Influencing Factors of Limestone Sorption and Its Usage in
Advanced Wastewater Treatment for Phosphorus Removal
122Li Linyong, Zhang Hua, Wang Dunqiu 5
(1. School of Road&Bridge, Zhejiang Institute of Communication, Hangzhou 311112; 2. College of Environmental Science and Engineering , Guilin University of Technology, Guilin
Abstract: The aim of this article was to investigate the influencing factors of limestone(LS) adsorption
10 for phosphorus(P) removal and the effect of treating the effluent from a municipal wastewater treatment plant(MWTP). Firstly a series of batch experiments were conducted to study the influencing factors of LS for P removal. Consequently, the P removal efficency increased with the temperature and was higher during the initial 3 h; the efficency was as high as 75% even at initial P content 50 mg/L or corresponding P/LS 3 mg/g; smaller LS particle size enhanced the adsorption of limestone; the
15 efficiency was over 90% when pH was below 6.37 and decreased sharply with pH when it was above 8.15; sodium chloride as background electrolyte decreased the adsorption; organic acids including tartaric acid, oxalic acid, and citric acid suppressed the adsorption, and citric acid showed the strongest effect which proved to be effective material for the regeneration of saturated limestone. Then column study was conducted to evaluate the effect of the continuous vertical-flow limestone bed treating
20 effluent from a MWTP with varying hydraulic retention time(HRTs). Over 80 days the effluent pH was between 7 and 9 which was suitable for growing plant; running period while effulent TP?0.5 mg/L
increased with HRTand shorter HRTs such as 1 or 1.5h was recommended. It showed that LS as a effective absorbent, was suitable for the substrate in constructed wetland for advanced treating effluent from MWTPs.
25 Key words: Limestone; Environmental Engineering; Phosphorus; Substrate
Phosphorus(P) is one of the main triggering nutrients responsible for eutrophication of shallow freshwater lakes. Many waters in China are now troubled by eutrophication. The State
30 Department of Environment Protection had declared that in some areas around important lakes
and rivers, the total P content of effluent from municipal wastewater treatment plant(MWTP) should be less than 0.5 mg/L since 2006, according to the state standard. So it is very urgent for those related old plants to upgrade their treatment process to meet the demand in recent years.
P removal by Constructed wetland (CW) has been widely accepted due to its prominently
35 ecological service, effective and eco-friendly technology and reliable operating condition. Results from several studies have showed that P removal in CW occurs through substrate adsorption, chemical precipitation, bacterial action, plant and algal uptake and incorporation into organic
matter. Further, more research showed that the major mechanisms for removing phosphorus from eutrophic water by CW are chemical adsorption and sedimentation by substrates, rather than
[2,3]40 plant uptake and microbe removal. So it was very important to select appropriate substrate
material. As calcium ions can form stable and insoluble products with phosphate, calcium-based materials are considered to be one of the potential sorbents for phosphorus removal. C. Vohla et al.
 found a significant positive Spearman Rank Order Correlation between the P retention and CaO and Ca content of filter materials. Ca-bound P was also found to be more available for plants
[5-6]45 than Al- or Fe-bound P . On the other hand, the cost of a constructed wetland depends mainly
on the substrates filled within it. All these indicated that limestone(LS) would be the appropriate
Foundations: National Critical Patented Projects in the Control and Management of Polluted Waters Grant (NO.2008zx070317-02-03)
Brief author introduction:Li Linyong(1975-),male, lectuer,majored in water pollution and control. E-mail: firstname.lastname@example.org
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material for it both contains high Ca content, and is cheap or easy to get in China as well as in
 many other countries. B. Guan et al. found that while treating lake water by CW, among the
three inexpensive substrates such as LS, cinder and loess, LS had the highest phosphorus removal
 50 and inhibition ability due to its highest calcium content. B.Q. Shan et al. showed that LS is one
of the ideal substrates for removing P-polutants from wastewater of low P content such as storm water. The main purpose of this study was to investigate the effect and adsorption of LS as
val, and to establish the parameter for the design of CW using LS as absorbing material for P remo
the main substrate when treating effluent from municipal wastewater treatment plant.
