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Fly ash based low cost method for COD removal from domestic waste water
 Rani Sahu rani_sahu@yahoo.com Vijender Sahu*, R. P. Dahiya, K. Gadgil Center for Energy Studies Indian Institute of Technology, New Delhi *District Science specialist Hisar (Haryana)
Abstract Domestic wastewater is analysed for determining its major pollutants. Combating potential of fly ash and a commercial grade activated carbon is determined to remove COD from the wastewater. For this purpose a system of standardised batch absorbers under steady state conditions is used to study the effect of these media. The influence of treatment time, adsorbent dose, pH of the media, initial COD concentration, agitation speed and adsorbent particle size on the rate of per cent COD reduction is evaluated. Fly ash has shown quite effective adsorbent capacity for COD reduction from the domestic wastewater. Though its capacity is lower than that of commercial grade activated carbon, the low material cost makes it an attractive option for the treatment of domestic wastewater. Introduction Pollution of water by organic and inorganic chemicals is of serious environmental concern. Domestic wastewater differs in characteristics from the industrial wastewater. In domestic wastewater the organic load mainly due to the processes like food processing, washing of floor, cloths, utensils, animals, bathing and sewage. The main components of domestic wastewater are proteins, carbohydrates, detergents, tannins, lignin, humic acid, fulvic acid, melanic acid and many other dissolved organic compounds (Rebhun and Manka, 1971; Manka et al.; 1974). The organic content of wastewater is traditionally measured using lumped parameters such as BOD, COD and TOC. These parameters as such do not show any chemical identity of organic matter. A number of conventional treatment technologies have been considered for treatment of wastewater contaminated with organic substances. Among them, adsorption process is found to be the most effective method. Adsorption as a wastewater treatment process has aroused considerable interest during recent years. Commercial activated carbon is regarded as the most effective material for controlling the organic load. However due to its high cost and about 10-15 % loss during regeneration, unconventional adsorbents like fly ash, peat, lignite, bagasse pith, wood, saw dust etc. have attracted the attention of several investigations and adsorption characteristics have been widely investigated for the removal of refractory materials (Pandey et al., 1985) for varying degree of success. Thus the removal of organic material by adsorption onto low cost waste material has recently become the subject of considerable interest. This approach offers a potentially simple and economic “End of Pipe” solution to the challenges set by new legislation covering effluent discharges. Several investigations (Nelson and Guarino, 1969; Eye and Basu, 1970; Johnson et al., 1965; Deb et al., 1966; Gupta et al., 1988, 1990; Mott and Weber, 1992; Viraraghavan and Alfaro, 1994) explored the use of fly ash as an adsorbent for the treatment of wastewater to remove a variety of organic compounds and color. Pandey et al., 1985, investigated the removal of copper from wastewater by taking fly ash as an adsorbent. Gupta et al., 1990, used fly ash for the removal of chrome dye from aqueous solutions and found that the mixture of fly ash and coal (1:1) may substitute the activated carbon. Each of them concluded that fly ash has a significant capacity for adsorption of organic compounds from aqueous solutions. It was reported that (Banerjee et al., 1995) the carbon content of fly ash plays a significant role during the adsorption of organic compounds by fly ash. The adsorption capacity increases with the increasing carbon content of fly ash. An identical trend was observed by other investigation (Mott and Weber, 1992; Mancy et al., 1964). However, a review of the literature showed that very little investigation has been conducted to find out the suitability of fly ash for the removal of COD from the domestic wastewater. Objective of the research was to demonstrate the use of fly ash as an alternative media over activated carbon, to gain an understanding of the adsorption process. Fly ash is a residue that results from the combustion of coal in power plants. One of the main advantages of COD removal by using fly ash over the other chemical treatment methods is that it is in abundance and easy availability makes it a strong choice in the investigation of an economic way of COD removal. Other advantage is that it could easily be solidified after the pollutants are adsorbed because it contains pozzolanic particles that react with lime in the presence of water, forming cementation calcium-silicate hydrates. In the present study various parameters affecting adsorption like contact time between the waste water and the adsorbent, adsorbent dose, pH of the sample, initial COD concentration, agitation speed and size of the adsorbent particles have been investigated and data on adsorption isotherms have been presented.
Methodology Wastewater samples were collected from the urbanized village. The pH and EC of the samples were measured on the site and the other parameters were analysed in the lab according to the APHA (1989). Samples were stored at temperature below 3oC to avoid any change in the physic-chemical characteristics. The COD of the samples were estimated before and after adsorption giving different treatment. Fly ash was obtained from Faridabad thermal power plant, Haryana. The fly ash was derived out of the bituminous coal obtained from the Siyal and Gaddi coal mines of Bihar (India). The sample received was washed with distilled water to remove surface dust and was dried in sun. Fly ash samples were stored in the laboratory in airtight plastic container. The physico-chemical characterisation of fly ash was carried out using standard procedures. In addition physical properties such as density and surface area were also determined. The major components of fly ash are alumina, silica, iron oxide, calcium oxide and residual carbon. However, the constituents of fly ash vary according to the type of coal used and degree of combustion.
All the experiments were carried out at ambient temperature in batch mode. Batch mode was selected because of its relative simplicity. The batch experiments were run in different glass flask of 250 ml capacity using average speed shaker. Prior to each experiment, a predetermined amount of adsorbent was added to each flask. The stirring was kept constant for each run throughout the experiment ensuring equal mixing. The desired pH was maintained using dilute NaOH/ HCl solutions. Each flask was filled with a known volume of sample having desired pH commenced the stirring. The flask containing the sample was withdrawn from the shaker at the predetermined time interval, filtered through whatmann No. 44 filter paper. The experiments were carried out under different experimental conditions.
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