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Nitrogen Removal Improvement by Adding Peat in Deep Soil of Subsurface Wastewater Infiltration System
Journal of Integrative Agriculture 2014, 13(5): 1113-1120
May 2014
RESEARCH ARTICLE
Nitrogen Removal Improvement by Adding Peat in Deep Soil of Subsurface Wastewater Infiltration System CHEN Pei-zhen1, CUI Jian-yu1, HU Lin1, 2, ZHENG Miao-zhuang1, CHENG Shan-ping1, HUANG Jiewen1 and MU Kang-guo1 1 2
College of Resources and Environmental Sciences, China Agricultural University, Beijing 100193, P.R.China National Energy R&D Center for Non-Food Biomass of China, China Agricultural University, Beijing 100193, P.R.China
Abstract In order to enhance the nitrogen removal, a subsurface wastewater infiltration system (SWIS) was improved by adding peat in deep soil as carbon source for denitrification process. The effects of addition of carbon source in the underpart of the SWIS on nitrogen removal at different influents (with the total nitrogen (TN) concentration 40 and 80 mg L-1, respectively) were investigated by soil column simulating experiments. When the relatively light pollution influent with 40 mg L-1 TN was used, the average concentrations of NO3--N and TN in effluents were (4.69±0.235), (6.18±0.079) mg L-1, respectively, decreased by 32 and 30.8% than the control; the NO3--N concentration of all effluents was below the maximum contaminant level of 10 mg L-1; as high as 92.67% of the TN removal efficiency was achieved. When relatively heavy pollution influent with 80 mg L-1 TN was used, the average concentrations of NO3--N and TN in effluents were (10.2±0.265), (12.5±0.148) mg L-1 respectively, decreased by 20 and 21.2% than the control; the NO3--N concentration of all effluents met the grade III of the national quality standard for ground water of China (GB/T 14848-1993) with the values less than 20 mg L-1; the TN removal efficiency of 94.1% was achieved. In summary, adding peat in the underpart of the SWIS significantly decreased TN and NO3-N concentration in effluents and the nitrogen removal efficiency improved significantly. Key words: peat addition, nitrogen removal efficiency, denitrification, subsurface wastewater infiltration system
INTRODUCTION Currently China showed an increasing trend in wastewater discharge and the domestic sewage accounted for the major part. In 2010, the total discharge of wastewater of China was 61.73 billion ton, of which 61.5% was the domestic wastewater. The big problem caused by sewage discharge is eutrophication in lakes (reservoirs) and the main pollutants are total nitrogen (TN) and total phosphorus (TP) (Ministry of Environmental Protection of the People’s Republic of China 2011). If domestic sewage
can be properly used as water source for agricultural irrigation, it could be helpful not only to alleviate the shortage of agricultural water but also to provide N, P nutrients for crops as well. However, to avoid pollution on soil and groundwater by irrigation of domestic sewage, the sewage need to be treated before being used as groundwater recharge. So, on-site land treatment technologies of domestic sewage in China are particularly important. However, up to now, both research and engineering application in such area are inadequate. Subsurface wastewater infiltration system (SWIS), as one of on-site land wastewater treatment systems, is considered to be an efficient
Received 1 November, 2012 Accepted 16 February, 2013 CHEN Pei-zhen, Mobile: 15822916682, E-mail: [email protected]; Correspondence HU Lin, Tel: +86-10-62733729, E-mail: [email protected]
© 2014, CAAS. All rights reserved. Published by Elsevier Ltd. doi: 10.1016/S2095-3119(13)60401-3
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and economic ecological process for decentralized domestic wastewater treatment in villages, small towns and scattered residential areas (USEPA 2002). The removal rate is generally satisfactory in terms of chemical oxygen demand (CODMn), biological oxygen demand (BOD5), TP and suspended solid (SS) (Kadam et al. 2009). However, with high NO3--N concentration in effluents, the nitrogen removal is inadequate in most of the present operations (Lloréns et al. 2011), and there is a risk of groundwater contamination. Nitrification-denitrification is considered to be an important way to remove nitrogen in SWIS (Zhang et al. 2005). Whelan and Barrow (1984) found that nitrification process completed within 50 cm of the unsaturated up soil. Zheng et al. (2011) also reported that the ammonia nitrogen (NH4+-N) concentrations in the SWIS effluents were very low and the NH4+-N removal rate was up to 99%. It indicated that the SWIS is sufficient for the accomplishment of the nitrification. However, in the deep soil with aerobic and low-carbon source, the denitrifying process was not effective (Beal et al. 2005). For successful denitrification process, two prerequisites must be met in the soil wastewater treatment system. One is the anoxic condition (with redox potential lower than 300 mV)；the another is sufficient carbon（Meek and Grass 1975; Ye and Li 2009). Zhang et al. (2005) reported that at 25 cm depth of SWIS, the redox potential was lower than 250 mV. Ten Have et al. (1994) studied the nitrification and denitrification process in an activated sludge plant with separate reactors and Del Pozo and Diez (2003) investigated the biological nitrogen removal in a sequencing batch reactor（SBR）in laboratory. Both of them found that carbon source scarceness is the major reason for the inadequate denitrification process. Therefore, we present a study suppose here that in the deep soil of the SWIS, carbon source scarceness may be the limiting factor for the denitrification process. If the supplemental carbon source was added to the lower part anoxic soil of SWIS, the denitrification process may be improved and the NO3--N concentration in effluents would decrease. If such a research suppose is proved to be correct, it will provide new ideas and new methods to improve the effect of nitrogen removal in SWIS. In this paper, we added peat in the soil at the
depth of 50 cm to 80 cm of the SWIS to provide supplemental carbon source for denitrification process. Lightly polluted and heavily polluted influent with TN concentration about 40 and 80 mg L-1 were used. The index of NO3--N concentration in effluents was chosen to assess the risk of groundwater pollution against the international standard of the maximum contaminant level (MCL) with less than 10 mg L -1 for drinking water (USEPA 2000; WHO 2004) and the national quality standard of groundwater of China for grade III (GB/T 14848-1993) with NO3--N concentration less than 20 mg L-1.
RESULTS Effects of deeply adding peat on NO 3 - -N concentration in effluents The concentration variation of NO 3--N in effluents during the trial period was shown in Fig. 1. When the influent TN concentration was 40 mg L-1, the NO3--N concentration in effluents was in the range of 2.63 to 13.07 mg L-1 in treatment 40-CK and in the range of 1.67 to 9.65 mg L-1 in treatment 40-PM. The NO3--N concentration of (4.69±0.235) mg L-1 on average in treatment 40-PM was significantly lower than that of (6.89±0.160) mg L-1 on average in treatment 40CK, decreased by 32%. In order to further examine and reflect the efficacy of adding peat in deep soil in SWIS treating domestic wastewater, higher nitrogen concentration (80 mg L-1) influent treatment was set up. When the influent TN concentration was 80 mg L-1, the NO3--N concentration in effluents was in the range of 12.8 to 56 mg L-1 in treatment 80-CK and in the range of 3.50 to 19.62 mg L-1 in treatment 80PM. The NO3--N concentration of (10.2±0.265) mg L-1 on average in treatment 80-PM was significantly lower than that of (25.2±0.259) mg L-1 on average in treatment 80-CK, decreased by 20%. It is obviously that the NO 3--N concentration in effluents can be significantly reduced by deeply adding carbon source under different nitrogen concentration of influent. In the whole experiment period, total 89 times influent loading was carried out and total 67 times effluents were collected. The proportion of NO3--N
© 2014, CAAS. All rights reserved. Published by Elsevier Ltd.
