AEROBIC THERMOPHILIC AND ANAEROBIC MESOPHILIC TREATMENT OF SWINE WASTE

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1 PII: S (00) Wat. Res. Vol. 34, No. 10, pp. 2747±2753, Elsevier Science Ltd. All rights reserved Printed in Great Britain /00/$ - see front matter AEROBIC THERMOPHILIC AND ANAEROBIC MESOPHILIC TREATMENT OF SWINE WASTE KRISHNA R. PAGILLA 1 *, HYUNGJIN KIM 1 and TAPANA CHEUNBARN 2 1 Department of Chemical and Environmental Engineering, Illinois Institute of Technology, Chicago, IL 60616, USA and 2 Faculty of Science, Maejo University, Chiang-Mai, 50290, Thailand (First received 1 July 1999; accepted in revised form 1 November 1999) AbstractÐLaboratory experiments were conducted to investigate two-stage aerobic thermophilic and anaerobic mesophilic treatment of swine waste. The two-stage system included a 1-day sludge retention time (SRT) aerobic thermophilic reactor operating at 628C and with 1.0 mg dissolved oxygen/l, followed by a 5, 9, and 14-day SRT anaerobic mesophilic digester operating at 378C. A single stage anaerobic mesophilic digester operating at 6, 10, and 15-day SRT and 378C was used as the control. Feed swine waste slurry average composition included total solids (TS)=4.3%; volatile solids (VS)=67% of TS; supernatant chemical oxygen demand (COD)=14,330 mg/l; fecal coliform density= MPN/g TS; ph=7.1; and alkalinity=2700 mg CaCO 3 /l. Two-stage system operating at 6, 10, and 15-day system SRT reduced VS by 46, 54, and 61%, respectively, and was signi cantly better than the control at each SRT. Supernatant COD reduction by the two-stage system (56±67%) was signi cantly better than that obtained in the control (44±60%). Fecal coliform density was reduced to <10 3 MPN/g TS at all SRT by the two-stage system, whereas, the control did not reduce the fecal coliform density below 10 5 MPN/g TS at all SRT. The two-stage system anaerobic digester produced 0.56±0.64 m 3 CH 4 /kg VS destroyed compared to lower levels of 0.47±0.51 m 3 CH 4 /kg VS destroyed by the control, both operating at 6, 10, and 15-day system SRT. The methane gas production by the twostage system of 0.26, 0.32, and 0.39 m 3 /kg VS fed at 6, 10, and 15-day system SRT, respectively, was signi cantly higher than that by the control system (0.17, 0.22, and 0.25 m 3 /kg VS fed at 6, 10, and 15- day SRT, respectively). The biogas produced by the two-stage system anaerobic digester contained 353±387 ppm (v/v) H 2 S content compared to 569±609 ppm (v/v) H 2 S content in the biogas from the control anaerobic digester. The time-to- lter values of the product sludge from the two-stage system (245, 197, and 158 s at 6, 10, and 15-day system SRT, respectively) were about 50% lower than those of the product sludge from the control (355, 295, and 250 s at 6, 10, and 15-day system SRT, respectively), indicating better dewaterability of the two-stage system product sludge Elsevier Science Ltd. All rights reserved Key wordsðswine waste, thermophilic, two-stage treatment INTRODUCTION The shift in animal production from small familyowned farms to large, concentrated, industrial-type facilities has forced changes in animal waste processing. Land spreading and lagoons were common methods of waste treatment and disposal at small cattle, swine, and poultry farms. The large-scale farms provide improved economy of scale for animal production; however, the land requirements to treat and dispose of the waste by traditional methods are excessive. The concentrated waste streams from these large-scale farms cannot be treated e ectively by land-based treatment systems, and they often produce nuisance odors and uncontrolled *Author to whom all correspondence should be addressed. Tel.: ; fax: ; pagilla@iit.edu release of nutrients to the watershed. Hence, animal waste treatment is shifting to high rate unit processes such as anaerobic digestion, activated sludge process, and attached growth processes. These engineered systems can be adopted for animal waste treatment based on the experience in treating other concentrated industrial and municipal waste. Since the key aspect of animal production is sustainability, the waste processed should provide products that can be recycled. For example in swine production, the solids recovered from the waste processing are reused as bedding material in the animal production units or added to the feed as a nutrient source (Livestock Waste Facilities Handbook, 1985). Hence the requirements for engineered animal waste systems include reclamation of chemical and biologically safe nutrient-rich solids, processed water, energy in the form of biogas, and reduction of odor and nutrient releases to the environment. 2747

