Research Article - (2021) Volume 11, Issue 11
Received: 06-Oct-2021
Published:
30-Nov-2021
, DOI: 10.37421/2165-784X.2021.11.425
Citation: Daggupati, Sridhar, Srinivas K, Meghana Rao P, Aakanksha K and S.S.V. Varsha. “Efficiency of Macrophytes for the Removal of Pollutants Using Constructed Wetlands.” J Civil Environ Eng 11 (2021): 425
Copyright: © 2021 Daggupati S, et al. This is an open-access article distributed
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Water scarcity and waste water management are serious problems experiencing in developing countries like India. So, waste water recycling should be encouraged where ever it is economical and safe. There are several techniques to treat waste water but constructed wet lands are one of the most efficient methods of treatment. Constructed wetlands for waste water have been effective economical and sustainable alternative for conventional waste water treatment technologies. A small-scale preliminary study is conducted on three constructed wetland units that have been constructed in NMREC, Divya Nagar, in which the influent is taken from STP Nalla Cheruvu, Uppal. The constructed wetland units have dimensions of 60 × 40 × 30 cm. The performance of these vertical flow wet land units is estimated with a withholding time of 48hours. The macrophytes used in the study are Colocasia, Cyperus rotundus and Cocos nucifera in three different units. Three field trials were carried out for the collected samples at the time of project for various characteristics like pH, DO, BOD, COD, T.S.S, T.D.S, Conductivity, Acidity, Alkalinity of the effluent are studied.
Constructed wetlands • Treatment • Macrophytes
Waste water generation
The rapid population growth in the major cities of developing country like India is increasing the load on limited freshwater resources. The population growth not only increased the problem of fresh water but also increased the waste water discharge. So, the availability of water resources in India is getting depleted day by day due to huge demand of water for various purposes. Some of the figures, officially state that 78% of the sewage generated is remained untreated and is disposed of in rivers, lakes or ground water [1]. So, we need some alternatives to treat the waste water. In India mainly wastewater comes from the following sources:
1. Municipal sewage
2. Industrial effluents
3. Wastewater from agricultural sources
4. Wastewater from mining activities.
Most of the surface water sources are being contaminated and makes them as unfit for human consumption. It is estimated that around 61754 Million Liters per Day (MLD) of sewage is generated in India whereas only 22963 MLD is treated [2]. The untreated sewage is dangerous for water resources and also for human health. It may lead to various water borne diseases which includes cholera, typhoid, shigella, polio, meningitis, hepatitis A, B and blue baby disease. The wastewater also damages the aquatic ecosystem, flora and fauna present in that eco system [3].
Problems due to wastewater
On an average a developed city generates 20000 MLD of wastewater every day. This wastewater pollutes the existing natural resources like rivers, lakes, aquifers and other sources. According to UNO over 80% of wastewater is not collected or treated and left into surface sources. The study also states that urban areas are the main sources of pollution. This wastewater which is flowing into lakes, rivers and highly productive coastal zones are threatening the health, access to safe drinking water and food security.
This waste water problem adversely affects habitats and living beings present in the habitat but the local institutional & Governments are not opting for waste water treatment because it is high cost, energy intensive, requires skilled labor & also rigorous operation & maintenance.
Various methods of wastewater treatment adopted in India are:
1. Sewage Treatment Plant.
2. Common Effluent Treatment Plant.
These methods are found to be effective for wastewater treatment but require skilled labor, high cost and also high operation & maintenance. According to United Nations organization only 25% of the total generated sewage is treated throughout the world and remaining sewage is left directly into the streams. The existent treatment systems are functioning effectively but they consume more man power, machinery and electrical energy these treatment systems are also not eco-friendly. So, we need an alternative solution for the treatment of wastewater which can be eco-friendly effective [4].
Wetlands are natural method of treatment of wastewater which is most effective and economical method of wastewater treatment (Figure 1).
Figure 1. Longitudinal cross section of constructed wetland unit.
