Research on the nitrogen removal efficiency and mechanism of deep subsurface wastewater infiltration systems by fine bubble aeration

Ecological Engineering 107 (2017) 33–40

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Research on the nitrogen removal efficiency and mechanism of deep subsurface wastewater infiltration systems by fine bubble aeration Hongqiang Wang b , Lieyu Zhang a,∗ a b

State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of Environmental Sciences, Beijing 100012, China College of Environmental and Safety Engineering, University of South China, Hengyang 421001, China

a r t i c l e

i n f o

Article history: Received 14 November 2016 Received in revised form 14 June 2017 Accepted 3 July 2017 Keywords: Deep subsurface wastewater infiltration system Fine bubble aeration Nitrogen removal Simulated livestock wastewater

a b s t r a c t To respond to the reality that it is difficult and expensive to deal with wastewater rich in ammonia nitrogen (NH4 + -N) in the countryside, fine bubble aeration technology and a deep subsurface wastewater infiltration system are coupled to investigate their nitrogen removal efficiency and mechanism. The results show that fine bubble aeration can improve the effectiveness of a deep subsurface wastewater infiltration system in treating wastewater that is rich in ammonia nitrogen, and effective nitrogen degradation can be attained by limited aeration. The effluent Chemical Oxygen Demand (COD), NH4 + -N and total phosphorus (TP) can reach the first grade A standard required by “Discharge standard of pollutants for municipal wastewater treatment plants” (GB18918-2002). The effect of the fine bubble aeration technology is beneficial when it is used to strengthen land-treatment technology. © 2017 Published by Elsevier B.V.

1. Introduction Livestock wastewater is a combination of livestock excretions and water from discharge facilities (Kim et al., 2016). Therefore, it contains high levels of COD, BOD5 , nitrogen, phosphorus, and suspended solids (Kim et al., 2014). The discharge of livestock wastewater into the ecological system has a negative impact on the water bodies that receive it (Tak et al., 2015). To reduce the nutrient loading into the environment, clarification treatment should be implemented (Zheng et al., 2013). A common goal in livestock waste management is the need for ‘closed-loop’ systems; making full use of the residual value of livestock waste in ways that do not impact negatively on the environment and that are both socially and economically acceptable (Harrington and McInnes, 2009). Subsurface wastewater infiltration systems (SWISs) are the most commonly used systems for the treatment and disposal of onsite wastewater (Zheng et al., 2016). Over the past 20 years, the SWIS has gained popularity as an effective and low-cost alternative for wastewater treatment, especially in villages and small communities (Li et al., 2015). In SWI treatment, the wastewater is first treated by conventional physico-chemical and/or biological methods and then allowed to infiltrate through

∗ Corresponding author. E-mail addresses: [email protected] (H. Wang), [email protected] (L. Zhang). http://dx.doi.org/10.1016/j.ecoleng.2017.07.005 0925-8574/© 2017 Published by Elsevier B.V.

an aerated unsaturated zone, where it is purified through processes such as filtration, adsorption, chemical reaction and biodegradation (Li et al., 2011). The removal percentages are generally satisfactory in terms of chemical oxygen demand (COD), biological oxygen demand (BOD), total phosphorus (TP) and suspended solids (SS) (Pan et al., 2013). However, the removal percentage of nitrogen is deficient in most of the present operating SWISs, especially with a high pollution load, which poses a risk of polluting the receiving water or groundwater (Arve et al., 2006; Zhang et al., 2011). Artificial aeration (AA) has been proposed as a solution to create an aerobic environment conducive to nitrification with the additional benefit of the ability to be retrofitted into an existing horizontal subsurface flow constructed wetlands systems (Butterworth et al., 2013). Choi et al. (2009) believed that the introduction of microbubbles is a potentially feasible means for the aerobic biodegradation of organic compounds. The use of microbubbles has been shown to successfully enhance the aerobic biodegradation of many organic compounds (Rothmel et al., 1998; Mulligan and Eftekhari, 2003). Micro-nano bubble (MNB) injection seems to be an effective technique for increasing the oxygen in water compared with traditional air sparging technology with macrobubbles. Micro-nano bubbles have a larger interfacial area, higher inner pressure and density, and lower rising velocity in water compared to macrobubbles (Li et al., 2014a). However, the application is only in the beginning stages and must still be intensively studied (Li et al., 2014b).

