Applied and Environmental Soil Science
Volume 2019 (2019), Article ID 1214368, 11 pages
https://doi.org/10.1155/2019/1214368

Research Article

Evaluation of Long-Term Wastewater Treatment Performances in Multi-Soil-Layering Systems in Small Rural Communities

Kuniaki Sato, Toshiyuki Wakatsuki, Noriko Iwashima, and Tsugiyuki Masunaga

Faculty of Life and Environmental Science, Shimane University, Matsue 690-8504, Japan

Correspondence should be addressed to Kuniaki Sato; ksato@life.shimane-u.ac.jp

Received 19 August 2018; Accepted 19 December 2018; Published 23 January 2019

Academic Editor: Rafael Clemente

Copyright © 2019 Kuniaki Sato et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Multi-soil-layering (MSL) wastewater treatment systems consist of soil units (soil mixture blocks, SMB) arranged in a brick-like pattern surrounded by permeable layers of zeolite or alternating particles of homogeneous sizes that allow for a high hydraulic loading rate. This study evaluated the performances of MSL systems that have been operating for 17 to 20 years in small rural communities. Even though 20 years had passed since this system was installed, high organic matter treatment performance continued. Nitrogen removal was higher than with conventional soil systems. Two of the MSL systems continued to show high phosphorus removal performances, whereas in the third system, the adsorbing capacity was relatively low, requiring further investigation. Treatment performances were largely dependent upon the structure of the MSL systems. It appeared that improving the structure to enhance the contact efficiency between the wastewater and the soil in SMB was important for enhancing treatment performances. The combined use of existing wastewater treatment systems with the MSL system was effective for preventing environmental pollution over a long period.

1. Introduction

Water treatment systems incorporating soil have been widely used as on-site wastewater treatment systems in the U.S. [1], Australia [2], and Europe [3]. It has been reported that approximately 21% of American homes are served by on-site sewage disposal systems, and 95% of these are septic tank field systems [4]. Septic tank systems are generally composed of a septic tank and a soil drain field. Sedimentation of solids in the wastewater and anaerobic decomposition of organic materials occur within the septic tank. Afterward, the wastewater is treated in the soil drain field by infiltrating and percolating into the soil. Although septic tank systems have been used in urban fringe and rural areas that are difficult to connect to centralized wastewater collection systems, several adverse impacts have been identified, including the spread of diseases, contamination of ground and surface water, degradation of soil and vegetation, decreases in amenities due to odors and insects, and potential litigation [5]. Previous studies have shown that not all soil types have the capacity to provide adequate treatment and dispersal of sewage effluent [6]. On-site soil characteristics such as texture, permeability, cation exchange capacity, and groundwater level have a great influence on wastewater treatment performance.

To maximize the water-purifying function of soil, a multi-soil-layering (MSL) method has been investigated. MSL systems consist of soil units (soil mixture blocks, SMB) arranged in a brick-like pattern surrounded by permeable layers (PL) of zeolite or alternating particles of homogeneous sizes that allow for a high hydraulic loading rate (HLR). The MSL system has the added advantage that its performance does not depend strongly on soil properties at the site because the water purification function of the SMB can be enhanced by mixing materials such as sawdust, jute, iron metal, and charcoal with the soil [7].

Based on previous studies, the following wastewater treatment mechanisms have been proposed [8, 9]. Biological oxygen demand (BOD) and chemical oxygen demand (COD) components are trapped in the SMB. The trapped organic materials are decomposed subsequently by microbial activities. Organic matter removal can be enhanced by mixing charcoal with the SMB. Ammonium nitrogen (NH₄-N) can be adsorbed onto exchange sites of soil and zeolite and then can be oxidized to nitrate nitrogen (NO₃-N) by nitrification. Subsequently, NO₃-N is translocated to the SMB, which are relatively anaerobic due to percolating wastewater, and can be reduced to N₂ gas. Organic materials within the SMB, as well as the organic components of the wastewater, can supply carbon to enhance denitrification processes. Iron and aluminum hydrous oxides and aluminosilicate clays can adsorb phosphate ions; to enhance phosphate adsorption, metal iron particles can be mixed with the SMB.

