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Ozonolysis pre-treatment of waste activated sludge for solubilization and biodegradability enhancement
Journal of Environmental Chemical Engineering 7 (2019) 102945
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Ozonolysis pre-treatment of waste activated sludge for solubilization and biodegradability enhancement
T
⁎
Benton Otienoa, , Seth Apollob,c, John Kabubac, Bobby Naidooa, Geoffrey Simated, Aoyi Ochienge a
Department of Chemistry, Vaal University of Technology, Private Bag X021, Vanderbijlpark, South Africa Department of Chemistry, University of Embu, P.O. Box 6-60100, Embu, Kenya c Department of Chemical Engineering, Vaal University of Technology, Private Bag X021, Vanderbijlpark, South Africa d Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, South Africa e Botswana International University of Science and Technology, Private Bag 16, Palapye, Botswana b
A R T I C LE I N FO
A B S T R A C T
Keywords: Ozone Waste activated sludge Biodegradability enhancement Cell wall Solubilization
Handling of waste activated sludge (WAS) has been a great problem due to its low biodegradability which is contributed by the biorecalcitrant bacterial cells from the activated sludge process. Moreover, the sludge is poorly soluble which makes it difficult to subject to a further biological treatment downstream. This study investigated the application of ozonolysis pre-treatment for the solubilization and biodegradability enhancement of municipal WAS. Ozonolysis led to sludge solubilization as was shown by a 50% reduction in total suspended solids (TSS). Subsequently, the dissolved organic carbon (DOC) increased by 25%, and sludge biodegradability increased two-folds. The mechanism of sludge solubilization was explained through cell content analysis which revealed biomass cell wall rapture during ozonolysis leading to the release of the cellular contents including humic substances, proteins, phosphates, nitrates and carbohydrates into the supernatant. The soluble proteins decreased by 98% while carbohydrates increased four-fold. Additionally, sulphates in the supernatant increased from 18 mg/L to 45 mg/L while nitrates increased from 0 mg/L to 85 mg/L indicating oxidation of the organic nitrogen and phosphorus which are constituents of the biomass cell protein; this can provide nutrient balance in the downstream biological treatment method. Thermogravimetric (TGA) analysis showed that the weight loss for ozone-treated WAS was 10% while that for the raw WAS was 20% further indicating that ozonolysis had solubilized the volatile and organic components of the treated WAS. Ozonolysis is a promising technology for the pre-treatment of WAS before anaerobic digestion for stabilization and energy recovery.
1. Introduction Municipal wastewater (MWW) generated from different domestic sources such as toilets, showers, and the kitchen is widely treated through activated sludge treatment (AST) process. Waste activated sludge (WAS) is a major by-product of the AST process and is generated in large amounts [1,2]. The WAS consists majorly of biomass from cell growth and decay during AST process as well as traces of odorous and volatile organic compounds, and toxic chemicals like heavy metals, thus posing the risk of secondary environmental pollution if not stabilized [3–5]. The treatment and management of the excess WAS accounts for between 25–60% of the total costs of a municipal wastewater treatment plants [2,6]. Moreover, the conventional treatment methods such as landfilling, incineration, soil utilization and use as feedstock (based on the inherent minerals in sludge after pyrolysis) for making ceramics, activated carbon and cement paste, are restricted by various ⁎
extents in practice [7–9]. Recently the enhancement of anaerobic digestion (AD) process for WAS stabilization, reduction, and energy recovery in the form of methane gas has aroused great interest [10]. However, AD of WAS suffers high retention times, low methanogenic production and an overall 30–35% dry solids degradation [11]. The insoluble and biorecalcitrant proteinaceous biomass cell walls and extracellular polymeric substances (EPS) in WAS hinders the initial rate determining hydrolysis step of AD. During hydrolysis, enzyme-mediated solubilization of insoluble organic materials, the rapture of hard cell walls and degradation of EPS occur releasing biodegradable organic compounds for the ensuing acidogenesis step [12]. A long retention time resulting in the consumption of a large amount of energy is, therefore, required for the hydrolysis of WAS [3,11,13], limiting the effective application of AD in the treatment and stabilization of WAS [14]. To improve the AD process and meet the strict regulation on
Corresponding author. E-mail address: [email protected] (B. Otieno).
https://doi.org/10.1016/j.jece.2019.102945 Received 11 December 2018; Received in revised form 1 February 2019; Accepted 5 February 2019 Available online 07 February 2019 2213-3437/ © 2019 Elsevier Ltd. All rights reserved.
Journal of Environmental Chemical Engineering 7 (2019) 102945
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environmental protection, implementation of a pre-treatment method, that would enhance the hydrolysis step is necessary [2,6]. Several methods for sludge solubilization including chemical treatment using acids and alkalis, mechanical disintegration, and advanced oxidation processes (AOPs) have been applied to date with varying degrees of success [14–17]. Of these methods, ozonolysis, which is an AOP is considered most effective for sludge disintegration, because it does not lead to a significant increase in salt concentration and has no chemical residues in comparison to other pre-treatment techniques [18]. Through ozonolysis, organic matter can be released from the sludge into the solution as a result of the breaking and solubilization of the hard cellular membrane [14]. The role of ozonolysis pre-treatment is to partially oxidize the biorecalcitrant component of WAS to release biodegradable products [4]. Through ozonolysis, low soluble volatile organic compounds (VOCs), can be oxidized and solubilized as well thus contributing to the improved biodegradability [5,19]. To avoid overoxidation of the released biodegradable components, the ozonolysis dose and time have to be optimized [20]. The bacteria in the activated sludge (AS) process are known to simultaneously remove organic compounds and major inorganic nutrients present in municipal wastewater such as sulphates, nitrates, and phosphates through assimilation or accumulation within the hard biomass cell walls forming part of the WAS [21,22]. Also, phosphorus removal during the AS process through accumulation in the cell biomass may lead to its increased concentration in the WAS [23]. Subsequently, during ozonolysis, the disintegration of the cell biomass may release phosphorous compounds in excessive amounts [22]. Similarly, the assimilated nitrates/nitrogen which forms an important component of cell wall proteins may be released as nitrates, ammonia or nitrites [22–24]. It is thus significant to understand the impact of ozonolysis on the degradation of organic or bound phosphorus, sulphate, and nitrogen to determine if it leads to excessive release of the cell-bound nutrients or adequate amounts that are suitable for an ensued biological process. The aim of the current study was to carry out ozonolysis pre-treatment of WAS for sludge solubilization and to improve biodegradability. Of major interest was the understanding of the ozonolysis process based on the rapture of the proteinaceous cell wall and EPS leading to nutrients (proteins, carbohydrates, nitrates, and phosphates) speciation, their release into the supernatant and suitability for a possible ensued AD process. Also, the sludge properties were determined before and after ozonolysis through thermogravimetric and infrared spectroscopic studies.
