3.1.1 Organics and nutrients removal
After the system's acclimatisation, the VGCWs were operated at three different HRTs of 6, 12 and 24 h to evaluate the removal efficiencies of COD, SCOD, TKN and TP. Table 3 shows the removal efficiencies of TCOD, SCOD, TKN and TP at each HRT during steady-state conditions. The TCOD, SCOD, TKN, and TP removal efficiencies at the HRT of 24 h were 85 ± 7.3, 70 ± 6.1, 90 ± 2.5 and 80 ± 4.2%, respectively, significantly higher than those in the HRT operations of 6 and 12 h (p < 0.05). The TCOD and SCOD removal efficiencies were found to increase with increasing HRT because there was sufficient time for the microorganisms to biodegrade the organic matters. According to Masunaga et al (2007), sufficient retention time provided in the systems also enhanced the adsorption, filtration and nitrification reactions, resulting in improved wastewater treatment efficiency (Guan et al., 2012). The long period of retention time supports plant uptake of nutrients and nitrification, essential for TKN removal. Moreover, phosphorus was mainly reduced by physical and chemical processes, such as adsorption, precipitation on the media, whose removal efficiency depends on retention time. The above information explains the high treatment efficiencies of the operation at the 24-h HRT. Up to 70% of organic materials (or SCOD) contained in the wastewater were biodegraded by the vast arrays of microorganisms in the VGCW beds. The reduction of easily degradable organic compounds is the interactions with combined chemical, physical and biological processes driven by microorganisms (Nicomrat et al., 2006; Tietz et al., 2007).
Table 3
Organics and nutrients removal efficiencies of the VGCWs at various HRTsa
Parameters
|
HRTs (h)
|
6
|
12
|
24
|
TCOD
|
Removal efficiency (%)
|
35 ± 11.0
|
70 ± 8.8
|
85 ± 7.3
|
SCOD
|
Removal efficiency (%)
|
20 ± 8.8
|
40 ± 6.3
|
70 ± 6.1
|
TKN
|
Removal efficiency (%)
|
65 ± 14.7
|
85 ± 2.6
|
90 ± 2.5
|
TP
|
Removal efficiency (%)
|
35 ± 10.8
|
65 ± 6.1
|
80 ± 4.2
|
Table 4
Organics and nutrients removal efficiency of the mVGCW at various HRTsa
Parameters
|
HRTs (h)
|
12
|
24
|
36
|
TCOD
|
Removal efficiency (%)
|
72.92 ± 8.6
|
83.66 ± 4.6
|
91.38 ± 3.1
|
TKN
|
Removal efficiency (%)
|
70.03 ± 5.0
|
79.94 ± 3.7
|
97.26 ± 0.6
|
TP
|
Removal efficiency (%)
|
64.52 ± 2.9
|
75.48 ± 1.2
|
93.49 ± 1.2
|
*Significant differences (p < 0.05) |
The mVGCWs were operated at three different HRTs of 12, 24 and 36 h to evaluate the removal performance of TCOD, TKN and TP, as shown in Table 4. The average TCOD efficiencies at HRTs of 12, 24 and 36 h were 72.92 + 8.6, 83.66 + 4.6, and 91.38 + 3.1%, respectively. The TKN removal at the HRTs of 12, 24 and 36 h were 70.03 + 5.0, 79.94 + 3.7, and 97.26 + 0.6%, respectively. The TP removal at the HRTs of 12, 24 and 36 h were 64.52 + 2.9, 75.48 + 1.2, and 93.49 + 1.2%, respectively. These data suggest that the mVGCW could remove the organics and nutrients better than the VGCWs.
Table 5
Organics and nutrients removal efficiency of the mpVGCW at 36-h HRT
Parameters
|
Plants
|
Spider Ivy
|
Coleus
|
Selaginella frosty
|
TCOD
|
Removal efficiency (%)
|
91.37 ± 3.3
|
93.39 ± 2.9
|
92.87 ± 2.7
|
TKN
|
Removal efficiency (%)
|
99.32 ± 0.1
|
99.32 ± 0.1
|
99.32 ± 0.1
|
TP
|
Removal efficiency (%)
|
88.58 ± 3.9
|
87.18 ± 2.9
|
87.50 ± 2.4
|
*Significant differences (p < 0.05) |
The mpVGCW performance in removing TCOD, SCOD, TKN, and TP at the HRT of 36 h is shown in Table 5. The average TCOD efficiencies of Spider Ivy, Coleus, and Selaginella frosty were 91.37 ± 3.3, 93.39 ± 2.9, and 92.87 ± 2.7%, respectively. The TKN removal of Spider Ivy, Coleus, and Selaginella frosty were similar at 99.32 ± 0.1%. The TP removal of Spider Ivy, Coleus, and Selaginella frosty were also similar at 88.58 ± 3.9, 87.18 ± 2.9, and 87.50 ± 2.4%, respectively. Due to their long, extended roots and high durability, these plants are recommended for use in the mVGCWs to treat septic tank effluent.
3.1.2 Solids removal
Because the main mechanisms for solids removal are sedimentation, adsorption and filtration, the TSS removal efficiency in the VGCW were found to increase with increasing HRTs, similar to those of organic and nutrients removal (Table 6). According to Manios et al (2003), CW media consisting of soil, gravel and sand were found to be very effective for TSS removal. The high removal efficiencies of 95% for TSS suggest the strong potential for using the VGCWs for polishing septic tank effluent and minimizing water pollution problems.
Table 6
Solid removal efficiency of the VGCW at various HRTsa
Parameters
|
HRTs (h)*
|
6
|
12
|
24
|
TSS
|
Removal efficiency (%)
|
50 ± 15.1
|
75 ± 10.5
|
95 ± 3.7
|
*Significant differences (p < 0.05) |
Table 7
Solids and TKN removal efficiencies of the mpVGCW at 36-h HRTa
Parameters
|
Plants*
|
Spider Ivy
|
Coleus
|
Selaginella frosty
|
TSS
|
Removal efficiency (%)
|
89.52 ± 1.83
|
92.28 ± 0.88
|
90.93 ± 1.81
|
*Significant differences (p < 0.05) |
As shown in Table 7, the average TSS removal efficiencies of Spider Ivy, Coleus, and Selaginella frosty at the 36-h HRT were 89.52 ± 1.83, 92.28 ± 0.88, and 90.93 ± 1.81%, respectively. The TKN removal efficiencies of Spider Ivy, Coleus, and Selaginella frosty were 89.73 ± 1.59, 92.40 ± 1.42, and 92.26 ± 1.85%, respectively. These results suggest the potentials of using these plants in the VGCW system to effectively treat septic tank effluent.