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Performance of a membrane bioreactor used for the treatment of wastewater contaminated with heavy metals
Bioresource Technology 102 (2011) 4325–4332
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Performance of a membrane bioreactor used for the treatment of wastewater contaminated with heavy metals Evina Katsou, Simos Malamis ⇑, Maria Loizidou Unit of Environmental Science and Technology, School of Chemical Engineering, National Technical University of Athens, 9 Iroon Polytechniou St., Zographou Campus, PC 157 73, Athens, Greece
a r t i c l e
i n f o
Article history: Received 19 June 2010 Received in revised form 24 October 2010 Accepted 25 October 2010 Available online 31 October 2010 Keywords: Membrane bioreactor Fouling Heavy metals Toxicity Vermiculite
a b s t r a c t In this work the performance of a Membrane bioreactor (MBR) was assessed for the removal of 3–15 mg/l of copper, lead, nickel and zinc from wastewater. The average removal efficiencies accomplished by the MBR system were 80% for Cu(II), 98% for Pb(II), 50% for Ni(II) and 77% for Zn(II). The addition of 5 g/l vermiculite into the biological reactor enhanced metal removal to 88% for copper, 85% for zinc and 60% for nickel due to adsorption of metal ions on the mineral, while it reduced biomass inhibition and increased biomass growth. The metal ions remaining in soluble form penetrated into the permeate, while those attached to sludge flocs were effectively retained by the ultrafiltration membranes. The average heterotrophic biomass inhibition was 50%, while it reduced to 29% when lower metal concentrations were fed into the reactor in the presence of vermiculite. The respective autotrophic biomass inhibition was 70% and 36%. The presence of heavy metals and vermiculite in the mixed liquor adversely impacted on membrane fouling. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Membrane bioreactor (MBR) is successfully employed for the secondary treatment of municipal wastewater. MBR offers significant advantages compared to conventional activated sludge systems, including superior effluent quality, compactness and the ability to operate at high solids retention times (SRT) without operational problems (Liu et al., 2004; Malamis and Andreadakis, 2009; Tazi-Pain et al., 2002). MBR has also been employed for the treatment of specific industrial wastewater streams such as textile and electroplating (Blöcher et al., 2004; Lubello et al., 2007; Petrinic´ et al., 2009). It has been documented that heavy metal removal in activated sludge systems depends on the Mixed Liquor Suspended Solids (MLSS) concentration, the solids retention time and the pH. These factors control the distribution of metals between the liquid and the solid phases (Nelson et al., 1981). Malamis et al. (2010) and Katsou et al. (2010b) found that Ultrafiltration (UF) membranes combined with sludge resulted in a notable Cu(II) and Ni(II) removal from industrial wastewater, achieving removal efficiencies of 59–78% and 23–50% respectively, depending on the MLSS concentration. Increased levels of MLSS enhanced the metal removal process since more biosorption sites were available. The UF membranes were able to retain suspended solids. ⇑ Corresponding author. Tel: +30 210 772 3108; fax: +30 210 772 3285. E-mail addresses: [email protected] (E. Katsou), [email protected] (S. Malamis), [email protected] (M. Loizidou). 0960-8524/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2010.10.118
Consequently, the metal ions attached to sludge flocs were effectively retained by the UF membranes. Several wastewater treatment plants often treat a mixture of industrial and municipal wastewater. The resulting effluent streams may contain metals in concentrations which can be above the limits specified for wastewater reuse applications. Since MBR systems have been introduced for the biological treatment of wastewater, it is important to evaluate their performance with respect to metal removal. Battistoni et al. (2007) found that an MBR system employing UF membranes for the treatment of wastewater increased metal removal efficiency by 40–50% compared to the conventional activated sludge system. Fatone et al. (2008) examined the removal of heavy metals in an MBR and found that it could effectively remove copper and chromium, while nickel was removed to a lower extent. Dialynas and Diamadopoulos (2009) found that Pb(II) and Ni(II) were completely removed by an MBR system, indicating that these two metals were in particulate form, while Cr(III) and Cu(II) were removed by 89% and 49% respectively. However, the influent heavy metal concentrations in the aforementioned works were very low (<1 mg/l) since municipal wastewater was treated. The treatment of aquaculture wastewater containing very low heavy metal concentrations by an MBR was efficient since metal concentrations in the treated effluent satisfied the recommended limits for fish culture (Sharrer et al., 2010). Bolzonella et al. (2010) reported that the MBR system improved the removal efficiency for heavy metals by 10–15% compared to the conventional activated sludge system. Santos and Judd (2010)
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compared the metal removal accomplished by MBR and activated sludge systems and concluded that MBR consistently, but not dramatically achieved higher metal removal efficiencies (64–92% as opposed to 51–87%). However, their analysis was focused on low influent metal concentrations. Various mechanisms are involved in metal removal by activated sludge. These include physical trapping of precipitated metals into sludge flocs, binding, adsorption and complexation of soluble metal ions with extracellular polymeric substances (EPS), surface adsorption and diffusion of metal ions in sludge flocs (Arican et al., 2002; Kelly et al., 2004). In addition, the alkaline nature of wastewater further enhances metal removal through precipitation. Copper, lead, nickel and zinc are among the most common polluting metal ions in industrial wastewater and are also associated with toxicity problems. Lead, nickel and their compounds are classified as priority substances, while zinc, copper and their compounds are classified as potential main pollutants (Commission, European, 2000). Moreover, limit values for heavy metals have been recommended for wastewater reuse for irrigation purposes (USEPA 2004), while heavy metal limit values have also been set for the discharge of industrial wastewater into municipal sewers (USEPA, 2005). Therefore, the evaluation of heavy metal removal from industrial wastewater through the application of MBR technology is of particular concern. It is also important to consider the MBR performance when it is subjected to shock loads due to the inflow of wastewater characterized by significant heavy metal concentrations. Although some studies of heavy metal removal by MBR systems have been conducted, these focus on municipal wastewater containing low metal concentrations, while there is limited knowledge on MBR performance for the removal of heavy metals from industrial wastewater, where metal concentrations are usually high. This work investigated the potential removal of Ni(II), Cu(II), Pb(II) and Zn(II) from wastewater containing significant concentrations of these metals. In particular, the aim was to test the system performance when it is subjected to shock loads due to the inflow of wastewater containing significant heavy metal concentrations. The biomass activity in the biological reactor was investigated to determine the level of inhibition occurring due to heavy metal accumulation. Finally, membrane fouling was assessed. 2. Methods Primary treated municipal wastewater was enriched with Cu(II), Pb(II), Ni(II) and Zn(II) in multi-metal solutions and was fed to a pilot-scale MBR system. Sulphate salts of the above metals were supplied by Merck [CuSO4, NiSO46H20, ZnSO4H20] and Sigma–Aldrich [PbSO4]. Heavy metal enrichment of influent wastewater was conducted in a 2 m3 equalization tank where the influent wastewater was temporarily stored. The wastewater in the equalization tank was kept under continuous agitation. The wastewater enriched with heavy metals was fed to the biological reactor only for a time period of 6 h in order to avoid excessive heavy metal accumulation inside the reactor, resulting from the long-term system operation. However, to achieve the required metal concentration inside the reactor at the initiation of the 6-h experimentation period, it was usually necessary to add metals directly inside the reactor, depending on their concentrations in the mixed liquor. Hence dilution of heavy metals inside the reactor was avoided. In order to assess the MBR performance for the treatment of various influent metal concentrations, a range of 3–15 mg/l was examined. The sampling procedure for the measurement of all the parameters took place at the end of the 6-h period of metal enrichment. In the remaining time period the MBR was fed with municipal wastewater. The pH in the biological reactor was maintained above 7.0 in all the experimental runs using NaOH.
