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Anaerobic digestion and post-treatment of swine wastewater using IC–SBR process with bypass of raw wastewater
Process Biochemistry 41 (2006) 965–969 www.elsevier.com/locate/procbio
Short communication
Anaerobic digestion and post-treatment of swine wastewater using IC–SBR process with bypass of raw wastewater Liang-Wei Deng a,b, Ping Zheng a,*, Zi-Ai Chen b a
Department of Environmental Engineering, Zhejiang University, Hangzhou, China b Asia-Pacific Regional Biogas Research and Training Center, Chengdu, China
Received 14 May 2005; received in revised form 19 October 2005; accepted 20 October 2005
Abstract A combined system consisting of Internal Circulation (IC) anaerobic reactor and Sequencing Batch Reactor (SBR) was used to treat swine wastewater in order to establish a cost-efficient wastewater treatment system. IC reactor could remove about 80% of COD with organic loading rate of 6–7 kg COD/(m3 day) in treating raw swine wastewater. Performance of direct post-treatment of digested swine wastewater was very poor with COD removal 7.54% and NH3-N removal 81.2% because of the low biodegradability and improper carbon/nitrogen ratio. Performance of posttreatment using SBR with the addition of raw wastewater was good with effluent COD less than 300 mg/L and effluent NH3-N less than 10 mg/L. The removal rates of COD, BOD5, NH3-N and TN were 95.5%, 99.6%, 99.4% and 94.3%, respectively, in the IC–SBR system with total hydraulic retention time of 5–6 days. # 2005 Elsevier Ltd. All rights reserved. Keywords: IC anaerobic reactor; Sequencing Batch Reactor (SBR); Swine wastewater treatment; Nitrogen removal
1. Introduction The swine wastewater is widely used as fertilizer in many countries because of its high organic, nitrogen and phosphorus content. If a large herd of pig is raised in a restricted area, it is very difficult to landspread all the swine waste. Moreover, the swine waste possibly leads to local soil pollution and eutrophication of water body. So, many countries are paying attention to the pollution resulted from livestock farms, and have tighten legislation and discharging standards. As far as swine waste treatment is concerned, anaerobic digestion is important alternative to land application, because it reduces pollution and recovers methane. A number of studies and applications have been reported for anaerobic digestion of swine waste [1–4]. However, because of the longer hydraulic retention time (HRT) and low organic loading rate, large reactor volume and high investment are needed. Internal Circulation (IC) reactor provides a promising way to solve the problem since it is an efficient anaerobic reactor [5,6]. The effluent from anaerobic digestion contains high COD, nitrogen and phosphorus concentration. It is necessary to set up
* Corresponding author. Tel.: +86 571 86971709; fax: +86 571 86949320. E-mail address: [email protected] (P. Zheng). 1359-5113/$ – see front matter # 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.procbio.2005.10.022
a post-treatment to meet the discharge standards. However, previous studies showed that the efficiency of direct posttreatment was not satisfactory because of the poor biodegradability and low COD/NH3-N ratio in effluent [7–11]. Thus, new treatment strategy should be developed. The objective of this study was to establish a cost-efficient treatment system for swine wastewater and to test the feasibility of anaerobic digestion and post-treatment using IC–SBR process with bypass of raw wastewater. 2. Material and method 2.1. Experimental system The IC–SBR process is shown in Fig. 1. 2.1.1. IC reactor As shown in Fig. 1, IC reactor was made of stainless steel with side length 320 mm, height 1300 mm and total volume 133 L (working volume 120 L). Two gas–liquid–solid separators were located in the middle and upper parts of the reactor. A gas collector was connected with the two separators and a distributor was installed on the reactor bottom. The IC reactor was fed by pump (model SP-950), and the effluent was overflowed from weir to water container. It was seeded with 35.0 L anaerobic sludge obtained from a full-scale digester treating swine wastewater, and was filled with tap water and raw swine wastewater. After 3 days’ settling period, the IC reactor was fed with raw
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Fig. 1. Schematic diagram of IC–SBR process.