Table 1 Physicochemical characteristics of municipal waste activated sludge. Parameter
Units
pH TSS CODT CODS DOC BOD5 TS VSS BOD5:COD Sulphate Phosphate Chloride Soluble proteins
mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L
Value 6.7 12.4 6860 1408 122 901 65.3 42.1 0.1 18 67 55 20
2.3. Ozonolysis experiments The waste activated sludge was treated in a 4 L ozone reactor batchwise for sludge solubilization and improved biodegradability. For every batch experiment, 2 L of WAS was transferred to the reactor then subjected to ozonolysis. Gaseous ozone was generated from air using an ozone generator coupled to an air compressor. The ozone generator employed the corona discharge principle, where a high voltage corona mechanism converts the oxygen in air to ozone [25,26]. The resulting ozone air mixture was then bubbled in an up-flow mode in the ozone reactor via a gas diffuser. The ozone concentration in the mixture was determined following a volumetric method based on the reaction between potassium iodide (KI) and ozone [27]. To obtain different ozone dosages, the ozone flow rate was controlled using a manual valve and the flow measured using an air rotameter. For instance, an air flow rate of 2 L/min generated air-ozone mixture with a dosage of 45 mg O3 per minute supplied to the reactor. Air flow rates of 4, 6 and 8 L/min generated dosages of 90, 135 and 180 mg O3 per minute. The unutilized ozone gas from the reactor was destroyed by passing through the KI solution. The schematic diagram of the ozonolysis process is given in Fig. 1. 2.4. Average oxidation state
2. Materials and methods
The average oxidation state (AOS) of the WAS during ozonolysis was used to estimate the degree of oxidation of the substrate containing the oxidation by-products as well as the initial compounds. The AOS was calculated as;
2.1. Materials and methods
AOS = 4 ×
Sodium thiosulphate, phenol, bovine serum albumin (BSA), methanol, phosphoric acid, Bradford reagent, Coomassie brilliant blue, potassium iodide (KI), sulphuric acid, hydrochloric acid, silver sulfate, potassium dichromate, starch, and sodium hydroxide were all purchased from Merck Limited, South Africa. The chemicals were of analytical grade and were used as received. All standard and working solutions were prepared using ultra-pure distilled water.
where CODT and TOC are the total chemical oxygen demand and total organic carbon, respectively. The values for AOS vary from −4 (for CH4 the most reduced state for C) to +4 (for CO2 the most oxidized state for C). In an oxidation process, however, the increment observed in AOS (ΔAOS), can be calculated by;
TOC −
12 32
× CODT
TOC
CODTf ⎞ CODTi 12 ΔAOS = AOSf − AOSi = 4 × ⎜⎛ − ⎟ × DOCf ⎠ 32 ⎝ DOCi
(1)
(2)
where i and f, respectively, refer to initial and final values. The ΔAOS is of more importance as it indicates the degree of oxidation [28].
2.2. Waste activated sludge The waste activated sludge (WAS) was sourced from a secondary settler of a local municipal wastewater treatment (WWT) plant in Vanderbijlpark, Gauteng-South Africa. The WAS was filtered through a 450 μm microfilter to remove large solid particles then further analyzed to determine its physical and chemical characteristics. Table 1 gives the determined characteristics of the sludge.
2.5. Ozone transfer The amount of ozone in the inlet and outlet gas streams was determined for the different flow rates by titrating the ozonated KI solutions with potassium thiosulphate solution. Ozone transfer, which is the amount of ozone used up in the reaction was determined by; 2
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Fig. 1. Schematic representation of the ozonolysis unit.
O3 T =
O3 U X 100 O3 S
COD value of 600 mg/L was expected. However, the higher soluble COD value of 1408 mg/L reported was partly contributed to by the oxidation of ammonia, soluble proteins (20 mg/L) and other soluble inorganic compounds present in the WAS. The BOD5:COD ratio of 0.1 confirmed the poor biodegradability of the WAS. The low biodegradability is attributed to the proteinaceous biomass cell wall which hinders the rate determining hydrolysis step of AD [11]. Therefore, ozonolysis should be applied for the solubilization of the biorecalcitrant biomass cells.
(3)
where, O3 T , O3 S and O3 U are the ozone transfer (%), supplied and utilized ozone (mg), respectively. The utilized ozone was determined from the difference obtained between the ozone supplied to the reactor and the ozone coming out of the reactor (off gas). 2.6. Chemical analysis Aliquot samples were withdrawn and analyzed for pH, total organic carbon (TOC), COD, BOD, total suspended solids (TSS), total solids (TS), dissolved organic carbon (DOC) total volatile solids (VSS), nitrates, phosphates, proteins, carbohydrates, chlorides, and aromaticity (UV254). The COD, BOD, TSS, TS, and VSS were analyzed according to the standard methods of analysis [27]. Aromaticity was determined by UV absorption measurement (T80 + UV/vis Spectrometer, PG Instruments Ltd) at the maximum absorption wavelength (λ) of 254 nm [29], while the cations concentration was determined from Ion Chromatography (882 Compact IC plus – Cation: intelligent ion chromatograph for determining anions and cations without chemical suppression). Fourier transform infrared, FTIR, analysis (Perkin- Elmer FT-IR/NIR Spectrometer, PerkinElmer, Waltham, MA, USA) was used in determining the presence of functional groups in the WAS. Differential thermal gravimetric (DTG) and thermal gravimetric (TGA) analyses were performed on Perkin-Elmer Thermogravimetric analyzer TGA 400 instrument, to determine the thermal decomposition behavior of the dried WAS before and after ozonolysis. The amount of unutilized ozone captured in KI iodide solution was determined by titrimetric method [30,31]. The concentration of carbohydrates and proteins in the supernatant was determined using the Bradford assay and phenol-sulphuric acid methods, respectively [32,33].
3.2. Effect of ozone dosage The effect of ozone dosage at different ozone flowrates is given in Fig. 2. Selecting an appropriate ozone dosage is very important in optimizing the ozonolysis process. Low ozone dose would achieve low solubilization while an over-dose would lead to mineralization of already solubilized organics and a high concentration of ozone in the offgas. As a result, the process will not be economical. After 60 min of ozonolysis, the highest TSS reduction of 53% (12.4–5.6 mg/L) was achieved at a flow rate of 90 mg/min, while the lowest flow rate of 45 mg/min removed 47% of the TSS (Fig. 2a). A corresponding observation was made with the evolution of DOC with the highest increase of 25% (from 120 to 152 mg/L) attained at the 90 mg/min flow rate (Fig. 2b). An increase in ozone flow rate from 45 to 90 mg/min led to an increase in ozone concentration in the reactor which adequately dissolved the organic suspended solids leading to the reduced TSS. However, as the flow rate increased further, the TSS reduction decreased despite the increasing amount of supplied O3 molecules. At high ozone flow rates above the optimal dose, the supplied gas forms larger bubbles which significantly reduces gas hold-up resulting in reduced contact time between O3 and the suspended solids leading to reduced efficiency [34,35].
3. Results and discussion 3.3. Effect of initial pH 3.1. Characteristics of waste activated sludge Highest TSS reduction of 47% (12.4–6.5 mg/L) was obtained at pH 11 as compared to 36% and 19% reductions at pHs of 7 and 3, respectively (Fig. 3). Similarly, the highest increase in DOC evolution of 55% (122–190 mg/L) was achieved at pH 11 and the lowest value of 23% at pH 3. The reduction in TSS and increase in DOC are attributable to the solubilization of the suspended organic matter. Under acidic conditions, ozone selectively attacks parts of organic compounds
The physical and chemical characteristics of the obtained waste activated sludge are given in Table 1. The WAS had a soluble COD (CODS) of 1400 mg/L which was 20% of the total COD (CODT) indicating that up to 80% of the COD was in the suspended solids. The DOC value of 122 mg/L also indicated a low concentration of the organic compounds in soluble form. With the DOC of 122 mg/L, a soluble 3
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Fig. 2. Effect of ozone dosage on; (a) TSS reduction after 60 min of ozonolysis and (b) evolution of DOC at different flow rates of 45 (■), 90 (○), 135 (□) and 180 (●) mg/min.
