Domestic Wastewater Treatment Using Constructed Wetland with Para Grass Combined with Sludge Adsorption, Case Study in Vietnam: An Efficient and Alternative Way

Abstract

The environmental pollution due to wastewater, especially domestic wastewater, is becoming more serious. Thus, this study was performed to assess the removal of pollutants by the combination of wetland technology, specifically constructed wetland (CW), with Para grass (Brachiaria mutica) together with the adsorption of red sludge. With an organic loading rate (OLR) of 120 (kg COD/ha/day), the treatment efficiency for wastewater in this study is quite high and the concentrations of parameters in the effluent were lower than the limits established by the QCVN 14:2015/BTNMT for domestic wastewater, which indicated the feasibility of this combination. These results are expected to open the possibilities of using environmentally friendly processes for wastewater treatment in urban and industrial areas.

1. Introduction

Among different CW types, horizontal flow (HF) is the most common method, which has been used to treat domestic, municipal, industrial/agricultural wastewaters, and landfill minewater [1]. The hybrid constructed wetland system, which is one of the modern CW types and the combination of HF and vertical flow, shows the highest efficiency for treating domestic wastewater. Furthermore, it also achieves significant removal of nitrogen through the control of oxygen conversion conditions in the system [2,3].

In horizontal subsurface flow (HSSF-CWs), the polluted stream gently flows through the layers of filter, which are below the surface, and interacts with anaerobic, aerobic, and anoxic sections of the active system. When wastewater moves along the root, pollutants are removed by mechanical processes, biodegraded by microorganisms, and absorbed by the plants [4]. A previous study has shown that HSSF-CWs exhibited effective treatment ability for domestic effluents based on several parameters such as COD, BOD₅, and SS, yet the nitrogen- and phosphorus-adsorbing and -metabolizing mechanisms have not been determined due to the shortage of oxygen in the filtration layer having low nitrification [1]. Also, the phosphorus removal is restricted by the low absorption ability of the materials, which are gravel and crushed rock. Para grass, which originates from Brazil and Africa, is also found in many tropical countries [5]. It was first grown in the south of Vietnam in 1875, then in 1930, spread out to the central part and finally introduced in Northern Vietnam. Thus, this plant is grown in all parts throughout Vietnam and especially thrives in moist condition of summertime [4,6,7]. It is a common source of food for livestock due to its soft leaves and trunk.

Research in treating domestic wastewater by CWs has been conducted in Northern Cairo, Egypt. Different plants such as Canna or Canna Lily, common reed (Phragmites australis), and Cyprus papyrus or simply papyrus was grown in three wetland unit types, which were then experimented on with municipal effluent (flow rate of 20 m³/day). The rate of surface loading is between 26.2 and 76.5 kg BOD/ha/day and the time for detention is 7.7 days. The data revealed that 88% COD, 90% BOD, and 92% TSS (by average) were efficiently removed from the initial effluent. In addition, the residual values were 30.60, 13.20, and 8.50 (mg/L) in the conducted effluent, respectively. Moreover, the total and fecal coliforms was significantly reduced [2].

The condition under which this experiment on grass was conducted for elimination of nitrogen element in polluted water is an irrigation water rate of 98 (mm/day); 5 days per week; and an average water loss of 4.6 mm per day. After that, the total amount of removed nitrogen ranges from 130 to 2600 kg/ha/year. This includes 79% which was absorbed by the grass, 29% which was missed due to the reduction of nitrate and 3% which was retained during filtration. After experimenting, the residual nitrogen concentration was 10 mg/L [8]. In another study, this plant was grown and improved with three strains of endophytic bacterial, resulting in a decrease of COD, BOD₅ and chromium content by 82%, 94% and 95%, respectively. However, the TP and TN removal values remain relatively high from 19.8 to 9.4 mg/L and from 752 to 148 mg/L, respectively [9].