swine wastewater. The effluent from IC reactor was treated by SBR. The temperature for the experiment was maintained at 20–25 8C. 2.1.2. Sequencing Batch Reactor (SBR) SBR was also made of stainless steel with side length 320 mm, height 320 mm and total volume 32.7 L (working volume 18 L). Air was supplied by air compressor (model ACO-001), and was released through a stone diffuser on the bottom of SBR (Fig. 1). A mini-pump (model SP-1000) was used for FILL and DISCHARGE. After SBR was inoculated with 5.0 L sludge cultivated in our laboratory, the reactor was filled to final volume with effluent from IC reactor, and was then operated at hydraulic retention time of 3 days and biological solid retention time (SRT) of 30 days. The temperature was also maintained at 20–25 8C. The sequencing scheme consisted of 3-h AERATION, 3-h IDLE, 3-h AERATION, 1-h SETTING, 1-h DECANT and 1-h FILL. FILL, REACT, SETTLE, DECANT and IDLE were controlled by programmable timer. Sludge was drawn by siphon manually. No forced agitation was applied.
2.2. Materials Swine wastewater for the study was obtained from a local pig farm. It was allowed to settle for 6–12 h to remove some suspended solids. The COD, BOD5, NH3-N and TN concentrations were 3000–15,000 mg/L, 1700–8500 mg/L, 400–1400 mg/L and 600–2100 mg/L, respectively.
2.3. Analytical methods Samples were taken to determine COD, NH3-N and SV30 every day, and to monitor BOD5, SS, TN and TP once a month. Analysis of COD, BOD5, SS, NH3-N, TN and TP was carried out according to the standard methods issued by the China National Environmental Protection Agency [12].
3. Results and discussion 3.1. Performance of pretreatment with IC reactor At the beginning of experiment, 40 L swine wastewater was fed into IC reactor each day, and a hydraulic retention time of 3 days was controlled. As shown in Fig. 2, COD removal went up to 80% or higher within 2 weeks when the organic loading rate was 1–2 kg COD/(m3 day). Then, the organic loading rate was increased by means of enhancing influent concentration or shortening hydraulic retention time. In the end of experiment (the 21st–22nd week), the loading rate was 6–7 kg COD/ (m3 day), the volumetric biogas production rate was about 3 m3/(m3 day) and COD removal rate was about 80%. It is
Fig. 2. Performance of IC reactor: (A) loading rate and volumetric biogas production rate and (B) COD and NH3-N removal.
L.-W. Deng et al. / Process Biochemistry 41 (2006) 965–969 Table 1 Performance of SBR treating digested effluent of swine wastewater Parameters
Influent (mg/L)
Effluent (mg/L)
Removal rate (%)
COD NH3-N
955 281 702 95.6
883 289 132 48.8
7.54 81.2
reported that the organic loading rate was 3–4 kg COD/ (m3 day), and volumetric biogas production rate was 1–1.5 m3/ (m3 day) to treat the same wastewater with other kind of anaerobic reactors under similar conditions [1–3,13,14]. Therefore, IC reactor is very competent to treat swine wastewater. It can be seen from Fig. 2 that NH3-N concentration in effluent was little higher than that in influent, which implies that organic nitrogen was partly converted into ammonia nitrogen (NH3-N) during the pretreatment. 3.2. Performance of direct post-treatment with SBR SBR was operated to treat the effluent from IC reactor directly. After run for about 2 months, the performance became very poor. As listed in Table 1, the removal rates of COD and NH3-N were only 7.54% and 81.2%, respectively. The COD concentration (>800 mg/L) and NH3-N concentration (>100 mg/L) in SBR effluent were high. The results were similar to those observed by Su et al. [11]. They reported that COD removal rate was only 10.4–43.6% and TKN removal was 42.5–71.1% when SBR was used to treat digested swine wastewater. In order to find out the reason of poor removal efficiency of COD and NH3-N, the characteristics of influent and effluent of IC reactor were analyzed. The ratio of BOD5/COD is often used as an index to evaluate biodegradability of wastewater. BOD5/ COD > 0.45 indicates that biodegradability is very