EFFECTS OF BRONOPOL ON ANAEROBIC STABILIZATION OF SEWAGE SLUDGE AND BIOGAS PRODUCTION

Transcription

1 EFFECTS OF BRONOPOL ON ANAEROBIC STABILIZATION OF SEWAGE SLUDGE AND BIOGAS PRODUCTION TEREZA DOKULILOVA, TOMAS VITEZ, JAN KUDELKA Department of Agricultural, Food and Environmental Engineering Mendel University in Brno Zemedelska 1, Brno CZECH REPUBLIC Abstract: Antimicrobial preservatives (as a bronopol) are widely used in cosmetics and toiletries to prevent the spoilage of products due to microbial contamination than can be presented in wastewaters and in sewage sludge and may inhibit the process of sludge anaerobic stabilization. The aim of this study is to specify inhibitory effect of bronopol on anaerobic stabilization of sewage sludge and biogas production. Anaerobic fermentation test was carried out in 24 batch fermenters (the volume of 5 dm 3 ) for 21 days at 38 ± 0.2 C. Bronopol was used as toxic substance in 7 different concentrations; 25, 50, 75, 100, 125, 150 and 175 mg/l. The quantity and quality of produced biogas were monitored during hydraulic retention time. Hypothesis, which predicts presence of inhibitory effect of bronopol on anaerobic microorganisms, mainly on methanogenic Archaea, was confirmed. The lowest concentration of bronopol which causes significant inhibition of biogas production is 75 mg/l. This concentration leads to reduction of 5.8 ± 2.3% in the biogas yield. The lowest concentration of bronopol which causes significant inhibition of methane production is 75 mg/l. The reduction in biogas and methane production after addition of the highest bronopol concentration (175 mg/l) is 51.5 ± 0.9% and 76.9 ± 2.9%, respectively. Which means that methanogens are more inhibited by bronopol than other groups of anaerobic microorganism. Key Words: 2-bromo-2-nitropropan-1,3-diol, anaerobic fermentation, municipal wastewater sludge, inhibitory effect, methane yield INTRODUCTION Sewage sludge is produced as the by-product of wastewater treating at every wastewater treatment plant. The anaerobic stabilization of sewage sludge reduces its volume, stabilizes organic matter and reduces the quantity of pathogenic microorganisms. Anaerobic stabilization consists of 4 microbial steps (hydrolysis, acidogenesis, acetogenesis and methanogenesis) that are carried out by different groups of microorganism. Methanogens are the most sensitive and final group of anaerobic microorganism which convert organic matter to biogas, the mixture of methane and carbon dioxide. One of the main disadvantages of anaerobic stabilization is its lower resistance to toxicants than aerobic treatment. Many different toxicants can be presented in wastewaters than in sludge and may inhibit the process of sludge anaerobic stabilization (Chen et al. 2014). For example, antimicrobial preservatives (as a bronopol) are widely used in cosmetics and toiletries to prevent the spoilage of products due to microbial contamination (Kajimura et al. 2008). Bronopol (2-bromo-2-nitropropan-1,3-diol) is a white odourless crystalline substance well soluble in water, lower alcohols, diethyl ether, ethylacetate and acetic acid. Bronopol is used as preservative and antiseptic agent in liquid medication forms, cosmetic creams, gels and aerosols, deodorants, foot powders, liquid soap, shampoo, household detergents, cleaning pastes, polishing compounds, toilet water deodorants ect. (Legin 1996). Bronopol is also used as a biocide in industrial processes, for example textiles, paper mills and cooling water systems (USEPA 2005). Bronopol is toxic for a wide spectrum of microorganisms, including Gram-negative species, resistant to lots of antibacterial agents (Kajimura et al. 2008). The antimicrobial behaviour of bronopol is caused by electron-deficient bromine atoms in the molecules, which have oxidation properties. 773

