Water covering over 70% of the earth, is undoubtedly one of the most precious natural resource of the world. Without the very presence of water, life on earth will be non-existent. However in spite of such large quantity of water present over the Earths surface, only 0.4 % is available for use. Ninety seven per cent of the earths water is the salt water of oceans and seas whereas most of the remaining 3 per cent is captured in polar ice caps, glaciers, atmosphere or underground. Despite so much water being out of reach, still 4, 300, 000 cubic kilometers of water is available to sustain the plants, animals and humans living on the planet Earth.
Growing population, increased economic activity and industrialization has not only created an increased demand for fresh water but also resulted in severe misuse of this natural resource. Water resources all over the world are threatened not only by over exploitation and poor management but also by ecological degradation. According to a survey conducted by UNEP, 20% of world’s population lacks access to safe drinking water and 50% of the world’s population lacks access to safe sanitation. Further studies show that about 830 million people in South Asia lack access to safe drinking water and more than two billion lack proper sanitation. Polluted water is estimated to affect the health of about 1200 million people and contribute to the death of 15 million children under the age of five every year.
With increased industrial growth and urbanization, the volume of domestic and industrial effluent, agricultural waste and urban runoffs is steadily growing. Water bodies have an inherent capability to dilute the pollutants, which enter the system. However, indiscriminate dumping of untreated sewage and chemical wastes directly into rivers, lakes, and drains have made these water bodies unable to cope up with the pollutant load. The steady increase in the amount of water used and wastewater produced by urban communities and industries throughout the world also poses potential health and environmental problems. The contaminated waters disrupt the aquatic life and reduce their reproductive capability.
COLOR AND TURBIDITY IN WASTEWATER
Wastewater disposal is the major problem being faced by us. In developing countries, like India presently, only about 10% of the wastewater generated is treated; the rest is discharged as it is into our water bodies. The most commonly faced problem in disposal of wastewaters is their color and turbidity. Finely dispersed suspended and colloidal particles are responsible for the color and turbidity of the wastewaters. Color in water results from the presence of natural metallic ions, humus and peat materials, plankton, weeds and industrial wastes. Suspended and colloidal matter such as clay, silt, finely divided organic and inorganic matter, and plankton and other microscopic organisms are responsible for turbid waters.
Liquid wastes from paper mills, leather industry effluent have 10 to 20% total solids with a Chemical Oxygen Demand (C.O.D.) content of 25000 to 75000 mg/l. Slaughter house effluent has total suspended solids in the range of 3000 to 4000 mg/l and a COD content of 6000 to 8000 mg/l. The effluent generated by anaerobic digestion of municipal market wastes has 2- 3% solids. Wastewaters from textile, food, cosmetic, paper and leather industries contain dyes which being recalcitrant in nature are difficult to degrade. These dyes are highly colored compounds and leads to reduction of sunlight penetration in rivers, lakes or lagoons that in turn decrease photosynthetic activity and reduces the dissolved oxygen concentration. This brings about detrimental effect on the aquatic life.
THE COAGULATION PROCESS
Coagulation and flocculation are the processes used to remove the particles responsible for turbidity and color. The colloidal particles present in wastewaters generally carry a negative electrical charge. Their diameter may range between 10-4 to 10-6 mm. These particles are surrounded by an electrical double layer (due to attachment of positively charged ions from the ambient solution) and thus inhibit the close approach of each other. They remain finely divided and dont agglomerate. Due to their low specific gravity, they dont settle out.
Coagulation is accomplished by the addition of ions having the opposite charge to that of the colloidal particles. In coagulation, a coagulant (generally positively charged) is added which causes compression of the double layer and thus the neutralization of the electrostatic surface potential of the particles. The resulting destabilized particles stick sufficiently together when contact is made. Rapid mixing (a few seconds) is important at this stage to obtain uniform dispersion of the chemical and to increase the opportunity for particle-to-particle contact. Flocculation, which follows coagulation, consists of slow gentle stirring. During flocculation, the microscopic coagulated particles aggregate with each other to form larger flocs. These flocs then are able to aggregate with suspended polluting matter. These flocs are large enough to settle rapidly under the influence of gravity, and may be removed from suspension by filtration.
Aluminum and iron salts are commonly used as chemical coagulants. They form insoluble material i.e. aluminum and ferric hydroxides when they react with calcium and manganese hydrogen carbonates, which are almost always present in water. The formation of the insoluble hydroxides depends on the pH. Whereas turbidity is best removed within a pH range of 5.7 to 8.0, color removal is generally obtained at pH range of about 4.4 to 6.0. Alum (aluminum sulphate) is most widely used chemical coagulant whereas ferric chloride, potash alum, ammonia alum, ferrous sulphate are also some of the chemical coagulants which are not extensively used.
