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Comparative study of nitrogen removal and bio-film clogging for three filter media packing strategies in vertical flow constructed wetlands
Ecological Engineering 74 (2015) 1–7
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Ecological Engineering journal homepage: www.elsevier.com/locate/ecoleng
Comparative study of nitrogen removal and bio-film clogging for three filter media packing strategies in vertical flow constructed wetlands Xinshan Song a , Yi Ding a,b , Yuhui Wang a,∗ , Wei Wang a , Gang Wang b , Bin Zhou a a College of Environmental Science and Engineering, State Environmental Protection Engineering Center for Pollution Treatment and Control in Textile, Donghua University, Shanghai 201620, China b Beijing Academy of Environmental Sciences, Beijing 100037, China
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Article history: Received 4 March 2014 Received in revised form 18 July 2014 Accepted 8 August 2014 Keywords: Vertical flow constructed wetland Filter packing strategy Nitrogen removal Bio-film accumulation Clogging
a b s t r a c t This study investigated the discrepancies in nitrogen removal and clogging for vertical flow constructed wetlands (VFCWs) using different packing strategies. Parallel lab-scale VFCWs reactors were created and packed with quartz sand filter media. Three packing strategies were adopted: increasing-sized (I-packing), decreasing-sized (D-packing) and uniform-sized (U-packing) packing. The clogging rate coefficient and biomass accumulation rate were defined to assess clogging. The results demonstrated that the nitrogen removal was highest for the I-packing reactor at 43%. The ammonia and COD removal rates reached 56% and 64%, respectively. I-packing reactor presented an aerobic-to-anoxic transition area, where DO ranged from 4.6 to 0.3 mg/L. Such condition is ideal for the nitrification and denitrification reaction. Clogging is primarily the result of the rapid reduction of the effective porosity due to bio-film growth. Clogging is related to the position of biomass accumulation, while the reduction of the internal void space is not the determining factor for the I-packing reactor. The clogging rate of the I-packing reactor much lower than D-packing and U-packing reactors. Bed resting between operations can be used for clogging mitigation and system recovery, but this approach was not effective for D-packing. The I-packing reactor was proven to be the most efficient for nitrogen removal and showed the strongest clogging prevention capability. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Nitrogen is one of the principal pollutants contributing to eutrophication by depleting dissolved oxygen (DO) (Smith et al., 1999; Dokulil et al., 2000). High levels of nitrate in drinking water are harmful to public health, which has become a great concern. However, removing nitrogen via conventional solutions (e.g. centralized wastewater treatment plant) has been recognized as inefficient. Compared with centralized treatment plants, constructed wetlands (CWs) are efficient systems that require low energy and less manipulation for on-site and small communities wastewater treatment (Vymazal et al., 1998; Wallace and Knight, 2006). In recent decades, vertical flow CWs (VFCWs) have been designed and engineered to utilize the natural processes of nitrogen removal (Cooper, 1999; Chang et al., 2012). Due to the advantages of a small footprint, low construction and relatively greater oxygen transfer capacity (Brix and Arias, 2005; Cui et al., 2012; Verhoeven
∗ Corresponding author. Tel.: +86 18817551968. E-mail address: [email protected] (Y. Wang). http://dx.doi.org/10.1016/j.ecoleng.2014.08.008 0925-8574/© 2014 Elsevier B.V. All rights reserved.
and Meuleman, 1999), VFCWs are gaining popularity (Bruch et al., 2011; Vymazal, 2005) in various wastewater treatments. The removal of nitrogen is primarily facilitated by microorganisms via the nitrification-denitrification processes. As a result, studies (Ye et al., 2012a,b; Nivala et al., 2013; Sasikala et al., 2009) indicate that the inner filter oxygen conditions, media-specific surface area, etc. are the determining factors. Thus design and operation of VFCWs could be optimized to provide sufficient oxygen for ammonia oxidation or creating anoxic condition for denitrification. Usually, the batch operation of drainage has been used for re-oxygenation. While packing with small filter media can provide anoxic conditions for denitrification. However, VFCWs can be confronted with clogging after long-term wastewater treatment (Grismer et al., 2003; Knowles et al., 2011). The accumulation of suspended solids via the adhesion of biofilms due to microorganism growth (bio-clogging) is one of the contributing factors to clogging (Hua et al., 2010; Zhao et al., 2009). Clogging is an inherent and progressive process, and some degree of clogging is inevitable (De la Varga et al., 2013; Nivala et al., 2012). The degree of clogging is highly related to the media structure and organic load operation. Organic load largely determines the microbial growth
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Table 1 Influent water quality of low and high organic load (mean ± SD, n = 6), mg/L. Operations
NH4 + –N
NO3 − –N
TN
COD
DO
pH
Low organic load 1 High organic load Low organic load 2
21.79 ± 2.32 36.87 ± 5.39 23.58 ± 3.20
3.55 ± 1.98 3.72 ± 1.05 3.02 ± 1.21
26.49 ± 3.21 52.58 ± 7.21 29.64 ± 4.69
111.7 ± 13.8 235.8 ± 23.2 112.9 ± 20.4
3.34 ± 1.22 3.1 ± 1.25 3.38 ± 0.98
7.6 ± 0.1 7.4 ± 0.2 7.6 ± 0.2
Table 2 Mean pollutant removal performances of the wetland reactors (mean ± SD, n = 6) concerning different packing strategies and organic loads, mg/L. Notice: “–”means the removal rate is negative. Influent wastewater characteristics were listed in Table 1. Operation
Parameter
I-packing reactors Effluent
Low organic load 1
High organic load
Low organic load 2
NH4 –N NO3 –N TN COD DO NH4 –N NO3 –N TN COD DO NH4 –N NO3 –N TN COD DO
11.08 4.06 15.22 50.64 0.27 26.45 5.58 40.49 128.6 0.28 12.75 4.79 15.83 54.89 0.29
± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
3.21 2.08 4.82 12.34 0.15 9.56 4.53 8.95 32.7 0.13 6.21 3.55 3.95 15.6 0.16
D-packing reactors Mean removal (%)
Effluent
49 – 43 55 / 28 – 23 46 / 46 – 47 51 /