Several basic and applied studies on MSL systems have been undertaken in Japan, Thailand, Morocco, China, Taiwan, and the U.S. Basic studies of water movement inside the system [10, 11], wastewater treatment mechanisms [9], material composition of SMB and PL [7,12–14], aeration effects [13], horizontal flow MSL system [15], and the relationship between treatment capacities, HLR, and contamination levels [16, 17] have been conducted on these systems. More applied studies have included water purification for river water [18], domestic wastewater [18, 19], livestock wastewater [20], textile wastewater [21], leachate from unsanitary landfills [22], and olive mill wastewater [23] and have been carried out using regional resources. Based on the results of these basic and applied studies, some MSL systems have been put into practical use for the treatment of river water and wastewater from toilets in parks and small communities.

However, little experimental work has been done on the evaluation of long-term wastewater treatment performance in MSL systems. Luanmanee et al. [8] evaluated the efficiency of an MSL system for treating domestic wastewater during its ninth and tenth years in operation; however, this research was also a pilot-scale experiment on individual domestic wastewater treatment, and long-term performance was not evaluated on a practical application level [24]. Research on long-term wastewater treatment performance and system lifespan will be useful for creating design guidelines for MSL systems.

This study investigated the wastewater treatment performances of three MSL systems that have been operating for 17 to 20 years in small rural communities. We calculated the mass balances of BOD, nitrogen, and phosphorus and evaluated the effects of the structural differences between the three MSL systems on wastewater treatment efficiencies.

2. Materials and Methods

2.1. Experimental Apparatus and Description of Study Sites

This research was conducted in three domestic wastewater treatment facilities with MSL systems that were installed in small rural communities in Shimane Prefecture, Japan. Figure 1 shows the flow diagram of wastewater treatment. First, the wastewater from these small rural communities was treated in biological contact aeration equipment (BCAE). All three sites have used the same BCAE system. The BCAE consisted of settling and separation tanks 1 and 2, contact aeration tanks 1 and 2, another settling tank, and a pump tank. When a water level detection sensor detected a fixed water level in the pump tank, the submerged pump initiated operations, and water treated by the BCAE was discharged to the MSL system. Therefore, treated water was loaded intermittently to the MSL system and was treated by the MSL system in unsaturated vertical flow conditions. The water treated by the MSL system was discharged to the river by gravity flow along a natural slope after chlorine sterilization.

Figure 1: Flow diagram of wastewater treatment in the small rural community.

Figure 2 shows the structure and piping diagram of the MSL system at each site. The truncated square pyramid holes were excavated, and the vinyl sheets were laid out over the entire surface of holes to avoid groundwater pollution. The bottom 30 cm (SITE 1 and SITE 2) or 20 cm (SITE 3) was filled with gravel rock (4–10 cm diameter). The porous drainage pipe (15 cm diameter) was installed in the bottom of the gravel rock layer. The surface of the gravel layer was covered with a plastic net, then the MSL structures, consisting of three SMB layers, were constructed on the plastic net. The SMB contained a volcanic ash soil that was rich in organic matter and was classified as an Andisol. The PL (the void spaces between each soil block and the block sides) contained a weathered sandy granite soil (Entisol) and perlite in a ratio of 10 : 1 on a dry weight basis. The SMB were wrapped in jute cloth and placed 10 cm high and 100 cm wide; SMB were arranged in a brick pattern surrounded by the PL. The depth of SMB varied depending on the site. Although the scale of the MSL systems was nearly the same for SITE 1 and SITE 3, the depth of SMB at SITE 1 was deeper than that at SITE 3. The soil blocks at SITE 3 were divided further than those at SITE 1 (Figure 2). The intervals between soil blocks in the horizontal and vertical directions were 20 cm and 10 cm, respectively. In the upper part of the MSL structure, a porous inlet pipe (5 cm diameter) was installed and filled with gravel rocks approximately 4 cm in diameter. The surface of the gravel layer was covered with a plastic net and, to enhance permeability, the top of the system was covered by a soil mixture containing weathered granitic soil, volcanic ash soil, and perlite in a ratio of 10 : 1 : 1 on a dry weight basis.

(a)

(b)

(c)

(d)

Figure 2: The structure and piping diagram of multi-soil-layering system at each site: (a) SITE 1, (b) SITE 2, (c) SITE 3, and (d) enlarged view of MSL structure.

