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Wastewater treatment by using kenaf in paddy soil and effect of dissolved oxygen concentration on efficiency
e c o l o g i c a l e n g i n e e r i n g 2 9 ( 2 0 0 7 ) 125–132
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/ecoleng
Wastewater treatment by using kenaf in paddy soil and effect of dissolved oxygen concentration on efficiency Kaoru Abe a,∗ , Yasuo Ozaki b a b
Soil Biochemistry Laboratory, National Institute for Agro-Environmental Science, 3-1-3 Kannondai, Tsukuba 305-8604, Japan Akita Prefectural University, 241-7 Shimoshinjo, Akita City 010-0195, Japan
a r t i c l e
i n f o
a b s t r a c t
Article history:
We previously reported that kenaf (Hibiscus cannabinus L.) planted in a zeolite-bed filter-ditch
Received 30 July 2005
system provided highly effective treatment of wastewater. Here we compared that system
Received in revised form
with treatment in fallow paddy fields irrigated in different ways in a greenhouse. Paddy soil
15 February 2006
was a useful alternative to zeolite as the bed filter material. The efficiency of removal of N
Accepted 26 February 2006
and P under furrow irrigation and flooding was 82–92% of that of the zeolite system. Most kenaf roots were distributed in water with a high dissolved oxygen (DO) concentration and a high redox potential; few roots grew in reducing soil under water. The roots distributed in
Keywords:
the water contributed most to wastewater treatment. A low DO concentration (0.3 mg L−1 )
Hibiscus cannabinus
decreased the efficiency of N and P removal. However, nightly low DO concentration (near
Nitrogen
0 mg L−1 ) alternating with daily high DO concentration did not seriously restrict the effi-
Phosphorus
ciency. An increase of alpha-naphthylamine oxidation activity in kenaf roots at low DO
Removal
concentration is discussed in regard to induction of an oxygen-protective enzyme.
Furrow irrigation
© 2006 Elsevier B.V. All rights reserved.
Roots oxidation activity
1.
Introduction
The increase in wastewater production due to increases in rural populations is triggering increased eutrophication of aquatic ecosystems (Fujita et al., 1986; Hidaka, 1990). In recent years, considerable attention has been directed toward wastewater treatment systems consisting of bed filters planted with emergent macrophytes (generally referred to as constructed wetlands), because of their low cost and ease of operation (Wolverton, 1986; Cooper and Hobson, 1989; Brix, 1993; Kivaisi, 2001; Vymazal, 2005). We think that using crops for wastewater treatment in constructed wetland systems will help to recycle nitrogen (N) and phosphorus (P) in rural areas. As the majority of the crops are terrestrial species, we designed bed filter ditches where both aquatic and terrestrial plants could grow well and remove N
∗
Corresponding author. Tel.: +81 29 838 8315; fax: +81 29 838 8199. E-mail address: [email protected] (K. Abe). 0925-8574/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ecoleng.2006.02.003
and P from wastewater (Abe et al., 1997; Ozaki, 1997; Abe and Ozaki, 1998, 1999). We found that kenaf (Hibiscus cannabinus L.) is highly effective for wastewater treatment in comparison with other crops (Abe et al., 1997; Abe and Ozaki, 1998, 1999) and is relatively tolerant of waterlogging. Further, kenaf is viewed as an ecofriendly crop because it can replace wood as a paper source (Robinson, 1988). In Japan, the Ministry of Agriculture, Forestry and Fisheries has moved to limit excess rice production by decreasing the area of rice cultivation (and encouraging the planting of other crops). Thus, the area of fallow paddy fields has been increasing. Kenaf would thus appear to be both a suitable substitute crop and a cost-effective tool for wastewater treatment. In this study, we compared the wastewater treatment efficiency of our previous zeolite-bed filter-ditch system with a
126
e c o l o g i c a l e n g i n e e r i n g 2 9 ( 2 0 0 7 ) 125–132
paddy treatment system planted with kenaf and irrigated in different ways. We also considered the influence of dissolved oxygen (DO) concentration on the efficiency of N and P removal by kenaf.
2.
Materials and methods
2.1. Experiment 1: N and P removal efficiency of paddy soil/kenaf and zeolite/kenaf ditch systems 2.1.1.
Experimental system design
Plant-bed filter ditches were prepared by setting baskets filled with filter material – zeolite (particle size 6–8 mm) or paddy soil (Gray Lowland soil) – in small lysimeters (1 m × 0.5 m × 0.4 m deep) (Fig. 1). The filter material surface was about 10 cm higher than the water level. Paddy treatment systems were prepared by filling lysimeters (1 m × 0.5 m × 0.4 m deep) directly with paddy soil (Fig. 1). Table 1 shows the experimental systems prepared. In 1999, four types of plant-bed filter ditch were prepared to compare paddy soil with zeolite as bed filter material. In 2000, the N and P removal efficiencies of paddy treatment system under furrow irrigation were compared with those of kenaf–zeolite-bed filter ditch. The effect of ridge height (3 or 10 cm higher than water level) in furrow irrigation on wastewater treatment was also examined. In 2001, the effect of irrigation method (continuous furrow irrigation, continuous flooding, or intermittent irrigation) on wastewater treatment was examined.
2.1.2.
composition and loading rate of the artificial wastewater. It contained 2.5–8.0 mg L−1 N and 0.7–0.9 mg L−1 P. The loading rates were 220–500 L m−2 d−1 water, 1.2–1.6 g m−2 d−1 N and 0.17–0.27 g m−2 d−1 P. Experiments were performed in a greenhouse with open windows in Tsukuba, Japan, from May to November.
2.1.3.