NO3--N concentration (mg L-1)
Nitrogen Removal Improvement by Adding Peat in Deep Soil of Subsurface Wastewater Infiltration System 14 12 10 8 6 4 2 0 60 50
A
B
40-CK 40-PM
80-CK 80-PM
40
30 20 10 0
3/29 4/19 5/7 7/13 8/9 8/29 9/25 10/16 11/3 11/2112/10 Operation time (mon/d)
Fig. 1 Effect of deeply adding peat on NO3--N concentration in effluents. A, NO3--N concentration when influent was 40 mg L-1 TN. B, NO3--N concentration when influent was 80 mg L-1.
concentration in effluents which met the standard was counted according to the international MCL of 10 mg L-1 NO3--N (USEPA 2000; WHO 2004) and the grade III of the national quality standard for ground water of China (GB/T 14848-1993 1993) of 20 mg L-1. When the influent TN concentration was 40 mg L-1, the effluents of treatment 40-CK and treatment 40-PM both met the criteria grade III of GB/T 148481993 (1993); 86.57% effluents of the treatment 40-CK was below the international MCL, and the proportion below the international MCL of the effluents of treatment 40-PM was 100%, increased by 13.43%. When the influent TN concentration was 80 mg L-1, only 3% effluents of treatment 80-CK was below the international MCL and 40.3% met grade III of GB/ T 14848-1993 (1993). However, 49.25% effluents of the treatment 80-PM was below the international MCL and 100% effluents met the grade III of GB/T 14848-1993 (1993). It indicates that deeply adding carbon source can improve the compliance rates of the effluents which meet both international MCL and the national quality standard for ground water of China, increased by 46.25 and 59.7% respectively.
Effects of deeply adding carbon source on TP and CODMn removal TP is an important reason for eutrophication(Liu 2001; Li and Peng 2003). In this paper, the effect of deeply adding peat as carbon source on TP and CODMn
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removal was also investigated (Fig. 2). Average TP concentrations in effluents of treatments 40-CK, 40PM, 80-CK and 80-PM were (0.027±0.010), (0.012 ±0.001), (0.025±0.001) and (0.019±0.019) mg L -1, respectively. The average concentration of TP in the effluents of treatment 40-PM was lower than that of 40-CK, reduced by 55.6%. And the average concentration of TP in the effluents of treatment 80-PM was lower than that of 80-CK, reduced by 24%. Many research results have reported that soil has strong ability to remove TP. Phosphorus removal mechanisms in soil include soil adsorption, chemical precipitation and biological processes (including microbial and plant uptake). The most important way is chemical precipitation, and biological processes only play a weak role in phosphorus removal (Pratt et al. 2007; Hu and Shan 2009). In this paper, our research also proved the strong ability to remove TP from waste water through soil. The concentration of TP in the effluents of any treatment was very low. The difference of TP removal in the SWIS between peat addition and no peat addition was very little. As the results showed in Fig. 2-C and D, the infiltration system with deeply adding peat also has a better ability on CODMn removal. Average CODMn concentrations in effluents of treatments 40-CK, 40-PM, 80-CK and 80-PM were (6.38±0.098), (4.64±0.17), (7.57±1.15), (6.33±0.31) mg L-1, respectively. During the trial period, the CODMn concentrations in effluents of the deeply adding peat SWIS all met the standards of grade IV of GB/T 14848-1993 with the value 10 mg L-1. The treatments of deeply adding peat in SWIS can significantly lower the CODMn concentrations in effluents no matter the influent TN concentration was 40 or 80 mg L-1, the CODMn concentrations in effluents decreased by 27.2 and 16.3% respectively. In SWIS, the main organic pollutants removal mechanism is biodegradation. Better CODMn removal performance in this study may imply that extra carbon sources in deep soil may be helpful to enhance the microbial activities.
Effects of adding peat in deep soil on TN and NH4+-N removal of the SWIS Fig. 3-A and B showed the variation of NH 4 + -N concentrations in effluents during the experiment. The © 2014, CAAS. All rights reserved. Published by Elsevier Ltd.
CHEN Pei-zhen et al.
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A
0.030
10 40-CK 40-PM
8
0.015 0.010 0.005 0.000 0.08 0.07 0.06
B
80-CK 80-PM
0.05
40-CK 40-PM
6 4 2 0 14 12
D
80-CK 80-PM
10 8
0.04
6
0.03
4
0.02
2
0.01 0.00
CODMn concentration (mg L-1)
TP concentration (mg L-1)
0.025 0.020
C
3/29 4/19 5/7 7/13 8/9 8/29 9/22 10/13 10/31 11/18 12/7
0
3/29 4/19 5/7 7/13 8/9 8/29 9/22 10/13 10/31 11/18 12/7 Operation time (mon/d)
Operation time (mon/d)
Fig. 2 Effects of deeply adding peat of SWIS on TP and CODMn concentrations in effluents. A and B, TP concentrations; C and D, CODMn concentrations.
average NH4+-N concentration in effluents of treatment 40-CK and treatment 40-PM were (0.197±0.016) and (0.151±0.018) mg L-1, respectively. The average NH4+-N concentration in effluents of treatment 80CK and treatment 80-PM were (0.029±0.034) and (0.022±0.015) mg L-1. It can be seen that adding peat in deep soil of the SWIS had little influence on NH4+-N concentration in effluents. However, peat addition in deep soil had significant impact on TN removal in the SWIS. The result was showed in Fig. 3-C and D. The TN concentration in effluents of treatment 40PM was significantly lower than that of treatment 40CK. The former was (6.18±0.079) mg L-1 on average, decreased by 30.8% than the latter with the TN concentration (8.93±0.197) mg L-1 on average. The TN concentration of (12.5±0.148) mg L-1 on average in effluents of treatment 80-PM was also significantly lower than that of (29.7±0.756) mg L-1 on average in treatment 80-CK. The former decreased by 21.2% than the latter. The results indicated that adding peat as supplemental carbon source can significantly reduce TN concentration in effluents of the SWIS. A s s h o w n i n T a b l e 1 , t h e N H 4+- N r e m o v a l efficiencies of all treatments were higher than 99.7% and the NH 4+-N removal amount per unit area was not significant different. Strictly speaking, NH4+-N removal rate does not mean it is truly removed from
the system, because the presence of nitrogen in soil can be in various forms. And NH4+-N could disappear with converting to other forms of nitrogen, nitrate and nitrite, for example. So, it is worth paying more attention to TN removal. The TN removal efficiency and removal amount per unit area of treatment 40PM were (92.67±0.15)% and (52.13±0.20) g m -2 , which were higher than that of treatment 40-CK. The removal efficiency of (94.11±0.67)% and removal amount of (101.65±0.67) g m-2 in treatment 80-PM were significantly higher than that of treatment 80CK, increased by 11.35 and 16.21%, respectively. It indicated that peat adding in deep soil of SWIS can effectively enhance the TN removal. The difference on TN removal rates between treatment 80-CK and treatment 80-PM showed more significance than that between treatment 40-CK and treatment 40-PM.
