved with the natural pH. A culture of growing bacteria (Thiobacillus ferrooxidans) was added to two preacidified bottles and to two non-acidified bottles.

- With addition of a mixed substrate (S/Fe, S/Fe/Acid, S/Fe/Culture, S/Fe/Acid/Culture):

These experiments were identical to those applied for bioleaching with ferrous iron as substrate. But instead of ferrous iron solely, a mixture of elemental sulfur and ferrous iron was applied.

After collection, all the samples were centrifuged at 4000 rpm during 20 minutes. The supernatant was filtered by paper filter S&S black ribbon (12-25 m) and subsequently analyzed in terms of heavy metals by ICP-MS (Marchioretto et al., 2002).

Results and discussions

Table 2 shows that bioleaching is well able to mobilize copper and zinc, but it is not as effective as chemical leaching with sulfuric acid to solubilize lead and chromium, even at the lowest pH achieved (around 1.9) by the microorganisms, after 15 days. Comparing sulfuric acid with hydrochloric acid, the second one is able to solubilize higher amounts of metals than the first one.

Table 2. Bioleaching and chemical leaching of heavy metals from anaerobically digested sludge

	Bioleaching^*				Chemical leaching
Extraction	Fe/Acid	Fe/Acid	S/Fe/Acid/Culture	S/Fe/Acid/Culture	H2SO4	HCl
(%)	13 days	15 days	13 days	15 days	2 days	1 day
	pH= 2.5	pH= 2.4	pH= 2.4	pH= 1.9	pH= 1.0	pH= 1.0
Cr	18.4	25.5	21.0	33.5	64.8	70.8
Cu	65.5	61.7	58.4	58.7	57.9	96.2
Pb	0	0	0	0	51.5	100.0
Zn	79.6	73.3	69.9	66.1	79.6	98.7

^*Only results which gave high extraction yields are shown.
Lead

Kieken and Cottenie (1984) reported that the threshold of lead mobilization was nearly the pH value of 2. This is in agreement with the results of the present work that at pH values higher or around 2, no lead solubilization was observed by bioleaching. Marchioretto et al. (2001) showed that about 45% of lead are present in the residual fraction, in which minerals may hold metals in their crystal structure. Therefore, lead can only be extracted into the solution at very low pH values. Marchioretto et al. (2001) also reported that 80% of lead could be bound to the inorganic matter and/or inorganic precipitates. It may be more difficult to solubilize lead from this fraction than from the organic fraction because bioleaching occurs on the microorganism cells. Probably lead solubilization would start with a pH value lower than 1.9, after 15 days of bioleaching.

Copper

Copper solubilization highly improved after 9 days of bioleaching, achieving comparable values to that obtained with chemical leaching. The affinity of copper to organic matter is very strong and, therefore, copper extraction requires a prolonged time at a very low pH to occur (Marchioretto et al., 2001).

Chromium

Chromium required prolonged time at low pH value to be solubilized. This may be explained by the its speciation in the sludge. According to Marchioretto et al. (2001), similar to lead, 55% of chromium is bound to inorganic matter and/or inorganic precipitates. Besides, this metal (65%) was found incorporated in organic matter and organic mineral aggregates, which might explain the improvement in chromium solubilization after 9 days.

Zinc

Zinc extraction efficiency with bioleaching was equivalent to that of leaching with sulfuric acid. It was the least resistant metal to the effect of pH decreasing. This might be due to zinc speciation in the sludge, which shows that this metal is mostly located in Fe/Mn oxides fraction (Marchioretto et al., 2001).

Conclusions

Comparing to chemical leaching with sulfuric acid, bioleaching was satisfactory for zinc and copper but not for chromium and lead. Even at higher pH values in bioleaching, zinc and copper extractions were similar to those achieved with chemical leaching. In addition, the amount of acid required for bioleaching was much lower than that for chemical leaching. Hydrochloric acid, however, is still the most effective option for metals solubilization from the present sludge.

The present work suggests that a combined treatment consisting of bioleaching (previous acidification step) followed by chemical leaching with hydrochloric acid (second acidification step) with simultaneous oxidation (e.g. aeration) would promote a very satisfactory heavy metals extraction from the sludge. This alternative would be able to reduce favorably the cost of the chemical treatment and its inherent harms to the environment.
Aknowledgements - This work was supported by "Conselho Nacional de Desenvolvimento Científico e Tecnológico - CNPq" (Project n^o 200808/98-2), an entity from the Brazilian Government for the Development of Science and Technology.
References

Kiekens L. and Cottenie A. (1984). Report of results of the interlaborartory comparison: determination of the mobility of heavy metals in soils, in Processing and use of sewage sludge. L’Hermite P., and Ott H., Eds., D. Reidel, Dordrecht, the Netherlands, 140.

Marchioretto M. M., Bruning H., Loan N. T. P. and Rulkens W. H. (2001). Heavy metals extraction from anaerobically digested sludge. In: Specialised Conference on Sludge Management: Regulation, Treatment, Utilisation and Disposal. Proceedings. October 25-27, 2001. Acapulco, Mexico.

Marchioretto M.M., Bruning H. and Rulkens W.H. (2002). Optimization of chemical dosage in heavy metals precipitation in anaerobically digested sludge. In: XXVIII Interamerican Congress of Environmental Sanitary Engineering. Proceedings. October 27-November 1, 2002. Cancun, Mexico.

SDU (1991). Besluit Overige Organishe Meststoffen (BOOM). Decree 613:1-45 (In Dutch).

Tichy R., Rulkens W. H., Grotenhuis J. T. C., Nydl V., Cuypers C. and Fajtl J. (1998). Bioleaching of metals from soil sediments. Wat. Sci. Tech., 37(8),119-127.

ALTERNATIVE USES OF SEWAGE SLUDGE BY MEANS OF STABILIZATION/SOLIDIFICATION

90

Panagiota Boura⁽¹⁾ , Margarita Katsioti ⁽²⁾, Alexandra Katsiri <sup>(1)

(1)</sup> School of Civil Engineering, Division of Water Resources, National Technical University of Athens, 10 Iroon Polytechniou Street, 15780 Zografou, Athens
⁽²⁾ School of Chemical Engineering, Laboratory of Analytical and Inorganic Chemistry, National Technical University of Athens, 10 Iroon Polytechniou Street, 15780 Zografou, Athens

ABSTRACT

The main objective of this work is to investigate a viable alternative for the final disposal of sewage sludge from urban wastewater treatment plants by its use as an additive in developing new construction materials. For this purpose, several mixtures of sludge - cement and sludge-cement and jarosite/alunite were prepared. Jarosite/ alunite is a waste product of a new hydrometallurgical process. Two kinds of sludge were used: primary sludge from Psyttalia Wastewater Treatment Plant, which receives a considerable amount of industrial waste, and biological sludge from Metamorphosis Wastewater Treatment Plant. Various percentages of these sludges, both in wet and dry condition, were stabilized/solidified with Portland cement and Portland cement with jarosite/alunite. The specimens were tested by determination of compressive ad tensile strength according to the methods described by European Standard EN 196. Furthermore, in order to investigate the environmental compatibility of these new materials, leaching tests for heavy metals, substances with a great impact on the environment, were carried out. The immobilization of the heavy metals contained in sewage sludge and its binding mechanism was also studied by means of Electron Microscope Analysis. Finally, the possibility of using the produced materials as bases or sub-bases of roads, airfields, parking areas etc. was examined and discussed.

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1 Corresponding author. E-mail: [e-mail]