within the first 60 min. Based on these findings, the duration of ozonolysis should be minimized to reduce excessive energy consumption and depletion of carbon if anaerobic digestion is to be applied downstream to the ozonolysis process. [43–45]. As observed in Fig. 4b, the ozone transfer efficiency decreased with increasing duration of ozonolysis after every 20 min interval. In the first three intervals (60 min of ozonolysis) more than 90% transfer was achieved and was considered efficient. As the ozonolysis continued beyond the 60th minute, more of the target organic contaminants were already degraded leading to reduced ozone transfers of 84, 81 and 76% in the 4th, 5th and 6th intervals, respectively. Improved biodegradability due to ozonolysis was confirmed by the increases in AOS from −1.5 to 0.9 as shown in Fig. 4c. Also, in Fig. 4d, the BOD5:COD ratio increased from 0.1 to 0.21 while the pH decreased from 11 to 9.68. The increase in oxidation state could be attributed to the solubilization of the organic solids by degradation of the aromatic groups and formation of aliphatic structures with functional groups such as eCOOH, eOH and eCHO as indicated by the FTIR analysis shown in Fig. 6b. Aliphatic structures contain higher oxidation state carbons than aromatic carbons. Moreover, the decrease in basicity could be due to the formation of the easily biodegradable acidic intermediate compounds such as acetic acids and humic substances [46].
containing C]C double bonds, aromatic rings, and negatively charged centers/sites containing atoms such as N, P, O, S [36]. The selective attack leads to partial solubilization hence the low increase in DOC [37]. The performance of ozone is superior under alkaline conditions because its half-life increases from 15 min, which is the case in an acidic/neutral condition, to 25 min after which it decomposes leading to the formation of the highly reactive hydroxyl radicals (OH%). The OH% radicals unselectively react with all the organic compounds hence the highest increase in DOC at occurred pH 11 [38–40]. 3.4. Change in COD, biodegradability, and ozone transfer Ozonolysis of the WAS was carried out at the optimal conditions of pH 11 and ozone flow rate of 90 mg/min. Fig. 4 shows the changes in total COD (CODT) and soluble (CODS), pH, BOD5:COD ratio, AOS, and ozone transfer. The CODT decreased by 42% after 2 h of ozonolysis (Fig. 4a). Consequently, the CODS increased by 41% after 60 min of ozonolysis then reduced with additional ozonolysis time. The decrease in CODT and the subsequent increase in CODS was attributed to the solubilization of the suspended solids (sludge) present in the WAS. However, the extended ozonolysis period past the 60th minute could have led to mineralization of the already solubilized organic matter hence the observed decrease in CODS [41,42]. Also, the low reduction in CODT during the extended period from the 80th minute indicates that most of the oxidizable organic solids had already been solubilized
Fig. 3. Effect of pH on ozonolysis of WAS on; (a) TSS reduction after 60 min of ozonolysis and (b) evolution of DOC at different pHs of 3 (Δ), 7 (□) and 11 (○) at ozone dosage of 90 mg/min. 4
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Fig. 4. Change in; (a) CODT (○) and CODS (●), and (b) BOD5:COD (□) and pH (▲), and (c) AOS, and (d) ozone transfer during ozonolysis at pH 11 and 90 mg/min ozone dosage.
the aqueous phase as a result of the disintegration of EPS and solubilization of the organic suspended matter [49]. The slight reduction in UV-254 absorption after 60 min could have resulted from the mineralization of the already solubilized organic constituents.
3.5. Sludge solubilization mechanism The mechanism of WAS solubilization was monitored through changes in UV-254 absorption, and cations, proteins and carbohydrates concentration in the supernatant as shown in Fig. 5. The WAS organic fraction majorly consists of biomass cell with a structure that includes hard cell walls, membrane cell nuclei, and extracellular polymeric substances (EPS). The hard cell walls and EPS which are the major constituents of the WAS organic fraction are mainly composed (up to 80%) of polysaccharides (PS), proteins (PS) and humic substances. Moreover, during the AST process, the microorganisms absorb nutrients such as phosphates, sulphates, and nitrates leading to the development of the hard cell walls [16,22,47,48]. The ozonolysis of WAS caused the hard cell walls to rapture and solubilized the EPS thereby releasing the cellular contents. This led to the observed increase in sulphates (18 to 45 mg/L) and nitrates (0 to 85 mg/L) as shown in Fig. 5a, and carbohydrates from 0 to 102 mg/L (Fig. 5b). Whereas an increase in protein concentration was expected as reported by Zang et al. [16], the observed decrease from 20 to 0.5 mg/L (Fig. 5b) could have resulted from the oxidation of the released protein by ozone into nitrates hence the significant increase in the dissolved nitrate concentration observed (Fig. 5a). Ozonolysis did not affect the chloride while the phosphate concentration reduced from 67 to 37 mg/L despite an expected increase given the release of the cellular phosphorus content. Metal elements such as Al, Fe, Mg and Ca present in WAS could have combined with the phosphate ions forming poorly soluble compounds hence the decrease [22]. Organic compounds such as the humic substances that commonly exist in wastewater strongly absorb UV radiation at λ of 254 nm [27]. Thus, a correlation may exist between their concentration and UV absorption. In the first 60 min of ozonolysis (Fig. 5c) the UV-254 absorption increased signifying the increase of the organic constituents in
3.6. Thermal gravimetric and FTIR analyses of WAS The effect of ozonolysis on sludge solubilization was further monitored through characterization of raw (before ozonolysis) and treated (after ozonolysis) WAS using FTIR and TGA analyses (Fig. 6). The WAS substrates were first dried at 100 °C for 1 h before analysis. Fig. 6a gives the thermogravimetric behavior of the WAS before and after ozonolysis, heated from 30 to 1000 °C at a constant heating rate of 30 °C per minute. Table 2 gives the weight loses at the different phases of combustion. The TGA curves were almost similar from 30 °C to around 420 °C where they intersected. Beyond 420 °C the curve for WAS before ozonolysis was greater than that after ozonolysis indicating that the residues had a greater mass for the former than the later [50]. The reduced mass of residues of the WAS after ozonolysis was caused by the solubilization of the organic content of the sludge such as the hard cell walls, which mainly decomposed in the temperature range between 180 and 480 °C. In phase I of combustion (Table 2) for the sludge before and after ozonolysis, weight loses of 34, and 39% were observed, respectively. Ozonolysis had led to the rapture of cells allowing for the release of the pore and bound water during thermal treatment hence the higher weight loss for the WAS after ozonolysis than the WAS before ozonolysis in this phase. In the subsequent phase II, the WAS before ozonolysis had 20% weight loss as compared to 10% for the WAS after ozonolysis. The higher weight loss of 20% was attributed to the thermal decomposition of proteins and cellulose in the WAS. In the ozonated WAS, however, ozonolysis had already led to the decomposition and 5
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Fig. 5. Change in (a) cations concentration sulfate (▲), nitrate (□), phosphate (Δ) and chloride (○), (b) total proteins (■) and carbohydrates (□) concentration, and (c) UV-254 absorbance.
release of the organic compounds such as cellulose and proteins hence the significantly reduced weight lose during the thermal treatment [51,52]. The FTIR spectra of the sludge before and after ozonolysis were almost similar with broad bands between 3300 and 3500 cm−1, attributed to NH or OH stretching vibrations as shown in Fig. 6b. The bands around 2900, 1660, 1550, 1040 and 600 cm−1, respectively, corresponded to CeH stretching, CO2 antisymmetric stretching, NH3+ stretching in amino acids, C]O stretching, and CeO stretching in carbohydrates [33]. In the ozonated sludge spectrum, the peaks attributed to the proteins and carbohydrates were reduced indicating their release from the sludge into the aqueous phase during ozonolysis.