DISCUSSION The main mechanism of NO3--N removal by microbe is that nitrate transforms to nitrogen through the process of denitrification. NO3--N can also be changed to nitrite and nitrous oxide gas by anaerobic reactions (Knowles 1982). Electrons needed for denitrification can originate from the oxidation of organic carbon by microbe (denitrification bacteria use organic carbon as © 2014, CAAS. All rights reserved. Published by Elsevier Ltd.
Nitrogen Removal Improvement by Adding Peat in Deep Soil of Subsurface Wastewater Infiltration System
0.5
18
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16 14
40-CK 40-PM
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TN concentration (mg L-1)
NH4+-N concentration (mg L-1)
C 40-CK 40-PM
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3/29 4/19 5/7 7/13 8/9 8/29 9/22 10/13 10/3111/18 12/7 Operation time (mon/d)
Operation time (mon/d)
Fig. 3 Effect of adding peat in deep soil on NH4+-N and TN concentrations in effluents. A and B indicating NH4+-N concentrations, C and D indicating TN concentrations in effluents. Table 1 Nitrogen removal effect of different treatments Items +
NH4 -N
TN
Treatments 40-CK 40-PM 80-CK 80-PM 40-CK 40-PM 80-CK 80-PM
Input (g m-2) Wastewater 39.1 39.1 76.4 76.4 52.2 52.2 103 103
Rainfall 0.590 0.590 0.590 0.590 3.37 3.37 3.37 3.37
Output (g m-2) Effluent 0.096±0.013 0.089±0.006 0.139±0.027 0.075±0.014 4.02±0.61 3.46±0.20 15.08±2.07 A 4.72±0.67 B
Removal amount per unit area (g m-2) 39.6±0.013 39.6±0.006 76.9±0.027 76.9±0.014 51.57±0.61 52.13±0.20 91.29±2.07 A 101.65±0.67 B
Removal efficiency (%) 99.7±0.044 99.7±0.012 99.7±0.034 99.8±0.020 91.28±1.39 92.67±0.15 80.98±2.18 A 94.11±0.67 B
Different letters in the same column indicate significant differences (P<0.01).
the electron donor). Several studies revealed that over 80% of the organic matter is biodegraded within the 0-25 cm layer in the infiltration systems (Reemtsma et al. 2000). Lack of organic carbon is identified as the major factor that limits the denitrification rate in aquifers. The relationship between nitrate and organic matter in the denitrification process is given by the chemical reaction formula below (Jørgensen et al. 2004): 5CH2O+4NO3-=2N2+4HCO3-+CO2+3H2O In other words, poor denitrification process means more NO 3--N remain in effluents. NO 3--N concentration in effluents reflects the denitrification intensity of the system. In this study, the peat as carbon source was added at the soil depth of 0.5-0.8 m in the SWIS. The results showed that adding peat in deep soil as supplemental carbon source can enhance the denitrification process of SWIS.
As showed in Fig. 1-B, NO3--N concentration in effluents above international MCL of drinking water in treatment 80-PM happened mainly in July, August and December. Firstly, it was caused by the relatively high TN loading in influent with 80 mg L-1. Usually, the typical TN loading in SWIS is about 40 mg L-1. Secondly, the SWIS was operated outdoor and can be affected by rainfall. Precipitation in July and August accounted for 42.3% of total rainfall during the experiment (Fig. 4). The soil of SWIS could be at saturated or supersaturated condition during heavy rain and the hydraulic retention time for influent was shortened in SWIS (Rubin et al. 2010). The NO3--N in SWIS had no enough time to complete the denitrification process in such a situation. In addition, NO3--N with a negative charge was not adsorbed by the soil particles, which resulted in the heavy leaching of NO3--N. Thirdly, low temperature was also other © 2014, CAAS. All rights reserved. Published by Elsevier Ltd.
CHEN Pei-zhen et al.
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reason for higher NO3--N concentration in effluents. The average temperature in December in Beijing area dropped to -2.2°C. It was reported that the optimal temperature for denitrification is between 25 and 35°C and the proliferation rate and metabolic rate of denitrifying bacteria reduced when the temperature is lower than 15°C and it cause NO3--N concentration in effluents increased (Beal et al. 2005). Nitrogen removal efficiency of SWIS is generally characterized by the TN removal efficiency. The level of TN removal efficiency also reflects the degrees of nitrification and denitrification in the system. In this
Precipitation (mm)
Mean monthly temperature
100
25 20
80
15
60
10
40
5
20 0
30
0 Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec.
Mean monthly temperature (°C)
Precipitation
120
-5
Fig. 4 Rainfall and mean monthly temperature during the experiment.
study, the TN removal efficiency of peat adding in deep soil treatments were all more than 90% and much higher than that from some other studies (Table 2). The above results proved that the improved SWIS by adding carbon source in deep soil for denitrification process significantly enhanced the nitrogen removal capacity. However, what is the optimal C/N ratio for the SWIS remains unclear. In this paper, the domestic sewage with TN concentration less than 100 mg L-1 was used for the investigation. It should be valuable and interesting to probe what will happen if use wastewater with very high concentration of nitrogen as influent, livestock wastewater following the biogas project, for example, which often contains the TN more than 1 000 mg L-1.
CONCLUSION In this study, the SWIS was improved by adding peat
Table 2 Removal efficiency of TN in some studies Removal efficiency (%) 80.7 62.3 70.1 77.7 65.1 92.7 94.1
References Li et al. (2011a) Nakhla and Farooq (2003) Li et al. (2011b) Zhang et al. (2005) Wang et al. (2010) 40-PM (this study) 80-PM (this study)
as carbon source in deep soil and had successfully operated outdoor for 10 months. For the treatments of adding peat in deep soil, the concentrations of NO3--N and TN in effluents were significantly decreased and the TN removal efficiency was significantly increased. For the treatments of 40 mg L-1 TN in influent, all of the NO3--N concentration in effluents was below the international MCL for drinking water (less than 10 mg L-1). For the treatments of 80 mg L-1 TN in influent, the NO3--N concentration in effluents met grade III of national quality standard of China for groundwater (GB/T 14848-1993) with the value less than 20 mg L-1. The improved SWIS also has better performance on TP and CODMn removal. The research results proved that adding peat as the supplemental carbon source in deep soil of the SWIS is an effective way to enhance the denitrification process and TN removal.
