9264
Ind. Eng. Chem. Res. 2005, 44, 9264-9272
Occurrence and Removal of Potentially Toxic Metals and Heavy Metals in the Wastewater Treatment Plant of Fusina (Venice, Italy) F. Busetti,*,† S. Badoer,‡ M. Cuomo,‡ B. Rubino,‡ and P. Traverso† Department of Environmental Sciences, University Ca’Foscari of Venice Calle Larga S. Marta 2137, I-30123 Venice, Italy, and VESTA SpA Cannaregio 462, I-30121 Venice, Italy
This study addresses the issue of whether it is possible to accurately predict the removal efficiencies of metals of environmental concern (i.e., Al, Ag, As, B, Ba, Cd, Cr, Fe, Mn, Hg, Ni, Pb, Cu, V, and Zn) in a wastewater treatment plant. The plant in question (at Fusina, Venice, Italy) is fed by mixed wastes from municipal and industrial sources (∼300 000 equivalent inhabitants) and discharges the treated effluent into the Venice lagoon. The year-long sampling campaign (2001-2002) yielded a substantial amount of analytical data and relatively wide ranges of concentrations of metals in the influent samples, which made it possible to study the removal efficiencies by plotting the terms (inlet concentration - outlet concentration) vs (inlet concentration) for each metal investigated. The data in the plots were fitted using the linear regression model Y ) BX. The slope rates (terms B), which were estimated by the least-squares method, have been adopted as the removal efficiencies, and they can be considered as constants in the concentration ranges recorded in this work. The relative abundance of metals in the raw wastewaters feeding Fusina WWTP followed the order Al > Fe > B > Zn > Ba > Mn > Cu > Pb > Hg ) Ni > Cr ) As > V > Ag > Cd, while in the effluent the order was Fe > Al > Zn > Mn > Ba > Ni > Cu > Pb > Cr > Ag > As > Hg ) V > Cd. The removal percentages (%) of the metals were Al ) 92 ( 1; Ag ) 94 ( 1; As ) 76 ( 3; B ) n.d.; Ba ) 85 ( 2; Cd ) 85 ( 2; Cr ) 87 ( 1; Fe ) 90 ( 1; Mn ) 61 ( 2; Hg ) 93 ( 1; Ni ) 50 ( 3; Pb ) 92 ( 1; Cu ) 93 ( 1; V ) 74 ( 2; and Zn ) 75 ( 3. 1. Introduction At present, wastewater treatment plants (WWTPs) serving both municipal and industrial districts receive complex mixtures of nutrients and organic and inorganic micropollutants, which are treated by conventional plants so that they do not reach and impact the environment in high doses and concentrations.1 Most wastewater treatment plants serving communities around the world have been designed and are regulated to remove nutrients from wastewaters, but it is also known that large amounts of potentially toxic elements such as metals and heavy metals from anthropogenic emissions end up in wastewater.2 Until recently, however, the concentrations of organic and inorganic micropollutants in the WWTPs influents and effluents were not routinely tested, which was also because of the high costs of carrying out analyses and extensive sampling campaigns. Consequently, there is limited information on micropollutant concentrations in the influent and effluent wastewaters and on their removal by conventional WWTPs. However, in the last fifteen years, the EU directive 91/271/EEC3 and a number of guidelines and regulations from national and local authorities have come into force to protect the environment from the adverse effects of discharging micropol* To whom correspondence may be addressed. Current address of Dr. Francesco Busetti: Water Chemistry Research Group, Centre for Applied Organic Geochemistry, Curtin University of Technology, GPO Box U1987, Perth, WA, 6845. Tel.: +61 (8) 9266 3273. Fax: +61 (8) 9266 2300. E-mail:
[email protected]. † University Ca’Foscari of Venice Calle Larga S. Marta 2137. ‡ VESTA SpA Cannaregio 462.