good; BOD5/COD = 0.3–0.45, biodegradability is good; BOD5/ COD = 0.2–0.3, biodegradability is poor; BOD5/COD < 0.2, biotreatment is unsuitable. The ratio calculated from Table 2 indicated that the BOD5/COD ratio of raw swine wastewater (IC influent) was 0.66, and its biodegradability was very good. After anaerobic pretreatment, most of biodegradable organic pollutants in raw swine wastewater were degraded. The BOD5/ COD ratio of digested effluent (IC effluent) fell to 0.18, and its biodegradability became very poor. The suitable ratio of carbon, nitrogen and phosphorus for microbial growth is BOD5:N:P = 100:5:1, while the ratio of BOD5:N:P of raw swine wastewater is about 20.4:4.6:1 (calculated from Table 2), that means the raw swine wastewater was already maladjust-
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ment for microorganism growth. The BOD5:N:P ratio of digested swine wastewater was changed into 3.1:10.2:1, that means the digested swine wastewater is seriously short of organic matter. These factors would influence aerobic posttreatment of digested swine wastewater. pH in SBR dropped sharply during ‘AERATION’ when NH3-N was oxidized. In the end of the first ‘AERATION’, pH dropped to 6.0 or so. In the stage of ‘IDLE’ (3–6 h), pH rose a little due to denitrification. In the end of the second ‘AERATION’ (3–9 h), pH fell below 5.5, which may inhibit the bacterial activity. This was reason behind very poor removal efficiency of COD and NH3-N to treat IC reactor effluent with SBR directly. In the stage of ‘FILL’ (11–12 h), digested swine wastewater was fed and pH only went up to about 6.8 (less than initial pH of the cycle). The NOx-N (the sum of NO2 -N and NO3 -N) at the end of cycle was higher than that at the beginning of cycle, indicating accumulation of NOx-N (Fig. 3). It could be attributed to feeble denitrification because there was not enough biodegradable organic substance in digested swine wastewater [9]. Judged from above-mentioned results, the performance of conventional anaerobic–aerobic process treating swine wastewater is poor and its stability is unstable. New strategy for posttreatment of digested effluent should be tried. 3.3. Performance of post-treatment using SBR with addition of raw wastewater In order to improve post-treatment of digested swine wastewater, raw swine wastewater was added into digested swine wastewater. The ratio of raw wastewater to digested swine wastewater was about 1:2. As a result, both the ratio of BOD5/COD and the proportion of BOD5:N:P in SBR influent were increased. The removal rate of COD and NH3-N arose largely. As listed in Table 2, the COD (less than 300 mg/L) and BOD5 (less than 20 mg/L) in SBR effluent were low and such low effluent COD and BOD, especially in digested swine wastewater treatment, have been rarely reported so far. The NH3-N removal was also very good in SBR treating digested swine wastewater with addition of raw wastewater. The influent NH3-N was as high as 1400 mg/L with average concentration of 721 mg/L, while the effluent NH3-N was lower than 10 mg/L with removal rate more than 99%. It has been confirmed that the high NH3-N removal rate mainly resulted from biological nitrification rather than air stripping [15]. As shown in Fig. 3, during ‘AERATION’, about 70 mg/L of NH3-N was converted into nitrite/nitrate while pH value was always kept at higher than 7.0 in SBR treating digested swine
Table 2 Overall removal efficiencies of pollutants by IC–SBR process Parameters
Influent of IC
Effluent of IC
Influent of SBR
Effluent of SBR
Overall removal (%)
COD (mg/L) BOD5 (mg/L) NH3-N (mg/L) TN (mg/L)
6372 1639 4210 353 707 162 918 164
1229 397 223 77.8 727 167 739 136
2811 825 1293 333 721 166 793 144
288 59.2 15.6 6.2 3.89 5.48 51.9 0.1
95.5 99.6 99.4 94.3
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Fig. 3. Variations of main parameters during a cycle in SBR treating digested effluent before and after addition of raw wastewater. Profile of: (A) pH and (B) NH3-N and NOx-N (NO2 -N + NO3 -N).