2 The antimicrobial mechanism of bronopol in the cross-linking of sulfohydrids of enzymes existing on the microbial cells surface. Adaptation to bronopol is theoretically impossible because of its nonspecific mechanism. The main advantage of bronopol is high antimicrobial activity combinated with relatively low toxic effect to hot-blooded animals (Legin 1996). Dissolution of bronopol in aqueous solution at warm temperature and higher ph may lead to release of formaldehyde that is converted into formic acid (Trivisano et al. 2015). In aqueous solutions, bronopol rapidly degrades to many transformation products like 2-bromo-2-nitroethanol, bromonitromethane, tri(hydroxymethyl)nitromethane, nitromethane, 2-bromoethanol, formaldehyde ect. However, its antimicrobial behaviour is still acceptable (Cui et al. 2011). The aim of this study is to specify inhibitory effect of bronopol on anaerobic stabilization of sewage sludge and biogas production. Hypothesis predicts presence of inhibitory effect of bronopol on anaerobic microorganisms, mainly on methanogenic Archaea. This study is the first evaluation of inhibitory effect of bronopol on anaerobic stabilization of sewage sludge and biogas production. MATERIAL AND METHODS According to Czech Standard Method CSN EN ISO , sludge sample was taken directly from the anaerobic stabilization tank at the WWTP Brno - Modřice, PE, Czech Republic and transported to the laboratory immediately. According to Czech Standard Method CSN EN 15934, fresh sample was dried at 105 ± 5 ºC in the laboratory oven EcoCELL 111 (BMT Medical Technology Ltd., Czech Republic) to determine the sludge total solids (TS) content. According to Czech Standard Method CSN EN 15169, volatile solids (VS) content was determined by incineration of the samples in a muffle furnace (LMH 11/12, LAC, Ltd., Czech Republic) at 550 ± 5 ºC. In accordance with Czech Standard Method CSN EN 12176, ph, conductivity and redox potential of sludge were determined by using ph/cond meter 3320 (WTW GmbH, Germany). In order to measure biogas quantity and quality, anaerobic fermentation test was hold at temperature 38 ± 0.2 ºC, according to German Standard VDI Three systems, which each consists of eight batch fermenters of volume 5 dm 3, were used. All 24 batch fermenters were filled up with 3 dm 3 of anaerobic sludge. In this study, 8 ml of glycerine were used as a carbon and energy source for microbial growth. One fermenter was used as a blank in each system. To achieve 8 different concentrations of bronopol (25, 50, 75, 100, 125, 150, 175 mg/l), bronopol stock solution was added into remaining fermenters. All concentrations were tested in triplicate. Hydraulic retention time was 21 days. The biogas was collected in wet gas meters and was measured daily over this period. Biogas quality (content of methane, carbon dioxide, hydrogen and hydrogen sulphur) was analysed during the test 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 were converted to biogas and methane yield, by expressing them as m 3 per kg of VS of the substrate. All measurements were done in triplicate. All measured values are expressed as arithmetic mean ± standard deviation. RESULTS AND DISCUSSION Anaerobic fermentation is a complex process in which many mechanisms such as antagonism, synergism, acclimation and complexing could affect the inhibition level. This is reason why the inhibition levels reported in literature show significant variation (Chen et al. 2008). The potential toxicity of chemical compounds is significantly affected by the physical and chemical conditions in which they are present, e. g. ph, redox potential, conductivity, DM, ODM of sludge. Therefore, the characteristics of used sewage sludge are shown in Table

