Phytoremediation - The Clean Up Technology for the Future

Keywords: Bio Accumulation; Macrophytes; Phytoremediation; Urban Wastes

Introduction

One of the burning problems of our industrial society is the high consumption of water and the high demand for clean drinking water. Numerous approaches have been taken to reduce water consumption, but in the long run it seems only possible to recycle wastewater into high quality water [1]. Phytoremediation has probably a large potential for treatment of pollutants in the environment, even if today, plants are not widely used. Studies were conducted in order to determine technical and economical feasibility of phytoremediation processes for full-scale treatment, including rhizofiltration (use of plant to accumulate compounds from aqueous solutions into roots), phytostimulation (use of plant to stimulate naturally occurring microbial degradation), phytostabilization (use of plant to prevent compounds from mobilizing or leaching in soil) and phytoextraction (use of plant to remove contaminants from soils into plant roots or shoots). One of the greatest advantages of phytoremediation is its lower cost than other competing technologies. In addition to cost, phytoremediation offers other advantages: it is a non destructive in-situ technology applicable to a variety of contaminants; it is capable of remediating the bioavailability fraction of pollutants and accumulating heavy metals (Cu, Co, Ni, Zn, Cd, and Pb). On the other hand, treatment time is the biggest disadvantage, and 5 to 20 years may be needed in some cases for soil clean-up. Plants are seasonally dependent and hyper accumulator plant species can also have a very low growth rate, making necessary to select new varieties capable of hyper accumulation with high biomass production. Among disadvantages are the fact that phytoremediation is not applicable to mixed wastes or to high concentrations of pollutants. It also requires large available surface area and it is applicable only to surface soils. One of the major problems is the need, in some cases, to harvest the biomass and to dispose it as hazardous waste. There is also a lack of recognized economic performance data and the potential market seems to be very confidential today. Further research and development efforts are then necessary to increase remediation performances and to reduce treatment time, especially for high concentration pollutions or complex pollutions with mixtures of heavy metals and/or organic compounds.

Alicia et al. [2] reported that the roots of some aquatic plants could retain both coarse and fine particulate organic materials present in water bodies supporting their growth. Plants sustain large microbial population in the rhizosphere by rhizo-deposition, root cap cells, which protect the root from abrasion, may be lost to the soil at a rate of 10000 cells per plant. In addition, root cells excrete mucigel, a gelatinous substance that is a lubricant for root penetration through the soil during growth and microbes in the root zone can help to solublise insoluble nutrients and recycle organically bound nutritive elements [3-5].

Phytoremediation was used as a promising technology by many researchers in removing various pollutants from the industrial effluents. BOD from the industrial effluents has been effectively removed by phytoremediation [6]. The average overall treatment efficiency for COD removal was high with wetland treatment technology for industrial effluents [7-9]. Gudekar and Trivedi [10] reported 59.54 per cent reduction of turbidity in treatment of engineering industry waste with water hyacinth. Reduction of total dissolved solids from acid mine drainage from a uranium deposit and pulp and paper industrial wastewater by means of a natural wetland [11-12]. The effluent nutrients also can be effectively removed by constructed wetland technology [13]. Karavaiko [14] reported reduction of total β-activity, from influent 2.80 Bq/l to less than 0.5 Bq/l of radionuclide polluted mine waters by aquatic macrophytes.

Materials and Methods

Five artificial wetland plots were constructed in the Department using plastic crates. The sizes of the crates were 18 x 18 x 24 cm. These crates were filled with soil from the wetland and water up to a level of 9 cm. The crates filled with soil were kept for one month for stabilization. After stabilization, the experimental plants were planted.

The plants used for the experiment are: Emergent plants like Typha sp., floating plants like Lemna and Azolla. Effluents from various factories were collected viz. TTP (Travancore titanium products), Trivandrum, Kerala, India. IRE (Indian Rare Earths) Kollam, Kerala, India. The former is a titanium dioxide producing factory which uses sulphuric acid for the cleaning process and the latter is an ilmanite separating factory.