wastewater with addition of raw wastewater. It indicated that the SBR system had a larger buffering capacity than the SBR for direct post-treatment. During ‘AERATION’ (0–3 h and 6– 9 h), NOx-N increased as NH3-N was oxidized. During ‘IDLE’ stage (4–6 h and 10–12 h), however, NOx-N decreased due to denitrification. In SBR treating digested swine wastewater without addition of raw wastewater, removed NOx-N in denitrification was less than that produced in nitrification, resulting in NOx-N accumulation and alkalinity shortage. In SBR treating digested swine wastewater with addition of raw wastewater, removed NOx-N in denitrification was far more than that produced in nitrification because there was greater biodegradable organic substance in mixed swine wastewater [9], resulting in more alkalinity production in denitrification than that consumed in nitrification. As a result, pH value (7.58) in the end of cycle was higher than initial pH (7.16) and the nitrification rate was better. 3.4. Performance of the IC–SBR system The overall performance of the IC–SBR system is shown in Table 2. The pollutants were efficiently removed with COD removal 95.5%, BOD5 removal 99.6%, NH3-N removal 99.4% and TN removal 94.3%, respectively. These parameters were better than those from anoxic aerobic SBR (Ng [10], Edgerton et al. [16] and Fernandes et al. [18]) or combined anaerobic–aerobic SBR (Bernet et al. [17]) or hydrolysis reactor and SBR (Deng [15]). The IC–SBR system is also superior to the system only using anaerobic process or the system only using SBR. If swine wastewater is treated only with anaerobic process, the concentration of pollutants was so high that it cannot meet the discharge standard. If
using SBR treats raw swine wastewater directly, it takes very long HRT (about 9–16 days [17,19]), and requires a large reactor. Moreover, it consumes a lot of energy. The HRT of IC–SBR system with bypass of raw wastewater is 5–6 days and has shown a great advantage. 4. Conclusions The IC–SBR process for treatment of swine wastewaters was tested; following conclusions can be drawn based on the present study: IC reactor could remove about 80% of COD with organic loading rate of 6–7 kg COD/(m3 day) in treatment of raw swine wastewater, while the concentration of NH3-N in effluent was little higher than that in influent. Performance of direct post-treatment of digested swine wastewater was very poor because of the low biodegradability and improper carbon and nitrogen ratio. It is not feasible to treat swine wastewater with conventional anaerobic–aerobic process. Performance of post-treatment using SBR with addition of raw wastewater was good. The removal rate of COD, BOD5, NH3-N and TN were 95.5%, 99.6%, 99.4% and 94.3%, respectively, in the IC–SBR system. Based on the experiment, IC–SBR process with bypass of raw wastewater is developed to treat swine wastewater. In the new process, the raw wastewater is divided into two parts, i.e. one part (about 70%) was fed into anaerobic reactor, while the other part (about 30%) was bypassed into SBR to mix with digested wastewater.
L.-W. Deng et al. / Process Biochemistry 41 (2006) 965–969
Acknowledgments Ministry of Science and Technology, China, supported this study work financially. The chemical analysis work of Miss Chen Gemin is also greatly appreciated.