9.58 9.35 20.53 45.1 0.43 22.49 4.46 27.12 121.1 0.41 10.95 10.87 22.05 51.43 0.41
and accumulation on the substrate surface (Tanner and Sukias, 1995a), while the bio-film results in large morphological differences (Kim et al., 2010) due to the heterogeneity of porous media. Hence, the variation in the bio-film accumulation in VFCWs needs to be determined with regard to clogging. Clogging can be remedied by improving the design and operation and is very sensitive to the media size, distribution and structure. Filter media grain size is known negatively correlates with the specific surface area for bio-film establishment, causing rapid clogging (Wallace and Knight, 2006). To date, media structure design has not been investigated for their difference in treatment performance and clogging effect (Langergraber et al., 2003; Nguyen, 2000; Wu et al., 2011; Zhao et al., 2004). A comparative study of the removal efficiency and clogging of VFCWs using different media packing strategies is needed. An anti-clogging filter media packaging strategy that efficiently removes nitrogen needs to be developed to improve the nitrogen removal efficiency and reduce the filter media clogging in VFCWs. This experiment was designed to compare the nitrogen removal and clogging for three media packing strategies under sequential operation of low high organic loads. The objectives of this study were as follows: (i) to test the overall and interlaminar efficiency of the system performances in terms of nitrogen removal; (ii) to examine the treatment stability with sequential low and high organic load operation; and (iii) to evaluate the ability of the system for clogging mitigation using different packing strategies. Thus, the outcomes will provide a useful reference for the anti-clogging media packing strategy in VFCWs design. 2. Materials and methods 2.1. VFCWs reactors Six identical VFCWs reactors are manufactured and divided into two parallel groups. The reactors were placed on the campus of Donghua University, Shanghai. A reactor sketch and experimental setups are shown in Fig. 1. The filter column of each reactor was
± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
4.45 3.32 5.23 10.82 0.35 3.55 3.98 5.82 26.4 0.31 2.55 7.67 4.43 12.6 0.24
U-packing reactors Mean removal (%)
Effluent
56 – 22 60 / 39 – 48 49 / 54 – 26 54 /
10.01 6.40 17.78 57.0 0.35 24.43 3.55 28.34 144.8 0.55 12.99 7.28 20.73 61.97 0.36
± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
Mean removal (%) 5.43 4.25 4.57 14.8 0.17 5.93 2.37 5.25 37.6 0.43 4.69 4.85 3.05 20.1 0.23
54 – 33 49 / 34 – 46 39 / 45 – 30 45 /
Fig. 1. Section sketch of VFCWs reactor group with different media packing strategies. Notice: (A) increasing-sized packing (I-packing); (B) decreasing-sized packing (D-packing); (C) uniform-sized packing (U-packing).
manufactured from opaque PVC pipes that were 40 cm in radius and 90 cm in height. The filter column is vertically divided into four layers. Height of each layer is 20 cm. Quartz sand with stable chemical and neutral properties was employed as the filter medium. Four particle sizes were employed: an extra-large size of 8–9 mm(ϕ), a large size of 6–7 mm(ϕ), a medium size of 5–6 mm(ϕ) and a small size of 3–4 mm(ϕ). Three filter packing strategies were adopted: (a) increasing-sized packing (I-packing): the filter media was packed in a vertically increasing particle size manner from top to bottom; (b) decreasing-sized packing (D-packing): the opposite of I-packing; (c) uniform-sized packing (U-packing): the filter media packing material was of medium size (ϕ = 5–6 mm). For each layer, a PVC perforated pipe (ϕ = 2.0 cm) was installed to the center of the column for sample collection. Before experiment, the reactors were
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Fig. 2. Vertical DO distribution in VFCWs for different packing strategies.
shaken several times for filter media compactation. Thus compactation during experimental period is very low and can be ignored (Suliman et al., 2007). An influent distribution device was installed on the top of the reactor. A peristaltic pump (BT600-2J) was used to ensure steady inflow. A harvesting device was placed at the bottom. C. indica was planted. Each packing strategy for each VFCWs reactor was duplicated twice. 2.2. Experimental influent Artificial wastewater was prepared and stored in a feed tank. The wastewater was pumped into the reactor through a distributer. Sequential batch operations were carried out via intermittent influent operation in order of low-high-low organic load. Each operation lasted 60 days. The water depth was maintained equal to the level of filter media surface. Hydraulic retention time (HRT) of 10 hr and drainage time of 2 hr were set. A bed resting time of 7 days was used to recover the system between the two operation periods. The column was recovered by flushing with tap water once every two days and air-dried for re-oxygenation. Table 1 lists the quality of influents in batch operations. The whole experiments were completed during a 9-month period at greenhouse conditions, for which the temperature ranged from 19 to 23 ◦ C. The reactors were stabilized for two weeks prior to starting their operation. 2.3. Water quality analysis Samples collected from influent, effluent and porous water in each layer were analyzed immediately in the lab. Ammonia (NH4 –N), nitrate (NO3 –N) and total nitrogen (TN) concentrations were measured using a gas-phase absorption spectrometer (GMA3230, Beiyu Co. Ltd., China). Chemical oxygen demand (COD) was tested using a digital reactor block (HACH, DRB200, USA). Dissolved oxygen (DO) content was measured simultaneously using a DO meter (HI9143, NANNA, Italy). Significance of removal efficiency for different packing strategies are checked at a probability level of p < 0.05 using SPSS 13.0 software. 2.4. Determination of bio-film clogging The biomass accumulation rate (BAR) in each layer and total BAR (TBAR) of the filter column were used to assess the biofilm growth. BAR is defined as the decrease in the media water storage volume over time due to bio-film accumulation, written as BAR = (V0 × i − Vt × i )/V0 , where V0 × i is the initial void in layer
Fig. 3. Interlaminar efficiency of system performances during the second low organic loadings.