MATERIALS AND METHODS Device description Fig. 5 shows the experiment device of the improved SWIS. It is a soil column made of PVC pipe with a diameter of 30 cm and height of 100 cm. The additional components include influent channel, perforated water distribution pipe and percolation tank. Sandy loam was filled in the column. The influent channel is a covered PVC cylinder with a diameter of 10 cm and height of 35 cm. The perforated water distribution pipe with a diameter of 1.5 cm is placed at 35 cm below the surface of soil. There are 12 laps of water hole on the surface of pipe and each lap has 8 holes of 0.2 cm in diameter, plum-like distributed. The percolation tank is a 25 cm×15 cm×15 cm PVC box with the top side open and a watertight bottom, which is connected to the influent channel. In the tank, there are three layers materials, with a 5-cm gravel layer in the bottom, a 5-cm coarse sand layer in the middle and a 5-cm sandy-loam-peat mixture (volume ratio 7:3) in the top. The overflow port and drainpipe are installed on the top and bottom of the soil column. At the bottom of the soil column is a 5-cm layer of gravel and a 5-cm layer coarse sand immediately above. Four peat © 2014, CAAS. All rights reserved. Published by Elsevier Ltd.
Nitrogen Removal Improvement by Adding Peat in Deep Soil of Subsurface Wastewater Infiltration System
Experimental method and treatment The experiment was conducted from March (turfgrass began to turn green) to December in 2010 (icebound), lasted 10 months. During the experimental period, the soil columns were in open air and both the volume and quality of the precipitation were measured after each rain. The TN concentrations in influent were adjusted to approximately 40 and 80 mg L-1. The sewage with TN concentration of 80 mg L-1 is the typical high nitrogen domestic wastewater in China. Four treatments were set up: (1) adding peat in deep soil and with TN concentration about 40 mg L-1 in influent (40-PM); (2) no adding peat in deep soil and with TN concentration about 40 mg L-1 in influent as the control (40CK); (3) adding peat in deep soil and with TN concentration about 80 mg L-1 in influent (80-PM); and (4) no adding peat in deep soil and with TN concentration about 80 mg L-1 in influent (80-CK). Three soil columns were used as the replicates for each treatment.
Analytical methods Both influent and effluent water samples were collected and all the samples were stored at 4°C in a refrigerator for less than 24 h before the water samples were determined. Alkaline potassium persulfate digestion ultraviolet spectrophotometric method was used for TN test; ultraviolet spectrophotometry method was used for NO3-
Sand loam-peat mixture 25 cm 5 cm coarse sand layer 5 cm gravel layer
Sand loam
Peat column
10 cm
40 cm
100 cm
Characteristics of the wastewater as influent The raw wastewater for this study was obtained from the septic tank close to the research building on the west campus of China Agricultural University, Beijing, China. The TN concentration in influent was diluted artificially to approximately 40 and 80 mg L-1. Since the influent was from actual sewage and the experiment was conducted outdoor and only the index of TN was under the considering of artificial adjustment, the concentrations of other indexes of influent fluctuated with season. For TN concentration in influent being approximately 40 mg L-1, the concentration of main water quality index were: TN, 31.28-49.31 mg L-1; NO3--N, 0.13910.46 mg L-1; NH4+-N, 19.26-46.39 mg L-1; TP, 1.06-10.13 mg L-1; CODMn, 14.62-55.44 mg L-1. For TN concentration in influent being approximately 80 mg L-1, the concentration of main water quality index were: TN, 71.06-89.75 mg L-1; NO3-N, 0.261-6.24 mg L-1; NH4+-N, 33.33-84.18 mg L-1; TP, 1.9913.21 mg L-1; CODMn, 21.99-121.41 mg L-1.
Influent channel
35 cm 15 cm infiltration trench
columns with diameter of about 3 cm and height of 30 cm are settled down at the depth of 50 to 80 cm from the surface of soil, 15 cm below from the bottom of the percolation tank. The devices for CK treatment are the same except for peat columns setting. The physical and chemical properties of the sandy loam and peat for the experiment are shown in Table 3.
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5 cm coarse sand layer 5 cm gravel layer
Drain pipe 30 cm
Fig. 5 The longitudinal profile of the experiment device. Table 3 Physical and chemical properties of tested soil and peat Sand loam Organic matter (g kg-1) TN (g kg-1) Available phosphorous (mg kg-1) Potassium (g kg-1)
0.721
pH
8.83
2.92 0.15 2.98
Peat Color Organic matter (%) Humic acid (%) Cation exchange capacity (cmol kg-1) pH
Brown 96 58.3 125 4.93
-N determination; and nessle’s reagent spectrophotometry method was used for NH4+-N test, ammonium molybdate spectrophotometry was used for TP determination and acidic potassium permanganate method was used for COD Mn determination (State Environmental Protection Administration 2002). SAS statistical software (SAS Institute Inc., Cary, NC, USA) was used for statistical analysis, with P<0.01 considered statistically significant. The pollutant removal efficiency was calculated as follows:
η1=
∑Cinfluent×Vinfluen-∑Cenfluent×Venfluent ∑Cinfluent×Vinfluent
×100%
Where, η1 is removal efficiency, Cinfluent is the concentration of influent (including sewage and rainfall, mg L-1), Ceffluent is the concentration of effluents (mg L-1), Vinfluent is the volume of influent (including sewage and rainfall, L) Veffluent is the volume of effluents (L).
Acknowledgements The study was supported by the Key Technologies R&D
© 2014, CAAS. All rights reserved. Published by Elsevier Ltd.
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Program of China during the 11th Five-Year Plan period (2008BADC4B17 and 2006 BAD16B09) and the Beijing Key Discipline Construction Project of Biomass Engineering Interdisciplinary.
References Beal C D, Gardner E A, Menzies N W. 2005. Process, performance, and pollution potential: A review of septic tank-soil absorption systems. Australian Journal of Soil Research, 43, 781-802. Del Pozo R, Diez V, 2003. Organic matter removal in combined anaerobic-aerobic fixed-biofilm bioreactors. Water Research, 37, 3561-3568. GB/T 14848-1993. 1993. Quality standard for ground water. Standardization administration of the People’s Republic of China. (in Chinese) Hu C M, Shan B Q. 2009. Phosphorus removal performance and mechanisms of a constructed horizontal subsurface flow wetlands treating reclaimed water. Environmental Engineering Science, 26, 1097-1106. Jørgensen P R, Urup J, Helstrup T, Jensen M B, Eiland F, Vinther F P. 2004. Transport and reduction of nitrate in clayey till underneath forest and arable land. Journal of Contaminant Hydrology, 73, 207-226. Kadam A M, Nemade P D, Oza G H, Shankar H S. 2009. Treatment of municipal wastewater using literate-based constructed soil filter. Ecological Engineering, 35, 10511061. Knowles R. 1982. Denitrification. Microbiology Review, 46, 43-70. Li Y M, Peng, Y A. 2003. The pollution feature of Dianchi Lake and its control countermeasure. Yunnan Geographic Environment Research, 15, 32-38. (in Chinese). Li Y H, Li H B, Sun T H, Wang X. 2011a. Study on nitrogen removal enhanced by shunt distributing wastewater in a constructed subsurface infiltration system under intermittent operation mode. Journal of Hazardous Materials, 189, 336-341. Li Y H, Li H B, Sun T H, Wang X. 2011b. Effects of hydraulic loading rate on pollutants removal by a deep subsurface wastewater infiltration system. Ecological Engineering, 37, 1425-1429. Liu L P. 2001. Analysis on eutrophication of Dianchi Lake and its control measures. Yunnan Geographic Environment Research, 20, 25-27. (in Chinese) Lloréns M, Pérez-Marín A B, Aguilar M I, Sáez J, Ortuno J F, Meseguer V F. 2011. Nitrogen transformation in two subsurface infiltration systems at pilot scale. Ecological Engineering, 37, 736-743. Nakhla G, Farooq S. 2003. Simultaneous nitrificationdenitrification in slow sand filters. Journal of Hazardous Materials, 96, 291-303. Meek B D, Grass L B. 1975. Redox potential in irrigated desert soils as an indicator of aeration status. Soil Science Society of America Journal, 39, 870-875.