Wetlands
a) Wetlands are the natural ecosystems which are inundated by water and consist of vegetation & aquatic plants. They perform various functions like water purification, water storage, processing of nutrients and as habitat to plants and animals.
b) These are two types of wetlands:
i. Natural wetlands
ii. Constructed wetlands
Natural wetlands in India: According to directory of Asian wetlands (1989) natural wetlands occupy around 18.4% of country’s area [5-10]. Other than rivers,
Constructed wetlands: Construed wetlands are the manmade wetlands units which used to treat wastewater. Constructed wetlands can treat various types of wastewaters like storm water runoff, municipal and industrial wastewater etc. Construed wetlands are the eco-friendly engineered system that as the natural functions like vegetation, soil and microorganisms is to treat wastewater.
The Constructed wetlands are divided into two types:
i. Surface flow wetlands and
ii. Subsurface flow Constructed wetlands.
Functions of constructed wetlands
Constructed wetlands provide a number of functions and values such as:
i. Water quality improvement
ii. Cycling of nutrients
iii. Habitat for flora & fauna
iv. Aesthetic & landscape enhanced
v. Controlling runoff
vi. Reduce soil erosion
vii. Decrease the organic load on natural streams
Objective
The main objectives of the study are:
1. Relate and Compare the efficiency of constructed wetlands over conventional treatment methods.
2. Comparison of efficiency of different macrophytes in constructed wetlands to uptake the pollutants present in wastewater.
Study area
The waste water samples have been collected from the inlet of primary treatment tank of sewage treatment plant located in Uppal at Hyderabad, Telangana. This area has experienced a highest Temperature of 45.5Ë?C and least temperature of 16.1Ë?C.
Various characteristics of wastewater evaluated in the study are:
I. pH
II. Turbidity
III. TSS
IV. TDS
V. BOD
VI. COD
VII. Acidity
VIII. Alkalinity
IX. Dissolved oxygen
X. Conductivity
Study of efficiency of existing system
Initial step includes the study of efficiency of existing system. Existing system includes a conventional sewage wastewater treatment plant with an equalization tank, aeration tank, sedimentation tank, filtration chamber. Efficiency of the system is analyzed by the collection and characterization of wastewater samples at inlet and outlets. Its working procedure includes the following steps. Initially all the wastewater is collected and stored into the equalization tank provided. Then the water is fed into aeration tank in which biological reaction take place to form the floc. It is then passed to the sedimentation tank where the sludge settles at the bottom and is then filtered in the filtration chamber. Finally the treated water coming out from the treatment plant is stored in the storage tank. It is drained off in the rainy season, and is used for gardening when there is a shortage of water.
Finding the efficiency of the existing treatment facility includes the characterization of two wastewater samples which is collected before and after treatment from domestic wastewater treatment plant.
Design of pilot scale wetland unit
The pilot scale wetland units have been designed based on EPA Manual and CPCB. The wetland units are designed according to Darcy’s law. Three wetland units are constructed to operate an inlet discharge of 0.02 m3/day with a retention time of 48 hrs.
Design consideration
In general, there is no any design criteria for constructed wetlands though to obtain good quality of water suggest the following guidelines for creating successful constructed wetlands units. Keep the design as sample to avoid failure. Design the system such that no power in needed for operation i.e., to use natural energies such as gravity flow. Design the unit for extreme climate conditions.
Experimental site
Three units of constructed wetlands have been prepared at Nalla Malla Reddy Engineering College. The maximum temperature at day time is 42°C and minimum temperature of 21°C. The wetland units prepared in the study are subsurface vertical flow wetland units. In which the water passes through gravity. The pilot scale units possess the following features with an impermeable plastic layer on all side:
Soil: The soil used in this wetland is red soil which is porous in nature allows the influent to percolate deep into the soil; it is obtained from a nearby local field. The soil layer consists of 10 mm thickness.
Sand: The second layer consists of sand. It was obtained from a local merchant. The sand used in the study is of uniform size and passing through 2.36 mm sieve. Sand used is washed and saturated thoroughly before putting into the units.
Gravel: The gravel layer is situated at the bottom of the unit which ranges from 10 mm to 20 mm. The gravel used is passing through 20 mm and retained on 10 mm sieves. It provides support for the above layers and also helps in pollutants removal mechanism.