Sampling and analysis
The volume of wastewater inflow was measured by integration flow meter. The volume of outflow was measured by tippingwater flow meter. The water quality was analyzed once a week. Total N concentration was measured with a Mitsubishikasei T-N analyzer, and total P concentration was measured by the ascorbic acid method (APHA, AWWA, and WPFC, 1989) after persulfate digestion. Removal rate was calculated monthly as the difference between the rates of N and P in the wastewater entering and leaving the systems divided by lysimeter area. Redox potential (Eh ) was measured at 3 and 8 cm below the water level in the high-ridge system. Platinum-tipped electrodes were inserted into the soil. The potential was measured against an AgCl reference electrode with an ORP meter RM10P (DDK-TOA, Tokyo, Japan) and was converted to potential against a hydrogen electrode. The DO concentration of surface water in the continuously flooded system was monitored with a fluorescent O2 analyzer FC960 (ASR, Tokyo, Japan).
2.2. Experiment 2: effect of DO concentrations on removal of N and P by kenaf
Operational conditions
Twelve kenaf seedlings, 2 weeks after sowing, were transplanted into the lysimeter in May every year (Fig. 1). Some systems were left plant-free as a control (Table 1). Artificial wastewater simulating eutrophic water (influenced by domestic wastewater inflow) was continuously added at one end of the lysimeter and flowed out from the opposite end. Undiluted solution for preparation of artificial wastewater was automatically diluted with tap water to 1/1000 in a Kanneki diluter (Taiyo-kogyo) and supplied to each lysimeter at a constant flow rate. Table 2 shows the
2.2.1.
Experimental system design
Six containers (volume 20 L) were settled in the growth chamber (3 m × 3 m × 1.8 m high) under natural light. Artificial wastewater was added at one end of the container in continuous flow and drained out at the opposite end. To make the N and P concentration in the containers uniform, the container solution was mixed continuously by circulation (Fig. 2). Wastewater containing 5.5 mg L−1 N (NO3 -N: 5.0 mg L−1 ; NH4 -N: 0.5 mg L−1 ) and 1.0 mg L−1 P (PO4 -P: 1.0 mg L−1 ) was
Fig. 1 – Experimental system (small lysimeter).
127
2001 2001 Nothing Kenaf Paddy soil Intermittent irrigation/plant-free Intermittent irrigation/kenaf
Intermittent irrigation. Irrigation was stopped and field was drained for 1–2 days a week
2001 2001 Continuous flooding. Water depth 10 cm Paddy soil Flooding/plant-free Flooding/kenaf
Nothing Kenaf
2000, 2001 2000, 2001 Furrow irrigation. Ridge height 10 cm higher than water level. Water depth 10 cm High-ridge/plant-free High-ridge/kenaf
Paddy soil
Nothing Kenaf
2000 2000
1999
Furrow irrigation. Ridge height 3 cm higher than water level. Water depth 10 cm
Nothing Kenaf
Table 2 – Artificial wastewater compositions and loading rates by year
Paddy soil
Paddy soil Soil/plant-free Soil/kenaf
Low-ridge/plant-free Low-ridge/kenaf Paddy treatment system
1999 1999 Nothing Kenaf
1999 1999, 2000 Nothing Kenaf Setting baskets filled with filter material. Filter surface 10 cm higher than water level. Water depth 30 cm Zeolite Zeolite/plant-free Zeolite/kenaf Plant-bed filter ditch
System type
Table 1 – Experimental design
System name
Bed material
Irrigation condition
Plantings
Experimental year
e c o l o g i c a l e n g i n e e r i n g 2 9 ( 2 0 0 7 ) 125–132
2000
2001
T-N NO3 -N NH4 -N
−1
3.0 mg L 2.5 mg L−1 0.5 mg L−1
−1
5.0 mg L 4.0 mg L−1 1.0 mg L−1
8.0 mg L−1 7.0 mg L−1 1.0 mg L−1
T-P PO4 -P
0.7 mg L−1 0.7 mg L−1
0.9 mg L−1 0.9 mg L−1
0.7 mg L−1 0.7 mg L−1
Hydraulic loading rate 500 L m−2 d−1 300 L m−2 d−1 220 L m−2 d−1
supplied to each test container. The loading rate of wastewater was 24–48 L m−2 d−1 .
2.2.2.
Operational conditions
Five seedlings of kenaf, 3 weeks after sowing, were transferred to each test container in the growth chamber under natural light and were precultured for 4 weeks at 25 ◦ C. After preculture, the DO concentration was regulated to three levels: low (0.3 mg L−1 ), medium (2 mg L−1 ), and high (4–7 mg L−1 ), for 18 days, through the supply of N2 and air. DO concentration was controlled by switching the air and N2 lines on and off according to a feedback signal from the polarographic O2 sensor (Mettler Toledo). Air and water temperatures were maintained at 25 ◦ C during the experiment.
2.2.3.
Sampling and analysis
The N and P concentrations in the containers and the volume of inflow and outflow were measured. The total N and P concentrations were measured as in Experiment 1. The DO concentration of water was measured by polarographic O2 sensor (Mettler Toledo). After the experiment finished, fresh roots were cut into 2cm pieces, and oxidation activity was measured by the alphanaphthylamine oxidation method (Futami, 1990). The average removal rate was calculated during the last 4 days of the DO regulation period. The removal rate per biomass was calculated as the removal rate divided by the plant dry weight (80 ◦ C air dry).
3.
Results
3.1. Experiment 1: N and P removal efficiency of several paddy soil/kenaf systems and zeolite/kenaf ditch system Kenaf improved N and P removal efficiency in all systems examined. N and P concentrations in the effluents from the soil/kenaf system and from the paddy with furrow irrigation (both high-ridge/kenaf and low-ridge/kenaf) were only a little higher than that in the zeolite/kenaf system during the first 2 months. Thereafter, concentrations in those systems became similar (Figs. 3 and 4). The average rates of N and P removal in the furrow irrigation/kenaf system (low-ridge/kenaf and high-ridge/kenaf) were 0.98 ± 0.33–1.05 ± 0.38 g m−2 d−1 N and 0.130 ± 0.031–0.138 ± 0.054 g m−2 d−1 P, equal to 87–92% and 84–90% of that in the zeolite/kenaf system (Tables 3 and 4). The effect of ridge height (3 or 10 cm above water level) was not clear.
