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Soil Science and Plant Nutrition

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Characterization of Treatment Processes andMechanisms of COD, Phosphorus and NitrogenRemoval in a Multi-Soil-Layering System

Kuniaki Sato , Tsugiyuki Masunaga & Toshiyuki Wakatsuki

To cite this article: Kuniaki Sato , Tsugiyuki Masunaga & Toshiyuki Wakatsuki (2005)Characterization of Treatment Processes and Mechanisms of COD, Phosphorus and NitrogenRemoval in a Multi-Soil-Layering System, Soil Science and Plant Nutrition, 51:2, 213-221, DOI:10.1111/j.1747-0765.2005.tb00025.x

To link to this article: http://dx.doi.org/10.1111/j.1747-0765.2005.tb00025.x

Published online: 17 Dec 2010.

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Soil Sci. Plant Nutr., 51 (2), 2 I3 - 22 I , 2005 213

Characterization of ’Ikeatment Processes and Mechanisms of COD, Phosphorus and Nitrogen Removal in a

Multi-Soil-Layering System

Kuniaki Sato, Tsugiyuki Masunaga and Toshiyuki Wakatsuki”

Laboratory of Soils and Ecological Engineering, Faculty of Life and Environmental Science, Shimane University, Matsue, 690-8504 Japan; and *Faculty of Agriculture. Kinki University. Nara, 631 -8505 Japun

Received July 2,2004; accepted in revised form December 15, 2004

Characteristics of the treatment processes inside a MSL system were investigated by using a laboratory-scale MSL system, which was set up in a D10 x W60 x H73 cm acrylic box enclosing “soil mixture blocks” alternating with permeable zeolite layers. For the study of the treatment processes inside the system, wastewater, with mean concentrations (mg L-’) of COD: 70, T-N: 12, T-P: 0.9, was introduced into the system at a loading rate of 1,000 L m-2 d-I. lkeatment processes in the MSL system were different for the COD, P and N pollutants. Eighty percent of COD was removed in the 1st soil layer among the 6 layers, and the removal rate increased as water moved down and finally reached 90% in the last layer of the system. Phosphorus concentration was lower under the soil mixture layers than under the permeable layers, presumably because P was adsorbed mainly by soil and mixed iron particles. The P concentration in water gradually decreased in the lower layers of the sys- tem. The concentration of PO,3--P was generally lower in the aerated MSL system than in the non-aerated one. NH,+-N was adsorbed and nitrified in the upper part of the system. The NO,--N concentration was lower in water under the soil mixture layers than under the permeable layers, indicating that denitrification mainly occurred in the soil mixture layers.

Key Words: Multi-Soil-Layering method, nitrogen and phosphorus removal, organic matter removal, processes of wastewater treatment.

The 21st century is generally considered to be the century with a growing concern about the preservation of the environment and water resources. Water pollution has become a serious problem in advanced as well as developing countries, and the low quality of the water supplied is a serious problem worldwide.

Water purification based on soil ecology is carried out widely all over the world, because of the ubiquitous presence of soil (Bhamidimarri 1988; Kaplan 1988; Per- kins 1989; Ho and Mathew 1993; Reed et al. 1995; Mara 1996; Lantzke et al. 1999; William 2000). Soil displays various characteristics related to the environ- ment, including size of pore space, oxidation-reduction and hydrophilicity-hydrophobicity. As a result, various species of microorganisms are able to live in soil. There- fore, a kind of purification process occurs simultaneous- ly in soil. Since we were concerned about such environ- mental cleanup function of soil, we developed and stud- ied a water purification system, the Multi-Soil-Layering (MSL) system. The MSL system consists of soil units arranged in a brick-like pattern surrounded by layers of

zeolite or alternating particles of homogeneous sizes that allow a high hydraulic loading rate. The MSL sys- tem is effective for the prevention of clogging and short- cuts which are the main constraints in the conventional soil-based wastewater treatment systems (Wakatsuki et al. 1993, 1998, 1999; Masunaga et al. 1998, 2002,2003; Sat0 et al. 2002,2005).