For treatments, the respective plants which were maintained in the tanks were collected, cleaned and blotted. Approximately 250g each experimental plant used for the study, each occupying half of craits, were carefully introduced into the treatment containers. Duplicate of each experimental setup was maintained. 500ml each of water and effluent samples from the respective treatment sets were collected periodically for analyzing the changes in its physico-chemical characteristics. Initial analysis of plant, soil and effluent were done according to Standard Methods [15]. The effluents were added in various concentrations to the constructed wetlands. Two replicates were used for each experiment. After 30 days of  effluent addition to CWS, plants, water and soil samples were analyzed for physico-chemical and biological characteristics viz pH, conductivity, temperature, turbidity, COD(Chemical Oxygen Demand), BOD (Biochemical Oxygen Demand), TOC (Total Organic Carbon), TSS (Total Suspended Solids), TDS (Total Dissolved Solids), NNO3 and bemission of plants. The variations in the values of plant, soil and water samples from the primary data were noted.

Results of the Study

The effluent characteristics before and after treatment in both emergent plant based CWS and free-floating plant based CWS was noted. The results are presented hereunder.

Efficiency of Typha sp. (Emergent) based CWS

The results of the study of changes in IRE effluent characteristics using Typha sp. is provided in Table I. The system showed an overall BOD removal of 70. 84%. 75. 87% of total solids were removed. The pH was also reduced for alkaline nature to near neutral. The changes in TTP effluent due to remediation using Typha based CWS is given in Table II. This system was found to work more effectively with TTP effluent compared to IRE effluent. The TSS removal was upto 92%. 89% of BOD and COD were removed using this system. The pH of TTP effluent (initial) was highly acidic. But after treatment, pH increased to 5.1.

Efficiency of Azolla (free-floating) based CWS

Changes in IRE effluent characteristics when treated with azolla are provided in Table III. A moderate reduction in the pollutant concentration was observed. 54.8% of TSS, 48.6% of COD and 51% of TDS was reduced. The TTP effluent parameters showed a drastic change after treatment. 86.5% of TSS and 88% BOD were reduced. Azolla proved to be a positive candidate for organic matter removal. Changes in TTP effluent characteristics when treated with Azolla are provided in Table IV.

Efficiency of Lemna (free-floating) based CWS

Lemna showed moderate removal of pollutants in the IRE effluent (Table V). 58% of COD and TSS, 56 % of COD and 49% of TDS were removed by the system. However, Lemna was found to be more efficient in the treatment of TTP effluent (Table. VI). 93% COD, 90% BOD and 80% TDS were removed using this system.

Radioactivity Studies

Polluted waters were treated by means of a natural wetland. The wetland was characterized by pistia and emergent species like Typha latifolia, in the wetland vegetation and a diverse micro flora. An efficient removal of pollutants was achieved in the wetland. Changes in b emission C14 of IRE effluent was studies using both emergent and free floating CWS. After treatment, the radioactivity was found to decrease in the effluent and it was found to increase in the plants. The percentage radioactivity increase was maximum with pistia (27.40) and minimum with azolla. However, the percentage decrease of radioactivity in the effluent was high when treated with Typha (13.17) followed by Pistia (Table. VII).

Discussion

Constructed or engineered wetlands often using cattails to treat effluents. For treating contaminated waste water, the phytoremediation plants are grown in a bed of inert granular substrate, such as sand pea gravel, using hydroponic techniques. The waste water supplemented with nutrients if necessary trickles through this bed which is ramified with plant roots that function as a biological filter and contaminant up take system.