The membrane module was supplied by GE Water and Process Technologies (ZW-10). The module was that of hollow fibres and was directly immersed in the biological reactor. The nominal pore size of the membranes was 0.04 lm, while the membrane material was Polyvinylidene Fluoride (PVDF). The membrane surface area was 0.93 m2. The system was allowed to develop its own biomass and subsequently three experimental runs were conducted in series. In the 1st run the system operated for a period of 34 days without any metal addition and under steady-state conditions. This run represented the control conditions. In the 2nd run (days 35–77) heavy metal enrichment of the feed wastewater took place, while in the 3rd run (days 78–170) heavy metal enrichment was combined with the addition of 5 g/l vermiculite. Chemical cleaning of the membrane module took place periodically by immersing the module in NaOCl solution (1000 mg/l Cl2) for 8 hours and afterwards in 4000 mg/l of citric acid solution for 4 hours. The operating conditions of the 2nd and 3rd runs were maintained the same as those of the 1st run. Vermiculite was the aluminosilicate mineral that was employed to reduce the permeate heavy metal concentration through the processes of ion exchange and adsorption. Vermiculite was supplied by Mathios Refractories S.A. and was used in its natural form without any chemical modification. It was supplied in a grain size of 2.0–5.0 mm which was accordingly ground and sieved to the size of 0.18 mm and was subsequently added into the biological reactor. A fixed mass of vermiculite was added every day into the reactor to replenish the amount lost through sludge wasting and thus maintain its concentration constant in the biological reactor. The chemical composition of vermiculite is given in previous work. Its cation exchange capacity was 120.65 meq/ 100 g (Malamis et al., 2010). The biological reactor had a working volume of 210 l and treated daily approximately 450 l of influent wastewater. The transmembrane pressure was continuously recorded using the digital recorder EASYLOG 40KY, with an accuracy of 0.25 kPa which was connected to a pressure transducer (S-10 WIKA). The temperature was also recorded using the data logger EASYLOG 40NSW with an accuracy of 0.1 °C. Laboratory analyses were conducted to determine the characteristics of influent wastewater, activated sludge and permeate. More specifically, COD, TN, NH4–N, NO3–N were determined using suitable Merck kits, while Cu(II), Pb(II), Ni(II) and Zn(II), were determined using the Atomic Absorption Spectrometer of Varian AA240FS. The method detection limits were 0.05 mg/l for Pb(II), 0.01 mg/l for Cu(II), 0.005 mg/l for Zn(II) and 0.02 mg/l for Ni(II). To determine the total metal concentration in activated sludge the samples were digested at 440 °C using H2O2 and HNO3 and then filtered through GF/C Whatman filters. The soluble metal concentration in the biological reactor was determined by directly filtering the sludge through Whatman membranes with a pore size of 0.45 lm. The percentage removal (R) of each metal was calculated using the equation:
ð%ÞR ¼
C in C out 100 C in
ð1Þ
where Cin is the influent concentration and Cout is the permeate concentration (mg/l) of the metal ions. Total Suspended Solids (TSS) and Volatile Suspended Solids (VSS) were determined using Standard Methods (APHA, 1998). The maximum Oxygen Uptake Rate (OUR) of biomass was determined using primary wastewater and sodium acetate as substrates. More specifically, 250 ml of fresh primary wastewater was added to 250 ml of biomass and the mixture was kept under continuous aeration at room temperature (25 °C ± 2), while the mixture pH was maintained at 7.3 ± 0.2. At the time intervals of 10 and 30 min after the mixing had taken place, samples were collected, placed under mild agitation in BOD flasks and the reduction of the Dissolved Oxygen (DO) with time was recorded. The
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maximum OUR was taken as the maximum DO reduction among the two cases of 10 and 30 min. The maximum OUR was also measured when excess sodium acetate was added as substrate. The initial concentration of sodium acetate in biomass was 250 mg/l and the DO reduction with time was recorded. The procedure followed was similar to the one where primary wastewater was employed as substrate. In all OUR experiments 10–12 mg/l allyl-thiourea were added to inhibit nitrification. In the 2nd and 3rd runs, the biomass contained significant heavy metal concentrations. To avoid dilution, the measured heavy metal concentrations in sludge, were also added into the fresh wastewater and the sodium acetate solution. In each experimental period 10 repetitions of the OUR experiments were conducted. The OUR was always normalized by dividing it with the VSS of the mixture. The heterotrophic biomass inhibition was determined as the percentage difference between the measured maximum OUR of biomass prior and after the enrichment of feed wastewater with heavy metals:
ð%ÞHeterotrophic inhibition ¼
OUR1 OUR2 100 OUR1
ð2Þ
where OUR1 and OUR2 (mgO2/gVSS-h) are the maximum OURs of biomass prior (i.e. 1st run) and after (i.e. 2nd and 3rd runs) the addition of heavy metals respectively. Nitrification inhibition was determined by conducting Ammonium Uptake Rate (AUR) experiments. Biomass (500 ml) was collected from the bioreactor and was placed inside a suitable flask under continuous aeration. Ammonium chloride (NH4Cl) solution was added to the biomass so that the initial NH4-N concentration would be approximately 40 mg/l. The biomass was maintained at room temperature (25 ± 2 °C) and the DO concentration was maintained above 4 mg/l. At fixed time intervals 10–15 ml of biomass was collected, centrifuged at 3000g and filtered through Whatman membranes with pore size 0.45 lm. The filtrate was measured for its NO3-N concentration. The biomass pH was periodically recorded during the experiment and was adjusted using NaOH in order to be within the range of 7.2–7.6. To avoid dilution, in the 2nd and 3rd runs the measured heavy metal concentrations in sludge were also added into the NH4Cl solution that was added to biomass. The AUR was always normalized by dividing it with the VSS of the mixture. In total 10 repetitions were conducted for each cycle. The autotrophic biomass inhibition was determined as the percentage difference between the measured AUR of biomass prior and after the enrichment of feed wastewater with heavy metals:
AUR1 AUR2 ð%ÞAutotrophic inhibition ¼ 100 AUR1
ð3Þ
where AUR1 and AUR2 (mgNO3–N/gVSS-h) are the Ammonium Uptake Rates of biomass prior and after the addition of heavy metals respectively. Membrane fouling was assessed by calculating membrane permeability with time corrected at 20 °C using the following equation:
L20 ¼
J 20 TMP
ð4Þ
where L20 is the membrane permeability corrected at the reference temperature of 20 °C (l/m2-h-bar), J20 is the filtration flux corrected at the reference temperature of 20 °C (l/m2-h) and TMP is the transmembrane pressure (bar). The filtration flux was corrected at the reference temperature of 20 °C using the following equation (Fan et al., 2006):
J 20 ¼ J T 1:025ð20TÞ
ð5Þ
where JT is the filtration flux at the operating temperature of T (°C) SEM and EDX analysis were performed in used membrane fibres using the Quanta 200 of FEI in order to investigate element depositions on the membrane surface.
3. Results and discussion The MBR performance was assessed with respect to organics, ammonium nitrogen, copper, lead, nickel and zinc removal from wastewater. In addition, heterotrophic and autotrophic biomass activity and membrane fouling were evaluated. 3.1. Wastewater and system operating characteristics Table 1 summarizes the influent wastewater characteristics for the three experimental runs that were conducted. In the 1st run the feed wastewater was municipal, while in the 2nd and 3rd runs influent wastewater was enriched with Cu(II), Pb(II), Ni(II) and Zn(II), with vermiculite being added in the 3rd run. The 1st run served as a control run and was compared with the other two runs to assess the impact of heavy metal addition on biomass activity. The feed heavy metal concentrations ranged from 3.4–9.1 mg/l for copper, 3.9–14.7 mg/l for lead, 4.3–14.7 mg/l for nickel and 3.2–12.1 mg/l for zinc during the 2nd and 3rd experimental runs. Such concentrations are frequently met in industrial wastewater. The aforementioned metal concentrations were fed to the system for a limited amount of time (6 h) in order to limit metal accumulation inside the reactor. In industries that employ combined industrial and municipal wastewater treatment systems, it is essential to determine whether direct biological treatment is feasible and therefore whether the shock loads experienced occasionally due to the inflow of significant heavy metal concentrations can be tolerated by the MBR system. Table 2 summarizes the biological and filtration characteristics of the system. The filtration and backwash fluxes were kept constant at 22.3 and at 26.8 l/m2-h respectively, while the system operated at steady-state conditions with SRT = 15 d and Hydraulic Retention Time (HRT) = 10.3 h. 3.2. MBR performance and biomass activity 3.2.1. COD removal TSS removal was complete as the UF membranes formed a barrier to suspended solids. COD removal in the 1st run was very high Table 1 Influent wastewater characteristics. Parameter
1st Run (1–34 days)
2nd Run (35–77 days)
3rd Run (78–170 days)
TSS (mg/l) VSS (mg/l) COD (mg/l) NH4-N (mg/l) TN (mg/l) Cu(II) (mg/l) Pb(II) (mg/l) Ni(II) (mg/l) Zn(II) (mg/l)
244 (140–360) 193 (100–300) 535 (460–602) 53 (48–59) 72 (65–80) <0.1 <0.1 <0.1 <0.2
226 (155–310) 170 (115–230) 508 (438–617) 44 (34–57) 58 (48–73) 5.7 (3.4–6.7) 13.1 (11.8–14.7) 12.2 (9.7–14.7) 9.9 (7.8–11.4)
267 (150–415) 200 (95–310) 515 (395–719) 48 (29–67) 68 (53–76) 6.2 (4.2–9.1) 6.4 (3.9–8.9) 9.9 (4.3–14.1) 6.5 (3.2–12.1)
Table 2 MBR operating characteristics. Parameter
1st Run (1–34 days)
2nd Run (35–77 days)
3rd Run (78–170 days)
SRT (d) HRT (h) Mixed liquor pH MLSS (g/l) MLVSS (g/l) Filtration flux (l/m2-h) Backwash flux (l/m2-h) F/M (d-1)
15 10.3 7.51 (7.25–7.77) 5.84 (5.40–6.68) 4.82 (4.38–5.63) 22.3
15 10.3 7.38 (7.02–7.62) 5.77 (4.90–6.15) 4.20 (3.65–4.91) 22.3
15 10.3 7.47 (7.00–7.78) 10.33 (9.00–12.55) 5.24 (4.47–6.20) 22.3
26.8
26.8
26.8
0.24
0.26
0.25
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ranging from 95–97% and the permeate COD was very low (19– 24 mg/l). The addition of metals resulted in lower, but satisfactory COD removal which ranged from 83–91% with an average of 88% during the 2nd run and from 86–95% with an average of 92% in the 3rd run. In Fig. 1a, the influent and treated effluent COD concentrations are given, with the permeate COD ranging from 42– 78 mg/l (average of 59 mg/l) in the 2nd run and from 27–68 mg/l (average of 43 mg/l) in the 3rd run. Despite the partial inhibition of heterotrophic biomass the treated effluent COD remained at relatively low levels, but was higher than the effluent COD obtained when the system was not fed with heavy metals (i.e. control conditions). In the 3rd run the permeate COD was lower than that of the 2nd run (Fig. 1a) since the heterotrophic biomass inhibition was limited due to the lower influent metal concentrations. More importantly, the addition of vermiculite provided a suitable carrier material for the development of biomass at its surface, thus increasing the system’s ability to oxidize organic matter. Also, vermiculite adsorbed part of the heavy metals, thus reducing their toxic effects on biomass. During the last 20 days of system operation the permeate COD was very low and similar to that of the 1st run. This is attributed to the low nickel and zinc concentrations in influent wastewater, that resulted in lower inhibition of heterotrophic biomass and thus more effective COD removal.
(a)
Days of system operation (d)
MLSS / MLVSS (g/l)
MLSS st 12 1 Run
2nd Run
MLVSS
3rd Run
(b)
8 4 0 0
50 100 150 Days of system operation (d) Influent NH4-N
Effluent NH4-N
200
NH4-N / NO3-N (mg/l)
2nd Run
3rd Run
(c)
60 40 20 0
0
3.2.3. Nitrification Full (100%) nitrification was observed during the control conditions since metals were not added into the MBR system. The addition of metals inside the biological reactor during the 2nd and 3rd runs impacted on the nitrification process. In particular, nitrification was 22–54% during the 2nd run and 33–96% during the 3rd run. The higher nitrification observed in the 3rd run was partly attributed to the lower Zn(II), Ni(II) and Pb(II) inlet concentrations and partly to the addition of vermiculite. The mineral adsorbed part of Zn(II), Ni(II) and Cu(II) on its surface, reducing the adverse impact of metals on microorganisms and it also enhanced microbial growth at its surface. Fig. 1c shows the variation of influent and effluent NH4–N and effluent NO3–N concentrations, where it is seen that significant NH4–N concentrations were present in the effluent, in the 2nd and 3rd runs. Jn average the effluent NH4–N in the 2nd run was 26 mg/l (18–35 mg/l) with the 95% confidence interval falling within the range of 23–29 mg/l. In the 3rd run the effluent ammonium nitrogen concentration was on average 16 mg/l (2–30 mg/l) with the 95% confidence interval falling within the range of 12–20 mg/l. From the above analysis it is evident that there is a significant difference between the ammonium nitrogen effluent concentrations in the 2nd and 3rd runs. In the last 20 days of system operation, the ammonium nitrogen in the treated effluent was low (<4 mg/l) and nitrification was high. During these days the influent nickel and zinc concentrations were lower than the average values (<5.4 and <6.3 mg/l respectively) and this enhanced the nitrification process.