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[9] Poo KM, Jun BH, Lee SH, Im JH, Woo HJ, Kim CW. Treatment of strong nitrogen swine wastewater in a full-scale sequencing batch reactor. Water Sci Technol 2004;49(5–6):315–23. [10] Ng WG. Aerobic treatment of swine wastewater with the sequencing batch reactor. Bio Waste 1987;22:285–94. [11] Su JJ, Lian WC, Wu JF. Studies on swine wastewater treatment by a fullscale sequencing batch reactor after anaerobic fermentation. Chung-Hua Nungxue Huibao 1999;188:47–58. [12] Editor Committee of Monitoring and Analysis Method for Water and Wastewater. Monitoring and analysis method for water and wastewater, 3rd ed., Beijing: China Environmental Science Press, 1989. [13] Wellinger A, Kaufmann R. Psychropilic methane generation from pig manure. Process Biochem 1982;17:26–30. [14] Zeeman G, Sutter H, Vens T, Koster M, Wellinger A. Psychropilic digestion of diary cattle and pig manure: start-up procedure of batch, fed-batch and CSTR-type digesters. Biol Wastes 1988;26:15–31. [15] Deng LW. Treatment of swine waste with hydrolysis–SBR process. Chin Water Wastewater 2001;17(3):8–11 [in Chinese]. [16] Edgerton BD, McNevin D, Wong CH, Menoud P, Barford JP, Mitchell CA. Strategies for dealing with swine effluent in Australia: the sequencing batch reactor as a solution. Water Sci Technol 1999;41(1):123–6. [17] Bernet N, Delgenes N, Akunna C, Delgenes JP, Moletta R. Combined anerobic–aerobic SBR for the treatment of swine wastewater. Water Res 2000;34(2):611–9. [18] Fernandes L, Mckyes E, Warith M, Barrington S. Treatment of liquid swine manure in the sequencing batch reactor under aerobic and anoxic condition. Can Agric Eng 1991;33:373–9. [19] Tilche A, Bacilieri E, Bortone G, Malaspina F, Piccinini S, Stante L. Biological phosphorus and nitrogen removal in a full scale sequencing batch reactor treating piggery wastewater. Water Sci Technol 1999;40(1): 199–206.
Short communication
Anaerobic digestion and post-treatment of swine wastewater using IC–SBR process with bypass of raw wastewater Liang-Wei Deng a,b, Ping Zheng a,*, Zi-Ai Chen b a
Department of Environmental Engineering, Zhejiang University, Hangzhou, China b Asia-Pacific Regional Biogas Research and Training Center, Chengdu, China
Received 14 May 2005; received in revised form 19 October 2005; accepted 20 October 2005
Abstract A combined system consisting of Internal Circulation (IC) anaerobic reactor and Sequencing Batch Reactor (SBR) was used to treat swine wastewater in order to establish a cost-efficient wastewater treatment system. IC reactor could remove about 80% of COD with organic loading rate of 6–7 kg COD/(m3 day) in treating raw swine wastewater. Performance of direct post-treatment of digested swine wastewater was very poor with COD removal 7.54% and NH3-N removal 81.2% because of the low biodegradability and improper carbon/nitrogen ratio. Performance of posttreatment using SBR with the addition of raw wastewater was good with effluent COD less than 300 mg/L and effluent NH3-N less than 10 mg/L. The removal rates of COD, BOD5, NH3-N and TN were 95.5%, 99.6%, 99.4% and 94.3%, respectively, in the IC–SBR system with total hydraulic retention time of 5–6 days. # 2005 Elsevier Ltd. All rights reserved. Keywords: IC anaerobic reactor; Sequencing Batch Reactor (SBR); Swine wastewater treatment; Nitrogen removal
1. Introduction The swine wastewater is widely used as fertilizer in many countries because of its high organic, nitrogen and phosphorus content. If a large herd of pig is raised in a restricted area, it is very difficult to landspread all the swine waste. Moreover, the swine waste possibly leads to local soil pollution and eutrophication of water body. So, many countries are paying attention to the pollution resulted from livestock farms, and have tighten legislation and discharging standards. As far as swine waste treatment is concerned, anaerobic digestion is important alternative to land application, because it reduces pollution and recovers methane. A number of studies and applications have been reported for anaerobic digestion of swine waste [1–4]. However, because of the longer hydraulic retention time (HRT) and low organic loading rate, large reactor volume and high investment are needed. Internal Circulation (IC) reactor provides a promising way to solve the problem since it is an efficient anaerobic reactor [5,6]. The effluent from anaerobic digestion contains high COD, nitrogen and phosphorus concentration. It is necessary to set up
* Corresponding author. Tel.: +86 571 86971709; fax: +86 571 86949320. E-mail address: [email protected] (P. Zheng). 1359-5113/$ – see front matter # 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.procbio.2005.10.022
a post-treatment to meet the discharge standards. However, previous studies showed that the efficiency of direct posttreatment was not satisfactory because of the poor biodegradability and low COD/NH3-N ratio in effluent [7–11]. Thus, new treatment strategy should be developed. The objective of this study was to establish a cost-efficient treatment system for swine wastewater and to test the feasibility of anaerobic digestion and post-treatment using IC–SBR process with bypass of raw wastewater. 2. Material and method 2.1. Experimental system The IC–SBR process is shown in Fig. 1. 2.1.1. IC reactor As shown in Fig. 1, IC reactor was made of stainless steel with side length 320 mm, height 1300 mm and total volume 133 L (working volume 120 L). Two gas–liquid–solid separators were located in the middle and upper parts of the reactor. A gas collector was connected with the two separators and a distributor was installed on the reactor bottom. The IC reactor was fed by pump (model SP-950), and the effluent was overflowed from weir to water container. It was seeded with 35.0 L anaerobic sludge obtained from a full-scale digester treating swine wastewater, and was filled with tap water and raw swine wastewater. After 3 days’ settling period, the IC reactor was fed with raw
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Fig. 1. Schematic diagram of IC–SBR process.
