LEACHATE TRANSPORT FROM FLY ASH SLURRY POND

Abstract

Huge quantities of fly ash, generated from combustion of low rank coal at various thermal power plants, are being disposed off on large piece of land. Utilization of fly ash is the one way to solve the problem of its disposal, but only 30% of fly ash is being utilized in India and rest of the unused fly ash is dumped in the ash pond. Fly ash contains several toxic elements such as Pb, Zn, Cd, Ni, Co and As. Leaching of these toxic elements in ash pond poses great environmental hazard due to its potential toxicity. The adverse impact includes contamination of surface and subsurface water with toxic heavy metals present in the coal ashes, loss of soil fertility around the plant sites. Water • infiltrating through ash pond dissolves such heavy metals and soluble salts and carries it downward through the unsaturated zone. Such pollutants traveling through the • unsaturated zone join the water table and may affect the water quality adversely. The problem of safe disposal of large quantities of ash which remain unutilized has received little attention in the past and is area of focus of this research work. The physical and chemical properties of coal ashes are dependent on coals geological origin, combustion conditions, efficiency of particulate removal and degree of weathering before final disposal. Mineralogy along with morphology gives an important index about utilization of fly ashes or it can be also useful to monitor post disposal scenario. Physical and chemical properties are characterized with the help of various instruments such as XRF, XRD, SEM, AFM, SEM-EDAX and Laser Particle size Analyzer. To study to the field leaching behavior of fly ashes cascade leaching test has been performed at pH= 7 (120 hours) and pH= 4 (288 hours). Cascade leaching test simulates natural leaching of fly ash. The leaching trend was observed as, an initial or early peak concentration of the metals in leachate followed by an exponential decrease of concentration with increase in time or L/S ratio or pore volumes. Scanning electron Micrographic studies of post leached fly ashes reveals that surface of fly ash particles is etched. Single and composite exponential decay models were fitted between two parameters, i.e. metal concentrations in leachate and `n' stage of leaching, and can be used for the prediction of concentrations of metals in leachate. Based on the desorption ii phenomenon and laboratory experiments performed, leachate prediction model is formulated where it is assumed that distribution coefficient is constant but, it is observed that it is not constant and it decreases exponentially with W. A model considering that the distribution coefficient, changes from stage to stage of leaching has been developed. The distribution coefficient is linearly proportional to the leacahte concentration and it decreases with decrease in leachate concentration. Soil column leaching tests conducted in the laboratory showed that the pH of the effluent is persistent for the study period of 44 days. The release of metals appears to be adsorption controlled, due to which attenuation of heavy metals took place. Attenuated heavy metals, however, are not biodegradable. They are stored in the soil and may be remobilised and migrate into groundwater when the soil's adsorption capacity. is exceeded or its properties are changed. To protect unsaturated zone and groundwater provision of liners is essential. Top soil layer of surrounding of the Nashik ash pond is occupied by fly ash and it may be due to wind blown particles of fly ash. Analysis of groundwater from open dugwells at Kotamgaon and Hinganvedhe village found that the groundwater quality is deteriorated due to the presence of fly ash ions such as Fe, Mn Cd and Ni. Apart from input from ash pond, use of ash pond effluent for irrigation, seepage from Eastern and Southern effluent channel and leaching of top soil (contaminated due to fly ash dispersal) contributes to groundwater. There is significant potential exists for environmental impacts from uncontrolled seepage of rainwater and slurry water through ash deposits in the absence of liner, once the operating life of ash pond is over. Provision of low cost liner is necessary to protect the groundwater contamination from ash pond. The past several decades have seen considerable progress in the conceptual understanding and mathematical description of water flow and solute transport processes in the unsaturated zone. Mathematical models can be used to predict changes in ambient soil and water quality due to transport of pollutant through the underlying unsaturated zone below the ash pond. Once the leachate joins the water table, the solute transport would take place through advection and dispersion. Predicting water quality impact of a single discharge , that is due to storage of flyash slurry, can be done quickly and sufficiently accurately with simple models. In this study, using Green and Ampt iii infiltration theory, the arrival time of the pollutants at water table has been derived for the case when a low permeable liner is placed at the bottom of the ash pond. Provision of liner is essential. The brick lining is inadequate for restricting leachate infiltration. A liner having conductivity of the order of 3.6x104 cm /hour is preferable to brick lining. Once the operating life of ash pond is over, gradual reclamation of ash pond should be practiced. Iv