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An Evaluation of an Autothermal Aerobic Digestion System
0957–5820/02/$10.00+0.00 # Institution of Chemical Engineers Trans IChemE, Vol 80, Part B, March 2002
AN EVALUATION OF AN AUTOTHERMAL AEROBIC DIGESTION SYSTEM D. W. RILEY1 and C. F. FORSTER2 1
United Utilities plc, Wastewater Technical Support Facility, Ellesmere Port Sewage Treatment Works, Cheshire, UK 2 School of Civil Engineering, University of Birmingham, Edgbaston, Birmingham, UK
T
he speci cations of the Safe Sludge Matrix mean that there is a need to examine treatment processes which achieve a degree of digestion. There are a number of processes which operate in the thermophilic range. This study examines one such option, an autothermal thermophilic aerobic digester (ATAD) which uses a high aspect ratio design and a hyperbolic stirrer. Mixing times were found to range from 4 to 10 minutes and oxygenation capacities from 10 to 30 kg h¡1. Using a hydraulic retention time of 7 days, autothermal conditions did occur with a speci c heat yield of 15.68 MJ kg¡1 oxygen. A comparison of a range of air:liquid ratios (v=v) showed that a ratio of 0.5:1 would be recommended. This gave a reduction in volatile solids of 35%. Keywords: sewage sludge; aerobic digestion; thermophilic; autothermal; performance.
INTRODUCTION
The objectives of the work being reported in this paper were:
The amount of sludge generated in the UK during the biological treatment of sewage is of the order of 1,746,000 dry tonnes, a gure which is predicted to rise to 2,155,000 tonnes by 20061. Therefore, considerable efforts are being made to derive process streams which will minimize the cost of treating and processing waste sludges. In addition, attention is being focused on the quality of the end product, particularly when the sludge is to be used on agricultural land. The ‘Safe Sludge Matrix’, which de nes the type of treatment required for different types of land use, has introduced the concept of ‘Enhanced Treatment’2. Sludges which have received enhanced treatment will be free of Salmonella and have had a 6 log reduction in pathogens. The European Directive on the use of sewage sludge on agricultural land is also under review and the third draft of the new directive contains similar speci cations3. As a result, there is considerable interest in sludge treatment processes which operate at temperatures of 55± C or higher. Two processes which fall into these categories are thermophilic aerobic digestion (TAD) and autothermal thermophilic aerobic digestion (ATAD). TAD is frequently used as the initial part of a two-stage process in which the second stage is mesophilic anaerobic digestion4,5. This approach has certainly been shown to produce a sludge which will meet the criteria for the US EPA Class A solids6. ATAD is currently being considered as a single-stage treatment and it has been reported that the ATAD process could compete on economic grounds with anaerobic digestion7. It has also been stated that, unless pasteurization became a requirement for the disposal of sludge to agricultural land, thermophilic aerobic digestion was unlikely to nd wide use in the UK8. This requirement is now, in part, a reality. There are a limited number of TAD=ATAD equipmentmanufacturers in the world. One of them is the Swiss company, Alpha International Ltd9.
² To characterize the ATAD reactor (Alpha Environmental Technology Ltd.) in terms of its main physical parameters. ² To determine the characteristics of the digested sludge. ² To determine how effectively sludge could be treated at a hydraulic retention time of 7 days. This retention time was chosen as it had been recommended by previous workers10–12. The criteria used for assessing treatment were the fraction of volatile solids destroyed (FVSD) and the reduction in pathogenic and indicator bacteria.
