e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 576–581 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/ecoleng Phosphorus removal ability of three inexpensive substrates: Physicochemical properties and application Baohua Guan a,b , Xin Yao b , Jinhui Jiang a , Ziqiang Tian c , Shuqing An a,∗ , Binhe Gu d , Ying Cai a a The State Key Laboratory of Pollution Control and Resource Reuse, School of Life Sciences, Nanjing University, Nanjing 210093, PR China Key Laboratory of Lake Science & Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences, Nanjing 210008, PR China c The Institute of Water Science, Chinese Academy of Environmental Science, Beijing, PR China d University of South Florida, St. Petersburg, FL, USA b a r t i c l e i n f o a b s t r a c t Article history: Eutrophication of shallow freshwater lakes is a severe ecological problem all over the world. Received 4 October 2007 Phosphorus is one of the main triggering nutrients responsible for eutrophication of shallow Received in revised form freshwater lakes. Constructed wetlands provide an effective means of phosphorus removal 9 April 2008 from enriched waters. The cost of a constructed wetland depends mainly on the sub- Accepted 15 April 2008 strates ﬁlled within it. Here we used three inexpensive substrates including loess, cinder and limestone to construct a wetland along the shore of Lake Taihu, and then tested the total phosphorus (TP) removal ability of the wetland, the chemical and physical character- Keywords: istics, and the abilities of phosphorus adsorption and inhibition of substrates. We found Constructed wetland that the artiﬁcial wetland constructed with layered loess, cinder, and limestone but without Substrates selection hydrophytes and mesophytes had high phosphorus removal ability during the 44-day test, Physicochemical properties especially in the ﬁrst 10 days. The average removal rate for TP was 41% for the overall testing Phosphorus adsorption ability time. Chemical properties of the substrates had stronger impacts on phosphorus removal Phosphorus inhibition ability than physical properties did. Among the three substrates, limestone had the highest phosphorus removal and inhibition ability due to its highest calcium content. This suggests that more attention should be paid to chemical composition during the selection and assembly of substrates for both constructed wetland study and its engineering applications. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Phosphorus is a key nutrient element responsible for the eutrophication of freshwaters (Schindler, 1974; Baldy et al., 2007). Sufﬁcient phosphorus removal from enriched lake waters is an effective method for overcoming the eutrophication problem (Schindler, 1974). The constructed wetland (CW) is a common technique for removing phosphorus from eutrophic water bodies instead of the natural wetland in areas where natural wetlands are in short supply or have ∗ been destroyed (Vymazal, 2006). Compared to natural wetlands, the CW needs a smaller surface to treats the same wastewaters, so it is easily applied in speciﬁc cases (Prochaska and Zouboulis, 2006). CWs have signiﬁcant efﬁciencies in removing total phosphorus (TP), on average at least 40%, from the wastewater (Verhoeven and Meuleman, 1999; Vymazal, 2006). Furthermore, more and more scientists have found that the major mechanisms for removing phosphorus from eutrophic water by CWs are chemical adsorption and sedimentation by sub- Corresponding author. Tel.: +86 25 83594560; fax: +86 25 83594560. E-mail addresses: [email protected] (B. Guan), [email protected] (S. An). 0925-8574/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ecoleng.2008.04.015 e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 576–581 strates, rather than plant uptake and microbe removal (Tanner et al., 1999; Mitsch and Gosselink, 2000). Thus, the selection and assembly of substrates are very important for wetland construction. For cost concerns, basic substrates such as loess, cinder, and limestone are widely used in CWs (Dinges, 1982). In order to construct a high efﬁciency wetland with relatively inexpensive substrates, scientists try to correlate the substrates’ physical and chemical characteristics with the efﬁciency of phosphorus removal (Gray et al., 2000). Studies have shown that the physical characteristics of substrates, including granule size, speciﬁc surface area, consistency and porosity, signiﬁcantly impact phosphorus removal effectiveness; at the same time, the chemical composition of the substrates has a great impact on the adsorption and inhibition efﬁciency of phosphorus in constructed wetlands (Dinges, 1982; McEldowney et al., 1993; Garcia et al., 2004). Thus, it is necessity to test which one, the physical or chemical characteristics of the applied substrates, mainly decides the phosphorus removal efﬁciency of CW. Lake Taihu is a large