Effluent NO3-N
80 1st Run
3.2.2. MLSS and MLVSS variation Fig. 1b shows the variation of MLSS and MLVSS with time. Comparing MLVSS prior and after metal addition (i.e. 1st and 2nd runs), a 13% reduction was observed when metals were added due to partial biomass inhibition. Despite the accumulation of heavy metals inside the biological reactor, the biomass development was satisfactory with the MLVSS being stable in the range of 3.7–4.9 g/l. The addition of 5 g/l vermiculite during the 3rd run increased MLSS from an average of 5.8 g/l (2nd run) to an average of 10.3 g/l, while the average MLVSS increased by 20% in the 3rd run compared to the MLVSS of the 2nd run. Two reasons can account for this increase of MLVSS: vermiculite provided a support medium for the growth of microorganisms on its surface, thus increasing the MLVSS concentration. Thus, apart from the microorganisms under suspension, biofilm also developed onto the mineral surface. Consequently, during the 3rd run a combined suspended and attached growth process was observed. Furthermore, the lower influent metal concentration resulted in enhanced biomass development. Vermiculite is purely inorganic and did not contribute as a material to the increase of MLVSS. The MLSS concentration during the 3rd run was lower than the expected one, showing that some of the vermiculite had actually settled in the bioreactor.
50 100 150 Days of system operation (d)
200
Fig. 1. Variation of (a) influent and effluent COD concentration (b) MLSS and MLVSS in the biological reactor, (c) influent and effluent NH4–N and effluent NO3–N concentration with time.
3.2.4. Biomass activity Heterotrophic biomass activity was assessed by comparing the maximum OUR obtained for the control conditions (i.e. no metal addition) with the maximum OUR obtained for the experimental runs in which heavy metals were added into the system. Fig. 2a shows the maximum OUR measured for the different experimental conditions in which primary wastewater and sodium acetate were used as substrates. A notable reduction in heterotrophic biomass activity was observed when metals were added. In particular, during the 2nd run biomass inhibition was on average 50% and 45% for primary wastewater and sodium acetate respectively, while during the 3rd run it was 29% and 25% respectively (Fig. 2a). In the latter case, the reduction in the inhibition of heterotrophic biomass was partly attributed to the lower influent concentrations of heavy metals and partly to the addition of 5 g/l vermiculite. The mineral
E. Katsou et al. / Bioresource Technology 102 (2011) 4325–4332
OURmax (mg/gVSS-h)
adsorbed part of the metal ions found in soluble form, thus reducing the toxic effect of heavy metals on biomass and it also provided a suitable carrier for the development of biofilm on its surface. Fig. 2b shows the maximum OUR (measured using primary wastewater as substrate) as a function of the heavy metal concentration in the biological reactor. Lead was not included in the figure since the vast majority of the metal formed precipitates in the reactor and did not significantly impact on biomass activity. It is seen that the increase of metal concentration resulted in higher heterotrophic biomass inhibition. Lead had the highest accumulation since its concentration in influent wastewater was higher than that of the other metals, while low concentrations were detected in the effluent since the majority of lead ions formed precipitates at the operating pH
24
(a)
Primary wastewater
18
Sodium acetate
12 6 0
AUR (mg NO3-N/gVSS-h)
OURmax (mg/gVSS-h)
1st run 21
Cu(II)
3rd run
Zn(II)
(b)
Ni(II)
17 13 9 5 0.00
0.05
0.10 0.15 Metal concentration (mM)
0.20
4
(c) 3 2 1 0
1st run AUR (mgNO3-N/gVSS-h)
2nd run
2nd run
3rd run
4 Cu(II)
Zn(II)
(d)
Ni(II)
3 2 1 0 0.00
0.05
0.10
0.15
0.20
Metal concentration (mM) Fig. 2. (a) Maximum OUR measured using primary wastewater and sodium acetate as substrates (error bars represent standard deviation), (b) variation of maximum OUR with the heavy metal concentration in the biological reactor, (c) AUR measured for the three experimental runs (error bars represent standard deviation) and (d) variation of AUR with the heavy metal concentration in the biological reactor.
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range. Copper was characterized by the lowest concentrations in the reactor due to its low influent concentration. The inhibition of autotrophic biomass was determined through AUR experiments and was found to be on average 70% and 36% for the 2nd and 3rd runs respectively compared to the control conditions (Fig. 2c). The lower inhibition obtained in the 3rd run is partly attributed to the lower influent metal concentration and partly to the addition of vermiculite. Fig. 2d shows the AUR versus the heavy metal concentration in the biological reactor. A reduction in autotrophic biomass activity was observed with increasing metal concentration. As it is observed the average heterotrophic biomass inhibition was lower than the respective autotrophic biomass inhibition, indicating that nitrifying bacteria were more sensitive to toxic pollutants than heterotrophic microorganisms. These findings are in accordance with the results of Madoni et al. (1999) and of Principi et al. (2006) who found that nitrifiers were more sensitive than aerobic heterotrophic microorganisms when exposed to certain heavy metals. Biomass toxicity can be limited by maintaining the pH at an alkaline environment (pH = 8.0–8.5) that is also suitable for biomass development in order to enhance metal removal through precipitation. However, this was not conducted in the present study since the principal aim was to investigate metal removal in the pH operating range of 7.0–7.8. The role of vermiculite in the reduction of heavy metal toxicity was beneficial, but further investigation is required under more stable laboratory conditions. In this work the operating MLSS was not very high. However, most MBR systems operate at higher MLSS that may exceed 10 g/l. In such cases the risk of biomass inhibition is greater even for lower influent metal concentrations due to higher heavy metal accumulation. 3.2.5. Metal removal In Fig. 3a–d the influent and effluent concentrations of Cu(II), Pb(II), Ni(II) and Zn(II) are given respectively for the two runs in which metals were added into the reactor, while in Table 3 the system removal efficiencies obtained for each metal are summarized. The copper and lead concentrations in the treated effluent were always lower than 1.5 and 1.2 mg/l respectively. Zinc concentration in the effluent was higher and ranged from 0.3–3.9 mg/l, while nickel concentration was even higher ranging from 2.7–7.5 mg/l. Comparing the heavy metal concentrations in the treated effluent with the USEPA (2004) recommended limits for irrigation of reclaimed water it is deduced that for lead the long-term reuse limit was always satisfied. For copper and zinc the short-term reuse limit was always met, while the long-term limit was occasionally satisfied for copper and frequently met for zinc (particularly in the 3rd run). The nickel concentration in the final effluent could not meet any of the reuse limits. Thus, particular attention is required when the influent wastewater is characterized by high nickel concentrations. Metal removal achieved by the MBR system followed the sequence Pb(II) > Cu(II) > Zn(II) > Ni(II). The heavy metals were removed through various processes which included: (i) precipitation of metals inside the biological reactor, (ii) biosorption of metal ions on sludge flocs, (iii) retention of the insoluble metal species by the UF membranes and (iv) adsorption of metal ions on vermiculite. The latter process occurred during the 3rd run when 5 g/l vermiculite were added into the reactor. The pH inside the biological reactor ranged from 7.0–7.8 and thus significant precipitation occurred. Previous work has shown that precipitation is observed at pH = 7.0–8.0 in multi-metal solutions, particularly for lead and copper and to a lower extent for zinc and nickel (Malamis et al., in press). It is also important to notice that the MBR system can also remove the insoluble metal species that remain in suspension as opposed to the conventional activated sludge system. The metal removal process was enhanced through the biosorption of metal
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Influent
Effluent
10
Cu(II) (mg/l)
2nd
3rd
Run
(a)
Run
8 5 3 0 50
70
90
110
130
150
170
Days of system operation (d) Influent
2nd Run
Effluent
3rd Run
(b)
15 10
0 50
70
90
110
130
150
170
Days of system operation (d) Influent
Effluent
Ni(II) (mg/l)
16
(c) 12
2nd run
10
3rd run
(a)
8 6
R² = 0.827
4
R² = 0.870
2 0 0
8
2
4
6
8
10
12
14
MLSS (g/l) 4
2nd Run
3rd Run
0 50
70
90
110
130
150
170
Days of system operation (d) Influent
Zn(II) (mg/l)
Cu(II) removed (mg/l)
5
Effluent
(d)
12 8 2nd Run
Pb(II) removed (mg/l)
Pb(II) (mg/l)
20
added since this mineral is very effective for nickel and zinc adsorption (Katsou et al., 2010a,b). As it is seen in Table 3 the addition of 5 g/l vermiculite enhanced the removal of nickel, zinc and copper through adsorption. In particular, copper removal increased from 80% to 88%, zinc removal from 77% to 85% and nickel removal from 50% to 60%. Vermiculite did not impact on lead removal since this metal was mainly removed through precipitation and thus there was a low amount of lead available for adsorption on vermiculite. Vermiculite addition significantly increased the MLSS concentration. This drawback was counterbalanced by the advantages of lower biomass inhibition, greater microorganism growth (both attached and suspended) and higher metal removal. During the last 20 days of the 3rd run, Ni(II) removal was very low (28–37%) and Zn(II) removal was quite low (74–79%) compared to the average removal efficiencies obtained during the 3rd run. As it has been previously explained, nitrification was very high due to
2nd run
16
R² = 0.568
(b)
12 8 4
R² = 0.308
0 0
3rd Run
2
4
90
110
130
150
170
Days of system operation (d) Fig. 3. Variation of influent and effluent (a) Cu(II), (b) Pb(II), (c) Ni(II) and (d) Zn(II) concentrations.
Table 3 Metal removal efficiencies achieved by the MBR system.
Ni(II) removed (mg/l)
70
8
10
12
14
2nd run
(c)
8
R² = 0.643
6
R² = 0.881
4 2 0 0
2
4
Percentage removal (%) 2nd Run (35–77 days)
3rd Run (78–170 days)
80 98 50 77
88 98 60 85
(71–86) (90–>99.9) (31–58) (66–85)
3rd run
10
6
8
10
12
14
MLSS (g/l)
(77–97) (95–>99.9) (45–68)* (74–95)
*
The stated range does not include the very low nickel removal obtained during the last 20 days of operation. An explanation for these very low Ni(II) removal efficiencies is given in the text.
ions on the sludge flocs. Nickel exhibited the worst performance showing that significant part of the metal ions remained in soluble form and thus penetrated through the UF membranes. To increase MBR performance with respect to metal removal, vermiculite was
2nd run Zn(II) removed (mg/l)
50
Cu(II) Pb(II) Ni(II) Zn(II)
6
MLSS (g/l)
4 0
Metal
3rd run
3rd run
12
(d)
9 6
R² = 0.813
R² = 0.890
3 0 0
2
4
6
8
10
12
14
MLSS (g/l) Fig. 4. Impact of MLSS concentration on (a) Cu(II), (b) Pb(II), (c) Ni(II) and (d) Zn(II) removal.
