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Denitrification on upflow-anaerobic filter filled with coconut shells (Cocos nucifera)
Ecological Engineering 82 (2015) 474–479
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Denitrification on upflow-anaerobic filter filled with coconut shells (Cocos nucifera) Jenifer Clarisse Pereira da Silva, Adriano Luiz Tonetti* , Lays Paulino Leonel, Aline Costa School of Civil Engineering, Architecture and Urbanism, FEC/UNICAMP, Avenida Albert Einstein, 951, Cidade Universitária “Zeferino Vaz”, Caixa Postal 6021, CEP: 13083-852 Campinas, SP, Brazil
A R T I C L E I N F O
A B S T R A C T
Article history: Received 22 January 2015 Received in revised form 17 April 2015 Accepted 23 May 2015 Available online 9 June 2015
This paper studied the denitrification in an upflow-anaerobic filter filled with coconut shells (Cocos nucifera). We sought to remove nitrogen in decentralized systems, reducing the diffuse contamination of groundwater and water bodies by nitrate. The upflow-anaerobic filter was operated under hydraulic retention time (HRT) of 16 h and on the bottom, there was an inflow of a combination of raw sewage and nitrified effluents from an intermittent sand filter. The research was divided into 5 stages in which there was an inflow of the following combination of raw sewage and nitrified effluent: Stage 1: 100.0%/0.0%; Stage 2: 87.5%/12.5%; Stage 3: 75.0%/25.0%; Stage 4: 62.5%/37.5%, and Stage 5: 50.0%/50.0%. The upflowanaerobic filter promoted denitrification in all stages with a quick acclimatization of the denitrifying biomass. In the most critical situation (Stage 5), in which there was an inflow of 46.9 mg L 1 of nitrate, a transformation of 98% of such compounds was found. On the effluent, the nitrate concentration obtained did not exceed 1.0 mg L 1. So we found that a simple recirculation of nitrified effluent to anaerobic filters can contribute to the reduction of diffuse pollution of nitrate in remote areas of large cities or in rural areas. ã2015 Elsevier B.V. All rights reserved.
Keywords: Sewage Biofilm reactor Treatment Onsite Nitrate Nitrogen
1. Introduction Disposing nitrogen-rich effluents into water bodies is a public health and environmental contamination issue. In the form of nitrite combined with secondary amines, nitrogen may create nitroamines, which are considered carcinogenic, teratogenic and mutagenic products (USEPA, 1993). In the form of nitrate, it may lead to a disease known as the blue baby syndrome. In the environment, it causes nutrient enriching, leading to an excessive growth of algae (Qin et al., 2012). A form of preventing it from being disposed into water bodies would be the wastewater treatment techniques that combine the removal of nitrogen and carbonaceous matter in anaerobic reactors. Such technology would be viable for developing countries with warm climates. However, denitrification and methanogenesis are mediated by different microbial populations requiring distinct environmental conditions and consequently, an integration of these processes might be problematic (Hendriksen and Ahring, 1996; Andalib et al., 2011). However, the occurrence of
* Corresponding author. Tel.: +55 19 35212369. E-mail address: [email protected] (A.L. Tonetti). http://dx.doi.org/10.1016/j.ecoleng.2015.05.007 0925-8574/ ã 2015 Elsevier B.V. All rights reserved.
denitrification in methanogenic systems is not well documented (Andalib et al., 2011). Baloch et al. (2006) and Tai et al. (2006) reported that the production of alkalinity during the denitrification process can be beneficially used to improve the stability of anaerobic reactors by counteracting the decrease in pH, usually associated with acidogenesis. However, in order for the denitrification to occur satisfactorily, one must pay attention to the influent COD/NO3 N ratio on the reactor. For Bernet et al. (1996), the complete denitrification will only occur when the COD/NO3 N ratio is greater than 3.4. Karim and Gupta (2003) found that denitrification and methanogenesis may occur simultaneously with COD/ NO3 N ratios higher than 4. The Brazilian Basic Sanitation Research Program (PROSAB) has studied the sewage treatment of small and decentralized human agglomerates using anaerobic filters. Anaerobic filters filled with easily acquired packing materials were studied, such as bamboo rings (Camargo and Nour, 2001), coconut shells (Cruz et al., 2013), ceramic bricks and ground-up tires (Barros et al., 2011). Cruz et al. (2013) verified that coconut shells showed a high resistance to biological degradation and empty bed volume (81.3 2.7%) and a specific surface area (100.3 14.8 m2 m 3) compatible with synthetic materials. In addition, the effluent
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produced in anaerobic filters filled with such shells reached a 77 50 mg L 1 COD, with an efficiency of 81 38%. There has been a considerable effort to couple denitrification and methanogenesis in a single reactor in both suspended and attached growth systems (Andalib et al., 2011). However, the great majority of the papers on denitrification in anaerobic reactors are related to UASB reactors. As for example, Tai et al. (2006) found that the synchronous production of methane and nitrogen gases demonstrated the potential of combined methanogenesis and denitrification in a UASB reactor. The operation of the integrated system demonstrated that a combined carbon and nitrogen removal was technically feasible with more than 87 and 96% of total COD and BOD removal, 46–79% of TN removal, and 49–86% of denitrification efficiency, depending on the HRTs and recycle ratios. Tai et al. (2006) concluded that combined carbon and nitrogen removal can occur in the UASB reactor without addition of an external carbon source. Cruz et al. (2013) stated that nevertheless, the anaerobic filters are more easily constructed than a UASB reactor, requiring only a tank and packing material. The packing material makes it difficult to washout the sludge and particulate organic matter, ensuring better effluent quality even with the existence of large fluctuations in the flow, characteristic of small treatment systems. The packing material also assists in the proper sewage flow distribution, hampering the formation of short circuits. In comparison, the UASB reactor requires the gas–liquid separator, which requires greater care in its design and construction. Additionally, in such a system, if the sewage distribution system is not built and maintained correctly, there will be the risk of forming preferential flow channels, impairing treatment. Therefore, the objective of this paper was to evaluate the denitrification on an upflow-anaerobic filter using coconut shells as the packing material. For such, several nitrified effluent and raw sewage ratios were used as the influent for the reactor. 2. Material and methods The research was developed at the School of Civil Engineering, Architecture and Urbanism of Unicamp. The raw sewage (RS) came
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from the university and was directed toward the up-flowanaerobic filters (Fig. 1). The effluent was directed to sand filters, whose role was to create the nitrified effluent and which were built according to the study developed by Tonetti et al. (2012). To build the sand filters, cylindrical boxes with an inner size of 1.00 m were used. The bed was constituted in three layers. The first was 0.20 m deep and consisting of gravel with an effective size (ES) of 16.12 mm and uniformity coefficient (UC) of 1.89. Right on top, there was another layer consisting of 0.05 m of gravel (ES of 7.51 mm and UC of 1.66). The last layer consisted of 0.75 m of sand (ES of 0.17 mm and UC of 3.14). The sand filters received a 200 L m 2 day 1 hydraulic loading rate. The anaerobic filter and sand filter were only used to produce the anaerobic effluent and nitrified effluent. These reactors have been investigated in other studies (Cruz et al., 2013; Tonetti et al., 2012). In this research, the strategy of adding synthetic nitrate to raw sewage was not adopted. This is because in the future, a recirculating sand filter effluent to the anaerobic reactor will be studied. 3. Denitrifying upflow-anaerobic filter (DAF) As shown in Fig. 1, the raw sewage (RS) and the effluent produced by intermittent sand filters (ISF) were sent to a denitrifying anaerobic filter (DAF). In this reactor, the packing material used was coconut shells from the Cocos nucifera species. Before they were placed inside the cylinder, such shells were divided into four sections (Fig. 2). The denitrifying anaerobic filter was built with stainless steel shaped as a cylinder and with a total volume of 500 L (Fig. 3). It was 1.68 m high, and its diameter was 0.76 m. The conic-shaped bottom was separated from the section filled with coconut shells by a bamboo grid. Denitrification was evaluated in this anaerobic filter. In the study, different combinations of raw sewage and nitrified effluents from intermittent sand filter were applied (Table 1). The hydraulic retention time used in the denitrifying anaerobic filter (DAF) was 16 h.