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Fig. 1. Soil column setup.

The objective of this study was to combine fine bubble aeration technology with a deep subsurface wastewater infiltration system and evaluate the nitrogen removal and degradation mechanism through a more than eight month experiment. The combined systems were installed in the Field Research Base of Groundwater, Chinese Research Academy of Environmental Sciences, which is located in the Shunyi District of Beijing.

Table 1 Results of physical property analysis of the soil employed in the column experiments. soil depth(cm)

Porosity (%)

unit weight(g/cm3 )

pH (1:1)

CEC (cmol/kg)

0–50 50–100 100–150 150–200

30.5 29.4 27.8 25.4

1.43 1.56 1.63 1.76

6.6 5.9 6.5 6.9

11.31 8.49 4.65 3.12

2. Materials and methods 2.1. Pilot system description The column pilot system, which is made of organic glass with a height of 2.4 m and a radius of 0.3 m, was manufactured and mounted in a steel frame. This column is made of four segments that are connected by flanges. To maintain a watertight seal, a gasket was placed between these segments before securing them together. Fig. 1 shows a schematic drawing of the soil column setup. Water sampling points were provided at seven points from the top of the column, as shown in Fig. 1, on the left side. These sampling tubes extended to the bottom of the column’s cross-section. A soil moisture sampler (Rhizon SMS 19.21.01) was attached to each water sampling point. The soil used in the experiment was collected from the Shunyi District of Beijing and stratified into the organic glass column with the same depth. The physical characteristics of each soil are summarized in Table 1. Along the depth, the column was packed to

a density of 1.32 g cm−3 in the top 50 cm, 1.38 g cm−3 from 50 to 100 cm, 1.46 g cm−3 from 100 to 150 cm, and 1.50 g cm−3 from 150 to 200 cm. To simulate a natural interface, each interface was brushed. In addition, from the top down, 10 cm of small gravel and 5 cm of cobblestone were packed at the bottom of the soil column. After filling, the column was wrapped with black plastic tarps to shut out light and discourage algae growth during the experiment. A variable-speed peristaltic pump was used to deliver wastewater to the column through a silicone tube. The column was initially set up and soaked with tap water for 30 days, after which an analysis of a 24-h composite of percolate confirmed that no leachable constituents were of concern (TN < 1 mg L−1 , COD < 4 mg L−1 , TP < 0.01 mg L−1 ). Throughout the test period, the soil column was continuously fed with synthetic domestic wastewater at a hydraulic loading of 10 cm/d.

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Table 2 Quality of simulated livestock wastewater with high ammonia nitrogen. parameters

COD (mg/L)

BOD5 (mg/L)

NH4 + -N (mg/L)

NO3 — N (mg/L)

TN (mg/L)

TP (mg/L)

SS (mg/L)