As described by Attanandana et al. (2000), Boonsook et al. (2003) and Luanmanee et al. (2001, 2002a, b), the MSL system can be developed mainly from locally available resources, such as soil, iron particles, jute or saw dust, charcoal, and zeolite or alternative materials. Iron particles mixed in soil to enhance the phosphate adsorption and denitrification functions act as reducing agents. Organic materials are added as foods for the microorganisms and as electron donors for denitrifica- tion. Charcoal contributes to the improvement of organic matter decomposition as an adsorbent and habitat for the microorganisms. Although appropriate amount and tim- ing of aeration are necessary (Luanmanee et al. 2002a), the maintenance of the MSL system is simple and it was

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2 14 K. SATO, T. MASUNAGA, and T. WAKATSUKI

found that the MSL system can contribute to the improvement of the water quality under a wide range of conditions, even in developing countries (ex. Masunaga et al. 2002). The MSL system enables to enhance and control various biological activities by regulating soil physical and chemical properties.

Although various types of wastewater treatments have been applied successfully using the MSL system in Japan, Thailand and Indonesia, the processes of waste- water treatment by the MSL system have not been fully elucidated. This is because the treatment processes have been mainly estimated by comparing wastewater and treated water and the material balance of the system before and after wastewater treatment so far. Although experiments in which water was collected inside the MSL system had been conducted (Wakatsuki et al. 1990), quantitative evaluation of the treatment processes in the MSL systems has not been achieved because of the shortage of water sample points. Quantitative char- acterization of the treatment mechanisms in the MSL system enables to further improve the efficiency of the MSL system by appropriate adjustment of the structure and material composition. Therefore we conducted labo- ratory-scale experiments in the present study to analyze quantitatively the processes of wastewater treatment in the respective layers of the MSL system. In the present study, the positions inside the MSL system (i.e. soil mixture layers, permeable layers and depth of the sys- tem), changes in the treatment processes with the pas- sage of time and the effect of aeration were system- atically and comprehensively investigated.

MATERIALS AND METHODS

Description of MSL system in the present study. Figure 1 shows the structure of MSL system. For the MSL system, an acrylic box D10 X W50 X H73 cm in size was used to enclose zeolite and soil mixture layers forming an alternate brick layer-like pattern. Twenty five sets of water collection pipes were installed in the zeolite layers between each soil mixture layer. The soil mixture layers were composed of soil (Andisol, i.e. humus-rich volcanic ash soil, Kurohoku), saw dust, granular iron and charcoal at the ratios of 7 1.6%, 10.5%, 11.9% and 6.0%, respectively, on a dry weight basis. The void spaces (permeable layers) between each block and block sides were filled with zeolite particles 3- 5 mm in diameter. A detailed description of the MSL system used in this study can be found in Fig.1 of the report of Sat0 et al. (2005).

Research on quantitative processes of treat- ment in the respective layers of the system. Domestic wastewater from a nearby community dispos-

Aeration pipe (Air supply:3.3 X 104 L m3 day-')

MSL system MSL system in the under aeration absence of aeration

v Outlet of water collection pipe : front side

Outlet of water collection pipe : back side

Fig. 1. Comparison of the MSL system under aeration and in the absence of aeration in the studies on treatment processes. In the MSL system under aeration, an aeration pipe was installed.

a1 plant was diluted three times with tap water and intro- duced into the system for studying water purification processes inside the MSL system. Two types of MSL systems were prepared (Fig. I) . The effect of aeration was determined by comparing the MSL system under aeration and without aeration. Loading rate of 1,OOO L m-? d-' was continuously applied for 212 d (28 May 2002-26 December 2002), except during the period for the preparation of the wastewater and maintenance of the water pump. Mean chemical properties of the inflow water were as follows : COD 70 mg L-', T-N 12 mg L-' and T-P 0.9 mg L-' . An aeration pipe (1 .O cm in diame- ter) was installed between the first and the second soil mixture layers (MSL under aeration). Rate of aeration of 3.3 X lo4 L mp3 d-' was the same as that used in a MSL system for domestic wastewater treatment (Wakatsuki et al. 1993).

Water samples under the wastewater treatment pro- cess in the system were collected from each water col- lection pipe of the MSL system under aeration and without aeration, starting from the bottom to the upper layer one by one, to prevent the influence of water sam- pling on the flow of water in the subsequent layers. A sufficient amount of water was collected for the analysis (at least over 60 mL) for about 1-50 h. Except for water sampling, the water collection pipes were closed by a plug. The water samples, wastewater and treated water (i.e. final effluent of the system) were analyzed for the COD concentration by the potassium dichromate meth- od, NH,+-N concentration by the Nessler method, NO,--N and N02--N concentrations by ion chromatog- raphy (DIONEX OX-120) and for the PO4"-P concen- tration by the ascorbic acid method (APHA 1992).

