water movement characteristics in a multi-soil-layering system

Soil Sci. Plant Nutr., 61 ( I ), 75 - 82, 2005 75

Water Movement Characteristics 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 Universiiy, Nakamachi, Nara, 631 -8505 Japan

Received July 2,2004; accepted in revised form December 7,2004

The Multi-Soil-Layering (MSL) system consists of soil units arranged in a brick-like pattern that are surrounded by layers of zeolite or alternating particles with a homogeneous size that allow a high hydraulic loading rate. Characteristics of the changes in the water movement, hydraulic retention time (HRT) and system weight during the wastewater treatment in the MSL system were investigated using a laboratory-scale MSL system (D10 x W50 x H73 cm). As the loading rate increased from 1,000 to 5,500 L m-2 d-', wastewater preferen- tially flowed into the permeable layers in the MSL, which decreased the contact of the wastewater with the soil mixture layers. HRT was inversely related to the loading rate. HRT decreased from 20 to 1 h, as the loading rate increased from 250 to 6,000 L m-2 d-l. As an indicator of the system condition, the weight variation of the system was determined during the wastewater treatment. When the weight was stable, input and output of wastewater and decomposition of organic matter appeared to be equilibrated. When the weight increased, the system started to clog. Due to clogging, the emciency of COD and phosphorus removal decreased, while the efficiency of nitrate removal increased.

Key Words: clogging, hydraulic retention time (HRT), Multi-Soil-Layering method, water movement characteristics, wastewater treatment.

In terrestrial ecosystems, soil supports not only biological production, but also acts as an ecosphere where these products are finally decomposed. Traditionally, the physical, chemical, and biological properties of soil have been used for wastewater treatment over a long period of time in various areas in the world. However, traditional soil-based wastewater treatment systems display several limitations. Clogging is the major limitation even in an on-site septic tank and in soil leachline systems (Bhamidimarri 1988; Kaplan 1988; Perkins 1989; Ho and Mathew 1993; Reed et al. 1995; Mara 1996; Lantzke et al. 1999; William 2000). Although soil exhib- its a high purification function, the function strongly depends on the properties of the respective soil types. Therefore, even for similar systems, soil-based water treatment systems did not always perform with the same level of wastewater treatment efficiency. Some soils and sites were not suitable for soil-based traditional water treatment systems. However, soil is potentially a highly suitable material for water purification because it is available all over the world.

The author's group attempted to address the above problems and to develop a new soil-based water treatment system, designated as Multi-Soil-Layering (MSL) system. The MSL system consists of soil mixture layers

alternating with permeable layers in a brick-like layer arrangement. The MSL system possesses mechanisms for the prevention of clogging and enhancement of the purification functions of soil resources (Wakatsuki et al. 1993, 1998, 1999; Masunaga et al. 1998, 2002, 2003; Luanmanee et al. 2001, 2002a, b; Sat0 et al. 2002a, b; Boonsook et al. 2003). BOD and COD components are trapped in the soil mixture layers. The trapped organic materials are subsequently decomposed by microbial activities. NH4+-N can be adsorbed on to exchange sites of zeolite and soil and oxidized to NO,--N by nitrifica- tion. Subsequently, NO,--N is translocated to the soil mixture layers which are relatively anaerobic due to per- colating wastewater and can be reduced to N, gas. Organic materials within the soil mixture blocks as well as the organic components of wastewater can supply carbon for enhancing the denitrification process. Iron and aluminium hydrous oxides and aluminosilicate clays can adsorb phosphate ions (Nyle and Ray 2002). In order to enhance the phosphate adsorption, metal iron particles can be mixed with the soil mixture layers. Unno et al. (2003) showed that the MSL system

enables to treat 30 mg L-' BOD (on the average) in river water and to reduce the amount up to 2 mg L-' (on the average) with very small fluctuations throughout a year