2 2748 Krishna R. Pagilla et al. Swine production facilities generate about 0.21 kg biochemical oxygen demand (BOD 5 ), kg total nitrogen, and kg phosphorus per 68 kg nishing pig, and the water use is about 40±75 l/ pig/day (Livestock Waste Facilities Handbook, 1985). In addition, swine waste also contains viral, bacterial, and protozoan pathogens that can cause disease in both animals and humans (Black et al., 1982; Lund and Niessen, 1983; Marti et al., 1983). Several mesophilic anaerobic treatment methods have been successfully demonstrated for the treatment of concentrated swine wastes (Angelidaki and Ahring, 1993; Gorecki et al., 1993; Hansen et al., 1998; Hashimoto, 1983; Yang et al., 1993; Yang and Kuroshima, 1995). Conventional anaerobic treatment methods reduce the organic content of the waste while producing energy in the form of biogas. The major drawbacks of the mesophilic anaerobic treatment for concentrated wastes include long retention time (20±30 days or longer) requirement, and inability to inactivate pathogenic bacteria and viruses present in the waste. Enteric pathogens can be e ectively inactivated at treatment temperatures much higher than the internal body temperature (358C) of warm-blooded animals. Thermophilic temperature (>508C) processes are well known for enteric pathogen inactivation, and require much lower retention time than the mesophilic processes (Aitken and Mullennix, 1993; Hashimoto, 1983). However, complete treatment of the waste at thermophilic temperatures is energy intensive, and may not be economically viable. Two-stage treatment methods combining short retention time thermophilic step and longer retention time mesophilic step are known to provide both pathogen control and e ective organic matter treatment (Ghosh et al., 1995; Han and Dague, 1997; Pagilla et al., 1996; Zhao and Kugel, 1996). Municipal waste sludge processing by aerobic thermophilic treatment at 1-day retention time followed by 14-day retention time anaerobic mesophilic treatment was successful in reducing pathogens and vector attraction potential to levels that allow unrestricted reuse of the processed solids (Pagilla et al., 1996). Furthermore, both volatile solids (VS) reduction and methane gas production were much higher in the two-stage thermophilic-mesophilic system than those in the control single stage anaerobic mesophilic digester, both operating at the same overall retention time. Similarly, anaerobic thermophilic and anaerobic mesophilic two-stage treatment was able to provide improved treatment of waste sludge in terms of VS reduction and gas production compared to the single stage mesophilic treatment (Han and Dague, 1997). It has been also stated that a two-stage digestion process is better than a single stage process because the two-stage process separates faster acidogenesis reactions in the rst stage from the slower methanogenesis reactions in the second stage (Fox and Pohland, 1994). Such kinetic separation of di erent steps in anaerobic digestion enhances the overall process by enhancing the individual steps. The main objective of the research described in this paper was to investigate treatment of swine waste slurry by a two-stage aerobic thermophilic and anaerobic mesophilic treatment process. The speci c objectives were to determine the ability of the two-stage process to: (1) increase VS reduction; (2) decrease fecal pathogens; (3) increase biogas production; (4) reduce H 2 S in the biogas; (5) reduce supernatant chemical oxygen demand (COD); and (6) to improve product sludge dewaterability during swine waste treatment. MATERIALS AND METHODS Laboratory scale experiments were conducted using swine waste slurry from Dumoulin Farms, Hampshire, Illinois, USA. The lab scale experimental treatment system (ATP system) consisted of a 2.5 l aerobic thermophilic (ATP) reactor (working volume=2 l) followed by a 12 l anaerobic mesophilic digester (working volume=10 l), and the lab scale control system consisted of 12 l anaerobic mesophilic digester (working volume=10 l). Typical size and operating parameters of all lab scale systems are shown in Table 1. The aerobic thermophilic reactor was operated at 628C and a dissolved oxygen (DO) concentration