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the lower Ni(II) and Zn(II) feed concentrations. The increase in nitrification resulted in a subsequent pH reduction in the biological reactor (from 7.6 to 7.0–7.2). It seems that the solubility of the metals increased and hence higher amount of nickel and zinc ions could penetrate through the UF membranes. The pH range of 7.0–7.5 affected Zn(II) and Ni(II) removal (Malamis et al., in press). On the contrary, for Cu(II) and Pb(II) the removal remained very high as this pH reduction did not alter significantly the metal solubility. The impact of MLSS on metal removal was also investigated. The correlation between MLSS and metal removal is shown in Fig. 4a–d. This correlation is considered separately for the 2nd and 3rd run since biomass had different characteristics, due to the addition of vermiculite in the 3rd run. It is observed that there was a significant correlation between the MLSS concentration and metal removal. The increase in MLSS resulted in more sites being available for the sorption process and consequently more metal ions were removed. The obtained correlation was low for Pb(II) since the majority of metal ions were removed through precipitation rather than biosorption. It must be mentioned that pH variation was not significant (with the exception of the last 20 days of the 3rd run) and this enabled the determination of a correlation between MLSS and metal removal. The excess sludge produced contained significant heavy metal concentrations. To assess sludge toxicity it is important to consider heavy metal accumulation in the bioreactor in the long run. In particular, the system operation for several days could result in the production of non-hazardous sludge since the overall heavy metal accumulation could be low, as significant influent metal concentrations are usually observed only for a specific time period. If the sludge is hazardous then it must be subjected to adequate treatment prior to its final disposal. The amount of produced sludge could be reduced by applying dissolved air flotation to separate the activated sludge from vermiculite. Vermiculite, having a specific gravity of 2–2.6, is heavier than sludge flocs and can settle easily. The obtained vermiculite could be regenerated and reused to reduce the operational cost of the system.
1st Run
3.3. Fouling investigation The impact of heavy metal and vermiculite addition on membrane fouling was assessed. In Fig. 5a the reduction in membrane permeability corrected at the reference temperature of 20 °C is given versus operation time. From this diagram it is possible to determine the rate of permeability reduction (dL/dt 20 °C) which was 1.44–1.47 l/m2-h-bar-d for the 1st run, 1.68–1.76 l/m2-h-bard for the 2nd run and 1.88–2.01 l/m2-h-bar-d for the 3rd run. It is evident that feed wastewater with significant metal concentrations adversely impacted on membrane fouling, as dL/dt increased by approximately 18% compared to the control conditions. The addition of vermiculite into the biological reactor further increased fouling by approximately 33% compared to the control conditions. SEM-EDX analysis revealed significant deposits of vermiculite on the membrane surface (Fig. 5b,c). The EDX analysis detected significant amounts of Si on the membrane surface and to a lower extent Al and Fe. Si is a major element of vermiculite, which also contains significant amounts of Al and Fe. Furthermore, EDX analysis showed the presence of heavy metal deposits (mainly lead). It seems that coarse bubble aeration could not effectively remove such deposits from the membrane surface. The increase of membrane fouling due to vermiculite addition in activated sludge was not expected since previous work in batch systems (Katsou et al., 2010a) has shown that vermiculite did not adversely impact on membrane fouling. It seems that in the continuous operation of an MBR system the suspended solids of the mineral could attach to the membrane fibres. However, this type of inorganic fouling is reversible and thus suitable measures can be taken for its reduction. It can be limited by increasing the intensity of coarse bubble aeration and/or increasing the duration or the flux of backwash. These two techniques are usually very effective for the removal of suspended solids. Another solution would be to add granular vermiculite instead of powder since the results have shown that granular vermiculite performed better than powder in terms of membrane fouling, but it was less effective in terms of metal removal (Katsou et al., 2010a).
2nd Run
3rd Run
L20oC (l/m2-h-bar)
120 dL/dt= -1.88
dL/dt = -1.47
100
dL/dt = -2.01
(a)
80 60 dL/dt = -1.44 dL/dt = -1.76 40 dL/dt = -1.68
20
dL/dt = -1.90
0 0
(b)
20
40
60 80 100 120 140 Days of system operation (d)
160
180
(c)
Fig. 5. (a) Variation of membrane permeability with time (b) EDX analysis and (c) SEM micrograph of used membrane fibre.
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3.4. Assessment of the role of vermiculite The addition of vermiculite improved the removal efficiencies for copper, zinc and nickel by approximately 8–10% for each metal. Thus, significant increase of metal removal was obtained for the three metals. It also reduced to some extent biomass inhibition and enhanced biomass growth, as it provided suitable surface for biofilm development. The negative impacts of vermiculite were the increase in MLSS and membrane fouling. These drawbacks can be counterbalanced by the advantages of lower biomass inhibition, greater microorganism growth and higher metal removal. 4. Conclusions The MBR system was able to achieve heavy metal removal efficiencies of 80% for Cu(II), 98% for Pb(II), 50% for Ni(II) and 77% for Zn(II). The addition of 5 g/l vermiculite in wastewater containing lower influent metal concentrations enhanced metal removal efficiencies for copper (88%), zinc (85%) and nickel (60%) due to adsorption of metal ions on the mineral, reduced biomass toxicity and enhanced the growth of microorganisms. Inhibition of heterotrophic biomass was 50%, and reduced to 29% when lower feed metal concentrations and vermiculite were added into the system. The respective inhibition of autotrophic biomass was on average 70% and 36%. A significant correlation was found between MLSS concentration and heavy metal removal. Fouling investigation revealed deposits of metals and vermiculite on the membrane surface that adversely impacted on the rate of membrane permeability reduction. References APHA, 1998. Standard methods for the Examination of Water and Wastewater, 20th ed. American Public Health Association, American Water Works Association, Water Environment Federation, Washington, DC, USA. Arican, B., Gokcay, C.F., Yetis, U., 2002. Mechanistics models of nickel sorption by activated sludge. Process Biochem. 37, 1307–1315. Battistoni, P., Cola, E., Fatone, F., Bolzonella, D., Eusebi, A.L., 2007. Micropollutants removal and operating strategies in ultrafiltration membrane systems for municipal wastewater treatment: Preliminary results. Ind. Eng. Chem. Res. 46, 6716–6723. Blöcher, C., Bunse, C., Seßler, B., Chmiel, H., Janke, H.D., 2004. Continuous regeneration of degreasing solutions from electroplating operations using a membrane bioreactor. Desalination 162, 315–326. Bolzonella, D., Fatone, F., Di Fabio, S., Cecchi, F., 2010. Application of membrane bioreactor technology for wastewater treatment and reuse in the Mediterranean region: focusing on removal efficiency of non-conventional pollutants. J. Environ. Manage. 91, 2424–2431.
Commission, European, 2000. Directive 2000/60/EC of the European Parliament and of the Council establishing a framework for Community action in the field of water policy. Off. J. Eur. Communities L 327, 1–73. Dialynas, E., Diamadopoulos, E., 2009. Integration of a membrane bioreactor coupled with reverse osmosis for advanced treatment of municipal wastewater. Desalination 238, 302–311. Fan, F., Zhou, H., Husain, H., 2006. Identification of wastewater sludge characteristics to predict critical flux for membrane bioreactor processes. Water Res. 40, 205–212. Fatone, F., Eusebi, A.L., Pavan, P., Battistoni, P., 2008. Exploring the potential of membrane bioreactors to enhance metals removal from wastewater: Pilot experiences. Water Sci. Technol. 57, 505–511. Katsou, E., Malamis, S., Haralambous, K.J., 2010a. Examination of zinc uptake in a combined system using sludge, minerals and ultrafiltration membranes. J. Hazard. Mater. 182, 27–38. Katsou, E., Malamis, S., Haralambous, K.J., Loizidou, M., 2010b. Use of ultrafiltration membranes and aluminosilicate minerals for nickel removal from industrial wastewater. J. Membr. Sci. 360, 234–249. Kelly, C.J., Tumsoroj, N., Lajoie, C.A., 2004. Assessing wastewater metal toxicity with bacterial bioluminescence in a bench-scale wastewater treatment system. Water Res. 38, 423–431. Liu, R., Chen, L., Wen, X., Qian, Y., 2004. Operational performance of a submerged membrane bioreactor for reclamation of bath wastewater. Process Biochem. 40, 125–130. Lubello, C., Caffaz, S., Mangini, L., Santianni, D., Caretti, C., 2007. MBR pilot plant for textile wastewater treatment and reuse. Water Sci. Technol. 55, 115–124. Madoni, P., Davoli, D., Guglielmi, L., 1999. Response of SOUR and AUR to heavy metal contamination in activated sludge. Water Res. 33, 2459–2464. Malamis, S., Andreadakis, A., 2009. Fractionation of proteins and carbohydrates of extracellular polymeric substances in a membrane bioreactor system. Bioresour. Technol. 100, 3350–3357. Malamis, S., Katsou, E., Stylianou, M., Haralambous, K.J., Loizidou, M., 2010. Copper removal from sludge permeate with ultrafiltration membranes using zeolite, bentonite and vermiculite as adsorbents. Water Sci. Technol. 61, 581–589. Malamis, S., Katsou, E., Haralambous, K.J. Study of Ni(II), Cu(II), Pb(II), and Zn(II) Removal Using Sludge and Minerals Followed by MF/UF. Water Air Soil Pollut. 1–12, in Press. Nelson, P.O., Chung, A.K., Hudson, M.C., 1981. Factors affecting the fate of heavy metals in the activated sludge process. J. Water Pollut. Control Fed. 53, 1323– 1333. ˇ urlin, M., Racyte, J., Simonicˇ, M., 2009. Textile wastewater treatment Petrinic´, I., C with membrane bioreactor and water re-use. Tekstil. 58, 11–19. Principi, P., Villa, F.M., Bernasconi, E., Zanardini, E., 2006. Metal toxicity in municipal wastewater activated sludge investigated by multivariate analysis and in situ hybridization. Water Res. 40, 99–106. Santos, A., Judd, S., 2010. The fate of metals in wastewater treated by the activated sludge process and membrane bioreactors: a brief review. J. Environ. Monitor. 12, 110–118. Sharrer, M.J., Rishel, K., Summerfelt, S.T., 2010. Evaluation of a membrane biological reactor for reclaiming water, alkalinity, salts, phosphorus and protein contained in a high-strength aquacultural wastewater. Bioresour. Technol. 101, 4322– 4330. Tazi-Pain, A., Schrotter, J.C., Bord, G., Payreaudeau, M., Buisson, H., 2002. Recent improvement of the BIOSEPÒ process for industrial and municipal wastewater treatment. Desalination 146, 439–443. USEPA, 2004. Guidelines for Water Reuse. EPA/625/R-04/108, U.S Agency for Inter. Development, Washington, DC, USA. USEPA, 2005. Streamlining the General Pretreatment Regulations for Existing and New Sources of Pollution 40 CFR Parts 9, 122 and 403.