Fig. 1. Schematic of the studied system.
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matter or denitrification. The shells only served as support for the biofilm. 4. Results
Fig. 2. Coconut shells of the Cocos nucifera species (Cruz et al., 2013).
Fig. 3. Upflow-anaerobic filter schematic (Tonetti et al., 2012; Cruz et al., 2013).
The following samples were collected twice a week: raw sewage, intermittent sand filter effluent and denitrifying anaerobic filter effluents. The raw sewage and intermittent sand filter effluent sampling was conducted immediately before the input on the denitrifying anaerobic filter. The analyses of pH, chemical oxygen demand (COD), total Kjeldahl nitrogen (TKN), ammoniacal nitrogen, nitrate, nitrite and alkalinity were performed according to Standard Methods for the Examination of Water and Wastewater (APHA et al., 2012). The coconut shells were inside the reactor for over two years. This may be due to the consumption of easily degradable compounds (Cruz et al., 2013). The portion of the husk consisting of fibers has its degradation hampered by the existence of cellulose and woody material, which confers high stability to the biological action (Ohmiya et al., 2005). Therefore, this material was stabilized and did not interfere in the biological process of removing organic
In Stage 1 of the study, 100% of raw sewage was applied to the denitrifying upflow-anaerobic filter (DAF). The objective was to evaluate the performance of the anaerobic reactor without the nitrified effluent. The reactor showed a similar behavior (Table 2) to the upflow-anaerobic filters studied by Cruz et al. (2013) and Camargo and Nour (2001). In this case, a removal of 63% of COD was found, which is close to the typical values for anaerobic reactors (Foresti, 2002). In Stage 2, there was an inflow of nitrified effluent together with raw sewage. The study considered as DAF influent the result of the raw sewage and nitrified effluent combination (RS + ISF combination). It was observed that even by increasing the nitrate concentration throughout the stages, the DAF was highly efficient in transforming such a compound. In Stage 5, where there was an inflow of a combination of 50% raw sewage and 50% nitrified effluent, the influent had a nitrate concentration of 47.7 11.4 mg L 1, while the effluent had a concentration of only 0.89 1.0 mg L 1. This refers to a 98.1% nitrate transformation. Even with such an increase of nitrate concentration on the influent, in every stage the DAF reached a stable state after a short period of time. In Stage 2, 95% reduction of the concentration of NO3 N was reached after 60 h of the first nitrified effluent application. This shows the quick acclimatization of the denitrifying biomass. Akunna et al. (1994) found a greater period for such acclimatization, reaching 120 h. A possible explanation for this rapid acclimatization of biomass may be the fact that a reactor that was in operation for over two years was used, treating the same wastewater. Moreover, according Achak et al. (2009), even with the sand filter having a large air capacity, denitrification can occur in anaerobic microcosms. When the reactor is supplied with wastewater the process of denitrification takes place in the anoxic zones. Thus, the nitrified effluent could already have a small amount of denitrifying bacteria adapted to the wastewater. When these bacteria reached the anaerobic filter, they encountered conditions for their rapid development. As a consequence of the nitrate transformation, the total nitrogen concentration was also reduced (Fig. 4). In Stage 2, the influent Total-N concentration was 90.8 14.6 mg L 1 and the effluent concentration was 68.5 4.7 mg L 1, with an average reduction of 23.3 9.3%. In Stage 5, the average reduction reached 59.4 11.4%. Huang et al. (2007) found that the combined UASB–AS reactor system achieved an efficient removal of TKN (100%) and TN (54–77%) from piggery wastewater. Ahn et al. (2007) combined an anaerobic upflow bed filter with a membrane-aerated bioreactor and obtained a 46% removal of nitrogen. During each stage, there was no significant difference between the influent and effluent concentration of N–NH3 (Fig. 5). This indicates that an assimilatory reduction, in which the nitrate is transformed into ammoniacal nitrogen, did not occur. Such results are in agreement with the research developed by Akunna et al. (1994). The authors found that 24 h after introducing the nitrate into an anaerobic reactor, the ammonia production was
Table 1 Nitrified effluent and raw sewage application stages in the denitrifying anaerobic filter (DAF). Influent source
Stage 1
Stage 2
Stage 3
Stage 4
Stage 5
Raw sewage (%) Nitrified effluent (%) Raw sewage to nitrified effluent ratio
100 0 –
87.5 12.5 7.0/1.0
75.0 25.0 3.0/1.0
62.5 37.5 1.7/1.0
50.0 50.0 1.0/1.0
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477
Table 2 Mean values for the evaluated parameters. Stage
Effluent
COD (mg O2 L
1
RS ISF DAF RS ISF Combination DAF RS ISF Combination DAF RS ISF Combination DAF RS ISF Combination DAF
837 295 49 32 291 127 643 171 41 18 568 149 173 79 926 384 22 11 700 288 139 58 1224 437 39 13 779 273 172 66 1428 843 55 41 742 420 121 30
2
3
4
5
(RS + ISF)
(RS + ISF)
(RS + ISF)
(RS + ISF)
1
)
TKN-N (mg L 98.5 19.5 15.2 9.2 84.8 12.6 91.6 18.8 1.8 2.2 80.4 16.5 68.0 6.3 93.7 18.5 0.7 0.4 82.1 16.2 51.8 4.7 90.4 28.5 0.4 0.1 79.1 24.9 43.0 5.0 91.4 26.9 1.2 1.4 80.2 23.5 33.8 3.7
1
)
NO3
N (mg L
1
)
3.1 1.0 67.1 19.8 1.5 0.8 2.3 1.0 57.8 12.1 9.3 2.1 1.1 1.9 2.2 1.1 64.0 9.3 17.7 2.4 1.2 1.1 2.8 1.0 76.3 9.7 30.4 3.9 1.4 0.4 2.8 3.0 91.1 21.5 47.7 11.4 0.89 1.0
NO2
N (mgL
1
)
0.1 0.1 0.7 1.3 0.1 0.1 0.1 0.1 1.8 2.4 0.2 0.3 0.1 0.2 0.1 0.1 0.3 0.4 0.1 0.1 0.1 0.1 0.1 0.1 1.3 1.3 0.5 0.5 0.1 0.1 0.1 0.1 0.7 0.4 0.3 0.2 0.1 0.2
RS: Raw Sewage; ISF: Intermittent sand filter; DAF: Denitrifying upflow-anaerobic filter; TKN: Total Kjendal nitrogen.