3 Table 1 Sewage sludge sample characteristics Sample ph [-] Redox potential [mv] Conductivity [S/m] Dry matter [%] Organic dry matter [%] Sewage sludge 7.21 ± ± ± ± ± 0.12 The curve of cumulative biogas production during 21 days hydraulic retention time is shown in Figure 1. Specific biogas yield after same time is shown in Table 2. The lowest concentration of bronopol which causes significant inhibition of biogas production is 75 mg/l. This concentration leads to reduction of 5.8 ± 2.3% in the biogas yield. The reductions of 35.2 ± 4.6%, 53.4 ± 1.1%, 53.2 ± 1.5, 51.5 ± 0.9% in biogas production can be observed after addition of 100, 125, 150 and 175 mg/l, respectively. There are no significant differences among inhibitions caused by bronopol in concentrations 125, 150 and 175 mg/l. As can be seen in Figure 1, anaerobic microorganisms need longer adaptation time after addition of bronopol in higher concentration. Thus, curves of biogas production are affected after addition of bronopol in all tested concentrations. However, there are no significant differences in final biogas yield among blank and bronopol in concentrations till 50 mg/l. Figure 1 Cumulative biogas yield 0,25 [m3/kgvs] 0,2 0,15 0,1 0, [Day] Blank 25 mg/l 50 mg/l 75 mg/l 100 mg/l 125 mg/l 150 mg/l 175 mg/l Table 2 Biogas yield after 21 days hydraulic retention time Sample Specific biogas production [m 3 /kg VS] Relative biogas production [%] Blank ± ± mg/l ± ± mg/l ± ± mg/l ± ± mg/l ± ± mg/l ± ± mg/l ± ± mg/l ± ± 0.9 The composition of biogas generated during 21 days hydraulic retention time is shown in Figure 2. The curve of cumulative methane production during same time is shown in Figure 3. Specific methane yield after 21 days is shown in Table 3. The lowest concentration of bronopol which causes significant inhibition of methane production is 75 mg/l. This concentration leads to reduction of 10.2 ± 4.5% in the methane yield. The reductions of 54.8 ± 5.1%, 79.4 ± 4.4%, 79.6 ± 5.6%, 76.9 ± 2.9% in biogas production can be observed after addition of 100, 125, 150 and 175 mg/l, respectively. There are no significant differences among inhibitions caused by bronopol in concentrations 125, 150 and 175 mg/l. As can be seen in Figures 2 and 3, methanogens need longer adaptation time after addition of bronopol in higher concentration. The curves of biogas production are affected after addition of bronopol in all tested concentrations. However, there is no significant difference in cumulative 775

4 methane production after 21 days after addition of bronopol in concentration 25 mg/l and blank. The methane yield after addition of bronopol in concentration 50 mg/l seems to be the highest, but there is the highest standard deviation, so the difference between blank and 50 mg/l is not significant, see Table 3. Figure 2 Methane concentration in biogas [%vol] 80,00 70,00 60,00 50,00 40,00 30,00 20,00 10,00 0, [Day] Blank 25 mg/l 50 mg/l 75 mg/l 100 mg/l 125 mg/l 150 mg/l 175 mg/l Figure 3 Cumulative methane yield [m 3 /kg VS ] 0,1 0,09 0,08 0,07 0,06 0,05 0,04 0,03 0,02 0, [Day] Blank 25 mg/l 50 mg/l 75 mg/l 100 mg/l 125 mg/l 150 mg/l 175 mg/l Table 3 Methane yield after 21 days hydraulic retention time Sample Specific methane production [m 3 /kg VS] Relative methane production [%] Blank ± ± mg/l ± ± mg/l ± ± mg/l ± ± mg/l ± ± mg/l ± ± mg/l ± ± mg/l ± ± 2.9 The comparison of Tables 2 and 3 shows that reduction in biogas and methane production after addition of the highest bronopol concentration (175 mg/l) is 51.5 ± 0.9% and 76.9 ± 2.9%, respectively. Which means that methanogens are more inhibited by bronopol than other groups of anaerobic microorganism. The lowest concentration of bronopol which causes significant inhibition of biogas and methane production is 75 mg/l. This concentration leads to reduction of 5.8 ± 2.3% and 10.2 ± 4.5% in the biogas and methane yield, respectively. Which is higher than minimum effective concentration of bronopol 776