range mean value

464–506 481.4

215–259 235.6

184–226 203.5

14–21 18.3

220–267 242.4

16–25 23.2

14–32 20.4

Table 3 Operation mode of each column. serial number

D column

E column

F column

G column

H column

mean value range aeration mode

2 mg/L ±1.39 fine bubble aeration

4 mg/L ±1.02 fine bubble aeration

6 mg/L ±0.68 fine bubble aeration

8 mg/L ±1.52 ordinary aeration

0.52 mg/L ±0.41 non-aeration

2.2. Quality of raw sewage Simulated livestock wastewater with high ammonia nitrogen and containing dissolved pollutants was used to minimize the variability in the experiment; it did not contain solids. The simulated livestock wastewater with high ammonia nitrogen was composed of 200 mg L−1 C12 H22 O11 , 30 mg L−1 CON2 H4 , 570 mg L−1 NH4 Cl, 91 mg L−1 NaNO3 ·H2 O, 1.9 mg L−1 CaCl2 ·2H2 O, 7.8 mg L−1 ZnSO4 ·5H2 O, 62 mg L−1 MgCl2 ·6H2 O, 0.8 mg L−1 CuCl2 ·2H2 O, 7.8 mg L−1 MnCl2 ·4H2 O, and 5 mL L−1 leach liquor of pig dung. The experiment began in April and lasted for more than eight months in 2014. The quality of the simulated livestock wastewater is shown in Table 2. The ambient temperature was controlled at 18–29 ◦ C. The dissolved oxygen (DO) concentration of each column is shown in Table 3. According to the results of a preliminary experiment, the optimal concentration of dissolved oxygen is no greater than 6 mg/L. Therefore, the dissolved oxygen concentrations were 2 mg/L, 4 mg/L, and 6 mg/L, for columns D, E, and F, respectively. In addition, columns G and H were used as control groups for ordinary aeration and non-aeration, respectively. The fine bubble aeration equipment was purchased from the state (Beijing) nanometer science and technology Co., LTD, its model was B&W-15. 2.3. Sampling and analytical methods Samples were collected from the influent, at each sampling point, and from the effluent of the system. Samples were collected once a month after the system was allowed to run steadily. The TN, NH4 + –N, NO3 − –N, NO2 − –N and TP were determined using standard methods (E.P.A. Chinese, 2012). The DO and pH were determined using a glass electrode. Statistical analyses were conducted with SPSS 19.0. 3. Results and discussion 3.1. Degradation of COD The inflow COD concentration was 461 mg/L throughout the test period. The variation of the effluent COD concentration and environmental temperature with time is shown in Fig. 2. The effluent COD concentrations are 1.87 mg/L, 1.23 mg/L, 0.77 mg/L, 0.77 mg/L, and 2.08 mg/L for columns D, E, F, G, and H, respectively, and the corresponding removal efficiencies are as high as 99.71%, 99.79%, 99.84%, 99.84%, and 99.84%, respectively. The effluent water quality is better than that mandated by “Discharge standard of pollutants for municipal wastewater treatment plants” (GB18918-2002) the first grade A standard (Table 4). The average removal percentage of COD increases with the dissolved oxygen concentration, and the effluent quality of the aeration soil column is stable and is better than that of the nonaeration column, which shows that aeration could improve the

Table 4 Cities Sewage Treatment Plant Pollutant Discharge Standard” (GB18918-2002). items

CODcr

BOD5

SS

TN

NH3 -N

TP

effluent (mg/L)

≤50

≤10

≤10

≤15

≤5(8)

