Study on quantity and quality of leachate of matuail sanitary landfill

Abstract

Municipal solid waste management, from 2005 Dhaka City Corporation has given emphasis on introducing sanitary landfill technology rather than open dumping. A master plan was formulated by Dhaka City Corporation, with the technical assistance of Japan International Cooperation Agency (JICA) in 2005 for a target year of 2015 to make Dhaka City Clean. Under the Landfill Improvement project, Dhaka City Corporation (DCC) transformed an open dump to in a sanitary landfill at Matuail; the construction period was March 2006 to October 2007. It is the first sanitary landfill site in Bangladesh. Semi-aerobic system of waste decomposition has been installed at the landfill site. Aerated lagoons have been also constructed to treat the leachate. To operate the leachate treatment system efficiently, it is essential to know about the variations in the quantity and quality of leachate throughout the year. As a result, a study was conducted to monitor the effectiveness of the installed semi-aerobic system on leachate quality and differentiate between semi-aerobic system and boundary system for both in dry and wet season. The estimation of quantity of leachate from rainfall and find out relationship between leachate flow rates with rainfall intensity were also important to analyze both for semi-aerobic and boundary system. For completing the study objectives, a rain gauge (18 in x 8 in) was placed at landfill site to record rainfall data and leachate production was estimated on monthly basis by using water balance method (Fenn et al. 1975). To obtain a relationship between leachate flow rate and rainfall intensity; leachate flow was also measured for a number of rainy days and dry period both in semi-aerobic and boundary system. Semi-aerobic system involves installation of a combined use of the perforated pipe network and gravel pack below the solid wastes layers for collection of leachate produced in the landfill as well as supply of oxygen into the deposited waste. Oxygen in air is led into the landfill through the leachate collection pipe by heat convection resulting from differences between the inner temperature and outside air temperature (Hanashima et al. 1981a). In boundary system depletion of oxygen supply biodegradable organic matter eventually is subjected to anaerobic breakdown. This anaerobic decomposition is biologically much the same as that in the anaerobic digestion of sewage sludge. Microbial organisms responsible for anaerobic decomposition include both facultative and obligate anaerobes. In the study total rainfall was found 1816 mm for wet season and maximum rainfall and intensity were 650 mm and 55.90 mm/hr in August and September, 2008 respectively. The higher leachate flow rate occurred in rainy months of June, July and August, 2008 and maximum value of total leachate flow rate was 26.90 m3/hr on 27th August, 2008 with rainfall intensity of 34.60 mm/hr. It was observed that in dry period collection efficiency of leachate was more than wet period at Matuail and the flow rates were within the range 3.2 – 5.0 m3/hr for dry period and 12.7- 19.49 m3/hr in rainy season. Two regression equations were obtained to correlate flow rate with rainfall intensity. A linear relationship was found both for semi-aerobic and boundary system. The equations were y = 0.245x + 3.214, r2 = 0.856 and y = 0.325x + 5.153, r2 = 0.784 for semi-aerobic and boundary system respectively. From regression analysis it was also seen that during dry period leachate flow occoured 3.20 m3/hr in semi-aerobic system and 5.0 m3/hr in boundary system which was approximately similar to flow rate found from field observations. Production of leachate was increased with the increasing rainfall but collection efficiency degraded in wet period. For the existing operating landfill area of 31.12 acres it was estimated that maximum leachate production was 70,819 m3 in August, 08 generated from waste and rainfall. In dry period contribution of leachate generation from waste was 4795 m3 and observed flow was 5.32m3/hr in November, 2008. Total excess leachate generated from rainfall was not collected through installed leachate pipe networks due to unmanaged cleaning operation during rainy days as well as number of leachate pipes might not sufficient to carry the excess generated leachate lead to contaminate soil and ground water. For the month of May, June, July, September, October, November, 2008 and January 09, the leachate productions were 19,400 m3, 24,634 m3, 52,130 m3, 20,218 m3, 24,314 m3 , 4795 m3and 4717 m3 respectively. For monitoring seasonal variations of leachate quality parameters, two types of leachate samples; one from semi-aerobic and another from boundary system were collected for laboratory tests. The characterization of leachate at Matuail has shown that the concentation of different parameter varies within a wide range. It was due to non homogeneity of wastes. It typically contains a wide range of dissolved organic and inorganic contaminants, including heavy metals. Seasonal variations of COD, BOD, EC, NH3-N, alkalinity, Cl- and heavy metals contents were less in semi-aerobic system than boundary system except pH. The ratios of BOD to COD of the collected samples were 0.51–0.72 and 0.16–0.40 in semi-aerobic system and 0.56–0.72 and 0.14–0.42 in boundary system for the rainy and dry seasons respectively. The collected effluent suitable to be treated only by biological treatment methods in rainy season and in dry period effluent needs to be treated by physical or chemical treatment. Semi-aerobic system is effective in improving leachate quality compared with boundary system.