ABSTRACT
Three aquatic macrophytes were used to treat wastewater using a pilot-constructed wetland (CW) system to determine the most efficient plants for removing contaminants from wastewater. The three macrophytes are water hyacinth (Eichhornia crassipes), water lettuce (Pistia stratiotes), and duckweed (Lemna minor). Three 150 L capacity tanks with sand and gravel as substrates were used as the pilot CW for each plant. Upon initial examination, the raw wastewater was not compliant with standard discharge limits. The wastewater samples were collected every 7 days for 3 weeks for treatment. From the findings, at 14 days hydraulic retention time (HRT), E. crassipes and P. stratiotes achieved the highest total phosphorus (TP) and chemical oxygen deman (COD) reductions of 99.3 and 99.4%, respectively. E. crassipes indicated better biological oxygen demand removal efficiency of 91.3%, COD (85.0%), electrical conductivity (90.4%), total dissolved solids (89.7%), and total coliforms (66.0%). Albeit, P. stratiotes indicated better results for total suspended solids (96.2%), TP (7.55%), and E. coli (94.4%), while L. minor was better with 90.8% total nitrogen removal. The overall analysis showed E. crassipes to be more efficient than the three macrophytes. However, the other two plants are replaceable options and large-scale implementation of this project in the community would be a major contributor to actualizing SDG number 6.
HIGHLIGHTS
Constructed wetlands are sustainable technologies for wastewater treatment in local communities.
Local aquatic macrophytes are vital tools for maintaining water and environmental quality and securing health.
Phytoremediation capacity of macrophytes may be affected by climatic conditions.
Water hyacinth, water lettuce, and duckweed are efficient for phytoremediation of wastewater in tropical climates.
INTRODUCTION
In most developing regions, wastewater is released into freshwater bodies without adequate monitoring/treatment, and in the end, its constituents may build up contaminants thereby causing harm to man and affecting aquatic biodiversity (Olukanni & Aremu 2008; Omole & Ndambuki 2014; Chen et al. 2016; Justin et al. 2022). While the world focuses on protecting the environment and ensuring sustainability, conventional sewage treatment systems may not be feasible for widespread application in rural areas due to the high cost and technicality of operating them. Thus, low-cost and less technically demanding constructed wetland systems (CWSs), capable of eliminating pollutants and pathogens from wastewater, are now preferred over conventional treatment methods (Olukanni & Ducoste 2011; Tan et al. 2019; Ji et al. 2020; Holtman et al. 2022). These are robust and sustainable technologies with significant capacity to treat wastewater and are considered viable decentralized systems to manage wastewater in developing nations (Olukanni & Kokumo 2013; Tan et al. 2019; Bui et al. 2019; Maine et al. 2022). Albeit, the functionality of their most important component, macrophytes, which are plants that predominantly grow and spend most or all of their life cycle in water, that varies from one species to another is intriguing. It has been shown that different macrophytes have different levels of pathogen removal, adaptations to diverse climatic conditions, and resistance to physicochemical variances (Alufasi et al. 2017; Tan et al. 2019; Rahman et al. 2020; Justin et al. 2022). Hence, the results of a particular geographical location may not be generalized.
Water hyacinth (Eichhornia crassipes), water lettuce (Pistia stratiotes), and duckweed (Lemna minor) are invasive, free-floating aquatic macrophytes. The first two have broad, thick, and glossy ovate leaves and long-hanging roots in water. Due to the characteristics and the economic values of these plants, they have attracted interest globally for use in phytoremediation of polluted waters over the years (Ajibade et al. 2013; Swarnalatha & Radhakrishnan 2015; Rezania et al. 2016; Priya & Senthamil 2017).
In separate studies, water lettuce showed great ability to stabilize electrical conductivity (EC), and pH and decreased total solids (TS), nitrogen, and Se (selenium) concentrations (Lu 2009, 2010; Uka & Chukwuka 2011). Duckweed (L. minor) has proved resilient and successful in remediating diverse chemical and organic pollutants (Mohedano et al. 2012; Grijalbo et al. 2016; Ekperusi et al. 2019). Precisely, Tufaner (2018) reported over 82.0% removal rates for chemical oxygen demand (COD), biochemical oxygen demand (BOD), total nitrogen (TN), ammonium nitrate, and phosphate using L. minor.
