Efficiency of Autothermal Thermophilic Aerobic Digestion Under Two Different Oxygen Flow Rates Sebnem Koyunluoglu Aynur1*, Sonali Dohale1, Muriel Dumit1, Rumana Riffat1, Mohammad Abu-Orf2, Ralph Eschborn2, Sudhir Murthy3 1
Civil and Environmental Engineering Department, George Washington University, Washington, DC 20052, USA 2 AECOM Water (Formerly Metcalf and Eddy, Inc.), Philadelphia, PA, USA 3 DC Water and Sewer Authority, 5000 Overlook Ave, SW, Washington, DC 20032, USA * E-mail:
[email protected] ABSTRACT This paper evaluates two dual digestion processes in comparison with conventional digestion. The dual digestion process consists of an autothermal thermophilic aerobic digestion (ATAD) process ahead of an anaerobic digestion process. All the three processes evaluated had the same overall 10 day SRT. The objective of the work is to observe the differences between two different oxygen flow rates (0.105 v/v/h and 0.210 v/v/h) for an ATAD process using blended municipal solids with the help of profile experiments that were conducted for both conditions by sampling every 2 hours for duration of 24 hours. The two processes were compared with the conventional process in terms of VS removal, biogas yield and pathogen destruction. Oxygen utilization per volatile solids removed were found to be 1.20 lb/lb VS removed for the oxygen flow rate of 0.210 v/v/h and 0.83 lb/lb VS removed for 0.105 v/v/h. For the oxygen flow rate 0.210 v/v/h, higher (50% greater) ATAD effluent ammonia concentration was observed compared to the lower airflow rate. Furthermore, for the 0.201 v/v/h flow rate, there was no further increase in ammonia in the subsequent anaerobic step, suggesting that the hydrolytic reactions were complete for this ATAD reactor within the 2.25 day SRT. For the ATAD reactor with oxygen supply of 0.210 v/v/h, higher VS destruction of 23.8% was achieved when compared to 17.8% VS removal for oxygen flow rate of 0.105 v/v/h. However, the two different oxygen flow rates applied to the ATAD reactor did not affect the overall (ATAD + anaerobic digestion) VS removal efficiency, suggesting that the lower oxygen application rates were sufficient to produce a stable digestion process with an excess of 50% overall VS destruction in a relatively short 10 day overall SRT. Thus, there does not appear to be an obvious advantage for completion of the hydrolytic reactions within the ATAD process. The use of the higher oxygen flow rate should be solely considered for producing more heat to maintain thermophilic conditions and not for overall VS removal. Final effluent of both conditions, met the 40CFR Part 503 regulations with undetectable FC levels. The biological heat of oxidations were calculated to be 14,300 J/g VS removed and 15,900 J/g VS removed for the oxygen flow rates of 0.105 v/v/h and 0.210 v/v/h, respectively. KEYWORDS ATAD, Class A biosolids, ORP, oxygen utilization, heat of biological oxidation.
INTRODUCTION The main objectives of municipal solids treatment are stabilization and pathogen reduction in order to produce biosolids with a quality suitable for meeting the requirements for agricultural land application. Anaerobic digestion is the traditional method to reduce solids volume but it requires long retention times with most digesters operated at mesophilic temperatures. Anaerobic digestion can be enhanced using various pretreatment options, resulting in higher VS (Volatile Solids) destruction, and increased pathogen destruction using shorter SRT (Solids Retention Time) (Borowski and Szopa, 2007). Autothermal thermophilic aerobic digestion (ATAD) is a solids treatment process where heat is released by the aerobic microbial degradation of organic matter (Layden, 2007). In ATAD, the heat released by the digestion process is the major heat source used to achieve the desired operating temperature (EPA, 1990). An ATAD process with a relatively short residence time is also used as a pretreatment step to the mesophilic anaerobic digestion that is termed dual digestion (Ward et al., 1998, Zabranska et al., 2003). In the ATAD step, solids are pretreated by solubilization and partial acidification resulting in enhanced digestion together with improved pathogen destruction (Nosrati et al., 2007; Borowski and Szopa, 2007). Thermophilic aerobic digestion step through biological oxidation of volatile solids, by solubilizing organic solids, provides hydrolyzed and homogenized solids, that improve volatile solids destruction in the downstream anaerobic digester (McIntosh and Oleszkiewicz, 1997). ATAD is operated under oxygen limiting condition that in conjunction with short HRT, results in the formation of volatile fatty acids (VFAs) through the fermentative metabolism of thermophilic bacteria (Borowski and Szopa, 2007, Ward et al., 1998). ATAD reactor provides consistent feed with high VFA concentration, to the anaerobic