METHODS AND MATERIALS Sludge Samples The ATAD sludge was obtained from a 30 m3 pilot plant (Alpha International Ltd.) being operated by United Utilities plc (Figure 1). The basic dimensions of the reactor, which was manufactured from 1.4306 stainless steel, are: ² 14 m overall height. ² 10.5 m reactor chamber height. ² 2 metres reactor chamber diameter (this gave an aspect ratio of 5:1). ² Process working volume of 30 m3. ² Freeboard to allow foam expansion of approximately 1 metre (this was expanded by reducing active volume to accommodate excess foaming during the trial). ² Mixing was provided by a motor-driven hyperbolic stirrer with a gear box and a drive shaft supported by a foot bearing. Intermediate paddle stirrers were employed at 1=3 and 2=3 levels. ² A rotating vane compressor, capable of supplying 120 m3 h¡ 1 at 1.1 bar (gauge), provided the air supply. 100
EVALUATION OF AN AUTOTHERMAL AEROBIC DIGESTION SYSTEM
101
The oxygen transfer measurements used three dissolved oxygen probes within the vessel located at the top, intermediate and approximately 1 metre above the hyperbolic stirrer in the base of the reactor. Phox Systems probes were used in the lower and middle zones and an ATI Corporation probe was used in the top zone. The probes were monitored via a three-channel logger situated at the top of the vessel. Dissolved oxygen concentrations (Ct) were measured every minute in the case of the lower probes and every 30 seconds for the upper probe. The vessel was lled with 30 m3 of good quality nal ef uent and purged of oxygen by the introduction of sodium sulphite (100 gm¡3) and cobalt chloride (0.2 gm¡3) prior to starting the aeration trials. The slopes of semi-logarithmic plots of (C*–Ct) against time gave values for the mass transfer coef cient, KLa, which were then used to calculate the oxygenation capacities, OC (kg O2h¡ 1): ¤ V 10¡3 OC = KL a20C20
where KLa20 = mass transfer coef cient at 20± C, h¡1 ¤ C20 = saturated dissolved oxygen concentration, mg l¡1 V = reactor volume, m3 Statistical analyses were done with the Analysis ToolPak in Microsoft Excel. RESULTS AND DISCUSSION
Figure 1. Schematic diagram of the ATAD pilot plant.
² Off-gas management was by air blowers designed to extract 180 m3 h¡1 at 12 mbar (gauge) and treatment of the off-gas was achieved with a compost style bio lter. The feed for this reactor was co-settled sludge and its characteristics are summarized in Table 1. The reactor was operated in a semi-continuous mode (one addition of feed per day) with a hydraulic retention time of 7 days. Air ow rates were varied from 11 to 49 m3 h¡1. This meant that, on an hourly basis, the air:liquid ratio (v=v) varied from 0.5 to 2.5. Analytical Methods
The results of the aeration trials are presented in Table 2 (as mass transfer coef cients) and Figure 2 (as oxygenation Table 2. Effect of stirrer speed and aeration rates on the oxygen transfer and mixing in the ATAD reactor. Stirrer speed, rpm
Aeration rate, m3 h¡ 1
KLa, h¡ 1
Mixing time, min
42 42 42 84 84
140 70 – 70 –
38.79 28.18 – 46.87 –
4 – 10 – 8
Total solids’ and volatile solids’ concentrations were measured on site by the standard gravimetric methods13. Capillary suction times (CST) were measured with a Triton (Type 65) automatic system. Other analyses were carried out at the NAMAS accredited laboratories at Lingley Mere operated by United Utilities plc. These included elemental analyses, calori c value measurements and bacteriological counts. Tracer studies were done with Rhodamine B using a Jenoway uorimeter to measure the dye concentration. Table 1. Characteristics of the feed sludge (mg l¡ 1). Parameter
Mean § standard deviation
TS VS Amm-N o-PO4 pH COD BOD
54,180§ 13,000 79.96%§ 0.95 192.3§ 48.1 244.4§ 100.7 5.04§ 0.16 66,585§ 22,920 20,103§ 6752
Trans IChemE, Vol 80, Part B, March 2002
Figure 2. Oxygenation capacities of the ATAD at 70 m3 h¡ 1 and stirrer speeds of 42 and 84 rpm.