swine wastewater. The effluent from IC reactor was treated by SBR. The temperature for the experiment was maintained at 20–25 8C. 2.1.2. Sequencing Batch Reactor (SBR) SBR was also made of stainless steel with side length 320 mm, height 320 mm and total volume 32.7 L (working volume 18 L). Air was supplied by air compressor (model ACO-001), and was released through a stone diffuser on the bottom of SBR (Fig. 1). A mini-pump (model SP-1000) was used for FILL and DISCHARGE. After SBR was inoculated with 5.0 L sludge cultivated in our laboratory, the reactor was filled to final volume with effluent from IC reactor, and was then operated at hydraulic retention time of 3 days and biological solid retention time (SRT) of 30 days. The temperature was also maintained at 20–25 8C. The sequencing scheme consisted of 3-h AERATION, 3-h IDLE, 3-h AERATION, 1-h SETTING, 1-h DECANT and 1-h FILL. FILL, REACT, SETTLE, DECANT and IDLE were controlled by programmable timer. Sludge was drawn by siphon manually. No forced agitation was applied.
2.2. Materials Swine wastewater for the study was obtained from a local pig farm. It was allowed to settle for 6–12 h to remove some suspended solids. The COD, BOD5, NH3-N and TN concentrations were 3000–15,000 mg/L, 1700–8500 mg/L, 400–1400 mg/L and 600–2100 mg/L, respectively.
2.3. Analytical methods Samples were taken to determine COD, NH3-N and SV30 every day, and to monitor BOD5, SS, TN and TP once a month. Analysis of COD, BOD5, SS, NH3-N, TN and TP was carried out according to the standard methods issued by the China National Environmental Protection Agency [12].
3. Results and discussion 3.1. Performance of pretreatment with IC reactor At the beginning of experiment, 40 L swine wastewater was fed into IC reactor each day, and a hydraulic retention time of 3 days was controlled. As shown in Fig. 2, COD removal went up to 80% or higher within 2 weeks when the organic loading rate was 1–2 kg COD/(m3 day). Then, the organic loading rate was increased by means of enhancing influent concentration or shortening hydraulic retention time. In the end of experiment (the 21st–22nd week), the loading rate was 6–7 kg COD/ (m3 day), the volumetric biogas production rate was about 3 m3/(m3 day) and COD removal rate was about 80%. It is
Fig. 2. Performance of IC reactor: (A) loading rate and volumetric biogas production rate and (B) COD and NH3-N removal.
