Chapter 4 Effects of mixing cooked potato on the treatment of potato
5.3 Results and discussion
5.3.1 Characteristics of dissolved organic matter formed in aerobic and anaerobic
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Based on the determined δ13C, the mass fractions of potato and WAS in their mixtures remaining after treatment were estimated according to the following two source model (Li et al., 2013):
fp + fw = 1 㻌 㻌 㻌 㻌 㻌 㻌 㻌 㻌 (17) fp · Xp + fw · Xw = Xs 㻌 㻌 㻌 㻌 㻌 㻌 㻌 㻌 㻌 㻌 㻌 㻌 (18)
where, Xp and Xw are the determined δ13C for fresh potato and WAS, respectively; Xs
is the determined δ13C for the mixed feedstock after MFC treatment, and fp and fw are the corresponding mass fractions of potato and WAS to be estimated.
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(biodegradable and less or non-biodegradable) contained in water, wastewater or various biomass samples, UV260 only reflects those possessing UV-absorbing features and, in most environmental samples, humic and fulvic acids that are considered not biodegradable.
Fig. 21 The observed and calculated profiles of TOC in the aerobic (a) and anaerobic (b) reactors operated at 20 qC with the TS concentrations of 1.2, 2.3 and 5.2 %.
The concentrations of TOC and UV260 at the end of digestion for 61 days are used to compare the differences of DOM resulting from different digestion conditions and are plotted in Fig. 23. And, the values of SUVA are plotted in Fig. 24. At 20 ºC, the values of TOC with anaerobic digestion were obviously higher than those with aerobic
0 100 200 300 400 500 600 700
0 10 20 30 40 50 60 70
500 1000 1500 2000 2500 3000 3500
Observed
(b)
20 㼛㻯㻘㻌㼀㻿㻌㻩㻌㻝㻚㻞㻌㻑 20 㼛㻯㻘㻌㼀㻿㻌㻩㻌㻞㻚㻟㻌㻑 20 㼛㻯㻘㻌㼀㻿㻌㻩㻌㻡㻚㻞㻌㻑 Calculated
(a)
TOC (mg/L)
Time (day)
80
digestion. However, at both 5 and 35 ºC, the TOC differences between the aerobic and anaerobic conditions were becoming smaller.
Fig. 22 The observed and calculated profiles of TOC in the aerobic (a) and anaerobic (b) reactors under the temperature of 5, 20 and 35 ℃ with the TS concentration of 1.2 %.
For UV260, the value differences between the aerobic and anaerobic conditions were less apparent. A general trend of increases of TOC and UV260 with increases of either the TS concentration or the temperature was revealed except for the anaerobic condition with TS of 1.2 % and temperature of 5 ºC.
0 20 40 60 80 100 120 140
0 10 20 30 40 50 60 70
0 80 160 240 320
400 㼀㻿㻌㻩㻌㻝㻚㻞㻌㻑, 5 䉝
㼀㻿㻌㻩㻌㻝㻚㻞㻌㻑, 20 䉝㻌 㼀㻿㻌㻩㻌㻝㻚㻞㻌㻑, 35 䉝
(a)
Calculated Observed
(b)
TOC (mg/L)
Time (day)
81
Fig. 23 The concentrations of TOC and UV260 at the end of digestion for 61 days in the aerobic and anaerobic reactors under different TS concentrations and temperatures. *The value for day 46.
For SUVA, except for the anaerobic condition with the TS of 1.2 % and the temperature of 5 and 35 qC, its values with the aerobic digestion were significantly higher than those with the anaerobic digestion, suggesting that more humic and/or humic-like substances were formed and remaining during aerobic digestion of EAS. An exception existed for the condition at 5 qC, where higher SUVA was observed for anaerobic digestion.
TS = 1.
2 %, 35
0C••
TS = 1.2 % , 20
0C••
20
0C, TS = 5.
2 %
20
0C, TS = 2.
3 %
TS = 1.
2 %, 5
0C••
20
0C, TS = 1.2 %
TS = 1.
2 % , 35
0C••
TS = 1.2 % , 20
0C••
20
0C, TS = 5.
2 %
20
0C, TS = 2.