Where many plant species exist, comparisons were made to evaluate the efficiency of the better macrophytes in pollutant removal. For instance, an examination of hyacinth, water lettuce, and vetiver grass has been carried out to establish which one is best at remediating TS, EC, BOD, COD, TN, and phosphorus (Gupta et al. 2012; Rezania et al. 2016). This report revealed that all the macrophytes successfully reduced the pollutant indicators to minimal levels, with water hyacinth being the most effective. Elsewhere, Edaigbini et al. (2015) compared the remediating abilities of water hyacinth and water lettuce in water obtained from a detention pit in the Niger Delta region of Nigeria. With standard discharge limits as a baseline, the analysis showed some positive results, and water hyacinth was found to be better in remediation of the produced wastewater. However, the majority of the parameters were above the permissible limits. The authors recommended improving the growth of macrophytes using fertilizers and occasional removal of dead plants to maintain optimum plant density and prevent contaminants reoccurrence in the treatment system. While there are several studies in the subject area, information on the phytoremediation efficiencies of different macrophyte species is limited in climatic regions of Nigeria.
Covenant University can be described as a community with a growing population (Isiorho et al. 2014), and consequently a high wastewater generating rate. As the university drives toward sustainability, wastewater treatments such as the application of CWSs are indispensable. While the university community has fully adopted this technology, only water hyacinth (E. crassipes) has been used in CWs for wastewater treatment (Olukanni 2013; Isiorho et al. 2014). Meanwhile, other local macrophytes, especially water lettuce (P. stratiotes) and duckweed (L. minor), are readily available for this purpose and may be more effective. Moreover, the basis for selecting macrophyte species could be clearly stated. In addition, a complete evaluation of the treated effluent with standard effluent discharge limits may help to provide the reliability of effluents released for safeguarding the environment.
Therefore, this research aims to evaluate the wastewater treatment efficiency of three aquatic plants (water hyacinth, water lettuce, and duckweed) using pilot CWSs in the Covenant University community. This aim was achieved by first treating the wastewater from the university's treatment plant using pilot CWs with the three plants. Comparing the treatment efficiencies of the three plants and then determining the reliability of the treated effluents with the NESREA (National Environmental Standards and Regulations Enforcement Agency) effluent discharge limits (Owusu-Ansah et al. 2015).
METHODOLOGY
Study location
This research was conducted at the Department of Civil Engineering, Covenant University, in Canaan Land, Ota, Ogun State, Nigeria between August and November 2022. It has a population of about 10,000 people with daily water requirements of over 136 L/C/day, and an estimated total wastewater generation rate of 800,000 L/day (Olukanni & Kokumo 2013; Omole et al. 2019). The site lies on coordinates; latitude 6.67°N and longitude 3.16°E at 56.1 m height above mean sea level. The community manages its wastewater biologically (CW that utilizes water hyacinth for phytoremediation) before discharging it into river Atuwara which flows down to the Atlantic Ocean.
Plant collection and cultures
Duckweed (L. minor) was collected in the Ogudu area in Lagos and was acclimatized to the study region for 1 week. Water hyacinth (E. crassipes) was already available at Covenant University, while water lettuce (P. stratiotes) was sourced outside of the university at Iju River, Ota. All the plants were allowed to multiply under favorable conditions and nutrients, but for maintenance purposes, the leaves of the plants were harvested from time to time to ensure active plant shoots.
Experimental setup
The materials for constructing the wetland include:
(i) Four 150 L treatment tanks (non-corrosive) with drainage built on each tank.
(ii) Substrates consisted of sand (3.50 cm depth) and gravel (1.50 cm depth) for adequate porosity to enhance oxidation (Ting et al. 2018).
(iii) Aquatic plants (water hyacinth, water lettuce, and duckweed plants) whose root system is essential for maximizing phytoremediation (Gupta et al. 2012).
(iv) Influent wastewater was collected from Covenant University's primary treatment system (septic tank).
Experimental procedure
As a precautionary measure, the tanks were inspected for leakage and other structural defects that may affect the results, while the bottom slope was maintained at 0.500% (Wu et al. 2015). Wastewater of equal volume was then introduced into the reactors at a hydraulic loading rate of 17.84 L/m2/day to improve oxygen circulation (Wu et al. 2015; Ting et al. 2018). The three aquatic plants were transplanted in each tank at an optimum plant density of 80.0% of the total surface area of the tanks (Wu et al. 2015; Olguín et al. 2017).
