Heavy metals removal from anaerobically digested sludge

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 H2O2 was also investigated. The best results achieved with the most effective acid (HCl) were: Cr: 85 % with H2O2, Cu: 100 % with H2O2, Pb and Zn: 100 % with aeration and H2O2. Moreover, 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 H2SO4 and HCl. With addition of ferrous iron, the 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 H2SO4 to solubilize all the metals studied. 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 Na2S, both separately and in 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 Na2S to the filtered liquid at a 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 H2O2. In the second 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.