shallow lake in eastern China, which is now troubled by eutrophication (Chen and Mynett, 2003; Qin et al., 2007). It is urgent to seek an inexpensive and highly efﬁcient CW as one of the main means to abate inﬂuent phosphorus loadings and purify the enriched lake water. In order to establish a stable and effective constructed wetland, we used three facile substrates along Lake Taihu, including loess, cinder and limestone, to construct an experimental and demonstration wetland. The physicochemical properties, adsorption and inhibition ability of different substrate were studied after analyzing the phosphorus removal efﬁciency of the CW. The aims of this paper are to propose useful information on choice and application of substrates in constructed wetlands for which the goal is phosphorus removal, and to provide some helpful information for loading abatement and wastewater treatment of Lake Taihu. 2. Materials and methods 2.1. Research site and basic circumstance Lake Taihu is one of the ﬁve largest freshwater lakes in China. It is located in the centre of the Yangtze River Delta, Eastern China (30◦ 55 40 N to 31◦ 32 58 N, 119◦ 52 32 E to 120◦ 36 10 E). With optimum climate (Li et al., 2008), there are approximately 32 million inhabitants (867.33 capitals per km2 ) and about 75% of the arable land is used for rice cultivation in the densely populated and intensively cropped Lake Taihu drainage basin. This area is also one of the most economically developed and rapidly urbanized areas in China (Zhang et al., 2003; Niu et al., 2004). The eutrophication of the lake is accelerating with economic growth and urbanization (Chen and Mynett, 2003). The mean TP of the whole lake surface water was generally more than 0.14 ± 0.03 mg L−1 after the year 2000 (Niu et al., 2004). 2.2. Setup of a constructed wetland A subsurface ﬂow CW with 140 m × 10 m area was constructed as an experimental and demonstration wetland along the 577 Fig. 1 – Flow chart of water cycle across the constructed wetland and Lake Taihu. shore of Lake Taihu. Eutrophic lake water was pumped via polyvinyl chloride (PVC) pipe into an artiﬁcial trench along the CW and then ﬂowed into the CW. There was another parallel artiﬁcial trench dug along the opposite side of the CW to discharge the puriﬁed water (Fig. 1). The three layer substrates ﬁlled in the CW from bottom to surface were limestone, cinder, and loess with the thickness of 20, 20 and 10 cm, respectively. In order to avoid loss of loess along the interspaces of limestone and cinder, a layer of fabric was laid under the layers loess. A pump working about 10 h a day (from 7:00 h to 17:00 h) with a hydraulic loading rate of 11 m3 s−1 was used to pump enriched lake water to the CW. Total phosphorus concentration of water pumped from Lake Taihu was 0.63 ± 0.22 mg L−1 . 2.3. CW Measurement of phosphorus removal rate for the After pumping 2 h each day, samples were collected from the inﬂuent and efﬂuent trenches of the wetland, and the sample days were the 1st, 2nd, 7th, 11th, 20th, 43rd and 44th day. Both the inﬂuent and efﬂuent trenches had six sample sites, each of which was twin-sampled four times with 2-h intervals from 9:00 h to 15:00 h for each day. These samples were transported to the laboratory in polyethylene (PE) bottles and analyzed within 24 h. The total phosphorus was determined by molybdate reagent colorimetry after HClO4 –H2 SO4 digestion (Marr and Cresser, 1983). 2.4. Measurement of physicochemical properties for the substrates The particle size of these three substrates was measured following the method of Drizo et al. (1999). A pycnometer was used to determine the true and bulk densities (Lee et al., 1997). The porosity of substrates was determined by the Standard Soil 578 e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 576–581 Fig. 2 – Daily dynamic (left) and longtime TP removal rate (right) by the wetland constructed with loess, cinder and limestone. Science Procedure based on estimations of bulk density and true density (Klute, 1986). Speciﬁc surface areas were determined through the N2 gas adsorption method (−195.698 ◦ C) (Davis and Kent, 1990). The collected samples for loess, cinder and limestone from the CW were ground and then sieved using a 0.25 mm sieve mesh. The powders of these three substrates were washed two times with dilute hydrochloric acid, and then dried 48 h in an oven at 80 ◦ C. The presences of chemical composition of the substrates was determined with X-ray ﬂuorescence diffraction analysis. Measured contents were expressed as per unit dry weight. These were performed by the Modern Analysis Centre of Nanjing University. Measured contents were expressed as ratio of mass. (BG), 2.000 g of the ground sediment powder was put into a taper bottle, and then was covered with 100 mL of distilled water. Three other treatments were to put 2.000 g sediment into each bottle, and then covered by 1 g loess, cinder or limestone, respectively before ﬁlled with distilled water. Each treatment had three replications. A period of 48 h allowed phosphate to be released from the substrates and further transferred into the added distilled water under 25 ◦ C. The total phosphorus concentration of the covering water was then measured. Higher phosphorus concentration in the covering water means lower phosphorus inhibition ability. The data were statistically analyzed by using SPSS (Ver. 12.0) if needed, with the signiﬁcant level at 0.05 by using post hoc with Duncan-test to indicate the differences. 