≤0.5

COD removal. However, because the removal percentage of COD in the non-aeration column is already 99.16%, the opportunity for improvement by aeration is not large. The COD removal is also influenced by the temperature. The peak values of the removal efficiency of COD are in July and August, and there is a clear increase in the effluent COD concentration when the room temperature decreases. The statistical study (a paired T test) revealed that the COD effluent of fine bubble aeration (columns E, F) were no significant difference with that of ordinary aeration (columns G) (P > 0.05). It also shows that fine bubble aeration was more energy-efficient. 3.2. Degradation of NH4 + -N The inflow NH4 + -N concentration is 186–203 mg/L throughout the test period. The variations of the effluent NH4 + -N concentration and environmental temperature with time are shown in Fig. 3. The effluent NH4 + -N concentrations are 0.197 mg/L, 0.175 mg/L, 0.139 mg/L, 0.150 mg/L, and 0.163 mg/L for columns D, E, F, G, and H, respectively, and the corresponding removal efficiencies of NH4 + N are 99.90%, 99.91%, 99.93%, 99.92%, and 99.91%, respectively. The effluent quality is superior to that of the “Discharge standard of pollutants for municipal wastewater treatment plants” (GB189182002), the first grade A standard. The ammonia nitrogen removal ability of each column system does not show a significant difference. Because the removal method of ammonia nitrogen is through physical and chemical adsorption, although the aeration process for ammonia nitrogen adsorption and biodegradation has a certain effect, the deep subsurface wastewater infiltration system reduces the difference under different working conditions and makes them tend to homogeneity. Because the effluent ammonia nitrogen concentration is very low, the effect of temperature on the ammonia nitrogen removal is insignificant. The results indicate that good removal efficiency can be achieved for wastewater with high ammonia nitrogen using deep subsurface wastewater infiltration systems. 3.3. Degradation of TN The inflow TN concentration is 231–260 mg/L throughout the test period. The variation of the effluent TN concentration and environmental temperature with time is shown in Fig. 4. The effluent TN concentrations are 55.2 mg/L, 49.0 mg/L, 67.6 mg/L, 81.5 mg/L, and 86.9 mg/L for columns D, E, F, G, and H, respectively, and the corresponding removal efficiencies of TN are 82.2%, 85.4%, 78.2%, 76.8%, and 73.5%, respectively.

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Fig. 2. Effluent COD concentrations under different operational conditions.

Fig. 3. Effluent NH4 + -N concentrations under different operational conditions.

The effluent qualities of all of the aeration column systems are significantly better than that of the non-aeration column system, which shows that aeration can enhance the TN removal. The mean removal efficiency of TN rises at first but then goes down with the increasing dissolved oxygen concentration. When the dissolved oxygen concentration is 4 mg/L, the removal efficiency of TN reaches 85.4%, which is higher than that of the traditional soil infiltration system. The results show that the denitrification ability of the deep subsurface wastewater infiltration system can be strengthened by restricting the fine bubble aeration. From Fig. 4, the peak values of the removal percentage of TN are in July and August, and the effluent TN concentration fluctuates significantly with the temperature, especially in column H. The correlation coefficients of the removal percentage of TN and the environmental temperature are 0.875**, 0.718*, 0.872**, 0.816*, and 0.918** for columns D, E, F, G, and H, respectively. Clearly, the temperature and the removal percentage of TN have significant correlations in all column systems, with the correlation of column H (non-aeration) being the highest and that of column E (fine bub-

ble aeration) being the lowest. Therefore, the results show that the restriction of fine bubble aeration is effective not only to improve the efficiency of denitrification but also to improve the adaptability of the deep subsurface wastewater infiltration system for environmental temperature fluctuations, when the dissolved oxygen concentration is 4 mg/L. 3.4. Parameter variation of deep subsurface wastewater infiltration system with soil depth An in-depth study is needed to investigate the purification mechanism of the deep subsurface wastewater infiltration system. Therefore, we select three soil columns (E, G, and H) as the research object, and they represent the three different working systems of the restriction fine bubble aeration system, the ordinary aeration system, and the non-aeration system, respectively. The experiment began in September 2014 and lasted for one month. Samples were collected from the influent, at each sampling point (15 cm, 30 cm, 45 cm, 60 cm, 75 cm, 90 cm, 105 cm, 120 cm, 150 cm, and 180 cm),

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Fig. 4. Effluent TN concentrations under different operational conditions.