128
e c o l o g i c a l e n g i n e e r i n g 2 9 ( 2 0 0 7 ) 125–132
Fig. 2 – System used to regulate dissolved oxygen (DO) concentration.
Fig. 3 – Nitrogen concentration in influent (solid line) and effluents.
Fig. 4 – Phosphorus concentration in influents and effluents.
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e c o l o g i c a l e n g i n e e r i n g 2 9 ( 2 0 0 7 ) 125–132
Table 3 – Average nitrogen loading and removal rates from July to October Year
Plantings
Removal rate (g m−2 d−1 )
Loading rate
(g m−2
d−1 ) Zeolitebed filter
Soilbed filter
1999 2000 2001
Kenaf Kenaf Kenaf
1.57 1.55 1.76
1.34 1.13
1.25
1999 2000 2001
Plant-free Plant-free Plant-free
1.57 1.55 1.76
0.02
0.09
Lowridge
Highridge
0.99
1.05 0.98
0.87
0.61
0.16 0.19
0.19
0.17
0.208
Flooding
Intermittent irrigation
LSD (0.05) for removal rate (g m−2 d−1 ): 0.39 (1999), 0.40 (2000), 0.23 (2001).
Table 4 – Average phosphorus loading and removal rates from July to October Year
Plantings
Loading rate (g m−2 d−1 )
Removal rate (g m−2 d−1 ) Zeolitebed filter
Soil-bed filter 0.243
1999 2000 2001
Kenaf Kenaf Kenaf
0.369 0.278 0.167
0.263 0.154
1999 2000 2001
Plant-free Plant-free Plant-free
0.369 0.279 0.166
0.047
Lowridge
Highridge
Flooding
Intermittent irrigation
0.130
0.139 0.094
0.089
0.056
0.027 0.022
0.022
0.022
0.035 0.032
LSD (0.05) for removal rate (g m−2 d−1 ): 0.116 (1999), 0.062 (2000), 0.025 (2001).
In the experiment evaluating several irrigation methods in the paddy field in 2001, the average N and P removal rate of the high-ridge/kenaf system was the highest (Tables 3 and 4). Intermittent irrigation/kenaf removed less N and P than other systems containing kenaf (Figs. 3 and 4). This was because that sharp change between dry and wet conditions damaged the roots distributed in the soil surface (in the water in the flooded period). In contrast, under continuous flooding, kenaf seedlings grew well. The average N and P removal efficiencies of the flooding/kenaf system were 0.87 and 0.089 g m−2 d−1 (equal to 88% and 95% of that in the furrow irrigation/kenaf system) (Tables 3 and 4). Redox potential (Eh ) in the ridge soil and furrow water in the high-ridge/kenaf system was measured. Eh at 3 and 8 cm below water level in the ridge was low, −150 to 200 mV, but
that in the furrow water was high, 500–800 mV (Fig. 5). The major part (47%) of kenaf roots was distributed in the oxidative furrow water, and made a thick root mat. Only 15% was distributed in the soil under the water. Kenaf developed a root mat in the water in the flooding systems. Most roots (97%) were distributed in the water, and the rest in the soil. DO concentrations in the effluent changed synchronously with solar radiation, increasing to 8–10 mg L−1 during the daytime with strong solar radiation and decreasing to 0 mg L−1 at night (Fig. 6). However, DO in the water near the inflow gate (cock) varied from 1 to 4 mg L−1 , and showed no circadian variation (Fig. 6). We consider that algae in the paddy water supplied oxygen by photosynthesis during the daytime and consumed oxygen by respiration at night.
Fig. 5 – Redox potential (Eh ) in the water and ridge soil in the high-ridge/kenaf system.
130
e c o l o g i c a l e n g i n e e r i n g 2 9 ( 2 0 0 7 ) 125–132
Fig. 6 – Changes in DO concentration in water in flooding/kenaf system.
Fig. 8 – DO concentration in the solution and root oxidation activity (␣-naphthylamine oxidation rate).
3.2. Experiment 2: effect of DO concentrations on removal of N and P by kenaf The DO concentrations in the six containers were 4.41 ± 0.62 and 4.64 ± 0.66 mg L−1 at the high DO level, 1.60 ± 0.41 and 1.87 ± 0.08 mg L−1 at the medium level, and 0.32 ± 0.27 and 0.22 ± 0.10 mg L−1 at the low level. Exposing roots to low DO concentration (about 0.3 mg L−1 ) for 18 days tended to depress biomass production and wastewater treatment efficiency of young kenaf (Fig. 7). Chlorosis was observed in the upper leaves. The N and P removal rates at low DO concentration were 50–51% and 65–74% of those at high and medium DO concentrations. The N and P removal rates per kenaf biomass were also depressed at low DO concentration. The alpha-naphthylamine oxidation rate was high at low DO concentration (Fig. 8).
4.
Discussion
To evaluate the N and P removal efficiency of the zeolite/kenaf system and several paddy irrigation systems containing kenaf examined in different years, we expressed removal efficiency relative to the removal efficiency of the zeolite/kenaf system, which was defined as 100 (Fig. 9). As the zeolite-bed system was not examined in 2001, we calculated the index value of each system by referring to the removal rate of the
Fig. 9 – Comparison of the zeolite/kenaf system with other kenaf systems in N and P removal efficiency. (Removal rate of each system is expressed relative to that of the zeolite/kenaf system, defined as 100).
high-ridge/kenaf system in 2001 and the index value of the high-ridge/kenaf system in 2000. The index value of each system was calculated by the following equations: In 1999 and 2000: (index value of each system) = (removal efficiency of each system)/(removal efficiency of the zeolite/kenaf system) × 100. In 2001: (index value of each system) = (removal rate of each system in 2001)/(removal rate of high-ridge/kenaf system in 2001) × (index value of high-ridge/kenaf system in 2000).
