OPTIMIZATION OF CHEMICAL DOSAGE IN HEAVY METALS PRECIPITATION IN ANAEROBICALLY DIGESTED SLUDGE Marina Maya Marchioretto (*) PhD student (CNPq-200808/98-2) from the Sub-department of Environmental Technology, Wageningen University. MSc. from the Dept. of Hydraulics and Sanitation, University of São Paulo, campus of São Carlos, Brazil - EESC-USP (1999). Civil Engineer from the EESC-USP (1996). Harry Bruning Sub-department of Environmental Technology, Wageningen University. Wim H. Rulkens Sub-department of Environmental Technology, Wageningen University. (*): Sub-department of Environmental Technology, Wageningen University, P.O. Box 8129, 6700 EV Wageningen, The Netherlands. Phone: +31 (0)317 48-4134. Fax: +31 (0)317 48-2108. E-mail: marina.marchioretto@algemeen.mt.wau.nl. ABSTRACT The metals removal from anaerobically digested sludge was studied by sulfide and hydroxide precipitation (in single and combined ways) followed by filtration in bench scale. Before submitted to precipitation the sludge was aerated and acidified till the value equal to 1, in order to attain the best conditions for metals solubilization. The results showed that the combination of hydroxide and sulfide precipitation before physical separation was capable to promote an efficient removal of heavy metals from anaerobically digested sludge. Applying sodium hydroxide at equal to 4 and 5 with further addition of sodium sulfide at values of 7 and 8, respectively, decreased highly the dosage of the second precipitant, when it was exclusively applied. The best percentages achieved for metals removal were: lead - 100%, chromium - 99.9%, copper - 99.7%, and zinc - 99.9%. Keywords: Anaerobically digested sludge, heavy metals, sulfide precipitation, hydroxide precipitation. INTRODUCTION Activated sludge systems for wastewater treatment produce large amounts of sludge. Normally, this sludge is stabilized by anaerobic digestion and afterwards it is dewatered and disposed in landfills. However the feasibility of adequate sites and the risk of contamination of superficial and subterranean waters are some of the drawbacks of sludge disposal on land. A sustainable management of wastewater sludge, and in particular its beneficial reuse, is a growing practice that has gained public awareness throughout the world. Several attempts have been made to develop regulations that protect public health and the environment. In addition, some of those regulations are under review or improvement to further reduce the risks derived from recycling practices (Jiménez and Spinosa, 2001). The risks of the sludge utilization are mainly related to the presence of pathogens, persistent organic pollutants and heavy metals. Metals concentrations in the sludge vary from one site to another, depending on the contribution of domestic and industrial input into the system. Heavy metals can be tightly bound or incorporated in organic matter, organic mineral aggregates and inorganic particles, bound to carbonates or iron and manganese oxides, among other forms. This means that a mere physical separation of the major fractions of heavy metals would be difficult. Prior to the separation process, a pre-treatment focused on the dissolving heavy metals is necessary. Once solubilized metals can be precipitated and then removed by a physical separation process.
According to Tyagi et al., 1988, the solubility of metals is governed primarily by but other factors like redox potential of the sludge and the concentration of metals and ligands (negative ions and uncharged molecules) are also important. The authors state that the solubilization strategy requires optimum adjustment of and redox potential of the sludge in such a way that the chemical equilibria will be shifted in favour of dissolved metallic ion formation. Usually the anaerobic digested sludge contains sulfur or sulfide compounds, which results in formation of weakly soluble compounds, e.g. metal sulfides. Only diminishing the of the anaerobic sludge is not enough to promote a change of metals sulfides to the soluble ions form unless the acidification is preceded by a raise in the sludge redox potential, as observed by Hayes and Theis, 1978. The redox potential of anaerobic sludge can be increased either by means of chemical or biological oxidation (bioleaching). Chemical oxidation can be achieved with aeration. This process might be able to promote the oxidation of metal sulfides into soluble metal sulfates. This is followed by the release of metals and sulfuric acid (Maas and Miehlich, 1988). Hydroxide precipitation using lime or caustic is the most commonly used form of chemical precipitation at wastewater treatment plants. The second is sulfide precipitation, which is more advantageous than hydroxide precipitation, once it can reduce hexavalent chromium to the trivalent state under the same process conditions required for metals precipitation, it allows for the precipitation of metals when chelating agents are