and from the effluent of the system. The water quality parameters of COD, NH4 + -N, NO2 − -N and NO3 − -N, TN, TP, pH, and DO were determined. 3.4.1. Variation in pH and DO with soil depth The influent pH was from 8.02 to 8.20, which is alkaline. The pH decreased slowly and then increased slightly with the increase in soil depth. The pH of the column G surface soil was lower than that of the other soil columns, which may be because the column has a higher DO concentration, leading the ammonia nitrogen to be oxidized faster and consuming the alkalinity in the water (Chen, 2006). Because the degradation of NH4 + -N is accomplished mainly in shallow soil, within the 0–30 cm soil depth, the pH fluctuation directly reflects the oxidation percentage of NH4 + -N. When the soil depth is greater than 60 cm, the pH has an amplitude fluctuation, which is likely to be an important indicator of nitrification and denitrification processes. Finally, the effluent pH is 7.3-7.5, close to neutral. The oxygen concentration in all soil columns decreases quickly, especially in the 0–30 cm soil depth. The oxygen concentration exhibits significant differences, showing conditions that are aerobic, facultative anaerobic, and facultative anaerobic in columns G, E, and H, respectively. There is an anaerobic zone in the 45–75 cm depth of the soil column, and the dissolved oxygen concentration drops to less than 12 mmol/L when the soil depth increases to 90 cm. In the 120–180 cm soil depth, the dissolved oxygen concentration is close to zero. In addition, in the 180–200 cm soil depth, the dissolved oxygen concentration significantly increases because the bottom of the soil column has an outlet. The dissolved oxygen concentration of column G in 0–45 cm exhibits a rapid decline, and it was the same as in column E at 45 cm soil depth. The dissolved oxygen concentration of column H is less than those of columns E and G, which shows the transition from facultative anaerobic to anaerobic with the increase in soil depth. The dissolved oxygen concentration of column E exhibits a slow decline with increasing soil depth, and there is a wide range in the facultative anaerobic zone (Figs. 5 and 6). 3.4.2. Variation of COD with soil depth In all soil columns, the COD exhibits effective degradation. The degradation percentage of COD is more than 90% at a 120 cm soil

depth. The effluent water quality could reach “Discharge standard of pollutants for municipal wastewater treatment plants” (GB18918-2002), the first grade A standard, and the degradation percentage of COD can reach more than 95% at a 150 cm soil depth. The degradation percentages of COD in the three columns follow the order column G > column E > column H. The results indicate that aeration pretreatment, especially using saturated aeration technology, is able to effectively improve the processing capacity of deep subsurface wastewater infiltration systems for COD. In column G, the degradation percentage of COD can be broadly divided into three categories: a rapid degradation stage in 0–30 cm soil depth, a slow degradation stage in 30–90 cm soil depth, and a stable stage in 90–150 cm soil depth. In column E, the degradation percentage of COD is lower than that of column G in the 0–30 cm soil depth, and then there are two moderate zones in the 30–45 cm and 75–90 cm soil depths. However, an acceleration zone appears at the 45–60 cm and 90–120 cm soil depths. This could be because of the relatively obvious denitrification reaction that occurs in these zones. Denitrification and short-range denitrification reactions can consume organic matter and thus accelerate the degradation percentage of COD. In column H, the degradation percentage of COD exceeds 90% until the 120 cm soil depth, and the effluent water quality could reach the “Discharge standard of pollutants for municipal wastewater treatment plants” (GB18918-2002) first grade A standard.

3.4.3. Variation of nitrogen with soil depth The trend of various forms of nitrogen with soil depth is shown in Fig. 7. The removal efficiency of NH4 + -N was very high in all soil columns, and the effluent NH4 + -N concentration reached the ¨ Discharge standard of pollutants for municipal wastewater treat¨ ment plants (GB18918-2002) first grade A standard when the depth of the soil was 120 cm. This showed that the soil column had a large degradation capacity for ammonia nitrogen. The NO3 − - N concentrations first presented rapid increases and then decreased in the soil column. The NO3 − - N concentrations were essentially stable when the soil depth reached 150 cm, indicating that the denitrification reaction has been completed. NO2 − - N is the intermediate of the denitrification reaction and is an important landmark material (Zhang et al., 2006). The fluctuation of the NO2 − - N concentration was very large in the different

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Fig. 5. Changes in pH (left) and oxygen concentration (right) with soil column depth.