Fig. 4. Total nitrogen influent and effluent concentration in the denitrifying anaerobic filter (DAF).
Fig. 5. Influent and effluent concentration of ammoniacal nitrogen in the denitrifying anaerobic filter (DAF).
insignificant, and the reduction of N-nitrate into nitrogen gas prevailed. They stated that dissimilatory nitrate reduction to ammonia took place in anaerobic digestion only during fermentation and that nitrate conversion to ammonia was greatly minimized with non-fermentable organic carbon sources.
In Stage 1 (raw sewage application only), the COD removal was equal to 63% (Table 2). After introducing the nitrified effluent, there was an increase in efficiency, reaching 75 12% in Stages 2–5. Such results may indicate that the organic matter consumption by denitrifying bacteria is not inhibiting methanogenesis. In
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comparison, by using a sludge blanket reactor, Lee et al. (2004) found a reduction in the COD removal after adding the nitrified effluent. For Andalib et al. (2011), in biofilm processes, where diffusion gradients can evolve, resulting in distinct environmental conditions, methanogenesis and denitrification has reportedly been observed to coexist. Lin and Chen (1995) state that in a coimmobilized mixed culture system of denitrifying bacteria and methanogenic microbes contained in polyvinyl alcohol gel beads, it was demonstrated that methanogenesis was active inside the beads where nitrate is absent, but denitrifiers grew on the surface of the beads. Based on the calculation suggested by the USEPA (1993), in which a 1.00 g nitrate reduction is equivalent to a 2.86 g reduction of O2, it is believed that between 5 and 23% of the influent COD removal in DAF occurred during the reduction of NO3 N to N2. Such COD removal percentage grew at every stage, following the nitrified effluent proportion increase applied to DAF (Table 3). Therefore, in Stage 5, possibly 77% of organic matter removal in terms of COD was promoted during methanogenesis, while 23% was removed during the nitrate reduction. Such results are in agreement with the research developed by Huang et al. (2007). These authors concluded that the presence of methane in a UASB reactor showed that methanogenesis occurred at the same time as denitrification. For Huang et al. (2007), low values for the COD/NOx N ratio will tend to result in a prevalence of the microbial group responsible for denitrification, leading to a lack of substrate for methanogenic microorganisms. For Bernet et al. (1996), the total denitrification will only occur when the COD/NO3 N ratio is greater than 3.4. Karim and Gupta (2003) found that denitrification and methanogenesis may occur simultaneously with COD/ NO3 N ratios greater than 4. Akunna et al. (1992) and Rustrian et al. (1997) stated that denitrification increases as the COD/NO3 N relation is reduced. For these authors, the denitrifying activity is hampered when the COD/NO3 N relation is below 10 or at a nitrate concentration above 2000 mg L 1. This occurs due to the accumulation of intermediate denitrification products, such as NO2 and N2O. Such intermediate products may inhibit the methanogenic activity, compromising the system’s global efficiency (Andalib et al., 2011). In this research (Fig. 6), the COD/NO3 N relation varied between 15.64 7.30 and 62.05 16.16 averages. Therefore, such values were adequate for denitrification, leading to the creation of an effluent with low nitrate concentrations in every stage studied (Table 2). Akunna et al. (1992) and Andalib et al. (2011) reported methane production without denitrification at COD/NO3 N greater than 53, with glucose as substrate in a CSTR of combined acidogenesis and methanogenesis. In Stage 2, a mean COD/NO3 N ratio equal to 62.05 16.16 was found. Even so, no reduction in the denitrifying efficiency was found (Table 2). Therefore, we may say that the great variety of compounds in the raw sewage may favor denitrification, regardless of the COD/NO3 N ratio. Another parameter that may be used to evaluate denitrification is alkalinity. According to Metcalf and Eddy Inc. (2003), during the NO3 N reduction to N2, a theoretical gain of 3.57 mg CaCO3 per
Fig. 6. COD/nitrate ratio.