Sample collection
Wastewater samples were collected for analyses three times within 7 days, making a hydraulic retention time (HRT) of 7, 14, and 21 days (Wu et al. 2015; Olguín et al. 2017). Being a non-continuous feeding mode (i.e. batching system), the treated wastewater was replaced with a new batch of polluted wastewater after each HRT. All samples were collected and analyzed in situ and the laboratory using standard methods outlined in the American Public Health Association (APHA) 2012 (Rice et al. 2012). Analysis of the physicochemical parameters such as BOD5, COD, total suspended solids (TSS), total phosphorus (TP), TN, EC, E. coli, total coliforms (TC), ammonium, pH, and temperature of the raw wastewater was first conducted to determine the initial concentration before being introduced into the treatment systems.
RESULTS AND DISCUSSION
Raw wastewater quality
The results of the initial wastewater physicochemical parameters are presented in Table 1. The table shows the values of each parameter against standard effluent discharge limits. This was done to determine if the concentration of the parameters was within or above standard acceptable discharge limits, which is a prerequisite for treating the wastewater.
Comparison of raw wastewater (WW) with EPA/NESREA standard
Parameter . | Raw WW values . | NESREA discharge limits . |
---|---|---|
Temperature (°C) | 26.8 | <30.0 |
pH | 6.14 | 6.00–9.00 |
Total suspended solids (mg/L) | 186 | 50.0 |
Total phosphorus (mg/L) | 9.24 | 2.00 |
Turbidity (NTU) | 61.5 | 75.0 |
Total nitrogen (mg/L) | 101 | 50.0 |
Ammonia (NH3-N, mg/L) | 0.003 | 1.00 |
Biochemical oxygen demand (mg/L) | 285 | 50.0 |
Chemical oxygen demand (mg/L) | 1,720 | 250 |
Electrical conductivity (μS/cm) | 518 | 750 |
Total dissolved solids (mg/L) | 258 | 1,500 |
Dissolved oxygen (mg/L) | 1.81 | 1.00 |
Total coliforms (CFU/mL) | 49.0 | 400 |
E. coli (CFU/mL) | 28.0 | 10.0 |
Parameter . | Raw WW values . | NESREA discharge limits . |
---|---|---|
Temperature (°C) | 26.8 | <30.0 |
pH | 6.14 | 6.00–9.00 |
Total suspended solids (mg/L) | 186 | 50.0 |
Total phosphorus (mg/L) | 9.24 | 2.00 |
Turbidity (NTU) | 61.5 | 75.0 |
Total nitrogen (mg/L) | 101 | 50.0 |
Ammonia (NH3-N, mg/L) | 0.003 | 1.00 |
Biochemical oxygen demand (mg/L) | 285 | 50.0 |
Chemical oxygen demand (mg/L) | 1,720 | 250 |
Electrical conductivity (μS/cm) | 518 | 750 |
Total dissolved solids (mg/L) | 258 | 1,500 |
Dissolved oxygen (mg/L) | 1.81 | 1.00 |
Total coliforms (CFU/mL) | 49.0 | 400 |
E. coli (CFU/mL) | 28.0 | 10.0 |
From Table 1, it can be seen that six (6) parameters, namely TSS, TN, TP, BOD, E. coli, and COD, were over the required effluent standard discharge limits of NESREA. This indicates that the raw wastewater from the university community was not safe for discharge into the environment or for reuse purposes without further treatment. As such, comparative analyses were conducted on the treatment efficiency of the plants for only these parameters.
Effect of HRT on treatment efficiency of macrophytes
The effect of HRT of 7, 14, and 21 days on the removal of the physicochemical parameters by pilot CWS with the three aquatic macrophytes was examined (Table 2). Likewise, an analysis of the removal efficiencies of the three plants for the selected pollution indicators is presented in Table 3. It can be observed from Table 3 that the treated effluent concentration of the indicators decreased drastically to their lowest values as HRT increased from 7 to 21 days. Consequently, the treatment efficiencies increased with increasing HRTs (Table 3). At the end of the investigation, the average TSS concentrations were 149 mg/L in the control experiment, 21.3 mg/L in water hyacinth treatment, 7.00 mg/L in water lettuce treatment, and 12.0 mg/L in duckweed reactor.