2.5. Measurement of adsorption isotherms for the substrates 3. Results The dry and sieved substrates (2.000 g for each) were put into taper bottles (200 mL). Then, 100 mL of potassium phosphate monobasic (KH2 PO4 ), with a series of different concentrations from 2.5 to 40 mg L−1 , were put into taper bottles. A few drops of chloroform were put into the bottles to inhibit microbial bioactivity. Bottles were shaken on a rotating shaker (100 rpm) at constant temperature (25 ◦ C) for 48 h. Suspensions were then centrifuged and the covering waters were determined for phosphorus concentration. Phosphorus adsorption was calculated from the slightly modiﬁed Langmuir equation (Kuschk et al., 2003): 3.1. TP removal efﬁciency by the CW (C0 × V) − (Ct × V) M 3.2. Padsorption (mg kg−1 ) = The average TP concentration for all the 44 test days of the efﬂuent samples was 0.37 ± 0.13 mg L−1 , while the inﬂuent was 0.63 ± 0.22 mg L−1 ; thus, the total TP removal rate by the constructed wetland was 41%. The longtime average TP removal rate decreased from the 1st day to the 43rd day, and then had a stabilization (Fig. 2, right); the daily TP removal rate increased from 9:00 to 15:00 O’clock, and then decreased a little (Fig. 2, left). Physicochemical properties of substrates (1) where C0 is the original concentration of TP in the solution; Ct is the concentration at equilibrium; V is the volume of the solution; M is the mass of the substrate put into the bottle. 2.6. Measurement of phosphorus inhibition ability of the substrates Samples of hyper-enrichment sediment collected from Yangtze River were ground after it was air-dried, and then passed through a 0.25 mm mesh sieve. As the background The measured values of the physical parameters including true density, bulk density, porosity, diameter, and speciﬁc surface area are given in Table 1. Limestone had the highest true density, while cinder ashes had the lowest value. As for the bulk density, loess had the highest value, while cinder had the lowest one. Cinder had higher porosity, whereas limestone particles were lower in porosity than those of the other substrates. Except for cinder particles, limestone had the largest particle size among the other substrates in terms of the diameter. The speciﬁc surface area of the loess was much higher than those of cinder and limestone. 579 e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 576–581 Table 1 – Major physical properties of the three substrates used in the constructed wetland Substrates True density (g mL−1 ) Bulk density (g mL−1 ) Porosity (%) Diameter (mm) Speciﬁc surface area (m2 g−1 ) Loess Cinder Limestone 1.983 1.813 2.687 1.174 0.761 1.620 40.8 58.1 39.7 0.5–1.0 1.5–2.5 1.0–2.0 34.692 7.414 1.8912 Mean 2.129 1.224 43.4 0.8–1.6 11.979 Table 2 – Main chemical composition of the substrates used in the constructed wetland (mass %) SiO2 Al2 O3 Fe2 O3 CaO Ti2 O3 SO3 K2 O MgO Na2 O P2 O5 SrO Others Loess Cinder Limestone 37.2 46.4 1.36 10.9 30.9 0.38 5.2 8.96 0.13 28.5 2.2 35.5 0.77 1.3 0.02 1.5 1.0 44.3 2.5 1.3 0.08 2.9 0.48 0.20 0.36 0.37 <0.01 0.28 0.14 <0.01 0.13 0.08 0.36 <0.01 <0.01 <0.01 Mean 28.32 14.06 4.76 22.07 0.7 15.6 1.29 1.19 0.24 0.14 0.19 <0.01 The mean values for true density, bulk density, porosity, diameter and speciﬁc surface of the three substrates in constructing the wetland were 2.129 g mL−1 , 1.224 g mL−1 , 43.4%, 0.8–1.6 mm, 11.979 m2 g−1 , respectively. As for the main chemical composition of the substrates, loess had higher content of K2 O, MgO and P2 O5 , cinder occupied greater content of SiO2 , Al2 O3 , Fe2 O3 , Ti2 O3 , Na2 O, while limestone had more of CaO, SO3 , SrO than the other substrates (Table 2). The CW ﬁlled with loess, cinder and limestone had more than 10% of SiO2 , Al2 O3 , CaO and SO3 . 3.3. Adsorption isotherms of the substrates Limestone showed the greatest P adsorption among the three substrates, followed by loess and cinder (Table 3). The bonding capacity of cinder was the highest among the three substrates, following by the loess and limestone. P adsorption maxima varied between 2.002 and 0.890 mg g−1 , while bonding capacities varied between 0.254 and 0.100. The mean P adsorption and the bonding capacity of the wetland constructed by loess, cinder and limestone were 1.256 mg g−1 and 0.180. 