stage that would perform as the methane forming step. In addition, ammonification in the ATAD reactor produces a pH buffered feed to the anaerobic stage (Warakomski et al., 2007) In addition to enhancing efficiency of anaerobic digestion step, ATAD also provides better pathogen removal. In the short retention time thermophilic phase, high levels VFAs and ammonia produced results in reduction in pathogenic bacteria (Fukushi et al., 2003, Salsali et al., 2006, Smith Jr. et al., 2008). ATAD is a named process to further reduce pathogens (PFRP) used to achieve Class A biosolids according to 40CFR Part 503 that regulates biosolids pathogen density and requires vector attraction reduction before land application (EPA, 1999). The objective of the present work is to observe the differences between two different oxygen flow rates for an ATAD process using blended municipal solids from Blue Plains Advanced Wastewater Treatment Plant. Profile experiments were conducted for both conditions by sampling every 2 hours for duration of 24 hours. Comparison will be provided in terms of VS reduction, oxygen utilization, VFA production, ammonification, and pathogen destruction. The heat of biological oxidation will be determined from the difference between the power requirements for the conditions with and without biological reaction for both conditions. A comparison will be presented between conventional digestion and two dual digestion processes
(ATAD with two different oxygen flow rates as the pre-treatment steps) having the same overall HRT, in terms of VS removal, biogas yield and pathogen destruction. METHODOLOGY The feed for both the processes was collected from Blue Plains Advanced Wastewater Treatment Plant in Washington, DC. Blended primary and waste activated sludge (1:1 by volume) was thickened to 6.5% total solids (TS) using a laboratory centrifuge. High-density polyethylene batch fermentation reactor with 25 L total volume supplied by Hobby Beverage Equipment Company (Temecula, California) was used for the study. The reactor was heated using Thermolyne Heating Tape BSAT101-100 (USA). Heating tape was connected to a wattmeter measuring the input electrical power. Mixing was achieved by recirculating the headspace gas using compressor 7006VD/2, 3/E/AC supplied by Gardner Denver Thomas (USA). Pure oxygen (99%) was added to the gas recirculation system with a rate of 35 mL/min. To evaluate the different oxygen flow rates, this value was decreased by half (17.5 mL/min). The reactor was operated at 55 C with 2.5 d HRT with an active volume of 10 L. Figure 1 shows the set-up of the ATAD reactor. ATAD reactors with two different oxygen inputs were both followed by 7.5 d-HRT mesophilic anaerobic reactors as the final step of dual digestion process. A mesophilic digestion control was used and operated at 35 C with 10-day HRT.
Figure 1. Set-up of the ATAD reactor.
Total solids (TS), volatile solids (VS), chemical oxygen demand (COD), alkalinity of the feed and effluent were measured for all the reactors according to Standard Methods (APHA, 1998). pH was measured daily using Denver Instrument Model 250 pH meter. For ammonia test, solution phase of the samples were filtered through a 0.45 µm filter. Ammonia was tested using the Hach DR4000 Spectrophotometer. O2 content of the recirculation gas was measured by using a Maxtec Oxygen Analyzer OM-25 AE (USA). VFAs were measured on the solution phase of each reactor. Samples were filtered through a 0.45 µm filter and frozen prior to analysis. VFAs were measured using a Shimadzu Gas Chromatograph (Model GC-2010) with flame ionization detector (FID) together with Restek Stabilwaxfi-DA capillary column (USA). To evaluate pathogen destruction, the most probable number (MPN) test (EPA, 2006) for fecal coliform (FC) was conducted for the feed, final effluents and for digester effluent at different time intervals during the profile experiments. RESULTS AND DISCUSSION Pure oxygen was added to the headspace gas recirculation system with two different O2 flow rates, 17.5 mL/min and 35 mL/min. The corresponding oxygen inputs per volume of solids per hour are 0.105 v/v/h and 0.210 v/v/h, respectively. These two different flow rates resulted in two different oxygenation states; anaerobic aerated condition and anoxic condition (McIntosh and Oleszkiewicz, 1997). The oxygen input of 0.105 v/v/h corresponds to anaerobic aerated conditions (ORP levels less than -300 mV). On the other hand, the oxygen input of 0.210 v/v/h corresponds to anoxic conditions (ORP levels 0 to -250 mV). The ORP levels for both conditions are provided in Figure 2 for the 24 h testing period for two different oxygen flow rates. The oxygen flow rate of 0.105 v/v/h produced ORP values in the range between -356 mV and -260 mV, whereas oxygen flow rate 0.210 v/v/h produced ORP values that ranged between -232mV and -172 mV.