3 %
TS = 1.
2 %, 5
0C••
20
0C, TS = 1.2 %
Ae robic Anae robic
0 50 100 150 200 250
*
TOC (mg/L)
0 50 100 150 200 250
*
10 100 1400 1600 1800
0 50 100 150 200 250
UV
260(m
-1)
82
Fig. 24 The values of SUVA at the end of digestion for 61 days in the aerobic and anaerobic reactors under different TS concentrations and temperatures. *The value for day 46.
This probably indicated that, under lower temperature, the formation of humic and/or humic-like substances was relatively easier during anaerobic digestion. In addition, as a common feature, the values of SUVA for most TS and temperature conditions of both aerobic and anaerobic digestion were below 2.0 m-1/(mg/L). This suggests that the formed and remaining humic and/or humic-like substances were mainly consisted of molecules with relatively lower aromaticity (Ates et al., 2007). A general trend of either increases or decreases of SUVA with the increases of the TS concentration and temperature was not revealed.
0 1 2 3 4 5 6
SUVA (m
-1/(mg/L))
Aerobic Anaerobic
*
TS = 1.2 %, 35
0 C••
TS = 1.2 %, 20
0 C••
20
0 C, TS = 5.2 % 20
0 C, TS = 2.3 %
TS = 1.2 %, 5
0 C••
20
0 C, TS = 1.2 %
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Fig. 25 The destruction and lysis of microorganisms observed by SEM (×19000) during digestion in aerobic (a) and anaerobic (b) reactors operated at 20 qC with the TS concentration of 1.2 %.
The observed presence of the DOM in the liquid of digestion of sludge was a combined result of the destruction and lysis of the microorganisms and the breakdown of some of their released organic species. The occurrence of the destruction and lysis of the microorganisms during both the aerobic and anaerobic digestion processes could be clearly seen from the SEM images given in Fig. 25 as examples for the condition with the TS of 1.2 % and temperature of 20 qC. Elongated and rod shaped bacteria were evident in the initial period (day 2). However, after 31 days, they looked well swelled, with their shapes becoming irregular. Then, at the end of digestion for 61 days, their shapes became vague and can hardly be distinguished. For all reactors, the changes in the shape of bacteria were found more dramatic for anaerobic digestion, suggesting their faster destruction and lysis than digestion under aerobic condition. Disintegration of sludge flocs in anaerobic fermentation was also found by Pang et al. (2014). In association with the increases of DOM, for both aerobic and anaerobic conditions,
Day 2 Day 31 Day 61
(a)
(b) Pores of membrane
filter with 0.2 μm Bacteria
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increasing the digestion temperature from 5 to 35 qC significantly accelerated the destruction and lysis of microorganisms (SEM images not provided here). In a previous study, Climent et al. (2007) reported that higher temperatures could enhance the denaturation of proteins and the permeabilization of cell membrane, resulting in faster leakage of intracellular substances.
Table 12 The rate parameters of hydrolysis and degradation of DOM in aerobic and anaerobic digestion of excess activated sludge estimated using a sequential reaction model.
Aerobic kh kd Error
TS = 1.2 %
5 qC 0.0011 0.0587 0.03386
20 qC 0.0024 0.1106 0.00191
35 qC 0.0027 0.1163 0.04131
20 qC
TS = 1.2 % 0.0023 0.1027 0.00279 TS = 2.3 % 0.0024 0.1106 0.00191 TS = 5.2 % 0.0130 0.4655 0.00205
Anaerobic kh kd Error
TS = 1.2 %
5 qC 0.0011 0.0377 0.00142
20 qC 0.0036 0.0509 0.00002
35 qC 0.0355 0.5407 0.03661
20 qC
TS = 1.2 % 0.0037 0.0509 0.00002 TS = 2.3 % 0.0078 0.0749 0.05030 TS = 5.2 % 0.0142 0.0763 0.00191
kh: first-order hydrolysis rate parameter (day-1); kd: first-order degradation rate parameter (day-1); Error: the difference between calculated and observed TOC concentrations defined by equation (9).