i evaluated according to effluent wastewater volume. Vt × i refers to the water storage volume after t days of operation. The measurement can avoid destructive sampling as opposed to the biomass loss on ignition (LOI) method (Tanner and Sukias, 1995b; Ye et al., 2012a,b). To evaluate clogging, the clogging rate coefficient (CRC) was defined as a function of the change in the effluent flow rate in the following form: CRC = (Tt − T0 )/T0 , where T0 , s/L and Tt , s/L are the time cost of per liter of wastewater drainage from the reactor before and after t days of operation. If the clogging happened, CRC value would increase. The CRC and BAR were synchronously tested during each operation period. Evaporation and transpiration of plant were ignored in this experiment.
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Fig. 4. BAR in each layer for sequential low, high and low organic load operation, each operation took 12 weeks.
3. Results and discussion 3.1. Nitrogen and organic matter removal Table 2 summarizes the determined mean pollutant removal efficiency for VFCWs reactors with different packing strategies. Generally, nitrification and COD degradation were more effective in D-packing reactors than I-packing and U-packing reactors. The nitrogen removal rate generally was not as good as COD for all packing strategies (p < 0.05). Significant differences in the pollutant removal rate were detected among packing strategies (p < 0.01). For low organic load operation, the D-packing reactor showed an efficient ammonia oxidation rate of 56%, reaching a concentration of 9.58 NH4 –N mg/L in the effluent. The increased oxygen transport capacity in the upper layer due to the larger media size improved the aerobic conditions for nitrification. Ammonia can more easily converted to nitrate by nitrifier. This aerobic condition contributes to the nitrification in D-packing reactors by stimulating nitrate accumulation. However, the I-packing reactor enhances denitrification in the bottom layer. A lower effluent nitrate concentration and larger TN removal rate (43%) demonstrate that denitrification in the I-packing reactor is more efficient than in the D-packing and Upacking reactors (p < 0.01). Denitrification is an anoxic process that is sensitive to the DO level (Bachand and Horne, 1999; Chung et al., 2008; Matheson and Sukias, 2010). Accordingly, the I-packing reactor could enhance the TN removal rates in a single filter column
via the concurrent aerobic or anoxic/anaerobic conditions. This enhancement can be verified via the obvious downward trend of the oxygen distribution for I-packing reactors (Fig. 2). The packing strategy also positively influences the COD removal. The COD content (117.7 mg/L) decreased sharply to 45.1 mg/L for the D-packing reactor, while the COD removal rates were 55% and 49% for the I-packing and U-packing reactors, respectively. Ammonia removal is generally below 40% for high organic load operation. Instead, the D-packing reactor obviously differed from this standard because the TN removal rate increased approximately 10% compared to operation at a low organic load. This difference might be caused by the significant DO consumption for high organic loads, which enlarges the anoxic areas and thus enhances denitrification when sufficient carbon is available (Wang et al., 2013; Ding et al., 2012). 3.2. Vertical dissolved oxygen distribution The DO changes in each layer were measured after the reaction. The DO is primarily affected by the porous transfer efficiency. The curves of the vertical DO level changes are illustrated in Fig. 2. The DO shows a decreasing trend for all columns from top to bottom. The DO in the D-packing and I-packing reactors was more variable than in the U-packing reactor (p < 0.01). The rate of DO decrease reached 0.71 mg/L/10 cm for the I-packing reactor and 0.78 mg/L/10 cm for the D-packing reactor, which was higher than the value of 0.57 mg/L/10 cm for the U-packing reactor. The DO
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while the I-packing and U-packing reactors provided relative higher DO levels in all layers.
3.3. Nitrification and organics degradation The removal efficiency varied in the filter column. The nitrification and degradation of organics in different layers are presented in Fig. 3. The ammonia oxidation was highest in the upper layer of the I-packing reactor. The ammonia and COD removal rates in top layer of the I-packing reactor reached 56% and 64%, respectively. Ammonia removal gradually decreased to 27% with depth, accounting for 47% of the total in the top layer. The COD removal followed a trend identical to that of ammonia. The D-packing reactor was less effective for nitrification. The ammonia removal rate was 44% on average, and highest COD removal was achieved in the bottom layer. Despite high re-oxygenation in the D-packing reactor, the small specific surface area of the media in top layers reduced the interaction between nutrients and the bio-film due to the large particle size (Aim et al., 1997). As suggested, NH4 –N was adsorbed onto filter media and organic matter during loading and nitrified mainly between the feeding periods (McNevin et al., 1999). The rapid consumption of available N in the filter can only occur when the surrounding bio-film-stored oxygen is sufficient (Tanner et al., 1999). Furthermore, the study indicated that the growth of the ammonium oxidizer is faster than that of the nitrite oxidizer at comparatively lower DO levels, leading to the accumulation of nitrite resulting from partial nitrification (Zhang et al., 2011). The ammonia and COD removal did not significantly change in the U-packing reactor compared to the I-packing reactor, significant differences were not detected among layers (p > 0.05). The ammonia and organic removal were generally not as effective as in the I-packing reactor.