102
RILEY and FORSTER
capacities). The oxygenation capacity was greatest at the base of the reactor immediately above the hyperbolic stirrer and varied with the speed of mixing. This variation of the oxygenation capacity across the reactor followed a power relationship. Taking the value for the lower mixing speed (42 rpm) in the middle of the reactor, the daily transfer of oxygen is 185 kg which is in excess of that required to oxidize 40% of the mean COD of the raw sludge, assuming an a value of 0.6 which is a compromise value midway between the two extremes likely to be found14. The results of the mixing trials are given in Figure 3 and Table 2. These show, as would be expected, that better mixing is achieved with the higher stirrer speed and that aeration also improves mixing. The results overall show that the ATAD reactor is well mixed, with full mixing occurring in about 10 minutes in the worst situation, the slower speed and no aeration. The rst stage of the semi-continuous trials was to determine whether the system was capable of operating in an autothermal manner. The results in Figure 4 show clearly that autothermal conditions could be achieved. The reactor temperature did drop as the feed was introduced but over the daily cycle the temperature did increase, without any external energy input (other than that required for the stirrer), to values well within the thermophilic range. These results,
together with the data for the amount of oxygen added and the off-gas analyses, enabled the speci c heat yield to be calculated. The value which was obtained, 15.68 MJ kg¡1 oxygen, compares well with the mean value of 13.1 MJ kg¡1 reported by Messenger et al.15. During the semi-continuous tests, it was found that digestion did increase the pH in relation to the raw sludge, from a mean value of 5.04 to one of 8.26. However, the variation in the air:liquid ratio did not alter the pH to any great extent. The variations in the ammoniacal-nitrogen and ortho-phosphate concentrations are shown in Figure 5. Alterations to the air:liquid ratio had no apparent effect on these concentrations but a comparison with the data in Table 1 shows that the ATAD process caused an increase in the mean ammoniacal-nitrogen concentrations and a decrease in the mean ortho-phosphate concentrations. These differences were statistically signi cant (ANOVA; P < 0.05). It is presumed that the ammoniacal-nitrogen would be generated by degradation of proteinaceous matter in raw sludge. The decrease in ortho-phosphate can only be explained by presuming that there was an uptake by the sludge but further work would be needed to substantiate both these points. The ATAD performance was judged on the basis of the destruction of volatile solids. This was calculated as the fraction of volatile solids destroyed (FVSD) by the constant ash method. The results for the ve air:liquid ratios are presented in Figure 6 and show that there were variations in performance. To determine whether the differences in performance were signi cant, the data were examined statistically (ANOVA; P > or < 0.05). The results, which are summarized in Table 3, show that some of the FVSD values were signi cantly different from others. At the present time it is not known why some air:liquid ratios should give rise to better FVSD values than others. On the basis of these results it would be recommended that the lowest air:liquid ratio should be used as this will give the greatest FVSD value, 34.6 § 3.1%, with the lowest aeration cost. The results of the bacteriological analyses showed that the faecal coliform counts in the digested sludge,
Figure 3. Effect of varying the stirrer speed and aeration on mixing in the ATAD reactor (~ 42 rpm 140 m3 h¡ 1; ¨ 42 rpm no air; ² 84 rpm no air).
Figure 4. Typical ATAD temperature cycling.
Figure 5. Concentrations of ammoniacal-nitrogen (¨) and ortho-phosphate (²) in the ATAD treated sludge.
Trans IChemE, Vol 80, Part B, March 2002
EVALUATION OF AN AUTOTHERMAL AEROBIC DIGESTION SYSTEM
103
increased from 495 sec to 842 sec, an increase which was signi cant (ANOVA; P < 0.05). High CST values of thermally digested sludges have been noted previously17,18 and attributed to the small size of the sludge particles18. The elemental analyses of the raw and digested sludge (Table 4) show that digestion decreased the concentration of hydrogen in the volatile solids (ANOVA; P = 0.018). There was no signi cant change in the carbon, nitrogen, sulphur or phosphorus concentrations (ANOVA; P > 0.05). The reduction in hydrogen is compatible with the sludge having been treated in a highly oxidizing process. CONCLUSIONS Figure 6. The effect of varying the air:liquid ratio on the reduction in VS.
Table 3. Results of the ANOVA tests for the ATAD trials. Air:liquid ratio
0.5
1.0
1.5
2.2
2.5
0.5 1.0 1.5 2.2 2.5
– Yes No No Yes
Yes – Yes Yes No
No Yes – No Yes
No Yes No – Yes
Yes No Yes Yes –
Yes = Difference is signi cant. No = Difference is not signi cant.