55 1 Material and method
1.1 Water analysis and material
The limestone particles was collected from a quarry in Guilin, China and was then ground and sieved. The main component of the material CaCOweighed over 90%. All examined 3
parameters were analysed in accordance with Methods of Examination for Water and Wastewater
1.2 Batch study
The batch study, including a series of batch experiments, was conducted to study the influencing factors of P removal by limestone(LS), including contact time, temperature, initial PO-P concentration, pH, particle size, background electrolyte and organic acid. Throughtout the 4
65 study, LS particle size was no less than 0.2 mm (except the experiment on the effect of particlesize), and each dose of 0.5 g was put into a taper bottle (100 ml) containing 30 ml of
-1 potassium phosphate monobasic (KHPO), of which the initial P concentration was 50 mgL24
(except the experiment of initial P concentration). All bottles were shaken on a rotating shaker for
24 h (except the experiment of contact time) at a constant temperature of 25 ? (except the
70 experiment of temperature). Then the bottles were removed from the shaker and suspensions were centrifuged at a rotate speed of 4000 rpm for 10 min, and the supernatants were then through filter paper with pore size of 0.45 μm to be determined for P concentration.
To examine the effect of contact time and temperature, the bottles were shaken at the temperatures of 12 ?, 22 ?, 32 ? and 42 ?, and were removed from the shaker one by one at
75 intervals from 20 min to 24 h to be examined. In the experiment of initial PO-P concentration and 4-1pH, different initial P concentrations ranged from 2 to 50 mg L, and solution pHs were adjusted
from 3.25 to 12.03 with 0.1M HCl or NaOH. Different LS particle sizes were obtained through a series of different sieve meshes which included the sizes of 0-0.09 mm, 0.09-0.105 mm, 0.105-0.125 mm, 0.125-0.150 mm, 0.150-0.20 mm, 0.20-0.60 mm, 0.60-2.0 mm. Sodium chloride
80 (NaCl) was selected as the background electrolyte and its initial concentration in the KHPO 24
Solution ranged from 0.5 to 5 mmol/L. Concentrations of organic acids such as oxalic acid, citric acid and tartaric acid varied between 0.5 and 5mM in the experiment.
1.3 Column study
This study aimed to invistigate the effect of the continuous vertical-flow limestone bed in a
85 column for P removal. Wastewater sample were collected from the effluent of a secondary sedimentation tank in a municipal wastewater treatment plant(MWTP) in Guilin, China. The quality indexes of the effluent all accorded with the level 1A (Class A of level 1) of the national
stardard Discharge Standard pollutants for municipal wastewater treatment plant, except that the
total phosphorus (TP) concentration was 1 to 1.5 mg/L which is out of the limit of 0.5 mg/L. The
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90 effect of hydraulic retention time(HRT) on the column performance were studied. Four identical
columns, 45 mm diameter and 250 mm bed hight, filled with 800 g limestone particles whose size
was between 0.2-2mm, run with different HRTs of 1, 1.5, 2, 2.5 h respectively. The effluent pH
and TP concentration were determined once a day.
2 Results and discussion
95 2.1 Batch study
2.1.1 Effects of contact time and temperature
Removal(%)25 12? 22?
32? 42? 0
0 6 12 18 24 Contact Time(h) Fig. 1 Effects of contact time and temperature on P removal(%) Fig.1 showed the effects of contact time and temperature on the P removal efficency by
100 Limestone. It was observed that the P removal efficiency increased with contact time and temperature. After 24 h, the removal efficiency at 12, 22, 32 and 22 ? were respective 49%,
71%, 91%, 94%. At 12 ? and 22 ?, the amount of P adsorption by LS was relatively less and increased slowly with time. At 32 ? and 42 ?, the adsorption amount was higer and the process was in equilibrium after 18 h and 12 h, respectively. For the initial 20 min to 3h, the four
105 removal efficiencies were relatively high. For instance, after 1 h the removal efficiencies at 12, 22,
32 and 22 ? were 10, 16, 21, 22% respectively and the amount occupied 20-24% of that after 24h. Compared with other substrate like cinder，LS has higher Ca content and adsorption capacity
[8,11] in and its P adsorption is mainly a chemical process, so it can be used as the main substrate
wetland and has more longevity.
2.1.2 Effect of initial P concentration 110 100
700 10 20 30 40 50 initial concentration(mg/L) Fig. 2 Effect of initial P concentration on P removal(%)
Fig. 2 showed the effect of varying initial P concentration from 1 to 50 mg/L on the sorption -P concentration (1-2 capacity of LS at the temperature of 25 ?. It showed that lower initial PO4
115 Pmg/L，corresponding mass ratio of P/LS=0.06-0.12 mg/g) resulted in higher removal efficency
(over 95%) compared to the higher concentration (75% at 50 mg/L, corresponding P/LS=3 mg/g).