Red mud is a waste which is discharged from the Bayer’s process which transfers bauxite into alumina in alumina plants. The red color of the red mud is because of high a accumulation of iron oxide content, which could take up more than 50% of the mud. Nowdays, red mud is treated simply by transportation to a sludge pond, and it takes up a lot of land and also could causes air pollution. At the moment, applications of red mud are still being researched [10,11].

In 1997, B. Koumanova et al. studied the phosphate removal by using red mud or bauxite residue from Bayer’s process [12]. The results indicated that red mud, after being treated with concentrated sulfuric acid, could eliminate phosphate in water. The contact time showed insignificant impact on the red mud’s phosphate adsorption, whereas the activated red mud dose was more significant. Results also revealed that the activated red mud was suitable for eliminating diluted ${\mathrm{PO}}_4^{3-}$.

In 2006, Cengeloglu et al. used red mud activated by 20% HCl at 100 °C for 20 min to increase specific surface area and adsorb nitrate and fluoride with the treated concentration of 5.858 mmol nitrate/g [13].

In 2008, Huang et al. illustrated the effects of adsorption ability on the types of acid used and the heat treatment process [14]. The results showed that acid-activated red mud exhibited the most efficient phosphate adsorption of 0.58 (mg P/g) at pH of 5.5 and 400 °C.

According to Xie, in 2018, although the production of red mud was as substantial as 130 million tons, the utilization rate was only about 4%, which is such a waste of by-products [15]. In recent work, some of the most common applications of red mud could be the synthesis of bricks or ceramics [16,17] or other materials like polymers or subgrade materials [18,19,20]. Others than these viable applications, there are also studies in which scientist tried to utilize the red mud for environmental remediation.

In 2020, Kazak and Tor successfully prepared magnetic hydrochar from co-hydrothermal treatment of vinasse and red mud [21]. The experiments showed that adsorption of Pb (II) was favored at solution pH > 5.0 and the equilibrium was attained in 120 min. The main adsorption mechanisms are cation-exchange, precipitation, and complexation.

In the present study, the combination of constructed wetland and Para grass with adsorption using an absorbent modified from red sludge has been studied for application in domestic wastewater treatment. The efficiency of the treatment system has been evaluated through various pollution parameters, including pH, COD, TN, TP, BOD, BOD₅, TSS, and total coliform. This study will contribute to the development of wastewater treatment technologies with simple, environmentally friendly, high-efficiency processes, as well as contribute to the utilization of by-products.

2. Experimental Section

2.1. Constructed Wetlands Model

Transparent acrylic was used to make this type of wetland, with the dimensions of 1.2 × 0.4 × 0.8 (m × m × m). There were two models investigated including one model planted with this grass and a controlled one without vegetable. Gravel with thickness of 10–200 (mm) was used to fill the inlet area to avoid overflow on the surface. There were three layers in the treating area, including the lower, the middle, and the upper layer. Regarding the lower layer, it was 0.10 m high, filled with 10–30 mm gravel, and used for water collecting and supporting above material. As for the middle layer, its height was 0.15 (m) and the thickness of gravel bed, which was used for water filtration, ranged from 5 to 10 (mm). For the upper one, it was filled with 0.35 (m) layer of a mixture of fine and medium sand; this area was the main environment for plant roots to stick, grow, and also keep the suspension in the inlet liquid. The grass was obtained from the pond of the effluent-treating factories; it was divided into pieces with length of 50 (cm) and continued to grow in a pot containing liquid domestic pollutants, which had been reduced its concentration by three times with pure water. While the grass was adapting to the new conditions, the input pollution parameters such as pH, COD, TN (Total Nitrogen), TP (Total Phosphorus), TC (Total Coliform), TSS were tested. The value of each parameter ranges from: 5.7–7.4 (mg/L) for pH, 165.9–203.2 (mg/L) for COD, 100.2–113.13 mg/L for BOD5, 18.75–23.2 mg/L for TSS, 45–48 mgN/L for TN, 16.7–19.4 mgP/L for TP, and 2.4 × 10⁶ MPN/L for TC. In comparison, the values of those parameters regulated in National technical regulations on domestic wastewater of Viet Nam are: 6–9, 75, 30, 50, 30, 6, 50,000 respectively [22].