Table 4 Total alkalinity on influent and effluent DAF. Stages
Influent (mg CaCO3 L
1 2 3 4 5
292 54 164 36 114 25 146 29 149 24
1
)
Effluent (mg CaCO3 L
1
)
355 42 399 47 355 24 361 31 382 37
reduced mg NO3 N occurs. In this study, an increase in the alkalinity occurred in all stages (Table 4). Since denitrification produces gaseous nitrogen, there was a concern that the formation of such gas could cause the sludge on the anaerobic filter to be dragged off. Therefore, the concentration of suspended solids (TSS) was constantly monitored at the DAF effluent. In Stages 1, 2, 3, 4, and 5, the mean concentrations were 35 24; 22 11; 18 08; 23 10 and 15 06 mg L 1, respectively. No significant difference was found between these mean values. This indicates the non-interference of denitrification for the biomass to be dragged off in the upflow-anaerobic filter. Possibly, in UASB reactors, where the biomass remains scattered, such dragging may compromise the quality of the effluent. The nitrite concentration during the entire analytical period and at all collection points never exceeded 2.2 mg L 1. Therefore, there was no accumulation of nitrogen in the reactor that could compromise denitrification. Under normal conditions for nitrification, ammonia and nitrite oxidation rates are reported to be highly coupled, both in rates as in space, preventing nitrite accumulation (Philips et al., 2002). However, wastewater treatment plants frequently fail to establish stable nitrification, which is often attributed to the slow growth of nitrifying bacteria (Philips et al., 2002). For Hongwei et al. (2009) nitrite accumulation can be caused by microbial communities with different characteristics for nitrate and nitrite reduction. Schuch et al. (2000) observed that a higher alkaline pH (>7.8) resulted in a lower denitrification rate and in an increasing nitrite concentration in the effluent. On the other hand, Cao et al. (2013) concluded that nitrite accumulation was more serious at low pH
Table 3 COD consumption during denitrification. Stage
Influent COD (mg L 1)
Theoretical COD required for denitrification (mg L 1)
Removed COD (mg O2 L 1)
% of theoretical removal of COD during denitrification
2 3 4 5
568 149 700 288 779 273 742 420
26 33 89 136
395 151 561 291 607 282 620 410
5% 6% 13% 23%
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than at high pH, regardless whether the pH of the mixed liquor was stabilized during denitrification. The major importance of the pH at such, it usually influences other parameters playing even a greater role in nitrite accumulation; among others, the balance of ammonium and ammonia in water is dependent on the pH, and the effect of free ammonia seems to be more pronounced. In this study the pH was always near neutrality (7.12 0.48). This shows that the system was able to maintain optimum conditions for denitrification throughout the analytical period. 5. Conclusions The upflow-anaerobic filter with coconut shell as packing material (C. nucifera) was efficient in reducing the organic matter and denitrification. Even with an influent consisting of 50% sewage and 50% nitrified effluent, an effluent with low nitrate concentrations, never higher than 1.5 mg L 1 was produced. The biomass in the denitrifying upflow-anaerobic filter showed quick acclimatization, even when there was an increase in the nitrate load applied. During the denitrification process, no solids were dragged off from the reactor by the gases created. This shows the robustness of upflow-anaerobic filters and their feasibility to be used as a denitrifying reactor to treat wastewater. Acknowledgments The authors would like to thank CNPq (the Brazilian National Council for Scientific and Technological Development) for the scholarships granted, in addition to FAPESP (São Paulo Research Foundation) and FINEP (Studies and Projects Financing Agency) for financing this study. The authors would also like to acknowledge the service of the Writing Space/General Coordination of UNICAMP for helping translate the original manuscript. References Achak, M., Mandi, L., Ouazzani, N., 2009. Removal of organic pollutants and nutrients from olive mill wastewater by a sand filter. J. Environ. Manage. 90, 2771–2779. Ahn, Y.T., Kang, S.T., Chae, S.R., Lee, C.Y., Bae Shin, B.U., HS, 2007. Simultaneous highstrength organic and nitrogen removal with combined anaerobic upflow bed filter and aerobic membrane bioreactor. Desalination 202, 114–121. Akunna, J.C., Bizeau, C., Moletta, R., 1992. Denitrification in anaerobic digesters: possibilities and influence of wastewater COD/NOxN ratio. Environ. Technol. 13 (9), 825–836. Akunna, J.C., Bizeau, C., Moletta, R., 1994. Nitrate reduction by anaerobic sludge using glucose at various nitrate concentrations: ammonification, denitrification and methanogenic activities. Environ. Technol. 15, 41–49.
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Andalib, M., Nakhla, G., McIntee, E., Zhu, J., 2011. Simultaneous denitrification and methanogenesis (SMD): Review of two decades of research. Desalination 279, 1–14. APHA/AWWA/WEF, 2012. Standard Methods for the Examination of Water and Wastewater, 20a ed. American Public Health Associatio, Nova Iorque 1268 p.. Baloch, M.I., Akunna, J.C., Collier, P.J., 2006. Carbon and nitrogen in a granular bed baffled reactor. Environ. Technol. 27, 201–208. Barros, A.R., Adorno, M.A.T., Sakamoto, I.K., Maintinguer, S.I., Varesche, M.B.A., Silva, E.S., 2011. Performance evaluation and phylogenetic characterization of anaerobic fluidized bed reactors using ground tire and pet as packing materials for biohydrogen production. Bioresour. Technol. 102, 3840–3847. Bernet, N., Delgenés, N., Moletta, R., 1996. Denitrification by anaerobic sludge in piggery wastewater. Environ. Technol. 17, 293–300. Camargo, S.A.R., Nour, E.A.A., 2001. Bamboo as an anaerobic medium: effect of filter columm height. Water Sci. Technol. 44 (4), 63–70. Cao, X., Qian, D., Menga, M., 2013. Effects of pH on nitrite accumulation during wastewater denitrification. Environ. Technol. 