76 K. SAT0 et al.

for a high hydraulic loading rate of 4,000 L m-' d- ' in demonstration-scale systems D6 X W2 X H1.5 m in size. The MSL systems enabled to obtain a higher hydraulic loading rate than the conventional soil-based water treatment systems, because of their structural characteristics (Wakatsuki et al. 1993, 1998, 1999). However, quantitative evaluation of the wastewater movement characteristics in the MSL systems has not yet been performed. Therefore, in the present study, we conducted laboratory-scale experiments to determine ( 1) the water movement characteristics inside the system, (2) the hydraulic retention time, and (3) the relationship between the performance of the wastewater treatment and weight variation of the whole MSL system, which reflects the changes in the water content and the accumulation of suspended solids and/or biofilm in the system in relation to clogging.

MATERIALS AND METHODS

Experiment 1: Water movement characteristics inside the system. Figure 1 shows the structure of the laboratory-scale MSL system used in the present study. For the MSL system, an acrylic box D10 X W50 X H73 cm in size, forming an alternate brick layer-like pattern with zeolite and soil mixture layers, was used. Twenty-five sets of water collection pipes (2 X 2 cm V-shape with 15 cm length) were installed in the zeolite layer between each soil mixture layer, as shown

in Fig. 1. Exits of the water collection pipes were dis- tributed on both sides of the equipment so that the flow of the water in the system would not be biased (Fig. 1). Barriers for preventing a shortcut of the water flow along the wall surface were set up in the layers when the water collection pipes were installed. The soil mixture layers contained soil (Andisol, i.e. humus-rich volcanic ash soil, Kurohoku), saw dust, granular iron, and char- coal in ratios of 7 1.6, 10.5, 1 1.9, and 6.0%, respectively, on a dry weight basis. The void spaces (permeable layers) between each block and block sides were filled with zeolite 3-5 mm in diameter, as shown in Fig. 1. Three identical systems were prepared.

To study the water movement characteristics inside the system, tap water was introduced into the system at loading rates of l,OOO, 3,000, and 5,500L m-'d-', respectively, and the quantity of water that flowed out from each water collection pipe was measured. The outflow rate from each collection pipe was measured for 5 min, from the bottom to the upper layer, to prevent the effect of water sampling on the flow of water in subse- quent layers. Except when water was sampled, the water collection pipes were closed by a plug. The measure- ments were repeated 3 times using 3 systems (9 replica- tions in total).

Experiment 2: Relationship between the loading rate and hydraulic retention time (HRT). To determine HRT, a MSL system was con- structed using on acrylic box and the same materials at those shown in Fig. 1. In this system, a water collection

Fig. 1. Structure of Multi-Soil-Layering (MSL) system for studies on water movement inside the system.

Water Movement Characteristics inside MSL 77

MSL

Inflow

Fig. 2. MSL system on balance with digital record connected to a personal computer. Water collection pipes were not installed.

pipe was not installed to avoid the influence of the pipes on HRT. The system was set up on an electric balance (digital record) with a capacity of 100 kg measurement that recorded the MSL weight every 5 min. The mini- mum display and repeatability of the balance were 10 and ? 20g. respectively (A&D Company, Ltd., HW- 100K) (Fig. 2). The water content in the system was calculated as the difference between the weight before and during the water discharge for the treatment. Loading rates of the tap water were set at about 250, 500, l,OOO, 3,000, and 6,000 L m-2 d-', respectively. The water content was measured and the HRT was calculated. For a loading rate of 250 L rn-' d-', tap water was introduced for 3 d to allow the equilibration of water absorption of the system for the onset of this experiment. Thereafter, the tap water was introduced at loading rates of 500 and 1,OOO-6,OOO L m-' d-' for 2 and 1 d, respectively. When the loading rate was changed, the water inflow was interrupted for 1 d. This allowed the water content of the MSL system to decrease to the field capacity level. The subse- quent loading rates were applied after the gravitational water was discharged for 1 d.