over an eight month period, had a mean value of 23.9§ 2.6 cfu g¡ 1 DS. The counts of the viral indicator species (F+ phage and Somatic phage) were < 0.25 pfu g¡ 1 DS. ATAD also altered the calori c value of the sludge, decreasing it from a mean value of 23,450 kJ kg¡1 to one of 20,300 kJ kg¡ 1. This decrease was statistically signi cant (ANOVA; P < 0.05). This can be attributed to the oxidation of putrescible volatile matter present in the raw sludge. Certainly there was a trend (rather than any sound mathematical relationship) for the calori c value to decrease as the COD decreased and as the carbon in the volatile solids decreased. However, there were neither relationships nor trends to relate the calori c value to the volatile solids as has been suggested previously16. CST was only measured during the 1:1 air:liquid trial but the results showed that lterability was another parameter which was affected by the digestion. The mean CST
Table 4. Elemental concentrations (%DS) of the raw and digested sludges. Mean value
Variance
Element
Raw
Digested
Raw
Digested
Difference ANOVA, P < 0.05
Hydrogen Nitrogen Organic-C Sulphur Total-P
8.39 3.38 52.70 0.663 0.854
7.43 3.62 50.10 0.820 0.867
1.267 0.335 35.19 0.105 0.117
1.826 1.842 34.14 0.158 0.057
Yes No No No No
Trans IChemE, Vol 80, Part B, March 2002
² The ATAD reactor under examination was able to effect rapid mixing which, in conjunction with the hyperbolic stirrer, achieved oxygenation capacities ranging from 10 to 30 kg h¡1. ² Autothermal heating was achieved with hydraulic retention times of 7 days. ² An examination of the FVSD values achieved at a hydraulic retention time of 7 days and air:liquid ratios of 0.5:1, 1.5:1 and 2.2:1 were not statistically different and were greater than those achieved at ratios of 1.0:1 and 2.5:1. It is, therefore, suggested that the 0.5:1 ratio would be the most cost effective. ² Digestion lowered the calori c value and the hydrogen content of the sludge. This is indicative of carbonaceous matter being oxidized in the ATAD process.
REFERENCES 1. Boon, A. G. and Thomas, V., 1996, Resource or rubbish? The Chemical Engineer, May 30, 25–32. 2. Anon, 1999, Safe Sludge Matrix: Guidelines for the Application of Sewage Sludge to Agricultural Land (AMPU 1234=C) (ADAS). 3. Evans, T., 2001, A comparison of international standards for the bene cial use of biomaterials: can we see where we might be going? J CIWEM, 15: 64–71. 4. Hawash, S., Ibiari, N., Aly, F. H., El Diwani, G. and Hamad, M. A., 1994, Kinetic study of thermophilic aerobic stabilisation of sludge, Biomass and Bioenergy, 6: 283–286. 5. Pagilla, K. R., Craney, K. C. and Kido, W. H., 1996, Aerobic thermophilic pretreatment of mixed sludge for pathogen reduction and Nocardia control, Wat Environ Res, 68: 1093–1098. 6. Ward, A., Stensel, H. D., Ferguson, J. F., Ma, G. and Hummel, S., 1998, Effect of autothermal treatment on anaerobic digestion in the dual digestion process, Wat Sci Technol, 38(8–9): 435–442. 7. Deeny, K., Heidman, J. and Smith, J., 1985, Autothermal thermophilic aerobic digestion in the Federal Republic of Germany, Proc 40th Ind Waste Conf, Purdue Univ (Butterworths, Boston, USA), 959–968. 8. Edgington, R. and Clay, S., 1993, Evaluation and development of a thermophilic aerobic digestor at Castle Donnington sewage-treatment works, J IWEM, 7: 149–155. 9. Davies, W. J. and Messerli, P., 2000, Pre-pasteurisation and operating cost savings using thermophilic anaerobic digestion retro t to conventional mesophilic anaerobic digestion, 5th European Biosolids and Organic Residuals Conference. 10. Morgan, S. F. and Gunson, H. G., 1989, The development of an aerobic thermophilic digestion system in the UK, in: Treatment of Sewage Sludge, Bruce, A. M., Colin, F. and Newman, P. J. (Eds) (Elsevier Applied Science, London), 29–34. 11. Bruce, A. M., 1989, Other investigations of thermophilic aerobic digestion in the UK, in: Treatment of Sewage Sludge, Bruce, A. M., Colin, F. and Newman, P. J. (Eds) (Elsevier Applied Science, London), 39–43. 12. Department of the Environment, 1989, Code of Practice for Agricultural Use of Sewage Sludge (HMSO, London).