34 (1), 45–51. Cruz, L.M., Stefanutti, R., Coraucci Filho, B., Tonetti, A.L., 2013. Coconut shells as filling material of anaerobic filters. Springer Plus Eng. 2, 655. United States Environmental Protection Agency, 1993. Manual Nitrogen Control. USEPA, Washington. Foresti, E., 2002. Anaerobic treatment of domestic sewage: established technologies and perspectives. Water Sci. Technol. 45 (10), 181–186. Hendriksen, H.V., Ahring, B.K., 1996. Combined removal of nitrate and carbon in granular sludge: substrate competition and activities. Antonie Van Leeuwenhoek 69, 33–39. Hongwei, S., Qing, Y., Yongzhen, P., Xiaoning, S., Shuying, W., Shujun, Z., 2009. Nitrite accumulation during the denitrification process in SBR for the treatment of pretreated landfill leachate. Chin. J. Chem. Eng. 17 (6), 1027–1031. Huang, J.S., Chou, H.H., Chen, C.M., Chiang, C.M., 2007. Effect of recycle-to-influent ratio on activities of nitrifiers and denitrifiers in a combined UASB-activated sludge reactor system. Chemosphere 68, 382–388. Karim, K., Gupta, S.K., 2003. Continuous biotransformation and removal of nitrophenols under denitrifyingconditions. Water Res. 37, 2953–2959. Lee, C.-Y., Shin, H.-S., Hwang, S.-J., 2004. Characteristics of granular sludge in a single upflow sludge blanket reactor treating high levels of nitrate and simple organic compounds. Water Sci. Technol. 50 (8), 217–224. Lin, Y.F., Chen, K.C., 1995. Denitrification and methanogenesis in a co-immobilized mixed culture system. Water Res. 29 (1), 35–43. Metcalf and Eddy Inc, 2003. Wastewater Engineering treatment Disposal Reuse, 4. ed. McGraw – Hill Book, New York, pp. 1815. Ohmiya, K., Sakka, K., Kimura, T., 2005. Anaerobic bacterial degradation for the effective utilization of biomass. Biotechnol. Bioprocess. Eng. 10, 482–493. Philips, S., Laanbroek, H.J., Verstraete, W., 2002. Origin, causes and effects of increased nitrite concentrations in aquatic environments. Rev. Environ. Sci. Bio/ Technol. 1, 115–141. Qin, B.Q., Gao, G., Zhu, G.W., Zhang, Y.L., Song, Y.Z., Tang, X.M., Xu, H., Deng, J.M., 2012. Lake eutrophication and its ecosystem response. Chin. Sci. Bull. 58, 961–970. Rustrian, E., Delgenes, J.P., Bernet, N., 1997. Nitrate reduction in acidogenic reactor: Influence of wastewater COD/NNO3 ratio on denitrification and acidogenic activity. Environ. Techonol. 18, 309–315. Schuch, R., Gensicke, R., Merkel, K., Winter, J., 2000. Nitrogen and DOC removal from wastewater streams of the metal-working industry. Water Res. 34 (1), 295–303. Tai, C.S., Singh, K.S., Grant, S.R., 2006. Combined removal of carbon and nitrogen in an integrated UASB-jet loop reactor bioreactor system. J. Environ. Eng. 132 (6), 624–637. Tonetti, A.L., Coraucci Filho, B., Nicolau, C.E., Barbosa, M., Tonon, D., 2012. Tratamento de esgoto e produção de água de reúso com o emprego de filtros de areia. Revista Engenharia Sanitária e Ambiental 17 (1), 287–294.
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Ecological Engineering journal homepage: www.elsevier.com/locate/ecoleng
Denitrification on upflow-anaerobic filter filled with coconut shells (Cocos nucifera) Jenifer Clarisse Pereira da Silva, Adriano Luiz Tonetti* , Lays Paulino Leonel, Aline Costa School of Civil Engineering, Architecture and Urbanism, FEC/UNICAMP, Avenida Albert Einstein, 951, Cidade Universitária “Zeferino Vaz”, Caixa Postal 6021, CEP: 13083-852 Campinas, SP, Brazil
A R T I C L E I N F O
A B S T R A C T
Article history: Received 22 January 2015 Received in revised form 17 April 2015 Accepted 23 May 2015 Available online 9 June 2015
This paper studied the denitrification in an upflow-anaerobic filter filled with coconut shells (Cocos nucifera). We sought to remove nitrogen in decentralized systems, reducing the diffuse contamination of groundwater and water bodies by nitrate. The upflow-anaerobic filter was operated under hydraulic retention time (HRT) of 16 h and on the bottom, there was an inflow of a combination of raw sewage and nitrified effluents from an intermittent sand filter. The research was divided into 5 stages in which there was an inflow of the following combination of raw sewage and nitrified effluent: Stage 1: 100.0%/0.0%; Stage 2: 87.5%/12.5%; Stage 3: 75.0%/25.0%; Stage 4: 62.5%/37.5%, and Stage 5: 50.0%/50.0%. The upflowanaerobic filter promoted denitrification in all stages with a quick acclimatization of the denitrifying biomass. In the most critical situation (Stage 5), in which there was an inflow of 46.9 mg L 1 of nitrate, a transformation of 98% of such compounds was found. On the effluent, the nitrate concentration obtained did not exceed 1.0 mg L 1. So we found that a simple recirculation of nitrified effluent to anaerobic filters can contribute to the reduction of diffuse pollution of nitrate in remote areas of large cities or in rural areas. ã2015 Elsevier B.V. All rights reserved.
Keywords: Sewage Biofilm reactor Treatment Onsite Nitrate Nitrogen
1. Introduction Disposing nitrogen-rich effluents into water bodies is a public health and environmental contamination issue. In the form of nitrite combined with secondary amines, nitrogen may create nitroamines, which are considered carcinogenic, teratogenic and mutagenic products (USEPA, 1993). In the form of nitrate, it may lead to a disease known as the blue baby syndrome. In the environment, it causes nutrient enriching, leading to an excessive growth of algae (Qin et al., 2012). A form of preventing it from being disposed into water bodies would be the wastewater treatment techniques that combine the removal of nitrogen and carbonaceous matter in anaerobic reactors. Such technology would be viable for developing countries with warm climates. However, denitrification and methanogenesis are mediated by different microbial populations requiring distinct environmental conditions and consequently, an integration of these processes might be problematic (Hendriksen and Ahring, 1996; Andalib et al., 2011). However, the occurrence of
* Corresponding author. Tel.: +55 19 35212369. E-mail address: [email protected] (A.L. Tonetti). http://dx.doi.org/10.1016/j.ecoleng.2015.05.007 0925-8574/ ã 2015 Elsevier B.V. All rights reserved.
