TREATMENT AND NUTRIENT RECOVERY OF LANDFILL LEACHATE BY ALGAE AND DUCKWEED

Abstract

Currently, sustainable management of leachate produced from open waste dump sites, is one of the biggest concerns in developing countries and Pakistan is no exception. Aquatic plants and algae having potential to remove pollution and uptake nutrients from wastewater can be cost-effective and technically-feasible options for leachate treatment. Based on this fact, the overall objective of present study was to identify the optimum operational parameters (leachate concentration, pH, electrical conductivity (EC), plant density and heavy metal concentration in leachate) for algae-duckweed based leachate treatment system under climatic conditions of Islamabad, keeping in view the sustainable post usage of duckweed plants. For this purpose, series of six experiments were conducted by growing duckweed (Lemna minor) and mixed algae (comprising of three genera) on leachate. Out of this, five experiments were conducted in open environment using dumpsite leachate while, last experiment was performed under controlled conditions using synthetic leachate so as to compare the results of natural and artificial systems. Results indicated that 30 % initial leachate concentration (chemical oxygen demand (COD): about 1,700 mg L-1), pH 7.1, EC 1,000 μScm-1, initial duckweed density of 50% and harvesting frequency of about 2.5 days are optimum for COD reduction, nutrient removal & uptake, and growth of duckweed on leachate. Under these conditions, duckweed was able to reduce COD by 61-67% from leachate, which corresponded to removal rates of total Kjeldahl nitrogen at 152-187mg m-2 d-1) and total phosphorous at (90-109 mg m-2 d-1). The growth rates of duckweed were 5.5-6.8 g m-2 d-1 under optimum conditions. Mixed algae showed maximum biosorption capacity of 5.09, 5.85, 7.03, 3.34 and 5.73 mg g-1 for Fe, Cu, Pb, Cr and Zn respectively at 10 mg L-1 initial metal concentration in leachate with algal dose of 0.8 g L-1. A comparison of experiments on dumpsite and synthetic leachate revealed that duckweed decreased COD and nutrients more efficiently from dumpsite leachate under natural climatic conditions compared to grown on synthetic leachate under similar environmental conditions. However; the amount of N and P taken up by duckweed was about 14-18% and 34-36% more from synthetic leachate compared to dumpsite leachate. Duckweed growth rate (5.5 to 6.3g m-2 d-1) was also observed high at synthetic leachate. Results of this research provide a basis to establish an algae-duckweed based leachate treatment system by presenting the optimum working conditions for such system that can be cost effective and feasible option even the landfills are put in place in near future.