Fig. 7 – Effect of DO concentration on N and P removal efficiency and biomass production of kenaf.
e c o l o g i c a l e n g i n e e r i n g 2 9 ( 2 0 0 7 ) 125–132
We reported that a zeolite-bed filter ditch planted with kenaf is highly effective for wastewater treatment (Abe et al., 1997; Abe and Ozaki, 1998, 1999). The paddy soil/kenaf system exhibited high N and P removal efficiency, showing that paddy soil is a useful alternative to zeolite as bed material in plant-bed filter ditches. The N and P removal efficiencies of the furrow irrigation (high-ridge/kenaf) and continuous flooding (flooding/kenaf) systems were 82–92% of that of the zeolite/kenaf system (Fig. 9). These irrigation methods with kenaf are thus effective for wastewater treatment in paddies. Kenaf roots grew into a thick mat in water with a high DO concentration and high Eh , but few roots grew in reducing soil under water. This indicates that the roots growing in the water contributed to the wastewater treatment. The effect of DO concentration in the culture solution on plant growth had been studied in some hydroponic horticultural crops. The dry weight of tomato plants was depressed at 1–2 mg L−1 DO at 25 ◦ C, but that of cucumber was not (Guo and Tachibana, 1997). The dry weight of lettuce plants was decreased at 0.01 mM DO (0.32 mg L−1 ) but not at 0.1 mM DO (3.2 mg L−1 ) (Yoshida et al., 1997). Our DO regulation experiment (Experiment 2) suggests that kenaf is more tolerant of low DO concentration than tomato. Kenaf dry weight was not reduced at 2 and 4.5 mg L−1 DO, but was reduced at 0.3 mg L−1 DO. The N and P removal efficiency was clearly inhibited when DO was about 0.3 mg L−1 during several days, and the upper leaves became chlorotic. We consider that ATP production in kenaf root was not high enough to maintain normal N and P uptake at that DO concentration. In contrary, the alpha-naphthylamine oxidation rate was increased. alpha-Naphthylamine is oxidized by H2 O2 (Tatsumi, 1998). Thus, alpha-naphthylamine oxidation activity gives an indirect measure of peroxidase activity. The activity of superoxide dismutase (catalyzing the conversion of O2 − to H2 O2 and oxygen) was, paradoxically, higher under O2 depletion than under aerobic condition in the roots or rhizomes of some wetland plants (Lepidium latifolium L. and Iris pseudacorus L.) (Monk et al., 1987; Chen and Qualls, 2003). Those reports discuss the possible critical role of oxygenprotective enzymes during oxygen depletion during recovery from anoxic stress. The increase of alpha-naphthylamine oxidation activity in kenaf roots suggests the induction of oxygen-protective enzymes under hypoxia. In the kenaf/flooding system (Experiment 1), where kenaf grew well and removal efficiency was high, DO concentrations in the effluent increased to 8–10 mg L−1 during the daytime and decreased to 0 mg L−1 at night. Thus, this brief drop in DO concentration did not seriously restrict the wastewater treatment efficiency of kenaf. But when wastewater rich in organic compounds is applied, the DO concentration might drop to near 0 mg L−1 even during the daytime because of the decomposition of the organic compounds, so kenaf is not recommended for treatment of these kinds of wastewater.
5.
Conclusions
(1) Paddy soil was a useful alternative to zeolite as bed material in the plant-bed filter-ditch system. The N and P removal efficiencies of the furrow irrigation and flooding
131
systems were 82–92% of that of the zeolite-bed filter-ditch system. These methods were effective for wastewater treatment by kenaf in a paddy. (2) The major part of kenaf roots were distributed in water with a high DO concentration and a high Eh . These roots contributed most to wastewater treatment. (3) The N and P removal efficiency of kenaf was decreased at long-term low DO concentration (0.3 mg L−1 ). But the brief nightly drop in DO concentration to near 0 mg L−1 in the flooding/kenaf system did not seriously restrict the N and P removal efficiency.
references
Abe, K., Ozaki, Y., 1998. Comparison of useful terrestrial and aquatic plant species for removal of nitrogen and phosphorus from domestic wastewater. Soil Sci. Plant Nutr. 44, 599– 607. Abe, K., Ozaki, Y., 1999. Evaluation of useful plants for the treatment of polluted pond water with low N and P concentrations. Soil Sci. Plant Nutr. 45, 409–417. Abe, K., Ozaki, Y., Kihou, N., 1997. Introduction of fiber plants to plant-bed filter systems for wastewater treatment in relation to resource recycling. Soil Sci. Plant Nutr. 43, 35–43. APHA, AWWA, WPFC, 1989. In: Ciesceri, L.S., Greenberg, A.E., Trussell, R.R. (Eds.), Standard Methods for the Examination of Water and Wastewater. APHA, AWWA, WPFC, Washington, DC. Brix, H., 1993. Wastewater treatment in constructed wetland: system design, removal process, and treatment performance. In: Moshiri, G.A. (Ed.), Constructed Wetland for Water Quality Improvement. CRC Press Inc., Boca Raton, FL, pp. 9–22. Chen, H., Qualls, R.G., 2003. Anaerobic metabolism in the roots of seedlings of the invasive exotic Lepidium latifolium. Environ. Exp. Bot. 50, 29–40. Cooper, P.F., Hobson, A., 1989. Sewage treatment by reed bed systems: the present situation in the United Kingdom. In: Hammer, D.A. (Ed.), Constructed Wetlands for Wastewater Treatment. Lewis Publishers Inc., Chelsea, MI, pp. 153– 171. Fujita, T., Gomi, T., Kotani, M., 1986. Small irrigation ponds in Osaka Prefecture. J. JSIDRE 54, 623–630. Futami, K., 1990. Measurement of root activity. In: Editorial Board of Methods of Plant Nutrition Experiment (Ed.), Methods of Plant Nutrition Experiment. Hakuyusha, Tokyo, pp. 50–52. Guo, S.R., Tachibana, S., 1997. Effect of dissolved O2 levels in nutrient solution on the growth and mineral nutrition of tomato and cucumber seedlings. J. Jpn. Soc., Horti. Sci. 66 (2), 331–337. Hidaka, S., 1990. Studies on effect of irrigation water quality upon the rice growth and soil, and its usable marginal concentration. Bull. Sait. Agric. Exp. Stn., 44. Kivaisi, K.A., 2001. The potential for construct wetlands for wastewater treatment and reuse in developing countries: a review. Ecol. Eng. 16, 545–560. Monk, L.S., Fagerstedt, K.V., Crawford, M.M., 1987. Superoxide dismutase as anaerobic polypeptide. Plant Physiol. 85, 1016–1020. Ozaki, Y., 1997. Development of domestic wastewater treatment by using plants usable for human life with resource recycling and symbiosis. Jpn. J. Water Treat. Biol. 33, 97–107.