3.4. Bio-film accumulation
Fig. 5. Clogging rate coefficient (CRC) and total biomass accumulation rate (TBAC) for sequential low, high and low organic load operation. 7-day bed resting between two operations is marked using “↓”.
ranged from 4.6 to 0.3 and 6.9 to 2.2 for the I-packing and D-packing reactors, respectively. The DO concentration of the I-packing reactor was lower in the bottom layer, which indicated ideal anoxic conditions for denitrification. A DO level above 2.0 mg/L has been accepted as essential for nitrification and should be controlled below 0.5 mg/L for denitrification (Ouellet-Plamondon et al., 2006). In the upper layers, the DO is mainly consumed by the degradation of organic pollutant and nitrification. Atmospheric re-oxygenation is the predominant source for VFCWs. Over 99.8% of DO is used for the degradation and nitrification of organics (Ye et al., 2012a,b). Larger filters in the upper layer are advantageous for atmospheric oxygen transfer. But the root oxygen transfer is not significant. The I-packing reactor showed an aerobic-to-anoxic transition that was beneficial for both the nitrification and denitrification processes,
Because the reactors were alternately operated with low and high organic loads, they were allowed to rest for seven days between operations. Changes in the BAR in each layer were measured (Fig. 4). The BAR logarithmically increased during each operation. The bio-film accumulation in the column obviously differed (p < 0.01). In the bottom layer of the D-packing reactor, the BAR increased to 17% after a 60-days of low organic load operation. The biomass accumulation was 15% higher than in the I-packing and U-packing reactors. This difference indicates a rapid reduction in the effective porosity due to biomass growth. This increase also occurs in the first layer in the I-packing reactor probably in the form of a nitrifier. However, the DO in the upper layer of the I-packing reactor remains at a relatively high level despite bio-film accumulation. This finding indicates that atmospheric re-oxygenation still significantly to nitrification by serving as a major DO source. Moreover, the large quantity of biomass attached on the small particles strongly correlated with nitrification and the degradation of organics (R2 = 0.87). In other layers, the BAR gradually increased during the entire operation. However, the BAR in the upper layer for the I-packing and U-packing reactors increased to a maximum of 25% and 16%, respectively, for high organic loads. The bio-film accumulation was highest in the bottom layer of the D-packing reactor at 23%. The high organic load clearly induced rapid bio-film growth for the entire column. Bed resting appeared most effective in the middle layer. The clogging mitigation positively correlated with the depth for I-packing reactors and negatively correlated with the depth for D-packing reactors.
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3.5. Clogging and recovery Changes in the CRC and TBAR for sequential operations are shown in Fig. 5. The biomass growth can be reduced via bed resting. The TBAR of each reactor dropped during bed resting. The TBAR of the I-packing reactor increased faster than that of the D-packing reactor compared to the value of the CRC. The degree of clogging in the I-packing reactor did not markedly increased after bed resting (p > 0.01). The final CRC of the I-packing reactor increased 12%. Although the TBAR showed a slow increase after high organic load, the CRC of the D-packing and U-packing reactors rapidly increased to 35% and 23%, respectively, which caused serious clogging. Clogging appeared to increase 10% in the D-packing reactor after 28 days of low organic load operation, nearly 35 days earlier than the I-packing reactor. These data suggest that the entrapment and accumulation of sloughed biofilms rather than the local growth of biofilms accelerates the clogging of the media. The general trend of CRC indicated that high organic load operation lead to lower hydraulic conductivities. Moreover, bed resting is not effective for clogging mitigation in D-packing reactors because a small filter is used near the harvesting device. The D-packing reactor appeared to be less prone to bio-film washout through the small particles in the bottom layer. On the contrary, I-packing reactor can be recovered more easily. Thus, proper operation and packing design are essential to slow down bio-film accumulation.
4. Conclusions In this study, the evaluation of three different filter media packing strategies was used to assess the efficiency of nitrogen removal and clogging characteristics. I-packing reactors present an aerobicto-anoxic transition area for efficient TN removal. The D-packing and U-packing reactors do not appear to be efficient for both nitrification and denitrification. The CRC and BAR were evaluated. The formation of clogs is a function of the position of biomass accumulation. A dense biomass layer formed at the bottom layer of the D-packing reactor to result in severe clogging. The I-packing reactor can bear organic impact load and is suitable for long-term wastewater treatment. If properly operated, this type of filter packing structure offers several advantages, such as reduced bio-film accumulation and strong clogging mitigation potential after bed resting.
Acknowledgments The authors would like to thank the following entities for their support of this study: (1). National Natural Science Foundation of China (Fund key. 51309053); (2). Public Welfare Project of Ministry of Environmental Protection, China (Fund key. 2013467042); (3). Fundamental Research Funds for the Central Universities-DHU Distinguished Young Professor Program (Fund key. B201310); and Fundamental Research Funds for the Central Universities (Fund key. 14D111313).