5 reported by Legin (1996) that represents 25 mg/l to Gram-negative bacteria 50 mg/l to Gram-positive bacteria, 50 mg/l to yeast-like fungi. On the other hand, the minimum effective concentration of bronopol effective against mold-like fungi is 200 mg/l (Legin 1996). Study by Cui et al. (2011) presents following EC50 values for bronopol to Chlorella pyrenoidosa; 5.76 mg/l, 2.18 mg/l, 4.84 mg/l and 5.17 mg/l, after 24, 48, 72 and 96 hours, respectively. The toxicity potential (IC50) of bronopol to Vibrio fischeri reported by Wang et al. (2008) in Cui et al. (2011) is mg/l. Previous reported concentration of bronopol are significantly lower that concentrations which causes reduction of 50% in biogas and methane yield, 125 and 100 mg/l, respectively. CONCLUSION The inhibitory effect of bronopol on anaerobic stabilization of sewage sludge was studied using 24 batch anaerobic fermenters at temperature 38 C ± 0.2 ºC during hydraulic retention time 21 days. Bronopol (2-bromo-2-nitropropan-1,3-diol) was used as toxic substance in 7 different concentrations; 25, 50, 75, 100, 125, 150 and 175 mg/l. Cumulative biogas and methane production were used as the comparative parameters of bronopol inhibitory effect. Hypothesis, which predicts presence of inhibitory effect of bronopol on anaerobic microorganisms, mainly on methanogenic Archaea, was confirmed. The lowest concentration of bronopol which causes significant inhibition of biogas production is 75 mg/l. This concentration leads to reduction of 5.8 ± 2.3% in the biogas yield. The reductions of 35.2 ± 4.6%, 53.4 ± 1.1%, 53.2 ± 1.5, 51.5 ± 0.9% in biogas production can be observed after addition of 100, 125, 150 and 175 mg/l, respectively. The lowest concentration of bronopol which causes significant inhibition of methane production is 75 mg/l. This concentration leads to reduction of 10.2 ± 4.5% in the methane yield. The reductions of 54.8 ± 5.1%, 79.4 ± 4.4%, 79.6 ± 5.6%, 76.9 ± 2.9% in biogas production can be observed after addition of 100, 125, 150 and 175 mg/l, respectively. The reduction in biogas and methane production after addition of the highest bronopol concentration (175 mg/l) is 51.5 ± 0.9% and 76.9 ± 2.9%, respectively. Which means that methanogens are more inhibited by bronopol than other groups of anaerobic microorganism. ACKNOWLEDGEMENTS The research was financially supported by the Internal Grant Agency of the Faculty of AgriSciences, Mendel University in Brno, IP 13/2017. REFERENCES Chen, J.L., Ortiz, R., Steele, T.W.J., Stuckey, D. C Toxicants inhibiting anaerobic digestion: A review. Biotechnology Advances [Online], 32(8): Available at: [ ]. Chen, Y., Cheng, J.J., Creamer, K.S Inhibition of anaerobic digestion process: A review. Bioresource Technology [Online], 99: Available at: [ ]. Cui, N., Zhang, X., Xie, Q., Wang, S., Chen, J., Huang, L., Qiao, X., Li, X., Cai, X Toxicity profile of labile preservative bronopol in water: The role of more persistent and toxic transformation products. Environmental Pollution [Online], 159(2): Available at: gateway&_docanchor=&md5=b ccfc9c30159a5f9aeaa92ffb. [ ]. Czech Standards Institute Characterization of sludge - Determination of ph-value. CSN EN Praha: Czech Standards Institute. Czech Standards Institute Characterization of waste - Determination of loss on ignition in waste, sludge and sediments. CSN EN ISO Praha: Czech Standards Institute. Czech Standards Institute Water quality Sampling Part 13: Guidance on sampling of sludges. CSN EN ISO Praha: Czech Standards Institute. Czech Standards Institute Sludge, treated biowaste, soil and waste Calculation of dry matter fraction after determination of dry residue or water content. CSN EN Praha: Czech Standards Institute. 777

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