EFFECTS OF ZINC ON ANAEROBIC FERMENTATION OF SEWAGE SLUDGE AND BIOGAS PRODUCTION

Transcription

1 EFFECTS OF ZINC ON ANAEROBIC FERMENTATION OF SEWAGE SLUDGE AND BIOGAS PRODUCTION TEREZA DOKULILOVA, TOMAS VITEZ Department of Agricultural, Food and Environmental Engineering Mendel University in Brno Zemedelska 1, Brno CZECH REPUBLIC Abstract: Toxic metals can be present in municipal wastewaters sludge and may inhibit the process of anaerobic fermentation. According to literature zinc is one from toxic metals with the strongest inhibitory effect. Therefore, this article deals with effect of zinc on anaerobic fermentation of sewage sludge and biogas production. Inhibitory effect of zinc on anaerobic stabilization of sewage sludge was studied using batch anaerobic fermenters at temperature 42 C ± 1 ºC. Hydraulic retention time was 21 days. As toxic substance was used zinc chloride (ZnCl 2) in three different amounts: 75, 312 and 625 mg/l which represent 12, 50 and 100 mg Zn/l, respectively. Biogas and methane yield after 21 days hydraulic retention time were used as comparative parameters of inhibitory effect of zinc. There were no significant differences between biogas yields from all tested concentrations of zinc and blank. There was only one significant difference between methane yields from tested concentrations of zinc and blank. The reduction of 6.3 ± 2.5% in the cumulative methane production can be observed after addition of 100 mg Zn/l (625 mg ZnCl 2/l). Key Words: anaerobic stabilization, municipal wastewaters sludge, inhibitory effect, zinc chloride, methane yield INTRODUCTION Municipal wastewater treatment plants (WWTP) produce sewage sludge as a by-product of the physical, chemical and biological processes used during treatment. Current daily production of sewage sludge ranges from 60 to 90 g of dry solids per population equivalent (PE) in EU (Apples et al.2008). This sludge must undergo some treatment in order to reduce its volume, to transform organic matter into a relatively stable or inert organic and inorganic residue and to reduce amount of pathogenic microorganisms. The disposal of sludge may represent up to 40% of the capital costs and 50% of the operating costs of WWTP (Spellman 2009). There are many possible ways how to handle with sewage sludge. Because of energy-rich biogas production, anaerobic fermentation is economically sustainable way of sludge stabilization. The anaerobic fermentation involves a complex interaction of several groups of bacteria. Methanogens being the final group of microorganisms, which converts acetate and carbon dioxide with hydrogen into methane (Codina et al. 1998). Optimal conditions for microbial community are important to achieve the highest possible yield and quality of biogas. Toxic metals can be present in municipal wastewaters sludge and may inhibit the process of anaerobic fermentation. According to literary sources (for example Sarioglu et al. 2010) the relative toxicity of metals, obtained by using the inhibition of methanogenic activity assay, is Cu > Ni Zn > Pb. According to other authors inhibition effect of toxic metals is quite different: Zn > Cr > Cu > Cd > Ni > Pb (Mudhoo and Kumar 2013) or Zn > Cr > Ni Cd (Altaş 2009). Therefore, this article deals with effect of zinc on anaerobic fermentation of sewage sludge and biogas production. MATERIAL AND METHODS Sludge samples were taken at the WWTP Brno - Modřice, PE, Czech Republic. Sludge samples were collected directly from the anaerobic stabilization tank at the WWTP, according to Czech Standard 853 P age

2 Method CSN EN ISO After the collection, the sludge samples were transported to the laboratory immediately. To determine the sludge dry matter (DM) content according to Czech Standard Method CSN EN fresh samples were dried at 105 C ± 5 ºC, the laboratory oven EcoCELL 111 (BMT Medical Technology Ltd., Czech Republic), was used. Organic dry matter (ODM) content was determined by incineration of the samples in a muffle furnace at 550 C ± 5 ºC according to Czech Standard Method CSN EN 15169, using a furnace (LMH 11/12, LAC, Ltd., Czech Republic). Sludge s ph, redox potential and conductivity were determined by using ph/cond meter 3320 (WTW GmbH, Germany) in accordance with CSN EN standard. The content of zinc in dried sample was determined by handheld spectrometer and metal analyser DELTA PROFESSIONAL (BAS Rudice, Czech Republic). Biogas yield and quality was measured using batch anaerobic fermenters at temperature 42 C ± 1 ºC, according to German Standard VDI Two systems, which each consists of eight batch fermenters of volume 5 dm 3, were used. All sixteen batch fermenters were filled up with 3 dm 3 of sludge samples from anaerobic sludge stabilization. In this research, glycerine (7 ml) was used as a co-substrate for carbon and energy source for microbial growth. In both systems, two batch fermenters were used as a blank. Into remaining fermenters three different amounts of zinc chloride (ZnCl 2) were added to achieve required concentration of zinc (Table 1). All tests were done in duplicate in both systems. Table 1 Tested amounts and concentrations of zinc Concentration of Zn [mg/l] Amount of ZnCl 2 [mg/l] Blank The biogas produced was collected in wet gas meters over a defined period of 21 days and was measured daily. Methane (CH 4), carbon dioxide (CO 2), hydrogen (H 2) and hydrogen sulphur (H 2S) content was measured during the batch fermentation tests using gas analyser COMBIMASS GA-s (BINDER GmbH, Germany). Biogas production was converted to standard conditions (T 0 = 273 K, p 0 = Pa). The volume of biogas and methane produced by a sample was converted to biogas yield and methane yield, by expressing them as m 3 per kg of organic dry matter (ODM) of the substrate. All measurements were done in triplicate. All measured values are expressed as arithmetic mean ± standard deviation. RESULTS AND DISCUSSION Results of the toxicity effect of heavy metals on sewage sludge stabilization are quite different in literature. There are some possible reasons of this difference. For example the carbon sources used for anaerobic metabolism (glucose, volatile fatty acids etc.), measured evaluation parameter (methane or hydrogen production, chemical oxygen demand removal etc.), used reactors (batch or continuous), characteristics of anaerobic sludge, binding strength of a heavy metal ion to the anaerobic sludge (sorption, precipitation) (Sarioglu et al. 2010). The potential toxicity of heavy metals is significantly controlled by the physical and chemical environment in which they are present, and this is correlated to different ion-specific physicochemical parameters, e.g. standard redox potential, electronegativity, the solubility product of the corresponding metal-sulfide complex, the Pearson softness index, electron density and the covalent index (Workentine et al in Chen et al. 2014). Moreover the operating solids level significantly impacts the heavy metal toxicity in anaerobic digesters by providing protection from metal inhibitory effect (Hickey et al in Chen et al. 2008). Unfortunately, most of the literature only reported the inhibition concentration values in mg/l (Chen et al. 2008). For the above reasons it is necessary to specify characteristics of tested sewage sludge, which are shown in Table P age

