3.1 Moisture content
Moisture content for the three samples ranged between 25 to 31% and is represented in the Table 1. Moisture content was high in sample S3 (30.9 ± 0.6) and lowest in the sample S1 (26.4 ± 0.7). It was observed that moisture content increased with age and depth of the samples. The sample collection was conducted during the pre-monsoon season. The weather conditions were humid and hot with an average temperature ranging between 35 to 40 °C. Initial showers occurred for a short duration. The moisture content value is associated with amount of precipitation, percolation of water in waste piles, water holding capacity, and degradation activity of the organic waste [7]. A two-way ANOVA test indicated that there is no significant difference (p > 0.05) in the moisture content obtained from the different age and depth samples.
For incineration, moisture content of the feedstock should be less than 45% [40]. In India, the average moisture content was ranging from 31 to 65%. The percentage varies due the composition, seasonal variations, and climatic conditions. During the incineration plant design for mixed waste, secondary pit has to be designed to drain leachate under gravity. The leachate has to be collected and treated for disposal or application on land. Comingling of wastes with different moisture content is an alternative approach for reducing moisture in waste. For the aged waste, the combustible components (plastic and textile) are contaminated with soil-type material. Pre-heating followed by cleaning systems should be designed to remove moisture content and soil-type material. The percentage of moisture content is one of the critical parameters in the design of drying unit.
3.2 Physical composition of the waste
The mean percentage of waste composition (by dry weight) for the samples collected from the unlined landfill in the study area is represented in Table 2. It was observed that the soil-like material trends increase with the age of the sample. Drain silt and street sweeping waste are mixed with the waste leading to an increase in soil waste levels. High amount of ash is observed as the ragpickers at the dumping site burning the heaps of waste to recover metal and ferrous material. The plastic waste component of the waste also increases with the age of the waste. It was probably due to increase in the usage of the plastic products after 2000 [41]. The annual per capita plastic product consumption in India is estimated as 11 kg [42]. Plastic is one of the major components that can be harvested from a landfill. The plastic component mainly includes covers, wrappers and 3D films. Water bottles and recoverable plastics are being picked up by the ragpickers for recycling.
Glass, metal and ceramics are being collected by the ragpickers. The organic waste, paper, yard waste and other biodegradable wastes decompose under anaerobic conditions leading to the release of methane gas. In the aged samples 95% of the biodegradable component is decomposed. Table 2 also presents a comparative study on physical classification of the aged waste recovered from landfill. Two-way ANOVA showed a significant difference (p < 0.05) in soil-type material, plastics and textile among all waste samples. No significant difference (p > 0.05) between S1 and S2 was observed for glass, metal, and ceramics.
Composition of landfill waste varies based on geographic location, socio-economic conditions, dietary habits, seasonal variations, recycling rates, and informal sector activity. Designing a system based on the average composition values will reduce the efficiency of the plant. Physical composition of the landfill waste plays a pivotal role in designing an incineration unit to meet the needs of local conditions.
3.3 Proximate analysis
The proximate analysis results of the RDF samples are presented in Table 3. RDF sample (S1) had high volatile solids of 58.8 ± (1.3) % dry weight and low ash content of 11.6 ± (0.3) % dry weight and fixed carbon content of 3.2 ± (0.7)% dry weight as compared to other RDF samples. In the RDF samples the volatile matter ranged between 43% and 58% on a dry weight basis. The decrease in volatile matter from RDF Sample S1 to S3 suggests that the organic matter decreases with the age of the waste. Central pollution control board compiled a report on selection criteria for waste processing technologies [40]. For incineration, the volatile matter should be greater than 45%. In RDF samples S1 and S2 the volatile matter is higher than 45% while in S3 sample the volatile matter is 2.5 to 4.0% less compared to the desired value. The ash content of the RDF samples ranged between 11 and 18% on a dry weight basis. The permissible range of ash content to achieve high efficiencies in mass burning incinerators recommended by US EPA is 5-15% (dry basis). In RDF samples S1 and S2 the ash content is within the range while in S3 sample the ash content is 13-18% higher than the maximum permissible value. The fixed carbon ranged between 3 and 9% on a dry weight basis. The high percentage of fixed carbon indicates longer retention times for combustion in incinerator [44].
The volatile solids content of normal plastic (not landfill recovered) is 98.5%, the ash content is 1.2% and the fixed carbon less than 0.1% [7]. In the present study, the representative samples of RDF were prepared without pretreatment for landfill mined plastic and textile components. The impurities attached to the surface are not completely removed. The inert material and impurities from the landfill recovered waste components can be reduced trough a suitable pre-cleaning method during the RDF sample preparation. This will increase the volatile solids, heating value and reduces the ash content of the RDF. Table 3 presents the results obtained and comparative analysis of proximate analysis. The results obtained in the present study are within the range obtained from other similar researches [7, 44].
