Physicochemical Assessment of Leachate and Biogas Generation Potential in Landfill Sites of Nepal

Abstract

Leachate analysis is one of the vital aspects of landfill operation and management. However very limited studies are found conducted in Nepal in this regards. The fact that the existing literature has not examined major concern like physicochemical constituents which have caused major problems in making leachate treatment plants operational – all the plants are non-functional – speaks out why further study is required in Nepal. Understanding of the constituents in terms of their age, functionality, waste characteristics and seasons will generate data which in turn will prove useful to construct treatment plants based on the type of MSW site. This study is focused on qualitative characteristics of leachate and its variation due to age, functionality, seasons and feeding amount. Similarly the study of biogas generated from MSW site is significant from methodological perspective. Existing studies in Nepal have deployed ultimate analysis. The present study uses proximate analysis for estimating potential biogas generation. The objective of this research was to study the qualitative aspects of landfill leachate and to estimate potential biogas and energy generation from landfill sites. Leachate parameters were quantified in relation to seasons, waste composition, functionality and ages of landfill sites (LFS). The studied landfills were Sisdole (operational and intermediate age), Aletar (closed and young age), Pokhara (operational and intermediate age), Dang (operational and intermediate age) and Gokarna (closed and mature age) landfill sites of Nepal. Gas estimates were calculated for three operational landfills namely Sisdole, Dang and Pokhara. Leachate and waste samples were collected at 2 months interval for a year. Samples were collected on March, May, July, September, November of 2013 and January of 2014 from all studied sites. 1 litre leachate sample from the leachate collection pond and four trenches at each landfill were collected on the standard sampling bottles each time. Waste sample of 100 kg from 3 vehicles at Dang, 5 at Pokhara and 7 at Sisdole LFS was taken at each sampling time. Composition study of the waste was done using the waste reduction method on site. The wastes were segregated as organic waste, plastics, paper, glass, rubber/leather, textile, metal, construction and demolition waste and others. The fractional minute particles remained after the compositions were sampled for proximate analysis. Proximate analysis of the solid waste was done for moisture content and volatile solids percentage. Total Nitrogen was measured using Kjeldahl Digestion method. Organic matter was estimated using Walkley and Black method in which the organic matter / carbon in the sample was determined by wet oxidation method. C:N ratio was also calculated. Through moisture content and volatile solids biogas vii generation potential was calculated using MATLAB software. pH, DO and temperature were measured in field. 5ml conc.HNO3 was used for the preservation of metals and Chloride was used to preserve COD of the sample. Leachate samples were collected from the same locations at every sampling time. BOD, COD, BOD/COD ratio, Fe, Cu, Pb, Zn, Cd, Cr, Hg, Ni, Cl, Ca, NH3 and H2S were analyzed according to APHA - 2012. Data were assembled into four seasons namely pre-monsoon (March, April, May), monsoon (June, July August), post-monsoon (September, October, November) and winter seasons (December, January, February) of Nepal using Principle Component Analysis (PCA). A One way ANOVA tool was used in SPSS to analysis the data obtained from lab analysis. The results were presented in terms of age, functionality and seasons. The organic component of wastes was found high as 61.6%, 52.5% and 65% at Sisdole, Pokhara and Dang LFS respectively. The pH value ranged from 6.5 to 8.7 in studied sites. Highest pH was found in Gokarna and lowest at Pokhara. The BOD and COD value ranged from 85.2 mg/L to 1046 mg/L and 969.4 mg/L to 9153.2 mg/L respectively in studied sites. Highest BOD was found in Dang and lowest at Gokarna. Highest COD was found in Pokhara and lowest in Gokarna. The concentration of iron ranged from 0.96 – 5.28 mg/L in studied sites. Highest concentration was found in Sisdole and lowest in Gokarna. The concentration of Cu, Pb, Ni and