lutants along with the treated wastewaters into surface waters such as rivers, canals, lakes, and the sea.2 The most recent legislative acts on water pollution control and water quality improvement in Italy are the Legislative Decree 152/1999 (modified by Decree 258/2000) and the Ministerial Decree 30/07/99, which have been conceived to incorporate the European Directives 91/271 into Italian legislation.4 The existing WWTP at Fusina is a large facility with a capacity of ∼90 000 m3/d, treating prevalently municipal wastewaters, and it discharges the treated effluent into the lagoon of Venice. To obtain useful information on the occurrence of micropollutants in the influents to the Fusina WWTP and on their removal efficiencies by the existing plant, VESTA SpA, the company managing the treatment plant, carried out an intensive and thorough monitoring campaign from 2001 to 2002. This paper refers to the results obtained for the regulated metals (i.e., Al, Ag, As, Ba, B, Cd, Cr, Fe, Mn, Ni, Pb, Cu, V, Hg, and Zn) which resulted in Limit of Detection >(LODs) in the influent and effluent streams of the plant. Moreover, the compliance of treated effluent from Fusina WWTP with local regulation limits concerning metals release in the Venice Lagoon has been assessed. 2. Methodology 2.1. WWTP of Fusina. The Fusina WWTP is located close to the city of Venice (Italy) and receives ∼90 000 m3/d of raw wastewater from the major neighboring residential districts (Mirese area, Marghera, and the Southwest area of Mestre city) and ∼10 000 m3/d of pretreated industrial waste from the industrial area of Porto Marghera, Fusina. The treatment plant also treats part of the local urban runoff, which is mainly
10.1021/ie0506466 CCC: $30.25 © 2005 American Chemical Society Published on Web 10/18/2005
Ind. Eng. Chem. Res., Vol. 44, No. 24, 2005 9265
Figure 1. Flowchart of Fusina WWTP, sampling points: (1) raw wastewater influent; (2) screening; (3) oil and sand removal; (4) accumulation tanks; (5) sand collection sump; (6) oil collection sump; (7) equalization; (8) pretreated industrial wastewater influent; (9) tank-truck; (10) pretreatment for municipal/industrial wastes truck transported; (11) screw pumps; (12) denitrification; (13) oxidation and nitrification; (14) recycle of primary sludge; (15); recycle of secondary sludge; (16) sedimentation; (17) waste sludge; (18) disinfection; and (19) effluent.
composed of atmospheric deposition and traffic emissions deposited on the road surface, as well as ∼300 m3/d of untreated industrial waste, sewage from passenger ships, and sludge from Imhoff tanks, which are transported to the plant by truck-tankers. The treatment process includes mechanical and preliminary treatments (screening, gritting, and oil/grease removal), biological treatments (pre-denitrification assisted by dosing external carbon, oxidation, and nitrification), and secondary sedimentation assisted by dosing a chemical coagulant, followed by disinfection with peracetic acid. The primary sedimentation stage is not included in the wastewater treatment process. The waste sludge is anaerobically digested, thickened, and then dewatered with belt filter presses. The treated effluent is discharged into the Venice lagoon, while the stabilized sludge is deposited in municipal landfills. A flowchart of the treatment plant indicating the sampling points is shown in Figure 1. 2.2. Sampling. The wastewater sampling was conducted over a period of one year, from July 2001 to May 2002, as follows: - First sampling period: from 01 July to 30 August, 2001, ∼2 days per week (16 days in total); - Second sampling period: from 30 September to 20 November, 2001, ∼2 days per week (13 days in total); - Third sampling period: from 6 January to 26 February, 2002, ∼2 days per week (15 days in total); and - Fourth sampling period: from 5 April to 23 May, 2002, ∼2 days per week (15 days in total). The sampling strategy was designed to control both the influent quality and the removal efficiency of the plant in the different