Nitrogen Removal from Water Resource Recovery Facility Secondary Effluent Using a Bioreactor Wenping Cao*
ABSTRACT: Solid-phase denitrification technology can potentially be used to remove nitrogen compounds, such as total nitrogen and nitrate nitrogen (NO 3 -N), from wastewater. In this study, the authors made use of an internal-circulation baffled biofilm reactor in which filamentous bamboo acted as a biocarrier for the removal of nitrogen (N) from water resource recovery facility (WRRF) secondary effluent. A laboratory-scale experiment was conducted to assess the efficacy and mechanisms of N removal from the WRRF secondary effluent operated in continuous-flow mode. Results indicated that total nitrogen and NO 3 -N removal rates reached 66.58 to 75.23% and 75.6 to 85.6%, respectively. Infrared spectrum analysis indicated biodegradation in the filamentous bamboo. A comparison of this method with the use of filamentous plastics as biocarriers indicated that higher NO 3 -N removal (as volumetric loading) and lower nitrite nitrogen accumulation rates were obtained when filamentous bamboo was used as a biocarrier. A NO 3 -N removal volumetric loading of 2.09 mg/Lh was reached when using bamboo as a single solid carbon source. These results confirm that filamentous bamboo can be used as an alternative to inert biocarriers in WRRF secondary effluent treatment systems. Water Environ. Res., 88, 223 (2016). KEYWORDS: filamentous bamboo, solid phase denitrification, NO 2N accumulation, carbon source, nitrogen removal, functional groups. doi:10.2175/106143016X14504669767652
Introduction The rapid development of industry and urbanization and population growth in China have resulted in the development of increasingly serious water shortages and water pollution (Qian et al., 2007; Wang and Lan, 2011). High concentrations of total nitrogen in the effluents from urban secondary water resource recovery facilities (WRRFs) often present significant problems and the deficiency of advanced wastewater treatment systems for WRRF secondary effluents represents an important source of surface water pollution, particularly in the case of secondary effluents, that do not comply with health safety standards (Wang and Lan, 2011). To ameliorate the level of eutrophication in surface waters, appropriate wastewater treatment methods and technologies have been developed and applied in China (Qian et al., 2007). In general, WRRF secondary effluents are further treated in tertiary plants for the purpose of reuse. Various methods including membrane processes (Wang et al., 2008), advanced oxidation processes (Li et al., 2007), and adsorption and filtration (Wang et al., 2008) are used for purification or post-treatment (Safari et al., 2013; Seven et al., 2004; Yin and Ding, 2009). Biological technologies have many advantages in terms of being cost*
School of Environmental Engineering of Xuzhou Institute of Technology, Xuzhou 221111, China; e-mail:
[email protected] March 2016
effective and environmentally friendly while avoiding the production of secondary pollution. Biological technologies include activated sludge and biofilm processes. The latter methods have certain advantages, such as a limitation on excess sludge and lower administration costs, compared to the activated sludge processes. For these reasons, considerable research has focused on biofilm processes in China, which has resulted in the development of some novel bioreactors and biocarriers (Cao, 2012). Water resource recovery facility secondary effluents are characterized by high concentrations of total nitrogen and dissolved oxygen, low concentrations of organic matter, and low carbon-to-nitrogen ratios. Because of a paucity of suitable biodegradable organic carbon sources for denitrifying bacteria, the additional removal of total nitrogen, particularly the removal of nitrate nitrogen (NO 3 -N), from these effluents is difficult to achieve by means of traditional wastewater treatment methods (Wen et al., 2011; Zhao et al., 2013). Although advanced purification processes have been developed such as biological aerated filtration, biological contact oxidation, methods associated with fluidized bed bioreactors (FBRS) (Safari et al., 2013), natural treatment systems (Taebi and Droste, 2008; Zhao et al., 2013), and algal-based immobilization processes (He and Xue, 2010), none of these can effectively overcome the problems associated with an insufficient carbon source, as is required for total nitrogen removal. Recent attention has focused on the use of solid-phase biodegradable materials as biocarriers and carbon sources for the