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Robinson, F.E., 1988. Kenaf: a new fiber crop for paper production. Cal. Agric. 42, 31–32. Tatsumi, J., 1998. Root oxidation activity. In: Editorial Board of Dictionary of Root (Ed.), Dictionary of Root. Asakura Shoten, Tokyo, pp. 343–345. Vymazal, J., 2005. Horizontal and sub-surface flow and hybrid constructed wetlands systems for wastewater treatment. Ecol. Eng. 25, 478–490.
Wolverton, B.C., 1986. Artificial marshes for wastewater treatment. In: Reddy, K.R., Smith, W.H. (Eds.), Aquatic Plants for Wastewater treatment and Resource Recovery. Magnolia Publishing Inc., Orlando, FL, pp. 153–174. Yoshida, S., Kitano, M., Eguchi, H., 1997. Growth of lettuce (Lactuca sativa L.) under control of dissolved O2 concentration in hydroponics. Biotronics 26, 39–45.
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/ecoleng
Wastewater treatment by using kenaf in paddy soil and effect of dissolved oxygen concentration on efficiency Kaoru Abe a,∗ , Yasuo Ozaki b a b
Soil Biochemistry Laboratory, National Institute for Agro-Environmental Science, 3-1-3 Kannondai, Tsukuba 305-8604, Japan Akita Prefectural University, 241-7 Shimoshinjo, Akita City 010-0195, Japan
a r t i c l e
i n f o
a b s t r a c t
Article history:
We previously reported that kenaf (Hibiscus cannabinus L.) planted in a zeolite-bed filter-ditch
Received 30 July 2005
system provided highly effective treatment of wastewater. Here we compared that system
Received in revised form
with treatment in fallow paddy fields irrigated in different ways in a greenhouse. Paddy soil
15 February 2006
was a useful alternative to zeolite as the bed filter material. The efficiency of removal of N
Accepted 26 February 2006
and P under furrow irrigation and flooding was 82–92% of that of the zeolite system. Most kenaf roots were distributed in water with a high dissolved oxygen (DO) concentration and a high redox potential; few roots grew in reducing soil under water. The roots distributed in
Keywords:
the water contributed most to wastewater treatment. A low DO concentration (0.3 mg L−1 )
Hibiscus cannabinus
decreased the efficiency of N and P removal. However, nightly low DO concentration (near
Nitrogen
0 mg L−1 ) alternating with daily high DO concentration did not seriously restrict the effi-
Phosphorus
ciency. An increase of alpha-naphthylamine oxidation activity in kenaf roots at low DO
Removal
concentration is discussed in regard to induction of an oxygen-protective enzyme.
Furrow irrigation
© 2006 Elsevier B.V. All rights reserved.
Roots oxidation activity
1.
Introduction
The increase in wastewater production due to increases in rural populations is triggering increased eutrophication of aquatic ecosystems (Fujita et al., 1986; Hidaka, 1990). In recent years, considerable attention has been directed toward wastewater treatment systems consisting of bed filters planted with emergent macrophytes (generally referred to as constructed wetlands), because of their low cost and ease of operation (Wolverton, 1986; Cooper and Hobson, 1989; Brix, 1993; Kivaisi, 2001; Vymazal, 2005). We think that using crops for wastewater treatment in constructed wetland systems will help to recycle nitrogen (N) and phosphorus (P) in rural areas. As the majority of the crops are terrestrial species, we designed bed filter ditches where both aquatic and terrestrial plants could grow well and remove N
∗
Corresponding author. Tel.: +81 29 838 8315; fax: +81 29 838 8199. E-mail address: [email protected] (K. Abe). 0925-8574/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ecoleng.2006.02.003
and P from wastewater (Abe et al., 1997; Ozaki, 1997; Abe and Ozaki, 1998, 1999). We found that kenaf (Hibiscus cannabinus L.) is highly effective for wastewater treatment in comparison with other crops (Abe et al., 1997; Abe and Ozaki, 1998, 1999) and is relatively tolerant of waterlogging. Further, kenaf is viewed as an ecofriendly crop because it can replace wood as a paper source (Robinson, 1988). In Japan, the Ministry of Agriculture, Forestry and Fisheries has moved to limit excess rice production by decreasing the area of rice cultivation (and encouraging the planting of other crops). Thus, the area of fallow paddy fields has been increasing. Kenaf would thus appear to be both a suitable substitute crop and a cost-effective tool for wastewater treatment. In this study, we compared the wastewater treatment efficiency of our previous zeolite-bed filter-ditch system with a
126
e c o l o g i c a l e n g i n e e r i n g 2 9 ( 2 0 0 7 ) 125–132
paddy treatment system planted with kenaf and irrigated in different ways. We also considered the influence of dissolved oxygen (DO) concentration on the efficiency of N and P removal by kenaf.
