Abstract: Water quantity adjustment equipment was installed to balance the influent quantity of different paralleling treatment units in a WWTP of 400 000 m³/d to tackle the problem of poor treatment effect resulting from the uneven distribution of influent wastewater to the treatment units of the large-scale WWTP. Water level change and concentrations of NH₄⁺-N, NO₃^--N, NO₂^--N along the bio-reactor were compared before and after installing the water quantity adjustment equipment. Result shows that the influent water quantity of different paralleling treatment units, which is calculated from the water level, significantly differ. The maximum quantity of influent water is 1.72 times the minimum quantity of influent water. Uneven distribution of influent wastewater results in indistinct difference of biological treatment effect. The effluent NH₄⁺-N concentrations of the bio-reactor are 0.22 - 5.24 mg/L and 0.60 - 0.85 mg/L and NH₄⁺-N removal effieiencies are 69.7% -97% and 93.9% -95.4% , respectively, before and after installing the water quantity adjustion equipment. The water quantity is balanced and effluent quality is improved due to the equilibrium of water quantity through the adjustment equipment.