Vegetation: Vegetation plays most important role in the operation of the constructed wetlands. So, the macrophytes growing in the study are chosen carefully so that they are susceptible to any type of environmental conditions. The plants most often used in wetlands are bulrushes, spike rush, other sedges, common reed; Palm and Cattails. All wetland spices are not suitable in constructed wetlands. Since they need to tolerate the variation in flow and wastewater containing, high and variable concentration of pollutants. The plants used in the pilot scale wetlands are sedges (Cyperus rotundus), Palm (Cocos nucifera), plant (Colocasia) in each unit and, both combined in the third unit. The plants have been gathered from the local areas and are set out in the constructed wetland units, two weeks before the start of treatment. The vegetation is implanted manually and domestic water has been used to enlarge the macrophytes. Macrophytes will escalate the persistence time of water by decreasing velocity and increase settling time of the suspended particles. Macrophytes have been used to remove heavy metal, bacteria and toxic organic materials (Figure 2).
Figure 2. Pilot scale wetland units with different macrophytes. (colocasia, cyperusrotundus and cocos nucifera).
Inlet sampling point at STP: The water is collected by grab sampling with sampling bucket and filled into plastic cans of capacity of 20 litres rinsed with distilled water and transported to the wetland site 9 (Figures 3 and 4).
Figure 3. Inlet sampling at sewage treatment plant.
Figure 4. Inlet of sewage water to wetlands.
Outlet sampling: The treated outlet of STP has been collected at effluent after disinfection tank by grab sampling method in sampling bottles, similarly the outlet samples of wetland units are collected at outlet tap of wetland units.
Operation and maintenance
The water from sewage treatment plant at Uppal is given as inlet to constructed wetland units. And three field trials have been carried out in the site with a retention time of 48 hrs. The outlet from each unit is collected separately after 48 hrs with grab sampling method in sampling bottles and stored in suitable conditions for testing. The same trial has been carried out for three weeks and tested simultaneously for various waste water characteristics of waste water that were evaluated (Figure 5).
Figure 5. Operation of wetland units with wastewater.
Removal mechanism
Mechanism of suspended solids removal: Due to large surface area and low velocity of the media in wetlands, they are helpful in eliminating suspended solids. Wetlands help to conduct adsorption, gravity settling, and straining onto gravel and plant media (EPA, 1999). It is found that 61% - 76% percent of solids removal in wetland takes place in the top most one third of the wetland. One of the major issues occurs with wetlands is blocking the filter media. As suspended solids were passing through the soil media, they can block the pores and decrease the hydraulic conductivity of the media which can produce head losses in the top surface of the wetlands.
Mechanism of organic matter removal: The microbes use an energy source from the Organics which is made up of about 55% carbon. The group of microorganisms was adjusted to anaerobic soils or aerobic surface waters. The aerobic microbes take the oxygen and provide good amounts of biomass and energy. Methane was produced by the anaerobic microbes by degrading the organic matter. BOD is used to measure the amount of O2 that microbes are required to disintegrate the organic matter. It is very important that there should be enough amount of oxygen should be available present in the water available in the wetland, this oxygen is much helpful for the wetland plants and aquatic animals can endure.
Mechanism of heavy metals removal: The metals which include mercury, cadmium, and lead were mainly available in wastewater coming from industries. Heavy metals will cause much damage to health of human beings and animals. Eradication of these toxic metals from the wastewater in wetlands is removed by macrophytes uptake, rainfall and soil adsorption.
Mechanism of pathogens removal: The major concern with the constructed wetlands is their capability to separate infectious agent of helminthes, protozoan, bacteria, viruses and fungi. Sedimentation is the important process which is involved with the infectious agent’s removal. These sediments of wetlands gather and form large amounts of coliforms and bacteria. The pathogens are also filtered out through the roots of macrophytes in constructed wetlands.
Permissible limits: Physicochemical parameters of any water body, plays a vital role in maintaining the ecosystem of various life forms. The following table shows the allowable limits of the waste water which was discharged in to lakes, streams and ponds [11].
Evaluation of existing treatment facility
The existing treatment facility provided is Sewage treatment plant located at Nalla Cheruvu in Medchal district of Telangana state. The existing STP treats 30 M liters of wastewater daily. The inlet of this STP comes from diverting the water from river Musi. Hyderabad Metropolitan Water Supply and Sewerage Board (HMWS&SB) managing director G Asok Kumar said that the STP, built at a cost of Rs 12 crore, would treat the sewage coming from the catchment areas of Tarnaka, Malkajgiri, Kapra, Uppal, 10 percent of Cantonment and 90 per cent of Osmania University, Habisguda and Ramanthapur [10]. The treatment process involves preliminary removal of inorganic solids and grit, anaerobic sludges, blanket reactors, followed by aeration in facultative aerated lagoons. Finally, the effluent will be chlorinated and let out into river Musi [10]. The outlet of this sewage treatment plant is discharged into the river Musi by dilution. The inlet and outlet samples are collected and tested in the laboratory. Various parameters of concern are pH, BOD, COD, Total dissolved solids, Total solids, and Total suspended solids.