Experiment 3: Relationship between wastewater treatment and weight variation of the system. Wastewater was flowed into the same system that used for the HRT measurement in Experiment 2. Changes in the system weight were measured, and the wastewater and treated water were periodically collected and analyzed. In this experiment, the domestic wastewater from a nearby community disposal plant was used after three-time dilution with tap water. Average characteristics of the wastewater were as follows: SS 11.2 mg L-', COD 70 mg L-I, T-N 12 mg L-I, and T-P 0.9 mg L-I. Loading rate was set at 1,OOO L m-'d-'. This experiment was started on May 28,2002 and was termi- nated on Nov. 13,2002 (168th d), due to clogging in the top part of the system. The weight of the whole MSL system under operation was continuously measured up

to Dec. 20, 2002 (205th d). The wastewater and treated water were analyzed for

COD by the potassium dichromate method, NH,+-N by the Nessler method, NO,--N and NO,--N by ion chro- matography (DIONEX OX-120), and PO,'--P by the ascorbic acid method (APHA 1992).

RESULTS AND DISCUSSION

Experiment 1: Water movement characteristics inside the system

Figure 3 shows the relationship between the loading rate and detailed water flow characteristics in each layer of MSL. The outflow rate from each water collection pipe was determined. The position of each layer is shown as a matrix of (I, 11,111, IV, V) and (1,2,3,4,5) in Fig. 3. The outflow rates from the water collection pipes were higher under the permeable layers and increased in the lower layers of the system. Since the boundary at the bottom of the system was supported by a plastic net laid on the acrylic board in which the hole opened, the water flow slowed down. Therefore, it was considered that the water content increased in the lower layers of the system. As the loading rate increased, the outflow rate under the permeable layers significantly increased com- pared to that under the soil mixture layers. Figure 4 shows a schematic model of the relation between the water movement characteristics inside the system and the loading rate. That is, while the proportion of the water flowing into the permeable layers increased as the loading rate increased, the proportion in the soil mixture layers decreased. When the loading rate increased from 1,000 to 5,500 L rn-, d-I, the volume of the water flow into the soil mixture layers increased by 65%. while the water flow in the permeable layers increased by 446% (mean value, respectively). This showed that while the permeable layers enabled high-speed treatment com- pared to the conventional water purification method by soil, the contact of the wastewater with the soil mixture layers should be improved in order to maintain the purification capacity of the MSL system. In a previous paper (Masunaga et al. 2002), it was reported that the treatment efficiency could be improved by decreasing the size of the soil mixture layers and expanding the surface area.

In this experimental method, although the trend of the water movement inside the system could be observed, the lateral water movement inside the system and the effect associated with the water collection pipes installed in the system could not be fully elucidated. Therefore, a more detailed analysis for the quantifica- tion of the water movement inside the system should be conducted .

78 K. SAT0 et al.

n

Loading rate(L m-2 day-')

Fig. 3. Relationship between loading rate and water outflow from 25 water collection pipes.

I 000 L rn-' day.' 5500 L m-2 day.'

Fig. 4. Schematic model of water movement inside the system.

Experiment 2: Relationship between the loading rate and hydraulic retention time (HRT)

Figure 5 shows the changes in the water content inside the system at the loading rates of 250-6,000 L m-2 d-'. Because of the malfunction of the pump, at the loading rate of 539 L m-z d-I, there were some fluctuations. In Fig. 5 , the quantity of water in the MSL system under various loading operations was determined as the total gross weight of MSL minus the dry weight of the acrylic box, soil mixture, and permeable layers. The amount of water inside the system immediately increased when the inflow started to increase, and then rapidly decreased when the inflow stopped for all the loading rates examined. The quantity of water increased as the loading rate increased. However, 1 d after the dis- continuation of the inflow, the amount of water retained in the system did not increase proportionally to the

increase of the loading rate. The water drained for 24 h was considered to correspond to the gravitational water which flows in macropores and the water retained after 24 h was possibly kept in the micropores. As a result, it was shown that the quantity of water that flows in the macropores increased with the increase of the loading rate.