Table 2 Weight loss at different phases of combustion of dried WAS before and after ozonolysis. Phase
Before ozonolysis Temperature range (oC)
Weight loss (%)
After Ozonolysis Temperature range (oC)
Weight loss (%)
I II III IV
30–197 197–395 395–480 480–997
7.294 34.127 18.502 6.203
30–194 194–349 349–467 467–997
5.398 38.922 9.704 8.393
Total
30–-1000
66.126
30–1000
62.417
Fig. 6. (a) DTG and TGA curves, and (b) FTIR spectra of dried sludge before (−) and after (…) ozonolysis. 6
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4. Conclusion
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In this study, ozonolysis of waste activated sludge (WAS) obtained from the secondary settler of a municipal wastewater treatment plant has been found to be a feasible pre-treatment before AD. Optimum ozone dosage of 90 mg/min for ozonation duration of 60 min and initial pH of 11 were found significant to save on the energy requirements and carbon content of the pre-treated WAS. It was shown that prolonged ozonolysis beyond 60 min led to the mineralization of the already solubilized organic constituents. This may affect the performance of the ensuing anaerobic process. The study further showed that the ozonolysis mechanism occurred through the rapture of the bacterial cell envelope releasing proteins, humic substances, and particulate organic constituents into the aqueous phase. This was confirmed by thermogravimetric and infrared spectroscopic analysis, which showed the loss of the cellular contents in the ozonated WAS. The solubilization and release of the cellular contents led to an increase in the biodegradability of the WAS by 50%. The ozonolysis also oxidized the organic nitrogen and phosphorus in the protein component of the biomass into nitrates and phosphates, respectively, making the treated effluent nutrient rich and suitable for the next step of anaerobic treatment. Acknowledgment Financial support from the Water Research Commission (WRC, Project no. K5/2773/3), South Africa, is acknowledged. References [1] G. Manterola, I. Uriarte, L. Sancho, The effect of operational parameters of the process of sludge ozonation on the solubilisation of organic and nitrogenous compounds, Water Res. 42 (2008) 3191–3197, https://doi.org/10.1016/j.watres.2008. 03.014. [2] Y. Zhou, H. Zheng, B. Gao, Y. Gu, X. Li, B. Liu, A.M. Jiménez, Waste activated sludge (WAS) dewatering properties of an original hydrophobically modified polyacrylamide containing a cationic microblock structure, RSC Adv. 7 (2017) 28733–28745, https://doi.org/10.1039/c7ra02939j. [3] H. Yuan, B. Yu, P. Cheng, N. Zhu, C. Yin, L. Ying, Pilot-scale study of enhanced anaerobic digestion of waste activated sludge by electrochemical and sodium hypochlorite combination pretreatment, Int. Biodeterior. Biodegrad. 110 (2016) 227–234, https://doi.org/10.1016/j.ibiod.2016.04.001. [4] D. Dobslaw, A. Schulz, S. Helbich, C. Dobslaw, K.H. Engesser, VOC removal and odor abatement by a low-cost plasma enhanced biotrickling filter process, J. Environ. Chem. Eng. 5 (2017) 5501–5511, https://doi.org/10.1016/j.jece.2017.10. 015. [5] Z.S. Wei, H.Q. Li, J.C. He, Q.H. Ye, Q.R. Huang, Y.W. Luo, Removal of dimethyl sulfide by the combination of non-thermal plasma and biological process, Bioresour. Technol. 146 (2013) 451–456, https://doi.org/10.1016/j.biortech.2013. 07.114. [6] O. Demir, A. Filibeli, Fate of return activated sludge after ozonation: an optimization study for sludge disintegration, Environ. Technol. (United Kingdom) 33 (2012) 1869–1878, https://doi.org/10.1080/09593330.2011.650220. [7] S. Tang, Y. Tang, C. Zheng, Z. Zhang, Alkali metal-driven release behaviors of volatiles during sewage sludge pyrolysis, J. Clean. Prod. 203 (2018) 860–872, https:// doi.org/10.1016/j.jclepro.2018.08.312. [8] K.M. Smith, G.D. Fowler, S. Pullket, N.J.D. Graham, Sewage sludge-based adsorbents: a review of their production, properties and use in water treatment applications, Water Res. 43 (2009) 2569–2594, https://doi.org/10.1016/J.WATRES. 2009.02.038. [9] D.F. Lin, M.C. Tsai, The effects of nanomaterials on microstructures of sludge ash cement paste, J. Air Waste Manag. Assoc. 56 (2006) 1146–1154, https://doi.org/ 10.1080/10473289.2006.10464537. [10] G. Silvestre, B. Ruiz, M. Fiter, C. Ferrer, J.G. Berlanga, S. Alonso, A. Canut, Ozonation as a pre-treatment for anaerobic digestion of waste-activated sludge: effect of the ozone doses, Ozone Sci. Eng. 37 (2015) 316–322, https://doi.org/10. 1080/01919512.2014.985817. [11] F. Ali Shah, Q. Mahmood, M. Maroof Shah, A. Pervez, S. Ahmad Asad, Microbial ecology of anaerobic digesters: The key players of anaerobiosis, Sci. World J. 2014 (2014), https://doi.org/10.1155/2014/183752. [12] C.M. Braguglia, A. Gianico, A. Gallipoli, G. Mininni, The impact of sludge pretreatments on mesophilic and thermophilic anaerobic digestion efficiency: role of the organic load, Chem. Eng. J. 270 (2015) 362–371, https://doi.org/10.1016/j.cej. 2015.02.037. [13] C. Bougrier, C. Albasi, J.P. Delgenès, H. Carrère, Effect of ultrasonic, thermal and ozone pre-treatments on waste activated sludge solubilisation and anaerobic biodegradability, Chem. Eng. Process. Process Intensif. 45 (2006) 711–718, https:// doi.org/10.1016/j.cep.2006.02.005.
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[45] J.W. Lee, H.Y. Cha, K.Y. Park, K.G. Song, K.H. Ahn, Operational strategies for an activated sludge process in conjunction with ozone oxidation for zero excess sludge production during winter season, Water Res. 39 (2005) 1199–1204, https://doi. org/10.1016/j.watres.2004.10.004. [46] P.C. Sangave, P.R. Gogate, A.B. Pandit, Combination of ozonation with conventional aerobic oxidation for distillery wastewater treatment, Chemosphere 68 (2007) 32–41, https://doi.org/10.1016/j.chemosphere.2006.12.053. [47] G.P. Sheng, H.Q. Yu, X.Y. Li, Extracellular polymeric substances (EPS) of microbial aggregates in biological wastewater treatment systems: a review, Biotechnol. Adv. 28 (2010) 882–894, https://doi.org/10.1016/j.biotechadv.2010.08.001. [48] M. Anjum, N.H. Al-Makishah, M.A. Barakat, Wastewater sludge stabilization using pre-treatment methods, Process Saf. Environ. Prot. 102 (2016) 615–632, https:// doi.org/10.1016/J.PSEP.2016.05.022. [49] J. Pei, H. Yao, H. Wang, D. Shan, Y. Jiang, L. Ma, X. Yu, Effect of ultrasonic and ozone pre-treatments on pharmaceutical waste activated sludge’s solubilisation, reduction, anaerobic biodegradability and acute biological toxicity, Bioresour. Technol. 192 (2015) 418–423, https://doi.org/10.1016/j.biortech.2015.05.079. [50] S. Tang, C. Zheng, Z. Zhang, Effect of inherent minerals on sewage sludge pyrolysis: product characteristics, kinetics and thermodynamics, Waste Manag. 80 (2018) 175–185, https://doi.org/10.1016/j.wasman.2018.09.012. [51] Y. Lin, Y. Liao, Z. Yu, S. Fang, Y. Lin, Y. Fan, X. Peng, X. Ma, Co-pyrolysis kinetics of sewage sludge and oil shale thermal decomposition using TGA–FTIR analysis, Energy Convers. Manage. 118 (2016) 345–352, https://doi.org/10.1016/J. ENCONMAN.2016.04.004. [52] H. Ren, L. Ma, B. Wang, H. Gong, Z. Yuan, Z. Li, Systematic analysis of the biochemical characteristics of activated sludge during ozonation for lowering of biomass production, Ozone Sci. Eng. 39 (2017) 80–90, https://doi.org/10.1080/ 01919512.2016.1268948.