The inlet and outlet characteristics of constructed wetland units have been evaluated for three field trials with a retention time of 48 hrs. Various characteristics of wastewater has been tested in environmental engineering laboratory of NMREC for three successive weeks and yielded the following results.
Statistical analysis
Here we are comparing the different parameters of the waste water like pH, dissolved oxygen, BOD, COD, turbidity, TSS and TDS before and after the treatment. The process that affects the removal of pollutants includes nitrification, oxidation, fragmentation, mechanical adsorption etc.., with increase in the time the removal pollutants has been increased due to capacity of plants to uptake pollutants. From Tables 1-3, the value of dissolved oxygen which is nil in the influent has been increased to maximum value of 4.3 mg/l which states that the aquatic life [2,8] is possible in the effluent of wetland units. From Tables 4 and 5 the pH has been balanced when compared to STP the effluent of wetland units has balanced pH i.e., ranges from 7.4-8.5 is under permissible limits so the water is useful for domestic purposes and gardening. Similarly, from Tables 1-3 the effluent from all three wetland units has more than 60% removal on an Average of TDS, BOD, Alkalinity, Acidity, Turbidity and Conductivity [1,4,6,8] compared to STP Effluent.
| Parameter | Value | Units |
|---|---|---|
| Length | 60 | Cm |
| Width | 40 | Cm |
| Depth | 30 | Cm |
| Surface area | 0.24 | Sq. meters |
| Retention Time |
48 | Hours |
| Vegetation | 5 | Plants |
| Parameters | Permissible limits |
|---|---|
| pH | 6.5-8.5 |
| D.O | 30 mg/l |
| COD | 250 mg/l |
| BOD | 30 mgl |
| TS | 600 mg/l |
| TDS | 500 mg/l |
| TSS | 30 mg/l |
| Parameter | STP Inlet | STP Outlet |
Outlet of Colocasia |
Outlet of Cyperus rotundus |
Outlet of Cocos nucifera |
|---|---|---|---|---|---|
| pH | 7.32 | 7.88 | 7.43 | 7.59 | 7.91 |
| Turbidity(NTU) | 79.9 | 5.8 | 3.6 | 4.1 | 4.9 |
| DO (mg/l) | 0.18 | 4.4 | 5.1 | 4.9 | 3.6 |
| BOD (mg/l) | 134 | 23 | 12 | 29 | 28 |
| TSS (mg/l) | 362 | 43 | 11 | 18 | 21 |
| TDS (mg/l) | 756 | 470 | 350 | 390 | 420 |
| Alkalinity | 75 | 98 | 107 | 124 | 128 |
| Acidity | 675 | 116 | 41 | 44 | 42 |
| Conductivity | 0.76 | 0.87 | 0.98 | 0.92 | 0.86 |
| Parameter | STP Inlet | STP Outlet | Outlet of Colocasia | Outlet of Cyperus rotundus | Outlet of Cocos nucifera |
|---|---|---|---|---|---|
| pH | 7.1 | 7.9 | 7.43 | 7.59 | 8.2 |
| Turbidity (NTU) | 80.4 | 6.1 | 2 | 2.8 | 2.7 |
| DO (mg/l) | 0.14 | 4.1 | 5.2 | 5.1 | 3.8 |
| BOD (mg/l) | 131 | 18 | 10 | 28 | 27 |
| TSS (mg/l) | 360 | 38 | 10 | 15 | 17 |
| TDS (mg/l) | 720 | 420 | 355 | 385 | 415 |
| Alkalinity | 80 | 100 | 110 | 126 | 124 |
| Acidity | 590 | 112.9 | 39 | 37 | 39 |
| Conductivity | 0.74 | 0.88 | 1.13 | 1.13 | 1.1 |
| Parameter | STP Inlet | STP Outlet | Outlet of Colocasia | Outlet of Cyperus rotundus | Outlet of Cocos nucifera |
|---|---|---|---|---|---|
| pH | 7.35 | 7.87 | 7.56 | 7.61 | 8.2 |
| Turbidity (NTU) | 80.1 | 5.4 | 1.9 | 2.5 | 2.8 |
| D.O (mg/l) | 0.15 | 4.2 | 5.3 | 4.8 | 3.9 |
| B.O.D (mg/l) | 135 | 21 | 9 | 24 | 29 |
| TSS (mg/l) | 370 | 36 | 8 | 14 | 17 |
| TDS (mg/l) | 710 | 440 | 340 | 380 | 410 |
| Alkalinity | 78 | 110 | 110 | 103 | 158 |
| Acidity | 670 | 113.4 | 43 | 45 | 47 |
| Conductivity | 0.76 | 0.84 | 1.14 | 1.12 | 1.1 |
Comparison of results
The results of three field trials have been compared based on their characteristics (Figures 6-14).