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Performance of a membrane bioreactor used for the treatment of wastewater contaminated with heavy metals Evina Katsou, Simos Malamis ⇑, Maria Loizidou Unit of Environmental Science and Technology, School of Chemical Engineering, National Technical University of Athens, 9 Iroon Polytechniou St., Zographou Campus, PC 157 73, Athens, Greece
a r t i c l e
i n f o
Article history: Received 19 June 2010 Received in revised form 24 October 2010 Accepted 25 October 2010 Available online 31 October 2010 Keywords: Membrane bioreactor Fouling Heavy metals Toxicity Vermiculite
a b s t r a c t In this work the performance of a Membrane bioreactor (MBR) was assessed for the removal of 3–15 mg/l of copper, lead, nickel and zinc from wastewater. The average removal efficiencies accomplished by the MBR system were 80% for Cu(II), 98% for Pb(II), 50% for Ni(II) and 77% for Zn(II). The addition of 5 g/l vermiculite into the biological reactor enhanced metal removal to 88% for copper, 85% for zinc and 60% for nickel due to adsorption of metal ions on the mineral, while it reduced biomass inhibition and increased biomass growth. The metal ions remaining in soluble form penetrated into the permeate, while those attached to sludge flocs were effectively retained by the ultrafiltration membranes. The average heterotrophic biomass inhibition was 50%, while it reduced to 29% when lower metal concentrations were fed into the reactor in the presence of vermiculite. The respective autotrophic biomass inhibition was 70% and 36%. The presence of heavy metals and vermiculite in the mixed liquor adversely impacted on membrane fouling. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Membrane bioreactor (MBR) is successfully employed for the secondary treatment of municipal wastewater. MBR offers significant advantages compared to conventional activated sludge systems, including superior effluent quality, compactness and the ability to operate at high solids retention times (SRT) without operational problems (Liu et al., 2004; Malamis and Andreadakis, 2009; Tazi-Pain et al., 2002). MBR has also been employed for the treatment of specific industrial wastewater streams such as textile and electroplating (Blöcher et al., 2004; Lubello et al., 2007; Petrinic´ et al., 2009). It has been documented that heavy metal removal in activated sludge systems depends on the Mixed Liquor Suspended Solids (MLSS) concentration, the solids retention time and the pH. These factors control the distribution of metals between the liquid and the solid phases (Nelson et al., 1981). Malamis et al. (2010) and Katsou et al. (2010b) found that Ultrafiltration (UF) membranes combined with sludge resulted in a notable Cu(II) and Ni(II) removal from industrial wastewater, achieving removal efficiencies of 59–78% and 23–50% respectively, depending on the MLSS concentration. Increased levels of MLSS enhanced the metal removal process since more biosorption sites were available. The UF membranes were able to retain suspended solids. ⇑ Corresponding author. Tel: +30 210 772 3108; fax: +30 210 772 3285. E-mail addresses: [email protected] (E. Katsou), [email protected] (S. Malamis), [email protected] (M. Loizidou). 0960-8524/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2010.10.118
Consequently, the metal ions attached to sludge flocs were effectively retained by the UF membranes. Several wastewater treatment plants often treat a mixture of industrial and municipal wastewater. The resulting effluent streams may contain metals in concentrations which can be above the limits specified for wastewater reuse applications. Since MBR systems have been introduced for the biological treatment of wastewater, it is important to evaluate their performance with respect to metal removal. Battistoni et al. (2007) found that an MBR system employing UF membranes for the treatment of wastewater increased metal removal efficiency by 40–50% compared to the conventional activated sludge system. Fatone et al. (2008) examined the removal of heavy metals in an MBR and found that it could effectively remove copper and chromium, while nickel was removed to a lower extent. Dialynas and Diamadopoulos (2009) found that Pb(II) and Ni(II) were completely removed by an MBR system, indicating that these two metals were in particulate form, while Cr(III) and Cu(II) were removed by 89% and 49% respectively. However, the influent heavy metal concentrations in the aforementioned works were very low (<1 mg/l) since municipal wastewater was treated. The treatment of aquaculture wastewater containing very low heavy metal concentrations by an MBR was efficient since metal concentrations in the treated effluent satisfied the recommended limits for fish culture (Sharrer et al., 2010). Bolzonella et al. (2010) reported that the MBR system improved the removal efficiency for heavy metals by 10–15% compared to the conventional activated sludge system. Santos and Judd (2010)
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compared the metal removal accomplished by MBR and activated sludge systems and concluded that MBR consistently, but not dramatically achieved higher metal removal efficiencies (64–92% as opposed to 51–87%). However, their analysis was focused on low influent metal concentrations. Various mechanisms are involved in metal removal by activated sludge. These include physical trapping of precipitated metals into sludge flocs, binding, adsorption and complexation of soluble metal ions with extracellular polymeric substances (EPS), surface adsorption and diffusion of metal ions in sludge flocs (Arican et al., 2002; Kelly et al., 2004). In addition, the alkaline nature of wastewater further enhances metal removal through precipitation. Copper, lead, nickel and zinc are among the most common polluting metal ions in industrial wastewater and are also associated with toxicity problems. Lead, nickel and their compounds are classified as priority substances, while zinc, copper and their compounds are classified as potential main pollutants (Commission, European, 2000). Moreover, limit values for heavy metals have been recommended for wastewater reuse for irrigation purposes (USEPA 2004), while heavy metal limit values have also been set for the discharge of industrial wastewater into municipal sewers (USEPA, 2005). Therefore, the evaluation of heavy metal removal from industrial wastewater through the application of MBR technology is of particular concern. It is also important to consider the MBR performance when it is subjected to shock loads due to the inflow of wastewater characterized by significant heavy metal concentrations. Although some studies of heavy metal removal by MBR systems have been conducted, these focus on municipal wastewater containing low metal concentrations, while there is limited knowledge on MBR performance for the removal of heavy metals from industrial wastewater, where metal concentrations are usually high. This work investigated the potential removal of Ni(II), Cu(II), Pb(II) and Zn(II) from wastewater containing significant concentrations of these metals. In particular, the aim was to test the system performance when it is subjected to shock loads due to the inflow of wastewater containing significant heavy metal concentrations. The biomass activity in the biological reactor was investigated to determine the level of inhibition occurring due to heavy metal accumulation. Finally, membrane fouling was assessed. 2. Methods Primary treated municipal wastewater was enriched with Cu(II), Pb(II), Ni(II) and Zn(II) in multi-metal solutions and was fed to a pilot-scale MBR system. Sulphate salts of the above metals were supplied by Merck [CuSO4, NiSO46H20, ZnSO4H20] and Sigma–Aldrich [PbSO4]. Heavy metal enrichment of influent wastewater was conducted in a 2 m3 equalization tank where the influent wastewater was temporarily stored. The wastewater in the equalization tank was kept under continuous agitation. The wastewater enriched with heavy metals was fed to the biological reactor only for a time period of 6 h in order to avoid excessive heavy metal accumulation inside the reactor, resulting from the long-term system operation. However, to achieve the required metal concentration inside the reactor at the initiation of the 6-h experimentation period, it was usually necessary to add metals directly inside the reactor, depending on their concentrations in the mixed liquor. Hence dilution of heavy metals inside the reactor was avoided. In order to assess the MBR performance for the treatment of various influent metal concentrations, a range of 3–15 mg/l was examined. The sampling procedure for the measurement of all the parameters took place at the end of the 6-h period of metal enrichment. In the remaining time period the MBR was fed with municipal wastewater. The pH in the biological reactor was maintained above 7.0 in all the experimental runs using NaOH.