Pollutant removal of water hyacinth, lettuce, and duckweed with respect to HRT
HRT (days) . | Samp . | Temp . | pH . | TSS . | TP . | Turb . | TN . | Amn. . | BOD . | COD . | EC . | TDS . | DO . | TC . | E. coli . |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
7 | Con. | 24.5 | 6.10 | 151 | 6.88 | 53.7 | 78.9 | 0.003 | 231 | 1,215 | 240 | 120 | 1.91 | 49.0 | 26.0 |
Hya | 23.6 | 5.38 | 23.0 | 1.27 | 13.0 | 12.4 | 0.002 | 7.00 | 485 | 120 | 60.0 | 2.14 | 5.00 | 2.00 | |
Lett. | 23.9 | 4.91 | 8.00 | 0.410 | 4.88 | 19.5 | 5.93 | 5.00 | 470 | 110 | 60.0 | 2.40 | 7.00 | 1.00 | |
Duck. | 23.5 | 4.89 | 15.0 | 2.29 | 9.15 | 6.25 | 0.560 | 15.0 | 510 | 140 | 70.0 | 2.32 | 9.00 | 1.00 | |
14 | Con. | 26.6 | 5.23 | 148 | 5.95 | 48.4 | 75.5 | 0.004 | 255 | 1,155 | 300 | 150 | 2.40 | 71.0 | 22.0 |
Hya. | 26.4 | 5.80 | 21.0 | 0.080 | 9.70 | 41.1 | 0.480 | 8.00 | 12.0 | 20.0 | 10.0 | 4.10 | 39.0 | Nd | |
Lett. | 26.4 | 5.24 | 7.00 | 0.060 | 3.50 | 30.0 | 6.47 | 10.0 | 45.0 | 70.0 | 40.0 | 3.04 | 44.0 | Nd | |
Duck. | 26.3 | 5.18 | 11.0 | 2.11 | 7.40 | 15.5 | 5.76 | 11.0 | 82.0 | 110 | 60.0 | 3.13 | 62.0 | Nd | |
21 | Con. | 23.8 | 5.45 | 148 | 3.59 | 44.1 | 7.52 | 0.250 | 255 | 1,020 | 100 | 50.0 | 3.65 | 64.0 | 6.00 |
Hya. | 24.4 | 5.45 | 20.0 | 0.200 | 6.30 | 1.87 | <0.001 | 8.00 | 275 | 10.0 | 10.0 | 4.40 | 6.00 | Nd | |
Lett. | 24.8 | 5.60 | 6.00 | 0.210 | 3.85 | 5.91 | 0.640 | 10.0 | 389 | 10.0 | 10.0 | 4.00 | 12.0 | Nd | |
Duck. | 24.3 | 6.70 | 10.0 | 0.260 | 4.30 | 6.00 | <0.001 | 11.0 | 297 | 140 | 70.0 | 4.21 | 5.00 | Nd |
HRT (days) . | Samp . | Temp . | pH . | TSS . | TP . | Turb . | TN . | Amn. . | BOD . | COD . | EC . | TDS . | DO . | TC . | E. coli . |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
7 | Con. | 24.5 | 6.10 | 151 | 6.88 | 53.7 | 78.9 | 0.003 | 231 | 1,215 | 240 | 120 | 1.91 | 49.0 | 26.0 |
Hya | 23.6 | 5.38 | 23.0 | 1.27 | 13.0 | 12.4 | 0.002 | 7.00 | 485 | 120 | 60.0 | 2.14 | 5.00 | 2.00 | |
Lett. | 23.9 | 4.91 | 8.00 | 0.410 | 4.88 | 19.5 | 5.93 | 5.00 | 470 | 110 | 60.0 | 2.40 | 7.00 | 1.00 | |
Duck. | 23.5 | 4.89 | 15.0 | 2.29 | 9.15 | 6.25 | 0.560 | 15.0 | 510 | 140 | 70.0 | 2.32 | 9.00 | 1.00 | |
14 | Con. | 26.6 | 5.23 | 148 | 5.95 | 48.4 | 75.5 | 0.004 | 255 | 1,155 | 300 | 150 | 2.40 | 71.0 | 22.0 |