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5.3.1.2 Rate parameters of hydrolysis and degradation in WAS digestion
The calculated TOC profiles that best fitted the observed data are depicted in Fig. 21 and Fig. 22 as solid lines, and the searched kh and kd are summarized in Table 12.
Compared to aerobic digestion, anaerobic digestion was found faster for hydrolysis to take place. As shown in Table 2, for aerobic digestion, kh increased from 0.0011 day-1 to 0.0027 day-1 as temperature increased from 5 to 35 qC for reactors with the TS concentration of 1.2 %; and increased from 0.0023 day-1 to 0.0130 day-1 as TS increased from 1.2 to 5.2 % for the reactors at 20 qC. For anaerobic digestion, the corresponding increase of kh with the increase of temperate was from 0.0011 day-1 to 0.0355 day-1 and that with the increase of TS concentration was from 0.0036 day-1 to 0.0142 day-1, respectively; with the values being generally higher than those estimated for the aerobic digestion.
In regard of the degradation rate, kd for the aerobic digestion varied from 0.0587 to 0.1163 day-1 as temperature increased from 5 to 35 qCand from 0.1027 to 0.4655 day-1 as TS increased from 1.2 to 5.2 %; with its values being obviously higher than those for the anaerobic digestion (from 0.0377 at 5 qC to 0.0509 day-1 at 20 qC and 0.0509 to 0.0763 day-1 as TS increased from 1.2 to 5.2 %, respectively). The higher rate of hydrolysis and lower rate of degradation for anaerobic digestion, as compared to aerobic digestion, were the reason for accumulation of DOM in the reactors, resulting in not only higher values of TOC and UV260 but also different SUVA as discussed earlier.
5.3.1.3 Composition of DOM assessed by volatile fatty acids
Aerobic and anaerobic digestion of EAS result in different intermediate and final organic compounds (Martinez-Garcia et al., 2014; Fall et al., 2014). For all the TS and temperature conditions investigated in this study, the total concentration of VFAs detected in the anaerobic reactors was higher than that in the aerobic reactors and changed with the operation time (data not shown). In addition, compared to the aerobic reactors where, in most cases, only citrate was detected; in the anaerobic reactors,
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however, among all seven VFAs targeted (citrate, isobutyrate, acetate, propionate, butyrate, valerate and isovalerate), except for valerate, all other six species were detected even if their concentrations differed with the digestion time, TS concentrations and temperatures.
Fig. 26 The concentrations of VFAs detected at the end of digestion for 61 days in the aerobic and anaerobic reactors operated under different TS concentrations and temperatures. *The value for day 46.
At the end of 61 days’ operation, the results plotted in Fig. 26 showed that, in the aerobic reactors, as the only VAF species detected, citrate in the reactors with the TS concentrations of 1.2, 2.3 and 5.2 % fell in the range of 5.7 – 20.2 mg/L. In the reactors with the same TS concentration (1.2 %) but different temperatures, a trend of significant
0 10 20 30 40 50 60 70 80
TS = 1 .2 %, 35
0 C••
TS = 1.2 %, 5
0 C••
20
0 C, TS = 5
.2 %
20
0 C, TS = 1.2 %
*
VFAs (mg/L)
0 10 20 30 40 50 60 70 80
20
0 C, TS = 1.2 % 20
0 C, TS = 2
.3 % Propionate Isovalerate Isobutyrate Citrate
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increase of citrate from 7.3 to 12.4 mg/L was revealed as the temperature increased from 5 to 20 qC and reached 47.6 mg/L at 35 qC after 46 days. In the anaerobic reactors, in addition to citrate, isobutyrate was the main VFA species remaining and its concentration varied in 3.5 – 20.8 mg/L. A unique trend of either increases or decreases of citrate and isobutyrate with the increases of TS or temperature was not revealed.
5.3.1.4 Composition of DOM assessed by fluorescence EEM
The fluorescence EEM of DOM in the aerobic and anaerobic reactors operated under 20 qC with the TS concentration of 1.2 % at day 2, day 31 and day 61 are shown in Fig.
