Efficiency of toxic substance removal from aquaculture wastewater by duckweed (Lemna minor) and Bacteria (Bacillus sp)

Bamidele Omitoyin


The steady growth of global aquaculture industry is dependent on the continued supply of fish meal. However, the decline in fish catch from the wild has led to intensification on the search of alternative sources of cheap plant proteins. The use of fish meal and plants proteins are, however, limited by their high amount of nitrogen and phytate phosphorus, respectively, which pollute aquatic ecosystems. Bioremediation is an effective approach to reduce hazardous substances in wastewater effluents that are detrimental to fish survival. The experiment was designed to evaluate the effectiveness of duckweed (Td), Bacillus sp (Tb) and clean water, as a negative control (Tc), in reducing nutrient load and improving water quality in aquaculture wastewater from catfish farming. Triplicate group of duckweeds (3.3kg m3) of average weight of 49.59g and 15ml of bacterial inoculum were randomly assigned to tanks (0.390m x 0.276m x 0.26m) with 15 litres of the wastewater. Quadruplicate samples of wastewater were analyzed for water quality parameters. After 2 weeks of bioremediation, biomass of duckweed increased by 97.51%. There was a significant reduction (P<0.05) in level of phosphate, mg/L; sulphate, mg/L; nitrate, mg/L; ammonia, mg/L; biological oxygen demand BOD, mg/L; chemical oxygen demand COD, mg/L; and total soluble solids TSS, mg/L by duckweed, with reduction efficiencies of 70.70%, 90.96%, 40.18%, 77.78%, 96.66%, 96.26% and 36.94%, respectively, compared to the positive control (initial) wastewater (P<0.05). Reduction efficiency (RE) was lower for Bacilli sp and negative control compared to duckweed (P<0.05). The pH level in the wastewater increased in all treatments, with the highest values recorded for duckweed. However, the highest dissolved oxygen level (DO) of 4.81±0.03mg/l was recorded in the bacteria, followed by duckweed (4.45±0.06mg/l) and 4.09±0.06mg/l for control. In conclusion, duckweed can be used in commercial aquaculture to reduce the high load of waste to improve growth and survival of fish, as well as, sustain aquatic environments.
Key words: Aquaculture, Wastewater, Bioremediation, Duckweeds, Bacillus Sp.

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