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Ministry of Environmental Protection of the People’s Republic of China. 2011. China Environmental Quality Report. China Environmental Science Press, Beijing. pp.1-91. (in Chinese) Pratt C, Shilton A, Pratt S, Haverkamp R G, Bolan N S. 2007. Phosphorus removal mechanisms in active slag filters treating waste stabilization pond effluent. Environmental Science & Technology, 41, 3296-3301. Reemtsma T, Gnirss R, Jekel M. 2000. Infiltration of combined sewer overflow and tertiary municipal wastewater: an integrated laboratory and field study on nutrients and dissolved organics. Water Research, 34, 1179-1186. Rubin D, Clifford B, Fedler P E. 2010. Performance of a combined natural wastewater treatment system in West Texas. Journal of Irrigation and Drainage Engineering, 136, 204-209. State Environmental Protection Administration. 2002. Determination Methods for Examination of Water and Waste Water. China Environmental Science Press, Beijing. pp. 54-286. (in Chinese) Ten Have P J W, Willers H C, Derikx P J I. 1994. Nitrification and denitrification in an activated sludge system for supernatant from settled sow manure with molasses as an extra carbon source. Bioresource Technology, 47, 135-141. USEPA (United States Environmental Protcction Agency). 2000. Drinking Water Standards and Health Advisories. 822-B-00-001. Washington, D.C. p. 12. USEPA (United States Environmental Protcction Agency). 2002. On site Wastewater Treatment Systems Manual. EPA/625/R-00/008. Washington, D. C. Wang X, Sun T, Li H, Li Y, Pand J. 2010. Nitrogen removal enhanced by shunt distributing wastewater in a subsurface wastewater infiltration system. Ecological Engineering, 36, 1433-1438. Whelan B R, Barrow N J. 1984. The movement of septic tank effluent through sandy soils near Perth. I. Movementof nitrogen. Australian Journal of Soil Research, 22, 283-292. WHO (World Health Organization). 2004. Guidelines for Drinking Water Quality. 3rd ed. ISBN 92 4 154638 7, 20 Avenue Appia, 1211 Geneva 27, Switzerland. Ye F X, Li Y. 2009. Enhancement of nitrogen removal in towery hybrid constructed wetland to treat domestic wastewater for small rural communities. Ecological Engineering, 35, 1043-1050. Zhang J, Huang X, Liu C X, Shi H C, Hu H Y. 2005. Nitrogen removal enhanced by intermittent operation in a subsurface wastewater infiltration system. Ecological Engineering, 25, 419-428. Zheng M Z, Hu L, Dong R J, Wu D M, Chen S P, Cui J Y. 2011. Environmental effect of turf-soil system disposing nitrogen in biogas fertilizers. Transactions of the CSAE, 27, 294-299. (in Chinese) (Managing editor SUN Lu-juan) © 2014, CAAS. All rights reserved. Published by Elsevier Ltd.
May 2014
RESEARCH ARTICLE
Nitrogen Removal Improvement by Adding Peat in Deep Soil of Subsurface Wastewater Infiltration System CHEN Pei-zhen1, CUI Jian-yu1, HU Lin1, 2, ZHENG Miao-zhuang1, CHENG Shan-ping1, HUANG Jiewen1 and MU Kang-guo1 1 2
College of Resources and Environmental Sciences, China Agricultural University, Beijing 100193, P.R.China National Energy R&D Center for Non-Food Biomass of China, China Agricultural University, Beijing 100193, P.R.China
Abstract In order to enhance the nitrogen removal, a subsurface wastewater infiltration system (SWIS) was improved by adding peat in deep soil as carbon source for denitrification process. The effects of addition of carbon source in the underpart of the SWIS on nitrogen removal at different influents (with the total nitrogen (TN) concentration 40 and 80 mg L-1, respectively) were investigated by soil column simulating experiments. When the relatively light pollution influent with 40 mg L-1 TN was used, the average concentrations of NO3--N and TN in effluents were (4.69±0.235), (6.18±0.079) mg L-1, respectively, decreased by 32 and 30.8% than the control; the NO3--N concentration of all effluents was below the maximum contaminant level of 10 mg L-1; as high as 92.67% of the TN removal efficiency was achieved. When relatively heavy pollution influent with 80 mg L-1 TN was used, the average concentrations of NO3--N and TN in effluents were (10.2±0.265), (12.5±0.148) mg L-1 respectively, decreased by 20 and 21.2% than the control; the NO3--N concentration of all effluents met the grade III of the national quality standard for ground water of China (GB/T 14848-1993) with the values less than 20 mg L-1; the TN removal efficiency of 94.1% was achieved. In summary, adding peat in the underpart of the SWIS significantly decreased TN and NO3-N concentration in effluents and the nitrogen removal efficiency improved significantly. Key words: peat addition, nitrogen removal efficiency, denitrification, subsurface wastewater infiltration system
INTRODUCTION Currently China showed an increasing trend in wastewater discharge and the domestic sewage accounted for the major part. In 2010, the total discharge of wastewater of China was 61.73 billion ton, of which 61.5% was the domestic wastewater. The big problem caused by sewage discharge is eutrophication in lakes (reservoirs) and the main pollutants are total nitrogen (TN) and total phosphorus (TP) (Ministry of Environmental Protection of the People’s Republic of China 2011). If domestic sewage
can be properly used as water source for agricultural irrigation, it could be helpful not only to alleviate the shortage of agricultural water but also to provide N, P nutrients for crops as well. However, to avoid pollution on soil and groundwater by irrigation of domestic sewage, the sewage need to be treated before being used as groundwater recharge. So, on-site land treatment technologies of domestic sewage in China are particularly important. However, up to now, both research and engineering application in such area are inadequate. Subsurface wastewater infiltration system (SWIS), as one of on-site land wastewater treatment systems, is considered to be an efficient
Received 1 November, 2012 Accepted 16 February, 2013 CHEN Pei-zhen, Mobile: 15822916682, E-mail: [email protected]; Correspondence HU Lin, Tel: +86-10-62733729, E-mail: [email protected]
© 2014, CAAS. All rights reserved. Published by Elsevier Ltd. doi: 10.1016/S2095-3119(13)60401-3
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and economic ecological process for decentralized domestic wastewater treatment in villages, small towns and scattered residential areas (USEPA 2002). The removal rate is generally satisfactory in terms of chemical oxygen demand (CODMn), biological oxygen demand (BOD5), TP and suspended solid (SS) (Kadam et al. 2009). However, with high NO3--N concentration in effluents, the nitrogen removal is inadequate in most of the present operations (Lloréns et al. 2011), and there is a risk of groundwater contamination. Nitrification-denitrification is considered to be an important way to remove nitrogen in SWIS (Zhang et al. 2005). Whelan and Barrow (1984) found that nitrification process completed within 50 cm of the unsaturated up soil. Zheng et al. (2011) also reported that the ammonia nitrogen (NH4+-N) concentrations in the SWIS effluents were very low and the NH4+-N removal rate was up to 99%. It indicated that the SWIS is sufficient for the accomplishment of the nitrification. However, in the deep soil with aerobic and low-carbon source, the