denitrification in methanogenic systems is not well documented (Andalib et al., 2011). Baloch et al. (2006) and Tai et al. (2006) reported that the production of alkalinity during the denitrification process can be beneficially used to improve the stability of anaerobic reactors by counteracting the decrease in pH, usually associated with acidogenesis. However, in order for the denitrification to occur satisfactorily, one must pay attention to the influent COD/NO3 N ratio on the reactor. For Bernet et al. (1996), the complete denitrification will only occur when the COD/NO3 N ratio is greater than 3.4. Karim and Gupta (2003) found that denitrification and methanogenesis may occur simultaneously with COD/ NO3 N ratios higher than 4. The Brazilian Basic Sanitation Research Program (PROSAB) has studied the sewage treatment of small and decentralized human agglomerates using anaerobic filters. Anaerobic filters filled with easily acquired packing materials were studied, such as bamboo rings (Camargo and Nour, 2001), coconut shells (Cruz et al., 2013), ceramic bricks and ground-up tires (Barros et al., 2011). Cruz et al. (2013) verified that coconut shells showed a high resistance to biological degradation and empty bed volume (81.3 2.7%) and a specific surface area (100.3 14.8 m2 m 3) compatible with synthetic materials. In addition, the effluent
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produced in anaerobic filters filled with such shells reached a 77 50 mg L 1 COD, with an efficiency of 81 38%. There has been a considerable effort to couple denitrification and methanogenesis in a single reactor in both suspended and attached growth systems (Andalib et al., 2011). However, the great majority of the papers on denitrification in anaerobic reactors are related to UASB reactors. As for example, Tai et al. (2006) found that the synchronous production of methane and nitrogen gases demonstrated the potential of combined methanogenesis and denitrification in a UASB reactor. The operation of the integrated system demonstrated that a combined carbon and nitrogen removal was technically feasible with more than 87 and 96% of total COD and BOD removal, 46–79% of TN removal, and 49–86% of denitrification efficiency, depending on the HRTs and recycle ratios. Tai et al. (2006) concluded that combined carbon and nitrogen removal can occur in the UASB reactor without addition of an external carbon source. Cruz et al. (2013) stated that nevertheless, the anaerobic filters are more easily constructed than a UASB reactor, requiring only a tank and packing material. The packing material makes it difficult to washout the sludge and particulate organic matter, ensuring better effluent quality even with the existence of large fluctuations in the flow, characteristic of small treatment systems. The packing material also assists in the proper sewage flow distribution, hampering the formation of short circuits. In comparison, the UASB reactor requires the gas–liquid separator, which requires greater care in its design and construction. Additionally, in such a system, if the sewage distribution system is not built and maintained correctly, there will be the risk of forming preferential flow channels, impairing treatment. Therefore, the objective of this paper was to evaluate the denitrification on an upflow-anaerobic filter using coconut shells as the packing material. For such, several nitrified effluent and raw sewage ratios were used as the influent for the reactor. 2. Material and methods The research was developed at the School of Civil Engineering, Architecture and Urbanism of Unicamp. The raw sewage (RS) came
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from the university and was directed toward the up-flowanaerobic filters (Fig. 1). The effluent was directed to sand filters, whose role was to create the nitrified effluent and which were built according to the study developed by Tonetti et al. (2012). To build the sand filters, cylindrical boxes with an inner size of 1.00 m were used. The bed was constituted in three layers. The first was 0.20 m deep and consisting of gravel with an effective size (ES) of 16.12 mm and uniformity coefficient (UC) of 1.89. Right on top, there was another layer consisting of 0.05 m of gravel (ES of 7.51 mm and UC of 1.66). The last layer consisted of 0.75 m of sand (ES of 0.17 mm and UC of 3.14). The sand filters received a 200 L m 2 day 1 hydraulic loading rate. The anaerobic filter and sand filter were only used to produce the anaerobic effluent and nitrified effluent. These reactors have been investigated in other studies (Cruz et al., 2013; Tonetti et al., 2012). In this research, the strategy of adding synthetic nitrate to raw sewage was not adopted. This is because in the future, a recirculating sand filter effluent to the anaerobic reactor will be studied. 3. Denitrifying upflow-anaerobic filter (DAF) As shown in Fig. 1, the raw sewage (RS) and the effluent produced by intermittent sand filters (ISF) were sent to a denitrifying anaerobic filter (DAF). In this reactor, the packing material used was coconut shells from the Cocos nucifera species. Before they were placed inside the cylinder, such shells were divided into four sections (Fig. 2). The denitrifying anaerobic filter was built with stainless steel shaped as a cylinder and with a total volume of 500 L (Fig. 3). It was 1.68 m high, and its diameter was 0.76 m. The conic-shaped bottom was separated from the section filled with coconut shells by a bamboo grid. Denitrification was evaluated in this anaerobic filter. In the study, different combinations of raw sewage and nitrified effluents from intermittent sand filter were applied (Table 1). The hydraulic retention time used in the denitrifying anaerobic filter (DAF) was 16 h.
Fig. 1. Schematic of the studied system.
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matter or denitrification. The shells only served as support for the biofilm. 4. Results
Fig. 2. Coconut shells of the Cocos nucifera species (Cruz et al., 2013).
Fig. 3. Upflow-anaerobic filter schematic (Tonetti et al., 2012; Cruz et al., 2013).
