Occurrence and fate of selected heavy metals in the wastewater treatment plant of Thessaloniki, Greece

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Bioresource Technology, 72 (2000), 113-119 • Conclusions • Heavy metals were determined at all the different stages of an activated sludge WWTP at frequencies between 90-100% • Most abundant HM in wastewater was Ni while Zn was the most abundant metal in sludge • Least abundant metal was Cd • Removal rates ranged from 21% for Fe to 56% for Cu. These rates are considered to be satisfactory since no coaggulant is used in the plant Fe < Pb < Zn  Mn < Ni < Cr < Cd < Cu • Concentrations of H.Ms found for the Final sludge are below the maximum permissible concentrations for safe agricultural use Occurrence and fate of selected heavy metals in the wastewater treatment plant of Thessaloniki, Greece Michalis Karvelas, Athanasios Katsoyiannis, Constantini Samara Aristotle University of Thessaloniki, Department of Chemistry, Environmental Pollution Control Laboratory, Gr-54124 Thessaloniki, Greece Introduction Heavy metals are considered to be some of the most hazardous pollutants for the environment, due to the fact that they are toxic and some of them carcinogenic. They are not degradable; on the contrary they bio-concentrate and thus enter the trophic chain. In this work, the occurrence and fate of 8 heavy metals in the Wastewater Treatment Plant of Thessaloniki is studied. Major objectives of this work were to study the presence of heavy metals in this plant and to estimate their removal rates along the sewage treatment Heavy metals may end to a wastewater treatment plant mainly as industrial products. At the end of their life as useful industrial products, they are discharged in wastes. Products that contain high amounts of heavy metals and they are often disposed to wastes, are batteries, dental amalgams, thermometers, tubes, lamps etc. Heavy metals may also be lost in wastes during production and use phases. Abbreviations: WWTP, wastewater treatment plant; HMs, heavy metals; RW, raw wastewater; PSE, effluent from primary sedimentation tank; SSE, effluent from secondary sedimentation tank; PS, primary sludge; AS, activated sludge; FS, final sludge. Results and Discussion Table 1: Concentrations of Heavy Metals in the wastewater of the Thessaloniki WWTP Metal RW (μg/L) PSE (μg/L) SSE (μg/L) Mean Std Mean Std mean std Cu 79.2 34.9 58.1 36.8 32.9 5.8 Cr 39.6 12 24.5 11.8 20.4 3.5 Ni 766.7 199.9 604.1 272.4 424.5 97.1 Pb 38.9 9.4 31.1 11.9 26.8 3.6 Mn 67.2 12.3 25.7 4.3 19.3 2.4 Fe 476.6 86.7 450.2 77.9 377.5 46.5 Cd 3.3 1.1 2.3 0.9 1.49 0.7 Zn 465.7 144.4 380.6 49.9 269.8 52.9 Table 2: Concentrations of Heavy Metals in the sludges of the Thessaloniki WWTP Metal PS (μg/g d.m) AS (μg/g d.m) FS (μg/g d.m) Mean Std Mean Std Mean Std Cu 1037.1 211.9 909.2 165.9 1216.1 222.5 Cr 169.1 61.2 314.9 129.5 366.1 104.8 Ni 368.3 122.4 305.6 158.1 305.6 76.2 Pb 278.7 79 162.7 31.4 325.8 83.5 Mn 826.2 278.1 732.7 172.1 1141.5 180.5 Fe 2405.1 634.6 3582.7 626.9 3325.6 261.1 Cd 10.1 3.9 10.1 3.7 10.2 4.3 Zn 3516.1 31.4 4427.4 590.9 4465.8 445.7 Removal Rates of H.Ms Primary (%) Secondary (%) Total(%) Cu 26,63 43,37 58,45 Ni 21,21 29,72 44,63 Cr 38,24 16,66 48,53 Pb 19,92 13,96 31,10 Mn 23,45 24,89 42,50 Fe 5,54 16,17 20,81 Cd 30,31 34,79 54,55 Zn 18,27 29,10 42,06 COD 42,84 91,41 95,09 BOD 47,70 96,22 98,02 SS 50,22 96,44 98,23 TOC 0,70 73,70 73,88 Figure 3:Removal rates of all H.Ms in the WWTP of Thessaloniki • The WWTP of Thessaloniki - Sampling • The WWTP of Thessaloniki treats daily 120,000 – 150,000 tons of municipal wastewaters • About 5-10% of the total flow is contributed by industry • No use of coagulant is performed • Sampling was conducted in two one - week campaigns during November and December 2001 • Given that the residence time of wastewater in the WWTP was about 24 hours, this sampling rate enabled calculation of the mass balance of heavy metals throughout the treatment plant • Samples were collected from 6 points: RW, PSE, SSE, PS, AS, FS Better correlation in the removal rates appears for: Fe-Cu: 0.848 Pb-Cr: 0.813 Fe-Pb: 0.874 Cd-Pb: 0.831 Figure 1: Mean Concentrations of H.Ms in wastewater samples Figure 1: Flow chart of the Thessaloniki Wastewater Treatment Plant Figure 4: Correlation of the removal rates between all the individual parameters Figure 2: Mean Concentrations of H.Ms in sludge samples Table 3: Maximum permissible concentrations of H.Ms in sludge for use in agriculture CountryHeavy Metals (μg/g) Cd Cr Cu Pb Ni Zn Greece 20-40 1500 1750 1200 400 4000 European Union 1750 1200 400 USA 4300 840 420 Austria 3 250 500 250 100 1200 Belgium 5 200 500 1000 100 1500 Denmark 0,4 120 30 Germany 10 900 800 900 200 2500 France 20 1000 1000 800 200 3000 Luxembourg 1,5 100 100 150 50 400 Holland 1 50 75 100 30 200 Spain 10 400 50 300 120 1100 Italy 10 600 600 500 200 2500