Hya. | 26.4 | 5.80 | 21.0 | 0.080 | 9.70 | 41.1 | 0.480 | 8.00 | 12.0 | 20.0 | 10.0 | 4.10 | 39.0 | Nd | |
Lett. | 26.4 | 5.24 | 7.00 | 0.060 | 3.50 | 30.0 | 6.47 | 10.0 | 45.0 | 70.0 | 40.0 | 3.04 | 44.0 | Nd | |
Duck. | 26.3 | 5.18 | 11.0 | 2.11 | 7.40 | 15.5 | 5.76 | 11.0 | 82.0 | 110 | 60.0 | 3.13 | 62.0 | Nd | |
21 | Con. | 23.8 | 5.45 | 148 | 3.59 | 44.1 | 7.52 | 0.250 | 255 | 1,020 | 100 | 50.0 | 3.65 | 64.0 | 6.00 |
Hya. | 24.4 | 5.45 | 20.0 | 0.200 | 6.30 | 1.87 | <0.001 | 8.00 | 275 | 10.0 | 10.0 | 4.40 | 6.00 | Nd | |
Lett. | 24.8 | 5.60 | 6.00 | 0.210 | 3.85 | 5.91 | 0.640 | 10.0 | 389 | 10.0 | 10.0 | 4.00 | 12.0 | Nd | |
Duck. | 24.3 | 6.70 | 10.0 | 0.260 | 4.30 | 6.00 | <0.001 | 11.0 | 297 | 140 | 70.0 | 4.21 | 5.00 | Nd |
Con., control; Hya, water hyacinth; Lett., water lettuce; Duck., duckweed; Nd, not detected.
Removal efficiency of water hyacinth, lettuce, and duckweed with respect to HRT
Parameter . | HRT (days) . | Removal efficiency (%) . | ||
---|---|---|---|---|
Hyacinth . | Lettuce . | Duckweed . | ||
TSS (mg/L) | 7 | 87.6 | 95.7 | 91.9 |
14 | 88.7 | 96.2 | 94.1 | |
21 | 89.3 | 96.8 | 94.6 | |
TP (mg/L) | 7 | 86.3 | 95.6 | 75.2 |
14 | 99.1 | 99.4 | 77.2 | |
21 | 97.8 | 97.7 | 97.2 | |
TN (mg/L) | 7 | 87.8 | 80.6 | 93.8 |
14 | 59.3 | 70.3 | 84.6 | |
21 | 98.2 | 94.1 | 94.1 | |
BOD5 (mg/L) | 7 | 97.5 | 98.3 | 94.7 |
14 | 97.2 | 96.5 | 96.1 | |
21 | 97.2 | 96.5 | 96.1 | |
COD (mg/L) | 7 | 71.8 | 72.7 | 70.4 |
14 | 99.3 | 97.4 | 95.2 | |
21 | 84.0 | 77.4 | 82.7 | |
E. coli (CFU/L) | 7 | 92.9 | 96.4 | 96.4 |
14 | Nd | Nd | Nd | |
21 | Nd | Nd | Nd |
Parameter . | HRT (days) . | Removal efficiency (%) . | ||
---|---|---|---|---|
Hyacinth . | Lettuce . | Duckweed . | ||
TSS (mg/L) | 7 | 87.6 | 95.7 | 91.9 |
14 | 88.7 | 96.2 | 94.1 | |
21 | 89.3 | 96.8 | 94.6 | |
TP (mg/L) | 7 | 86.3 | 95.6 | 75.2 |
14 | 99.1 | 99.4 | 77.2 | |
21 | 97.8 | 97.7 | 97.2 | |
TN (mg/L) | 7 | 87.8 | 80.6 | 93.8 |
14 | 59.3 | 70.3 | 84.6 | |
21 | 98.2 | 94.1 | 94.1 | |
BOD5 (mg/L) | 7 | 97.5 | 98.3 | 94.7 |
14 | 97.2 | 96.5 | 96.1 | |
21 | 97.2 | 96.5 | 96.1 | |
COD (mg/L) | 7 | 71.8 | 72.7 | 70.4 |
14 | 99.3 | 97.4 | 95.2 | |
21 | 84.0 | 77.4 | 82.7 | |
E. coli (CFU/L) | 7 | 92.9 | 96.4 | 96.4 |
14 | Nd | Nd | Nd | |
21 | Nd | Nd | Nd |
Nd, not detected.