2.
Materials and methods
2.1. Experiment 1: N and P removal efficiency of paddy soil/kenaf and zeolite/kenaf ditch systems 2.1.1.
Experimental system design
Plant-bed filter ditches were prepared by setting baskets filled with filter material – zeolite (particle size 6–8 mm) or paddy soil (Gray Lowland soil) – in small lysimeters (1 m × 0.5 m × 0.4 m deep) (Fig. 1). The filter material surface was about 10 cm higher than the water level. Paddy treatment systems were prepared by filling lysimeters (1 m × 0.5 m × 0.4 m deep) directly with paddy soil (Fig. 1). Table 1 shows the experimental systems prepared. In 1999, four types of plant-bed filter ditch were prepared to compare paddy soil with zeolite as bed filter material. In 2000, the N and P removal efficiencies of paddy treatment system under furrow irrigation were compared with those of kenaf–zeolite-bed filter ditch. The effect of ridge height (3 or 10 cm higher than water level) in furrow irrigation on wastewater treatment was also examined. In 2001, the effect of irrigation method (continuous furrow irrigation, continuous flooding, or intermittent irrigation) on wastewater treatment was examined.
2.1.2.
composition and loading rate of the artificial wastewater. It contained 2.5–8.0 mg L−1 N and 0.7–0.9 mg L−1 P. The loading rates were 220–500 L m−2 d−1 water, 1.2–1.6 g m−2 d−1 N and 0.17–0.27 g m−2 d−1 P. Experiments were performed in a greenhouse with open windows in Tsukuba, Japan, from May to November.
2.1.3.
Sampling and analysis
The volume of wastewater inflow was measured by integration flow meter. The volume of outflow was measured by tippingwater flow meter. The water quality was analyzed once a week. Total N concentration was measured with a Mitsubishikasei T-N analyzer, and total P concentration was measured by the ascorbic acid method (APHA, AWWA, and WPFC, 1989) after persulfate digestion. Removal rate was calculated monthly as the difference between the rates of N and P in the wastewater entering and leaving the systems divided by lysimeter area. Redox potential (Eh ) was measured at 3 and 8 cm below the water level in the high-ridge system. Platinum-tipped electrodes were inserted into the soil. The potential was measured against an AgCl reference electrode with an ORP meter RM10P (DDK-TOA, Tokyo, Japan) and was converted to potential against a hydrogen electrode. The DO concentration of surface water in the continuously flooded system was monitored with a fluorescent O2 analyzer FC960 (ASR, Tokyo, Japan).
2.2. Experiment 2: effect of DO concentrations on removal of N and P by kenaf
Operational conditions
Twelve kenaf seedlings, 2 weeks after sowing, were transplanted into the lysimeter in May every year (Fig. 1). Some systems were left plant-free as a control (Table 1). Artificial wastewater simulating eutrophic water (influenced by domestic wastewater inflow) was continuously added at one end of the lysimeter and flowed out from the opposite end. Undiluted solution for preparation of artificial wastewater was automatically diluted with tap water to 1/1000 in a Kanneki diluter (Taiyo-kogyo) and supplied to each lysimeter at a constant flow rate. Table 2 shows the
2.2.1.
Experimental system design
Six containers (volume 20 L) were settled in the growth chamber (3 m × 3 m × 1.8 m high) under natural light. Artificial wastewater was added at one end of the container in continuous flow and drained out at the opposite end. To make the N and P concentration in the containers uniform, the container solution was mixed continuously by circulation (Fig. 2). Wastewater containing 5.5 mg L−1 N (NO3 -N: 5.0 mg L−1 ; NH4 -N: 0.5 mg L−1 ) and 1.0 mg L−1 P (PO4 -P: 1.0 mg L−1 ) was
Fig. 1 – Experimental system (small lysimeter).
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2001 2001 Nothing Kenaf Paddy soil Intermittent irrigation/plant-free Intermittent irrigation/kenaf
Intermittent irrigation. Irrigation was stopped and field was drained for 1–2 days a week
2001 2001 Continuous flooding. Water depth 10 cm Paddy soil Flooding/plant-free Flooding/kenaf
Nothing Kenaf
2000, 2001 2000, 2001 Furrow irrigation. Ridge height 10 cm higher than water level. Water depth 10 cm High-ridge/plant-free High-ridge/kenaf
Paddy soil
Nothing Kenaf
2000 2000
1999
Furrow irrigation. Ridge height 3 cm higher than water level. Water depth 10 cm
Nothing Kenaf
Table 2 – Artificial wastewater compositions and loading rates by year
Paddy soil
Paddy soil Soil/plant-free Soil/kenaf
Low-ridge/plant-free Low-ridge/kenaf Paddy treatment system
1999 1999 Nothing Kenaf
1999 1999, 2000 Nothing Kenaf Setting baskets filled with filter material. Filter surface 10 cm higher than water level. Water depth 30 cm Zeolite Zeolite/plant-free Zeolite/kenaf Plant-bed filter ditch
System type
Table 1 – Experimental design
System name
Bed material
Irrigation condition
Plantings
Experimental year
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2000
2001
T-N NO3 -N NH4 -N
−1
3.0 mg L 2.5 mg L−1 0.5 mg L−1
−1
5.0 mg L 4.0 mg L−1 1.0 mg L−1
8.0 mg L−1 7.0 mg L−1 1.0 mg L−1
T-P PO4 -P
0.7 mg L−1 0.7 mg L−1
0.9 mg L−1 0.9 mg L−1
0.7 mg L−1 0.7 mg L−1
Hydraulic loading rate 500 L m−2 d−1 300 L m−2 d−1 220 L m−2 d−1
supplied to each test container. The loading rate of wastewater was 24–48 L m−2 d−1 .