denitrifying process was not effective (Beal et al. 2005). For successful denitrification process, two prerequisites must be met in the soil wastewater treatment system. One is the anoxic condition (with redox potential lower than 300 mV)；the another is sufficient carbon（Meek and Grass 1975; Ye and Li 2009). Zhang et al. (2005) reported that at 25 cm depth of SWIS, the redox potential was lower than 250 mV. Ten Have et al. (1994) studied the nitrification and denitrification process in an activated sludge plant with separate reactors and Del Pozo and Diez (2003) investigated the biological nitrogen removal in a sequencing batch reactor（SBR）in laboratory. Both of them found that carbon source scarceness is the major reason for the inadequate denitrification process. Therefore, we present a study suppose here that in the deep soil of the SWIS, carbon source scarceness may be the limiting factor for the denitrification process. If the supplemental carbon source was added to the lower part anoxic soil of SWIS, the denitrification process may be improved and the NO3--N concentration in effluents would decrease. If such a research suppose is proved to be correct, it will provide new ideas and new methods to improve the effect of nitrogen removal in SWIS. In this paper, we added peat in the soil at the
depth of 50 cm to 80 cm of the SWIS to provide supplemental carbon source for denitrification process. Lightly polluted and heavily polluted influent with TN concentration about 40 and 80 mg L-1 were used. The index of NO3--N concentration in effluents was chosen to assess the risk of groundwater pollution against the international standard of the maximum contaminant level (MCL) with less than 10 mg L -1 for drinking water (USEPA 2000; WHO 2004) and the national quality standard of groundwater of China for grade III (GB/T 14848-1993) with NO3--N concentration less than 20 mg L-1.
RESULTS Effects of deeply adding peat on NO 3 - -N concentration in effluents The concentration variation of NO 3--N in effluents during the trial period was shown in Fig. 1. When the influent TN concentration was 40 mg L-1, the NO3--N concentration in effluents was in the range of 2.63 to 13.07 mg L-1 in treatment 40-CK and in the range of 1.67 to 9.65 mg L-1 in treatment 40-PM. The NO3--N concentration of (4.69±0.235) mg L-1 on average in treatment 40-PM was significantly lower than that of (6.89±0.160) mg L-1 on average in treatment 40CK, decreased by 32%. In order to further examine and reflect the efficacy of adding peat in deep soil in SWIS treating domestic wastewater, higher nitrogen concentration (80 mg L-1) influent treatment was set up. When the influent TN concentration was 80 mg L-1, the NO3--N concentration in effluents was in the range of 12.8 to 56 mg L-1 in treatment 80-CK and in the range of 3.50 to 19.62 mg L-1 in treatment 80PM. The NO3--N concentration of (10.2±0.265) mg L-1 on average in treatment 80-PM was significantly lower than that of (25.2±0.259) mg L-1 on average in treatment 80-CK, decreased by 20%. It is obviously that the NO 3--N concentration in effluents can be significantly reduced by deeply adding carbon source under different nitrogen concentration of influent. In the whole experiment period, total 89 times influent loading was carried out and total 67 times effluents were collected. The proportion of NO3--N
© 2014, CAAS. All rights reserved. Published by Elsevier Ltd.
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Nitrogen Removal Improvement by Adding Peat in Deep Soil of Subsurface Wastewater Infiltration System 14 12 10 8 6 4 2 0 60 50
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Fig. 1 Effect of deeply adding peat on NO3--N concentration in effluents. A, NO3--N concentration when influent was 40 mg L-1 TN. B, NO3--N concentration when influent was 80 mg L-1.
concentration in effluents which met the standard was counted according to the international MCL of 10 mg L-1 NO3--N (USEPA 2000; WHO 2004) and the grade III of the national quality standard for ground water of China (GB/T 14848-1993 1993) of 20 mg L-1. When the influent TN concentration was 40 mg L-1, the effluents of treatment 40-CK and treatment 40-PM both met the criteria grade III of GB/T 148481993 (1993); 86.57% effluents of the treatment 40-CK was below the international MCL, and the proportion below the international MCL of the effluents of treatment 40-PM was 100%, increased by 13.43%. When the influent TN concentration was 80 mg L-1, only 3% effluents of treatment 80-CK was below the international MCL and 40.3% met grade III of GB/ T 14848-1993 (1993). However, 49.25% effluents of the treatment 80-PM was below the international MCL and 100% effluents met the grade III of GB/T 14848-1993 (1993). It indicates that deeply adding carbon source can improve the compliance rates of the effluents which meet both international MCL and the national quality standard for ground water of China, increased by 46.25 and 59.7% respectively.
Effects of deeply adding carbon source on TP and CODMn removal TP is an important reason for eutrophication(Liu 2001; Li and Peng 2003). In this paper, the effect of deeply adding peat as carbon source on TP and CODMn
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removal was also investigated (Fig. 2). Average TP concentrations in effluents of treatments 40-CK, 40PM, 80-CK and 80-PM were (0.027±0.010), (0.012 ±0.001), (0.025±0.001) and (0.019±0.019) mg L -1, respectively. The average concentration of TP in the effluents of treatment 40-PM was lower than that of 40-CK, reduced by 55.6%. And the average concentration of TP in the effluents of treatment 80-PM was lower than that of 80-CK, reduced by 24%. Many research results have reported that soil has strong ability to remove TP. Phosphorus removal mechanisms in soil include soil adsorption, chemical precipitation and biological processes (including microbial and plant uptake). The most important way is chemical precipitation, and biological processes only play a weak role in phosphorus removal (Pratt et al. 2007; Hu and Shan 2009). In this paper, our research also proved the strong ability to remove TP from waste water through soil. The concentration of TP in the effluents of any treatment was very low. The difference of TP removal in the SWIS between peat addition and no peat addition was very little. As the results showed in Fig. 2-C and D, the infiltration system with deeply adding peat also has a better ability on CODMn removal. Average CODMn concentrations in effluents of treatments 40-CK, 40-PM, 80-CK and 80-PM were (6.38±0.098), (4.64±0.17), (7.57±1.15), (6.33±0.31) mg L-1, respectively. During the trial period, the CODMn concentrations in effluents of the deeply adding peat SWIS all met the standards of grade IV of GB/T 14848-1993 with the value 10 mg L-1. The treatments of deeply adding peat in SWIS can significantly lower the CODMn concentrations in effluents no matter the influent TN concentration was 40 or 80 mg L-1, the CODMn concentrations in effluents decreased by 27.2 and 16.3% respectively. In SWIS, the main organic pollutants removal mechanism is biodegradation. Better CODMn removal performance in this study may imply that extra carbon sources in deep soil may be helpful to enhance the microbial activities.