-150 0
4
8
12
16
20
24
ORP, mV
-200 -250 -300 -350 -400
Time, h 0.105 v/v/h
0.210 v/v/h
Figure 2. ORP change in the ATAD reactor during the profile experiments for two different oxygen flow rates.
As illustrated in Figure 2, ORP levels of the reactor continued to drop until the first 12 hours of the experiment and started to rise after this point. The drop in ORP levels in the first 12 hours of the experiment suggests that oxygen consumption increases during this period. Decreasing ORP levels can be explained with the increased oxygen uptake during this 12-hour period. This can be confirmed with the change in O2 concentration in the recirculation system during the profile experiment that can be observed from Figure 3. As shown in Figure 3, oxygen percentage in the gas recirculation system decreased from its initial value in the first 12 hours and started to increase again, showing that the oxygen requirement was the greatest in the first 12 hour period. It was reported by Scisson, Jr. (2003) that for ATAD with concentrated feed and slug loading, the oxygen requirement was the greatest in the first 6 hours when much of the waste is stabilized. After this period, the oxygen demand decreases. In our study this transition occurred after the first 12 hours with increasing oxygen percentage in the gas recirculation system.
Recirculation O2, %
40 35 30 25 20 15 10 0
4
8
12 16 Time, h 0.105 v/v/h 0.210 v/v/h
20
24
Figure 3. O2 concentration in the recirculation system during the profile experiments for two different oxygen flow rates. Table 1 gives the calculations of O2 utilization as lb/d for the two different oxygen flow rates. As provided in Table 1, as the O2 flow rate increased, the oxygen utilization expressed as lb O2/d also increased by almost two fold. Increased O2 utilization as lb O2/d resulted in increased VS reduction, VFA production and ammonification that will be explained further. VS reduction for oxygen flow rate of 0.105 v/v/h was 17.8 % and the VS reduction increases to 23.8 % for the oxygen flow rate of 0.210 v/v/h. Together with the calculations in Table 1, oxygen utilizations per volatile solids removed were found to be 1.20 lb/lb VS removed and 0.83 lb/lb VS removed for 0.210 v/v/h and 0.105 v/v/h oxygen flow rates, respectively.
Table 1. Oxygen Utilization at two different oxygen flow rates. Oxygen Utilization
0.210 v/v/h
0.105 v/v/h
O2 Flow rate, L/d
50.40
25.20
Vent Flow, L/d
33.39
22.53
Vent Purity, %
29.42
19.06
O2 Vent Flow, L/d
9.82
4.29
O2 Utilization, %
81.51
82.96
O2 Feed wt, lb/d
0.16
0.08
O2 Utilization, lb/d
0.13
0.07
Increased ammonia concentrations in the ATAD reactor can be explained by the hydrolysis of volatile solids. The ammonia results from the breakdown of proteinaceous material. As illustrated in Figure 4, when the increase in ammonia concentrations for both conditions is compared, higher ammonia concentrations are observed for the oxygen flow rate of 0.210 v/v/h. Ammonia increase was 114% for 0.105 v/v/h oxygen flow rate, whereas this increase was 132% for 0.210 v/v/h oxygen flow rate. Ammonia concentration of final effluent of the dual digestion process with ATAD operated at 0.105 v/v/h oxygen flow rate was 1400 mg/L when compared to 1015 mg/L ammonia concentration for ATAD effluent. This shows that that for the lower oxygen input, ammonia production is not complete in the ATAD reactor. On the other hand for the higher oxygen flow rate there is no further increase in ammonia in the anaerobic digester suggesting that hydrolysis reaction was nearly complete in the ATAD step. It can be concluded that oxygen flow rate in the range of 0.210 v/v/h is optimal to maximize hydrolytic reactions for the downstream anaerobic step for the solids used. 1750
NH3 -N, mg/L
1500 1250 1000 750 500 0
4
8
12
16
20
24
Time, h 0.105 v/v/h 0.210 v/v/h
Figure 4. Changes in ammonia concentration during the profile experiments for two different oxygen flow rates.