PLANT MATERIALS AS NATURAL COAGULANTS
The use of plant materials as natural coagulants to clarify turbidity of wastewaters is of common practice since ancient times. Powdered roasted grains of Zea mays were used by soldiers in Peru as a means of settling impurities in the 16th and 17th century. In India, ancient writings refer to the use of the seeds of the Nirmali tree Strychnos potatorum as a clarifier. The sap of tuna cactus (Opuntia fiscus indica) is widely used in Chili as water purifying agent. The commercially extracted products are known as Tunaflex A and B. Similarly, dried beans (Vicia faba) and peach seeds (Percica vulgaris) are widely used for this purpose in Bolivia and Peru. Tunaflex and Nirmali seeds have been successfully employed in municipal treatment plants in combination with alum.
Of all plant material investigated, seeds of Moringa oleifera are one of the most effective sources of primary coagulant for water treatment. The M. oleifera tree, commonly known as drumstick and horseradish, is a native of Northern India. It is widely grown throughout the tropics. The tree is drought resistant, fast growing and even grows in poor soils.
The traditional use of M. oleifera seeds for domestic water treatment is a common practice in rural areas of Sudan. The seedpods are allowed to dry naturally on the tree prior to harvesting. The seeds are then shelled, crushed and sieved. The crushed seed powder are mixed with turbid water that produces positively charged water-soluble proteins. These proteins bind to the suspended particles forming large agglomerated solids. The flocculated solids are allowed to settle prior to boiling and subsequent consumption of water.
Pilot scale and full-scale water treatment plants using M. oleifera seeds are successfully operated in Malawi. A pilot plant under the Ministry of Works and Supplies Water Department of the Malawi Government was commissioned in 1992. In 1994, the full-scale treatment plant was operated using M. oleifera solution as coagulant. The plant comprised of upflow contact clarifiers followed by rapid gravity filters and chlorinators. Alum solution was introduced into the incoming flow of 60 cubic metres per hour by simple gravity feed. Turbidities of 270 - 380 NTU were consistently reduced to below 4 NTU. This was the first time when M. oleifera was used successfully as a primary coagulant at such a scale with the treated water entering the water supply.
The results obtained from the use of M. oleifera as coagulating agents show:
Significant reduction of turbidity
No alteration in pH value of treated water
Initial reduction of bacterial count
Antibiotic effect of bacteria and fungi.
Tannins, essential oils, sap or adhesive agents contained in plants are the active agents that bring about coagulation. Natural polymers such as starch, gums, glues, alginates, etc., function as bridging flocculants. Investigations of M. oleifera seeds have revealed that the coagulant properties of the seeds are due to a series of low weight cationic proteins.
WETLANDS FOR WATER TREATMENT
Aquatic plants also prove to be effective in treatment of wastewater. Water hyacinth, (Eichhornia crassipes), cattails, tortora and duckweed are the species, which are being used for wastewater removal. Tortora and cattails are found in shallow lakes, rivers and impoundments. These are generally used in small lagoons, where they bring about sedimentation of suspended solids, biological decomposition of organic compounds, removal of nitrates, phosphates, heavy metals such as manganese, zinc, copper lead and reduction of pathogenic microorganisms. Water hyacinths are the most promising aquatic plants for removal of suspended solids and heavy metals from wastewater.
Wetlands both natural and artificial are receiving worldwide attention for their ability to improve water quality. Wetlands are frequently chosen as the best solution for wastewater treatment because they are often less expensive to build and have lower operation and maintenance costs than other treatment technologies.
Artificial wetlands are created wetlands and are mainly used for municipal and residential wastewater, storm water runoff, agriculture wastewater, landfill leachates, acid mine drainage and decontamination of polluted groundwater. Constructed wetlands can also provide excellent wildlife habitat, recreational opportunities, and an aesthetically pleasing treatment facility. Water hyacinth based and other wetland systems produce plant biomass that can be used as a fertilizer, animal feed supplement, or even as a source of methane.
The use of seed materials and aquatic plants are receiving attention for their effectiveness in wastewater treatment. The technologies involved are economical, traditional and easy to implement and ideal for rural areas. The process being biological in nature does not generate any non-treatable wastes. These processes are easy to operate and require little or no maintenance.
For the future development of the use of plant materials for wastewater treatment, other native plants and plant materials should be investigated as coagulants for color and turbidity removal. Algae derived substances, chitosan produced from shells of shrimps and lobsters and dough from millet bread are some of the potential natural coagulants. Future studies should focus upon the dosage of these materials to be used in water treatment. Efforts should be made to make these traditional technologies, which are already being used in rural areas, recognized and accepted globally.
 David Krantz and Brad Kifferstein, Water Pollution and Society
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