This is due to the greater relative availability of sorption sites on the sorbent for PO-P removal. In 4
the case of higher initial P concentration, the total number of available sorption sites was exceeded
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-P which caused the decrease in the phosphate adsorption rate. by the moles of PO4
1202.1.3 Effect of initial solution pH
3 5 7 9 11 13
Fig. 3 Effect of initial pH on P removal(%) ution pH from 3.25 to 12.03 on the P romoval Fig. 3 showed the effect of varying initial sol
by LS at 25 ?. There was a clear decline in removal efficiency with pH, especially in alkaline
125 area. The efficiency was as high as about 90% till pH surpassed 6.37, while as pH was over 8.15 it
dropped dramatically with pH. When pH was over 11, the efficiency was as low as 25%. It could -P. In acid wetlands, LS be concluded that LS is more suitable for acid wastewater to remove PO 4 could buffer the waters due to the dissolution of carbonate substrate materials which generates 2+ alkalinity and consumes protons. Additionally, the acids causes the release of Cawhich would  precipitate PO-P. Kim et al. reported that in phosphate-containing sediments of eutrophic 130 4 waters, limestone could suppress the dissolution of phosphate by organic acid and/or carbonic acid cause by the activity of anaerobic bacteria. So LS might be a useful measure to prevent deterioration of water quality through eutrophication, by breaking the internal loading of phosphates in eutrophic water bodies.
2.1.4 Effect of Limestone particle size 135
70 Removal(%) 65 0.105- 0.125- 0.15- 0.2-0.6 0.6-2 <0.09 0.09- 0.1250.150.20 0.105 Particle Size(mm) Fig. 4 P removal efficiency (%) of different particle sizes
Fig. 4 showed the effect of various LS particle size ranges on the sorption capacity. Theoretically, the smaller the particle size the more surface area and active sorption sites the
absorbent would have. Hence small particle size increased the adsorption capacity. It was seen that 140
there was a decline of removal efficiency from 78 to 70% as the particle size increased to 2 mm. Yet the difference was not as much as expected. However, too small particle may decrease the
porosity of filter layer in wetland and the substrate tended to be clogged. It was recommended that the size of LS particle used in filter not be too little. The LS particle used in the later column study
was above 0.2 mm. 145
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2.1.5 Effects of background electrolyte and organic acid
50 0 1 2 3 4 5 NaCl concentration(mmol/L) Fig. 5 Effect of background electrolyte on P removal (%) Electrolytes in real wastewater may interfered the P adsorption on substrate. All kind of
electrolytes provide a certain ionic strength which can affect the surface charge of adsorbents 150
particle, and there are also a series of complex interactions including competitive adsorption, precipitation and ion exchange. Here we chose NaCl as the background electrolyte to investigate
the effect on adsorption. As shown in Fig. 5, it was observed that there was a decline of removal efficiency from 73 to 64% with the increase of NaCl from 0 to 5mM, due to the competitive -155 adsorption on limestone between PO-P and Cl 4.
100oxalic acid acid tartaric 80 citric acid
40 removal(%) 20
0 1 2 3 4 5
concentration(mmol/L) Fig. 6 Effect of LMWOAs on P removal (%) In the substrate of a practical wetland, some low-molecular weight organic acids (LMWOAs) would be produced by the action of plant root and microbe. Here we chose three common acids
160 such as oxalic acid, citric acid and tartaric acid to investigate the effect of LMWOA on P removal.
As shown in Fig. 6, the three LMWOAs all supressed the LS adsorption as their concentrations increased. Among them citric acid show the strongest suppression and the P removal efficiency dropped to 7% at the concentration of 5 mM. Tartaric acid showed the weakest effect with the 57% removal efficiency at 5 mM, while oxalic acid with the 37% efficiency at 5 mM.