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Treatment Processes and Mechanisms inside MSL System 215

RESULTS AND DISCUSSION

Quantification of the degree of treatment in each layer of the MSL system

Figure 2 shows the fluctuations of the mean concen- trations of COD, PO,3--P, NH,+-N, NO,--N and NO,-- N of the treated water in each layer of the system during the 0- I80 d period of operation. Figures 3-5 show the treatment processes and mechanisms for the removal of organic matter (COD), phosphorus and nitrogen on the 7th and 170th d of the experimental period. The mean values of the concentrations in the soil mixture layers and permeable layers of the MSL system under aeration and without aeration are also shown in Figs. 3-5. The values on the 7th d represented the total mean in the MSL system under aeration and without aeration, because they did not differ at the initial stage.

Process of organic matter (COD) removal In the soil mixture layers, the values of COD were

lower than those in the permeable layers (Fig. 2a), which indicates that the efficiency of COD removal was higher in the soil mixture layers. Table 1 shows the mean outflow rate from each water collection pipe and cumulative COD removal rate in each layer. This cumu- lative COD removal rate was estimated by measuring the amount of water outflow from all the water collec- tion pipes and the COD concentration in each layer. Although the whole amount of the wastewater input was not collected from the pipes, the flow rate per total area occupied by the soil mixture layer and the permeable layer in each layer was estimated, based on the flow rate of the collection pipes and the area they occupied. Then after, the flow rate was corrected by the ratio the flow rate and the loading rate, assuming the existence of a uniform vertical flow without lateral water movement under the soil mixture layers and under the permeable layers. Most of the COD in the wastewater was removed in the top soil layer at the rate of 74.3% for the MSL system under aeration and 80.6% for the MSL system without aeration, and the removal rates increased as water moved down, and finally reached a value of 90%.

At the initial stage of the experiment, it was consid- ered that physicochemical reactions such as filtration and adsorption in the upper part of the system were the major processes. It appeared that the organic matter referred to as COD or BOD was easily trapped in the soil mixture layers, because of the amount of pore space, large surface area and enhanced hydrophobic properties by the addition of charcoal. After 170 d, the quality of the treated water remained satisfactory (Figs. 2a, 3). Biological process of COD removal seemed to occur, in addition to the physicochemical reaction. COD

values were lower on the 170th d than on the 7th d in the upper part of the system, indicating the existence of microbial activity, i.e. organic matter decomposition was enhanced with time through the accumulation of various microorganisms. On the effect of aeration on organic matter (COD) removal, the COD concentration of the treated water of the MSL system may not differ appre- ciably regardless of the presence or absence of aeration and the mean values were 5.5 and 6.6 mg L-', respec- tively. In contrast, in the 2nd-4th soil mixture layers, the COD values of the MSL system under aeration were lower than those in the MSL system without aeration (Figs. 2a, 3). This indicates that aeration enhanced the microbial activity for the decomposition of organic mat- ter in the soil mixture layers of MSL system. In the absence of aeration, the accumulation of organic matter may be enhanced in the upper layers of the MSL sys- tem, as shown in Fig. 3. In fact, in the absence of aera- tion, the MSL system was clogged on the 203rd d. The clogging due to the accumulation of organic matter, however, could be dissolved by interrupting the opera- tion during a period of 2 to 3 months (Wakatsuki et al. 1998, 1999). To avoid the formation of a thick biofilm, the control of aeration is important. The lifetime of the MSL system for organic matter treatment could be semi- permanent if the irreversible formation of impermeable layers could be avoided through the selection of suitable materials and appropriate construction of a non-com- pacted MSL structure (Luanmanee et al. 2002b).

Process of phosphorus removal The PO,"--P concentration was lower in the soil mix-

ture layers and fluctuated less than that in the permeable layers (Fig. 2b). The trend of PO,"--P concentration changed in the permeable layers, especially in the upper first and second layers, which showed a similar pattern to that of the fluctuations in wastewater. The concentra- tion in the soil mixture layers decreased abruptly even in the first layer, whereas the concentration in the perme- able layers gradually decreased in the lower layers. In both soil mixture layers and permeable layers, these were areas where PO,"--P could be adsorbed. The mechanism of phosphorus removal is based on phospho- rus adsorption on the active aluminum hydroxides con- tained in soil and also on ferric hydroxides formed from the metal irons added in the soil mixture layers. In the soil mixture layers, added metal irons dissolved and leached out as ferrous ions from the inner soil mixture layers under anaerobic conditions. And then, in an edge of the soil mixture layers close to the permeable layers under relatively aerobic conditions, ferrous ions were further oxidized to ferric hydroxides and were adsorbed on the clay particles. In the permeable layers, ferrous ions, which leached out from the soil mixture layers,