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Contents lists available at ScienceDirect
Ecological Engineering journal homepage: www.elsevier.com/locate/ecoleng
Comparative study of nitrogen removal and bio-film clogging for three filter media packing strategies in vertical flow constructed wetlands Xinshan Song a , Yi Ding a,b , Yuhui Wang a,∗ , Wei Wang a , Gang Wang b , Bin Zhou a a College of Environmental Science and Engineering, State Environmental Protection Engineering Center for Pollution Treatment and Control in Textile, Donghua University, Shanghai 201620, China b Beijing Academy of Environmental Sciences, Beijing 100037, China
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Article history: Received 4 March 2014 Received in revised form 18 July 2014 Accepted 8 August 2014 Keywords: Vertical flow constructed wetland Filter packing strategy Nitrogen removal Bio-film accumulation Clogging
a b s t r a c t This study investigated the discrepancies in nitrogen removal and clogging for vertical flow constructed wetlands (VFCWs) using different packing strategies. Parallel lab-scale VFCWs reactors were created and packed with quartz sand filter media. Three packing strategies were adopted: increasing-sized (I-packing), decreasing-sized (D-packing) and uniform-sized (U-packing) packing. The clogging rate coefficient and biomass accumulation rate were defined to assess clogging. The results demonstrated that the nitrogen removal was highest for the I-packing reactor at 43%. The ammonia and COD removal rates reached 56% and 64%, respectively. I-packing reactor presented an aerobic-to-anoxic transition area, where DO ranged from 4.6 to 0.3 mg/L. Such condition is ideal for the nitrification and denitrification reaction. Clogging is primarily the result of the rapid reduction of the effective porosity due to bio-film growth. Clogging is related to the position of biomass accumulation, while the reduction of the internal void space is not the determining factor for the I-packing reactor. The clogging rate of the I-packing reactor much lower than D-packing and U-packing reactors. Bed resting between operations can be used for clogging mitigation and system recovery, but this approach was not effective for D-packing. The I-packing reactor was proven to be the most efficient for nitrogen removal and showed the strongest clogging prevention capability. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Nitrogen is one of the principal pollutants contributing to eutrophication by depleting dissolved oxygen (DO) (Smith et al., 1999; Dokulil et al., 2000). High levels of nitrate in drinking water are harmful to public health, which has become a great concern. However, removing nitrogen via conventional solutions (e.g. centralized wastewater treatment plant) has been recognized as inefficient. Compared with centralized treatment plants, constructed wetlands (CWs) are efficient systems that require low energy and less manipulation for on-site and small communities wastewater treatment (Vymazal et al., 1998; Wallace and Knight, 2006). In recent decades, vertical flow CWs (VFCWs) have been designed and engineered to utilize the natural processes of nitrogen removal (Cooper, 1999; Chang et al., 2012). Due to the advantages of a small footprint, low construction and relatively greater oxygen transfer capacity (Brix and Arias, 2005; Cui et al., 2012; Verhoeven
∗ Corresponding author. Tel.: +86 18817551968. E-mail address: [email protected] (Y. Wang). http://dx.doi.org/10.1016/j.ecoleng.2014.08.008 0925-8574/© 2014 Elsevier B.V. All rights reserved.
and Meuleman, 1999), VFCWs are gaining popularity (Bruch et al., 2011; Vymazal, 2005) in various wastewater treatments. The removal of nitrogen is primarily facilitated by microorganisms via the nitrification-denitrification processes. As a result, studies (Ye et al., 2012a,b; Nivala et al., 2013; Sasikala et al., 2009) indicate that the inner filter oxygen conditions, media-specific surface area, etc. are the determining factors. Thus design and operation of VFCWs could be optimized to provide sufficient oxygen for ammonia oxidation or creating anoxic condition for denitrification. Usually, the batch operation of drainage has been used for re-oxygenation. While packing with small filter media can provide anoxic conditions for denitrification. However, VFCWs can be confronted with clogging after long-term wastewater treatment (Grismer et al., 2003; Knowles et al., 2011). The accumulation of suspended solids via the adhesion of biofilms due to microorganism growth (bio-clogging) is one of the contributing factors to clogging (Hua et al., 2010; Zhao et al., 2009). Clogging is an inherent and progressive process, and some degree of clogging is inevitable (De la Varga et al., 2013; Nivala et al., 2012). The degree of clogging is highly related to the media structure and organic load operation. Organic load largely determines the microbial growth
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Table 1 Influent water quality of low and high organic load (mean ± SD, n = 6), mg/L. Operations
NH4 + –N
NO3 − –N
TN
COD
DO
pH
Low organic load 1 High organic load Low organic load 2
21.79 ± 2.32 36.87 ± 5.39 23.58 ± 3.20
3.55 ± 1.98 3.72 ± 1.05 3.02 ± 1.21
26.49 ± 3.21 52.58 ± 7.21 29.64 ± 4.69
111.7 ± 13.8 235.8 ± 23.2 112.9 ± 20.4
3.34 ± 1.22 3.1 ± 1.25 3.38 ± 0.98
7.6 ± 0.1 7.4 ± 0.2 7.6 ± 0.2
Table 2 Mean pollutant removal performances of the wetland reactors (mean ± SD, n = 6) concerning different packing strategies and organic loads, mg/L. Notice: “–”means the removal rate is negative. Influent wastewater characteristics were listed in Table 1. Operation
Parameter
I-packing reactors Effluent
Low organic load 1
High organic load
Low organic load 2
NH4 –N NO3 –N TN COD DO NH4 –N NO3 –N TN COD DO NH4 –N NO3 –N TN COD DO
11.08 4.06 15.22 50.64 0.27 26.45 5.58 40.49 128.6 0.28 12.75 4.79 15.83 54.89 0.29
± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
3.21 2.08 4.82 12.34 0.15 9.56 4.53 8.95 32.7 0.13 6.21 3.55 3.95 15.6 0.16
D-packing reactors Mean removal (%)
Effluent
49 – 43 55 / 28 – 23 46 / 46 – 47 51 /
9.58 9.35 20.53 45.1 0.43 22.49 4.46 27.12 121.1 0.41 10.95 10.87 22.05 51.43 0.41