removal of NO 3 -N by heterotrophs from groundwater and drinking water (Aslan and T¨urkman, 2003; Boley et al., 2003; Ovez, 2006; Shen et al., 2013; Shen and Wang, 2011). Compared to solid-phase biodegradable materials, the disadvantages associated with liquor carbon sources relate to the need for a sophisticated process control, which is necessary to avoid overdosing risks, with a resultant deterioration of effluent water quality (Shen et al., 2013). To avoid these problems, solid carbon sources (which would have the capacity to replenish the carbon substrate base on which biodenitrification depends) are typically used as alternatives when carrying out denitrification procedures (Alvarez et al., 2007; Wang and Wang, 2009). These biofilm carrier systems are referred to solid-phase denitrification (SPD) systems. Various solids such as newspaper, unprocessed cotton fiber (Shen et al., 2013), the bark of various trees (Aslan and T¨urkman, 2003), hornbeam, pine shavings, sugar and sugar cane, water-insoluble biodegradable polymers, and synthetic polyester granules (Boley et al., 2003) have been evaluated as potential solid carbon sources. Previous studies have indicated certain disadvantages associated with these such as poor mechanical strength (in the case of wheat straw) (Shen and Wang, 2011), toxicity to microorganisms (Alvarez et al., 2007), or high costs (Aslan and T¨urkman, 2003; Shen et al., 2013; Alvarez et al., 2007). 223
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Figure 1—Photographs of filamentous bamboo (a) and filamentous bamboo with steady-state biofilm (b). The focus of most previous studies has been on the removal of NO 3 -N from groundwater and drinking water, in which solid-phase biodegradable materials were used as biocarriers and as a carbon source. There are, however, few reports on the removal of total nitrogen and NO 3 -N from WRRF secondary effluent using solid-phase biodegradable materials as biocarriers and as a carbon source. Compared to waterinsoluble biodegradable polymers and synthetic polyester granules, bamboo, which has high organic carbon content and is cost-effective, can potentially be used by denitrifying bacteria as an electron donor. The objectives of this study are as follows:
To assess the removal efficiency of nitrogen (N) pollutants from WRRF secondary effluent in terms of efficacy and performance, particularly NO 3 -N removal, when bamboo is used as a biocarrier and the functional groups on the surface of the bamboo change during the course of the experiments; To compare the advantages of using filamentous bamboo (a biodegradable material) or filamentous plastics (an inert
material) as biocarriers in terms of NO 3 -N removal efficiency; and To determine the characteristics of NO 3 -N removal performance when using filamentous bamboo as a single carbon source and as a biocarrier.
Materials and Methods Filamentous Bamboo. Samples of filamentous bamboo were obtained by cutting 20-mm 3 5-mm 3 1-mm pieces. The physical characteristics of filamentous bamboo (self-measurement) were as follows: porosity, 80.4%; specific surface area, 118.1 m2/m3 (Autosorb iQ; Quantachrome Corporation, Boynton Beach, Florida); and bulk density, 1.1 kg/L. Photographs of filamentous bamboo (Figure 1a) and filamentous bamboo with steady-state biofilm (Figure 1b) are illustrated in Figure 1. Bioreactor and Biofilm Formation. An internal-circulation baffled biofilm reactor (ICBBR) was used for treating WRRF secondary effluent, as shown in Figure 2. The biofilm reactor, which had a total liquid volume of 40 L, was comprised of the following components:
Figure 2—Schematic diagram of the internal circulation of an ICBBR. 224
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— Figure 3—Removal efficiency of total nitrogen, NHþ 4 -N, NO3 -N, and NO2 N.
Upflow and downflow sections (separated by a segregation board) of 15 L and The top and bottom sections of the reactor comprising an upper settling section of 7 L and a lower dilution section of 3 L for the inflow of air. Filamentous bamboo was placed in both the upflow and downflow sections. Raw water obtained from the WRRF secondary effluent (Yangzhou, China) was poured into a tank, from which it was pumped into the dilution section of the reactor. The flowrate was regulated using a peristaltic pump and the column was operated in an internal circulation mode. In addition, air was supplied to the bottom section of the reactor. The dissolved oxygen concentration in the top section of the ICBBR was 3.5 to 4.5 mg/L. The experimental study was conducted at a water temperature of 19.0 6 3 8C.