The conductivity of the effluents reduced considerably by treatment. Mahmood et al. [16] on working with Eichhornia sp. based constructed wetlands, reported 55.71 per cent reduction of conductivity after twelve days of treatment period. Though it is not a panacea, phytoremediation is well suited for application in low permeability soils, where most currently used technologies have a low degree of feasibility or success, as well as in combination with more conventional clean-up technologies. In appropriate situation, phytoremediation can be an alternative to the much harsher remediation technologies of incineration, thermal evaporation, and solvent washing which essentially destroy the biological component of the soil and can drastically alter its chemical as well as biological characteristics. Phytoremediation actually benefits the soil having an improved functional, soil ecosystem at costs estimated at approximately one tenth of those currently adopted technologies. The system micro-organism and plant are in the focus of sanitation of heavy metal contamination.

Wetlands are capable of achieving a high efficiency of suspended solids removal from the water column. Suspended matter in the water may contain a number of types of contaminants, such as nutrients, heavy metals and organic compounds. These contaminants may themselves be in particulate form, or they may be physically or chemically bound to the particulate matter. Thus, in cases where the bulk of the contaminant load is associated with particulate matter, physical settling of suspended solids can result in efficient removal of the contaminants from the water or wastewater stream. The removal percentages of TSS in the present study agree with the study of Haris [9].

The BOD of the effluent was reduced significantly after the treatment in all the effluents by the emergent plant. Tegegne et al. and Kirzhner et al. [17-18] noticed significance decrease in the concentration of BOD when the effluents from various industries were treated with constructed wetlands. Recent studies by Adeola et al. [6] reported significant reductions in the biochemical oxygen demand throughout the system with levels decreasing by up to 76.7 per cent across the constructed wetland cells. The reduction in BOD and COD can be attributed to many reasons. Aquatic plants have the unique feature of transporting oxygen from the aerial plant portions to the submerged parts of the plant and the oxygen transported by aquatic plants significantly increase the sub canopy oxygen content of the water [19].

The beta activity of the effluent was effectively remediate by Typha sp. by 13.17% followed by Pistia sp. Groudev et al., [11] during his study showed that the treatment of acid waters polluted with radioactive elements can be efficiently carried out by natural wetlands with a proper size and located in regions with suitable geological and hydro geological conditions and decrease was from 2.5 Bq/l to less than 0.5 Bq/l. Due to it providing green surfaces, the phytoremediation process is environmentally friendly and cost-effective. The constructed wetlands applied in waste water treatment indicate a new technical-technological solution for nature protection and environment conservation. The operation is inexpensive and energy-saving, since it is dominated by natural processes and does not require air or oxygen.

In most developing countries, there is very few waste water due to high costs of treatment process lack of effective environmental pollution control laws or law enforcement. An additional benefit gained by using wetlands for waste water treatment is the multipurpose sustainable utilization of the facility to uses such as swamp fisheries, biomass production, seasonal agriculture water supply public recreation, wild life conservation and scientific study [20]. Being low cost and low technology system wetlands are potential alternative or supplementary system for waste water treatment in developing countries.

Conclusion

Constructed wetlands have been implemented as waste water treatment facilities in many parts of the world, but to date the technology has been largely ignored in developing countries where effective, low cost waste water treatment facilities are critically needed. Constructed wetlands may be an economical option for secondary treatment of stabilization pond effluent, the most common treatment system in use in economically poor countries. Given the tropical location of many developing nation, Constructed wetlands may be successfully established with plant species acclimated to the tropical environment and able to be harvested for the use in secondary function like fuel production. Cost environmental impact and management consequences must be evaluated for each community considering the potential application of this treatment technology.

Since the dawn of industrial revolution, mankind has been introducing numerous hazardous compounds into the environment at an exponential rate. These hazardous pollutants consist of variety of organic compounds and heavy metals. Heavy metals are primarily a concern because they cannot be destroyed by degradation.

Acknowledgements

The authors are indebted to Professor and Head, Dept. of Environmental Sciences for her support and timely advice. Thanks are also due to University Grants Commission, New Delhi for providing financial assistance to the project and Central Pollution Control Board for timely advice.

References

***

Copyright © 2012, ECO Services International