The following samples were collected twice a week: raw sewage, intermittent sand filter effluent and denitrifying anaerobic filter effluents. The raw sewage and intermittent sand filter effluent sampling was conducted immediately before the input on the denitrifying anaerobic filter. The analyses of pH, chemical oxygen demand (COD), total Kjeldahl nitrogen (TKN), ammoniacal nitrogen, nitrate, nitrite and alkalinity were performed according to Standard Methods for the Examination of Water and Wastewater (APHA et al., 2012). The coconut shells were inside the reactor for over two years. This may be due to the consumption of easily degradable compounds (Cruz et al., 2013). The portion of the husk consisting of fibers has its degradation hampered by the existence of cellulose and woody material, which confers high stability to the biological action (Ohmiya et al., 2005). Therefore, this material was stabilized and did not interfere in the biological process of removing organic
In Stage 1 of the study, 100% of raw sewage was applied to the denitrifying upflow-anaerobic filter (DAF). The objective was to evaluate the performance of the anaerobic reactor without the nitrified effluent. The reactor showed a similar behavior (Table 2) to the upflow-anaerobic filters studied by Cruz et al. (2013) and Camargo and Nour (2001). In this case, a removal of 63% of COD was found, which is close to the typical values for anaerobic reactors (Foresti, 2002). In Stage 2, there was an inflow of nitrified effluent together with raw sewage. The study considered as DAF influent the result of the raw sewage and nitrified effluent combination (RS + ISF combination). It was observed that even by increasing the nitrate concentration throughout the stages, the DAF was highly efficient in transforming such a compound. In Stage 5, where there was an inflow of a combination of 50% raw sewage and 50% nitrified effluent, the influent had a nitrate concentration of 47.7 11.4 mg L 1, while the effluent had a concentration of only 0.89 1.0 mg L 1. This refers to a 98.1% nitrate transformation. Even with such an increase of nitrate concentration on the influent, in every stage the DAF reached a stable state after a short period of time. In Stage 2, 95% reduction of the concentration of NO3 N was reached after 60 h of the first nitrified effluent application. This shows the quick acclimatization of the denitrifying biomass. Akunna et al. (1994) found a greater period for such acclimatization, reaching 120 h. A possible explanation for this rapid acclimatization of biomass may be the fact that a reactor that was in operation for over two years was used, treating the same wastewater. Moreover, according Achak et al. (2009), even with the sand filter having a large air capacity, denitrification can occur in anaerobic microcosms. When the reactor is supplied with wastewater the process of denitrification takes place in the anoxic zones. Thus, the nitrified effluent could already have a small amount of denitrifying bacteria adapted to the wastewater. When these bacteria reached the anaerobic filter, they encountered conditions for their rapid development. As a consequence of the nitrate transformation, the total nitrogen concentration was also reduced (Fig. 4). In Stage 2, the influent Total-N concentration was 90.8 14.6 mg L 1 and the effluent concentration was 68.5 4.7 mg L 1, with an average reduction of 23.3 9.3%. In Stage 5, the average reduction reached 59.4 11.4%. Huang et al. (2007) found that the combined UASB–AS reactor system achieved an efficient removal of TKN (100%) and TN (54–77%) from piggery wastewater. Ahn et al. (2007) combined an anaerobic upflow bed filter with a membrane-aerated bioreactor and obtained a 46% removal of nitrogen. During each stage, there was no significant difference between the influent and effluent concentration of N–NH3 (Fig. 5). This indicates that an assimilatory reduction, in which the nitrate is transformed into ammoniacal nitrogen, did not occur. Such results are in agreement with the research developed by Akunna et al. (1994). The authors found that 24 h after introducing the nitrate into an anaerobic reactor, the ammonia production was
Table 1 Nitrified effluent and raw sewage application stages in the denitrifying anaerobic filter (DAF). Influent source
Stage 1
Stage 2
Stage 3
Stage 4
Stage 5
Raw sewage (%) Nitrified effluent (%) Raw sewage to nitrified effluent ratio
100 0 –
87.5 12.5 7.0/1.0
75.0 25.0 3.0/1.0
62.5 37.5 1.7/1.0
50.0 50.0 1.0/1.0
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Table 2 Mean values for the evaluated parameters. Stage
Effluent
COD (mg O2 L
1
RS ISF DAF RS ISF Combination DAF RS ISF Combination DAF RS ISF Combination DAF RS ISF Combination DAF
837 295 49 32 291 127 643 171 41 18 568 149 173 79 926 384 22 11 700 288 139 58 1224 437 39 13 779 273 172 66 1428 843 55 41 742 420 121 30
2
3
4
5
(RS + ISF)
(RS + ISF)
(RS + ISF)
(RS + ISF)
1
)
TKN-N (mg L 98.5 19.5 15.2 9.2 84.8 12.6 91.6 18.8 1.8 2.2 80.4 16.5 68.0 6.3 93.7 18.5 0.7 0.4 82.1 16.2 51.8 4.7 90.4 28.5 0.4 0.1 79.1 24.9 43.0 5.0 91.4 26.9 1.2 1.4 80.2 23.5 33.8 3.7
1
)
NO3
N (mg L
1
)
3.1 1.0 67.1 19.8 1.5 0.8 2.3 1.0 57.8 12.1 9.3 2.1 1.1 1.9 2.2 1.1 64.0 9.3 17.7 2.4 1.2 1.1 2.8 1.0 76.3 9.7 30.4 3.9 1.4 0.4 2.8 3.0 91.1 21.5 47.7 11.4 0.89 1.0
NO2
N (mgL
1
)
0.1 0.1 0.7 1.3 0.1 0.1 0.1 0.1 1.8 2.4 0.2 0.3 0.1 0.2 0.1 0.1 0.3 0.4 0.1 0.1 0.1 0.1 0.1 0.1 1.3 1.3 0.5 0.5 0.1 0.1 0.1 0.1 0.7 0.4 0.3 0.2 0.1 0.2
RS: Raw Sewage; ISF: Intermittent sand filter; DAF: Denitrifying upflow-anaerobic filter; TKN: Total Kjendal nitrogen.
Fig. 4. Total nitrogen influent and effluent concentration in the denitrifying anaerobic filter (DAF).
Fig. 5. Influent and effluent concentration of ammoniacal nitrogen in the denitrifying anaerobic filter (DAF).
insignificant, and the reduction of N-nitrate into nitrogen gas prevailed. They stated that dissimilatory nitrate reduction to ammonia took place in anaerobic digestion only during fermentation and that nitrate conversion to ammonia was greatly minimized with non-fermentable organic carbon sources.