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13. Greenberg, A. E., Clesceri, L. S. and Eaton, A. D., 1992, Standard Methods for the Examination of Water and Wastewater (18th Edition) (American Public Health Association (APHA), Washington, DC, USA). 14. Forster. C. F., 1985, Biotechnology and Wastewater Treatment (Cambridge University Press, Cambridge). 15. Messenger, J. R., de Villiers, H. A. and Ekama, G. A., 1993, Evaluation of the dual digestion system: Part 2: Operation and performance of the pure oxygen aerobic reactor, Water SA, 18: 193–200. 16. Thomas, J., 1975, Sludge incineration. New aspects of multiple hearth furnace and uidised bed incinerators, Prog Wat Technol, 7: 935–946. 17. Messenger, J. R., de Villiers, H. A., Laubscher, S. J. A., Kenmuir, K. and Ekama, G. A., 1993, Evaluation of the dual digestion system: Part 1: Overview of the Milnerton experience, Water SA, 18: 185–191.
18. Riley, D. W. and Forster, C. F., 2001, The physico-chemical characteristics of thermophilic aerobic sludges, J Chem Tech Biotech, 76: 862–866.
ADDRESS Correspondence concerning this paper should be addressed to Dr C. F. Forster, School of Civil Engineering, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK. E-mail: [email protected] The manuscript was received 24 August 2001 and accepted for publication after revision 18 February 2002.
Trans IChemE, Vol 80, Part B, March 2002
AN EVALUATION OF AN AUTOTHERMAL AEROBIC DIGESTION SYSTEM D. W. RILEY1 and C. F. FORSTER2 1
United Utilities plc, Wastewater Technical Support Facility, Ellesmere Port Sewage Treatment Works, Cheshire, UK 2 School of Civil Engineering, University of Birmingham, Edgbaston, Birmingham, UK
T
he speci cations of the Safe Sludge Matrix mean that there is a need to examine treatment processes which achieve a degree of digestion. There are a number of processes which operate in the thermophilic range. This study examines one such option, an autothermal thermophilic aerobic digester (ATAD) which uses a high aspect ratio design and a hyperbolic stirrer. Mixing times were found to range from 4 to 10 minutes and oxygenation capacities from 10 to 30 kg h¡1. Using a hydraulic retention time of 7 days, autothermal conditions did occur with a speci c heat yield of 15.68 MJ kg¡1 oxygen. A comparison of a range of air:liquid ratios (v=v) showed that a ratio of 0.5:1 would be recommended. This gave a reduction in volatile solids of 35%. Keywords: sewage sludge; aerobic digestion; thermophilic; autothermal; performance.
INTRODUCTION
The objectives of the work being reported in this paper were:
The amount of sludge generated in the UK during the biological treatment of sewage is of the order of 1,746,000 dry tonnes, a gure which is predicted to rise to 2,155,000 tonnes by 20061. Therefore, considerable efforts are being made to derive process streams which will minimize the cost of treating and processing waste sludges. In addition, attention is being focused on the quality of the end product, particularly when the sludge is to be used on agricultural land. The ‘Safe Sludge Matrix’, which de nes the type of treatment required for different types of land use, has introduced the concept of ‘Enhanced Treatment’2. Sludges which have received enhanced treatment will be free of Salmonella and have had a 6 log reduction in pathogens. The European Directive on the use of sewage sludge on agricultural land is also under review and the third draft of the new directive contains similar speci cations3. As a result, there is considerable interest in sludge treatment processes which operate at temperatures of 55± C or higher. Two processes which fall into these categories are thermophilic aerobic digestion (TAD) and autothermal thermophilic aerobic digestion (ATAD). TAD is frequently used as the initial part of a two-stage process in which the second stage is mesophilic anaerobic digestion4,5. This approach has certainly been shown to produce a sludge which will meet the criteria for the US EPA Class A solids6. ATAD is currently being considered as a single-stage treatment and it has been reported that the ATAD process could compete on economic grounds with anaerobic digestion7. It has also been stated that, unless pasteurization became a requirement for the disposal of sludge to agricultural land, thermophilic aerobic digestion was unlikely to nd wide use in the UK8. This requirement is now, in part, a reality. There are a limited number of TAD=ATAD equipmentmanufacturers in the world. One of them is the Swiss company, Alpha International Ltd9.