present and most metals can be removed to extremely low concentrations at a single. Limitations of the process involve the potential hydrogen sulfide gas evolution and the concern for sulfide toxicity. However eliminating sulfide reagent overdose prevents formation of the odor causing hydrogen sulfide (EPA, 1998). Nowadays, a combination of hydroxide and sulfide precipitation for optimal metals removal is being well considered. A common configuration is a two-stage process in which hydroxide precipitation is followed by sulfide precipitation with each stage followed by a separate solids removal step. This will produce the high quality effluent of the sulfide precipitation process while significantly reducing the volume of sludge generated and the consumption of sulfide reagent (EPA, 1998). In this way a promising method to remove heavy metals from anaerobically digested sludge might be comprised by a pretreatment of the sludge based on aeration and acidification by chemical leaching or bioleaching, separation of the suspended solids and the liquid fraction containing the dissolved metals by centrifugation or another separation technique, combined hydroxide and sulfide metals precipitation, and removal of the metals precipitates from the liquid by a physical method, e.g. flotation or membrane filtration. OBJECTIVES This research aims the assessment of metals precipitation in anaerobically digested sludge by sodium hydroxide, sodium sulfide and combination of both hydroxide and sulfide precipitations. The optimization of the dosage of the precipitating agents is evaluated. MATERIALS AND METHODS Anaerobically digested sludge The sludge applied in this research came from an anaerobic digester of a wastewater treatment plant located in Schijndel, The Netherlands. This plant receives both industrial and domestic contributions and it is based on activated sludge system followed by anaerobic digestion of the primary and secondary sludges. After collected from the anaerobic digestor, the sludge was stored at 4 C when not directly used. In order to evaluate the metals distribution in the solid and liquid parts of the sludge, this material was subjected to centrifugation during 20 minutes at 4000 rpm and the supernatant was analysed in terms of heavy metals. In addition, experiments concerning the physical fractionation of the sludge were performed based on wet-sieving. A vibrating sieving equipment (Retsch Labor-Siebmachine Type VIBRO) was supplied with stainless-steel sieves of hole sizes 5.0,
1.0, 0.5, 0.2, 0.09, 0.063, 0.032 and 0.02 mm. The portion collected in each sieve, as well as the remaining liquid (fraction lower than 0.02 mm) were analysed in terms of heavy metals. Table 1 shows some characteristics of the anaerobic sludge, the metals distribution in the several fractions, the percentage of metals content in the liquid part of the centrifuged sludge (supernatant) and the Dutch Standard for metals disposal on agriculture soils (BOOM), according to the National Environmental Policy Plan concerning the durable use ot the environment (SDU, 1991). According to the table 1, it is possible to observe that in almost all the fractions the amount of metals exceeded the Dutch Standard (BOOM), meaning that the sludge must be treated as a whole, without any previous fractionation. Furthermore, the heavy metals content in the liquid part of the centrifuged sludge (supernatant) is very low, compared to the heavy metals content in the total sludge (TS), leading to the conclusion that as mostly the metals are present in the solid fraction of the sludge, they must be dissolved in the liquid, before removed. TABLE 1. Some characteristics of the anaerobically digested sludge Parameter Dry Matter (DM ) Organic Matter Heavy Metal Content (mg/kg DM) (% DM) Cr Cu Pb Zn Total Sludge (TS) 25 g/l 60 300-500 700-900 150-200 1500-2000 5.0-1.0 mm 7.0 % TS 89 110 220 55 640 1.0-0.5 mm 3.8 % TS 82 180 300 90 815 0.5-0.2 mm 5.2 % TS 64 230 370 140 1230 0.2-0.09 mm 13.3 % TS 58 320 560 155 1340 0.09-0.063 mm 15.7 % TS 59 400 880 170 1715 0.063-0.032 mm 26.0 % TS 54 445 955 190 1930 0.032-0.02 mm 7.0 % TS 56 505 1100 200 2200 < 0.02 mm 22.0 % TS 54 415 950 170 210 BOOM (1991) - - 75 75 100 300 Supernatant - - 3.8 % TS 4.0 % TS 3.0 % TS 4.0 % TS Chemical characterization of the sludge Three sequential chemical extraction (SCE) schemes (Tessier, 1979, Veeken, 1998 and Sims & Kline, 1991) and two modified versions (Tessier and Veeken) were tested in this research, as detailed by Marchioretto et al., 2001. Although SCE is still an imperfect method referring to specificity and selectivity, it provides valuable information regarding the behaviour of the metals face to several conditions of temperature,, type of chemical reactions, etc. Table 2 summarizes some remarkable results obtained by the different schemes. It was taken into account only the highest percentages of metals content in each fraction, for the corresponding scheme.