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216 K. SATO, T. MASUNAGA, and T. WAKATSUKI

(d) tN0i-N (e) : NO,--N

Fig. 2. Dynamic mean concentrations of COD, PO,'--P, NH4+-N, NO,--N and NO,--N of the treated water under the process in each layer of the system. SML: soil mixture layers, PL: permeable layers.

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Treatment Processes and Mechanisms inside MSL System 217

able 1. system.

Mean outflow rate from each water collection pipe and cumulative COD removal rate (%) in the layers of the MSL

Mean outflow rate (mL h-I) Mean cumulative COD removal rate (%)

Aeration Absence of aeration Aeration Absence of aeration

Layer Under SML Under PL Under Sh4L Under PL

1 14.5 64.6 33.2 41.5 74.3 80.6 2 40.4 42.7 22.0 33.5 84.9 77.4 3 64.2 52.8 38.9 68.6 88.4 83.9 4 96.3 82.7 59.2 144.4 89.4 84.0 5 77.9 73.4 52.8 173.7 90.1 88.3

Treated water 92.1 90.6

SML: soil mixture layers. PL: permeable layers.

7th day 170th day

Fig. 3. Treatment process of COD on the 7th and 170th d. The mean values of the concentration (mg L-I) in both soil mixture lay- ers (circles) and permeable layers (squares) at each position of the MSL system under aeration and in the absence of aeration are also shown in the figure. The values on the 7th d represent the total mean in the MSL system under both conditions, because they did not differ at the initial stage.

were further oxidized to insoluble ferric hydroxides. Consequently, the surface area of ferric hydroxides expanded by these processes and was able to remove effectively phosphorus in the MSL system. The mecha- nisms of phosphorus removal were described in detail in

a previous report (Wakatsuki et al. 1993). It was eventu- ally concluded that the maintenance of proper redox conditions through appropriate aeration control is an important factor for effective phosphorus removal in the MSL system.

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218

7th day

K. SATO, T. MASUNAGA, and T. WAKATSUKI

170th day

aeration absence of aeration Fig. 4. Treatment process of phosphorus on the 7th and 170th d. The mean values of the concentration (mg L-I) in both soil mix- ture layers (circles) and permeable layers (squares) at each position of the MSL system under aeration and in the absence of aeration are also shown in the figure. The values on the 7th d represent the total mean in the MSL system under both conditions, because they did not differ at the initial stage.

Physicochemical reactions were the main processes involved in phosphorus removal and they took place from the initial stage of treatment for effective P remov- al. The effect of aeration appeared with the passage of time in the MSL system under aeration. The PO,3--P concentration of treated water was lower in the MSL system under aeration than in the MSL system without aeration during the later period of the experiment (Figs. 2b, 4). As mentioned above, it appeared that the ferrous ions which were dissolved from the inner soil mixture layers were more oxidized under aeration and became insoluble ferric hydroxides at the edge of the soil mix- ture layers and permeable layers, as shown in Fig. 4. However, it was reported that phosphorus removal by adsorption decreased because of the formation of a highly passive Fe,O, surface coating by excessive aera- tion (Nishiguchi 1990). Therefore, further improvements are necessary for the phosphorus treatment under aera- tion conditions. Since the main process of phosphorus removal consists of physicochemical adsorption, effec- tive contact between the wastewater and soil mixture

layers is a major factor for P removal. Reducing the size and increasing the number of soil mixture layers may enable to achieve such an effective contact by the increase of the surface area of the soil mixture layers.