of 1.0 mg/l, and the anaerobic digesters were operated at 378C. The units were placed on heavy duty combination magnetic stirrers/heaters to provide supplementary heat and mixing. Air was bubbled through the contents of the aerobic thermophilic reactor to provide 1.0 mg DO/l needed to promote heat generation through metabolic oxidation of the organic matter. The thermophilic reactor was started with 2 l of swine waste slurry, and allowed to undergo aerobic thermophilic treatment for 3 days in batch mode before daily feeding and product sludge withdrawal was initiated. Experimental and control anaerobic digesters were started with 10 l of Table 1. Size and operating parameters of laboratory scale treatment systems Reactor size Thermophilic reactor=2.5 l, working volume=2 l Anaerobic digesters=12 l, working volume=10 l Solids residence time Thermophilic reactor=1 day Experimental system anaerobic digester=5, 9, and 14 day Control anaerobic digester=6, 10, and 15 day DO concentration in thermophilic reactor mg/l Feed average TS content 4.3% Feed average VS content 67% of TS Temperature Thermophilic reactor=628c Experimental and control anaerobic digesters=378c

3 Treatment of swine waste 2749 swine waste, and were allowed to undergo anaerobic mesophilic digestion for 7 days in batch mode before daily feeding and sludge withdrawal was initiated. After this startup, the thermophilic reactor was batch fed twice a day after sludge withdrawal. A portion of the sludge withdrawn from the thermophilic reactor was simultaneously fed to the mesophilic anaerobic digester after the product sludge equal to the amount of feed thermophilic sludge was removed. Control anaerobic digester was fed two times a day and the same volume of anaerobic digester product sludge was taken out prior to feeding. Both experimental and control systems were operated for an initial period of 30 days prior to data collection for evaluation of the systems, and for three SRTs after each change in SRT (working volume/feed volume per day). The product sludge samples collected from the mesophilic anaerobic digesters of both experimental and control systems were analyzed for total solids (TS), VS, supernatant COD, and fecal coliform density according to Standard Methods (APHA, 1992). The thermophilic reactor e uent sludge was analyzed for VS content. Biogas production from the experimental and control mesophilic anaerobic digesters was measured by the water displacement method. Methane content of the sludge gas was measured by Sargent-Welch Scienti c Orsat apparatus (Model S ) based on Method 2720-B of Standard Methods (APHA, 1992), and hydrogen sul de was measured by Kitagawa toxic gas detector (Model 8014KA). DO was measured by YSI Model 5100 DO meter. The dewaterability of the product sludge from the experimental and control systems was determined by measuring the time-to- lter values according to the Method 2710-H of Standard Methods (APHA, 1992). RESULTS AND DISCUSSION The ATP system aerobic thermophilic reactor was operated at an SRT of 1.0 day, the anaerobic digester was operated at 5, 9, and 14-day SRT, and the combined two-stage ATP system SRT was 6, 10, and 15 days. The control anaerobic digester was operated at 6, 10, and 15-day SRT. Feed swine waste average composition included TS=4.3%, VS=67% of TS, supernatant COD=14,330 mg/l, fecal coliform density= MPN/g TS, ph=7.1, and alkalinity=2700 mg CaCO 3 /l. Statistical signi cance of the di erences in means was determined by student t-test at 95% con dence interval using eight measurements for each parameter. Volatile solids reduction The VS reduction of the feed swine waste by the ATP reactor, the two-stage ATP system, and control anaerobic mesophilic digester at 6, 10, and 15- day system SRT is shown in Fig. 1. The VS reduction was determined by the Van Kleeck equation method with no decant and no grit accumulation conditions (US EPA, 1992). The twostage ATP system reduced the average feed VS concentration of % by about 44±47% (average VS reduction=46%) compared to about 34±37% (average VS reduction=36%) by the control, both systems operating at a system SRT of 6 days. The average feed VS concentration during the 10-day system SRT experiments was %, and it was reduced by about 48±59% (average VS reduction=54%) by the two-stage ATP system and about 43±47% (average VS reduction=45%) by the control. At 15-day system SRT, average feed VS concentration of % was reduced