L. J. Wua,
Y. Qinb,
T. Hojoa and
Y. Y. Li*ab
aDepartment of Civil and Environmental Engineering, Graduate School of Engineering, Tohoku University, Aoba 6-6-06, Aramakizi, Aoba-ku, Sendai, 980-8579, Japan. E-mail: yyli@epl1.civil.tohoku.ac.jp; Fax: +81-22-795-7465; Tel: +81-22-795-7464
bDepartment of Frontier Science for Advanced Environment, Graduate School of Environmental Studies, Tohoku University, Aoba 6-6-20, Aramakizi, Aoba-ku, Sendai, 980-8579, Japan
First published on 30th July 2015
In order to upgrade the conventional mesophilic anaerobic digestion (MD) of waste activated sludge (WAS), a hyper-thermophilic (70 °C, stage I)–mesophilic (35 °C, stage II) temperature-phased anaerobic digestion system with a recycle system (TPAD-R) was constructed, with MD as a control. The accumulation of solids increased with shorter hydraulic retention time (HRT). Compared with MD, TPAD-R improved the reduction of solids by over 10%, and the removal rate of protein in TPAD-R increased by more than 20%. In stage I of TPAD-R an amount of organic matter was solubilized, in a range between 10 g L−1 and 20 g L−1, as soluble chemical oxygen demand (COD), and played an important role in enhancing hydrolysis and acidification. The specific hydrolysis and acidification rates of stage I reached their maximum values after a HRT of 10 days, 2.367 g COD per g VS per day and 1.120 g COD per g VS per day, respectively. Consequently, the methane yield in TPAD-R, primarily produced in stage II, was also improved, 29% higher than that in MD for a HRT of 10 days. Besides compensating for energy loss, TPAD-R obtained a higher net energy than MD, achieving a net energy which was 3.3 kJ g−1 VS more than that in MD for a HRT of 10 days. The TPAD-R proved to be efficient in upgrading the MD of WAS.
In the treatment of WAS, a waste with a high solids content, anaerobic digestion is generally limited by the rate of hydrolysis of particulates to soluble substrates.9 Therefore, the critical issue for upgrading the MD of WAS is to enhance the hydrolysis. Many pre-treatment methods, such as mechanical, thermal, chemical, biological, and a combination of these,10–12 have proved effective in making suspended substrates more accessible for the anaerobic bacteria, optimizing the methanogenic potential.3 Among these pre-treatment methods, thermal pre-treatment is suitable for not only the improvement of stabilization, but also the enhancement of dewatering of the sludge and the reduction of the number of pathogens at relatively low cost.11 However, higher temperature pre-treatment has high energy requirements and operating difficulties. Thus, thermal pre-treatment below 100 °C is accepted as appropriate.13 Pre-treatment at 70 °C prior to the MD showed positive effects on the enhanced solubilisation of WAS and the methane production in the subsequent MD.13,14 However, substantial acidification in the pre-treatment stage can possibly occur, and biogas production is nearly impossible in that stage.
A recycling of the effluent from the end stage to the front stage can supplement the alkalinity to the pre-treatment step, and reduce the serious acidification.15 Also, the introduction of the recycle system resulted in a diluted influent and microorganisms being supplemented from the end stage.16,17 Thus, it is possible that biogas production becomes available in the front stage, and further process upgrading is achieved. The process feasibility in treating high-solid waste has been demonstrated.16,18 Unfortunately, research utilizing a two-stage process with a recycle system to degrade WAS can hardly be found. In addition, the studies in the past, on the basis of a thermophilic–mesophilic two-stage process, focused on introducing the recycle system from the end stage to a thermophilic stage, and little attention was paid to hyper-thermophilic–mesophilic temperature-phased anaerobic digestion.
Based on the above, a hyper-thermophilic–mesophilic temperature-phased anaerobic digestion system with a recycle system (TPAD-R) was constructed, with MD as a control, to compare the performance and the degradation characteristics of TPAD-R with MD under different hydraulic retention times (HRTs) for treatment of concentrated WAS, and to evaluate the feasibility of utilizing TPAD-R to upgrade the treatment of WAS.