2.2.2.
Operational conditions
Five seedlings of kenaf, 3 weeks after sowing, were transferred to each test container in the growth chamber under natural light and were precultured for 4 weeks at 25 ◦ C. After preculture, the DO concentration was regulated to three levels: low (0.3 mg L−1 ), medium (2 mg L−1 ), and high (4–7 mg L−1 ), for 18 days, through the supply of N2 and air. DO concentration was controlled by switching the air and N2 lines on and off according to a feedback signal from the polarographic O2 sensor (Mettler Toledo). Air and water temperatures were maintained at 25 ◦ C during the experiment.
2.2.3.
Sampling and analysis
The N and P concentrations in the containers and the volume of inflow and outflow were measured. The total N and P concentrations were measured as in Experiment 1. The DO concentration of water was measured by polarographic O2 sensor (Mettler Toledo). After the experiment finished, fresh roots were cut into 2cm pieces, and oxidation activity was measured by the alphanaphthylamine oxidation method (Futami, 1990). The average removal rate was calculated during the last 4 days of the DO regulation period. The removal rate per biomass was calculated as the removal rate divided by the plant dry weight (80 ◦ C air dry).
3.
Results
3.1. Experiment 1: N and P removal efficiency of several paddy soil/kenaf systems and zeolite/kenaf ditch system Kenaf improved N and P removal efficiency in all systems examined. N and P concentrations in the effluents from the soil/kenaf system and from the paddy with furrow irrigation (both high-ridge/kenaf and low-ridge/kenaf) were only a little higher than that in the zeolite/kenaf system during the first 2 months. Thereafter, concentrations in those systems became similar (Figs. 3 and 4). The average rates of N and P removal in the furrow irrigation/kenaf system (low-ridge/kenaf and high-ridge/kenaf) were 0.98 ± 0.33–1.05 ± 0.38 g m−2 d−1 N and 0.130 ± 0.031–0.138 ± 0.054 g m−2 d−1 P, equal to 87–92% and 84–90% of that in the zeolite/kenaf system (Tables 3 and 4). The effect of ridge height (3 or 10 cm above water level) was not clear.
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Fig. 2 – System used to regulate dissolved oxygen (DO) concentration.
Fig. 3 – Nitrogen concentration in influent (solid line) and effluents.
Fig. 4 – Phosphorus concentration in influents and effluents.
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Table 3 – Average nitrogen loading and removal rates from July to October Year
Plantings
Removal rate (g m−2 d−1 )
Loading rate
(g m−2
d−1 ) Zeolitebed filter
Soilbed filter
1999 2000 2001
Kenaf Kenaf Kenaf
1.57 1.55 1.76
1.34 1.13
1.25
1999 2000 2001
Plant-free Plant-free Plant-free
1.57 1.55 1.76
0.02
0.09
Lowridge
Highridge
0.99
1.05 0.98
0.87
0.61
0.16 0.19
0.19
0.17
0.208
Flooding
Intermittent irrigation
LSD (0.05) for removal rate (g m−2 d−1 ): 0.39 (1999), 0.40 (2000), 0.23 (2001).
Table 4 – Average phosphorus loading and removal rates from July to October Year
Plantings
Loading rate (g m−2 d−1 )
Removal rate (g m−2 d−1 ) Zeolitebed filter
Soil-bed filter 0.243
1999 2000 2001
Kenaf Kenaf Kenaf
0.369 0.278 0.167
0.263 0.154
1999 2000 2001
Plant-free Plant-free Plant-free
0.369 0.279 0.166
0.047
Lowridge
Highridge
Flooding
Intermittent irrigation
0.130
0.139 0.094
0.089
0.056
0.027 0.022
0.022
0.022
0.035 0.032
LSD (0.05) for removal rate (g m−2 d−1 ): 0.116 (1999), 0.062 (2000), 0.025 (2001).
In the experiment evaluating several irrigation methods in the paddy field in 2001, the average N and P removal rate of the high-ridge/kenaf system was the highest (Tables 3 and 4). Intermittent irrigation/kenaf removed less N and P than other systems containing kenaf (Figs. 3 and 4). This was because that sharp change between dry and wet conditions damaged the roots distributed in the soil surface (in the water in the flooded period). In contrast, under continuous flooding, kenaf seedlings grew well. The average N and P removal efficiencies of the flooding/kenaf system were 0.87 and 0.089 g m−2 d−1 (equal to 88% and 95% of that in the furrow irrigation/kenaf system) (Tables 3 and 4). Redox potential (Eh ) in the ridge soil and furrow water in the high-ridge/kenaf system was measured. Eh at 3 and 8 cm below water level in the ridge was low, −150 to 200 mV, but
that in the furrow water was high, 500–800 mV (Fig. 5). The major part (47%) of kenaf roots was distributed in the oxidative furrow water, and made a thick root mat. Only 15% was distributed in the soil under the water. Kenaf developed a root mat in the water in the flooding systems. Most roots (97%) were distributed in the water, and the rest in the soil. DO concentrations in the effluent changed synchronously with solar radiation, increasing to 8–10 mg L−1 during the daytime with strong solar radiation and decreasing to 0 mg L−1 at night (Fig. 6). However, DO in the water near the inflow gate (cock) varied from 1 to 4 mg L−1 , and showed no circadian variation (Fig. 6). We consider that algae in the paddy water supplied oxygen by photosynthesis during the daytime and consumed oxygen by respiration at night.
Fig. 5 – Redox potential (Eh ) in the water and ridge soil in the high-ridge/kenaf system.
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Fig. 6 – Changes in DO concentration in water in flooding/kenaf system.