weather conditions (sun, rain, varying temperatures, different seasons, etc.) occurring in a year. This sampling strategy yielded wide concentration ranges of metals in the wastewater influent and effluent from the WWTP. In each campaign, wastewater samples were collected over 1 day (24 h composite samples). The mean hydraulic retention time in the Fusina WWTP was estimated based on the following calculations: total volume of the treatment plant ) 69 000 m3 (i.e., denitrification tanks (18 000 m3) + nitrification tanks (33 600 m3) + settlers (15 000 m3) + disinfection tank (2 400 m3)); and annual mean flow rate feeding the WWTP ) 96 650 m3/d. The mean hydraulic retention time was (69 000 × 24)/96 650 ) 17.1 h ≈ 17 h. Therefore, effluent samples started to be taken 17 h after the inlet samples. Water samples were collected from three points in the treatment plant (see Figure 1): Site A, raw wastewater from the sewerage of urban
districts of Venice; Site B, mixed wastewater; and Site C, disinfected treated effluent. In Site B, different influent flows are mixed, i.e., raw wastewater (1), pretreated industrial wastewater (8), and pretreated, truck-transported waste (10). Brown glass vessels with Teflon caps were used to collect the aqueous samples. The vessels were precleaned with HNO3 and deionized water according to standard methods APHA-AWWA-WPCF.5 All the samples were transported to the laboratories within the same day of collection and kept refrigerated at 4 °C until extraction and analysis procedures began ( Fe > B > Zn > Ba > Mn > Cu > Pb > Hg ) Ni > Cr ) As > V > Ag > Cd which is consistent with what was recently reported.7,8 As for the metal concentrations found in the influent to the biological reactor (Site B), aluminum was disregarded because it was added as a chemical coagulant to the denitrification tank. Thus, the order was
Fe > B > Zn > Ba > Cu > Mn > Ni > Pb > Ag > Cr > As > Hg > V > Cd which differs slightly from what was found in the sewage because of the contribution of industrial waste. The metal concentrations in the effluent (Site C) were in the following order: Figure 4. Variation in copper contents in influent and effluent wastewater.
sludge and septic tank wastes carried to the plant by tank-trucks (see Figure 1 and Figures 2-5). Figures 2-5 show the variations in the metal contents in the influent and effluent streams for a few selected compounds (e.g., Pb, Cr, Cu, and Ba, respectively). As previously reported,7 the magnitude of these variations is generally greater in the influent than in the effluent stream because the treatment processes applied to wastes are commonly able to remove a great portion of metals from influent wastewaters. At Fusina WWTP, which receives both municipal and industrial influents, the magnitude of variations is actually greater in the
Fe > Al > Zn > Mn > Ba > Ni > Cu > Pb > Cr > Ag > As > Hg ) V > Cd This result is consistent with those previously reported.7,8 The variations in the concentrations of metals that were observed in the raw wastewater samples can be considered characteristic of a sewerage receiving household effluents; drainage water and wastewater discharges from small-business activities (e.g., car washes, laundries, etc.), which are characterized by low metal concentrations;9 and unauthorized discharges containing high concentrations of one or more metals. Both the unauthorized discharges and the resuspension of pipe sediments deposited in the sewerage9 may be
Ind. Eng. Chem. Res., Vol. 44, No. 24, 2005 9267 Table 1. Metal and Heavy Metal Concentration Recorded at the Wastewater Treatment plant of Fusinaa raw wastewater influent (Site A) cmpd N units Al Ag As B Ba Cd Cr Fe Mn Hg Ni Pb Cu V Zn
59 56 57 58 57 22 57 59 57 40 52 59 58 40 55
min
mean
max
STD
µg/L 490 2570 29800 4060 µg/L 1.0 1.8 13.7 2.3 µg/L 0.3 5.8 31 4.6 µg/L 80 430 1140 226 µg/L 31 101 354 64 µg/L 0.2 0.6 1.8 0.4 µg/L 0.5 5.8 18.4 3.5 µg/L 210 1420 7400 1180 µg/L 16 56 168 24 µg/L 0.2 8.0 147 21.9 µg/L 1 8 39 6 µg/L 1.0 13.3 64 11.1 µg/L 5 52 430 58 µg/L 0.7 2.4 10.5 1.9 µg/L 61 206 833 147