Table 1 describes the three study sites in the small rural communities from July 2012 to June 2013. At the time of the study, these facilities had been in operation for 17 to 20 years since installation. SITE 1, SITE 2, and SITE 3 were installed in 1996, 1994, and 1993, respectively. Total pipeline lengths in these facilities ranged from 425 to 882 m, and the number of users associated with SITE 1, SITE 2, and SITE 3 were 30, 24, and 35, respectively. When the facilities were first installed, the numbers of associated users were 54, 30, and 52, respectively. These numbers have decreased by 20% to 44% over the operation periods due to depopulation in the region. The top area of the MSL system at SITE 1 (198 m²) was the largest, whereas the area at SITE 2 (139 m²) was the smallest. Mean water consumption in each community ranged from 4.84 to 8.76 m³·day⁻¹, and the HLR in the associated MSL systems ranged from 0.030 to 0.051 m³·m⁻²·day⁻¹. At SITE 1 and SITE 2, the MSL systems operated from May through October because the river water was utilized for rice farming during those months. Treated water from the BCAE was discharged directly into the river during other months. At SITE 3, the MSL system operated all year due to a defective flow selector valve, but it is unknown when the defect occurred.

Table 1: Description of the three study sites in small rural communities from July 2012 to June 2013.

Index
SITE 1
SITE 2
SITE 3

Duration of operation (years)
17
19
20

Total pipeline length (m)
882
701
425

Number of associated users
30
24
35

Top area of MSL system (m²)
198
139
173

Mean water consumption in each community (m³·day⁻¹)
5.96
4.84
8.76

Loading rate in MSL system (m³·m⁻²·day⁻¹)
0.030
0.035
0.051

Operation period of MSL
May–Oct.
May–Oct.
All year round

2.2. Water Sampling and Quality Measurement

At these three sites, wastewater and treated water were sampled about once per month from July 2012 until June 2013. Water samples were collected from (1) the settling and separation tank 2 in the BCAE as wastewater, (2) the pump tank in the BCAE as the treated water, and (3) the discharge pipe from the MSL system (before chlorine sterilization) as the treated water (Figure 2). The data from these samples were referred to as WW, TWBCAE, and TWMSL, respectively. At SITE 2, the samples could not be collected on February 8, 2013, due to heavy snow, and the WW sample could not be collected on April 15, 2013, due to maintenance on the BCAE. Collected water samples were analysed for the following: phosphate phosphorus (PO₄-P) concentrations using the ascorbic acid method, total phosphorus (T-P) concentrations using potassium peroxodisulfate digestion and the ascorbic acid method, pH and oxidation-reduction potential (ORP) using the electrode method, BOD using a dissolved oxygen (DO) meter, COD using the potassium dichromate method, NH₄-N concentrations using the Nesslerization method, NO₃-N and nitrite nitrogen (NO₂-N) concentrations using ion chromatography (DIONEX DX-120), and total nitrogen (T-N) concentrations using potassium peroxodisulfate digestion and the ultraviolet absorption method.

3. Results and Discussion

3.1. Phosphorus Removal

Figure 3 shows changes in the phosphorus components of wastewater and treated water from the BCAE and MSL systems. WW concentrations at SITE 1 were high and fluctuated throughout the year, particularly for forms (Total minus PO₄-P), which were regarded as organic . In TWBCAE from all sites, PO₄-P concentrations showed either no change or increased compared with those from WW samples, whereas concentrations decreased. Organic was decomposed and/or removed by sedimentation in the BCAE.

(a)

(b)

(c)

Figure 3: Changes in the phosphorus components of wastewater (WW) and treated water from the BCAE (TWBCAE) and MSL systems (TWMSL). At SITE 2, the samples could not be collected on February 8, 2013, and the WW sample could not be collected on April 15, 2013: (a) SITE 1, (b) SITE 2, and (c) SITE 3.