[37] M. Peña, M. Coca, G. González, R. Rioja, M.T. García, Chemical oxidation of wastewater from molasses fermentation with ozone, Chemosphere 51 (2003) 893–900, https://doi.org/10.1016/S0045-6535(03)00159-0. [38] N. Eqra, Y. Ajabshirchi, M. Sarshar, Effect of ozonolysis pretreatment on enzymatic digestibility of sugarcane bagasse, Agric. Eng. Int. CIGR J. 16 (2014) 151–156, https://doi.org/10.1016/j.biortech.2008.09.012. [39] M.B. Kasiri, N. Modirshahla, H. Mansouri, Decolorization of organic dye solution by ozonation; Optimization with response surface methodology, Int. J. Ind. Chem. Biotechnol. 4 (2013) 3, https://doi.org/10.1186/2228-5547-4-3. [40] D. Bögner, F. Schmachtl, B. Mayr, C.P. Franz, S. Strieben, G. Jaehne, K. Lorkowski, M.J. Slater, Sludge pre-treatment through ozone application: alternative sludge reuse possibilities for recirculating aquaculture system optimization, Ozone Sci. Eng. 00 (2018) 1–13, https://doi.org/10.1080/01919512.2018.1510765. [41] A.C. Gomes, L. Silva, R. Simões, N. Canto, A. Albuquerque, Toxicity reduction and biodegradability enhancement of cork processing wastewaters by ozonation, Water Sci. Technol. 68 (2013) 2214–2219, https://doi.org/10.2166/wst.2013.478. [42] J. Ariunbaatar, A. Panico, L. Frunzo, G. Esposito, P.N.L. Lens, F. Pirozzi, Enhanced anaerobic digestion of food waste by thermal and ozonation pretreatment methods, J. Environ. Manage. 146 (2014) 142–149, https://doi.org/10.1016/j.jenvman. 2014.07.042. [43] E. Fatoorehchi, S. West, G. Abbt-Braun, H. Horn, The molecular weight distribution of dissolved organic carbon after application off different sludge disintegration techniques, Sep. Purif. Technol. 194 (2018) 338–345, https://doi.org/10.1016/j. seppur.2017.11.047. [44] S.T. Yan, L.B. Chu, X.H. Xing, A.F. Yu, X.L. Sun, B. Jurcik, Analysis of the mechanism of sludge ozonation by a combination of biological and chemical approaches, Water Res. 43 (2009) 195–203, https://doi.org/10.1016/j.watres.2008. 09.039.
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Ozonolysis pre-treatment of waste activated sludge for solubilization and biodegradability enhancement
T
⁎
Benton Otienoa, , Seth Apollob,c, John Kabubac, Bobby Naidooa, Geoffrey Simated, Aoyi Ochienge a
Department of Chemistry, Vaal University of Technology, Private Bag X021, Vanderbijlpark, South Africa Department of Chemistry, University of Embu, P.O. Box 6-60100, Embu, Kenya c Department of Chemical Engineering, Vaal University of Technology, Private Bag X021, Vanderbijlpark, South Africa d Faculty of Engineering and the Built Environment, University of the Witwatersrand, Johannesburg, South Africa e Botswana International University of Science and Technology, Private Bag 16, Palapye, Botswana b
A R T I C LE I N FO
A B S T R A C T
Keywords: Ozone Waste activated sludge Biodegradability enhancement Cell wall Solubilization
Handling of waste activated sludge (WAS) has been a great problem due to its low biodegradability which is contributed by the biorecalcitrant bacterial cells from the activated sludge process. Moreover, the sludge is poorly soluble which makes it difficult to subject to a further biological treatment downstream. This study investigated the application of ozonolysis pre-treatment for the solubilization and biodegradability enhancement of municipal WAS. Ozonolysis led to sludge solubilization as was shown by a 50% reduction in total suspended solids (TSS). Subsequently, the dissolved organic carbon (DOC) increased by 25%, and sludge biodegradability increased two-folds. The mechanism of sludge solubilization was explained through cell content analysis which revealed biomass cell wall rapture during ozonolysis leading to the release of the cellular contents including humic substances, proteins, phosphates, nitrates and carbohydrates into the supernatant. The soluble proteins decreased by 98% while carbohydrates increased four-fold. Additionally, sulphates in the supernatant increased from 18 mg/L to 45 mg/L while nitrates increased from 0 mg/L to 85 mg/L indicating oxidation of the organic nitrogen and phosphorus which are constituents of the biomass cell protein; this can provide nutrient balance in the downstream biological treatment method. Thermogravimetric (TGA) analysis showed that the weight loss for ozone-treated WAS was 10% while that for the raw WAS was 20% further indicating that ozonolysis had solubilized the volatile and organic components of the treated WAS. Ozonolysis is a promising technology for the pre-treatment of WAS before anaerobic digestion for stabilization and energy recovery.
1. Introduction Municipal wastewater (MWW) generated from different domestic sources such as toilets, showers, and the kitchen is widely treated through activated sludge treatment (AST) process. Waste activated sludge (WAS) is a major by-product of the AST process and is generated in large amounts [1,2]. The WAS consists majorly of biomass from cell growth and decay during AST process as well as traces of odorous and volatile organic compounds, and toxic chemicals like heavy metals, thus posing the risk of secondary environmental pollution if not stabilized [3–5]. The treatment and management of the excess WAS accounts for between 25–60% of the total costs of a municipal wastewater treatment plants [2,6]. Moreover, the conventional treatment methods such as landfilling, incineration, soil utilization and use as feedstock (based on the inherent minerals in sludge after pyrolysis) for making ceramics, activated carbon and cement paste, are restricted by various ⁎
extents in practice [7–9]. Recently the enhancement of anaerobic digestion (AD) process for WAS stabilization, reduction, and energy recovery in the form of methane gas has aroused great interest [10]. However, AD of WAS suffers high retention times, low methanogenic production and an overall 30–35% dry solids degradation [11]. The insoluble and biorecalcitrant proteinaceous biomass cell walls and extracellular polymeric substances (EPS) in WAS hinders the initial rate determining hydrolysis step of AD. During hydrolysis, enzyme-mediated solubilization of insoluble organic materials, the rapture of hard cell walls and degradation of EPS occur releasing biodegradable organic compounds for the ensuing acidogenesis step [12]. A long retention time resulting in the consumption of a large amount of energy is, therefore, required for the hydrolysis of WAS [3,11,13], limiting the effective application of AD in the treatment and stabilization of WAS [14]. To improve the AD process and meet the strict regulation on
Corresponding author. E-mail address: [email protected] (B. Otieno).
https://doi.org/10.1016/j.jece.2019.102945 Received 11 December 2018; Received in revised form 1 February 2019; Accepted 5 February 2019 Available online 07 February 2019 2213-3437/ © 2019 Elsevier Ltd. All rights reserved.