2.2. Operation Period

The controlled model was operated at single organic loading rate (OLR) under consistent conditions to compare treatment efficiency. Four OLRs (30, 60, 90, and 120 kg COD/ha/day) were tested, the results of influent wastewater flow rates are 9.6, 14.4, 21.6, 28.8 and hydraulic retention times (L/day) are 10.73, 7.15, 4.76, 3.58 (days), respectively [22]. Domestic effluent was diluted to a desired concentration based on each mode of organic loading rate (OLR). The inlet stream was led into the model by a quantitative pump through a plastic tube (diameter of 6 mm), and the tube was approximately 10–20 cm higher than the surface of the mixed solid ground.

The coming solution was introduced into the system through a PVC tube with a flowmeter to adjust the flow rate. The water level in the experience was approximately 2–5 cm below the surface. Effluent was directly pumped into the seeping of CWs through the layer of sand–gravel–soil. Under the influence of the hydraulic flow, the contaminant in the sewage were gently exposed to filter material and the roots of studied plant, absorbing the required elements. Furthermore, this plant hosts the microorganisms which adsorbed and oxidized organic compounds in the sewage. Mechanical steps, such as sedimentation and filtration, were mainly investigated in the sand and gravel filter layer.

After retaining water, the pollutants in the wastewater were removed. The treated water was then crossed through a tube made from plastic (diameter of 21 mm) into a 30-L container. The samples were collected 2–3 times per week inlet and outlet points and then kept at low temperature (5 °C). After a week, analysis was carried out for further investigation.

2.3. Activation of Red Mud

Sludge mud was dried and activated with HCl 4M solution with a liquid/solid ratio of 2 mL/g. Then, it was stirred and kept heating at 950 °C for 2 h. The solution was then filtered to separate the water and solid residue. The residue was irrigated multiple times with distilled water, then left dry for 2 h. After that, it was calcined for 1 h at 3 different temperatures from 500 °C to 900 °C (500 °C–700 °C–900 °C) with 10 min⁻¹ set as the rate of heat flow. The morphology and composition of sludge were measured by SEM and EXD techniques.

2.4. Study on Domestic Wastewater Treatment Using Constructed Wetland Combined with Sludge Adsorption

The experiment was continuously performed at a load of 120 (kg COD/ha/day). Then, 0.05 (g) of material from the red sludge was added to the first 100 (mL) of water out of the constructed wetland models. They were agitated at 10 (rpm) for 30 (min). After adsorption, the mixture was isolated and the liquid was scrutinized for COD, TN, TP, TSS, and Coliform criteria.

2.5. Material Characterization

The X-ray diffraction (XRD) pattern of the red sludge was collected on a D2 PHASER (Bruker, Germany) equipped with a Cu ${\mathrm{K}}_{\mathsf{\alpha }}$ radiation source in the $2\mathsf{\theta }$ range of 20–80 degrees. The composition and distribution of elements in the activated red mud were recorded by scanning electron microscopy (SEM) coupled with S-3 energy-dispersive X-ray spectroscopy (EDX) mapping on a JEOF-JSM 6500F device with an acceleration voltage of 10 kV.