Phosphorus treatments by the MSL system were extremely efficient at SITE 3 and performed better overall than the system at SITE 1, although inflow concentrations (TWBCAE) were similar at both sites and the loading rate at SITE 3 was approximately 1.7 times higher than that at SITE 1. The mechanism for phosphorus removal was physicochemical adsorption by soil components, and the removal performance was largely influenced by the efficiency of the contact between wastewater and the soil in SMB. In a previous study, an MSL system with thinner and narrower SMB (i.e., larger SMB surface area) had higher removal rates for SS, BOD, COD, and T-P because of the enhanced contact efficiency between the wastewater and the SMB [25]. The results of this study indicate that the greater performance of the MSL system at SITE 3 was due to the enhanced division in the SMB structures versus those at SITE 1. Because of this increased division, the MSL system at SITE 3 had the larger SMB surface area.

The treatment efficiency of the MSL system at SITE 2 was lower than that at other sites. Previous studies have suggested that phosphorus removal is based mainly on phosphorus adsorption onto active aluminum hydroxides and ferric hydroxides contained in soils [7]. The ORP of the TWMSL from SITE 2 was lower than that from the other two sites (Figure 4), indicating that a relatively anaerobic condition was established inside that MSL system compared with the other sites. As a result, phosphorus adsorption on ferric hydroxides in the MSL system at SITE 2 most likely decreased because the presence of ferric hydroxides decreased. In addition, it is possible that shortcut flow occurred inside the MSL system due to an inflow pipe failure, which caused a decrease in the contact efficiency between the wastewater and the SMB in the system. In previous study, concentrated inflow caused a biased water flow (i.e., shortcut flow) towards a part of inside the system [10]. If TWBCAE was discharged only from a part of the inflow pipe due to the pipe failure, shortcut flow occurred inside the MSL system. This shortcut flow likely caused a decrease in organic matter decomposition and the ORP of the TWMSL.

(a)

(b)

Figure 4: Changes in (a) ORP and (b) pH of wastewater and treated water from the BCAE and MSL systems.

3.2. Removal of Organic Matter (BOD and COD)

Figure 5 shows changes in BOD and COD concentrations in WW, TWBCAE, and TWMSL at each site. The monthly BOD fluctuations in WW were high, and the concentrations in TWBCAE fluctuated slightly according to the WW concentrations. The TWMSL samples showed extremely low and stable concentrations that were highly purified. The treatment performance of the BCAE at SITE 1 was inferior to those at other sites. The BOD of TWMSL at SITE 1 was slightly higher than those at the other sites, partially due to the high BOD loading to the MSL system. At SITE 3, the treatment performances of the BCAE and MSL systems were consistently higher. According to the Act on Septic Tanks in Japan, a standard BOD value for sewage effluent is less than 20 mg·L⁻¹. Although the BCAE systems were not able to achieve these criteria alone, combined performance with the MSL systems did achieve the criteria throughout the year at all sites.

(a)

(b)

Figure 5: Changes in (a) BOD and (b) COD of wastewater and treated water from the BCAE and MSL systems.

Changes in COD concentrations showed similar trends to BOD concentrations. TWMSL at SITE 3 also showed the lowest COD concentrations. A possible cause for the better performance at SITE 3 is the difference between the MSL system structures at the different sites, as mentioned above. The SMB at SITE 3 were more divided than those at SITE 1 so that the wastewater could more efficiently infiltrate the SMB; therefore, it was likely that the organic matter was also trapped more efficiently in the SMB. On the contrary, COD removal capacity of the MSL system at SITE 2 was comparatively low, despite the high BOD removal capacity. COD includes both easily decomposable organic matter and slowly decomposable organic matter, whereas BOD represents only the easily decomposable organic matter. The results of phosphorus treatments indicated that the problems such as shortcut flow occurred in the MSL system at SITE 2. It was likely that the COD removal efficiency was easily affected by them, whereas the BOD removal efficiency was not.

3.3. Nitrogen Removal

Figure 6 shows changes in the nitrogen components of wastewater and treated water from the BCAE and MSL systems. The NH₄-N accounted for a large portion of the components in the wastewater, and the organic N (i.e., T-N minus inorganic N) was the second largest portion of that contaminant pool. The wastewater at SITE 1 contained some NO₃-N (2.7 mg·L⁻¹ on average) probably because the wastewater had a higher ORP (Figure 4) and was relatively aerobic compared with the other sites.

(a)

(b)

(c)

Figure 6: Changes in the nitrogen components of wastewater and treated water from the BCAE and MSL systems: (a) SITE 1, (b) SITE 2, and (c) SITE 3.