Run 1 | Run 2 | Run 3 | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
MD | TPAD-R | MD | TPAD-R | MD | TPAD-R | |||||||
I | II | I + II | I | II | I + II | I | II | I + II | ||||
Reactor volume (L) | 5 | 3 | 12 | 15 | 5 | 3 | 12 | 15 | 5 | 3 | 12 | 15 |
Temperature (°C) | 35 | 70 | 35 | — | 35 | 70 | 35 | — | 35 | 70 | 35 | — |
HRT (days) | 30 | 3 | 12 | 30 | 20 | 2 | 8 | 20 | 10 | 1 | 4 | 10 |
TS loading (g per L per day) | 1.6 | 12.2 | 2.6 | 1.6 | 2.2 | 20.1 | 4.0 | 2.2 | 5.0 | 42.5 | 10.0 | 5.0 |
VS loading (g per L per day) | 1.2 | 8.5 | 1.7 | 1.2 | 1.7 | 12.5 | 2.7 | 1.66 | 3.80 | 30.6 | 6.6 | 3.80 |
COD loading (g per L per day) | 1.8 | 13.6 | 3.4 | 1.8 | 2.7 | 19.4 | 4.6 | 2.7 | 5.8 | 48.4 | 11.4 | 5.8 |
Gas samples were taken using a 0.5 mL syringe equipped with metal hub needles to measure the gas composition. A volume of 0.4 mL of biogas was injected into a gas chromatograph (GC-8A, Shimazu Corporation, Japan) to measure the biogas composition. The temperatures of the injector, detector and column were set at 100 °C, 100 °C and 70 °C, respectively. The carrier gas was argon and its pressure was controlled at 100 kPa. The GC column used for the analysis detected the relative proportions of nitrogen, methane, and carbon dioxide.
The carbohydrate, protein and lipid content, as the main components of COD in WAS, was also analyzed. The measurement of carbohydrate and protein content utilized spectrophotography. The carbohydrate content was measured with a phenol-sulfate examination method.20 The protein content was measured with the traditional Folin-phenol method.21 A methanol–chloroform extraction and weight method was used to measure the lipid content.22
A GC with a flame ionization detector (FID) was utilized to detect volatile fatty acid (VFA) components. Six acids, namely acetic, propionic, iso-butyric, non-butyric, iso-valeric, and non-valeric acid were analyzed. When the total VFA was calculated, each individual VFA was converted to a value of acetic acid, namely HAc. The conversion ratios for acetic, propionic, butyric, and valeric acid were 1, 0.811, 0.682 and 0.588, respectively. To analyze the VFA, the filtrate after centrifugation was filtered through 0.45 μm filters. 0.5 mL filtrate was collected in a 1.5 mL GC vial, and 0.5 mL, 0.1 mol L−1 HCl solution was added to adjust the pH to acid. An Agilent 6890 GC with a FID and equipped with a 30.0 m × 530 μm × 1.00 μm DB-WAXetr column was utilized to analyze the content of the VFA. Nitrogen was the carrier gas and the flow-rate was 2 mL min−1. The injection port and the detector were maintained at 250 °C. The oven of the GC was programmed to begin at 125 °C and hold here for 5 minutes, then to increase the temperature at a rate of 2.5 °C min−1 to 180 °C, and to hold at 180 °C for 5 minutes. The sample injection volume was 1.0 μL.