Seed sludge was collected from a WRRF and fed into the reactor together with 2552 to 3621 mg/L of mixed liquor suspended solids (MLSS). This culture was maintained until a steady-state biomass loading on the filamentous bamboo was March 2016
achieved. Microorganisms that did not adhere to filamentous bamboo were discarded. Assessing the Treatment of Wastewater Plant Secondary Effluent. The reactor in which a steady-state biofilm was placed over the filamentous bamboo was fed with secondary effluent (from the wastewater plant) with the following characteristics: chemical oxygen demand (CODcr) (with K2Cr2O7 as oxidizer), 111 to 145 mg/L; 5-day biochemical oxygen demand, 18.7 to 36.2 mg/L; ammonia nitrogen (NHþ 4 -N), 11.4 to 13.6 mg/L; NO 3 -N, 6.7 to 8.2 mg/L; nitrite nitrogen (NO2 -N), 0.67 to 1.09 mg/L; total nitrogen, 22.4 to 25.3 mg/L; suspended solids, 72.5 to 144.5 mg/L; and pH, 7.49 to 7.96. The hydraulic retention time of the continuous flow reactor was 5 hours. The experiment aimed to monitor the concentration variations of N pollutants — — including total nitrogen, NHþ 4 -N, NO3 N, and NO2 N in the biofilm reactor and to determine the mechanism of N removal, particularly NO 3 -N. Performance when Using Bamboo as a Carbon Source and a Biocarrier for NO 3 -N Removal. Three 250-mL Erlenmeyer flasks were used as a batch reactor for denitrification. Flasks 225
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Figure 4—The CODcr removal efficiency. were sealed with rubber plugs to maintain anoxic conditions. The gas generated in reactors was discharged through exhaust pipes installed in rubber plugs. Argon striping was used to maintain dissolved oxygen concentrations below 1.5 mg/L before experiment startup and during sampling. Continuous acclimation resulted in the formation of stable biofilms of denitrifying bacteria on the filamentous bamboo, which stabilized the rate of denitrification. Seed sludge, obtained from a local municipal WRRF (with a concentration of 4200 mg/L MLSS), was domesticated with glucose as single carbon sourcefor the purpose of cultivating denitrifying bacteria. System A comprised 10 g of filamentous bamboo, 150 mL of synthetic wastewater, and 100 mL of domesticated sludge. System B comprised an inert biocarrier (filamentous plastics), 150 mL of synthetic wastewater, and 100 mL of domesticated sludge. The aim of the experimental step was to compare denitrification characteristics in two different systems. The batch experiment setup was as follows: initial NO 3 -N concentration, 100 mg/L; pH, 7.00; and influent volume, 240 mL. The synthetic wastewater used for the batch experiment contained about 100 mg/L of NO 3 -N and 20 -P) because of the addition of mg/L of phosphorus-P (PO3 4 monarkite (NaNO3) and monopotassium phosphate (KH2PO4). To further investigate the denitrifNO 3 ication characteristics, the batch experiment was operated under the following conditions when using filamentous bamboo as a single carbon source and a biocarrier: initial NO 3 -N concentration, 60 mg/L; pH, 7.00; and influent volume, 250 mL. Analytical Methods. Samples were collected at regular intervals and tested within 2 hours of collection. All data generated in the study were obtained from three replicate trials. Samples were filtered through a 0.45-lm membrane before analysis; the N content (including NHþ 4 -N, -N, and NO2 -N) was determined using an ion chromatograph analyzer (model PIC-10A; Puren 226
Instrument Co., Ltd., Qingdao, China); total nitrogen, CODcr, and suspended solids were assayed according to China State Environmental Protection Administration (SEPA) Standard Methods (SEPA, 2002); and the pH, dissolved oxygen, and water temperature values were monitored with a pH and oxygen meter (model Oxi300i; WTW GmbH, Oberbayem, Germany). Results Nitrogen Removal Efficacy. Figure 3 shows that the initial total nitrogen concentration range was 22.4 to 25.3 mg/L, the final total nitrogen concentration range was 5.87 to 7.95 mg/L, and the final corresponding total nitrogen removal rate range was 66.58 to 75.23%, with a mean value of 68.85%. The ranges of initial NHþ 4 -N, NO3 -N, and NO2 -N concentrations were 11.4 to 13.6 mg/L, 6.7 to 8.2 mg/L, and 0.67 to 1.09 mg/L, respectively. These resulted in final concentration