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environmental protection, implementation of a pre-treatment method, that would enhance the hydrolysis step is necessary [2,6]. Several methods for sludge solubilization including chemical treatment using acids and alkalis, mechanical disintegration, and advanced oxidation processes (AOPs) have been applied to date with varying degrees of success [14–17]. Of these methods, ozonolysis, which is an AOP is considered most effective for sludge disintegration, because it does not lead to a significant increase in salt concentration and has no chemical residues in comparison to other pre-treatment techniques [18]. Through ozonolysis, organic matter can be released from the sludge into the solution as a result of the breaking and solubilization of the hard cellular membrane [14]. The role of ozonolysis pre-treatment is to partially oxidize the biorecalcitrant component of WAS to release biodegradable products [4]. Through ozonolysis, low soluble volatile organic compounds (VOCs), can be oxidized and solubilized as well thus contributing to the improved biodegradability [5,19]. To avoid overoxidation of the released biodegradable components, the ozonolysis dose and time have to be optimized [20]. The bacteria in the activated sludge (AS) process are known to simultaneously remove organic compounds and major inorganic nutrients present in municipal wastewater such as sulphates, nitrates, and phosphates through assimilation or accumulation within the hard biomass cell walls forming part of the WAS [21,22]. Also, phosphorus removal during the AS process through accumulation in the cell biomass may lead to its increased concentration in the WAS [23]. Subsequently, during ozonolysis, the disintegration of the cell biomass may release phosphorous compounds in excessive amounts [22]. Similarly, the assimilated nitrates/nitrogen which forms an important component of cell wall proteins may be released as nitrates, ammonia or nitrites [22–24]. It is thus significant to understand the impact of ozonolysis on the degradation of organic or bound phosphorus, sulphate, and nitrogen to determine if it leads to excessive release of the cell-bound nutrients or adequate amounts that are suitable for an ensued biological process. The aim of the current study was to carry out ozonolysis pre-treatment of WAS for sludge solubilization and to improve biodegradability. Of major interest was the understanding of the ozonolysis process based on the rapture of the proteinaceous cell wall and EPS leading to nutrients (proteins, carbohydrates, nitrates, and phosphates) speciation, their release into the supernatant and suitability for a possible ensued AD process. Also, the sludge properties were determined before and after ozonolysis through thermogravimetric and infrared spectroscopic studies.
Table 1 Physicochemical characteristics of municipal waste activated sludge. Parameter
Units
pH TSS CODT CODS DOC BOD5 TS VSS BOD5:COD Sulphate Phosphate Chloride Soluble proteins
mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L mg/L
Value 6.7 12.4 6860 1408 122 901 65.3 42.1 0.1 18 67 55 20
2.3. Ozonolysis experiments The waste activated sludge was treated in a 4 L ozone reactor batchwise for sludge solubilization and improved biodegradability. For every batch experiment, 2 L of WAS was transferred to the reactor then subjected to ozonolysis. Gaseous ozone was generated from air using an ozone generator coupled to an air compressor. The ozone generator employed the corona discharge principle, where a high voltage corona mechanism converts the oxygen in air to ozone [25,26]. The resulting ozone air mixture was then bubbled in an up-flow mode in the ozone reactor via a gas diffuser. The ozone concentration in the mixture was determined following a volumetric method based on the reaction between potassium iodide (KI) and ozone [27]. To obtain different ozone dosages, the ozone flow rate was controlled using a manual valve and the flow measured using an air rotameter. For instance, an air flow rate of 2 L/min generated air-ozone mixture with a dosage of 45 mg O3 per minute supplied to the reactor. Air flow rates of 4, 6 and 8 L/min generated dosages of 90, 135 and 180 mg O3 per minute. The unutilized ozone gas from the reactor was destroyed by passing through the KI solution. The schematic diagram of the ozonolysis process is given in Fig. 1. 2.4. Average oxidation state
2. Materials and methods
The average oxidation state (AOS) of the WAS during ozonolysis was used to estimate the degree of oxidation of the substrate containing the oxidation by-products as well as the initial compounds. The AOS was calculated as;
2.1. Materials and methods
AOS = 4 ×
Sodium thiosulphate, phenol, bovine serum albumin (BSA), methanol, phosphoric acid, Bradford reagent, Coomassie brilliant blue, potassium iodide (KI), sulphuric acid, hydrochloric acid, silver sulfate, potassium dichromate, starch, and sodium hydroxide were all purchased from Merck Limited, South Africa. The chemicals were of analytical grade and were used as received. All standard and working solutions were prepared using ultra-pure distilled water.
where CODT and TOC are the total chemical oxygen demand and total organic carbon, respectively. The values for AOS vary from −4 (for CH4 the most reduced state for C) to +4 (for CO2 the most oxidized state for C). In an oxidation process, however, the increment observed in AOS (ΔAOS), can be calculated by;
TOC −
12 32
× CODT
TOC
CODTf ⎞ CODTi 12 ΔAOS = AOSf − AOSi = 4 × ⎜⎛ − ⎟ × DOCf ⎠ 32 ⎝ DOCi
(1)
(2)
where i and f, respectively, refer to initial and final values. The ΔAOS is of more importance as it indicates the degree of oxidation [28].
2.2. Waste activated sludge The waste activated sludge (WAS) was sourced from a secondary settler of a local municipal wastewater treatment (WWT) plant in Vanderbijlpark, Gauteng-South Africa. The WAS was filtered through a 450 μm microfilter to remove large solid particles then further analyzed to determine its physical and chemical characteristics. Table 1 gives the determined characteristics of the sludge.
2.5. Ozone transfer The amount of ozone in the inlet and outlet gas streams was determined for the different flow rates by titrating the ozonated KI solutions with potassium thiosulphate solution. Ozone transfer, which is the amount of ozone used up in the reaction was determined by; 2
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Fig. 1. Schematic representation of the ozonolysis unit.
O3 T =
O3 U X 100 O3 S
COD value of 600 mg/L was expected. However, the higher soluble COD value of 1408 mg/L reported was partly contributed to by the oxidation of ammonia, soluble proteins (20 mg/L) and other soluble inorganic compounds present in the WAS. The BOD5:COD ratio of 0.1 confirmed the poor biodegradability of the WAS. The low biodegradability is attributed to the proteinaceous biomass cell wall which hinders the rate determining hydrolysis step of AD [11]. Therefore, ozonolysis should be applied for the solubilization of the biorecalcitrant biomass cells.
(3)
where, O3 T , O3 S and O3 U are the ozone transfer (%), supplied and utilized ozone (mg), respectively. The utilized ozone was determined from the difference obtained between the ozone supplied to the reactor and the ozone coming out of the reactor (off gas). 2.6. Chemical analysis Aliquot samples were withdrawn and analyzed for pH, total organic carbon (TOC), COD, BOD, total suspended solids (TSS), total solids (TS), dissolved organic carbon (DOC) total volatile solids (VSS), nitrates, phosphates, proteins, carbohydrates, chlorides, and aromaticity (UV254). The COD, BOD, TSS, TS, and VSS were analyzed according to the standard methods of analysis [27]. Aromaticity was determined by UV absorption measurement (T80 + UV/vis Spectrometer, PG Instruments Ltd) at the maximum absorption wavelength (λ) of 254 nm [29], while the cations concentration was determined from Ion Chromatography (882 Compact IC plus – Cation: intelligent ion chromatograph for determining anions and cations without chemical suppression). Fourier transform infrared, FTIR, analysis (Perkin- Elmer FT-IR/NIR Spectrometer, PerkinElmer, Waltham, MA, USA) was used in determining the presence of functional groups in the WAS. Differential thermal gravimetric (DTG) and thermal gravimetric (TGA) analyses were performed on Perkin-Elmer Thermogravimetric analyzer TGA 400 instrument, to determine the thermal decomposition behavior of the dried WAS before and after ozonolysis. The amount of unutilized ozone captured in KI iodide solution was determined by titrimetric method [30,31]. The concentration of carbohydrates and proteins in the supernatant was determined using the Bradford assay and phenol-sulphuric acid methods, respectively [32,33].