and accumulation on the substrate surface (Tanner and Sukias, 1995a), while the bio-film results in large morphological differences (Kim et al., 2010) due to the heterogeneity of porous media. Hence, the variation in the bio-film accumulation in VFCWs needs to be determined with regard to clogging. Clogging can be remedied by improving the design and operation and is very sensitive to the media size, distribution and structure. Filter media grain size is known negatively correlates with the specific surface area for bio-film establishment, causing rapid clogging (Wallace and Knight, 2006). To date, media structure design has not been investigated for their difference in treatment performance and clogging effect (Langergraber et al., 2003; Nguyen, 2000; Wu et al., 2011; Zhao et al., 2004). A comparative study of the removal efficiency and clogging of VFCWs using different media packing strategies is needed. An anti-clogging filter media packaging strategy that efficiently removes nitrogen needs to be developed to improve the nitrogen removal efficiency and reduce the filter media clogging in VFCWs. This experiment was designed to compare the nitrogen removal and clogging for three media packing strategies under sequential operation of low high organic loads. The objectives of this study were as follows: (i) to test the overall and interlaminar efficiency of the system performances in terms of nitrogen removal; (ii) to examine the treatment stability with sequential low and high organic load operation; and (iii) to evaluate the ability of the system for clogging mitigation using different packing strategies. Thus, the outcomes will provide a useful reference for the anti-clogging media packing strategy in VFCWs design. 2. Materials and methods 2.1. VFCWs reactors Six identical VFCWs reactors are manufactured and divided into two parallel groups. The reactors were placed on the campus of Donghua University, Shanghai. A reactor sketch and experimental setups are shown in Fig. 1. The filter column of each reactor was
± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
4.45 3.32 5.23 10.82 0.35 3.55 3.98 5.82 26.4 0.31 2.55 7.67 4.43 12.6 0.24
U-packing reactors Mean removal (%)
Effluent
56 – 22 60 / 39 – 48 49 / 54 – 26 54 /
10.01 6.40 17.78 57.0 0.35 24.43 3.55 28.34 144.8 0.55 12.99 7.28 20.73 61.97 0.36
± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
Mean removal (%) 5.43 4.25 4.57 14.8 0.17 5.93 2.37 5.25 37.6 0.43 4.69 4.85 3.05 20.1 0.23
54 – 33 49 / 34 – 46 39 / 45 – 30 45 /
Fig. 1. Section sketch of VFCWs reactor group with different media packing strategies. Notice: (A) increasing-sized packing (I-packing); (B) decreasing-sized packing (D-packing); (C) uniform-sized packing (U-packing).
manufactured from opaque PVC pipes that were 40 cm in radius and 90 cm in height. The filter column is vertically divided into four layers. Height of each layer is 20 cm. Quartz sand with stable chemical and neutral properties was employed as the filter medium. Four particle sizes were employed: an extra-large size of 8–9 mm(ϕ), a large size of 6–7 mm(ϕ), a medium size of 5–6 mm(ϕ) and a small size of 3–4 mm(ϕ). Three filter packing strategies were adopted: (a) increasing-sized packing (I-packing): the filter media was packed in a vertically increasing particle size manner from top to bottom; (b) decreasing-sized packing (D-packing): the opposite of I-packing; (c) uniform-sized packing (U-packing): the filter media packing material was of medium size (ϕ = 5–6 mm). For each layer, a PVC perforated pipe (ϕ = 2.0 cm) was installed to the center of the column for sample collection. Before experiment, the reactors were
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Fig. 2. Vertical DO distribution in VFCWs for different packing strategies.
shaken several times for filter media compactation. Thus compactation during experimental period is very low and can be ignored (Suliman et al., 2007). An influent distribution device was installed on the top of the reactor. A peristaltic pump (BT600-2J) was used to ensure steady inflow. A harvesting device was placed at the bottom. C. indica was planted. Each packing strategy for each VFCWs reactor was duplicated twice. 2.2. Experimental influent Artificial wastewater was prepared and stored in a feed tank. The wastewater was pumped into the reactor through a distributer. Sequential batch operations were carried out via intermittent influent operation in order of low-high-low organic load. Each operation lasted 60 days. The water depth was maintained equal to the level of filter media surface. Hydraulic retention time (HRT) of 10 hr and drainage time of 2 hr were set. A bed resting time of 7 days was used to recover the system between the two operation periods. The column was recovered by flushing with tap water once every two days and air-dried for re-oxygenation. Table 1 lists the quality of influents in batch operations. The whole experiments were completed during a 9-month period at greenhouse conditions, for which the temperature ranged from 19 to 23 ◦ C. The reactors were stabilized for two weeks prior to starting their operation. 2.3. Water quality analysis Samples collected from influent, effluent and porous water in each layer were analyzed immediately in the lab. Ammonia (NH4 –N), nitrate (NO3 –N) and total nitrogen (TN) concentrations were measured using a gas-phase absorption spectrometer (GMA3230, Beiyu Co. Ltd., China). Chemical oxygen demand (COD) was tested using a digital reactor block (HACH, DRB200, USA). Dissolved oxygen (DO) content was measured simultaneously using a DO meter (HI9143, NANNA, Italy). Significance of removal efficiency for different packing strategies are checked at a probability level of p < 0.05 using SPSS 13.0 software. 2.4. Determination of bio-film clogging The biomass accumulation rate (BAR) in each layer and total BAR (TBAR) of the filter column were used to assess the biofilm growth. BAR is defined as the decrease in the media water storage volume over time due to bio-film accumulation, written as BAR = (V0 × i − Vt × i )/V0 , where V0 × i is the initial void in layer
Fig. 3. Interlaminar efficiency of system performances during the second low organic loadings.