ranges of 4.23 to 5.91 mg/L, 1.14 to 1.76 mg/L, and 0.09 to 0.13 mg/L, — respectively. The removal rates of NHþ 4 -N, NO3 N, and and þ NO2 -N were as follows: NH4 -N range of 50.96 to 66.43%, mean value of 58.29%;NO 3 -N range of 75.6 to 85.6%, mean value of -N range of 85.30 to 87.79%, mean value of 80.61%; and NO 2 86.43%. The initial total nitrogen and NHþ 4 -N did not meet the standard for Class-I(A) in terms of the emission standards of pollutants in urban WRRFs (GB18919-2002), but the final total nitrogen and NHþ 4 -N concentrations were much lower than those of the maximum contaminant level (15 mg/L and 8 mg/L, respectively) for Class-I(A) of the emission standard of pollutants in an urban WRRF (GB18919-2002). The final NO 3 -N concentration was much lower than the maximum allowable contaminant content (10 mg/L) set by the Standards for Drinking Water (GB5749-2006) of China. Removal Efficacy of Other Pollutants. As shown in Figure 4, the initial and final CODcr concentration ranges were 111 to 145 Water Environment Research, Volume 88, Number 3
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Figure 5—The changes in functional groups on the filamentous bamboo during the experiments, which provide an indication of the biodegradable performances of filamentous bamboo and its suitability as a solid-phase carbon source. The upper figure indicates functional groups on the raw filamentous bamboo and the bottom figure indicates functional groups on the filamentous bamboo after it had been used for 2 months. mg/L and 14 to 29 mg/L, respectively, while the initial and final average values were 125 and 20.3 mg/L, respectively. The corresponding removal rate of CODcr was in the range of 78.95 to 88.89% (mean value of 83.94%). The initial CODcr concentration did not comply with the emission standards of pollutants in an urban WRRF (GB18919-2002) and the average dissolved oxygen content increased from 3.85 to 7.78 mg/L. The advanced treatment for wastewater plant secondary effluent resulted in an March 2016
effluent with lower CODcr concentration and a higher dissolved oxygen being discharged into the receiving waterbodies, indicating improved water quality, which will contribute toward a healthier ecosystem and improved self-purification of water. In addition, the concentration of suspended solids had decreased from an initial range of 72.5 to 144.5 mg/L to less than 20 mg/L. Infrared Spectrum Analysis. Figure 5 shows changes in functional groups on the filamentous bamboo during the 227
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Figure 6—The effect of biocarrier species on NO 3 -N removal, in which System A includes 10 g of filamentous bamboo, 150 mL of synthetic wastewater, and 100 mL of domesticated sludge and System B includes an inert biocarrier (filamentous plastic filling), 150 mL of synthetic wastewater, and 100 mL of domesticated sludge. experiments, providing an indication of the biodegradable performances of filamentous bamboo and its suitability as a solid-phase carbon source. The upper figure indicates functional groups on the raw filamentous bamboo, and the bottom figure indicates functional groups on the filamentous bamboo after it had been used for 2 months Compared to the raw filamentous bamboo, the changed performance of the functional groups on filamentous bamboo after it had been used for 2 months was as follows: the intensity of the main functional groups [–OH (~3400/cm, ~1049/cm)], [–CH2 (~2920/cm)], and [–NH2 (~1650/cm)] on the filamentous bamboo was significantly reduced because of the biodegradation of cellulose, fat, protein, and polysaccharide in the filamentous bamboo tissue. Comparison of NO 3 -N Removal Performance Using Different Materials as Biocarriers. Results illustrated in Figure 6 indicate the effect of the carbon source on NO 3 -N removal efficiency. Compared to System B, the NO -N decreased rapidly 3 in System A, in which filamentous bamboo played an important role in the removal of NO 3 -N. According to Figure 6, the reduction of NO 3 -N reached zero within 3 days in System A because filamentous bamboo was used as a carbon source and a continuous electron donor was available. The NO 3 -N removal efficiency was, however, unfavorable in System B because no continuous carbon source for denitrification was available, although a small amount of carbon was supplied from the hydrolysis products of surplus sludge. In addition, the NO 2 -N concentrations in System A and System B were lower than 0.16 and 0.86 mg/L, respectively. Relevant statistics indicate that the weights of accumulated NO 2 -N were 1.40 and 6.52 mg, respectively, for each gram of NO -N that had 3 been removed. 228