Total VFA, mg/L as HAc
As shown in Figure 5, the concentration of VFAs increased with the higher oxygen supply when higher VS destruction was achieved as well. This can be explained with more oxygenated environment improving VS destruction by increasing the kinetics of the thermophilic aerobes (Scisson, Jr., 2003). Acetic acid constituted 70 to 80% of total concentration of VFAs produced. High acetic acid concentrations is desired, since in this study ATAD is evaluated as the pre-treatment step before anaerobic digestion and acetate is the preferred substrate for methanogenic bacteria. In addition, although hydrolytic reactions were complete for oxygen input of 0.210 v/v/h, enhanced acidification was also observed for oxygen input of 0.105 v/v/h in the following anaerobic step of dual digestion process with similar overall VFA produced. 8000 7000 6000 5000 4000 3000 0
4
8
12
16
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24
Time, h 0.105 v/v/h 0.210 v/v/h
Figure 5. Changes in total VFA concentration during the profile experiments for two different oxygen flow rates. As presented in Figure 6, the fecal coliform reduction was achieved in the first 12 hours of the ATAD step, simultaneous with the period when the oxygen requirement was the greatest. Comparably lower FC levels in the reactor for 0.210 v/v/h oxygen flow rate can be a result of the elevated VFA levels for this condition. Effluent of both conditions, met the 40CFR Part 503 regulations with FC MPN values of 1.73*102 MPN/g dry weight for 0.105 v/v/h and 3.99*101 MPN/g dry weight for 0.210 v/v/h oxygen supply rate. Fecal coliform level in the raw solids was 2.50*107.
MPN/g dry weight
1.00E+06 1.00E+05 1.00E+04 1.00E+03 1.00E+02 1.00E+01 1.00E+00 0
2
4
6
8
12
14
20
24
Time, h 0.105 v/v/h
0.210 v/v/h
Figure 6. Fecal coliform levels during the profile experiments for two different oxygen flow rates. In addition, to determine the heat of biological oxidation, the reactor was filled with 10L distilled water and the reactor was operated under the same conditions as the experiments with the municipal solids. Power needed to keep the water temperature in the reactor at 55 C was measured. The objective was to determine the heat of biological oxidation from the difference between the power requirements for the conditions with and without biological reaction. The energy balance in the absence of biological oxidation (no sludge) was obtained by operating the ATAD reactor with distilled water instead of sludge. Equation 1 presents the energy balance in the absence of biological oxidation: qh + qc = qe + qd + qg Eq.1 qh: Power input through electrical heater in the absence of biological oxidation qc: Power input from compressor qe: Power lost by evaporation qd: Power lost through reactor’s body qg: Power lost through gas stream (aeration) Equation 2 presents the energy balance in the presence of biological oxidation: qh* + qc + qb = qe + qd + qg
Eq.2
qh*: Power input through electrical heater in the presence biological oxidation qb: Power generated by biological process For the energy balance in the presence of biological oxidation, qc, qd, qg are the same as the energy balance in the absence of biological oxidation. qh will be lower compared to the experiment run with water since another parameter will be added to the equation for the contribution of the heat released from biological oxidation, qb. Therefore, the difference
between the power input of the electrical heater in two different conditions (with and without biological reaction) will be used to calculate the heat of biological oxidation. The biological heat of oxidations were calculated to be 14,300 J/g VS removed and 15,900 J/g VS removed for the oxygen flow rates of 0.105 v/v/h and 0.210 v/v/h, respectively. Studies of Nosrati et al. (2007) found biological heat of oxidation ranging from 16,313 to 17,347 J/g VS removed for oxygen flow rates ranging from 0.92 to 1.84 v/v/h, respectively that are much lower that the oxygen flow rated used in this study (0.105 and 0.210 v/v/h). Gemmel et al. (1999) reported biological heat of reaction of 16,600 J/g VS removed. Pitt and Ekama (1996) observed a biological heat of reaction of 17,000 J/g VS removed. These values, together with the values obtained from this study are smaller than the heat of combustion of cells, 21,000 J/gVSS destroyed. This difference can be explained by the fact that cells require energy for maintenance purposes even during the endogenous respiration phase. In addition, all parts of a cell cannot be assumed to be biodegradable matter that would contribute heat release by biological oxidation (Nosrati et al., 2007). Lower oxygen flow rates used in this study can explain the lower biological heat of reactions observed which would suggest that more energy is used for maintenance purposes. Table 2. Comparison of two dual digestion processes and a conventional digestion process in terms of VS removal, biogas production and pathogen reduction.
Parameter
0.105 v/v/h Anaerobic ATAD Digester
0.210 v/v/h Anaerobic ATAD Digester
Control Digester
HRT, d
2.5
7.5
2.5
7.5
10
Temperature, °C
55
35
55
35
35
VS removal, %
17.8
32.5
23.8
30.0
48
Methane Content, %
-
67
-
67
60
Biogas Yield, m3 CH4/kg VS fed
-
0.26
-
0.27
0.22
Biogas Yield, m3 CH4/kg VS removed
-
0.87
-
0.92
0.47
1.73*102