3.2. Effect of ozone dosage The effect of ozone dosage at different ozone flowrates is given in Fig. 2. Selecting an appropriate ozone dosage is very important in optimizing the ozonolysis process. Low ozone dose would achieve low solubilization while an over-dose would lead to mineralization of already solubilized organics and a high concentration of ozone in the offgas. As a result, the process will not be economical. After 60 min of ozonolysis, the highest TSS reduction of 53% (12.4–5.6 mg/L) was achieved at a flow rate of 90 mg/min, while the lowest flow rate of 45 mg/min removed 47% of the TSS (Fig. 2a). A corresponding observation was made with the evolution of DOC with the highest increase of 25% (from 120 to 152 mg/L) attained at the 90 mg/min flow rate (Fig. 2b). An increase in ozone flow rate from 45 to 90 mg/min led to an increase in ozone concentration in the reactor which adequately dissolved the organic suspended solids leading to the reduced TSS. However, as the flow rate increased further, the TSS reduction decreased despite the increasing amount of supplied O3 molecules. At high ozone flow rates above the optimal dose, the supplied gas forms larger bubbles which significantly reduces gas hold-up resulting in reduced contact time between O3 and the suspended solids leading to reduced efficiency [34,35].
3. Results and discussion 3.3. Effect of initial pH 3.1. Characteristics of waste activated sludge Highest TSS reduction of 47% (12.4–6.5 mg/L) was obtained at pH 11 as compared to 36% and 19% reductions at pHs of 7 and 3, respectively (Fig. 3). Similarly, the highest increase in DOC evolution of 55% (122–190 mg/L) was achieved at pH 11 and the lowest value of 23% at pH 3. The reduction in TSS and increase in DOC are attributable to the solubilization of the suspended organic matter. Under acidic conditions, ozone selectively attacks parts of organic compounds
The physical and chemical characteristics of the obtained waste activated sludge are given in Table 1. The WAS had a soluble COD (CODS) of 1400 mg/L which was 20% of the total COD (CODT) indicating that up to 80% of the COD was in the suspended solids. The DOC value of 122 mg/L also indicated a low concentration of the organic compounds in soluble form. With the DOC of 122 mg/L, a soluble 3
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Fig. 2. Effect of ozone dosage on; (a) TSS reduction after 60 min of ozonolysis and (b) evolution of DOC at different flow rates of 45 (■), 90 (○), 135 (□) and 180 (●) mg/min.
within the first 60 min. Based on these findings, the duration of ozonolysis should be minimized to reduce excessive energy consumption and depletion of carbon if anaerobic digestion is to be applied downstream to the ozonolysis process. [43–45]. As observed in Fig. 4b, the ozone transfer efficiency decreased with increasing duration of ozonolysis after every 20 min interval. In the first three intervals (60 min of ozonolysis) more than 90% transfer was achieved and was considered efficient. As the ozonolysis continued beyond the 60th minute, more of the target organic contaminants were already degraded leading to reduced ozone transfers of 84, 81 and 76% in the 4th, 5th and 6th intervals, respectively. Improved biodegradability due to ozonolysis was confirmed by the increases in AOS from −1.5 to 0.9 as shown in Fig. 4c. Also, in Fig. 4d, the BOD5:COD ratio increased from 0.1 to 0.21 while the pH decreased from 11 to 9.68. The increase in oxidation state could be attributed to the solubilization of the organic solids by degradation of the aromatic groups and formation of aliphatic structures with functional groups such as eCOOH, eOH and eCHO as indicated by the FTIR analysis shown in Fig. 6b. Aliphatic structures contain higher oxidation state carbons than aromatic carbons. Moreover, the decrease in basicity could be due to the formation of the easily biodegradable acidic intermediate compounds such as acetic acids and humic substances [46].
containing C]C double bonds, aromatic rings, and negatively charged centers/sites containing atoms such as N, P, O, S [36]. The selective attack leads to partial solubilization hence the low increase in DOC [37]. The performance of ozone is superior under alkaline conditions because its half-life increases from 15 min, which is the case in an acidic/neutral condition, to 25 min after which it decomposes leading to the formation of the highly reactive hydroxyl radicals (OH%). The OH% radicals unselectively react with all the organic compounds hence the highest increase in DOC at occurred pH 11 [38–40]. 3.4. Change in COD, biodegradability, and ozone transfer Ozonolysis of the WAS was carried out at the optimal conditions of pH 11 and ozone flow rate of 90 mg/min. Fig. 4 shows the changes in total COD (CODT) and soluble (CODS), pH, BOD5:COD ratio, AOS, and ozone transfer. The CODT decreased by 42% after 2 h of ozonolysis (Fig. 4a). Consequently, the CODS increased by 41% after 60 min of ozonolysis then reduced with additional ozonolysis time. The decrease in CODT and the subsequent increase in CODS was attributed to the solubilization of the suspended solids (sludge) present in the WAS. However, the extended ozonolysis period past the 60th minute could have led to mineralization of the already solubilized organic matter hence the observed decrease in CODS [41,42]. Also, the low reduction in CODT during the extended period from the 80th minute indicates that most of the oxidizable organic solids had already been solubilized
Fig. 3. Effect of pH on ozonolysis of WAS on; (a) TSS reduction after 60 min of ozonolysis and (b) evolution of DOC at different pHs of 3 (Δ), 7 (□) and 11 (○) at ozone dosage of 90 mg/min. 4
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Fig. 4. Change in; (a) CODT (○) and CODS (●), and (b) BOD5:COD (□) and pH (▲), and (c) AOS, and (d) ozone transfer during ozonolysis at pH 11 and 90 mg/min ozone dosage.
the aqueous phase as a result of the disintegration of EPS and solubilization of the organic suspended matter [49]. The slight reduction in UV-254 absorption after 60 min could have resulted from the mineralization of the already solubilized organic constituents.
3.5. Sludge solubilization mechanism The mechanism of WAS solubilization was monitored through changes in UV-254 absorption, and cations, proteins and carbohydrates concentration in the supernatant as shown in Fig. 5. The WAS organic fraction majorly consists of biomass cell with a structure that includes hard cell walls, membrane cell nuclei, and extracellular polymeric substances (EPS). The hard cell walls and EPS which are the major constituents of the WAS organic fraction are mainly composed (up to 80%) of polysaccharides (PS), proteins (PS) and humic substances. Moreover, during the AST process, the microorganisms absorb nutrients such as phosphates, sulphates, and nitrates leading to the development of the hard cell walls [16,22,47,48]. The ozonolysis of WAS caused the hard cell walls to rapture and solubilized the EPS thereby releasing the cellular contents. This led to the observed increase in sulphates (18 to 45 mg/L) and nitrates (0 to 85 mg/L) as shown in Fig. 5a, and carbohydrates from 0 to 102 mg/L (Fig. 5b). Whereas an increase in protein concentration was expected as reported by Zang et al. [16], the observed decrease from 20 to 0.5 mg/L (Fig. 5b) could have resulted from the oxidation of the released protein by ozone into nitrates hence the significant increase in the dissolved nitrate concentration observed (Fig. 5a). Ozonolysis did not affect the chloride while the phosphate concentration reduced from 67 to 37 mg/L despite an expected increase given the release of the cellular phosphorus content. Metal elements such as Al, Fe, Mg and Ca present in WAS could have combined with the phosphate ions forming poorly soluble compounds hence the decrease [22]. Organic compounds such as the humic substances that commonly exist in wastewater strongly absorb UV radiation at λ of 254 nm [27]. Thus, a correlation may exist between their concentration and UV absorption. In the first 60 min of ozonolysis (Fig. 5c) the UV-254 absorption increased signifying the increase of the organic constituents in
3.6. Thermal gravimetric and FTIR analyses of WAS The effect of ozonolysis on sludge solubilization was further monitored through characterization of raw (before ozonolysis) and treated (after ozonolysis) WAS using FTIR and TGA analyses (Fig. 6). The WAS substrates were first dried at 100 °C for 1 h before analysis. Fig. 6a gives the thermogravimetric behavior of the WAS before and after ozonolysis, heated from 30 to 1000 °C at a constant heating rate of 30 °C per minute. Table 2 gives the weight loses at the different phases of combustion. The TGA curves were almost similar from 30 °C to around 420 °C where they intersected. Beyond 420 °C the curve for WAS before ozonolysis was greater than that after ozonolysis indicating that the residues had a greater mass for the former than the later [50]. The reduced mass of residues of the WAS after ozonolysis was caused by the solubilization of the organic content of the sludge such as the hard cell walls, which mainly decomposed in the temperature range between 180 and 480 °C. In phase I of combustion (Table 2) for the sludge before and after ozonolysis, weight loses of 34, and 39% were observed, respectively. Ozonolysis had led to the rapture of cells allowing for the release of the pore and bound water during thermal treatment hence the higher weight loss for the WAS after ozonolysis than the WAS before ozonolysis in this phase. In the subsequent phase II, the WAS before ozonolysis had 20% weight loss as compared to 10% for the WAS after ozonolysis. The higher weight loss of 20% was attributed to the thermal decomposition of proteins and cellulose in the WAS. In the ozonated WAS, however, ozonolysis had already led to the decomposition and 5
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Fig. 5. Change in (a) cations concentration sulfate (▲), nitrate (□), phosphate (Δ) and chloride (○), (b) total proteins (■) and carbohydrates (□) concentration, and (c) UV-254 absorbance.