TABLE 2. Metals distribution in the anaerobically digested sludge according to SCE schemes Fraction Cr Cu Pb Zn Bound to Fe-Mn oxides 85 % c - reducing agent applied - Bound to inorganic matter and/or 55 % a, b 80 % a, b 60 % a, b inorganic precipitates - chelating agent applied - Incorporated in organic matter and organic mineral aggregates - oxidating agent applied - 65 % c, d, e c, d, e 85 % a Veeken Scheme, b Sims & Kline Scheme, c Tessier Scheme, d Modified Veeken Scheme, e Modified Tessier Scheme. Pretreatment of the sludge In order to achieve the best conditions for metals solubilization, samples of 1 L of the sludge were subjected to previous aeration, followed by acidification and centrifugation. Both aeration and acidification were applied during 24 hours, with continuous shaking (150 rpm), at 20 C. During aeration the air flow rate was 1.5 L/h. Acidification with 5 ml of hydrochloric acid (37 %) was applied to decrease the initial of the sludge from 8 till 1, as described in detail by Marchioretto et al., 2001. At the end the bottles were centrifuged at 4000 rpm during 30 minutes and the supernatant was submitted to the precipitation experiments. Table 3 shows the heavy metals content in the centrifuged liquid after aeration and acidification. The sludge was submitted to the pretreatment two times because the precipitation experiments were carried out in different days. According to table 3, the pretreatment conditions are efficient to solubilize mainly lead and zinc. Copper and especially chromium are more difficult. This fact might be due the chemical distribution of the metals in the sludge (see table 2). Mostly copper and chromium are entrapped in organic solids, intensifying the difficulty to be solubilized. Hayes et al., 1980, mentions that in anaerobic sludges copper is likely to form stable complexes with humic acid particles and chromium can be predominant as the trivalent hydroxide (inorganic precipitate). The unlikely observed behaviour of chromium is not easily explained. The authors also suggests that there might occur interferences of several mechanisms of heavy metal immobilization, such as complexation to organic solids, coprecipitation, protective organic entrapment of precipitates and intracellular uptake, exerting some control over metal solubilization in acidified sewage sludges. The results obtained suggest that, still,the pretreatment phase must be optimized, in order to improve chromium solubilization. TABLE 3. Heavy metals content after pretreatment with aeration and acidification followed by centrifugation Cr (mg/l) Cu (mg/l) Pb (mg/l) Zn (mg/l) Total Sludge (TS) 9.2 19.8 4.9 35.0 Centrifuged Liquid 1 6.1 17.4 4.8 33.7 Centrifuged Liquid 2 5.8 18.1 4.7 34.3 Precipitation experiments The precipitation experiments were carried out in three phases: hydroxide precipitation with NaOH, sulfide precipitation with Na 2 S and combination of hydroxide and sulfide precipitation. The first two parts were performed with the addition of the precipitating agent based on the values of 5, 7, 9 and 11 with continuous shaking (100 rpm) during 30 minutes. After each experiment the liquid was filtered by paper filter S&S black ribbon (12-25 µm) and two samples were collected and analyzed in terms of heavy metals (Cr, Cu, Pb and Zn).