3.4 Ultimate analysis
The ultimate analysis results of the RDF samples are presented in Table 4. RDF sample (S1) has a high percentage of carbon 71.7 ± (13.7) % dry weight as compared to other RDF samples due to the amount of plastic content. No trend was observed for the plastic waste with age of the samples. The amount of plastic content in the waste depends on the consumption rates, disposal practices and recycling systems in the urban cities. Site specific characterization studies provide in depth understanding on the plastic waste component across the globe. Compared to normal plastic waste, recovered plastic waste has lower carbon content attributed to the presence of impurities like soil, sand that are difficult to be cleaned.
In RDF samples S2 and S3 the percentage of sulphur was 24.6 ± (3.0) % and 33.0 ± (10.5) % on a dry weight basis. The values obtained in this study are representing a high value of sulphur content compared to the other studies in literature. The high amount of sulphur is attributed to the formation of oxides of sulphur, hydrogen sulphide and volatile malodorous organic compounds with the aerobic and anaerobic decomposition of the waste. During ageing process in landfill, organic material with low molecular weight are degraded and resistant material are converted to humus-like matter. In this, transformation process the redox buffer changes from reducing conditions to slightly oxidizing conditions creating a favorable condition for sulphide to dissolve. The leaching experiments showed that the sulphur content is high in aerated landfill compared to original landfill indicating dissolution of sulphide in landfill material is a slow process. Binding of sulphide with solid landfill material due to deposited sulphur has been reported [45].
The percentage of oxygen in all the samples ranged from 9 to 12%. The percentage of hydrogen and nitrogen was low. The percentage of hydrogen decreased with the age of the waste and no trend was observed in terms of nitrogen and oxygen content.
3.5 Calorific value
The calorific value results of the RDF samples are presented in the Table 5. The High Heating Value (HHV) on a dry basis was found to be the highest for the RDF sample (S1) or 20.5 ± (2.9) MJ kg-1 and lowest for the RDF sample (S3) of 11.1 ± (0.7). The variations were due to the physical composition of the samples. Using Dulong’s equation and elemental composition, HHV (theoretical) was determined for the RDF samples. The HHV (experimental) and HHV (theoretical) for the RDF sample (S3) were found to be matching. However, for the other two RDF samples a percentage variation of 5-15% was observed. As per SWM rules 2016, RDF samples prepared from solid waste are recommended to be utilized as fuel in incineration units, if the calorific value is greater than 6.3 MJ kg-1. In the present study, the calorific value of all the three RDF samples prepared from the waste recovered from landfill can be used as a feedstock in mass burn incineration plant. Table 5 presents the results obtained and comparative analysis of energy content. The results obtained in the present study are within the range obtained from similar researches [7, 40]. The calorific value of landfill recovered waste depends on the composition of the waste and percentage of impurities. In the present study, segregation of the waste in laboratory contributed to high calorific value. However, pre-cleaning was not performed to simulate current field practices. The average calorific value of the plastic material is 43.6 MJ kg-1 [7]. The calorific values obtained in this study are low compared to the normal plastics. Segregation and pre-cleaning of recovered waste will increase the calorific value of the RDF. Development of pre-treatment systems involves high capital and operation costs and require skilled personal for operation and maintenance [46].
The recovered landfill waste can be utilized to develop RDF or directly incinerated to harness thermal energy using the advancements in current technologies. RDF can mix with sawdust, rice husk, plastic waste, and other combustible components in certain ratio to increase the calorific value of feedstock. According to previous researchers, thermal treatment options are economical compared to chemical recycling methods [7]. Life cycle assessment studies should be conducted on designed treatment systems to develop environmentally sound solution [47, 48].
3.6 Heavy metal analysis in ash
The amount of copper, lead and cadmium in the water extract of ash samples are shown in Table 6. The ash generated is disposed into the landfill located 4 km from the waste to energy unit. The landfill is single liner facility, precautionary measures to identify leaching of heavy needs to be installed to protect soil and ground water from contamination. Assessment of heavy metals in ash and volatile gases plays a pivotal role in determining the application of RDF as source of fuel in industrial applications. The presence of heavy metals in RDF contributes to variations in mechanical properties and reactivity characteristics. The major sources of heavy metals include leather, power wires, metal cans and scrap material [49].