![]() | (1) |
The hydrolysis of the particulate organic matter in each system was characterized using the VS removal and PCOD hydrolysis rates. To obtain the specific rate, the rate of PCOD hydrolysis was divided by the mass of the VS concentration in the reactor as shown in the equation. Because of the difficulty in determining the active biomass concentration, the specific hydrolysis rates were normalized on the basis of the total VS.14,23
![]() | (2) |
![]() | (3) |
The heat requirements of the digesters consist of that to raise the temperature of the incoming sludge to that of the digestion tank and that to compensate for the heat losses through the walls, floor, and roof of the digester.24,25 The heat requirements are estimated using eqn (4).
q = CQΔT1 + UAΔT2 | (4) |
Pnet = Poutput − q | (5) |
![]() | ||
Fig. 2 Time profiles of the pH, TS, VS, biogas production rate and composition, NH4+–N and VFA in MD, and stage I and stage II of TPAD-R. |
There were analogous results indicated in TPAD-R like MD with a HRT of 30 days and 20 days. Conversely, in Run 3 although the solids concentration in stage II increased compared to that in Run 1 and Run 2, the VFA concentration remained in a low range, about 700 mg L−1 as HAc, and most of it could be consumed. It was inferred that the optimal HRT for TPAD-R to treat WAS should be between 10 days and 20 days. Whichever conditions were utilized, the pH in stage I and stage II of TPAD-R was relatively higher than that in MD. As to the higher pH in stage I, it was possible that the recycle system from stage II with high alkalinity and pH played an important part. Under the conditions of a HRT of 20 days, the pH in stage II of TPAD-R was especially high, attaining 7.61. It was believed that the decrease in alkalinity was mainly caused by acid formation in the anaerobic digestion, while most of the acidification in TPAD-R was accomplished in stage I. Thus, in the following stage II alkalinity utilization was reduced. In addition, the pH increase in stage I of TPAD-R as a result of the alkalinity being supplemented from the recycle system led to the improved pH in the influent of stage II. Because of the dilution from the recycle system and the higher decomposition level of TPAD-R, the solids concentration in each reactor of TPAD-R was lower than for MD. As a consequence, the gas production rate of TPAD-R was improved. Or rather, a higher gas production rate was observed in stage I, about 1.7 times faster than that in stage II, while in stage II the longer retention time (4 times longer than that in stage I) and higher methane content (above 70%) than stage I resulted in most of the methane in TPAD-R being produced in stage II. Through the time profile of the VFA in each reactor of TPAD-R, it could be seen that with the biogas produced in stage I, the VFA concentrations were also in a high range, 2500–4000 mg L−1 as HAc. This phenomenon was also reported in the previous study, which pointed out that increasing the temperature to more than 60 °C will often lead to an increased concentration of VFA in the completely mixed tank reactors.3 Another investigation into the effects of different thermophilic temperatures on the pretreatment of sewage sludge has shown that acidogenesis had the highest product formation when the temperature was at 70 °C or 75 °C, under the conditions of which a higher acidogenic activity was maintained and the methanogenic activity was inhibited.26 The VFA produced in stage I was almost consumed in the following stage II of TPAD-R. NH4+–N is usually formed in anaerobic treatment from the degradation of proteins or urea and its inhibitory concentrations may be approached in highly concentrated municipal waste sludge.27,28 It has been reported that when the NH4+–N concentration exceeds 3000 mg L−1, then the ammonium ion itself becomes quite toxic regardless of the pH and the process can be expected to fail.28 In this study the NH4+–N concentration in both reactors of TPAD-R was maintained in the range of 2000–2500 mg L−1, suggesting that the NH4+–N concentration in the reactors was appropriate to not cause inhibition, although when the HRT was changed to 10 days NH4+–N immediately accumulated for a time. The averages of the experimental results in the steady states of Run 1, Run 2 and Run 3 are summarized in Table 2.