mixed municipal and industrial wastewater influent (Site B)
max w/in 95% CI 5960 3.5 11.2 760 250 1.1 10.2 3060 92 39.9 16 30 105 4.7 474
min
mean
max
STD
830 4320 15450 2840 1.0 26 140 26 1.5 8.4 36 6.1 100 750 1900 440 68 160 800 120 0.2 1.0 3.1 0.6 3.0 17.8 89 15.2 1300 4650 21760 3000 30 91 220 29 0.1 5.3 45.4 10.6 7 46 537 72 7.9 31.1 131 22.3 29 108 608 93 1.2 4.4 13 1.9 116 356 1900 246
treated effluent (Site C)
max w/in 95% CI 8700 72 15.5 1500 370 1.4 46 8160 128 39.2 90 63 257 6.4 636
min
mean
max
STD
max w/in 95% CI
34 422 1940 390 0.6 3.3 12.2 2.8 0.5 3.0 9.2 1.6
450 6.5 5.1
10 30 82 15 0.1 0.3 1.6 0.3 0.4 3.7 8.2 2.0 120 940 3780 760 10 38 78 15 0.1 1.3 9.5 1.6 1 21 65 14 1.0 3.8 11 2.5 3 13 90 13 0.5 1.3 2.2 0.4 24 134 238 51
270 1.0 6.3 980 46 3.1 24 6.8 20 2.2 148
a N ) number of analytical data points used for calculations; min ) minimum metal concentration; mean ) mean metal concentration; STD ) standard deviation; max ) maximum metal concentration; max w/in 95% CI ) maximum metal concentration observed within the calculated 95% confidence interval.
responsible for the high concentrations of one or more metals recorded in the raw wastewater (i.e., the absolute maximum concentration of Hg was 147 µg/L in raw wastewater, while its average annual concentration was 8 µg/L). The contributions of drainage waters and, more generally, of nonhousehold effluents to the total flow of raw wastewaters into the Fusina WWTP has been estimated as ∼50%, based on the mean BOD (biological oxygen demand) ) 120 mg/L value (compared with the typical 220-240 mg/L in household wastewaters).10 A comparison of the ranges of metal contents in the raw wastewaters at Fusina with those in some other European plants is shown in Table 2: no relevant differences stand out, with the possible exceptions of As and Hg, for which the concentrations in the influent (Site A) were higher at Fusina WWTP than elsewhere, and of Cr, for which the concentration was lower. As far as the metals contents in the effluent were concerned, Cr was lower at Fusina than elsewhere, while the other metals were within the ranges reported. In conclusion, the concentration ranges of metals reported in Table 2 are representative of the magnitude of the metal contents in raw wastewaters feeding a number of European treatment plants, and the metal contents in raw wastewaters feeding Fusina WWTP are within these ranges. 3.2. Removal Efficiencies of Metals in the Fusina WWTP. The ability of Fusina WWTP to remove the metals contents from influent wastewaters is a major concern because it partially affects the water quality in the lagoon of Venice, a highly urbanized coastal zone which is subject to the heavy anthropogenic pressure of nutrients and pollutants. Hence, if metal removal was to remain unpredictable due to numerous factors (i.e., metal ion species and concentration, operating parameters, and physical, chemical, and biological factors), as has been pointed out by many researchers,11-16 there would be even greater cause for concern at Fusina WWTP because of the contribution of industrial wastewaters and industrial/septic truck-tank transported wastes, which increase the concentrations of metals in the influent to the biological treatment (see Figure 2 and Table 1). The pH in the oxidation basin was significantly constant (pH ) 7.5 ( 0.2) throughout the experimentation period, without requiring any chemical addition, thus avoiding the potential impact of these variations on removal efficiencies. The large number of
Table 2. Metal Contents in Raw Wastewater Influents and Treated Effluents from Some European Treatment Plants and from Fusina WWTP cmpd
country
raw wastewater influent (µg/L)
As
Spain Italy Poland Austria France Germany Greece Italy Italy Greece Austria Italy Italy Austria France Germany Greece Italy Italy Greece Austria Italy Italy Poland Austria France Germany Greece Italy Spain Italy Poland Austria Greece Italy Italy Poland Austria Greece Italy Italy
2.2 0.3-31 5-70