² To characterize the ATAD reactor (Alpha Environmental Technology Ltd.) in terms of its main physical parameters. ² To determine the characteristics of the digested sludge. ² To determine how effectively sludge could be treated at a hydraulic retention time of 7 days. This retention time was chosen as it had been recommended by previous workers10–12. The criteria used for assessing treatment were the fraction of volatile solids destroyed (FVSD) and the reduction in pathogenic and indicator bacteria.
METHODS AND MATERIALS Sludge Samples The ATAD sludge was obtained from a 30 m3 pilot plant (Alpha International Ltd.) being operated by United Utilities plc (Figure 1). The basic dimensions of the reactor, which was manufactured from 1.4306 stainless steel, are: ² 14 m overall height. ² 10.5 m reactor chamber height. ² 2 metres reactor chamber diameter (this gave an aspect ratio of 5:1). ² Process working volume of 30 m3. ² Freeboard to allow foam expansion of approximately 1 metre (this was expanded by reducing active volume to accommodate excess foaming during the trial). ² Mixing was provided by a motor-driven hyperbolic stirrer with a gear box and a drive shaft supported by a foot bearing. Intermediate paddle stirrers were employed at 1=3 and 2=3 levels. ² A rotating vane compressor, capable of supplying 120 m3 h¡ 1 at 1.1 bar (gauge), provided the air supply. 100
EVALUATION OF AN AUTOTHERMAL AEROBIC DIGESTION SYSTEM
101
The oxygen transfer measurements used three dissolved oxygen probes within the vessel located at the top, intermediate and approximately 1 metre above the hyperbolic stirrer in the base of the reactor. Phox Systems probes were used in the lower and middle zones and an ATI Corporation probe was used in the top zone. The probes were monitored via a three-channel logger situated at the top of the vessel. Dissolved oxygen concentrations (Ct) were measured every minute in the case of the lower probes and every 30 seconds for the upper probe. The vessel was lled with 30 m3 of good quality nal ef uent and purged of oxygen by the introduction of sodium sulphite (100 gm¡3) and cobalt chloride (0.2 gm¡3) prior to starting the aeration trials. The slopes of semi-logarithmic plots of (C*–Ct) against time gave values for the mass transfer coef cient, KLa, which were then used to calculate the oxygenation capacities, OC (kg O2h¡ 1): ¤ V 10¡3 OC = KL a20C20
where KLa20 = mass transfer coef cient at 20± C, h¡1 ¤ C20 = saturated dissolved oxygen concentration, mg l¡1 V = reactor volume, m3 Statistical analyses were done with the Analysis ToolPak in Microsoft Excel. RESULTS AND DISCUSSION
Figure 1. Schematic diagram of the ATAD pilot plant.
² Off-gas management was by air blowers designed to extract 180 m3 h¡1 at 12 mbar (gauge) and treatment of the off-gas was achieved with a compost style bio lter. The feed for this reactor was co-settled sludge and its characteristics are summarized in Table 1. The reactor was operated in a semi-continuous mode (one addition of feed per day) with a hydraulic retention time of 7 days. Air ow rates were varied from 11 to 49 m3 h¡1. This meant that, on an hourly basis, the air:liquid ratio (v=v) varied from 0.5 to 2.5. Analytical Methods
The results of the aeration trials are presented in Table 2 (as mass transfer coef cients) and Figure 2 (as oxygenation Table 2. Effect of stirrer speed and aeration rates on the oxygen transfer and mixing in the ATAD reactor. Stirrer speed, rpm
Aeration rate, m3 h¡ 1
KLa, h¡ 1
Mixing time, min
42 42 42 84 84
140 70 – 70 –
38.79 28.18 – 46.87 –
4 – 10 – 8
Total solids’ and volatile solids’ concentrations were measured on site by the standard gravimetric methods13. Capillary suction times (CST) were measured with a Triton (Type 65) automatic system. Other analyses were carried out at the NAMAS accredited laboratories at Lingley Mere operated by United Utilities plc. These included elemental analyses, calori c value measurements and bacteriological counts. Tracer studies were done with Rhodamine B using a Jenoway uorimeter to measure the dye concentration. Table 1. Characteristics of the feed sludge (mg l¡ 1). Parameter
Mean § standard deviation
TS VS Amm-N o-PO4 pH COD BOD
54,180§ 13,000 79.96%§ 0.95 192.3§ 48.1 244.4§ 100.7 5.04§ 0.16 66,585§ 22,920 20,103§ 6752
Trans IChemE, Vol 80, Part B, March 2002
Figure 2. Oxygenation capacities of the ATAD at 70 m3 h¡ 1 and stirrer speeds of 42 and 84 rpm.