The membrane module was supplied by GE Water and Process Technologies (ZW-10). The module was that of hollow fibres and was directly immersed in the biological reactor. The nominal pore size of the membranes was 0.04 lm, while the membrane material was Polyvinylidene Fluoride (PVDF). The membrane surface area was 0.93 m2. The system was allowed to develop its own biomass and subsequently three experimental runs were conducted in series. In the 1st run the system operated for a period of 34 days without any metal addition and under steady-state conditions. This run represented the control conditions. In the 2nd run (days 35–77) heavy metal enrichment of the feed wastewater took place, while in the 3rd run (days 78–170) heavy metal enrichment was combined with the addition of 5 g/l vermiculite. Chemical cleaning of the membrane module took place periodically by immersing the module in NaOCl solution (1000 mg/l Cl2) for 8 hours and afterwards in 4000 mg/l of citric acid solution for 4 hours. The operating conditions of the 2nd and 3rd runs were maintained the same as those of the 1st run. Vermiculite was the aluminosilicate mineral that was employed to reduce the permeate heavy metal concentration through the processes of ion exchange and adsorption. Vermiculite was supplied by Mathios Refractories S.A. and was used in its natural form without any chemical modification. It was supplied in a grain size of 2.0–5.0 mm which was accordingly ground and sieved to the size of 0.18 mm and was subsequently added into the biological reactor. A fixed mass of vermiculite was added every day into the reactor to replenish the amount lost through sludge wasting and thus maintain its concentration constant in the biological reactor. The chemical composition of vermiculite is given in previous work. Its cation exchange capacity was 120.65 meq/ 100 g (Malamis et al., 2010). The biological reactor had a working volume of 210 l and treated daily approximately 450 l of influent wastewater. The transmembrane pressure was continuously recorded using the digital recorder EASYLOG 40KY, with an accuracy of 0.25 kPa which was connected to a pressure transducer (S-10 WIKA). The temperature was also recorded using the data logger EASYLOG 40NSW with an accuracy of 0.1 °C. Laboratory analyses were conducted to determine the characteristics of influent wastewater, activated sludge and permeate. More specifically, COD, TN, NH4–N, NO3–N were determined using suitable Merck kits, while Cu(II), Pb(II), Ni(II) and Zn(II), were determined using the Atomic Absorption Spectrometer of Varian AA240FS. The method detection limits were 0.05 mg/l for Pb(II), 0.01 mg/l for Cu(II), 0.005 mg/l for Zn(II) and 0.02 mg/l for Ni(II). To determine the total metal concentration in activated sludge the samples were digested at 440 °C using H2O2 and HNO3 and then filtered through GF/C Whatman filters. The soluble metal concentration in the biological reactor was determined by directly filtering the sludge through Whatman membranes with a pore size of 0.45 lm. The percentage removal (R) of each metal was calculated using the equation:
ð%ÞR ¼
C in C out 100 C in
ð1Þ
where Cin is the influent concentration and Cout is the permeate concentration (mg/l) of the metal ions. Total Suspended Solids (TSS) and Volatile Suspended Solids (VSS) were determined using Standard Methods (APHA, 1998). The maximum Oxygen Uptake Rate (OUR) of biomass was determined using primary wastewater and sodium acetate as substrates. More specifically, 250 ml of fresh primary wastewater was added to 250 ml of biomass and the mixture was kept under continuous aeration at room temperature (25 °C ± 2), while the mixture pH was maintained at 7.3 ± 0.2. At the time intervals of 10 and 30 min after the mixing had taken place, samples were collected, placed under mild agitation in BOD flasks and the reduction of the Dissolved Oxygen (DO) with time was recorded. The
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maximum OUR was taken as the maximum DO reduction among the two cases of 10 and 30 min. The maximum OUR was also measured when excess sodium acetate was added as substrate. The initial concentration of sodium acetate in biomass was 250 mg/l and the DO reduction with time was recorded. The procedure followed was similar to the one where primary wastewater was employed as substrate. In all OUR experiments 10–12 mg/l allyl-thiourea were added to inhibit nitrification. In the 2nd and 3rd runs, the biomass contained significant heavy metal concentrations. To avoid dilution, the measured heavy metal concentrations in sludge, were also added into the fresh wastewater and the sodium acetate solution. In each experimental period 10 repetitions of the OUR experiments were conducted. The OUR was always normalized by dividing it with the VSS of the mixture. The heterotrophic biomass inhibition was determined as the percentage difference between the measured maximum OUR of biomass prior and after the enrichment of feed wastewater with heavy metals:
ð%ÞHeterotrophic inhibition ¼
OUR1 OUR2 100 OUR1
ð2Þ
where OUR1 and OUR2 (mgO2/gVSS-h) are the maximum OURs of biomass prior (i.e. 1st run) and after (i.e. 2nd and 3rd runs) the addition of heavy metals respectively. Nitrification inhibition was determined by conducting Ammonium Uptake Rate (AUR) experiments. Biomass (500 ml) was collected from the bioreactor and was placed inside a suitable flask under continuous aeration. Ammonium chloride (NH4Cl) solution was added to the biomass so that the initial NH4-N concentration would be approximately 40 mg/l. The biomass was maintained at room temperature (25 ± 2 °C) and the DO concentration was maintained above 4 mg/l. At fixed time intervals 10–15 ml of biomass was collected, centrifuged at 3000g and filtered through Whatman membranes with pore size 0.45 lm. The filtrate was measured for its NO3-N concentration. The biomass pH was periodically recorded during the experiment and was adjusted using NaOH in order to be within the range of 7.2–7.6. To avoid dilution, in the 2nd and 3rd runs the measured heavy metal concentrations in sludge were also added into the NH4Cl solution that was added to biomass. The AUR was always normalized by dividing it with the VSS of the mixture. In total 10 repetitions were conducted for each cycle. The autotrophic biomass inhibition was determined as the percentage difference between the measured AUR of biomass prior and after the enrichment of feed wastewater with heavy metals:
AUR1 AUR2 ð%ÞAutotrophic inhibition ¼ 100 AUR1
ð3Þ
where AUR1 and AUR2 (mgNO3–N/gVSS-h) are the Ammonium Uptake Rates of biomass prior and after the addition of heavy metals respectively. Membrane fouling was assessed by calculating membrane permeability with time corrected at 20 °C using the following equation:
L20 ¼
J 20 TMP
ð4Þ
where L20 is the membrane permeability corrected at the reference temperature of 20 °C (l/m2-h-bar), J20 is the filtration flux corrected at the reference temperature of 20 °C (l/m2-h) and TMP is the transmembrane pressure (bar). The filtration flux was corrected at the reference temperature of 20 °C using the following equation (Fan et al., 2006):
J 20 ¼ J T 1:025ð20TÞ
ð5Þ
where JT is the filtration flux at the operating temperature of T (°C) SEM and EDX analysis were performed in used membrane fibres using the Quanta 200 of FEI in order to investigate element depositions on the membrane surface.
3. Results and discussion The MBR performance was assessed with respect to organics, ammonium nitrogen, copper, lead, nickel and zinc removal from wastewater. In addition, heterotrophic and autotrophic biomass activity and membrane fouling were evaluated. 3.1. Wastewater and system operating characteristics Table 1 summarizes the influent wastewater characteristics for the three experimental runs that were conducted. In the 1st run the feed wastewater was municipal, while in the 2nd and 3rd runs influent wastewater was enriched with Cu(II), Pb(II), Ni(II) and Zn(II), with vermiculite being added in the 3rd run. The 1st run served as a control run and was compared with the other two runs to assess the impact of heavy metal addition on biomass activity. The feed heavy metal concentrations ranged from 3.4–9.1 mg/l for copper, 3.9–14.7 mg/l for lead, 4.3–14.7 mg/l for nickel and 3.2–12.1 mg/l for zinc during the 2nd and 3rd experimental runs. Such concentrations are frequently met in industrial wastewater. The aforementioned metal concentrations were fed to the system for a limited amount of time (6 h) in order to limit metal accumulation inside the reactor. In industries that employ combined industrial and municipal wastewater treatment systems, it is essential to determine whether direct biological treatment is feasible and therefore whether the shock loads experienced occasionally due to the inflow of significant heavy metal concentrations can be tolerated by the MBR system. Table 2 summarizes the biological and filtration characteristics of the system. The filtration and backwash fluxes were kept constant at 22.3 and at 26.8 l/m2-h respectively, while the system operated at steady-state conditions with SRT = 15 d and Hydraulic Retention Time (HRT) = 10.3 h. 3.2. MBR performance and biomass activity 3.2.1. COD removal TSS removal was complete as the UF membranes formed a barrier to suspended solids. COD removal in the 1st run was very high Table 1 Influent wastewater characteristics. Parameter
1st Run (1–34 days)
2nd Run (35–77 days)
3rd Run (78–170 days)
TSS (mg/l) VSS (mg/l) COD (mg/l) NH4-N (mg/l) TN (mg/l) Cu(II) (mg/l) Pb(II) (mg/l) Ni(II) (mg/l) Zn(II) (mg/l)
244 (140–360) 193 (100–300) 535 (460–602) 53 (48–59) 72 (65–80) <0.1 <0.1 <0.1 <0.2
226 (155–310) 170 (115–230) 508 (438–617) 44 (34–57) 58 (48–73) 5.7 (3.4–6.7) 13.1 (11.8–14.7) 12.2 (9.7–14.7) 9.9 (7.8–11.4)
267 (150–415) 200 (95–310) 515 (395–719) 48 (29–67) 68 (53–76) 6.2 (4.2–9.1) 6.4 (3.9–8.9) 9.9 (4.3–14.1) 6.5 (3.2–12.1)
Table 2 MBR operating characteristics. Parameter
1st Run (1–34 days)
2nd Run (35–77 days)
3rd Run (78–170 days)
SRT (d) HRT (h) Mixed liquor pH MLSS (g/l) MLVSS (g/l) Filtration flux (l/m2-h) Backwash flux (l/m2-h) F/M (d-1)
15 10.3 7.51 (7.25–7.77) 5.84 (5.40–6.68) 4.82 (4.38–5.63) 22.3
15 10.3 7.38 (7.02–7.62) 5.77 (4.90–6.15) 4.20 (3.65–4.91) 22.3
15 10.3 7.47 (7.00–7.78) 10.33 (9.00–12.55) 5.24 (4.47–6.20) 22.3
26.8
26.8
26.8
0.24
0.26
0.25
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ranging from 95–97% and the permeate COD was very low (19– 24 mg/l). The addition of metals resulted in lower, but satisfactory COD removal which ranged from 83–91% with an average of 88% during the 2nd run and from 86–95% with an average of 92% in the 3rd run. In Fig. 1a, the influent and treated effluent COD concentrations are given, with the permeate COD ranging from 42– 78 mg/l (average of 59 mg/l) in the 2nd run and from 27–68 mg/l (average of 43 mg/l) in the 3rd run. Despite the partial inhibition of heterotrophic biomass the treated effluent COD remained at relatively low levels, but was higher than the effluent COD obtained when the system was not fed with heavy metals (i.e. control conditions). In the 3rd run the permeate COD was lower than that of the 2nd run (Fig. 1a) since the heterotrophic biomass inhibition was limited due to the lower influent metal concentrations. More importantly, the addition of vermiculite provided a suitable carrier material for the development of biomass at its surface, thus increasing the system’s ability to oxidize organic matter. Also, vermiculite adsorbed part of the heavy metals, thus reducing their toxic effects on biomass. During the last 20 days of system operation the permeate COD was very low and similar to that of the 1st run. This is attributed to the low nickel and zinc concentrations in influent wastewater, that resulted in lower inhibition of heterotrophic biomass and thus more effective COD removal.
(a)
Days of system operation (d)
MLSS / MLVSS (g/l)
MLSS st 12 1 Run
2nd Run
MLVSS
3rd Run
(b)
8 4 0 0
50 100 150 Days of system operation (d) Influent NH4-N
Effluent NH4-N
200
NH4-N / NO3-N (mg/l)
2nd Run
3rd Run
(c)
60 40 20 0
0
3.2.3. Nitrification Full (100%) nitrification was observed during the control conditions since metals were not added into the MBR system. The addition of metals inside the biological reactor during the 2nd and 3rd runs impacted on the nitrification process. In particular, nitrification was 22–54% during the 2nd run and 33–96% during the 3rd run. The higher nitrification observed in the 3rd run was partly attributed to the lower Zn(II), Ni(II) and Pb(II) inlet concentrations and partly to the addition of vermiculite. The mineral adsorbed part of Zn(II), Ni(II) and Cu(II) on its surface, reducing the adverse impact of metals on microorganisms and it also enhanced microbial growth at its surface. Fig. 1c shows the variation of influent and effluent NH4–N and effluent NO3–N concentrations, where it is seen that significant NH4–N concentrations were present in the effluent, in the 2nd and 3rd runs. Jn average the effluent NH4–N in the 2nd run was 26 mg/l (18–35 mg/l) with the 95% confidence interval falling within the range of 23–29 mg/l. In the 3rd run the effluent ammonium nitrogen concentration was on average 16 mg/l (2–30 mg/l) with the 95% confidence interval falling within the range of 12–20 mg/l. From the above analysis it is evident that there is a significant difference between the ammonium nitrogen effluent concentrations in the 2nd and 3rd runs. In the last 20 days of system operation, the ammonium nitrogen in the treated effluent was low (<4 mg/l) and nitrification was high. During these days the influent nickel and zinc concentrations were lower than the average values (<5.4 and <6.3 mg/l respectively) and this enhanced the nitrification process.
