Bioprocess technology for coal washery effluent treatment by Lemna minor

Duckweed species are promising macrophytes that can be used in wastewater treatment due to their rapid growth, ease of harvest, low fiber feed potential, and high protein contents. Waste‐water contaminants that are likely to be generated during washing of coal are total suspended solids (TSS), chemical oxygen demand (COD), acidity or alkalinity (pH), and metallic contaminants. Bioprocesses were developed to evaluate the potential of duckweed (Lemna minor) to treat coal washery effluent (CWE) as well as to study the impact on the biochemical changes of the Lemna minor. CWE samples were diluted with distilled water (DW) in different ratios as follows: T1‐CWE:DW(20:80%), T2‐CWE:DW(40:60%), T3‐CWE:DW(60:40%), T4‐CWE:DW (80:20%), T5‐CWE (100%), and Control‐DW(100%). The electrical conductivity of the effluent treated with Lemna minor was 0.035 deciSiemens per meter (dS/m) in the control at Day 10 and substantially higher at treatment ratios of 100, 80, and 60 percent (1.754, 1.842, and 1.631 dS/m). The highest amount of TSS was observed at test ratio T5 (38,834 mg/L), followed by T4 (28,816 mg/L), T3 (26,970 mg/L), T2 (15,320 mg/L), T1 (4,524 mg/L), and control (424 mg/L). Total hardness was higher (820 mg/L) in T4 compared to the control (220 mg/L). Total hardness of the effluent decreased after 30 days of incubation in all the treatment aliquots. The CWE at 20 percent concentration increased the duckweed population and no adverse impacts on its growth were observed. At higher concentration (T5) total mortality of Lemna minor was observed. The chlorophyll production was determined to be inversely proportional to the effluent concentration. Based on this study, it is concluded that Lemna minor can be used for treating CWE after dilution with fresh water.