System A had a significantly higher NO 3 -N removal efficiency and a much lower accumulation of NO2—N compared to the values for these parameters in System B. Filamentous bamboo was, therefore, chosen as the most effective carbon source for the following experiments that we carried out. Denitrification Characteristics of Bamboo when Used as a Single Solid Carbon Source. An assessment of the denitrification characteristics of filamentous bamboo as a single carbon source was carried out at 30 8C and 100 rpm. Results illustrating the variations in NO 3 -N concentration in the bamboo system are provided in Figure 7. Figure 7 presents NO 3 -N removal efficacy using bamboo as single solid carbon source. The NO 3 -N volumetric loading range was 1.41 to 5.00 mg/Lh, with a mean value of 2.09 mg/Lh in the bamboo system. Unlike many other inert biocarriers, filamentous bamboo can be decomposed during water treatment, which results in the formation of a thicker biofilm on the bamboo (Boley et al., 2003). This provides an anaerobic environment, which facilitates the conversion of NO 3 -N into N2. Moreover, bacteria on the surface of filamentous bamboo facilitates decomposition into soluble matter, which is used for NO 3 -N removal. The reduction of each gram of NO3 -N resulted in an accumulation of 0.23 mg NO -N. 2 Conclusions Filamentous bamboo can be decomposed during water treatment, resulting in a thicker biofilm on the filamentous bamboo. The thicker biofilm provides anaerobic conditions that facilitate denitrification and organic matter biodegradation (Cao et al., 2012; Peng et al., 2008). Filamentous bamboo can, therefore, be regarded as a natural biocarrier that, because of its greater bioaffinity and lower biotoxicity, results in the presence of a greater biomass of nitrifying bacteria on Water Environment Research, Volume 88, Number 3
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Figure 7—The NO 3 -N removal efficacy using bamboo as a single solid carbon source. its surface. This results in a substantial decrease in the concentration of NHþ 4 -N. In the presence of total nitrogen and NO -N, filamentous bamboo was broken down into 3 soluble matter by surface bacteria and used for denitrification, resulting in a thicker biofilm that provided anaerobic conditions that facilitated the denitrification process. A significantly greater decrease in the levels of total nitrogen and NO 3 -N was noted on filamentous bamboo with its associated biofilm carriers compared to results obtained when using other inert materials. As a method for the advanced treatment of WRRF secondary effluent, the bioreactor with filamentous bamboo as a biofilm carrier and a carbon source provided an effective method for the removal of NHþ 4 -N, total nitrogen, NO2 -N, NO3 -N, CODcr, and suspended solids. This suggests that nitrogen removal from WRRF secondary effluent using biofilms on filamentous bamboo is feasible and effective. The variation in main functional groups on the filamentous bamboo indicated that this substrate can be biodegraded by microorganisms attached to its surface. A higher removal of NO 3 -N, together with a lower accumulation of NO 2 -N, was obtained than that in the nonsolid-phase carbon source system, resulting in NO 3 -N volumetric load removal reaching 2.09 mg/Lh when filamentous bamboo was used as a single carbon source. Filamentous bamboo can, therefore, be regarded as a potential carbon source for denitrification. Acknowledgments This study was supported by the State Key Laboratory of Pollution Control and Resource Reuse Foundation (No. PCRRF13021), Six Talent Summit of Jiangsu Province (No. JNHB-005), Qinglan Project of Jiangsu Province, Youth Fund of Xuzhou Institute of Technology (XKY2011213), and the National Spark Program of China (2013GA690421). March 2016
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