Fig. 8 – DO concentration in the solution and root oxidation activity (␣-naphthylamine oxidation rate).
3.2. Experiment 2: effect of DO concentrations on removal of N and P by kenaf The DO concentrations in the six containers were 4.41 ± 0.62 and 4.64 ± 0.66 mg L−1 at the high DO level, 1.60 ± 0.41 and 1.87 ± 0.08 mg L−1 at the medium level, and 0.32 ± 0.27 and 0.22 ± 0.10 mg L−1 at the low level. Exposing roots to low DO concentration (about 0.3 mg L−1 ) for 18 days tended to depress biomass production and wastewater treatment efficiency of young kenaf (Fig. 7). Chlorosis was observed in the upper leaves. The N and P removal rates at low DO concentration were 50–51% and 65–74% of those at high and medium DO concentrations. The N and P removal rates per kenaf biomass were also depressed at low DO concentration. The alpha-naphthylamine oxidation rate was high at low DO concentration (Fig. 8).
4.
Discussion
To evaluate the N and P removal efficiency of the zeolite/kenaf system and several paddy irrigation systems containing kenaf examined in different years, we expressed removal efficiency relative to the removal efficiency of the zeolite/kenaf system, which was defined as 100 (Fig. 9). As the zeolite-bed system was not examined in 2001, we calculated the index value of each system by referring to the removal rate of the
Fig. 9 – Comparison of the zeolite/kenaf system with other kenaf systems in N and P removal efficiency. (Removal rate of each system is expressed relative to that of the zeolite/kenaf system, defined as 100).
high-ridge/kenaf system in 2001 and the index value of the high-ridge/kenaf system in 2000. The index value of each system was calculated by the following equations: In 1999 and 2000: (index value of each system) = (removal efficiency of each system)/(removal efficiency of the zeolite/kenaf system) × 100. In 2001: (index value of each system) = (removal rate of each system in 2001)/(removal rate of high-ridge/kenaf system in 2001) × (index value of high-ridge/kenaf system in 2000).
Fig. 7 – Effect of DO concentration on N and P removal efficiency and biomass production of kenaf.
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We reported that a zeolite-bed filter ditch planted with kenaf is highly effective for wastewater treatment (Abe et al., 1997; Abe and Ozaki, 1998, 1999). The paddy soil/kenaf system exhibited high N and P removal efficiency, showing that paddy soil is a useful alternative to zeolite as bed material in plant-bed filter ditches. The N and P removal efficiencies of the furrow irrigation (high-ridge/kenaf) and continuous flooding (flooding/kenaf) systems were 82–92% of that of the zeolite/kenaf system (Fig. 9). These irrigation methods with kenaf are thus effective for wastewater treatment in paddies. Kenaf roots grew into a thick mat in water with a high DO concentration and high Eh , but few roots grew in reducing soil under water. This indicates that the roots growing in the water contributed to the wastewater treatment. The effect of DO concentration in the culture solution on plant growth had been studied in some hydroponic horticultural crops. The dry weight of tomato plants was depressed at 1–2 mg L−1 DO at 25 ◦ C, but that of cucumber was not (Guo and Tachibana, 1997). The dry weight of lettuce plants was decreased at 0.01 mM DO (0.32 mg L−1 ) but not at 0.1 mM DO (3.2 mg L−1 ) (Yoshida et al., 1997). Our DO regulation experiment (Experiment 2) suggests that kenaf is more tolerant of low DO concentration than tomato. Kenaf dry weight was not reduced at 2 and 4.5 mg L−1 DO, but was reduced at 0.3 mg L−1 DO. The N and P removal efficiency was clearly inhibited when DO was about 0.3 mg L−1 during several days, and the upper leaves became chlorotic. We consider that ATP production in kenaf root was not high enough to maintain normal N and P uptake at that DO concentration. In contrary, the alpha-naphthylamine oxidation rate was increased. alpha-Naphthylamine is oxidized by H2 O2 (Tatsumi, 1998). Thus, alpha-naphthylamine oxidation activity gives an indirect measure of peroxidase activity. The activity of superoxide dismutase (catalyzing the conversion of O2 − to H2 O2 and oxygen) was, paradoxically, higher under O2 depletion than under aerobic condition in the roots or rhizomes of some wetland plants (Lepidium latifolium L. and Iris pseudacorus L.) (Monk et al., 1987; Chen and Qualls, 2003). Those reports discuss the possible critical role of oxygenprotective enzymes during oxygen depletion during recovery from anoxic stress. The increase of alpha-naphthylamine oxidation activity in kenaf roots suggests the induction of oxygen-protective enzymes under hypoxia. In the kenaf/flooding system (Experiment 1), where kenaf grew well and removal efficiency was high, DO concentrations in the effluent increased to 8–10 mg L−1 during the daytime and decreased to 0 mg L−1 at night. Thus, this brief drop in DO concentration did not seriously restrict the wastewater treatment efficiency of kenaf. But when wastewater rich in organic compounds is applied, the DO concentration might drop to near 0 mg L−1 even during the daytime because of the decomposition of the organic compounds, so kenaf is not recommended for treatment of these kinds of wastewater.
5.
Conclusions
(1) Paddy soil was a useful alternative to zeolite as bed material in the plant-bed filter-ditch system. The N and P removal efficiencies of the furrow irrigation and flooding
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systems were 82–92% of that of the zeolite-bed filter-ditch system. These methods were effective for wastewater treatment by kenaf in a paddy. (2) The major part of kenaf roots were distributed in water with a high DO concentration and a high Eh . These roots contributed most to wastewater treatment. (3) The N and P removal efficiency of kenaf was decreased at long-term low DO concentration (0.3 mg L−1 ). But the brief nightly drop in DO concentration to near 0 mg L−1 in the flooding/kenaf system did not seriously restrict the N and P removal efficiency.
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