In the BCAE, the removal efficiency was not very high, and the removal rate was only around 20%. The distribution ratios of the components differed among the three SITEs, although T-N concentrations were similar across the sites. At SITE 1, the proportion of NH₄-N was high and NO₃-N was low. The nitrification activity was restricted; pH was higher than that at the other two sites (Figure 4). On the contrary, the proportion of NO₃-N was higher at SITE 3. The BCAE at SITE 3 indicated higher nitrification activity resulting in an accumulation of NO₃-N, and the pH of the treated water was lower than the pH of the wastewater.

In the MSL system at SITE 1, the nitrogen removal efficiency was lower compared with the other sites, and the NH₄-N occupied a large portion of the nitrogen components. In the MSL system at SITE 2, the T-N concentrations were less than 10 mg·L⁻¹. The NO₃-N concentrations were extremely low (nearly 0 mg·L⁻¹), whereas NH₄-N occupied a large portion of the nitrogen components. In the MSL system at SITE 3, the average T-N concentration was extremely low at 1.7 mg·L⁻¹. The pH values in the MSL systems at SITE 1 and SITE 2 decreased compared with those in the BCAE, whereas the pH in the MSL system at SITE 3 increased compared with that in the BCAE. This is most likely because the denitrification activity in the MSL system at SITE 3 was higher than that in the MSL systems at SITE 1 and SITE 2, efficiently removing NO₃-N from the system.

In this study, the NO₃-N concentrations in treated water from MSL systems were extremely low at all sites; therefore, it is likely that the inside of the MSL systems remained relatively anaerobic, allowing denitrification activities to occur easily. The gravel-sized materials, such as zeolite and pumice, have been used as PL materials for high-speed treatments and clogging prevention [18]. In this study, a mixture of weathered granite soil and perlite was used for the PL materials. The application of weathered granite soil to the PL increased the water content inside of the MSL system compared with an MSL system using gravel-sized materials for the PL. The inside of the MSL system, particularly inside the SMB, became anaerobic, elongating the residence time of nitrogen components. As a result, denitrification activity probably took place easily in the MSL systems in this study. In the case of a high-speed treatment, such as 2000 to 4000 L·m⁻²·day⁻¹ for a river water treatment or a final tertiary wastewater treatment, the application of gravel-sized materials for the PL is needed to maintain water permeability and to prevent clogging; however, the application of weathered granite soil was likely more effective for denitrification activity in a case with low HLR, such as in this study. In addition, there were indications that the higher proportion of NO₃-N in treated water from the BCAE at SITE 3 led to increased denitrification activity in the MSL system, ultimately removing most of the nitrogen from the treated water. However, introducing excess NO₃-N to the MSL system may cause leaching of NO₃-N because soil particles generally have a negative charge. To increase the nitrogen treatment performance in the MSL system, it will be necessary to establish an appropriate operation management technique for the BCAE according to the incurrent wastewater quality and quantity.

3.4. Mass Balances of BOD, COD, T-N, and T-P

Table 2 shows the mass balances of BOD, COD, T-N, and T-P from July 2012 to June 2013 at each site. The mass balance data were calculated from the concentration of each parameter and volume of water consumption at each site.

Table 2: Mass balances of BOD, COD, T-N, and T-P from July 2012 to June 2013.

Index
BOD
COD
T-N
T-P

SITE 1
SITE 2
SITE 3
SITE 1
SITE 2
SITE 3
SITE 1
SITE 2
SITE 3
SITE 1
SITE 2
SITE 3

Total load (from wastewater)
159.1 (100)
172.4 (100)
223.5 (100)
363.3 (100)
330.9 (100)
427.5 (100)
77.6 (100)
63.0 (100)
113.8 (100)
10.5 (100)
6.6 (100)
11.7 (100)

Removal in BCAE
75.6 (48)
125.0 (72)
171.9 (77)
196.0 (54)
216.5 (65)
286.0 (67)
15.8 (20)
13.1 (21)
19.9 (17)
3.0 (29)
0.6 (9)
1.8 (16)