It has been known that in soil, plant root would secrete LMWOAs to dissolve sparingly 165
soluble phosphate and incorporate P into its own nutrient when soluble P is poor. Kpomblekou  et al.found that at the same concentration, some LMWOAs(oxalic acid, citric acid) had even
more P-dissolving ability than mineral acid such as sulphuric acid, nitric acid and hydrochloric acid. As to a wetland of which substrate adsorption is exhausted, it might be a better choice to
regenerate it by LMWOAs produced by plants. Since the kinds of LMWOAs are mainly related to 170
a special plant, it is advisable to choose those plant which secrete such LMWOAs as citric acid for
the regenaration of wetland substrate composed of limestone. For example, the root of White lupin
(Lupinus albus L.) would mainly exude citric acid to acquire P in from a P-deficient calcareous
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175 2.2 Column study
90 HRT=1h HRT=1.5h HRT=2h 60 HRT=2.5
0 0.1 0.2 0.3 0.4 0.5
Fig. 7 Effluent TP content at different HRTs by running days The column experiment lasted over 80 days. The temperature of influent was 22 to 27 ?. and 9 for all columns over running time, which is a suitable pH The effluent pH was between 7
180 range for plant to grow. Fig. 7 showed the time for the effluent TP content from 4 columns to reach a certain concentration of 0.5 mg/L, which is the limit of 1A level of the national standard. It
was clear that the effluent TP content gradually increased with the influent volume or the running
time. For the effluent from 4 columns at HRTs of 1, 1.5, 2, 2.5 h，it took 50, 61, 71, 82 days respectively to reach 0.5 mg/L When the flow rate increases, the residence time in the bed
185 decreases which results in lower bed utilization. Thus, the column saturation time and the bed
capacity decreased with increased flow rate. So, shorter HRT results in higher flow rate, higher water yield capacity and shorter column exhaustion time. It could be estimated that the yield volume of the column at 1h HRT was nearly 1.6 times that of column at 2.5 HRT. However, longer HRT means longer contact time between LS and P and higher utilizing efficiency of the
sorbent. Considering that limestone is inexpensive it was advisable to running at shorter HRTs 190 such as 1 or 1.5h.
3 Conclusion A series of batch experiments showed the influencing factors of P removal by limestone. The P removal efficiency increased with the temperature and contact time, and decreased with initial
concentration; smaller LS particle size enhanced the adsorption of limestone. the efficiency was 195
over 90% when pH was below 6.37 and decreased with pH when it was above 8.15; background electrolyte such as sodium chloride decreased the adsorption; organic acids such as tartaric acid,
oxalic acid, and citric acid suppressed the adsorption, and citric acid especially showed to be effective materials for the regeneration of saturated substrate. Column study showed the effect of
the continuous vertical-flow limestone bed treating effluent from a MWTP. The effluent pH was at 200
the scope of 7 to 9 which was suitable for growing plant, and it took 50 to 82 days for the effluent TP content to reach 0.5 mg/L from 4 columns at varying HRTs. Shorter HRT such as 1 or 1.5 h
was recommended. It showed that LS as a effective absorbent, was suitable for the substrate in constructed wetland for advanced treating effluent from MWTP to meet the demand of 1A level of
the national standard. 205
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水深度处理中的应用 122 李林永，张华，王敦球 ，1. 路桥分院，浙江交通职业技术学院，杭州 311112；
2. 环境科学与工程学院，桂林理工大学，桂林 541004！245 摘要;本文研究了石灰石吸附除磷的影响因素及其用于城市污水处理厂出水的深度处理效
果。首先用一系列静态批处理试验研究了石灰石除磷的影响因素，结果表明，石灰石对磷 的去除率随温度升高, 在反应的前 3 h 内去除率相对很高；在初始磷浓度 50mg/L，对应质量 比磷(石灰石=3 mg/g 下，磷去除率仍高达 75%；在 pH 小于 6.37 去除率在 90%以上而当 pH
大于 8.15 时去除率随 pH 急剧下降；NaCl 作为背景电解质降低了磷吸附；三种有机酸草 250 酸、酒石酸和柠檬酸均抑制了磷的去除，尤其是柠檬酸抑制最强，这表明它是一种有效的 湿地基质再生材料。随后进行了小试规模的石灰石反应柱连续流试验，以评价石灰石在不 同水力停留时间下处理污水厂出水的效果，80 余天的运行中各出水 pH 处于适宜植物生长 的范围(pH=7-9)，出水总磷低于 0.5 mg /L 的运行天数随水力停留时间(HRT)延长，建议采
用短 HRT 如 1 h 或 1.5 h 来运行。本研究表明，石灰石作为合适的人工湿地的基质材料可进 255
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