The third part of the experiments was accomplished with NaOH addition till the values of 4 and 5 were achieved. After this phase the liquids were filtered, analyzed in terms of heavy metals and submitted to Na 2 S addition in such dose to reach the values of 5, 5.5, 6, 7 and 8. All the experiments were carried out in duplicates following the same procedure for the single precipitation. Heavy metals analysis The heavy metals (Cr, Cu, Pb and Zn) were analysed by the Inductively Coupled Plasma - Mass Spectrometry Method (ICP-MS), which is described in the Standard Methods. After the precipitation experiments samples of 1 ml were dilluted 10 times with HNO 3-0.14 M, before the metals analysis. In addition, following the procedure described by Veeken, 1998, two samples of 15 ml of the total sludge were previously digested in the microwave with addition of aqua regia (HCl:HNO 3-3:1) before final dillution for ICP-MS measurement. RESULTS 1) Single precipitation Figure 1 and 2 show the percentage of the remaining metals in the filtered liquids resulted from the precipitation experiments, according to variations. The initial metals content in the pretreated sludge is formerly shown in table 3. According to figure 1, Cu removal is very insignificant for all the values. This might be due to the formation of colloidal copper hydroxides forms, which were not retained by the 12-25µm filter or because the precipitation reaction was very slow. At value equal to 5 zinc removal was low but at higher values it improved. Comparing both figures, it is possible to observe that in general sulfide precipitation is more efficient for all the metals together than hydroxide precipitation. Besides the advantages of sulfide precipitation over hydroxide precipitation when single applied, it is important to notice that high dosages of the precipitating agents must be applied to achieve the suitable. Metals content (%) 5 6 7 8 9 10 11 Pb Cr Cu Zn 100 %: Initial metals concentration 5 6 7 8 9 10 11 Figure 1. Precipitation with NaOH (NaOH (100 %) doses (g/l): =5: 10.7, =7: 11.4, =9: 13.2, =11: 16.4) Figure 2. Precipitation with Na 2 S (Na 2 S (35 %) doses (g/l): =5: 33.3, =7: 53.1, =9: 75.1, =11: 101.4). Pb below detection limit. 2) Combined precipitation Figure 3 and 4 show the percentage of the remaining metals in the filtered liquids resulted from the precipitation experiments, with previous application of NaOH to achieve values of 4 and 5 respectivelly, followed by Na 2 S dosing. These initial values were chosen because according to the Figure 2, sulfide precipitation is already efficient at equal to 5. The initial metals content in the pretreated sludge is presented in table 3.
From Figure 3 it is observed that with initial value of 4, the metals are removed at lower with the application of Na 2 S than in the situation shown in figure 4, when a higher NaOH dosage is applied. The more hydroxide precipitation is applied, the more difficult is the removal of the metals by the subsequent sulfide precipitation, especially for copper and zinc. The explanation might be again because of the slowness of the precipitation reactions, associated to the occurrence of colloidal copper hydroxides forms passing through the 12-25 µm filter. In addtion, it can be seen that when is raised to 4 or 5 by NaOH, it should be increased another 3 -units by Na 2 S to get a better metal removal. The first is better if the objective is to minimize NaOH dose. The second option is more suitable if the main goal is to reduce Na 2 S dosage and increase the value of zinc removal. For the main goal of this research it is appropriate to find a solution that fits to the four metals involved. Comparing the chemical doses required for single sulfide precipitation and the amount of Na 2 S needed for the combined precipitation, in the second case less precipitant is needed to achieve the same metals removal. Metals content (%) 4 5 6 7 8 Pb Cr Cu Zn 100 %: Initial metals concentration 4 5 6 7 8 Figure 3. Precipitation with Na 2 S =4 (NaOH (100 %) dose: 10.3 g/l) (Na 2 S (35 %) doses (g/l): =5: 0.12, =5.5: 0.13, =6: 0.14, =7: 0.18, =8: 0.27) Figure 4. Precipitation with Na 2 S =5 (NaOH (100 %) dose: 10.7 g/l) (Na 2 S (35 %) doses (g/l): =5.5: 0.008, =6:, =7: 0.04, =8: 0.16) CONCLUSIONS Anaerobic sludge pretreated with aeration and acidification followed by hydroxide and sulfide precipitation with further physical separation process is a feasible option to remove heavy metals from anaerobically digested sludge. The combination of NaOH ( equal to 4 and 5) and Na 2 S ( equal to 7 and 8 respectively) is able to reduce considerably the dosage of the second precipitant, when it is solely applied. Considering the two best situations, the percentages of metals removal are Pb: 100 %, Cr: 99.9 %, Cu: 99.7 % and Zn: 99.7-99.9 %. RECOMMENDATIONS The remaining amount of sodium in the liquid after the physical separation process, due to the addition of the precipitating agents NaOH and Na 2 S, must be considered. Redirecting the effluent to the beginning of the treatment plant might be one solution, but other alternatives must be evaluated, including the promising application of the biogenic sulfide generation (Gilbert et al. and Tabak et al., 2002) as a substitute to the chemical sulfide. Optimization of the pretreatment of the sludge (aeration and acidification) in order to improve chromium solubilization. Investigation of the physical separation step. The suggested processes are flotation (induced-air flotation and dissolvedair flotation), membrane filtration and sedimentation, which are now under investigation.
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