by about 59±63% (average VS reduction=61%) by the twostage ATP system and by about 49±53% (average VS reduction=50%) by the control. The two-stage ATP system gave about 10% higher VS reduction than the control at 6, 10, and 15-day system SRT. The di erence in VS reduction between the ATP system and the control was found to be signi cant according to paired comparison student t-test at 95% con dence interval. The thermophilic reactor of the ATP system operating at 1-day SRT reduced the feed VS concentration by about 15±33% (average VS reduction=24%) in all three sets of experiments whose results are shown in Fig. 1. Supernatant COD reduction Supernatant COD reduction in the treated waste along with VS reduction indicates the level of waste treatment achieved in the process. The feed waste soluble COD is due to the soluble organic matter present in the waste, whereas the treated waste supernatant COD re ects both the untreated portion of the feed waste soluble COD and hydrolyzed particulate organic matter. The supernatant COD reduction by the two-stage ATP system was 56, 61, 67% of the feed waste supernatant COD, while that in the control system was 44, 55, 60% of the feed waste supernatant COD at 6, 10, and 15-day system SRT, respectively. The average feed swine waste supernatant COD was 15, , 16, , and 15, mg/l during 6, 10, and 15-day SRT experiments, respectively. The two-stage ATP system was better than the control in terms of supernatant COD reduction by 6±12% of the feed supernatant COD. The supernatant COD reduction increased with increasing SRT in both systems, and the di erences were found to be statistically signi cant (student t-test with 95% con dence limit). Pathogen reduction Fecal coliforms are considered as indicator pathogens in municipal sludge, and a level of 10 3 MPN/g TS is considered to be su cient for unrestricted reuse of the treated sludge (US EPA, 1992). Since no such guidelines or regulations currently exist for indicator pathogens in swine waste, fecal coliform density of 10 3 MPN/g TS was used as the reference level for treated swine waste sludge. Figure 2 shows the fecal coliform density in the feed swine waste, and treated swine waste from the ATP system and the control system operating at 6, 10, and 15-day system SRT. Average fecal coliform density in the feed swine waste was MPN/ g TS. It can be seen that the two-stage ATP system was able to reduce the swine waste fecal coliform density below 10 3 MPN/g TS at all times at all

4 2750 Krishna R. Pagilla et al. Fig. 1. Volatile solids reduction in the aerobic thermophilic pretreatment reactor, two-stage ATP system, and control anaerobic digester. SRT, while the control anaerobic digester could reduce it to no less than 10 5 MPN/g TS at all SRT. This improvement of >3-log reduction in fecal coliform density by the two-stage ATP system over that of the control can be attributed to the thermophilic step of the two-stage process. It was reported that 1-day SRT thermophilic reactor was capable of reducing the fecal coliform density of the municipal waste sludge from about 10 8 to about 10 4 MPN/g TS (Cheunbarn and Pagilla, 1999). Gas production The average methane gas production by the mesophilic anaerobic digesters of the two-stage ATP system and the control are shown in Table 2. The average methane gas production ranged from 0.56±0.64 m 3 /kg VS destroyed for the two-stage ATP system, and that from the control ranged from 0.47±0.51 m 3 /kg VS destroyed. The two-stage ATP system anaerobic digester produced about 19, 27, and 25% higher methane at 6, 10, and 15-day system SRT, respectively, over that produced by the control anaerobic digester. Although the di erences in methane production/per kg VS destroyed between the two-stage ATP system and the control were statistically signi cant at all SRT, the increase in methane production/kg VS destroyed due to SRT increase was statistically not signi cant in

5 Treatment of swine waste 2751 Fig. 2. Fecal coliform density in the feed sludge, and product sludge from two-stage ATP system and control anaerobic digester. Table 2. Average methane gas production from the experimental and control anaerobic digesters at 6, 10, and 15-day system SRT System SRT Methane production, m 3 Experimental anaerobic digester Control anaerobic digester per kg VS destroyed per kg VS fed per kg VS destroyed per kg VS fed 6 day a day day a Mean2standard deviation (no. of observations=8).