Items | Run 1 | Run 2 | Run 3 | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
WAS | MD | TPAD-R | WAS | MD | TPAD-R | WAS | MD | TPAD-R | ||||||
I | II | I | II | I | II | |||||||||
a *‘T’ is ‘Total’. **‘S’ is ‘Soluble’. ***‘HAc’ is ‘acetic acid’. ****‘N.D.’ is ‘not detected’. | ||||||||||||||
Digested sludge | pH | 5.80 ± 0.04 | 7.06 ± 0.07 | 7.37 ± 0.09 | 7.55 ± 0.02 | 5.82 ± 0.01 | 7.22 ± 0.02 | 7.50 ± 0.03 | 7.61 ± 0.02 | 5.92 ± 0.05 | 7.03 ± 0.03 | 7.14 ± 0.03 | 7.28 ± 0.05 | |
TS (%) | 4.66 ± 0.13 | 3.19 ± 0.11 | 3.16 ± 0.14 | 2.67 ± 0.06 | 4.30 ± 0.07 | 3.25 ± 0.07 | 3.17 ± 0.02 | 2.73 ± 0.06 | 5.00 ± 0.05 | 3.99 ± 0.09 | 3.82 ± 0.10 | 3.50 ± 0.07 | ||
VS (%) | 3.46 ± 0.20 | 2.16 ± 0.12 | 2.08 ± 0.18 | 1.64 ± 0.12 | 3.32 ± 0.03 | 2.17 ± 0.05 | 2.15 ± 0.04 | 1.69 ± 0.02 | 3.80 ± 0.16 | 2.75 ± 0.04 | 2.64 ± 0.10 | 2.32 ± 0.08 | ||
T*-COD (g L−1) | 53.3 ± 0.9 | 31.3 ± 1.0 | 40.4 ± 2.6 | 28.4 ± 2.1 | 53.7 ± 0.8 | 33.6 ± 0.9 | 37.1 ± 0.6 | 26.9 ± 0.6 | 58.1 ± 0.9 | 42.1 ± 1.1 | 45.7 ± 1.3 | 38.6 ± 1.1 | ||
SCOD (g L−1) | 7.2 ± 0.7 | 3.3 ± 0.1 | 11.4 ± 0.5 | 4.9 ± 0.2 | 6.9 ± 0.4 | 4.1 ± 0.4 | 15.3 ± 0.5 | 6 ± 0.8 | 2.7 ± 0.5 | 4.5 ± 0.6 | 19.6 ± 0.4 | 10.1 ± 0.3 | ||
T-carbohydrate (g L−1) | 5.5 ± 0.7 | 3.1 ± 0.1 | 3.9 ± 0.4 | 2.8 ± 0.2 | 4.8 ± 0.3 | 3.6 ± 0.6 | 3.6 ± 0.2 | 3.0 ± 0.1 | 7.9 ± 0.6 | 5.6 ± 0.2 | 5.6 ± 0.4 | 5.0 ± 0.3 | ||
S**-carbohydrate (g L−1) | 0.6 ± 0.0 | 0.3 ± 0.0 | 1.0 ± 0.1 | 0.6 ± 0.0 | 0.5 ± 0.0 | 0.5 ± 0.0 | 1.4 ± 0.2 | 0.8 ± 0.1 | 0.2 ± 0.0 | 0.5 ± 0.0 | 1.8 ± 0.1 | 1.1 ± 0.1 | ||
T-protein (g L−1) | 16.8 ± 1.3 | 11.7 ± 0.5 | 10.8 ± 1.0 | 8.2 ± 0.4 | 14.5 ± 1.0 | 10.2 ± 0.9 | 7.1 ± 0.7 | 5.7 ± 0.5 | 14.6 ± 1.0 | 9.9 ± 0.4 | 7.1 ± 0.7 | 6.9 ± 0.6 | ||
S-protein (g L−1) | 1.3 ± 0.0 | 0.9 ± 0.1 | 2.4 ± 0.2 | 1.0 ± 0.1 | 1.1 ± 0.1 | 1.4 ± 0.1 | 2.5 ± 0.2 | 1.4 ± 0.2 | 0.3 ± 0.0 | 1.1 ± 0.0 | 2.5 ± 0.2 | 1.6 ± 0.1 | ||
T-lipid (g L−1) | 4.0 ± 0.2 | 2.8 ± 0.5 | 3.4 ± 0.2 | 3.2 ± 0.3 | 4.9 ± 0.3 | 3.4 ± 0.2 | 4.3 ± 0.1 | 3.5 ± 0.1 | 4.9 ± 0.4 | 3.3 ± 0.2 | 4.3 ± 0.1 | 3.7 ± 0.2 | ||
S-lipid (g L−1) | 3.1 ± 0.1 | 1.6 ± 0.4 | 1.8 ± 0.5 | 1.8 ± 0.2 | 1.8 ± 0.1 | 1.8 ± 0.4 | 1.9 ± 0.2 | 1.8 ± 0.2 | 1.0 ± 0.1 | 1.2 ± 0.4 | 2.4 ± 0.3 | 1.7 ± 0.1 | ||
VFA (mg HAc*** L−1) | 1420 ± 280 | N.D.**** | 2590 ± 170 | N.D. | 1840 ± 230 | N.D. | 2790 ± 290 | N.D. | 850 ± 190 | 1640 ± 220 | 3840 ± 230 | 702 ± 40 | ||
Alkalinity (mg CaCO3 L−1) | 1020 ± 100 | 4190 ± 160 | 4920 ± 380 | 6860 ± 110 | 930 ± 70 | 4850 ± 80 | 5760 ± 160 | 7570 ± 120 | 660 ± 70 | 3780 ± 90 | 4150 ± 120 | 5530 ± 320 | ||
NH4+–N (mg L−1) | 430 ± 40 | 1280 ± 170 | 2100 ± 110 | 2220 ± 60 | 330 ± 20 | 1700 ± 50 | 2350 ± 90 | 2580 ± 60 | 260 ± 50 | 1200 ± 100 | 2420 ± 100 | 2100 ± 80 | ||
Biogas | Production rate (L per L per day) | — | 0.45 ± 0.01 | 0.63 ± 0.03 | 0.39 ± 0.04 | — | 0.58 ± 0.01 | 1.15 ± 0.01 | 0.63 ± 0.01 | — | 1.06 ± 0.02 | 2.08 ± 0.02 | 1.19 ± 0.06 | |
