Having taken all aspects into account, the pre-treatments clearly increased the soluble fraction of organic matter.
This stems mainly from oxidative properties of the pre-treatments employed in this study that disrupt cell walls and EPS [ 17 , 19 ]. More organic matter was solubilized when higher concentrations of pre-treatment reagents were used. When it comes to effectiveness of the pre-treatments in solubilizing different types of organic matter, the pre-treatments were much more influential in solubilizing protein than polysaccharide. The amount of soluble fraction of organic matter is enhanced slightly in the control reactor after pre-treatment time, suggesting some solubalisation with no chemical additives and pH control.
Biogas Production: Pretreatment Methods in Anaerobic Digestion
The improvement of organic solubalisation is consistent with but slightly higher than the results achieved in previous studies, in which soluble fraction of organic matter was analogously augmented [ 9 , 18 , 25 , 28 , 29 , 30 ]. This can be attributed to sludge characteristics and the concentration of the substances used in this study. Whether alone or combined, FNA and Fenton pre-treatments enhanced methane production, affirming effectiveness of these pre-treatments in improving methane production from anaerobic digestion process.
This stems from the release of more readily biodegradable organic matter in pre-treated bioreactors Figs. The higher concentrations of the pre-treatment constituents resulted in enhanced biogas production attributable to providing more soluble organic matter for anaerobic organisms. This could be because the free radicals released by each pre-treatment hydrolyze different types of organic matter in the substrate that resulted in enhanced biodegradability of organic matter and methane production in biochemical methane potential reactors.
From an economic and environmental perspective, the enhancement of methane production is of great significance because not only could higher renewable energy be generated, but also methane emission into the atmosphere as a major greenhouse gas could be decreased from anaerobic digestion of sewage sludge [ 31 ]. The difference can be mainly attributed to the different digestion time, characterizations of the substrate and the inoculum employed in this study. The amount of methane enhancement caused by singular Fenton pre-treatment was Compared to combined FNA and thermal pre-treatment used in Wang et al.
However, different sludge specifications and using higher dosages of FNA should be considered for more precise comparison.
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- 1. Introduction.
The COD of digested sludge is one of the most important characteristics in applying the sludge to agricultural lands and forests. The enhanced COD removal in this study is of paramount importance because it lowers the amount of sludge and reduces the associated costs of transport. It also paves the way for shaping an integrated, sustainable system for treating sewage sludge by applying sludge to agricultural lands and forests.
Despite the significant results this research represents, applying new systems to anaerobic digestion of sewage sludge entails a great deal of money and time. Therefore, in future studies, a precise economic assessment for full-scale application of combined FNA and Fenton pre-treatments seems indispensable. Additionally, the influence of the pre-treatments on the microbial community should be put into perspective to provide deep insight into the mechanism of these pre-treatments and possible long-term side-effects on microbial behaviour.
This study investigated the feasibility of enhancing anaerobic digestion of mixed primary sludge and waste activated sludge through combined FNA and Fenton pre-treatment. Combined FNA and Fenton pre-treatments were shown to increase soluble fractions of organic matter considerably more than these pre-treatments alone, resulting in enhanced biodegradability of organic matter, biogas production, methane production, and COD removal during the anaerobic digestion process.
The improved methane production is of paramount importance, not only because higher amounts of renewable energy are obtained from the anaerobic digestion process, but also because lower methane emission, a major greenhouse gas, is released to the atmosphere. The improved COD of the digested sludge paves the way for having a more integrated and sustainable sludge treatment process, as sludge transport expenditures are reduced and the digested sludge achieves a higher potential application to agricultural lands.
Additionally, combined FNA and Fenton pre-treatment is potentially an economically attractive and environmentally friendly technology, particularly considering that both are obtainable as by-products from anaerobic digestion of sewage sludge. Biopolymers proteins and polysaccharides were measured in the soluble phase before and after pre-treatment.
Phenol with sulfuric acid was also used for measuring polysaccharide [ 35 ]. These methods have been widely used to determine the proteins and polysaccharides concentration, even in sludge pre-treatment studies to determine the effect of pre-treatments on sludge properties [ 9 , 13 , 25 , 26 , 36 , 37 ]. The volume of biogas was measured by liquid displacement method [ 38 ].
The biogas was collected from the bioreactors in Tedlar gas bags before using in GC measurements. One-liter reactors were used when carrying out FNA pre-treatment. The considered FNA concentration is obtained from economic assessments from literature review [ 18 , 40 , 41 , 42 ].
With the FNA concentration, the nitrite salt concentration can be calculated.
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In this experiment, the pH of the mixed sludge was set at 5. Fenton pre-treatments were also conducted in 1-L reactors. The designated concentrations of H 2 O 2 for the Fenton reaction were considered from the literature review [ 19 , 23 , 30 , 31 ]. First, the pH of the mixed sludge was set to three by H 2 SO 4 [ 22 ].
For combined pre-treatments, the FNA pre-treatment was first conducted, and after 5-h exposure time, the Fenton pre-treatment with 1-h exposure time was conducted. To keep the pH of the reactors stable, the pH was regularly measured during the treatment process. During FNA pre-treatment, the pH of the mixture remained stable at 5. Biochemical methane potential system is shown in Fig.
A Review of the Processes, Parameters, and Optimization of Anaerobic Digestion
The ratio between inoculum and mixed sludge was 2 for proper performance of bioreactors, according to Boulanger et al. A control test mixed sludge without chemical additives and pH control was also conducted. After mixing the substrate and inoculum, the reactors were completely sealed to provide a strict anaerobic environment for better performance of anaerobic organisms. Schematic of experimental system. For assessing the influence of nitrite on inoculum performance, two sets of blanks were considered.
Blank I, in which inoculum without chemical additives and substrate was used in biochemical methane potential tests. Assessing bioreactors I and II, it was shown that the concentration of nitrite used in this study has no a significant effect on inoculum performance. This is in agreement with previous studies, in which no significant influences on inoculum performance were analogously observed [ 9 , 18 , 26 , 28 ].
The methane production from mixed sludge was obtained by subtracting measured biogas production in an experimental bioreactor from that measured in blank I. Data analysis and graph processing were carried out with Microsoft Excel software Low-temperature thermal pre-treatment of municipal wastewater sludge: process optimization and effects on solubilization and anaerobic degradation.
Water Res. Advanced sludge treatment affects extracellular polymeric substances to improve activated sludge dewatering. J Hazard Mater. Influence of low temperature thermal pre-treatment on sludge solubilisation, heavy metal release and anaerobic digestion.
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