Effluent NO3-N
80 1st Run
3.2.2. MLSS and MLVSS variation Fig. 1b shows the variation of MLSS and MLVSS with time. Comparing MLVSS prior and after metal addition (i.e. 1st and 2nd runs), a 13% reduction was observed when metals were added due to partial biomass inhibition. Despite the accumulation of heavy metals inside the biological reactor, the biomass development was satisfactory with the MLVSS being stable in the range of 3.7–4.9 g/l. The addition of 5 g/l vermiculite during the 3rd run increased MLSS from an average of 5.8 g/l (2nd run) to an average of 10.3 g/l, while the average MLVSS increased by 20% in the 3rd run compared to the MLVSS of the 2nd run. Two reasons can account for this increase of MLVSS: vermiculite provided a support medium for the growth of microorganisms on its surface, thus increasing the MLVSS concentration. Thus, apart from the microorganisms under suspension, biofilm also developed onto the mineral surface. Consequently, during the 3rd run a combined suspended and attached growth process was observed. Furthermore, the lower influent metal concentration resulted in enhanced biomass development. Vermiculite is purely inorganic and did not contribute as a material to the increase of MLVSS. The MLSS concentration during the 3rd run was lower than the expected one, showing that some of the vermiculite had actually settled in the bioreactor.
50 100 150 Days of system operation (d)
200
Fig. 1. Variation of (a) influent and effluent COD concentration (b) MLSS and MLVSS in the biological reactor, (c) influent and effluent NH4–N and effluent NO3–N concentration with time.
3.2.4. Biomass activity Heterotrophic biomass activity was assessed by comparing the maximum OUR obtained for the control conditions (i.e. no metal addition) with the maximum OUR obtained for the experimental runs in which heavy metals were added into the system. Fig. 2a shows the maximum OUR measured for the different experimental conditions in which primary wastewater and sodium acetate were used as substrates. A notable reduction in heterotrophic biomass activity was observed when metals were added. In particular, during the 2nd run biomass inhibition was on average 50% and 45% for primary wastewater and sodium acetate respectively, while during the 3rd run it was 29% and 25% respectively (Fig. 2a). In the latter case, the reduction in the inhibition of heterotrophic biomass was partly attributed to the lower influent concentrations of heavy metals and partly to the addition of 5 g/l vermiculite. The mineral
E. Katsou et al. / Bioresource Technology 102 (2011) 4325–4332
OURmax (mg/gVSS-h)
adsorbed part of the metal ions found in soluble form, thus reducing the toxic effect of heavy metals on biomass and it also provided a suitable carrier for the development of biofilm on its surface. Fig. 2b shows the maximum OUR (measured using primary wastewater as substrate) as a function of the heavy metal concentration in the biological reactor. Lead was not included in the figure since the vast majority of the metal formed precipitates in the reactor and did not significantly impact on biomass activity. It is seen that the increase of metal concentration resulted in higher heterotrophic biomass inhibition. Lead had the highest accumulation since its concentration in influent wastewater was higher than that of the other metals, while low concentrations were detected in the effluent since the majority of lead ions formed precipitates at the operating pH
24
(a)
Primary wastewater
18
Sodium acetate
12 6 0
AUR (mg NO3-N/gVSS-h)
OURmax (mg/gVSS-h)
1st run 21
Cu(II)
3rd run
Zn(II)
(b)
Ni(II)
17 13 9 5 0.00
0.05
0.10 0.15 Metal concentration (mM)
0.20
4
(c) 3 2 1 0
1st run AUR (mgNO3-N/gVSS-h)
2nd run
2nd run
3rd run
4 Cu(II)
Zn(II)
(d)
Ni(II)
3 2 1 0 0.00
0.05
0.10
0.15
0.20
Metal concentration (mM) Fig. 2. (a) Maximum OUR measured using primary wastewater and sodium acetate as substrates (error bars represent standard deviation), (b) variation of maximum OUR with the heavy metal concentration in the biological reactor, (c) AUR measured for the three experimental runs (error bars represent standard deviation) and (d) variation of AUR with the heavy metal concentration in the biological reactor.
4329
range. Copper was characterized by the lowest concentrations in the reactor due to its low influent concentration. The inhibition of autotrophic biomass was determined through AUR experiments and was found to be on average 70% and 36% for the 2nd and 3rd runs respectively compared to the control conditions (Fig. 2c). The lower inhibition obtained in the 3rd run is partly attributed to the lower influent metal concentration and partly to the addition of vermiculite. Fig. 2d shows the AUR versus the heavy metal concentration in the biological reactor. A reduction in autotrophic biomass activity was observed with increasing metal concentration. As it is observed the average heterotrophic biomass inhibition was lower than the respective autotrophic biomass inhibition, indicating that nitrifying bacteria were more sensitive to toxic pollutants than heterotrophic microorganisms. These findings are in accordance with the results of Madoni et al. (1999) and of Principi et al. (2006) who found that nitrifiers were more sensitive than aerobic heterotrophic microorganisms when exposed to certain heavy metals. Biomass toxicity can be limited by maintaining the pH at an alkaline environment (pH = 8.0–8.5) that is also suitable for biomass development in order to enhance metal removal through precipitation. However, this was not conducted in the present study since the principal aim was to investigate metal removal in the pH operating range of 7.0–7.8. The role of vermiculite in the reduction of heavy metal toxicity was beneficial, but further investigation is required under more stable laboratory conditions. In this work the operating MLSS was not very high. However, most MBR systems operate at higher MLSS that may exceed 10 g/l. In such cases the risk of biomass inhibition is greater even for lower influent metal concentrations due to higher heavy metal accumulation. 3.2.5. Metal removal In Fig. 3a–d the influent and effluent concentrations of Cu(II), Pb(II), Ni(II) and Zn(II) are given respectively for the two runs in which metals were added into the reactor, while in Table 3 the system removal efficiencies obtained for each metal are summarized. The copper and lead concentrations in the treated effluent were always lower than 1.5 and 1.2 mg/l respectively. Zinc concentration in the effluent was higher and ranged from 0.3–3.9 mg/l, while nickel concentration was even higher ranging from 2.7–7.5 mg/l. Comparing the heavy metal concentrations in the treated effluent with the USEPA (2004) recommended limits for irrigation of reclaimed water it is deduced that for lead the long-term reuse limit was always satisfied. For copper and zinc the short-term reuse limit was always met, while the long-term limit was occasionally satisfied for copper and frequently met for zinc (particularly in the 3rd run). The nickel concentration in the final effluent could not meet any of the reuse limits. Thus, particular attention is required when the influent wastewater is characterized by high nickel concentrations. Metal removal achieved by the MBR system followed the sequence Pb(II) > Cu(II) > Zn(II) > Ni(II). The heavy metals were removed through various processes which included: (i) precipitation of metals inside the biological reactor, (ii) biosorption of metal ions on sludge flocs, (iii) retention of the insoluble metal species by the UF membranes and (iv) adsorption of metal ions on vermiculite. The latter process occurred during the 3rd run when 5 g/l vermiculite were added into the reactor. The pH inside the biological reactor ranged from 7.0–7.8 and thus significant precipitation occurred. Previous work has shown that precipitation is observed at pH = 7.0–8.0 in multi-metal solutions, particularly for lead and copper and to a lower extent for zinc and nickel (Malamis et al., in press). It is also important to notice that the MBR system can also remove the insoluble metal species that remain in suspension as opposed to the conventional activated sludge system. The metal removal process was enhanced through the biosorption of metal
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Influent
Effluent
10
Cu(II) (mg/l)
2nd
3rd
Run
(a)
Run
8 5 3 0 50
70
90
110
130
150
170
Days of system operation (d) Influent
2nd Run
Effluent
3rd Run
(b)
15 10
0 50
70
90
110
130
150
170
Days of system operation (d) Influent
Effluent
Ni(II) (mg/l)
16
(c) 12
2nd run
10
3rd run
(a)
8 6
R² = 0.827
4
R² = 0.870
2 0 0
8
2
4
6
8
10
12
14
MLSS (g/l) 4
2nd Run
3rd Run
0 50
70
90
110
130
150
170
Days of system operation (d) Influent
Zn(II) (mg/l)
Cu(II) removed (mg/l)
5
Effluent
(d)
12 8 2nd Run
Pb(II) removed (mg/l)
Pb(II) (mg/l)
20
added since this mineral is very effective for nickel and zinc adsorption (Katsou et al., 2010a,b). As it is seen in Table 3 the addition of 5 g/l vermiculite enhanced the removal of nickel, zinc and copper through adsorption. In particular, copper removal increased from 80% to 88%, zinc removal from 77% to 85% and nickel removal from 50% to 60%. Vermiculite did not impact on lead removal since this metal was mainly removed through precipitation and thus there was a low amount of lead available for adsorption on vermiculite. Vermiculite addition significantly increased the MLSS concentration. This drawback was counterbalanced by the advantages of lower biomass inhibition, greater microorganism growth (both attached and suspended) and higher metal removal. During the last 20 days of the 3rd run, Ni(II) removal was very low (28–37%) and Zn(II) removal was quite low (74–79%) compared to the average removal efficiencies obtained during the 3rd run. As it has been previously explained, nitrification was very high due to
2nd run
16
R² = 0.568
(b)
12 8 4
R² = 0.308
0 0
3rd Run
2
4
90
110
130
150
170
Days of system operation (d) Fig. 3. Variation of influent and effluent (a) Cu(II), (b) Pb(II), (c) Ni(II) and (d) Zn(II) concentrations.
Table 3 Metal removal efficiencies achieved by the MBR system.
Ni(II) removed (mg/l)
70
8
10
12
14
2nd run
(c)
8
R² = 0.643
6
R² = 0.881
4 2 0 0
2
4
Percentage removal (%) 2nd Run (35–77 days)
3rd Run (78–170 days)
80 98 50 77
88 98 60 85
(71–86) (90–>99.9) (31–58) (66–85)
3rd run
10
6
8
10
12
14
MLSS (g/l)
(77–97) (95–>99.9) (45–68)* (74–95)
*
The stated range does not include the very low nickel removal obtained during the last 20 days of operation. An explanation for these very low Ni(II) removal efficiencies is given in the text.
ions on the sludge flocs. Nickel exhibited the worst performance showing that significant part of the metal ions remained in soluble form and thus penetrated through the UF membranes. To increase MBR performance with respect to metal removal, vermiculite was
2nd run Zn(II) removed (mg/l)
50
Cu(II) Pb(II) Ni(II) Zn(II)
6
MLSS (g/l)
4 0
Metal
3rd run
3rd run
12
(d)
9 6
R² = 0.813
R² = 0.890
3 0 0
2
4
6
8
10
12
14
MLSS (g/l) Fig. 4. Impact of MLSS concentration on (a) Cu(II), (b) Pb(II), (c) Ni(II) and (d) Zn(II) removal.