Effects of adding peat in deep soil on TN and NH4+-N removal of the SWIS Fig. 3-A and B showed the variation of NH 4 + -N concentrations in effluents during the experiment. The © 2014, CAAS. All rights reserved. Published by Elsevier Ltd.
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Fig. 2 Effects of deeply adding peat of SWIS on TP and CODMn concentrations in effluents. A and B, TP concentrations; C and D, CODMn concentrations.
average NH4+-N concentration in effluents of treatment 40-CK and treatment 40-PM were (0.197±0.016) and (0.151±0.018) mg L-1, respectively. The average NH4+-N concentration in effluents of treatment 80CK and treatment 80-PM were (0.029±0.034) and (0.022±0.015) mg L-1. It can be seen that adding peat in deep soil of the SWIS had little influence on NH4+-N concentration in effluents. However, peat addition in deep soil had significant impact on TN removal in the SWIS. The result was showed in Fig. 3-C and D. The TN concentration in effluents of treatment 40PM was significantly lower than that of treatment 40CK. The former was (6.18±0.079) mg L-1 on average, decreased by 30.8% than the latter with the TN concentration (8.93±0.197) mg L-1 on average. The TN concentration of (12.5±0.148) mg L-1 on average in effluents of treatment 80-PM was also significantly lower than that of (29.7±0.756) mg L-1 on average in treatment 80-CK. The former decreased by 21.2% than the latter. The results indicated that adding peat as supplemental carbon source can significantly reduce TN concentration in effluents of the SWIS. A s s h o w n i n T a b l e 1 , t h e N H 4+- N r e m o v a l efficiencies of all treatments were higher than 99.7% and the NH 4+-N removal amount per unit area was not significant different. Strictly speaking, NH4+-N removal rate does not mean it is truly removed from
the system, because the presence of nitrogen in soil can be in various forms. And NH4+-N could disappear with converting to other forms of nitrogen, nitrate and nitrite, for example. So, it is worth paying more attention to TN removal. The TN removal efficiency and removal amount per unit area of treatment 40PM were (92.67±0.15)% and (52.13±0.20) g m -2 , which were higher than that of treatment 40-CK. The removal efficiency of (94.11±0.67)% and removal amount of (101.65±0.67) g m-2 in treatment 80-PM were significantly higher than that of treatment 80CK, increased by 11.35 and 16.21%, respectively. It indicated that peat adding in deep soil of SWIS can effectively enhance the TN removal. The difference on TN removal rates between treatment 80-CK and treatment 80-PM showed more significance than that between treatment 40-CK and treatment 40-PM.
DISCUSSION The main mechanism of NO3--N removal by microbe is that nitrate transforms to nitrogen through the process of denitrification. NO3--N can also be changed to nitrite and nitrous oxide gas by anaerobic reactions (Knowles 1982). Electrons needed for denitrification can originate from the oxidation of organic carbon by microbe (denitrification bacteria use organic carbon as © 2014, CAAS. All rights reserved. Published by Elsevier Ltd.
Nitrogen Removal Improvement by Adding Peat in Deep Soil of Subsurface Wastewater Infiltration System
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Fig. 3 Effect of adding peat in deep soil on NH4+-N and TN concentrations in effluents. A and B indicating NH4+-N concentrations, C and D indicating TN concentrations in effluents. Table 1 Nitrogen removal effect of different treatments Items +
NH4 -N
TN
Treatments 40-CK 40-PM 80-CK 80-PM 40-CK 40-PM 80-CK 80-PM
Input (g m-2) Wastewater 39.1 39.1 76.4 76.4 52.2 52.2 103 103
Rainfall 0.590 0.590 0.590 0.590 3.37 3.37 3.37 3.37
Output (g m-2) Effluent 0.096±0.013 0.089±0.006 0.139±0.027 0.075±0.014 4.02±0.61 3.46±0.20 15.08±2.07 A 4.72±0.67 B
Removal amount per unit area (g m-2) 39.6±0.013 39.6±0.006 76.9±0.027 76.9±0.014 51.57±0.61 52.13±0.20 91.29±2.07 A 101.65±0.67 B
Removal efficiency (%) 99.7±0.044 99.7±0.012 99.7±0.034 99.8±0.020 91.28±1.39 92.67±0.15 80.98±2.18 A 94.11±0.67 B
Different letters in the same column indicate significant differences (P<0.01).
the electron donor). Several studies revealed that over 80% of the organic matter is biodegraded within the 0-25 cm layer in the infiltration systems (Reemtsma et al. 2000). Lack of organic carbon is identified as the major factor that limits the denitrification rate in aquifers. The relationship between nitrate and organic matter in the denitrification process is given by the chemical reaction formula below (Jørgensen et al. 2004): 5CH2O+4NO3-=2N2+4HCO3-+CO2+3H2O In other words, poor denitrification process means more NO 3--N remain in effluents. NO 3--N concentration in effluents reflects the denitrification intensity of the system. In this study, the peat as carbon source was added at the soil depth of 0.5-0.8 m in the SWIS. The results showed that adding peat in deep soil as supplemental carbon source can enhance the denitrification process of SWIS.
As showed in Fig. 1-B, NO3--N concentration in effluents above international MCL of drinking water in treatment 80-PM happened mainly in July, August and December. Firstly, it was caused by the relatively high TN loading in influent with 80 mg L-1. Usually, the typical TN loading in SWIS is about 40 mg L-1. Secondly, the SWIS was operated outdoor and can be affected by rainfall. Precipitation in July and August accounted for 42.3% of total rainfall during the experiment (Fig. 4). The soil of SWIS could be at saturated or supersaturated condition during heavy rain and the hydraulic retention time for influent was shortened in SWIS (Rubin et al. 2010). The NO3--N in SWIS had no enough time to complete the denitrification process in such a situation. In addition, NO3--N with a negative charge was not adsorbed by the soil particles, which resulted in the heavy leaching of NO3--N. Thirdly, low temperature was also other © 2014, CAAS. All rights reserved. Published by Elsevier Ltd.
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reason for higher NO3--N concentration in effluents. The average temperature in December in Beijing area dropped to -2.2°C. It was reported that the optimal temperature for denitrification is between 25 and 35°C and the proliferation rate and metabolic rate of denitrifying bacteria reduced when the temperature is lower than 15°C and it cause NO3--N concentration in effluents increased (Beal et al. 2005). Nitrogen removal efficiency of SWIS is generally characterized by the TN removal efficiency. The level of TN removal efficiency also reflects the degrees of nitrification and denitrification in the system. In this
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study, the TN removal efficiency of peat adding in deep soil treatments were all more than 90% and much higher than that from some other studies (Table 2). The above results proved that the improved SWIS by adding carbon source in deep soil for denitrification process significantly enhanced the nitrogen removal capacity. However, what is the optimal C/N ratio for the SWIS remains unclear. In this paper, the domestic sewage with TN concentration less than 100 mg L-1 was used for the investigation. It should be valuable and interesting to probe what will happen if use wastewater with very high concentration of nitrogen as influent, livestock wastewater following the biogas project, for example, which often contains the TN more than 1 000 mg L-1.