Discharge from BCAE
83.5 (52)
47.4 (28)
51.7 (23)
167.3 (46)
114.4 (35)
141.5 (33)
61.7 (80)
49.8 (79)
93.9 (83)
7.4 (71)
6.1 (91)
9.8 (84)

Discharge into river from BCAE (Nov–Apr)
34.3 (22)
13.5 (8)
—
76.3 (21)
39.8 (12)
—
24.5 (32)
17.2 (27)
—
2.7 (26)
2.1 (31)
—

Load into MSL
49.2 (31)
33.9 (20)
51.7 (23)
91.1 (25)
74.6 (23)
141.5 (33)
37.3 (48)
32.6 (52)
93.9 (83)
4.7 (45)
4.0 (60)
9.8 (84)

Load per unit area in MSL (g·m⁻²)
248.0
243.5
299.5
459.3
535.8
819.6
188.0
234.1
543.9
23.7
28.7
56.8

Removal in MSL
40.6 (26)
31.2 (18)
49.3 (22)
71.8 (20)
57.8 (18)
121.3 (28)
19.4 (25)
24.1 (38)
88.4 (78)
3.7 (36)
1.4 (21)
9.1 (78)

Removal rate in MSL (%)a
82.4
92.0
95.3
78.8
77.5
85.8
52.2
73.8
94.2
80.0
35.2
93.0

Discharge from MSL
8.6 (5)
2.7 (2)
2.4 (1)
19.3 (5)
16.8 (5)
20.2 (5)
17.8 (23)
8.5 (14)
5.5 (5)
0.9 (9)
2.6 (39)
0.7 (6)

Total discharge
42.9 (27)
16.2 (9)
2.4 (1)
95.6 (26)
56.5 (17)
20.2 (5)
42.3 (55)
25.8 (41)
5.5 (5)
3.7 (35)
4.7 (70)
0.7 (6)

Unit = kg (the data in parenthesis are expressed as relative values to total loading amount from wastewater (%)). aRemoval rate in MSL (%) = removal in MSL/load into MSL from BCAE × 100.

The BOD removal performance in the BCAE at SITE 1 was lower than the other sites, and the removal rate was 48%, whereas the removal rates at SITE 2 and SITE 3 were 72% and 77%, respectively. During the period in which MSL operation ceased, 8% and 22% of the total BOD loads from wastewater at SITE 1 and SITE 2, respectively, were discharged to the river directly. The BOD removal efficiencies in the MSL systems were high, and the removal rates were greater than 80%. In particular, the MSL system at SITE 3 had the highest BOD removal rate (95.3%), even though the total BOD load and BOD load per unit area were the highest because the number of associated users was the highest, and there was no cessation period during the year. As mentioned above, the larger SMB surface area of the MSL system at SITE 3 was most likely related to that high removal rate. The complete water treatment system at SITE 3 removed 99% of the BOD from the wastewater because the MSL system worked year-round without the discharge of treated water from the BCAE directly into the river. The COD removal rates of the MSL system were lower than the BOD removal rates because COD includes both easily decomposable organic matter and slowly decomposable organic matter. The MSL system at SITE 3 had the highest COD removal rate, similar to its BOD removal rate. Assuming that the removal rates for the MSL systems are not affected by variable operation periods, treating BCAE-treated water with an MSL system with no period of cessation removed more than 90% of both BOD and COD from wastewater at each site.

The T-N removal rates for the BCAE at each site were approximately the same at 17% to 21%. On the contrary, the rates from the MSL system varied across sites, ranging from 52.2% to 94.2%. The MSL system at SITE 3 had an extremely high removal rate. It has been reported that nitrogen removal rates for conventional septic tank systems may vary from 0% to 35% [26]. Removal rates in an MSL system are improved dramatically over conventional treatment systems. Additionally, it has been proposed that a water-dosing method provides a more uniform effluent distribution to the drain field, increasing the denitrification potential of an on-site wastewater disposal system [26]. A previous study showed that the structure of an MSL system provided effective water dispersion inside the system [10]; therefore, improving the flow of water most likely increased the efficiency of nitrogen removal. Moreover, the previous study also indicated that increasing the number of SMB in the system by reducing their widths helped to enhance the dispersion of water flow [10]. Greater nitrogen removal performance in the MSL system at SITE 3 compared with that at SITE 1 was most likely due to the larger number of SMB in the SITE 3 system that enhanced the dispersion of water flow. Although there are limits for subdividing SMB due to the technical restrictions on construction, improvements in nitrogen removal performances were expected with modification of the MSL structures. In addition, it is important to consider optimal system construction based on controlling the nitrogen components in the BCAE, as well as the aerobic-anaerobic conditions within the MSL system.