Figure 6. Comparison of pH of inlet and outlet samples.
Figure 7. Comparison of D.O of inlet and outlet samples.
Figure 8. Comparison of conductivity of inlet and outlet samples.
Figure 9. Comparison of alkalinity of inlet and outlet samples.
Figure 10. Comparison of turbidity of inlet and outlet samples.
Figure 11. Comparison of B.O.D of inlet and outlet samples.
Figure 12. Comparison of acidity of inlet and outlet samples.
Figure 13. Comparison of TSS of inlet and outlet samples.
Figure 14. Comparison of TDS of inlet and outlet samples.
The constructed wetlands are proved to be the most economical and efficient method for the treatment of municipal wastewater. The result of this work shows that the load on the existing treatment facility can be reduced by the effective use of constructed wetlands. A well-designed constructed wetland would perform efficient than the natural wetlands because of easier management and control. Removal efficiencies of constructed wetland units differ with their vegetation. In this work it is found that the removal efficiency of constructed wetland unit with sedges (Colacasia) is greater than the other units and STP. From this study following conclusions are made:
1. The unit with Colacasia has the greater average removal efficiency of 96.32% for turbidity, 92.23% for BOD, 97.33% for TSS, 96.32% for acidity respectively.
2. The unit with Cyperus rotundus has the average removal efficiency of 96.07% for turbidity, 79.72% for BOD, 95.68% for TSS, and 93.81% for Acidity respectively.
3. The unit Cocos nucifera has the average removal efficiency of 96.19% for turbidity, 78.99% for BOD, 94.95% for TSS, and 93.62% for Acidity respectively.
4. The DO of outlet of three wetland units has been above 3 mg/l which states that aquatic life is safe in the effluent and it can be discharged into the streams without further treatment.
5. pH of the effluent of three wetland units is under 8.5 stating that the wetlands can bring down the pH of wastewater.
Thus, the wetland treatment can be used effectively in the treatment of wastewater, so as to bring down the organic loading rate and make the final output fit enough for reuse.
The wastewater treatment system at NMREC using constructed wetlands has created knowledge on environmental issues to the students and staff. The treated effluent obtained was under the limits of Central Pollution Control Board for wastewater discharge. This technology can be implemented for wastewater treatment in houses, industries with grater organic load.
The wetlands can also be used for effective land management of barren lands, mining lands also. Constructed wetlands primarily act like biological filters which are very beneficial in removing Turbidity, BOD, COD, TSS and TDS. Wetlands will provide eco-friendly nature and it doesn’t works on any harmful and difficult technologies in removing pollutants from the polluted water.
The successful completion of any task would be incomplete without mentioning of names of the people, who’s constant and encouragement crown all the efforts with success.
First and foremost, we express our all gratitude to Mr. D. SRIDHAR, Asst. professor, department of Civil engineering, Nalla Malla Reddy Engineering College, for enabling me to complete this work successfully, it is my duty to convey sincere thanks to my project guide.
We wish to acknowledgement the continuous support and encouragement received from our head of department of Civil engineering, Nalla Malla Reddy Engineering College, DR. SUDHAKAR REDDY, during this project.
We express our heartfelt thanks to DR. Divya Nalla, Director of Nalla Malla Reddy Engineering College, for giving us this opportunity for the successful completion of our project.
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