Although the water content inside the system at each loading rate seemed to increase with the lapse of time, HRT was estimated by considering that the equilibrium state had been reached during the observation time. The relationship between the hydraulic retention time and loading rate is shown in Fig. 6, indicating that, a higher loading rate resulted in a shorter hydraulic retention time. As the water content in the system did not increase proportionally to the increase of the loading rate, the hydraulic retention time was inversely propotional to the loading rate.

Experiment 3: Relationship between wastewater treatment and weight variation of the system

The fluctuations of the net system weight during the 205-d period of wastewater inflow is shown in Fig. 7. This graph indicates the value obtained when the dry weight of the materials composing the system, such as the acrylic box and soil (based on dry weight) was sub- tracted from the total weight including the system, water


13.5

12.5

11.5

10.5

9.5

27.0L da .I 539L m2 da I ,3.5 52.2L da I l o w . m-2 da . ' I

HRT=I 0.4h HRT=S.Sh ~

2 bY 2 13,5 l50.6L day-' (3012L m-' day-') 13,5 3O4.2L d a y ' (6083L m-' d a y 0 3 I

c c 11.5

HRT=Z.Oh HRT=I. I h I I 2 9.5

- - . . ____

I5

5

0 0 2000 4000 6000 8OOO

Loading late (L m? day')

Fig. 6. retention time (HRT).

Inverse relation between loading rate and hydraulic

and accumulated materials inside the system. Periodical decrease of the weight was due to the interruption of the inflow for the preparation of wastewater or maintenance of the water pump for 0.5 to 3 h.

The weight increased until about the 30th d, as shown in Fig. 7. It was considered that the suspended solids in the wastewater accumulated in the system by adsorption and filtration. Additionally, the permeability decreased by the accumulation of the materials and increase of the number of microorganisms, which increased the water content in the system. Based on the SS and COD concentrations and the loading rate of the wastewater, the increase in the water content due to the decrease in the permeability seemed to contribute more significantly to the increase in the weight than the accumulation of the materials. The weight was almost stable from the 30th to lOOthd. It was assumed that the rate of organic matter decomposition was equilibrated with the input of organic matter through wastewater loading. After the 100th d, the weight in the system rose gradually, indicating that the input of organic matter seemed to exceed the decomposition. During this period, the accumulation of organic materials and the water content in the system increased. From the 126thd, the fluctuations in the weight were substantial. Before the 126th d, the weight decrease associated with the interruption of the inflow for maintenance purpose returned to the weight before the interruption of the inflow when the inflow was

Fig. 6. and quantity of water in the system.

Relationship between loading rate

resumed. From the 126thd, however, the weight which once exceeded the weight before the interruption of the inflow, started to decrease and became eventually stable. These fluctuations were repeated every time for maintenance purpose and the weight of the system increased gradually after the 126thd. Except for these fluctuations, the weight seemed to increase linearly from around the lOOthd. The inflow to the system was dis- continued on the 168th d, because the inflow water stagnated on the surface of the top of the system due to clogging of the system. Such weight fluctuations proba- bly occurred with the following sequence. 1) Air pene- trated into the system when the inflow interrupted was trapped in the pore spaces in the layers where organic materials accumulated, which decreased the permeability of the system. This prevented smooth water movement and water stagnated in the layers until water accumulated to some extent upon the resumption of the water inflow. 2) The water pressure that increased by the accumulation of the water inflow generated the shortcut and the weight of the system rapidly decreased. When the shortcut occurred, it was assumed that water flowed down only where it could easily pass through the permeable layers, especially on the wall surface of the system. 3) Thereafter, the weight was likely to stabilize, when the inflow and permeability rate were balanced. Addi- tionally, shrinkage and swelling of some part of the accumulated organic materials by drying and rewetting with the interruption and resumption of the inflow seemed to contribute to the decrease in permeability.