Composition | CH4 (%) | — | 60.6 ± 0.2 | 36.7 ± 1.1 | 70.9 ± 0.2 | — | 62.8 ± 0.3 | 35.8 ± 1.1 | 72.4 ± 0.2 | — | 60.0 ± 0.4 | 36.4 ± 0.4 | 70.1 ± 1.5 | |
CO2 (%) | — | 38.9 ± 0.5 | 62.1 ± 1 | 28.3 ± 0.7 | — | 36.7 ± 0.3 | 63.0 ± 1.5 | 27.5 ± 0.2 | — | 39.4 ± 0.7 | 62.4 ± 0.5 | 29.4 ± 1.3 | ||
H2S (ppm) | — | 1180 ± 20 | 2010 ± 10 | 600 ± 0 | — | 1340 ± 110 | 2360 ± 110 | 1300 ± 80 | — | 1520 ± 120 | 1860 ± 170 | 1060 ± 140 |
Removal rate (%) | Run 1 | Run 2 | Run 3 | |||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
MD | TPAD-R | MD | TPAD-R | MD | TPAD-R | |||||||
I | II | I + II | I | II | I + II | I | II | I + II | ||||
TS | 31.5 | 13.8 | 15.5 | 42.6 | 24.4 | 9.8 | 13.9 | 36.5 | 20.2 | 10.1 | 8.4 | 30.0 |
VS | 37.6 | 18.4 | 21.2 | 52.5 | 34.6 | 14.2 | 21.4 | 49.1 | 27.6 | 13.7 | 12.1 | 38.9 |
T-COD | 41.3 | 1.1 | 29.7 | 46.7 | 37.4 | 7.9 | 27.5 | 49.9 | 27.5 | 5.5 | 15.5 | 33.6 |
T-carbohydrate | 43.6 | 6.0 | 28.2 | 50.0 | 24.2 | 8.3 | 16.0 | 37.4 | 29.3 | 13.0 | 11.2 | 37.0 |
T-protein | 30.4 | 13.6 | 24.1 | 50.9 | 29.7 | 29.7 | 19.7 | 60.7 | 32.2 | 34.0 | 2.8 | 52.7 |
T-lipid | 30.0 | 5.6 | 5.9 | 18.7 | 30.6 | 0.0 | 18.6 | 28.6 | 32.7 | 0.0 | 14.0 | 24.5 |
With the VS degradation, different solid organic matter was firstly solubilized and further degraded and converted into biogas. Actually, a large number of microbial species are capable of utilizing organic substrates such as carbohydrates, proteins, and lipids to produce VFA and other soluble carbon compounds via a variety of anaerobic metabolic pathways. But since complex biopolymers cannot penetrate the cell membrane, bacteria excrete enzymes that hydrolyze the particulate substrate into small transportable molecules.29–31 In general, polysaccharides are converted into simple sugars, protein is hydrolyzed to amino acids, and lipid metabolism results in the production of fatty acids and a variety of other organic compounds.32 Thus, the organic matter reduction and solubilization could always reflect the degradation process of anaerobic digestion. In the hydrolysis, the rate-limiting step of WAS digestion,9 organic matter is firstly solubilized to become small transportable components. On one hand, the concentration of the solubilized organic matter indicates the hydrolysis capability, even the digestion capability of reactors for WAS; on the other hand, with the solubilization, soluble organic matter is rapidly metabolized into biogas. Consequently, there is a balance between formation and consumption of organic matter.