i evaluated according to effluent wastewater volume. Vt × i refers to the water storage volume after t days of operation. The measurement can avoid destructive sampling as opposed to the biomass loss on ignition (LOI) method (Tanner and Sukias, 1995b; Ye et al., 2012a,b). To evaluate clogging, the clogging rate coefficient (CRC) was defined as a function of the change in the effluent flow rate in the following form: CRC = (Tt − T0 )/T0 , where T0 , s/L and Tt , s/L are the time cost of per liter of wastewater drainage from the reactor before and after t days of operation. If the clogging happened, CRC value would increase. The CRC and BAR were synchronously tested during each operation period. Evaporation and transpiration of plant were ignored in this experiment.
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Fig. 4. BAR in each layer for sequential low, high and low organic load operation, each operation took 12 weeks.
3. Results and discussion 3.1. Nitrogen and organic matter removal Table 2 summarizes the determined mean pollutant removal efficiency for VFCWs reactors with different packing strategies. Generally, nitrification and COD degradation were more effective in D-packing reactors than I-packing and U-packing reactors. The nitrogen removal rate generally was not as good as COD for all packing strategies (p < 0.05). Significant differences in the pollutant removal rate were detected among packing strategies (p < 0.01). For low organic load operation, the D-packing reactor showed an efficient ammonia oxidation rate of 56%, reaching a concentration of 9.58 NH4 –N mg/L in the effluent. The increased oxygen transport capacity in the upper layer due to the larger media size improved the aerobic conditions for nitrification. Ammonia can more easily converted to nitrate by nitrifier. This aerobic condition contributes to the nitrification in D-packing reactors by stimulating nitrate accumulation. However, the I-packing reactor enhances denitrification in the bottom layer. A lower effluent nitrate concentration and larger TN removal rate (43%) demonstrate that denitrification in the I-packing reactor is more efficient than in the D-packing and Upacking reactors (p < 0.01). Denitrification is an anoxic process that is sensitive to the DO level (Bachand and Horne, 1999; Chung et al., 2008; Matheson and Sukias, 2010). Accordingly, the I-packing reactor could enhance the TN removal rates in a single filter column
via the concurrent aerobic or anoxic/anaerobic conditions. This enhancement can be verified via the obvious downward trend of the oxygen distribution for I-packing reactors (Fig. 2). The packing strategy also positively influences the COD removal. The COD content (117.7 mg/L) decreased sharply to 45.1 mg/L for the D-packing reactor, while the COD removal rates were 55% and 49% for the I-packing and U-packing reactors, respectively. Ammonia removal is generally below 40% for high organic load operation. Instead, the D-packing reactor obviously differed from this standard because the TN removal rate increased approximately 10% compared to operation at a low organic load. This difference might be caused by the significant DO consumption for high organic loads, which enlarges the anoxic areas and thus enhances denitrification when sufficient carbon is available (Wang et al., 2013; Ding et al., 2012). 3.2. Vertical dissolved oxygen distribution The DO changes in each layer were measured after the reaction. The DO is primarily affected by the porous transfer efficiency. The curves of the vertical DO level changes are illustrated in Fig. 2. The DO shows a decreasing trend for all columns from top to bottom. The DO in the D-packing and I-packing reactors was more variable than in the U-packing reactor (p < 0.01). The rate of DO decrease reached 0.71 mg/L/10 cm for the I-packing reactor and 0.78 mg/L/10 cm for the D-packing reactor, which was higher than the value of 0.57 mg/L/10 cm for the U-packing reactor. The DO
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while the I-packing and U-packing reactors provided relative higher DO levels in all layers.
3.3. Nitrification and organics degradation The removal efficiency varied in the filter column. The nitrification and degradation of organics in different layers are presented in Fig. 3. The ammonia oxidation was highest in the upper layer of the I-packing reactor. The ammonia and COD removal rates in top layer of the I-packing reactor reached 56% and 64%, respectively. Ammonia removal gradually decreased to 27% with depth, accounting for 47% of the total in the top layer. The COD removal followed a trend identical to that of ammonia. The D-packing reactor was less effective for nitrification. The ammonia removal rate was 44% on average, and highest COD removal was achieved in the bottom layer. Despite high re-oxygenation in the D-packing reactor, the small specific surface area of the media in top layers reduced the interaction between nutrients and the bio-film due to the large particle size (Aim et al., 1997). As suggested, NH4 –N was adsorbed onto filter media and organic matter during loading and nitrified mainly between the feeding periods (McNevin et al., 1999). The rapid consumption of available N in the filter can only occur when the surrounding bio-film-stored oxygen is sufficient (Tanner et al., 1999). Furthermore, the study indicated that the growth of the ammonium oxidizer is faster than that of the nitrite oxidizer at comparatively lower DO levels, leading to the accumulation of nitrite resulting from partial nitrification (Zhang et al., 2011). The ammonia and COD removal did not significantly change in the U-packing reactor compared to the I-packing reactor, significant differences were not detected among layers (p > 0.05). The ammonia and organic removal were generally not as effective as in the I-packing reactor.