27 as examples. Peak 1 and 2 at the excitation/emission (Ex/Em) wavelengths of 485/514 and 340/428, reported to reflect humic acid-like substances (Chen et al., 2003), were two common peaks emerged during both aerobic and anaerobic digestion. Peak 3 and 4 at the Ex/Em of 280/344 and 225/340, which are reported to reflect aromatic proteins and tryptophan- and tyrosine-like aromatic amino acids (Hudson et al., 2008; Spencer et al., 2007; Wu et al., 2004), were two specific peaks emerged only during anaerobic digestion; while, Peak 5 and 6 at the Ex/Em of 240/436 and 270/441, reported to reflect fulvic acid- and humic acid-like substances, respectively (Chen et al., 2003), were two specific peaks emerged only during aerobic digestion.
This indicated that, for aerobic digestion of EAS, humic and fulvic acid-like substances that are less or not biodegradable hydrophobic acids primarily formed from the hydrolysis of EPS (Pang et al., 2014) were the major organic species formed and remaining in the liquids after treatment. For anaerobic digestion, however, aromatic proteins and tryptophan- and tyrosine-like aromatic amino acids as the hydrolysates of proteins and other substrates (Pang et al., 2014; Spencer et al., 2007) were the major organic species that constitute for the major fractions of DOM formed and remaining in the liquids after anaerobic digestion.
The values of the fluorescence intensity of all peaks remaining at the end of the digestion for 61 days are plotted in Fig. 28. The total fluorescence intensity was higher
88
for the WAS after anaerobic digestion, suggesting that relatively larger proportions of the hydrolysis products were remaining in the liquids due probably to their slower degradation rate as shown earlier in Table 2.
Fig. 27 The fluorescence EEM of DOM during digestion in the aerobic (a) and anaerobic (b) reactors operated under 20 qC with the TS concentration of 1.2 %.
This indicated that humic acid- and fulvic acid-like substances were dominant species to remain after aerobic digestion; and aromatic proteins and tryptophan- and tyrosine- aromatic amino acids were dominant species to remain after anaerobic digestion. This supports the previous report that proteins are the largest organic constituents of EAS (Pang et al., 2014) and more proteins are released under anaerobic condition (Ramdani et al., 2012). In addition, it is obvious to see that, during the whole operation, Peak 2 and 5 were mainly found in the aerobic reactors; and Peak 3 and 4 in the anaerobic reactors.
(a)
(b)
Em(nm) Ex(nm)
Em(nm) Ex(nm)
Day 2 Day 31
Em(nm) Ex(nm)
Day 61
Em(nm) Ex(nm)
Day 2
Em(nm) Ex(nm)
Em(nm) Ex(nm)
Day 61 Day 31
Peak 1
Peak 2 Peak 4
Peak 5
Peak 6 Peak 3
Peak7
Peak8
89
Fig. 28 Fluorescence intensity of all peaks of DOM in the aerobic and anaerobic reactors at the end of digestion for 61 days. *The value for day 46. Peak 1, 2, 6, 7: humic acid-like substances; Peak 3: aromatic proteins; Peak 4: tryptophan-like and tyrosine-like aromatic amino acids; Peak 5: fulvic acid-like substance and Peak 8:
peptides and proteins.
5.3.1.5 Composition of DOM assessed by molecular weight
The MW distributions of the DOM in the aerobic and anaerobic reactors operated under 20 qC with the TS concentration of 1.2 % at the end of digestion are depicted in Fig. 29 as examples. Compared to aerobic digestion that resulted in a MW distribution with a broader range and a distinct peak corresponding to the smallest MW organic
0 500 1000 1500 2000
*
Anae robic Ae robic
TS = 1.2 % , 35
0 C••
TS = 1.2 %, 20
0 C••
20
0 C, TS = 5.2 % 20
0 C, TS = 2.3 %
TS = 1.2 % , 5
0 C••
TS = 1.2 % , 35
0 C••
TS = 1.2 %, 20
0 C••
TS = 1.2 % , 5
0 C••
20
0 C, TS = 1.2 % 0 500 1000 1500 2000
Peak 8 Peak 7 Peak 6 Peak 5 Peak 4 Peak 3 Peak 2 Peak 1
90
fraction, anaerobic digestion resulted in a distribution that had a narrower MW range and positioned on the left side reflecting relatively larger molecular weight.
