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. 

Table.1: Physico-chemical analysis of domestic wastewater

S.No.	Parameters	Concentration	Maximum Permissible Limit
1.	pH	7.2	6.8-8.5
2.	Electrical Conductance-EC (m mhos/cm)	0.31	0.1
3.	Temperature (0C)	20.5	16-32
4.	Turbidity (NTU)	320	5-10
5.	Total Solids (mg/l)	990	500
6.	Total Suspended solids (mg/l)	341	10-50
7.	Total dissolved solids (mg/l)	690	450
8.	Chemical Oxygen Demand (mg/l)	1080	30-45
9.	Biochemical Oxygen Demand (mg/l)	783	3-4

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.

Table.2: Characterization of fly ash.

Chemical characteristics (%)		Proximate analysis (%)		General characteristics
Silicon dioxide (SiO2)	62.0	Ash	72.42	Surface area, m2/g	13.0
Aluminium oxide (Al2O3)	21.98	Loss on ignition	13.02	Bulk density, kg/ m3.	100 0.80
Iron oxide (Fe2O3)	7.20	Volatile matter	3.26	Particle size, mm.	2.8, 0.710
Calcium oxide (CaO)	3.20	Fixed carbon	10.09		0.075 0.053
Magnesium oxide (MgO)	1.10	Moisture	1.21
Phosphorus pentoxide (P2O5)	1.35
Titanium oxide (TiO2)	1.10
Alkali oxide (Na2O/ K2O)	2.10
Sulphur trioxide (SO3)	0.62

Adsorption studies
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.

Experimental conditions
Study for contact time: These studies were conducted by agitating 100ml sample with initial COD concentration 1080 ppm. And known amount of fly ash as an adsorbent agitated it for different time period, 30 - 300 minutes. After the predetermined time intervals, the sample were withdrawn, filtered and determined the residual COD concentration.

Study for adsorbent dose: the studies were conducted by varying the amount of adsorbent. A known volume of sample was treated with different doses of fly ash, 2.0- 7.0g/100ml. The samples were agitated for specific time, filtered and then analyzed for the residual COD.

Study for pH: pH effect was performed taking a specific concentration, adsorbent dose, & contact time and varying the pH values from 1-12 using dilute NaOH/ HCl solutions. The samples were agitated for specific time, filtered and then analysed for the residual COD concentration.

Study for initial COD concentration: These studies were performed by keeping all the conditions constant except changing the initial COD conc. by using simulated COD bearing solutions prepared by dissolving known amount of glucose in distilled water.

Studies for agitation time were: performed by varying agitation speed from 100 rpm to 700 rpm and keeping all conditions constant. Finally analysed the sample for residual COD concentration.

Similarly, studies for adsorbent particle size were performed by keeping all conditions constant and varying the particle size. The residual COD concentration was determined after each run. The removal of COD was quoted (as %) relative to the values measured for the untreated effluent.

Similar kinds of experiments were performed for standard commercial activated carbon as an adsorbent for the shake of comparison.

Results and Discussion
The results observed after the physico-chemical analysis of the wastewater as depicted in table: 1 showed that the domestic waste water is highly polluted with the organic load and suspended matter. Organic load is depicted in terms of COD and BOD values. The COD concentration is much higher than the permissible limit.

The composition of typical Indian fly ash in table: 2 depicted that the fly ash is predominantly silicious followed by the insoluble oxides of aluminium, iron, calcium, magnesium, titanium, alkali oxides and a negligible amount of phosphorus pentoxide and sulphur oxides. In the case of fly ash as an adsorbent the metal salts hydrolyses in the presence of natural alkalinity to form metal hydroxides. The multivalent cations present in fly ash can reduce the zeta potential while the metal hydroxides are good adsorbents. They form monomolecular layer on the surface of suspended organic matter and removes it by enmeshing them and settling.



Fig. 1 represents the percent removal of COD for different contact time both by the fly ash and commercial activated carbon. It seems that COD removal has been achieved to the extent of more than 67% by fly ash at a maximum time period of 250 minute and the trend of percent COD reduction with fly ash was comparable to that of commercial activated carbon. From the removal curve (fig. 1), it has been seen that equilibrium attained in 250 minutes. The smooth and independent nature of curve indicates formation of monolayer cover of the adsorbate on the outer surface of the adsorbent. The adsorption process for the studied adsorbent follows first order kinetics and Freundlich adsorption pattern.