3. Results and Discussion

3.1. Domestic Wastewater Treatment Using Constructed Wetland

3.1.1. COD Removal Efficiency

Figure 1 shows the input and post-treatment COD concentrations of the HSSF-CWs using Para grass. It shows that the removal of organic matter depended on the organic load and the retention time of the system. The organic matter removal in the tectonic wetland system was mainly conducted by aerobic and anaerobic decomposition of microorganisms, combined with the sedimentation and filtration between layers of materials in the system. The efficiency of organic matter treatment in a laboratory-scale tectonic wetland model for growing Western grass ranged from 78–83.6%. The output concentration of the model depends on the input concentration and the initial load applied to the model. At a load of 30 kg COD/ha/day and an input concentration of 67.94–89.63 (mg/L), removal efficiency fluctuates from 74.45% to 76.92%. During the loading period of 60 kg COD/ha/day, the plants were fully adapted, and the microflora was well developed in combination with sedimentation treatment, so the model achieved high treatment efficiency. The fluctuation of the load of 90 (kg COD/ha/day) is because of the large fluctuations in the input concentration of the wastewater and the specific high concentration of 132.48–239.01 (mg/L), the efficiency reaching 81.63–82.74%. At the load of 120 (kg COD/ha/day), the treatment efficiency decreased slightly to 80.78–81.62%. The reason for this is that when increasing the load, the retention time in the model is low, resulting in a slight decrease in the processing efficiency. In general, the output wastewater concentration of the wetland system through different loads meets the requirements in domestic wastewater treatment column A, according to the National technical regulations on domestic wastewater in Vietnam (QCVN 14:2015/BTNMT).

3.1.2. Nitrogen Total Removal Efficiency

The total nitrogen (TN) removal efficiency by the horizontal subsurface flow constructed wetlands (HSSF CWs) using Para grass vegetation through each load is shown in Figure 2. The initial concentration of wastewater in the horizontal subsurface flow constructed wetlands (HSSF CWs) using Para grass in this study ranged from 11.3 to 58.5 mg/L. Although the inputs concentration of TN through different loads is different, the processing efficiency fluctuates at 57.09–74.16%. This is explained by the strong growth and development of the TN treatment of the grass at the survey condition, with loads from 60 to 120 kg COD/ha/day, combined with nitrifying and denitrifying microorganisms. The survey results show that the mechanism of organic matter, phosphorus, and nitrogen removal in wastewater by Para grass is correlated with the absorption N and P from wastewater as a source of nutrients for the plant. The nitrification performed by bacteria also contributes to NO³⁻ removal. The root system represents a substrate for active microorganisms [23]. A previous study by Linda L. Handley et al. [8] explains this mechanism: in 1981, they used Para grass to remove nitrogen from domestic wastewater. The concentration of TN in the model is mostly reduced by denitrification. Furthermore, TN can also be partially eliminated by (1) plant assimilation, (2) growth of anaerobic and aerobic microorganisms, and (3) the removal of suspended solids. Plants can assimilate <10% of the discarded TN, whereas microorganisms can utilize 15–20% of the TN. Treatment efficiency thrived at a load of 60 kg COD/ha/day and was 63.72–74.16%, which was slightly decreased to 57.09–60.12% as the organic load increased up to 120 kg COD/ha/day because of grass maturation during this period. In general, although the treatment efficiency was varied, the TN parameter in the output water quality achieved the National technical regulations on domestic wastewater Vietnam (QCVN 14:2015/BTNMT), with TN = 30 mg/L.

3.1.3. Phosphorous Total Removal Efficiency

Figure 3 shows the efficiency and performance variation of total phosphorous (TP) in the wastewater treatment of the HSSF CWs using Para grass. At the load of 30 kg COD/ha/day, the treatment efficiency was 88.58–89.45%. As the loads increased from 60 to 120 kg COD/ha/day, the treatment efficiency fluctuated in the range of 79.14–84.57% due to the increased concentration of wastewater inlet higher than the original, and the retention time of water in the model was reduced. In general, the concentration of TP in the output wastewater of the model is within the allowable limit in the domestic wastewater treatment water, according to national technical regulations on domestic wastewater Vietnam (QCVN 14:2015/BTNMT), with TP < 6 mg/L.