3 Table 2 Sewage sludge sample characteristics Sample ph [-] Sewage sludge Redox potential [mv] Conductivity [S/m] Dry matter [%] Organic dry matter [%] Content of Zn [mg/l] 7.32 ± ± ± ± ± ± 3.60 Specific biogas yield generated after 21 days hydraulic retention time is shown in Figure 1 and Table 3. There are no significant differences between yields from all tested concentrations of zinc and blank. Figure 1 Cumulative biogas yield during fermentation Table 3 Biogas yield after 21 days hydraulic retention time Sample Specific biogas production [m 3 per kg of ODM] Relative biogas production [%] Blank ± ± mg Zn/l ± ± mg Zn/l ± ± mg Zn/l ± ± 0.4 Biogas composition generated after 21 days hydraulic retention time is shown in Figure 2. Specific methane yield after same time is shown in Table 4. There is only one significant difference between methane yields from tested concentrations of zinc and blank. The reduction of 6.3 ± 2.5% in the cumulative methane production can be observed after addition of 100 mg Zn/l (625 mg ZnCl 2/l). These results are quite different from findings of other researches. For example when Altaş (2009) used unacclimatized (no adaptation to zinc) granular anaerobic sludge as a substrate, the inhibiting concentration of zinc that causes a 50% reduction in the cumulative methane production relative to the control sample over a fixed period of exposure time (24 h) was 7.5 mg/l. With whey as a substrate, Zayed and Winter (2000) observed 50% inhibition of methanogenesis in the presence of ZnCl 2 40 mg/l. According to their research, addition of zinc chloride at concentrations of 60 mg/l, 120 mg/l and 200 mg/l led to an inhibition of methane formation of 80%, 90% and 94%, respectively. On the other hand, Sarioglu et al. (2010) used anaerobic sludge taken from an up-flow anaerobic sludge blanket reactor treating the wastewaters of Pakmaya Yeast Factory, cumulative methane gas 855 P age

4 production decreased to 55 and 43%, respectively for 500 and 1000 mg Zn/l. Lin and Chen (1999) tested sludges that were obtained from an up-flow anaerobic sludge blanket reactor treating winery wastewater, the concentration at which zinc caused 50% inhibition of methane production was 690 and 270 mg Zn/l, respectively at hydraulic retention time 1 and 2 days. At these examples is shown that the dosage of zinc for a 50% inhibition of methanogenesis during wastewater sludge anaerobic stabilization is not clear from the literature. Precipitated chemical forms of heavy metals, as sulfides, carbonates and hydroxides, may be sorbed onto either biomass or inert particulate matter and forming of complexes in system. Therefore, it is difficult to understand and interpret the heavy metal behaviour in anaerobic stabilization of sewage sludge (Sarioglu et al. 2010). Figure 2 Methane and carbon dioxide concentration in biogas 70 gas concentration [% vol ] Blank CH 4 12 mg Zn/l CH 4 50 mg Zn/l CH mg Zn/l CH 4 Blank CO 2 12 mg Zn/l CO 2 50 mg Zn/l CO mg Zn/l CO day Table 4 Methane yield after 21 days hydraulic retention time Sample Specific methane production [m 3 per kg of ODM] Relative methane production [%] Blank ± ± mg Zn/l ± ± mg Zn/l ± ± mg Zn/l ± ± 2.5 CONCLUSION Inhibitory effect of zinc on anaerobic stabilization of sewage sludge was studied using batch anaerobic fermenters at temperature 42 C ± 1 ºC. Hydraulic retention time was 21 days. As toxic substance was used zinc chloride (ZnCl 2) in three different amounts: 75, 312 and 625 mg/l which represent 12, 50 and 100 mg Zn/l, respectively. Biogas and methane yield after 21 days hydraulic retention time were used as comparative parameters of inhibitory effect of zinc. There were no significant differences between biogas yields from all tested concentrations of zinc and blank. There was only one significant difference between methane yields from tested concentrations of zinc and blank. The reduction of 6.3 ± 2.5% in the cumulative methane production can be observed after addition of 100 mg Zn/l (625 mg ZnCl 2/l). ACKNOWLEDGEMENTS The research was financially supported by the Internal Grant Agency of the AgriSciences faculty, Mendel University in Brno, IP_17/ P age

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