TN was distinct for the different soil columns. TN removal in the traditional soil-infiltration system mainly relies on the reaction of anaerobic ammonia oxidation and the denitrification reaction, whose depths were 0–60 cm and 30–120 cm, respectively. TN removal in the saturated aeration system mainly relies on whole and partial denitrification reactions and partly relies on the reaction of anaerobic ammonia oxidation, whose depths are 60–90 cm, 60–120 cm and 0–60 cm, respectively. The effects of the two techniques on TN removal have no significant difference, but the oxygen environment in the aeration system is better. 3.4.4. Variation of TP with soil depth In all soil columns, the removal percentages of TP are greater than 90% at a 90 cm soil depth, so the effluent water quality could ¨ reach the Discharge standard of pollutants for municipal wastewater treatment plants” (GB18918-2002) first grade A standard. The removal percentage of TP reaches 99% at a 120 cm soil depth. In columns E and G, the TP is removed quickly in the 0–60 cm soil depth, and the removal percentage of TP exceeds 95% at 60 cm depth, at which point the concentration of TP begins a slow decline and is nearly removed at 60 cm depth. In column H, the degradation percentage of TP is the highest of the three columns in the 0–30 cm soil depth before significantly decreasing in the 30–90 cm depth. However, the removal percentage of TP ultimately surpasses 99% at the 120 cm depth (Fig. 8). Fig. 6. Changes in COD with soil column depth.

4. Conclusions soil columns, but the NO2 − - N concentrations tended to be zero at 180 cm soil depth, achieving a safe discharge standard. The TN concentration dropped stepwise with the increase in soil depth under different operational conditions and finally, at the 150 cm depth, they tended to be stable. The removal method of

(1) When using the combined system to treat simulated livestock wastewater, the removal percentages of COD, NH4 + -N and TP were as high as 95.12%, 98.52% and 99.98%, respectively, and the effluent water quality could reach the “Discharge stan-

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Fig. 7. Changes in four forms of nitrogen (NH4 + -N, TN, NO3 − - N and NO2 − - N) with soil column depth.

dard of pollutants for municipal wastewater treatment plants” (GB18918-2002) first grade A standard.

(2) Fine bubble aeration technology can effectively improve the nitrogen removal ability and low temperature adaptation ability of the deep-soil-infiltration system. In the limited situation

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in the National Science & Technology Pillar Program, and grant 201109024 from the National Environmental Protection Public Welfare Science and Technology Research Program of China. In addition, we thank the anonymous reviewers for their careful review of this manuscript and their valuable suggestions. References

Fig. 8. Changes in TP with soil column depth.

in which the inflow dissolved oxygen is 5 mg/L, the nitrogen removal effect is the greatest, up to 85.4%. However, the increase in oxygen saturation in pre-aeration is not conducive to TN removal. (3) Saturated aeration greatly reduces the required infiltration bed depth by decreasing the COD of wastewater to the first grade A standard. Under any condition, ammonia nitrogen can be effectively degraded from the depth of 0–90 cm. The total nitrogen (TN) shows a different stepwise degradation trend, while the phosphorus removal capacity of the strengthened aeration system is relatively stable. (4) The limited fine bubble aeration system, under the premise of maintaining the reaction activity of anaerobic ammonia oxidation in 0–30 cm, constructs a facultative anaerobic environment within the range of 45–90 cm and realizes the accumulation of nitrite nitrogen (NO2 − -N), which showed a high specific activity in the partial denitrification reaction. Moreover, by using a residual carbon source, the process of denitrification can be realized and efficient nitrogen removal can be achieved in the soil column, in which three functional areas of nitrogen removal are delineated. Acknowledgements This study was financially supported by grant 51208485 from the Natural Science Foundation, grants 2012BAJ21B06 and 2012BAJ21B04 as well as 2015BAL04B01 from the Key Projects

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