Abstract: The long-distance water conveyance system was simulated by a ductile iron pipe circulation device to study the changes of the main water quality indexes of the high-quality surface water source Wanlv Lake raw water after normal and separately contaminated by algae, NH₄⁺-N and NO₂^- -N. The test results showed that when the raw water was not contaminated, the DO showed a downward trend, and NH₄⁺-N, NO₂^- -N and turbidity were all maintained within the limits of the Surface Water Standard of Class I requirements; when subjected to algae pollution, the DO concentration decreased and turbidity, NH₄⁺-N and NO₂^- -N increased along the course. When being polluted by NH₄⁺-N and NO₂^- -N, due to biodegradation of the biofilm of the pipeline, it showed great purification ability on NH₄⁺-N and NO₂^- -N, with removal efficiency respectively being 61.79% and 68.05%. In summary, the Wanlv Lake suffered from poor water quality stability after being contaminated by algae, NH₄⁺-N and NO₂^- -N, and all have decreased DO and residual chlorine concentration and obvious accumulation of NO₂^- -N.

Key words: stability of water quality, sudden pollution, surface water of class I, long-distance conveyance