Leaching and Eco-Toxicity Studies of Metals in Sewage Sludge and Fly Ash Mixtures

Abstract

Leaching is the process of release of certain materials from a carrier into a liquid. Leaching and eco-toxic studies in different sewage sludge and fly ash mixtures were carried out. The 5 g mixture of FA/SS of three different ratios (1:1, 1:4 and 4:1) were taken and mixed with 100 mL of deionised water. The samples were then place on the rotatroy shaker at 120 rpm for 10 h at controlled temperature conditions of 26° ± 1C. The samples were then allowed to settle down and the supernantant was vaccum filtered with the help of 0.45 μm. The leachate was tested for Pb, Cd, Ni, Cu and Co using atomic absorption spectrophotometer (ECIL, India, Model AAS 4129). The leachate was also tested for different water quality parameters such as electrical conductivity, sulphate, chloride, total hardness, permanent hardness, chemical oxygen demand, total kjeldahl nitrogen (TKN), total solids, total dissolved solids, turbidity etc. It was observed that the leachate from the mixture with higher amounts of sewage sludge showed more metal contents and higher contaminants of different water quality parameters, whereas the leachate samples from the mixtures with lesser content of sewage sludge and higher content of fly ash (samples with FA/SS 4:1 ratio) showed very less or no leaching. Experiments were performed with different fly ash and sewage sludge mixtures (1:1, 4:1 and 1:4) to achieve the optimised conditions for leaching of metals. It was observed that with the fly ash-sewage sludge mixture (1:4) leaching of metal ions was the maximum for copper metal (1.5 ppm), and minimum for the Pb (0.2 ppm). For other metals it was 0.0 ppm for Cd and 1.0 ppm for Ni. A decrease in leaching of metals was observed with increase in fly ash content in FA/SS mixture (1:1) which was 0.1 ppm for Pb, 0.0 ppm for Cd, 0.1 ppm for Ni, and 0.3 ppm for Cu, respectively. A decreasing trend in leaching of metal ions was observed with increased fly ash content in the fly ash-sewage sludge mixture. In FA/SS 4:1 ratio the decreased leaching of metal ions (0.0 ppm for Pb, 0.0 ppm for Cd, 0.1 ppm Ni, 0.2 ppm Cu) was observed in comparison to 1:4 and 1:1 FA/SS mixtures. v It was confirmed from the experimental data that FA/SS mixture (4:1) was the optimum dose (the selected dose of FA:SS mixture for lesser leaching from the aqueous solution) for the minimum leaching of heavy metal ions into the aqueous medium. Optimum number of washings for the maximum removal of the metals was checked with different samples of fly ash and sewage sludge mixtures. The samples were tested for leaching of metal ions at acidic pH conditions followed by different number of washings with distilled water. Different washing were given with distilled water to FA/SS mixture in order to analyse leaching from the mixture. It was observed that leaching of metal ions decreased up to three washings only. No further decrease was observed in the fourth wash. So, three washings were considered as the optimum number of washings. Selection of optimum time of shaking was done by analyzing the leaching of metals with optimized FA/SS ratio (4:1) at different hours of shaking time. It was observed that the initial metal contents of 0.3 ppm for Pb, 0.1 ppm for Cd, 0.5 ppm for Ni and 0.9 ppm for Cu-metal decreased after shaking time of 30 minutes to 0.3 ppm (Pb), 0.1 ppm (Cd), 0.4 ppm (Ni), 0.7 ppm (Cu), respectively. No substantial change in leaching of metal ions was observed after a shaking time of 60 and 90 minutes. After shaking the FA/SS (4:1, optimized) mixture for 120 minutes, it was observed that no metal ion leached into the aqueous solution which confirmed that all the metals were leached up to 120 minutes. So, 120 minutes was considered as the optimum shaking time. Experiments were conducted with optimized FA/SS ratio (4:1), varying the pH from 2-8. It has been observed from the experimental results that minimum leaching of heavy metal ion takes place at pH 8 conditions. So, after conducting several experiments with different fly ash and sewage sludge mixtures the effect of fly ash as adsorbent of heavy metal was analyzed from sewage sludge leaching in various FA/SS mixture, it was observed that FA/SS 4:1 dose, three liquid wash, 2 hours shaking time and pH 8 conditions were the optimized conditions to arrest the leaching of metal ions from sewage sludge. Different experiments were carried out with the optimised dose of FA/SS 4:1 ratio shaken with synthetic solutions of metals at different concentrations of 5, 10, 15 and vi 20 ppm and it was observed that maximum removal was achieved with the optimised dose of FA/SS mixture in 5 ppm concentration with 100% removal efficiency. Experiments carried out with different doses of FA/SS mixture revealed that the leachate with higher dose of fly ash contained lesser microbial count. These results were supported by the eco-toxicity studies carried out separately. Eco-toxicity studies were carried out on 2 fish species zebra and guppy fish with three different age groups 60, 90 and 120 day old fish. The eco-toxicity studies of fish was analyzed by checking their survival rate after 96 h time period in optimised FA/SS mixture, different sewage sludge samples, and control sample. Ten fish per chamber of 5 L capacity were added in control sample (no FA/SS mixture), optimised FA/SS (4:1) mixture, and in four different concentrations (5, 10, 15 and 20%) of sewage sludge. The survival rate was checked after 96 hour time period. It was observed that no death of fish was observed in the control sample and the optimised fly ash-sewage sludge (4:1) mixture. As the sewage sludge concentration increased from 5% to 20%, the metal content found in the fish sample (on acid digestion of the dried fish sample) also increased and death of all the fish was observed in samples with 20% weight by volume of sewage sludge. So, it is clear from the experimental data that FA/SS 4:1 ratio is the best dose for arresting the leaching of heavy metal ions in to the aqueous medium. ***