Process of nitrogen removal At the initial stage of the experiment, although NH,+-

N was detected in the permeable layers in the upper part of the system, the concentration was very low in the soil mixture layers (Fig. 2c), presumably because both NH,+-N adsorption and nitrification took place in the soil mixture layers. The concentration of NO,--N was higher in the soil mixture layers than in the permeable layers in the top layers, especially at the onset of the operation before 60d, due to previous increase of the nitrification activity in the soil mixture layers. In addi- tion to physicochemical adsorption by soil and zeolite, the nitrification activity developed with time in the upper permeable layers, as evidenced by the decrease in the NH,+-N concentration and increase in the NO,--N concentration in the MSL system under aeration and

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Treatment Processes and Mechanisms inside MSL System 219

Fig. 6. Treatment process of nitrogen on the 7th and 170th d. The mean values of the concentration (mg L-I) in both soil mixture layers (circles) and permeable layers (squares) at each position of the MSL system under aeration and in the absence of aeration are also shown in the figure. The values on the 7th d represent the total mean in the MSL system under both conditions, because they did not differ at the initial stage.

without aeration at the later stage of the experiment, compared with the initial stage (Figs. 2c, e, 5). Since the NH,+-N concentration decreased in the upper part of the system, the nitrification of NH4+ mostly occurred in the upper part. NH,+ adsorption by zeolite and soil was also likely to have contributed to the decrease in the NH,+-N concentration and the enhancement of nitrification in the MSL system associated with the increase of the duration of the presence of NH,+-N in the system. NO,--N, the intermediate product of transformation of ammonia to nitrate, was generated in the upper layers only at the ini- tial stage of the experiment (Fig. 2d).

At the initial stage, NO,--N was formed in the upper part of the system and the concentration increased fur- ther in the middle part and remained at the same level in the lower part. With the passage of time, the NO,--N concentration decreased in the lower part, as shown in Fig. 5 and Fig. 2e, indicating that denitrification pro- ceeded. The concentration of NO,--N was significantly

low in the soil mixture layers, especially in the absence of aeration, indicating that denitrification mainly occurred in the soil mixture layers. It is suggested that the supply of NO,--N, decrease of the oxygen concen- tration and/or optimum supply of organic matter (i.e. added saw dust and organic matter from wastewater) in the upper part of the system may enable to enhance the denitrification process in the lower layers.

Absence of aeration of the system enhanced the nitro- gen removal. The concentration of NO,--N in the MSL system without aeration was lower than that in the MSL under aeration, especially in the lower part of the sys- tem. As shown in Fig. 5, it was assumed that an anaero- bic environment was generated and that organic matter as an electron donor accumulated more in the MSL sys- tem without aeration, as reflected in the COD concentra- tion (Fig. 3). These conditions were suitable for denitrification. Since nitrification was satisfactory in the MSL system both under aeration and without aeration,

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220 K. SATO, T. MASUNAGA, and T. WAKATSUKI

Table 2. Comparison of the relative concentration of each pollutant at each position of the system under soil mixture layers and permeable layers.

7th d 170th d

Aeration Absence of aeration Part of system SML PL

SML PL SML PL - < < - - - - - - - COD Upper Lower

<< << << < < << PO,-P Lower

< - -

NH,-N Upper Lower

- - - > - Upper NO,-N Middle < < <

< - - - Lower -

SML: soil mixture layers. PL: permeable layers.

denitrification was the limiting factor for nitrogen removal in this MSL system. This was because of the insufficient availability of organic matter as an electron donor, due to the high decomposition rate of organic matter, as evidenced by the effective COD removal (Wakatsuki et al. 1990). The implication is that the MSL system could introduce oxygen by natural gas exchange through the top of the system even in the absence of aer- ation.

Conclusion The results of the present study showed that the pro-

cesses of the treatment of organic matter, phosphorus and nitrogen were different (Figs. 3-5 and Table 2). Treatment process of COD was relatively simple, while that of phosphorus and nitrogen was more complicated. The location where each pollutant was removed was also different in the MSL layers. COD and phosphorus were removed mainly in the upper part of the system, while nitrogen was removed throughout the upper to lower parts of the system because two steps, i.e. nitrifi- cation and denitrification were involved. Moreover, the treatment conditions such as aeration affected the treat- ment process and effectiveness of each pollutant. Some of the mechanisms underlying the treatment processes in the MSL system were elucidated in the present study, as described above. However, for the practical application of the results obtained in this study, it is still necessary to accumulate more quantitative data and analyses of the treatment process of each pollutant. This includes stud- ies on the effect of structural differences in the MSL system such as size, shape and material composition of the soil mixture layers, and differences in the treatment conditions of the system, such as wastewater quality, loading rate and aeration, as shown in the present study.

These studies are currently being conducted or planned. Based on the results obtained, we plan to set up guide- lines for the planning and design of operational condi- tions and structure of the MSL system depending on the quality and quantity of target wastewater as well as treatment target in the near future.

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