3.1.4. Total Suspended Solids (TSS) Removal Efficiency

Figure 4 illustrated that the removal efficiency decreased slightly in response to the increasing load. At a load of 30 kg COD/ha/day, the average input TSS concentration was 13.52 ± 1.37 (mg/L), the average output concentration was 0.77 ± 0.26 (mg/L), and the average treatment efficiency was 94.63 ± 2.45%. In subsequent studies, the growing grassroots showed an adsorption capacity in the filter material in which they are adhered to the grass roots, thereby enhancing the suspended solids’ treating capacity. The treatment efficiency decreased gradually from 92.07% to 86.74% as the loads increased from 60 to 120 kg COD/ha/day, which was lower than that of the load of 30 kg COD/ha/day since the input concentration in the following survey loads being 2.3 times higher than the initial load and the water retention time in the model is reduced. In general, the output wastewater sample meets the standard of domestic wastewater treatment of class A, according to the National technical regulations on domestic wastewater Vietnam (QCVN 14:2015/BTNMT) with TSS < 50 mg/L.

3.1.5. Coliform Removal Efficiency

Figure 5 shows that the total coliform of wastewater entering the horizontal subsurface flow constructed wetlands (HSSF CWs) using Para grass in this study is about 11.5 × 10⁴–23.5 × 10⁴ (MPN/100 mL). The concentration of coliform in the outlet wastewater ranges from 50–3750 (MPN/100 mL). It can be concluded that the coliform output after treatment of the wetland system has a high treatment efficiency of over 90%, meeting class A standards according to national technical regulations on domestic wastewater Vietnam (QCVN 14:2015/BTNMT) with coliform < 5000 MPN/100 mL.

3.2. Domestic Wastewater Treatment Using Constructed Wetland Combined with Adsorption of Red Sludge

The sample of HCl-activated red mud calcined at 700 °C shows consistent results with the adsorption mechanism. After the sample is activated and calcined, the components in the sludge were transformed mainly into aluminum and iron oxides. Furthermore, the surface of the material formed was rough which enhanced red mud’s adsorption capacity which is shown in Figure 6. The results of HSSF CWs using Para grass combined with sludge adsorption material are shown in

Table 1. Treatment efficiency of domestic wastewater treatment using constructed wetland combined with sludge adsorption.

Organic Loading		120 kgCOD/ha/day
No.		1	2	3	4	5
pH	Input wastewater	7.3	7.4	7.4	7.5	7.5
	Output wastewater after treatment 1	7.6	7.6	7.1	7.2	7.2
	Output wastewater after treatment 2	7.6	7.6	7.1	7.2	7.2
COD (mg/L)	Input wastewater	139.54	160.16	174.72	180.54	188.62
	Output wastewater after treatment 1	26.82	29.56	32.12	33.4	35.6
	Output wastewater after treatment 2	15.2	18.85	20.06	18.24	27.64
TN (mg/L)	Input wastewater	58.5	50.35	53	55.67	55.67
	Output wastewater after treatment 1	25.1	21.6	21.25	22.32	22.2
	Output wastewater after treatment 2	19.4	16.9	16.7	17.2	18.5
TP (mg/L)	Input wastewater	15.2	17	13.9	17.5	20.9
	Output wastewater after treatment 1	2.7	2.8	2.3	2.9	3.4
	Output wastewater after treatment 2	N/A	N/A	N/A	N/A	N/A
TSS (mg/L)	Input wastewater	37.65	27.75	15.75	12.57	18.35
	Output wastewater after treatment 1	3.2	2.8	2.3	2.1	3.01
	Output wastewater after treatment 2	0.1	N/A	N/A	N/A	N/A
Coliform (MPN/100 mL)	Input wastewater	240,000	230,000	240,000	240,000	230,000
	Output wastewater after treatment 1	1910	1855	2520	3110	3750
	Output wastewater after treatment 2	1910	1855	2520	3110	3750

¹ Output wastewater after treatment by HSSF CWs using Brachiaria mutica; ² Output wastewater after treatment by HSSF CWs using Brachiaria mutica combined with Sludge Adsorption.

.