6 2752 Krishna R. Pagilla et al. both the two-stage ATP system and the control anaerobic digesters. Table 2 also shows the methane production per kg of VS fed to the two-stage ATP system and the control anaerobic digesters. The two-stage ATP system anaerobic digester produced 0.26, 0.32, and 0.39 m 3 methane/kg VS fed at 6, 10, and 15-day system SRT, respectively, whereas, the control produced 0.17, 0.22, and 0.25 m 3 methane/kg VS fed at 6, 10, and 15-day SRT, respectively. The two-stage ATP system increased the methane production per kg VS fed by an average of 53, 45, and 56% over that found in the control. It should be noted that in the case of the two-stage ATP system, a certain portion of the organic matter is used up in the aerobic thermophilic reactor for heat generation through metabolic oxidation. Hence, higher methane production coupled with higher VS destruction in the two-stage ATP system will result in substantially higher overall biogas production in the two-stage system over that in the control. The biogas from the two-stage ATP system anaerobic digester contains signi cantly lower H 2 S levels (353, 362, and 387 ppm (v/v) at 6, 10, and 15- day system SRT, respectively) than those in the biogas from the control anaerobic digester (569, 581, and 609 ppm (v/v) H 2 S at 6, 10, and 15-day system SRT, respectively). Hydrogen sul de levels in biogas indicate the corrosivity of the gas for reuse in electricity production, and the extent of H 2 S scrubbing needed for reuse. The lower the H 2 S levels in the biogas, the more suitable it is for energy production and environmental compliance. It was found that the lower H 2 S levels in the two-stage system biogas is due to air-stripping of H 2 S from the waste in the aerobic thermophilic reactor prior to the anaerobic digester (Cheunbarn and Pagilla, 1999). The exhaust gas from aerobic thermophilic reactor needs to be scrubbed to remove H 2 S prior to atmospheric discharge. Dewaterability of product sludge The time-to- lter value of the product sludge is indicative of its dewaterability. Time-to- lter values of the product sludge from the two-stage ATP system and the control anaerobic digester are shown in Table 3. The two-stage ATP system produces much better dewaterable product sludge as indicated by Table 3. Average time-to- lter values of the product sludge from the experimental and control systems operating at 6, 10, and 15- day SRT System SRT Product sludge time-to- lter, s Experimental system Control system 6 day a day day a Mean2standard deviation (no. of observations=8). its lower time-to- lter values compared to the control system product sludge. The average time-to- lter values of control system product sludge (355, 295, and 250 s at 6, 10, 15-day system SRT, respectively) were approximately 50% higher than the time-to- lter values of the two-stage ATP system product sludge (245, 197, and 158 s at 6, 10, and 15-day system SRT, respectively). The time-to- lter values of the product sludge from both systems signi cantly decrease with increasing SRT, indicating the dependence of dewaterability on digestion time. CONCLUSIONS Based on the results obtained from the laboratory scale experiments to investigate two-stage aerobic thermophilic and anaerobic mesophilic treatment of swine waste, the following conclusions can be made: 1. The two-stage ATP system reduced 46, 54, and 61% of the VS in swine waste compared to 36, 45, and 50% VS reduction by the control at 6, 10, and 15-day system SRT, respectively. 2. Average supernatant COD reduction by the twostage ATP system was 56, 61, and 67% of the feed compared to 44, 55, and 60% of the feed by the control at 6, 10, and 15-day system SRT, respectively. 3. Fecal coliform density was reduced from MPN/g TS in the swine waste to less than 10 3 MPN/g TS at all times by the two-stage ATP system at 6, 10, and 15-day system SRT, while the single stage control anaerobic digester could not reduce the same to less than 10 5 MPN/g TS at 6, 10, and 15-day system SRT. 4. The average methane gas production by the twostage ATP system anaerobic digester was 0.56, 0.61, and 0.64 m 3 /kg VS destroyed at 6, 10, and 15-day system SRT, respectively, compared to a methane gas production of 0.47, 0.48, and 0.51 m 3 /kg VS destroyed at 6, 10, and 15-day SRT, respectively, by the control digester. The experimental system methane production of 0.26, 0.32, and 0.39 m 3 /kg VS fed at 6, 10, and 15-day system SRT, respectively, was signi cantly higher than the methane production of 0.17, 0.22, and 0.25 m 3 /kg VS fed at 6, 10, and 15-day SRT, respectively, by the control anaerobic digester. 5. Average H 2 S content in the biogas from the twostage ATP system (353, 362, and 387 ppm (v/v) at 6, 10, and 15-day system SRT, respectively) was signi cantly lower than that in the biogas from the control anaerobic digester (569, 581, and 609 ppm (v/v) at 6, 10, and 15-day system SRT, respectively). 6. The time-to- lter values of the product sludge from the two-stage ATP system (245, 197, and 158 s at 6, 10, and 15-day system SRT, respectively) were signi cantly lower than that from the

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