Experiments on the efficiency, mechanism and hydraulic flow pattern of wastewater purification, especially the removal of N and P, in water peanut (Alternathera Philoxeroides) ponds are introduced in this paper. The results obtained show that water peanut pond advances over the symbiotic bacteria/algae systems pond and duckweed pond in removing COD, BOD5, TN, NH3–N, TP and phosphate salts at higher efficiencies. In comparison with water hyacinths (Eichhornia crassipes) pond, water peanut pond has higher removal efficiencies in removing BOD5, COD, TN and NH3–N and lower efficiencies in removing TP and PO43–.

The mechanism of the high pollutant removal efficiencies in the water peanut pond, which is proved through the experiments, is as follows: dominant bacteria such as Bacillus, Pseecomonas and Alcaligenes adhering to the root system of the plants help to achieve the efficient degradation of C, N and P containing matters into CO2, NH4–N and PO43−–P, which are then taken up by the roots for the growth and generation of the plants; this is the main mechanism of N and P removal from the wastewater. Due to the smaller number of nitrifying bacteria and algae existing in the pond, there will be no further concern of pH increase in the pond, hence the contribution of nitrification, denitrification, NH3release and settling of hydroxylopatite caused by the reaction of phosphate and calcium in N and P removal was negligible.

It was also determined through the experiments that the flow pattern in the rectangular water peanut pond was close to plug flow.

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