The T-P removal rates from the BCAEs varied between the three sites and ranged from 9% to 29%. The T-P removal rates from the MSL systems were highest at SITE 3, less at SITE 1, and lowest at SITE 2 (93.0%, 80.0%, and 35.2%, respectively).

Wakatsuki et al. [7] estimated that an Andisol can adsorb 1.0 g of PO₄-P per kg, whereas a weathered granite soil (quartz-rich sandy soil) can absorb 0.1 g of PO₄-P per kg in a conventional soil system. Based on the amount of materials (bulk densities of Andisol and weathered granite soil were 0.9 and 1.5 kg·L⁻¹) at each site in this study, it was estimated that the MSL systems at SITE 1, SITE 2, and SITE 3 can adsorb 49.9, 34.5, and 43.0 kg of PO₄-P, respectively. Total phosphorus load from the BCAE to the MSL systems at SITE 1, SITE 2, and SITE 3 from July 2012 to June 2013 were 4.67, 4.01, and 9.84 kg of PO₄-P, respectively. These amounts were calculated from T-P concentration of TWBCAE and volume of water consumption in each community. Assuming that the MSL system was able to remove 80% of the total phosphorus load from the BCAE, the lifespan of phosphorus purification in the MSL systems at SITE 1, SITE 2, and SITE 3 were calculated at 13.4, 10.8, and 5.5 years, respectively. However, these lifespans may be overestimated because the number of associated users was larger in past years.

Although all three MSL systems of this study surpassed the calculated lifespan for phosphorus purification, the MSL systems at SITE 1 and SITE 3 exhibited high phosphorus removal performances. Enhancing the contact efficiency between wastewater and the soil in SMB with the MSL structure seemingly enhanced the purification capabilities for phosphorus. As mentioned above, the MSL system at SITE 3 exhibited higher phosphorus removal performance than that at SITE 1, and these results indicate that the MSL structure greatly affected the phosphorus removal performance. In addition, Luanmanee et al. [13] indicated that reduced and eluted iron (in the form of ferrous ions) from the SMB under anaerobic conditions can then be oxidized to insoluble ferric hydroxide under aerobic conditions in the PL; the resulting ferric hydroxide is able to effectively remove phosphorus in an MSL system. These results indicate that an MSL system can adsorb and remove phosphorus greater than conventional soil systems, and because of these reasons, calculated lifespans will be greater than the above estimates. Since phosphorus removal is a physicochemical reaction, unlike the biological reaction such as organic matter and nitrogen treatment, enhancing the contact efficiency between wastewater and the soil in SMB with optimal SMB sizing and mixing iron particles with the SMB are important methods for prolonging the lifespan of the system.

In the MSL system at SITE 2, the removal performance was low, and the adsorption capacity of the soil may have reached its saturation point; however, the phosphorus load into the MSL system at SITE 2 was not greatly different from the loads at SITE 1 and SITE 3. Although the occurrence of anaerobic conditions and shortcut flow may have affected the low removal performance, as mentioned above, there may be other factors involved, and further investigation, such as soil analysis and pipeline inspection, is needed to confirm.

4. Conclusions

Even after 20 years since the installation of the MSL systems in this study, efficient organic matter and nitrogen removal are still occurring. Two of the MSL systems are exhibiting high levels of phosphorus removal; however, in the third system, the adsorption capacity of the soil might have reached its saturation point, and further investigation is needed. Wastewater treatment performances were largely dependent upon the structure of individual MSL systems. The results of this study indicate that the combined use of the existing wastewater treatment system (BCAE) with the MSL system was effective for preventing environmental pollution over the long term.

Data Availability

The data used to support the findings of this study are included within the article.

Conflicts of Interest

The authors declare that there are no conflicts of interest regarding the publication of this paper.

Acknowledgments

The authors thank the Faculty of Life and Environmental Science in Shimane University for help in financial supports for publishing this report.

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