L.-W. Deng et al. / Process Biochemistry 41 (2006) 965–969 Table 1 Performance of SBR treating digested effluent of swine wastewater Parameters
Influent (mg/L)
Effluent (mg/L)
Removal rate (%)
COD NH3-N
955 281 702 95.6
883 289 132 48.8
7.54 81.2
reported that the organic loading rate was 3–4 kg COD/ (m3 day), and volumetric biogas production rate was 1–1.5 m3/ (m3 day) to treat the same wastewater with other kind of anaerobic reactors under similar conditions [1–3,13,14]. Therefore, IC reactor is very competent to treat swine wastewater. It can be seen from Fig. 2 that NH3-N concentration in effluent was little higher than that in influent, which implies that organic nitrogen was partly converted into ammonia nitrogen (NH3-N) during the pretreatment. 3.2. Performance of direct post-treatment with SBR SBR was operated to treat the effluent from IC reactor directly. After run for about 2 months, the performance became very poor. As listed in Table 1, the removal rates of COD and NH3-N were only 7.54% and 81.2%, respectively. The COD concentration (>800 mg/L) and NH3-N concentration (>100 mg/L) in SBR effluent were high. The results were similar to those observed by Su et al. [11]. They reported that COD removal rate was only 10.4–43.6% and TKN removal was 42.5–71.1% when SBR was used to treat digested swine wastewater. In order to find out the reason of poor removal efficiency of COD and NH3-N, the characteristics of influent and effluent of IC reactor were analyzed. The ratio of BOD5/COD is often used as an index to evaluate biodegradability of wastewater. BOD5/ COD > 0.45 indicates that biodegradability is very good; BOD5/COD = 0.3–0.45, biodegradability is good; BOD5/ COD = 0.2–0.3, biodegradability is poor; BOD5/COD < 0.2, biotreatment is unsuitable. The ratio calculated from Table 2 indicated that the BOD5/COD ratio of raw swine wastewater (IC influent) was 0.66, and its biodegradability was very good. After anaerobic pretreatment, most of biodegradable organic pollutants in raw swine wastewater were degraded. The BOD5/ COD ratio of digested effluent (IC effluent) fell to 0.18, and its biodegradability became very poor. The suitable ratio of carbon, nitrogen and phosphorus for microbial growth is BOD5:N:P = 100:5:1, while the ratio of BOD5:N:P of raw swine wastewater is about 20.4:4.6:1 (calculated from Table 2), that means the raw swine wastewater was already maladjust-
967
ment for microorganism growth. The BOD5:N:P ratio of digested swine wastewater was changed into 3.1:10.2:1, that means the digested swine wastewater is seriously short of organic matter. These factors would influence aerobic posttreatment of digested swine wastewater. pH in SBR dropped sharply during ‘AERATION’ when NH3-N was oxidized. In the end of the first ‘AERATION’, pH dropped to 6.0 or so. In the stage of ‘IDLE’ (3–6 h), pH rose a little due to denitrification. In the end of the second ‘AERATION’ (3–9 h), pH fell below 5.5, which may inhibit the bacterial activity. This was reason behind very poor removal efficiency of COD and NH3-N to treat IC reactor effluent with SBR directly. In the stage of ‘FILL’ (11–12 h), digested swine wastewater was fed and pH only went up to about 6.8 (less than initial pH of the cycle). The NOx-N (the sum of NO2 -N and NO3 -N) at the end of cycle was higher than that at the beginning of cycle, indicating accumulation of NOx-N (Fig. 3). It could be attributed to feeble denitrification because there was not enough biodegradable organic substance in digested swine wastewater [9]. Judged from above-mentioned results, the performance of conventional anaerobic–aerobic process treating swine wastewater is poor and its stability is unstable. New strategy for posttreatment of digested effluent should be tried. 3.3. Performance of post-treatment using SBR with addition of raw wastewater In order to improve post-treatment of digested swine wastewater, raw swine wastewater was added into digested swine wastewater. The ratio of raw wastewater to digested swine wastewater was about 1:2. As a result, both the ratio of BOD5/COD and the proportion of BOD5:N:P in SBR influent were increased. The removal rate of COD and NH3-N arose largely. As listed in Table 2, the COD (less than 300 mg/L) and BOD5 (less than 20 mg/L) in SBR effluent were low and such low effluent COD and BOD, especially in digested swine wastewater treatment, have been rarely reported so far. The NH3-N removal was also very good in SBR treating digested swine wastewater with addition of raw wastewater. The influent NH3-N was as high as 1400 mg/L with average concentration of 721 mg/L, while the effluent NH3-N was lower than 10 mg/L with removal rate more than 99%. It has been confirmed that the high NH3-N removal rate mainly resulted from biological nitrification rather than air stripping [15]. As shown in Fig. 3, during ‘AERATION’, about 70 mg/L of NH3-N was converted into nitrite/nitrate while pH value was always kept at higher than 7.0 in SBR treating digested swine
Table 2 Overall removal efficiencies of pollutants by IC–SBR process Parameters
Influent of IC
Effluent of IC
Influent of SBR
Effluent of SBR
Overall removal (%)
COD (mg/L) BOD5 (mg/L) NH3-N (mg/L) TN (mg/L)
6372 1639 4210 353 707 162 918 164
1229 397 223 77.8 727 167 739 136
2811 825 1293 333 721 166 793 144
288 59.2 15.6 6.2 3.89 5.48 51.9 0.1
95.5 99.6 99.4 94.3
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Fig. 3. Variations of main parameters during a cycle in SBR treating digested effluent before and after addition of raw wastewater. Profile of: (A) pH and (B) NH3-N and NOx-N (NO2 -N + NO3 -N).