Similarly, after 3 weeks of treatment (HRT of 21 days), TP decreased from 9.24 to 0.200, 0.210, and 0.26 mg/L in water hyacinth, water lettuce, and duckweed treatment reactors, respectively. The results also show that CWS containing water hyacinth, water lettuce, and duckweed plants significantly reduced TN from the initial concentration of 101 to 1.87, 5.91, and 6.00 mg/L, respectively, at 21 days HRT. Information in the literature shows that HRT has a significant effect on plant growth rates, while long HRT and plant type significantly influence pollutant removal rate (Snow & Ghaly 2008; Adrados et al. 2018). Also, the physicochemical parameters in control tanks decreased, which could be attributed to the sedimentation, adsorption, and microbial activities within the substrates and the wastewater (Justin et al. 2022).
TSS removal rate by the macrophytes
Notwithstanding, the result obtained for water hyacinth was higher than those earlier reported 37.8–53.3% by Loan et al. (2014), 70.0% by Valipour et al. (2015), 44.3% by Tran & Van (2016), and 34.0% by Rezania et al. (2016). Similarly, the TSS results of water hyacinth were not better than other species as seen in previous studies such as in Loan et al. (2014). The significant reduction of TSS at a higher treatment period may be due to the plant's roots being fully formed in the reactors which enhanced the filtration and absorption capacity of the roots for suspended solids and dissolved nutrients.
TP removal
The primary sources of phosphorus in wastewater are human excreta, household detergents, and industrial and commercial (e.g. abattoir) effluents but occur as soluble and insoluble phosphates in varying forms (Kroiss et al. 2011). However, for this experiment, TP was considered a single parameter being a major pollutant indicated in the EPA/NESREA standard for effluent discharge/reuse. Its removal from domestic wastewater is necessary to lessen the possibility of eutrophication in receiving waters, being a mandate in many nations (Bunce et al. 2018).
TN removal
BOD5 removal
COD removal
E. coli removal
E. coli removal efficiency of water hyacinth, lettuce, and duckweed.
Average treatment capacities of the three macrophytes
The result of treated wastewater effluents showed that dissolved oxygen (DO) levels increased from 1.81 to 3.30 mg/L, and the temperature and pH were maintained within the acceptable standard values of less than 30.0 °C and 6.00–9.00, respectively (Table 1). As shown in Table 4, the average pollutant removal efficiency across the three HRTs by the three plants proved that water lettuce was better in TSS reduction (96.2%) than duckweed (93.6%) and water hyacinth (88.5%). Similarly, for TP removal, 97.6, 94.4, and 83.2% on average were achieved by water lettuce, hyacinth, and duckweed, respectively. Yet still, water lettuce was better at eliminating E. coli (94.4%) than water hyacinth treatment efficiency (92.5%).
Average removal efficiency of macrophytes
Parameters . | Effluent sample . | ||
---|---|---|---|
Hyacinth . | Lettuce . | Duckweed . | |
TSS | 88.5 | 96.2 | 93.6 |
TP | 94.4 | 97.5 | 83.2 |
Turbidity (NTU) | 84.3 | 93.4 | 88.7 |
TN | 81.7 | 81.7 | 90.8 |
BOD5 | 97.3 | 97.1 | 95.7 |
COD | 85.0 | 82.5 | 82.8 |
EC | 90.4 | 87.8 | 74.9 |
TDS | 89.7 | 85.8 | 74.2 |
DO | 96.0 | 73.9 | 77.9 |
TC | 66.0 | 57.1 | 48.3 |
E. coli | 92.6 | 96.4 | 96.4 |
Overall average | 87.8 | 86.3 | 82.4 |
Parameters . | Effluent sample . | ||
---|---|---|---|
Hyacinth . | Lettuce . | Duckweed . | |
TSS | 88.5 | 96.2 | 93.6 |
TP | 94.4 | 97.5 | 83.2 |
Turbidity (NTU) | 84.3 | 93.4 | 88.7 |
TN | 81.7 | 81.7 | 90.8 |
BOD5 | 97.3 | 97.1 | 95.7 |
COD | 85.0 | 82.5 | 82.8 |
EC | 90.4 | 87.8 | 74.9 |
TDS | 89.7 | 85.8 | 74.2 |
DO | 96.0 | 73.9 | 77.9 |
TC | 66.0 | 57.1 | 48.3 |
E. coli | 92.6 | 96.4 | 96.4 |
Overall average | 87.8 | 86.3 | 82.4 |
TN removal efficiency was better in duckweed (90.8%) than water hyacinth (81.7%) and water lettuce (81.7%). Nevertheless, water hyacinth was better at reducing most of the pollutants than water lettuce and duckweed. As can be observed in Table 4, the average removal efficiencies by water hyacinth such as BOD (97.3%), COD (85.0%), EC (90.4%), TDS (89.7%), and TC (66.0%) were all higher than the rest of the plants. The high BOD5 and COD reduction results of water hyacinth in this study further validate the report of Olukanni & Kokumo (2013). The overall average indicates that water hyacinth was the better of the three plants, and the high treatment efficiencies suggest that the three plants are very efficient in bioremediation of domestic wastewater. High treatment efficiencies of water lettuce and duckweed align well with those of (Gupta et al. 2012; Showqi et al. 2017; Wang et al. 2018).