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the lower Ni(II) and Zn(II) feed concentrations. The increase in nitrification resulted in a subsequent pH reduction in the biological reactor (from 7.6 to 7.0–7.2). It seems that the solubility of the metals increased and hence higher amount of nickel and zinc ions could penetrate through the UF membranes. The pH range of 7.0–7.5 affected Zn(II) and Ni(II) removal (Malamis et al., in press). On the contrary, for Cu(II) and Pb(II) the removal remained very high as this pH reduction did not alter significantly the metal solubility. The impact of MLSS on metal removal was also investigated. The correlation between MLSS and metal removal is shown in Fig. 4a–d. This correlation is considered separately for the 2nd and 3rd run since biomass had different characteristics, due to the addition of vermiculite in the 3rd run. It is observed that there was a significant correlation between the MLSS concentration and metal removal. The increase in MLSS resulted in more sites being available for the sorption process and consequently more metal ions were removed. The obtained correlation was low for Pb(II) since the majority of metal ions were removed through precipitation rather than biosorption. It must be mentioned that pH variation was not significant (with the exception of the last 20 days of the 3rd run) and this enabled the determination of a correlation between MLSS and metal removal. The excess sludge produced contained significant heavy metal concentrations. To assess sludge toxicity it is important to consider heavy metal accumulation in the bioreactor in the long run. In particular, the system operation for several days could result in the production of non-hazardous sludge since the overall heavy metal accumulation could be low, as significant influent metal concentrations are usually observed only for a specific time period. If the sludge is hazardous then it must be subjected to adequate treatment prior to its final disposal. The amount of produced sludge could be reduced by applying dissolved air flotation to separate the activated sludge from vermiculite. Vermiculite, having a specific gravity of 2–2.6, is heavier than sludge flocs and can settle easily. The obtained vermiculite could be regenerated and reused to reduce the operational cost of the system.
1st Run
3.3. Fouling investigation The impact of heavy metal and vermiculite addition on membrane fouling was assessed. In Fig. 5a the reduction in membrane permeability corrected at the reference temperature of 20 °C is given versus operation time. From this diagram it is possible to determine the rate of permeability reduction (dL/dt 20 °C) which was 1.44–1.47 l/m2-h-bar-d for the 1st run, 1.68–1.76 l/m2-h-bard for the 2nd run and 1.88–2.01 l/m2-h-bar-d for the 3rd run. It is evident that feed wastewater with significant metal concentrations adversely impacted on membrane fouling, as dL/dt increased by approximately 18% compared to the control conditions. The addition of vermiculite into the biological reactor further increased fouling by approximately 33% compared to the control conditions. SEM-EDX analysis revealed significant deposits of vermiculite on the membrane surface (Fig. 5b,c). The EDX analysis detected significant amounts of Si on the membrane surface and to a lower extent Al and Fe. Si is a major element of vermiculite, which also contains significant amounts of Al and Fe. Furthermore, EDX analysis showed the presence of heavy metal deposits (mainly lead). It seems that coarse bubble aeration could not effectively remove such deposits from the membrane surface. The increase of membrane fouling due to vermiculite addition in activated sludge was not expected since previous work in batch systems (Katsou et al., 2010a) has shown that vermiculite did not adversely impact on membrane fouling. It seems that in the continuous operation of an MBR system the suspended solids of the mineral could attach to the membrane fibres. However, this type of inorganic fouling is reversible and thus suitable measures can be taken for its reduction. It can be limited by increasing the intensity of coarse bubble aeration and/or increasing the duration or the flux of backwash. These two techniques are usually very effective for the removal of suspended solids. Another solution would be to add granular vermiculite instead of powder since the results have shown that granular vermiculite performed better than powder in terms of membrane fouling, but it was less effective in terms of metal removal (Katsou et al., 2010a).
2nd Run
3rd Run
L20oC (l/m2-h-bar)
120 dL/dt= -1.88
dL/dt = -1.47
100
dL/dt = -2.01
(a)
80 60 dL/dt = -1.44 dL/dt = -1.76 40 dL/dt = -1.68
20
dL/dt = -1.90
0 0
(b)
20
40
60 80 100 120 140 Days of system operation (d)
160
180
(c)
Fig. 5. (a) Variation of membrane permeability with time (b) EDX analysis and (c) SEM micrograph of used membrane fibre.
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3.4. Assessment of the role of vermiculite The addition of vermiculite improved the removal efficiencies for copper, zinc and nickel by approximately 8–10% for each metal. Thus, significant increase of metal removal was obtained for the three metals. It also reduced to some extent biomass inhibition and enhanced biomass growth, as it provided suitable surface for biofilm development. The negative impacts of vermiculite were the increase in MLSS and membrane fouling. These drawbacks can be counterbalanced by the advantages of lower biomass inhibition, greater microorganism growth and higher metal removal. 4. Conclusions The MBR system was able to achieve heavy metal removal efficiencies of 80% for Cu(II), 98% for Pb(II), 50% for Ni(II) and 77% for Zn(II). The addition of 5 g/l vermiculite in wastewater containing lower influent metal concentrations enhanced metal removal efficiencies for copper (88%), zinc (85%) and nickel (60%) due to adsorption of metal ions on the mineral, reduced biomass toxicity and enhanced the growth of microorganisms. Inhibition of heterotrophic biomass was 50%, and reduced to 29% when lower feed metal concentrations and vermiculite were added into the system. The respective inhibition of autotrophic biomass was on average 70% and 36%. A significant correlation was found between MLSS concentration and heavy metal removal. Fouling investigation revealed deposits of metals and vermiculite on the membrane surface that adversely impacted on the rate of membrane permeability reduction. References APHA, 1998. Standard methods for the Examination of Water and Wastewater, 20th ed. American Public Health Association, American Water Works Association, Water Environment Federation, Washington, DC, USA. Arican, B., Gokcay, C.F., Yetis, U., 2002. Mechanistics models of nickel sorption by activated sludge. Process Biochem. 37, 1307–1315. Battistoni, P., Cola, E., Fatone, F., Bolzonella, D., Eusebi, A.L., 2007. Micropollutants removal and operating strategies in ultrafiltration membrane systems for municipal wastewater treatment: Preliminary results. Ind. Eng. Chem. Res. 46, 6716–6723. Blöcher, C., Bunse, C., Seßler, B., Chmiel, H., Janke, H.D., 2004. Continuous regeneration of degreasing solutions from electroplating operations using a membrane bioreactor. Desalination 162, 315–326. Bolzonella, D., Fatone, F., Di Fabio, S., Cecchi, F., 2010. Application of membrane bioreactor technology for wastewater treatment and reuse in the Mediterranean region: focusing on removal efficiency of non-conventional pollutants. J. Environ. Manage. 91, 2424–2431.
Commission, European, 2000. Directive 2000/60/EC of the European Parliament and of the Council establishing a framework for Community action in the field of water policy. Off. J. Eur. Communities L 327, 1–73. Dialynas, E., Diamadopoulos, E., 2009. Integration of a membrane bioreactor coupled with reverse osmosis for advanced treatment of municipal wastewater. Desalination 238, 302–311. Fan, F., Zhou, H., Husain, H., 2006. Identification of wastewater sludge characteristics to predict critical flux for membrane bioreactor processes. Water Res. 40, 205–212. Fatone, F., Eusebi, A.L., Pavan, P., Battistoni, P., 2008. Exploring the potential of membrane bioreactors to enhance metals removal from wastewater: Pilot experiences. Water Sci. Technol. 57, 505–511. Katsou, E., Malamis, S., Haralambous, K.J., 2010a. Examination of zinc uptake in a combined system using sludge, minerals and ultrafiltration membranes. J. Hazard. Mater. 182, 27–38. Katsou, E., Malamis, S., Haralambous, K.J., Loizidou, M., 2010b. Use of ultrafiltration membranes and aluminosilicate minerals for nickel removal from industrial wastewater. J. Membr. Sci. 360, 234–249. Kelly, C.J., Tumsoroj, N., Lajoie, C.A., 2004. Assessing wastewater metal toxicity with bacterial bioluminescence in a bench-scale wastewater treatment system. Water Res. 38, 423–431. Liu, R., Chen, L., Wen, X., Qian, Y., 2004. Operational performance of a submerged membrane bioreactor for reclamation of bath wastewater. Process Biochem. 40, 125–130. Lubello, C., Caffaz, S., Mangini, L., Santianni, D., Caretti, C., 2007. MBR pilot plant for textile wastewater treatment and reuse. Water Sci. Technol. 55, 115–124. Madoni, P., Davoli, D., Guglielmi, L., 1999. Response of SOUR and AUR to heavy metal contamination in activated sludge. Water Res. 33, 2459–2464. Malamis, S., Andreadakis, A., 2009. Fractionation of proteins and carbohydrates of extracellular polymeric substances in a membrane bioreactor system. Bioresour. Technol. 100, 3350–3357. Malamis, S., Katsou, E., Stylianou, M., Haralambous, K.J., Loizidou, M., 2010. Copper removal from sludge permeate with ultrafiltration membranes using zeolite, bentonite and vermiculite as adsorbents. Water Sci. Technol. 61, 581–589. Malamis, S., Katsou, E., Haralambous, K.J. Study of Ni(II), Cu(II), Pb(II), and Zn(II) Removal Using Sludge and Minerals Followed by MF/UF. Water Air Soil Pollut. 1–12, in Press. Nelson, P.O., Chung, A.K., Hudson, M.C., 1981. Factors affecting the fate of heavy metals in the activated sludge process. J. Water Pollut. Control Fed. 53, 1323– 1333. ˇ urlin, M., Racyte, J., Simonicˇ, M., 2009. Textile wastewater treatment Petrinic´, I., C with membrane bioreactor and water re-use. Tekstil. 58, 11–19. Principi, P., Villa, F.M., Bernasconi, E., Zanardini, E., 2006. Metal toxicity in municipal wastewater activated sludge investigated by multivariate analysis and in situ hybridization. Water Res. 40, 99–106. Santos, A., Judd, S., 2010. The fate of metals in wastewater treated by the activated sludge process and membrane bioreactors: a brief review. J. Environ. Monitor. 12, 110–118. Sharrer, M.J., Rishel, K., Summerfelt, S.T., 2010. Evaluation of a membrane biological reactor for reclaiming water, alkalinity, salts, phosphorus and protein contained in a high-strength aquacultural wastewater. Bioresour. Technol. 101, 4322– 4330. Tazi-Pain, A., Schrotter, J.C., Bord, G., Payreaudeau, M., Buisson, H., 2002. Recent improvement of the BIOSEPÒ process for industrial and municipal wastewater treatment. Desalination 146, 439–443. USEPA, 2004. Guidelines for Water Reuse. EPA/625/R-04/108, U.S Agency for Inter. Development, Washington, DC, USA. USEPA, 2005. Streamlining the General Pretreatment Regulations for Existing and New Sources of Pollution 40 CFR Parts 9, 122 and 403.