release of the organic compounds such as cellulose and proteins hence the significantly reduced weight lose during the thermal treatment [51,52]. The FTIR spectra of the sludge before and after ozonolysis were almost similar with broad bands between 3300 and 3500 cm−1, attributed to NH or OH stretching vibrations as shown in Fig. 6b. The bands around 2900, 1660, 1550, 1040 and 600 cm−1, respectively, corresponded to CeH stretching, CO2 antisymmetric stretching, NH3+ stretching in amino acids, C]O stretching, and CeO stretching in carbohydrates [33]. In the ozonated sludge spectrum, the peaks attributed to the proteins and carbohydrates were reduced indicating their release from the sludge into the aqueous phase during ozonolysis.
Table 2 Weight loss at different phases of combustion of dried WAS before and after ozonolysis. Phase
Before ozonolysis Temperature range (oC)
Weight loss (%)
After Ozonolysis Temperature range (oC)
Weight loss (%)
I II III IV
30–197 197–395 395–480 480–997
7.294 34.127 18.502 6.203
30–194 194–349 349–467 467–997
5.398 38.922 9.704 8.393
Total
30–-1000
66.126
30–1000
62.417
Fig. 6. (a) DTG and TGA curves, and (b) FTIR spectra of dried sludge before (−) and after (…) ozonolysis. 6
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4. Conclusion
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In this study, ozonolysis of waste activated sludge (WAS) obtained from the secondary settler of a municipal wastewater treatment plant has been found to be a feasible pre-treatment before AD. Optimum ozone dosage of 90 mg/min for ozonation duration of 60 min and initial pH of 11 were found significant to save on the energy requirements and carbon content of the pre-treated WAS. It was shown that prolonged ozonolysis beyond 60 min led to the mineralization of the already solubilized organic constituents. This may affect the performance of the ensuing anaerobic process. The study further showed that the ozonolysis mechanism occurred through the rapture of the bacterial cell envelope releasing proteins, humic substances, and particulate organic constituents into the aqueous phase. This was confirmed by thermogravimetric and infrared spectroscopic analysis, which showed the loss of the cellular contents in the ozonated WAS. The solubilization and release of the cellular contents led to an increase in the biodegradability of the WAS by 50%. The ozonolysis also oxidized the organic nitrogen and phosphorus in the protein component of the biomass into nitrates and phosphates, respectively, making the treated effluent nutrient rich and suitable for the next step of anaerobic treatment. Acknowledgment Financial support from the Water Research Commission (WRC, Project no. K5/2773/3), South Africa, is acknowledged. References [1] G. Manterola, I. Uriarte, L. Sancho, The effect of operational parameters of the process of sludge ozonation on the solubilisation of organic and nitrogenous compounds, Water Res. 42 (2008) 3191–3197, https://doi.org/10.1016/j.watres.2008. 03.014. [2] Y. Zhou, H. Zheng, B. Gao, Y. Gu, X. Li, B. Liu, A.M. Jiménez, Waste activated sludge (WAS) dewatering properties of an original hydrophobically modified polyacrylamide containing a cationic microblock structure, RSC Adv. 7 (2017) 28733–28745, https://doi.org/10.1039/c7ra02939j. [3] H. Yuan, B. Yu, P. Cheng, N. Zhu, C. Yin, L. Ying, Pilot-scale study of enhanced anaerobic digestion of waste activated sludge by electrochemical and sodium hypochlorite combination pretreatment, Int. Biodeterior. Biodegrad. 110 (2016) 227–234, https://doi.org/10.1016/j.ibiod.2016.04.001. [4] D. Dobslaw, A. Schulz, S. Helbich, C. Dobslaw, K.H. Engesser, VOC removal and odor abatement by a low-cost plasma enhanced biotrickling filter process, J. Environ. Chem. Eng. 5 (2017) 5501–5511, https://doi.org/10.1016/j.jece.2017.10. 015. [5] Z.S. Wei, H.Q. Li, J.C. He, Q.H. Ye, Q.R. Huang, Y.W. Luo, Removal of dimethyl sulfide by the combination of non-thermal plasma and biological process, Bioresour. Technol. 146 (2013) 451–456, https://doi.org/10.1016/j.biortech.2013. 07.114. [6] O. Demir, A. Filibeli, Fate of return activated sludge after ozonation: an optimization study for sludge disintegration, Environ. Technol. (United Kingdom) 33 (2012) 1869–1878, https://doi.org/10.1080/09593330.2011.650220. [7] S. Tang, Y. Tang, C. Zheng, Z. Zhang, Alkali metal-driven release behaviors of volatiles during sewage sludge pyrolysis, J. Clean. Prod. 203 (2018) 860–872, https:// doi.org/10.1016/j.jclepro.2018.08.312. [8] K.M. Smith, G.D. Fowler, S. Pullket, N.J.D. Graham, Sewage sludge-based adsorbents: a review of their production, properties and use in water treatment applications, Water Res. 43 (2009) 2569–2594, https://doi.org/10.1016/J.WATRES. 2009.02.038. [9] D.F. Lin, M.C. Tsai, The effects of nanomaterials on microstructures of sludge ash cement paste, J. Air Waste Manag. Assoc. 56 (2006) 1146–1154, https://doi.org/ 10.1080/10473289.2006.10464537. [10] G. Silvestre, B. Ruiz, M. Fiter, C. Ferrer, J.G. Berlanga, S. Alonso, A. Canut, Ozonation as a pre-treatment for anaerobic digestion of waste-activated sludge: effect of the ozone doses, Ozone Sci. Eng. 37 (2015) 316–322, https://doi.org/10. 1080/01919512.2014.985817. [11] F. Ali Shah, Q. Mahmood, M. Maroof Shah, A. Pervez, S. Ahmad Asad, Microbial ecology of anaerobic digesters: The key players of anaerobiosis, Sci. World J. 2014 (2014), https://doi.org/10.1155/2014/183752. [12] C.M. Braguglia, A. Gianico, A. Gallipoli, G. Mininni, The impact of sludge pretreatments on mesophilic and thermophilic anaerobic digestion efficiency: role of the organic load, Chem. Eng. J. 270 (2015) 362–371, https://doi.org/10.1016/j.cej. 2015.02.037. [13] C. Bougrier, C. Albasi, J.P. Delgenès, H. Carrère, Effect of ultrasonic, thermal and ozone pre-treatments on waste activated sludge solubilisation and anaerobic biodegradability, Chem. Eng. Process. Process Intensif. 45 (2006) 711–718, https:// doi.org/10.1016/j.cep.2006.02.005.
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