The values of the weight-averaged molecular weight, Mw, and the polydispersity of DOM remaining at the end of digestion are plotted in Fig. 30. Except for the ones for the TS of 5.2 % and temperature of 5 qC, Mw was generally larger for the DOM formed and remaining after anaerobic digestion than after aerobic digestion. For both aerobic and anaerobic digestion, a unique trend of either increases or decreases of Mw with the increases of the TS or temperature was not revealed, suggesting that, judging from Mw, the composition of DOM did not change significantly with the changes of TS concentrations and temperatures.
Fig. 29 Molecular weight distributions of DOM at the end of digestion for 61 days in the 0
20 40 60 80 100 120
18 21 24 27 30 33 36 39 42
0 10 20 30 40 50 60
TS = 1.2 %, 20 oC TS = 2.3 %, 20 oC TS = 5.2 %, 20 oC TS = 1.2 %, 5 oC TS = 1.2 %, 35 oC Aerobic
*
Detecto r respo nse (mV)
Elution time (min)
Anaerobic
91
aerobic and anaerobic reactors under different TS concentrations and temperatures. *The value for day 46.
Fig. 30 The weight-averaged molecular weight (Mw) and polydispersity of DOM at the end of digestion for 61 days in the aerobic and anaerobic reactors under different TS concentrations and temperatures. *The value for day 46.
In regard of the heterogeneity of the DOM in molecular weight, the computed values of polydispersity indicated that, excluding the two exceptions mentioned above with the results of Mw, the constituting compounds of DOM formed and remaining after aerobic digestion were, in general, more heterogeneous than those formed and remaining after anaerobic digestion.
0 2000 4000 6000 8000
Mw (g/mol of PSS)
Aerobic Anaerobic
*
0.0 0.4 0.8 1.2 1.6 2.0
*
Po lydispersity (-)
TS = 1.2 % , 35
0 C••
TS = 1.2 %, 20
0 C••
20
0 C, TS = 2.3 % 20
0 C, TS = 1.2 %
20
0 C, TS = 5.2 %
TS = 1.2 % , 5
0 C••
92 5.3.1.6 Principal component analysis
For aerobic digestion, the component loading plots given in Fig. 31 (a) showed that the loadings of TOC, SCOD, Peak 3 and Peak 4 reflecting biodegradable organic constituents (including proteins and aromatic amino acids) as PC1 were higher; and the loadings of Peak 2, Peak 5 and Peak 7 reflecting the less biodegradable constituents of humic molecules as PC2 were higher. The score plots given in Fig. 31 (b) showed that for reactors R1 - R3 (TS 1.2 %, temperatures 5 - 35 °C), the score did not change obviously with the digestion time, suggesting the composition of DOM was similar during the aerobic digestion. For R4 (TS 2.3 %, temperature 20 °C), the score of PC2 showed slight increases, indicating that the fraction of humic substances was increasing with the digestion time. For R5 (TS 5.2 %, temperature 20 °C), the score of PC1 decreased from day 2 and day 15 to day 31 and then increased slightly until day 61; and the score of PC2 showed a consistent trend of increases. This indicated that for this reactor fed with the highest TS concentration (5.2 %), as digestion processed over time, the fraction of biodegradable DOM constituents gradually disappeared and in return, the less and non-biodegradable fraction gradually increased. For anaerobic digestion, as shown in Fig. 31 (c) and (d), the loadings of TOC, SCOD, VFAs, UV260, Peak 2, Peak 3 and Peak 4 as the PC1 were higher and the loadings of Peak 6, Peak 7 and Peak 8 as the PC2 were higher. Combination of the loading results with the corresponding score plots indicated that for R10 (TS 5.2 %, temperature 20 °C), both biodegradable and non-biodegradable DOM constituents increased from day 2 to day 15 and then started to decrease; for R8 (TS 1.2 %, 35 °C), the fraction of non-biodegradable constituents in DOM increased rapidly from day 2 to day 15 and then turned to a graduate decrease until day 61.
93
Fig. 31 The results of the principal component analysis (component loadings and component scores) using all observed data during the whole process of aerobic (a, b) and anaerobic (c, d) digestions under different TS concentrations and temperatures.