Zn were ranged from 0 – 0.5 mg/L in all studied sites in all seasons whereas concentration of Hg, Cr and Cd were almost not detectable. The concentration of Ca ranged from 94 – 454.9 mg/L. The yearly average amount of NH3 were found to be 69.035 mg/L, 99 mg/L, 108.08 mg/L, 130 mg/L and 23.75 mg/L in Aletar, Dang, Pokhara, Sisdole and Gokarna respectively. Highest concentration was found in Pokhara and lowest in Gokarna. The yearly average amount of H2S ranged from 18 - 105.25 mg/L. Highest concentration was found in Sisdole and lowest in Gokarna. Seasonally, pH was significantly difference in all sites except Gokarna which is closed and mature LFS. Gradual increase of pH from pre-monsoon to winter seasons and as per age was observed. BOD and COD significantly decrease from pre-monsoon to winter seasons in all except in Gokarna. It could also be due to increased age of the landfill too. Concentration of BOD and COD was higher where the amount of organic waste content was higher. BOD/COD ratio was 0.02 (mature) to 0.3 (young). Low ratio reflects the low biodegradability in landfills. Among metals and heavy metals, concentration of Fe decreases from pre-monsoon to winter. This could be due to increased age of landfill and increased pH as seasons changes. Others metals like Ni, Pb and Zn were detected in young and intermediate aged LFS. viii For old aged LFS, concentration of Fe, Cu, Cr, Hg, Ni, Pb, Cd and Zn were within the threshold values recommended by WHO (2012) guidelines. These parameters were leachout earlier as this landfill is running in stabilization phase. H2S and NH3 were high in intermediate than in young and mature aged LFS with no seasonal variation in all LFS. High H2S signifies higher anaerobic decomposition. High NH3 signifies the ammonification process in LFS. It can be concluded that age, seasonal variation, waste characteristics and feeding amount of the sanitary landfills had great effects on the quality of formed landfill leachate. Compared to the standard set by WHO (2012), all the physicochemical components except pH were observed to have significantly high value in young and intermediate aged LFS. In contrast to other researchers, the concentration of leachate in young aged but closed LFS behaves likes intermediate aged leachate. As the concentration of contaminants in the leachate is found to be high, fresh landfill leachate is recommended to use treatment prior to disposal. Characterization of the unprocessed leachate is the fundamental step leading to the selection of efficient treatment techniques. The present study found biological treatment methods to be effective for freshly produced leachate under the condition that the concentration of the metals suffices for the growth of microorganism. The results obtained have high concentration of the metals and metal ions as compared to the metals concentration threshold of inhibitory effect on heterotrophic organisms‘ growth. It can be concluded that due to the concentration of heavy metals in young, intermediate and operational landfill is not suitable for the biological treatment as primary treatment units. First municipality‘s authorities or plants operators need to bring the concentration of metals and accordingly operators can go for the biological treatment. Yearly average value of moisture content for Sisdole, Pokhara and Dang Landfill Site were 69.03%, 85%, 82% respectively. The moisture content is high in monsoon seasons in all LFS and these values were seasonally significantly difference (P < 0.05). The volatile solids ranged from 39.63% - 44.41% in studied sites. Dang has highest value of volatile solid and Pokhara has the lowest value. Average value of volatile solids for Sisdole, Pokhara and Dang LFS were 44.41%, 39.63%, 58.47% respectively. The difference was statistically insignificant (p > 0.05) in different seasons within the same LFS. Values of C:N ratio of Sisdole, Pokhara and Dang landfill site were 18.82, 19.03 and 19.62 respectively. The difference was statistically insignificant (p > 0.05). An equation was developed for biogas generation. Biogas generation in lit/kg/day can be expressed as; ((1-(α)/β) (γ)/β * Biogas Yield) Where α = moisture %, β = 100 (scale coefficient) and γ = volatile solids %. The biogas potential at these landfill sites were 12158 cum, 852 cum and 169 cum of biogas per day in Sisdole, Pokhara and dang LFS respectively.