Waste-water is the combination of liquid or water-carried wastes originating in houses, commercial or industrial units and institutions, in addition to any ground water, surface and storm water that may be present. Untreated wastewater generally contains high levels of organic materials, pathogenic microorganisms, as well as nutrients and toxic compounds. It thus entails environmental and health hazards and, must immediately be treated appropriately before disposal. The ultimate goal of wastewater management is the protection of the environment in a manner commensurate with public health and socio-economic concerns. Wastewater treatment is becoming even more critical due to increasing water stress and high operational cost of sewage treatment units.
Wastewater Treatment Systems
Natural systems for waste-water treatment are designed to take advantage of the physical, chemical and biological processes that occur in the natural environment when water, soil, plants, micro-organisms and the atmosphere interact. All natural treatment systems involve some form of mechanical pre-treatment for the removal of solid waste.
Constructed Wetlands
Wetlands are inundated land areas with water at shallow depths. Wetlands support the growth of plants such as cattail, bulrush, reeds and sedges. The vegetation provides surfaces for the attachment of bacteria that aids in the filtration and adsorption of pollutants, transfers oxygen into the water column and controls the growth of algae by restricting the penetration of sunlight. Wetlands also clean the water by filtering out sedimentation, decomposing organic matter and converting chemicals into useable form. The ability of wetlands to recycle nutrients makes them critical in the overall functioning of earth.
Floating aquatic plants
This system is similar to the FWS system except that the plants used are of the floating type, such as hyacinths and duckweeds. Water depths are greater than in the case of wetland systems. The floating plants shield the water from sunlight and reduce the growth of algae. It is effective in reducing BOD, nitrogen, metals and trace organics and in removing algae from lagoons and stabilization pond effluents. Supplementary aeration has been used with floating plant systems to increase treatment capacity and to maintain the aerobic conditions necessary for the biological control of mosquitoes.
Use of aquatic plants for water purification
Water plants have the ability to extract nutrients from the water in which they grow. The nutrient extraction possibilities of aquatic plants is the increasing awareness of the problems of water pollution as a consequence of population growth and industrial development, Many examples of the devastating consequences of the waste water on formerly clean and useful rivers and lakes have increased public and scientific awareness of the need not only to arrest the practice of direct dumping but to try and reverse it by extracting the pollutants. The remarkable ability of aquatic plants, particularly of the water hyacinth to extract compounds and elements has brought to light its capabilities in treatment.
Four species are considered suitable:
Eichhomiacrassipes (water hyacinth), Alternanthera philoxeroides(ponnanganni), Justicia Americana (water willow or kotakacalai) & Typha latifolia(Anaikkorai). Water hyacinth would be ideally suited for nutrient removal. As it floats on the surface and is not rooted, harvesting is easy. There would be considerable microbial activity beneath the hyacinths and nutrients would be absorbed by these organisms. In addition considerable organic matter would reach the water by the loss of root fragments and probably have a fairly high biological oxygen demand (BOD) and it might prove necessary to use conventional sewage holding ponds to reduce the BOD prior to final release. Altenanthera philoxeroides could probably be best harvested by draining the pond and then using modified forage harvesting equipment. Typha latifolia as an emergent species could be grown in ponds about 1 m deep so that the water remains anaerobic, allowing bottom soils to remove Phosphates. But it would be better, if space permitted, to grow the plants in water 1.5-20 cm deep to maximize soil absorption of Phosphates.
Aquatic plants for Natural remediation
Aquatic plant-based treatment systems using ponds or constructed wetlands are effective in water pollution control. Aquatic plants have proven effective in treating domestic wastewater and industrial effluents. Aquatic plants are considered for improving water quality of lakes and streams. It is seen to be better than the conventional system for removal of toxic metals from industrial effluents. The high nutrient concentrations in water can be substantially reduced by passage of water through aquatic plants. Nutrient removal is being influenced by temperature, biological activity and flow rate. The slower the flow and the longer the retention time, the greater is the removal. It is proven that out of various aquatic plant species Eichhorniacrassipes (water hyacinth) can be usefully employed to extract nutrients from sewage. It is also proved useful in treating effluents polluted with toxic heavy metals provided it is kept under strict control from spreading. Typha latifolia (Bulrush) is a perennial herbaceous plant which grows in tropical areas shows the maximum absorption of metal like Sodium. Thus on one hand aquatic plants show marked absorption of nutrients
like Nitrogen, Potassium, Sulphur, Calcium, Magnesium, etc. and metals like Sodium, Iron, Magnesium, Zinc, Copper etc. and on the other hand it is also necessary to simultaneously harvest at periodic intervals and remove them especially submerged plants from the lagoons otherwise they will die, decay and return their absorbed nutrients back to the water and thus the level of nutrients cannot be reduced to an acceptable level and will further degrade the water quality. – Mr. B. Saravanan