The fluctuations of the COD, PO,'--P, NH4+-N, NO,--N, and NO,--N concentrations of the treated water and wastewater are shown in Fig. 8. In the removal of organic matter, the COD concentration was low from the initial stage of the experiment until the 68th d. Accumulation by adsorption and filtration was considered to be the main treatment process at the initial stage, as indicated by the daily increase of the system weight for the first 30-d period depicted in Fig. 7, and the decomposition of organic matter occurred after about

80 K. SAT0 et al.

I I I I

0 50 100 150 200

day

Fig. 7. Changes in the weight of water and accumulated materials inside the system during wastewater treatment.

0 60 120 180 0 60 120 180 day day -

2 0.2 0 0 60 120 180 0 6OdaY120 180

day

+ Wastewater ? ++ Treated water 0" z

0 0 60 120 180

day

Fig. 8. Changes in COD, PO,3'-P, NH,+-N, N02--N, and NO,--N concentrations of the treated water and wastewater.

the 30thd. The COD concentration increased on the 108th d. This increase overlapped with the time at which the weight of the system increased. This overlap may indicate the decrease in the contact of the wastewater with the soil mixture layers by the shortcut, which gen- erally reduced the treatment efficiency of the system. The decomposition rate of organic matter may also decrease under the anaerobic conditions associated with the accumulation of organic materials. The PO,3--P concentration showed a similar trend to that of COD. The mechanism of phosphorus removal is based on phosphorus adsorption by active aluminum contained in soil and also by ferric oxides formed from the metal iron added to the soil mixture layers (Nyle and Ray 2002). After the 108th d, phosphorus removal seemed to decrease due to the decrease in the contact of the wastewater with the soil mixture layers associated with the accumulation of organic materials. Moreover, it was also assumed that the formation of ferric oxides was inhibited by the

anaerobic conditions in the system for the increase of the water content and by the accumulation of organic materials in the system. The concentrations of NH,+-N and NO,--N were consistently low. However, the concentration of NO,--N which was high at the initial stage, gradually decreased with time. From the 108th d when the weight in the system increased, NO,--N was hardly detected. This fact suggests that denitrification was enhanced by the accumulation of organic materials that acted as an electron donor and created anaerobic conditions in the system.

CONCLUSION

The studies on the water movement inside the system, revealed that the contact of the wastewater with the soil mixture layers decreased with increasing loading rate. This suggested that improvement of the contact by reducing the size of the soil mixture layers as much as possible could enlarge the surface area required to enhance the wastewater treatment efficiency at a high loading rate (Masunaga et al. 2002). Through the accumulation of quantitative data on the relationships between the size, surface area, and permeability of the soil mixture layers as well as treatment efficiency in relation to various parameters representing treatment conditions such as loading rate, and targeted quality of treated water and wastewater, we may eventually be able to design optimum MSL structures for the respective treatment conditions.

The relationship between the loading rate and HRT was inversely proportional. The amount of water hold in the system did not change appreciably with the changes in the loading rate. HRT could vary with the changes in the material composition and the structure of the MSL system. Measurement of the quantitative relationship between the material composition and structure of the system and HRT in connection with the quality and quantity of the target wastewater is necessary for deter- mining the optimum operational conditions of the MSL system in future. In addition, further investigations on


the HRT of the kinds of pollutants such as COD, P, and N in the system should be carried out. Various parameters including HRT, redox potential, adsorption, and filtration of the pollutants, and biological response should be considered in future studies.

Clogging has been an important issue to address in soil-based wastewater treatment systems. In the present study, the efficiency of the removal of organic matter and phosphorus decreased when the system started to clog, while, the efficiency of nitrate removal increased. However, if wastewater cannot pass through the system due to clogging, the treatment becomes impossible. Regarding clogging, various studies have been conducted and countermeasures have been applied (Siegrist 1987; Taylor and Jaffe 1990; Seki et al. 1998; Magesan et al. 2000). In the studies on Multi-Soil-Layering systems until now, it had been suggested that the aeration of the systems was effective to prevent clogging (Sato et al. 2002b). However, since various aspects related to the mechanism of clogging and countermeasures have not been fully evaluated, further investigations should be carried out in the future.

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water movement characteristics in a multi-soil-layering system

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