Fig. 2 indicates the effect of adsorbent dose on the percent COD reduction by fly ash and also compared its trend with that of commercial activated carbon. It was observed that maximum removal occur at the dose of 60 g/l for fly ash and 50 g/l for commercial activated carbon and that is 79.31 % and 98.59 % respectively. After that the equilibrium was set up by further addition of adsorbent dose. Fly ash shows fairly the same trend to that of commercial activated carbon. The trend of dose effect on percent COD reduction both by fly ash and activated carbon are also represented. The results showed the tremendous increase in percent COD removal with the increment of adsorbent dose, owing to the increase in the number of sites (Mancy et al., 1964). At lower doses, the significant small adsorption is possibly due to the saturation of surface active sites with the adsorbate molecules.



Fig. 3 depicts the effect of pH on percent COD reduction by fly ash and commercial activated carbon. The runs were taken at the constant initial COD concentration, adsorbent dose and the contact time. The results indicate that at all pH levels below 5.0, the fly ash has consistently higher adsorption capacity for COD. There is more than 80 % drop in percent COD by fly ash.

At high pH the capacity of the adsorbent get recessed. The reason for the better adsorption capacity observed at low pH levels may be attributed to the larger number of H+ ions present, which in turn neutralise the negatively charged adsorbent surface, thereby reducing hindrance to the diffusion of organics at higher pH. The reduction in adsorption may be possible due to the abundance of OH-ions, causing increased hindrance to diffusion of organics (contributing to COD) ions. Oxides of aluminium, calcium, silicon, iron etc. are abundant in fly ash. The hydroxylate surfaces of these oxides in aqueous medium is reported to undergo the following dissociation:

Similar observations have also been reported by the workers (Mott et al., 1992; Liskowitz et al., 1980; Mall et al., 1997)



Fig. 4 represents the effect of initial COD conc. on % COD reduction by fly ash and commercial activated carbon at the optimum pH, adsorbent dose and the contact time as predicted from table 3,4, and 5 respectively. The fly ash seems to be fairly active adsorbent even at higher initial concentrations. At lower initial concentrations, the ratio of the initial number of moles available to the adsorbent surface area is low and subsequently the fractional adsorption becomes independent of initial concentration. At higher concentrations, the available sites of adsorption become fewer and hence the % removal of COD depends upon the initial concentration. The COD removal of over 65% -81% obtained with fly ash within the concentration range investigated. The comparison in trend of % COD reduction by fly ash with respect to commercial activated carbon under this condition is depicted in the figure.



Fig. 5 indicates the effect of agitation speed on % COD removal by fly ash and commercial activated carbon. The agitation speed varied from 100 rpm to 800 rpm, keeping the initial conc., pH, contact time and adsorbent dose constant. The results indicate that there is a definite improvement in the extent of adsorption with increase in the speed of agitation and then equilibrium was set up after 700 rpm. And after that, decreased which the film covering the adsorbent surface due to the rate of adsorption owing to the mass transfer resistance contributes.



Fig. 6 represents the effect of particle size on % COD reduction. It was observed that the extent of adsorption decreased with increasing particle size. In general, the intra-particle mass transfer effect will increase with the increasing particle size. However, the surface area per unit mass of adsorbent as well as diffusional transport might be larger in case of smaller particles, which increases the adsorption rate. Rao et al., 2000, observed similar characteristics for the adsorption of COD on activated carbon. In case of 0.053 mm particles, 87.89% COD reduction was observed within 250 minutes, but it was only 75.31% for 2.8 mm particles.

Conclusions
It is revealed from the studies that the treatment of domestic wastewater can be done by fly ash generated from thermal power plant to reduce the organic load. It is observed that the COD can be reduced upto the extent of 87.89 % by use of fly ash. Trend of % COD removal by fly ash are fairly comparable to that of commercial activated carbon. It is physically viable and economically viable approach.

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