The reduction and solubilization of COD and its major fractions, carbohydrate, protein and lipid, are shown in Tables 3 and 2, respectively. Except for the results of total lipid reduction between MD and TPAD-R, the other organic matter reduction in TPAD-R was always higher than that in MD. The differences between the two systems for a HRT of 20 days were highest, and the improved percentages of COD, carbohydrate and protein were 12.5%, 13.2% and 31.0%, respectively. Moreover, under the conditions of a HRT of 20 days, while the lipid reduction in TPAD-R remained lower than that in MD, the values between the two systems were close to each other, about 30%. Actually, the COD reduction in stage I was always over 10% lower than that in stage II under whichever HRT condition was used, but the solubilized COD maintained a higher level, more than 2 times higher than that in stage II, which further proved that stage I in TPAD-R played the main role in the degradation of solids into soluble compounds. The experiment with 70 °C pretreatment to degrade WAS showed that solubilization was improved during pretreatment.14 Another study with pretreatment at 70 °C showed the dissolution of up to 40% of the particulate COD in waste sludge.33 Furthermore, with the HRT shortened, the concentrations of soluble COD in each reactor increased, and with a HRT of 10 days they accumulated most, 4.5 g L−1 for MD, 19.6 g L−1 for stage I and 10.1 g L−1 for stage II. From the effluents of MD and stage II, the conversions of soluble COD for a HRT of 20 days were more complete and appropriate for TPAD-R optimization, while for a HRT of 20 days the soluble COD in stage II, 6.0 g L−1, remained lower and close to the effluent of MD, 4.1 g L−1. For carbohydrate, the results of organic matter reduction and solubilization were consistent with those of COD. From the results of the WAS characterisation shown in Table 2, protein was always the primary component of the organic matter in WAS, about 15 g L−1 in the original WAS. Stage I still played an important role in protein solubilization, resulting in more soluble protein in the effluent of stage I than in stage II. The HRT seemed not to be influential to the protein solubilization under the experimental conditions in this study, or only a short retention time was needed to solubilize the protein. However, with a short HRT of 10 days for the TPAD-R system, the protein degradation in the following stage II was substantially affected, with only 2.8% removed. Therefore, for TPAD-R the retention time of 4 days in stage II was inappropriate. Lipid exhibited different degradation characteristics. The effects of TPAD-R on lipid degradation seemed weak, and stage II showed a stronger lipid removal ability than stage I. As reported, only carbohydrate and nitrogenous materials were extensively fermented in the acid stage, and lipid was fermented concurrently with the volatile acids in the methanogenic stage.9,34 The results of lipid reduction in Table 3 also show that almost no lipid was degraded in stage I of TPAD-R.