In Stage 1 (raw sewage application only), the COD removal was equal to 63% (Table 2). After introducing the nitrified effluent, there was an increase in efficiency, reaching 75 12% in Stages 2–5. Such results may indicate that the organic matter consumption by denitrifying bacteria is not inhibiting methanogenesis. In
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comparison, by using a sludge blanket reactor, Lee et al. (2004) found a reduction in the COD removal after adding the nitrified effluent. For Andalib et al. (2011), in biofilm processes, where diffusion gradients can evolve, resulting in distinct environmental conditions, methanogenesis and denitrification has reportedly been observed to coexist. Lin and Chen (1995) state that in a coimmobilized mixed culture system of denitrifying bacteria and methanogenic microbes contained in polyvinyl alcohol gel beads, it was demonstrated that methanogenesis was active inside the beads where nitrate is absent, but denitrifiers grew on the surface of the beads. Based on the calculation suggested by the USEPA (1993), in which a 1.00 g nitrate reduction is equivalent to a 2.86 g reduction of O2, it is believed that between 5 and 23% of the influent COD removal in DAF occurred during the reduction of NO3 N to N2. Such COD removal percentage grew at every stage, following the nitrified effluent proportion increase applied to DAF (Table 3). Therefore, in Stage 5, possibly 77% of organic matter removal in terms of COD was promoted during methanogenesis, while 23% was removed during the nitrate reduction. Such results are in agreement with the research developed by Huang et al. (2007). These authors concluded that the presence of methane in a UASB reactor showed that methanogenesis occurred at the same time as denitrification. For Huang et al. (2007), low values for the COD/NOx N ratio will tend to result in a prevalence of the microbial group responsible for denitrification, leading to a lack of substrate for methanogenic microorganisms. For Bernet et al. (1996), the total denitrification will only occur when the COD/NO3 N ratio is greater than 3.4. Karim and Gupta (2003) found that denitrification and methanogenesis may occur simultaneously with COD/ NO3 N ratios greater than 4. Akunna et al. (1992) and Rustrian et al. (1997) stated that denitrification increases as the COD/NO3 N relation is reduced. For these authors, the denitrifying activity is hampered when the COD/NO3 N relation is below 10 or at a nitrate concentration above 2000 mg L 1. This occurs due to the accumulation of intermediate denitrification products, such as NO2 and N2O. Such intermediate products may inhibit the methanogenic activity, compromising the system’s global efficiency (Andalib et al., 2011). In this research (Fig. 6), the COD/NO3 N relation varied between 15.64 7.30 and 62.05 16.16 averages. Therefore, such values were adequate for denitrification, leading to the creation of an effluent with low nitrate concentrations in every stage studied (Table 2). Akunna et al. (1992) and Andalib et al. (2011) reported methane production without denitrification at COD/NO3 N greater than 53, with glucose as substrate in a CSTR of combined acidogenesis and methanogenesis. In Stage 2, a mean COD/NO3 N ratio equal to 62.05 16.16 was found. Even so, no reduction in the denitrifying efficiency was found (Table 2). Therefore, we may say that the great variety of compounds in the raw sewage may favor denitrification, regardless of the COD/NO3 N ratio. Another parameter that may be used to evaluate denitrification is alkalinity. According to Metcalf and Eddy Inc. (2003), during the NO3 N reduction to N2, a theoretical gain of 3.57 mg CaCO3 per
Fig. 6. COD/nitrate ratio.
Table 4 Total alkalinity on influent and effluent DAF. Stages
Influent (mg CaCO3 L
1 2 3 4 5
292 54 164 36 114 25 146 29 149 24
1
)
Effluent (mg CaCO3 L
1
)
355 42 399 47 355 24 361 31 382 37
reduced mg NO3 N occurs. In this study, an increase in the alkalinity occurred in all stages (Table 4). Since denitrification produces gaseous nitrogen, there was a concern that the formation of such gas could cause the sludge on the anaerobic filter to be dragged off. Therefore, the concentration of suspended solids (TSS) was constantly monitored at the DAF effluent. In Stages 1, 2, 3, 4, and 5, the mean concentrations were 35 24; 22 11; 18 08; 23 10 and 15 06 mg L 1, respectively. No significant difference was found between these mean values. This indicates the non-interference of denitrification for the biomass to be dragged off in the upflow-anaerobic filter. Possibly, in UASB reactors, where the biomass remains scattered, such dragging may compromise the quality of the effluent. The nitrite concentration during the entire analytical period and at all collection points never exceeded 2.2 mg L 1. Therefore, there was no accumulation of nitrogen in the reactor that could compromise denitrification. Under normal conditions for nitrification, ammonia and nitrite oxidation rates are reported to be highly coupled, both in rates as in space, preventing nitrite accumulation (Philips et al., 2002). However, wastewater treatment plants frequently fail to establish stable nitrification, which is often attributed to the slow growth of nitrifying bacteria (Philips et al., 2002). For Hongwei et al. (2009) nitrite accumulation can be caused by microbial communities with different characteristics for nitrate and nitrite reduction. Schuch et al. (2000) observed that a higher alkaline pH (>7.8) resulted in a lower denitrification rate and in an increasing nitrite concentration in the effluent. On the other hand, Cao et al. (2013) concluded that nitrite accumulation was more serious at low pH
Table 3 COD consumption during denitrification. Stage
Influent COD (mg L 1)
Theoretical COD required for denitrification (mg L 1)
Removed COD (mg O2 L 1)
% of theoretical removal of COD during denitrification
2 3 4 5
568 149 700 288 779 273 742 420
26 33 89 136
395 151 561 291 607 282 620 410
5% 6% 13% 23%
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than at high pH, regardless whether the pH of the mixed liquor was stabilized during denitrification. The major importance of the pH at such, it usually influences other parameters playing even a greater role in nitrite accumulation; among others, the balance of ammonium and ammonia in water is dependent on the pH, and the effect of free ammonia seems to be more pronounced. In this study the pH was always near neutrality (7.12 0.48). This shows that the system was able to maintain optimum conditions for denitrification throughout the analytical period. 5. Conclusions The upflow-anaerobic filter with coconut shell as packing material (C. nucifera) was efficient in reducing the organic matter and denitrification. Even with an influent consisting of 50% sewage and 50% nitrified effluent, an effluent with low nitrate concentrations, never higher than 1.5 mg L 1 was produced. The biomass in the denitrifying upflow-anaerobic filter showed quick acclimatization, even when there was an increase in the nitrate load applied. During the denitrification process, no solids were dragged off from the reactor by the gases created. This shows the robustness of upflow-anaerobic filters and their feasibility to be used as a denitrifying reactor to treat wastewater. Acknowledgments The authors would like to thank CNPq (the Brazilian National Council for Scientific and Technological Development) for the scholarships granted, in addition to FAPESP (São Paulo Research Foundation) and FINEP (Studies and Projects Financing Agency) for financing this study. The authors would also like to acknowledge the service of the Writing Space/General Coordination of UNICAMP for helping translate the original manuscript. References Achak, M., Mandi, L., Ouazzani, N., 2009. Removal of organic pollutants and nutrients from olive mill wastewater by a sand filter. J. Environ. Manage. 90, 2771–2779. Ahn, Y.T., Kang, S.T., Chae, S.R., Lee, C.Y., Bae Shin, B.U., HS, 2007. Simultaneous highstrength organic and nitrogen removal with combined anaerobic upflow bed filter and aerobic membrane bioreactor. Desalination 202, 114–121. Akunna, J.C., Bizeau, C., Moletta, R., 1992. Denitrification in anaerobic digesters: possibilities and influence of wastewater COD/NOxN ratio. Environ. Technol. 13 (9), 825–836. Akunna, J.C., Bizeau, C., Moletta, R., 1994. Nitrate reduction by anaerobic sludge using glucose at various nitrate concentrations: ammonification, denitrification and methanogenic activities. Environ. Technol. 15, 41–49.
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