3.4. Bio-film accumulation
Fig. 5. Clogging rate coefficient (CRC) and total biomass accumulation rate (TBAC) for sequential low, high and low organic load operation. 7-day bed resting between two operations is marked using “↓”.
ranged from 4.6 to 0.3 and 6.9 to 2.2 for the I-packing and D-packing reactors, respectively. The DO concentration of the I-packing reactor was lower in the bottom layer, which indicated ideal anoxic conditions for denitrification. A DO level above 2.0 mg/L has been accepted as essential for nitrification and should be controlled below 0.5 mg/L for denitrification (Ouellet-Plamondon et al., 2006). In the upper layers, the DO is mainly consumed by the degradation of organic pollutant and nitrification. Atmospheric re-oxygenation is the predominant source for VFCWs. Over 99.8% of DO is used for the degradation and nitrification of organics (Ye et al., 2012a,b). Larger filters in the upper layer are advantageous for atmospheric oxygen transfer. But the root oxygen transfer is not significant. The I-packing reactor showed an aerobic-to-anoxic transition that was beneficial for both the nitrification and denitrification processes,
Because the reactors were alternately operated with low and high organic loads, they were allowed to rest for seven days between operations. Changes in the BAR in each layer were measured (Fig. 4). The BAR logarithmically increased during each operation. The bio-film accumulation in the column obviously differed (p < 0.01). In the bottom layer of the D-packing reactor, the BAR increased to 17% after a 60-days of low organic load operation. The biomass accumulation was 15% higher than in the I-packing and U-packing reactors. This difference indicates a rapid reduction in the effective porosity due to biomass growth. This increase also occurs in the first layer in the I-packing reactor probably in the form of a nitrifier. However, the DO in the upper layer of the I-packing reactor remains at a relatively high level despite bio-film accumulation. This finding indicates that atmospheric re-oxygenation still significantly to nitrification by serving as a major DO source. Moreover, the large quantity of biomass attached on the small particles strongly correlated with nitrification and the degradation of organics (R2 = 0.87). In other layers, the BAR gradually increased during the entire operation. However, the BAR in the upper layer for the I-packing and U-packing reactors increased to a maximum of 25% and 16%, respectively, for high organic loads. The bio-film accumulation was highest in the bottom layer of the D-packing reactor at 23%. The high organic load clearly induced rapid bio-film growth for the entire column. Bed resting appeared most effective in the middle layer. The clogging mitigation positively correlated with the depth for I-packing reactors and negatively correlated with the depth for D-packing reactors.
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3.5. Clogging and recovery Changes in the CRC and TBAR for sequential operations are shown in Fig. 5. The biomass growth can be reduced via bed resting. The TBAR of each reactor dropped during bed resting. The TBAR of the I-packing reactor increased faster than that of the D-packing reactor compared to the value of the CRC. The degree of clogging in the I-packing reactor did not markedly increased after bed resting (p > 0.01). The final CRC of the I-packing reactor increased 12%. Although the TBAR showed a slow increase after high organic load, the CRC of the D-packing and U-packing reactors rapidly increased to 35% and 23%, respectively, which caused serious clogging. Clogging appeared to increase 10% in the D-packing reactor after 28 days of low organic load operation, nearly 35 days earlier than the I-packing reactor. These data suggest that the entrapment and accumulation of sloughed biofilms rather than the local growth of biofilms accelerates the clogging of the media. The general trend of CRC indicated that high organic load operation lead to lower hydraulic conductivities. Moreover, bed resting is not effective for clogging mitigation in D-packing reactors because a small filter is used near the harvesting device. The D-packing reactor appeared to be less prone to bio-film washout through the small particles in the bottom layer. On the contrary, I-packing reactor can be recovered more easily. Thus, proper operation and packing design are essential to slow down bio-film accumulation.
4. Conclusions In this study, the evaluation of three different filter media packing strategies was used to assess the efficiency of nitrogen removal and clogging characteristics. I-packing reactors present an aerobicto-anoxic transition area for efficient TN removal. The D-packing and U-packing reactors do not appear to be efficient for both nitrification and denitrification. The CRC and BAR were evaluated. The formation of clogs is a function of the position of biomass accumulation. A dense biomass layer formed at the bottom layer of the D-packing reactor to result in severe clogging. The I-packing reactor can bear organic impact load and is suitable for long-term wastewater treatment. If properly operated, this type of filter packing structure offers several advantages, such as reduced bio-film accumulation and strong clogging mitigation potential after bed resting.
Acknowledgments The authors would like to thank the following entities for their support of this study: (1). National Natural Science Foundation of China (Fund key. 51309053); (2). Public Welfare Project of Ministry of Environmental Protection, China (Fund key. 2013467042); (3). Fundamental Research Funds for the Central Universities-DHU Distinguished Young Professor Program (Fund key. B201310); and Fundamental Research Funds for the Central Universities (Fund key. 14D111313).
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