102
RILEY and FORSTER
capacities). The oxygenation capacity was greatest at the base of the reactor immediately above the hyperbolic stirrer and varied with the speed of mixing. This variation of the oxygenation capacity across the reactor followed a power relationship. Taking the value for the lower mixing speed (42 rpm) in the middle of the reactor, the daily transfer of oxygen is 185 kg which is in excess of that required to oxidize 40% of the mean COD of the raw sludge, assuming an a value of 0.6 which is a compromise value midway between the two extremes likely to be found14. The results of the mixing trials are given in Figure 3 and Table 2. These show, as would be expected, that better mixing is achieved with the higher stirrer speed and that aeration also improves mixing. The results overall show that the ATAD reactor is well mixed, with full mixing occurring in about 10 minutes in the worst situation, the slower speed and no aeration. The rst stage of the semi-continuous trials was to determine whether the system was capable of operating in an autothermal manner. The results in Figure 4 show clearly that autothermal conditions could be achieved. The reactor temperature did drop as the feed was introduced but over the daily cycle the temperature did increase, without any external energy input (other than that required for the stirrer), to values well within the thermophilic range. These results,
together with the data for the amount of oxygen added and the off-gas analyses, enabled the speci c heat yield to be calculated. The value which was obtained, 15.68 MJ kg¡1 oxygen, compares well with the mean value of 13.1 MJ kg¡1 reported by Messenger et al.15. During the semi-continuous tests, it was found that digestion did increase the pH in relation to the raw sludge, from a mean value of 5.04 to one of 8.26. However, the variation in the air:liquid ratio did not alter the pH to any great extent. The variations in the ammoniacal-nitrogen and ortho-phosphate concentrations are shown in Figure 5. Alterations to the air:liquid ratio had no apparent effect on these concentrations but a comparison with the data in Table 1 shows that the ATAD process caused an increase in the mean ammoniacal-nitrogen concentrations and a decrease in the mean ortho-phosphate concentrations. These differences were statistically signi cant (ANOVA; P < 0.05). It is presumed that the ammoniacal-nitrogen would be generated by degradation of proteinaceous matter in raw sludge. The decrease in ortho-phosphate can only be explained by presuming that there was an uptake by the sludge but further work would be needed to substantiate both these points. The ATAD performance was judged on the basis of the destruction of volatile solids. This was calculated as the fraction of volatile solids destroyed (FVSD) by the constant ash method. The results for the ve air:liquid ratios are presented in Figure 6 and show that there were variations in performance. To determine whether the differences in performance were signi cant, the data were examined statistically (ANOVA; P > or < 0.05). The results, which are summarized in Table 3, show that some of the FVSD values were signi cantly different from others. At the present time it is not known why some air:liquid ratios should give rise to better FVSD values than others. On the basis of these results it would be recommended that the lowest air:liquid ratio should be used as this will give the greatest FVSD value, 34.6 § 3.1%, with the lowest aeration cost. The results of the bacteriological analyses showed that the faecal coliform counts in the digested sludge,
Figure 3. Effect of varying the stirrer speed and aeration on mixing in the ATAD reactor (~ 42 rpm 140 m3 h¡ 1; ¨ 42 rpm no air; ² 84 rpm no air).
Figure 4. Typical ATAD temperature cycling.
Figure 5. Concentrations of ammoniacal-nitrogen (¨) and ortho-phosphate (²) in the ATAD treated sludge.