According to Table 1, the concentration results of pollutants in the wastewater after being treated in combination with the tectonic wetland model of Long Tay grass and red mud adsorption material are very positive. Specifically, the TP treatment output is not detectable (according to the analytical method). Initial COD and TN concentrations ranged from 139.54–188.62 mg/L to 15.2–27.64 mg/L and 50.35–58.5 mg/L to 16.7–19.4 mg/L, respectively. Furthermore, there is no change in pH, showing that the red mud after activation does not affect the pH in water. Coliform is not changed because the adsorbent does not adsorb Coliform. The adsorption mechanism of red mud materials for TN and TP follows the ion exchange adsorption mechanism. In which M can be Al, Fe, Si.

The adsorption mechanism of red mud materials for TN ${\mathrm{NO}}_3^{-}$ and ${\mathrm{PO}}_4^{3-}$ is given as below in Equations (1) and (2) [13,24,25]:

$$=\mathrm{MOH}+{\mathrm{H}}^{+}\leftrightarrow ={\mathrm{MOH}}_2^{+}$$

(1)

$$={\mathrm{MOH}}_2^{+}+{\mathrm{NO}}_3^{-}\leftrightarrow ={\mathrm{MOH}}_2-{\mathrm{MNO}}_3\left({\mathrm{or}\mathrm{MNO}}_3+{\mathrm{H}}_2\mathrm{O}\right)$$

(2)

The adsorption mechanism of red mud for ${\mathrm{PO}}_4^{3-}$ in Equations (3) and (4):

$$\mathrm{MOH}+{\mathrm{H}}^{+}\leftrightarrow {\mathrm{MOH}}_2^{+}$$

(3)

$${\mathrm{MOH}}_2^{+}+{\mathrm{NO}}_3^{-}\leftrightarrow {[{\mathrm{MOH}}_2]}_{\mathrm{n}}-{\mathrm{A}\mathrm{or}\mathrm{MA}}^{-\mathrm{n}+1}+{\mathrm{H}}_2\mathrm{O}$$

(4)

Red sludge has an important role in absorbing organic substances, enhancing the efficiency of COD treatment in the system. In addition, the red mud activation process may have changed the phase structure of the red mud (goethite to hematite, and the substitution of aluminum element into the hematite lattice), with easily accessible polyhedral properties increasing the possibility of phosphate adsorption (Figure 7).

3.3. Analysis of Accumulated N, P Content in Para Grass

Table 2. Accumulated concentrations of N, P in Para grass.

Analytical Indicators	Grass Component	%	Concentration (mg/g)
TN	Grass root	4.02	40.15
	Grass stalks	3.86	38.61
	Grass leaves	2.12	21.24
TP	Grass root	0.31	3.11
	Grass stalks	0.41	4.07
	Grass leaves	0.28	2.82

and Figure 8a,b show that the accumulated nitrogen total content in roots is (40.2%), followed by stems (36.6%) and leaves (23.2%). The results show that the nitrogen content accumulated in the roots and stems was relatively equal. On the other hand, the accumulation of phosphorus in roots (31.1%) is less than in stems (40.7%). In general, the accumulation of TN and TP in the Para grass is mainly in the stalks.

4. Conclusions

The present study has evaluated a domestic wastewater treatment using a constructed wetland using Para grass, combined with sludge adsorption materials, as an efficient alternative system. Wastewater after passing through the wetland model yielded the COD, TN, TP, TSS, and Coliform average treating efficiencies of 80.63%, 63.43%, 84.66%, 90.47%, 99.47% respectively. Furthermore, the combination of high-level treatment with red mud adsorbent, activated by 4M HCl and calcined at 700 °C, resulted in high treatment efficiency as well as extremely high average efficiencies of COD (88.22%) and TN (67.52%). Furthermore, almost no TP was detected in all samples of wastewater. The excellent efficiency of the organic matter, phosphorus, and nitrogen removal in wastewater by Para grass in red sludge is likely to result from the ability of direct nitrogen and phosphorous absorption from wastewater as a source of nutrients and phosphate adsorption. The nitrification performed by bacteria also contributed to NO³⁻ removal. In addition, the organic pollutants can be absorbed by the roots and partially transformed into CO₂ and H₂O in the presence of microorganisms in the root system.