3.4. Phosphorus inhibition ability of substrates There was signiﬁcant difference of the water TP concentration between background (BG) and the other treatments covering with the substrates (p < 0.05, Fig. 3). Furthermore, The TP concentrations in the water column were signiﬁcantly different (p < 0.05) between different substrates adding to cover the sed- iment. As for the TP inhibited rate, limestone had the maximal value, and cinder had the minimal one. 4. Discussion 4.1. Phosphorus removal ability of the constructed wetlands Constructed wetlands are engineered systems that have been designed and constructed to utilize natural processes to remove pollutants from wastewater (Sakadevan and Bavor, 1998; Vymazal, 2006). There are many kinds of constructed wetlands in terms of the waterways ﬂowing within the wetland beds (Mitsch and Gosselink, 2000). Removal rate of total nitrogen in most types of constructed wetlands with hydrophytes varies between 40 and 55%, while total phosphorus is around 40–60%. Without hydrophytes in the cold season, the removal rate for TP of the CW would be about 10% (Tanner, 1996; Peng et al., 2005; Vymazal, 2006). The constructed wetland we created with three inexpensive substrates along Lake Taihu had a mean TP removal rate of 41% during the 44 tested days with the maximum rate of 48% on the 1st day. Since there were no hydrophytes or wetland plants rooted in the constructed wetland, the efﬁciency was relatively higher in removing total phosphorus comparing Table 3 – P adsorption maxima, bonding capacities and correlation coefﬁcient, derived from the Langmuir equation Substrate P adsorption maximum (mg g−1 ) Bonding capacity Correlation coefﬁcient (r) Loess Cinder Limestone 1.082 0.890 2.002 0.204 0.254 0.163 0.658 0.472 0.523 Mean 1.256 0.180 0.437 Fig. 3 – Total phosphorus (TP) concentration in the covering water, and the TP inhibition rate of different substrates. 580 e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 576–581 with other CW without hydrophytes (Peng et al., 2005). With some wetland lawn grass growing on it, the constructed wetland costed about 400 yuan RMB per cubic meter (7.5 yuan = 1 USD as of December, 2007). Compared to other CWs in China, the costs is lower (Liu et al., in press). 4.2. Phosphorus removal ability and the physicochemical properties of substrates The physical and chemical properties determine the P adsorption capacity of substrates (Tanner et al., 1999). Consequently, the P removal efﬁciency of the constructed wetland depends mostly on the physic-chemical properties of applied substrates (Drizo et al., 1999; Burley et al., 2001). At the same time, since the constructed wetland was established above the high eutrophic sediment of Lake Taihu, the inhibition ability of substrates was thus important too. To advance and maintain high phosphorus removal rate of the constructed wetlands, more attention should be paid to chemical composition in future engineering applications. Otherwise, selection of suitable substrates is useful in improving the performance of constructed wetland in removing pollutants, especially for the phosphorus (Sakadevan and Bavor, 1998). Thus, some less expensive substrates need to be selected and used to enhance the P adsorption capacity of the substrates, and to maintain the activity of the constructed wetlands. Substrates with high total calcium content can remove phosphorus effectively (UNEP, 2002). Among the three substrates, limestone has the highest calcium content (35.5% of mass), while cinders have the lowest. Cinders had highest mass content of iron (30.9%) and aluminum (8.96%). Since calcium has a higher adsorption ability than the iron and aluminum, the more the limestone was used in the constructed wetlands, the more TP would be removed. Among the three substrates, limestone would be the best substrate to construct a wetland due to the highest P-adsorption and P-inhibition ability. We found that the artiﬁcial wetland constructed with the three inexpensive substrates in our studies had high phosphorus removal ability, especially during the ﬁrst 10 days. Phosphorus removal rates of constructed wetlands generally decline after an initial equilibration period unless special substrates with high sorption capacity are used (Tanner et al., 1999). Phosphorus transformations during wastewater treatment in CWs include adsorption, de-adsorption, precipitation, dissolution, plant and microbial uptake, fragmentation, leaching, mineralization, sedimentation (peat accretion) and burial (Drizo et al., 1999). The major phosphorus removal processes are sorption, precipitation, plant uptake (with subsequent harvest) and peat/soil accretion. However, the ﬁrst two processes are saturable (Sakadevan and Bavor, 1998). Thus, wetland plants and macrophytes would be necessary to maintain a long lifespan CW (Vymazal, 2006). 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