CONCLUSION In this study, the SWIS was improved by adding peat
Table 2 Removal efficiency of TN in some studies Removal efficiency (%) 80.7 62.3 70.1 77.7 65.1 92.7 94.1
References Li et al. (2011a) Nakhla and Farooq (2003) Li et al. (2011b) Zhang et al. (2005) Wang et al. (2010) 40-PM (this study) 80-PM (this study)
as carbon source in deep soil and had successfully operated outdoor for 10 months. For the treatments of adding peat in deep soil, the concentrations of NO3--N and TN in effluents were significantly decreased and the TN removal efficiency was significantly increased. For the treatments of 40 mg L-1 TN in influent, all of the NO3--N concentration in effluents was below the international MCL for drinking water (less than 10 mg L-1). For the treatments of 80 mg L-1 TN in influent, the NO3--N concentration in effluents met grade III of national quality standard of China for groundwater (GB/T 14848-1993) with the value less than 20 mg L-1. The improved SWIS also has better performance on TP and CODMn removal. The research results proved that adding peat as the supplemental carbon source in deep soil of the SWIS is an effective way to enhance the denitrification process and TN removal.
MATERIALS AND METHODS Device description Fig. 5 shows the experiment device of the improved SWIS. It is a soil column made of PVC pipe with a diameter of 30 cm and height of 100 cm. The additional components include influent channel, perforated water distribution pipe and percolation tank. Sandy loam was filled in the column. The influent channel is a covered PVC cylinder with a diameter of 10 cm and height of 35 cm. The perforated water distribution pipe with a diameter of 1.5 cm is placed at 35 cm below the surface of soil. There are 12 laps of water hole on the surface of pipe and each lap has 8 holes of 0.2 cm in diameter, plum-like distributed. The percolation tank is a 25 cm×15 cm×15 cm PVC box with the top side open and a watertight bottom, which is connected to the influent channel. In the tank, there are three layers materials, with a 5-cm gravel layer in the bottom, a 5-cm coarse sand layer in the middle and a 5-cm sandy-loam-peat mixture (volume ratio 7:3) in the top. The overflow port and drainpipe are installed on the top and bottom of the soil column. At the bottom of the soil column is a 5-cm layer of gravel and a 5-cm layer coarse sand immediately above. Four peat © 2014, CAAS. All rights reserved. Published by Elsevier Ltd.
Nitrogen Removal Improvement by Adding Peat in Deep Soil of Subsurface Wastewater Infiltration System
Experimental method and treatment The experiment was conducted from March (turfgrass began to turn green) to December in 2010 (icebound), lasted 10 months. During the experimental period, the soil columns were in open air and both the volume and quality of the precipitation were measured after each rain. The TN concentrations in influent were adjusted to approximately 40 and 80 mg L-1. The sewage with TN concentration of 80 mg L-1 is the typical high nitrogen domestic wastewater in China. Four treatments were set up: (1) adding peat in deep soil and with TN concentration about 40 mg L-1 in influent (40-PM); (2) no adding peat in deep soil and with TN concentration about 40 mg L-1 in influent as the control (40CK); (3) adding peat in deep soil and with TN concentration about 80 mg L-1 in influent (80-PM); and (4) no adding peat in deep soil and with TN concentration about 80 mg L-1 in influent (80-CK). Three soil columns were used as the replicates for each treatment.
Analytical methods Both influent and effluent water samples were collected and all the samples were stored at 4°C in a refrigerator for less than 24 h before the water samples were determined. Alkaline potassium persulfate digestion ultraviolet spectrophotometric method was used for TN test; ultraviolet spectrophotometry method was used for NO3-
Sand loam-peat mixture 25 cm 5 cm coarse sand layer 5 cm gravel layer
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Characteristics of the wastewater as influent The raw wastewater for this study was obtained from the septic tank close to the research building on the west campus of China Agricultural University, Beijing, China. The TN concentration in influent was diluted artificially to approximately 40 and 80 mg L-1. Since the influent was from actual sewage and the experiment was conducted outdoor and only the index of TN was under the considering of artificial adjustment, the concentrations of other indexes of influent fluctuated with season. For TN concentration in influent being approximately 40 mg L-1, the concentration of main water quality index were: TN, 31.28-49.31 mg L-1; NO3--N, 0.13910.46 mg L-1; NH4+-N, 19.26-46.39 mg L-1; TP, 1.06-10.13 mg L-1; CODMn, 14.62-55.44 mg L-1. For TN concentration in influent being approximately 80 mg L-1, the concentration of main water quality index were: TN, 71.06-89.75 mg L-1; NO3-N, 0.261-6.24 mg L-1; NH4+-N, 33.33-84.18 mg L-1; TP, 1.9913.21 mg L-1; CODMn, 21.99-121.41 mg L-1.
Influent channel
35 cm 15 cm infiltration trench
columns with diameter of about 3 cm and height of 30 cm are settled down at the depth of 50 to 80 cm from the surface of soil, 15 cm below from the bottom of the percolation tank. The devices for CK treatment are the same except for peat columns setting. The physical and chemical properties of the sandy loam and peat for the experiment are shown in Table 3.
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5 cm coarse sand layer 5 cm gravel layer
Drain pipe 30 cm
Fig. 5 The longitudinal profile of the experiment device. Table 3 Physical and chemical properties of tested soil and peat Sand loam Organic matter (g kg-1) TN (g kg-1) Available phosphorous (mg kg-1) Potassium (g kg-1)
0.721
pH
8.83
2.92 0.15 2.98
Peat Color Organic matter (%) Humic acid (%) Cation exchange capacity (cmol kg-1) pH
Brown 96 58.3 125 4.93
-N determination; and nessle’s reagent spectrophotometry method was used for NH4+-N test, ammonium molybdate spectrophotometry was used for TP determination and acidic potassium permanganate method was used for COD Mn determination (State Environmental Protection Administration 2002). SAS statistical software (SAS Institute Inc., Cary, NC, USA) was used for statistical analysis, with P<0.01 considered statistically significant. The pollutant removal efficiency was calculated as follows:
η1=
∑Cinfluent×Vinfluen-∑Cenfluent×Venfluent ∑Cinfluent×Vinfluent
×100%
Where, η1 is removal efficiency, Cinfluent is the concentration of influent (including sewage and rainfall, mg L-1), Ceffluent is the concentration of effluents (mg L-1), Vinfluent is the volume of influent (including sewage and rainfall, L) Veffluent is the volume of effluents (L).
Acknowledgements The study was supported by the Key Technologies R&D
© 2014, CAAS. All rights reserved. Published by Elsevier Ltd.
1120
Program of China during the 11th Five-Year Plan period (2008BADC4B17 and 2006 BAD16B09) and the Beijing Key Discipline Construction Project of Biomass Engineering Interdisciplinary.
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