Trans IChemE, Vol 80, Part B, March 2002
EVALUATION OF AN AUTOTHERMAL AEROBIC DIGESTION SYSTEM
103
increased from 495 sec to 842 sec, an increase which was signi cant (ANOVA; P < 0.05). High CST values of thermally digested sludges have been noted previously17,18 and attributed to the small size of the sludge particles18. The elemental analyses of the raw and digested sludge (Table 4) show that digestion decreased the concentration of hydrogen in the volatile solids (ANOVA; P = 0.018). There was no signi cant change in the carbon, nitrogen, sulphur or phosphorus concentrations (ANOVA; P > 0.05). The reduction in hydrogen is compatible with the sludge having been treated in a highly oxidizing process. CONCLUSIONS Figure 6. The effect of varying the air:liquid ratio on the reduction in VS.
Table 3. Results of the ANOVA tests for the ATAD trials. Air:liquid ratio
0.5
1.0
1.5
2.2
2.5
0.5 1.0 1.5 2.2 2.5
– Yes No No Yes
Yes – Yes Yes No
No Yes – No Yes
No Yes No – Yes
Yes No Yes Yes –
Yes = Difference is signi cant. No = Difference is not signi cant.
over an eight month period, had a mean value of 23.9§ 2.6 cfu g¡ 1 DS. The counts of the viral indicator species (F+ phage and Somatic phage) were < 0.25 pfu g¡ 1 DS. ATAD also altered the calori c value of the sludge, decreasing it from a mean value of 23,450 kJ kg¡1 to one of 20,300 kJ kg¡ 1. This decrease was statistically signi cant (ANOVA; P < 0.05). This can be attributed to the oxidation of putrescible volatile matter present in the raw sludge. Certainly there was a trend (rather than any sound mathematical relationship) for the calori c value to decrease as the COD decreased and as the carbon in the volatile solids decreased. However, there were neither relationships nor trends to relate the calori c value to the volatile solids as has been suggested previously16. CST was only measured during the 1:1 air:liquid trial but the results showed that lterability was another parameter which was affected by the digestion. The mean CST
Table 4. Elemental concentrations (%DS) of the raw and digested sludges. Mean value
Variance
Element
Raw
Digested
Raw
Digested
Difference ANOVA, P < 0.05
Hydrogen Nitrogen Organic-C Sulphur Total-P
8.39 3.38 52.70 0.663 0.854
7.43 3.62 50.10 0.820 0.867
1.267 0.335 35.19 0.105 0.117
1.826 1.842 34.14 0.158 0.057
Yes No No No No
Trans IChemE, Vol 80, Part B, March 2002
² The ATAD reactor under examination was able to effect rapid mixing which, in conjunction with the hyperbolic stirrer, achieved oxygenation capacities ranging from 10 to 30 kg h¡1. ² Autothermal heating was achieved with hydraulic retention times of 7 days. ² An examination of the FVSD values achieved at a hydraulic retention time of 7 days and air:liquid ratios of 0.5:1, 1.5:1 and 2.2:1 were not statistically different and were greater than those achieved at ratios of 1.0:1 and 2.5:1. It is, therefore, suggested that the 0.5:1 ratio would be the most cost effective. ² Digestion lowered the calori c value and the hydrogen content of the sludge. This is indicative of carbonaceous matter being oxidized in the ATAD process.
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13. Greenberg, A. E., Clesceri, L. S. and Eaton, A. D., 1992, Standard Methods for the Examination of Water and Wastewater (18th Edition) (American Public Health Association (APHA), Washington, DC, USA). 14. Forster. C. F., 1985, Biotechnology and Wastewater Treatment (Cambridge University Press, Cambridge). 15. Messenger, J. R., de Villiers, H. A. and Ekama, G. A., 1993, Evaluation of the dual digestion system: Part 2: Operation and performance of the pure oxygen aerobic reactor, Water SA, 18: 193–200. 16. Thomas, J., 1975, Sludge incineration. New aspects of multiple hearth furnace and uidised bed incinerators, Prog Wat Technol, 7: 935–946. 17. Messenger, J. R., de Villiers, H. A., Laubscher, S. J. A., Kenmuir, K. and Ekama, G. A., 1993, Evaluation of the dual digestion system: Part 1: Overview of the Milnerton experience, Water SA, 18: 185–191.
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ADDRESS Correspondence concerning this paper should be addressed to Dr C. F. Forster, School of Civil Engineering, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK. E-mail: [email protected] The manuscript was received 24 August 2001 and accepted for publication after revision 18 February 2002.
Trans IChemE, Vol 80, Part B, March 2002