Comparison of treated wastewater EPA/NESREA effluent discharge limits
Comparison of treated wastewater EPA/NESREA effluent discharge limits.
CONCLUSION AND RECOMMENDATIONS
In this study, the investigation of wastewater treatment efficiency of three aquatic macrophytes such as water hyacinth, water lettuce, and duckweed plants using pilot CWSs was carried out. The pollutant removal efficiency of the three aquatic macrophytes was determined and the treated effluent samples were compared with EPA/NESREA standards for effluent discharge/reuse limits. The results showed that the three plants were effective in treating the wastewater after the 21-day treatment period. In the beginning, the influent wastewater was analyzed and some of the physicochemical parameters (TSS, TP, TN, BOD, COD, and E. coli) were above EPA/NESREA effluent discharge limits. The influent wastewater was subjected to 21 days of treatment in water hyacinth, water lettuce, and duckweed CWS and observed within 7 days of batching mode of operation. In the end, there was a drastic increment in treatment efficiency for the first two HRTs (7 and 14 days), without significant differences at 21 days HRT, especially for hyacinth and water lettuce.
Despite the outstanding results by all the plants, the comparison of the treatment performance of the three macrophytes suggested that water hyacinth attained higher efficiencies than water lettuce and duckweed plants, especially in BOD, COD, EC, TDS, and TC reductions. Upon correlation with the EPA/NESREA standard, all the examined wastewater parameters including turbidity, ammonia, EC, TDS, and TC had reduced to permissible limits, while the temperature and pH were maintained within the required standard limit. The DO level of the raw wastewater was improved from the influent concentration of 1.81 mg/L to an average of 3.30 mg/L in the treated effluents. All the plants produced high removal efficiencies of at least 70.0%, except for TC, 53.2% after 21-day treatment intervals.
From all the analysis and discussions, it may be safe to conclude that water hyacinth, water lettuce, and duckweed produced better results under 14 days HRT. The three macrophytes are capable and very efficient in the bioremediation of domestic wastewater. Higher treatment efficiencies of water hyacinth in most parameters put it in a better position than water lettuce and duckweed. The overall results suggest that wastewater treated with water hyacinth, water lettuce, and duckweed plants is safe for discharge/reuse. The completion of this project has helped to show the effectiveness of locally available resources; water hyacinth, water lettuce and duckweed plants, and the wetland substrates in the treatment of municipal wastewater. The implementation of this research on a large scale will contribute greatly to actualizing SDG 6 (Clean water and sanitation) in local communities.
Based on the literature review, site investigation, and analysis throughout this research, the following are recommended.
1. Shorter HRTs such as daily intervals may be investigated for more information on those short periods.
2. This study was carried out within a single climatic season. Hence, the effect of seasonal change on the treatment performance of these macrophytes may be investigated.
ACKNOWLEDGEMENTS
The authors appreciate the management of Covenant University, Ota, Nigeria for the enabling environment to carry out this research.
FUNDING STATEMENT
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.
CONFLICT OF INTEREST
The authors declare there is no conflict.