wastewater with addition of raw wastewater. It indicated that the SBR system had a larger buffering capacity than the SBR for direct post-treatment. During ‘AERATION’ (0–3 h and 6– 9 h), NOx-N increased as NH3-N was oxidized. During ‘IDLE’ stage (4–6 h and 10–12 h), however, NOx-N decreased due to denitrification. In SBR treating digested swine wastewater without addition of raw wastewater, removed NOx-N in denitrification was less than that produced in nitrification, resulting in NOx-N accumulation and alkalinity shortage. In SBR treating digested swine wastewater with addition of raw wastewater, removed NOx-N in denitrification was far more than that produced in nitrification because there was greater biodegradable organic substance in mixed swine wastewater [9], resulting in more alkalinity production in denitrification than that consumed in nitrification. As a result, pH value (7.58) in the end of cycle was higher than initial pH (7.16) and the nitrification rate was better. 3.4. Performance of the IC–SBR system The overall performance of the IC–SBR system is shown in Table 2. The pollutants were efficiently removed with COD removal 95.5%, BOD5 removal 99.6%, NH3-N removal 99.4% and TN removal 94.3%, respectively. These parameters were better than those from anoxic aerobic SBR (Ng [10], Edgerton et al. [16] and Fernandes et al. [18]) or combined anaerobic–aerobic SBR (Bernet et al. [17]) or hydrolysis reactor and SBR (Deng [15]). The IC–SBR system is also superior to the system only using anaerobic process or the system only using SBR. If swine wastewater is treated only with anaerobic process, the concentration of pollutants was so high that it cannot meet the discharge standard. If
using SBR treats raw swine wastewater directly, it takes very long HRT (about 9–16 days [17,19]), and requires a large reactor. Moreover, it consumes a lot of energy. The HRT of IC–SBR system with bypass of raw wastewater is 5–6 days and has shown a great advantage. 4. Conclusions The IC–SBR process for treatment of swine wastewaters was tested; following conclusions can be drawn based on the present study: IC reactor could remove about 80% of COD with organic loading rate of 6–7 kg COD/(m3 day) in treatment of raw swine wastewater, while the concentration of NH3-N in effluent was little higher than that in influent. Performance of direct post-treatment of digested swine wastewater was very poor because of the low biodegradability and improper carbon and nitrogen ratio. It is not feasible to treat swine wastewater with conventional anaerobic–aerobic process. Performance of post-treatment using SBR with addition of raw wastewater was good. The removal rate of COD, BOD5, NH3-N and TN were 95.5%, 99.6%, 99.4% and 94.3%, respectively, in the IC–SBR system. Based on the experiment, IC–SBR process with bypass of raw wastewater is developed to treat swine wastewater. In the new process, the raw wastewater is divided into two parts, i.e. one part (about 70%) was fed into anaerobic reactor, while the other part (about 30%) was bypassed into SBR to mix with digested wastewater.
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Acknowledgments Ministry of Science and Technology, China, supported this study work financially. The chemical analysis work of Miss Chen Gemin is also greatly appreciated.
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