Methane production | Run 1 | Run 2 | Run 3 | |||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
MD | TPAD-R | MD | TPAD-R | MD | TPAD-R | |||||||
I | II | I + II | I | II | I + II | I | II | I + II | ||||
Production rate (L per L per day) | 0.27 | 0.23 | 0.28 | 0.27 | 0.36 | 0.41 | 0.46 | 0.45 | 0.64 | 0.76 | 0.83 | 0.82 |
Yield (L per g VS added) | 0.23 | 0.03 | 0.16 | 0.23 | 0.22 | 0.03 | 0.17 | 0.27 | 0.17 | 0.02 | 0.13 | 0.22 |
Recovery rate (L per g VS degraded) | 0.63 | 0.15 | 0.75 | 0.44 | 0.63 | 0.23 | 0.79 | 0.55 | 0.61 | 0.18 | 1.04 | 0.55 |
Run 1 | Run 2 | Run 3 | |||||
---|---|---|---|---|---|---|---|
MD | TPAD-R | MD | TPAD-R | MD | TPAD-R | ||
Dosage of flocculant (mg per g TS) | 58.1 | 79.2 | 56.6 | 79.3 | 62.7 | 81.4 | |
Characteristics of the dewatering filtrate | COD (mg L−1) | 600 | 770 | 440 | 1050 | 970 | 1870 |
NH4+–N (mg L−1) | 1200 | 1510 | 1160 | 1850 | 980 | 1350 | |
VFA (mg HAc L−1) | N.D. | N.D. | N.D. | N.D. | 330 | 610 |
![]() | ||
Fig. 3 Mass balances in the MD and TPAD-R systems, and in stage I and stage II of TPAD-R for the different runs. |
As shown in Table 6, either hydrolysis, acidogenesis, or methanogenesis in TPAD-R was higher than those in MD, indicating that digestion was advanced in TPAD-R compared to MD. Or rather, in TPAD-R hydrolysis mainly happened in stage I rather than stage II. In particular, in stage II under a HRT of 10 days, there was almost no hydrolysis. It is safe to say that to a great extent stage I in TPAD-R served as the hydrolysis stage. In fact, with the hydrolysis in stage I, part of the soluble COD was also converted into VFA, leading to acidification, and in stage II the abundance of available biodegradable influent caused more methane production. Moreover, in eqn (1) the COD conversion ratio of acidogenesis included the item of the mass of CODCH4, which possibly resulted in the acidogenesis ratios between stage I and stage II being nearly the same. As a matter of fact, the results of the methanogenesis ratios shown in Table 6 also show that the methane producing capability of stage II was much higher than that of stage I.
Run 1 | Run 2 | Run 3 | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
MD | TPAD-R | MD | TPAD-R | MD | TPAD-R | ||||||||
I | II | I + II | I | II | I + II | I | II | I + II | |||||
Conversion ratios (%) | Hydrolysis | 34.2 | 21.0 | 10.7 | 45.2 | 39.9 | 34.2 | 5.2 | 53.8 | 36.0 | 36.6 | 0.1 | 55.6 |
Acidogenesis | 33.8 | 12.9 | 14.5 | 40.7 | 34.6 | 15.0 | 16.6 | 44.1 | 36.5 | 15.5 | 11.4 | 41.4 | |
Methanogenesis | 36.9 | 4.8 | 23.8 | 43.4 | 38.7 | 5.8 | 28.1 | 47.6 | 31.3 | 4.5 | 20.9 | 40.3 | |
Reaction rates (g COD per L per day) | Hydrolysis | 0.526 | 2.440 | 0.258 | 0.695 | 0.955 | 5.877 | 0.141 | 1.288 | 1.996 | 15.363 | 0.008 | 3.079 |
Acidogenesis | 0.573 | 1.700 | 0.436 | 0.689 | 0.871 | 2.889 | 0.663 | 1.109 | 2.072 | 7.268 | 1.118 | 2.348 | |
Methanogenesis | 0.656 | 0.657 | 0.800 | 0.771 | 1.040 | 1.177 | 1.303 | 1.278 | 1.816 | 2.163 | 2.383 | 2.339 | |
Specific reaction rates (g COD per g VS per day) | Hydrolysis | 0.024 | 0.117 | 0.016 | 0.040 | 0.044 | 0.273 | 0.008 | 0.072 | 0.220 | 2.367 | 0.000 | 0.127 |
Acidogenesis | 0.027 | 0.082 | 0.027 | 0.040 | 0.040 | 0.134 | 0.039 | 0.062 | 0.228 | 1.120 | 0.039 | 0.097 | |
Methanogenesis | 0.030 | 0.032 | 0.049 | 0.045 | 0.048 | 0.055 | 0.077 | 0.072 | 0.200 | 0.333 | 0.083 | 0.097 |
The COD conversion rates and specific conversion rates of hydrolysis, acidogenesis and methanogenesis, were further analyzed using eqn (2) and (3). The calculated results are also shown in Table 6. According to the results, it’s clear that in stage I of TPAD-R, especially under a HRT of 10 days, the hydrolysis and acidogenesis rate and specific rate were considerably increased. The hydrolysis rate and specific hydrolysis rate of stage I in Run 3 were 15.4 g COD per L per day and 2.37 g COD per g VS per day, respectively. Thus, for pre-fermentation at the temperature of 70 °C, in order to enhance the hydrolysis and acidogenesis of WAS anaerobic digestion, the retention time of 1 to 2 days had been adequate. Although the specific hydrolysis and acidogenesis rates in TPAD-R for a HRT of 10 days, caused by the accumulation of solids, were 0.093 g COD per g VS per day and 0.131 g COD per g VS per day lower than those in MD, respectively, which possibly resulted from the weak hydrolysis and acidogenesis rate and specific rate in stage II, because of the enhancement in stage I both the hydrolysis and acidogenesis rates in TPAD-R were improved to 3.079 g COD per g VS per day and 2.348 g COD per g VS per day, respectively. Furthermore, the introduction of the recycle system resulted in stage I of TPAD-R having a certain methanogenesis rate and specific rate. Consequently, the methanogenesis rate of the whole TPAD-R system was always faster than that of MD. In particular, for a HRT of 10 days, the methanogenesis rate in TPAD-R was 0.423 g COD per L per day faster than that in MD.
The calculated results show that the energy produced for every system under all conditions could always create a surplus energy output when the energy balance was obtained. The heat requirements increased with shorter HRTs. The heat requirements for MD increased from 15.0 kJ per g VS degraded in Run 1 to 18.6 kJ per g VS degraded, while the heat requirements for TPAD-R increased from 10.7 kJ per g VS degraded in Run 1 to 13.2 kJ per g VS degraded. The net energy obtained for MD decreased with shorter HRTs, and in Run 3 3.2 kJ per g VS degraded was left. The net energy for TPAD-R fluctuated in a range between 5.6 kJ per g VS degraded and 7.6 kJ per g VS degraded. Overall, TPAD-R achieved a higher net energy than MD. In particular, in Run 3 TPAD-R obtained 3.3 kJ per g VS degraded more than MD.
WWTP | Wastewater treatment plant |
WAS | Waste activated sludge |
HRT | Hydraulic retention time |
TS | Total solids |
VS | Volatile solids |
VFA | Volatile fatty acid |
PCOD | PCOD particulate-COD |
FID | Flame ionization detector |
MD | Conventional mesophilic anaerobic digestion |
TPAD-R | Temperature-phased anaerobic digestion with a recycle system |
This journal is © The Royal Society of Chemistry 2015 |