由于缺乏丝状微生物，活性污泥B反应器完全依靠微生物胞外聚合物，以保持其完整性。可能在实验过程中，受到较高浓度的Cr（VI）影响的细胞外聚合物后，会导致絮凝物的分离。

图.3在实验过程中SVI值在活性污泥处理厂A（对照组）和B（实验）的比较。

图.4实验期间在活性污泥处理厂A（对照组）和B（实验）的出口悬浮物浓度。

3.5对活性污泥中微生物群落的影响

在实验开始时，两个活性污泥系统的微生物是相似的，含有柄纤毛虫，自由游动纤毛虫和轮虫几个物种。和预期一样，使用模拟底物使微生物变异性逐渐减少。整个实验中，控制系统中的污泥中包括原生动物（钟虫属，盖虫属）和轮虫。另一方面，添加1 mg L-1的Cr（VI）导致轮虫的逐渐消失和的自由游动的纤毛虫在实验系统中的增加（B反应器）。抗絮凝阶段开始于添加3 mg L-1的Cr（VI）时，至5 mg L-1的Cr（VI）时，浓度进一步的增加导致活性污泥原虫的完全冲出。

有报道称，在各种重金属的存在时，原生动物物种在减少[38,39]。原生生物的重要性和作用在活性污泥的净化过程中是有据可查的[29]。这些微生物从废水中除去非絮凝细菌并通过产生粘液[17]而有助于生物絮凝。

关于Cr（VI）对活性污泥工艺中高等微生物的的影响，应该提及的是，活性污泥系统接收模拟底物，可能不会具有较强的恢复能力。尽管如此，上述观察揭示了在Cr（VI）存在的情况，不同微生物的敏感性。

4. 结论

本研究的结果表明，硝化微生物比异养微生物对Cr（VI）更敏感。即使是0.5 mg L-1的Cr（VI）便会显著抑制硝化过程，而只有浓度高达5 mg L-1的Cr（VI）引起CODdis去除略有下降。Cr（VI）的浓度为1 mg L-1引起了活性污泥中轮虫的消失，原生动物也受到影响。

研究的Cr（VI）对活性污泥絮凝物的尺寸和结构的结果表明，Cr（VI）会影响丝状微生物的丰度。Cr（VI）停止投加后，硝化作用会部分恢复。最后，在持续48h的5 mg L-1 Cr（VI）的冲击负荷下运行，以研究其对硝化作用的影响，有机物的去除和污泥沉降并没有受到明显抑制。

5. 致谢

Abstract：The effect of hexavalent chromium, Cr(VI), addition on various operating parameters of activated sludge process was evaluated. To accomplish this, two parallel lab-scale continuous-flow activated sludge plants were operated. One was used as a control plant, while the other received Cr(VI) concentrations equal to 0.5, 1, 3 and 5mgl-1. Cr(VI) concentrations of 0.5mgl-1 caused significant inhibition of the nitrification process (up to 74% decrease in ammonia removal efficiency). On the contrary, the effect of Cr(VI) on organic substrate removal was minor for concentrations up to 5mgl-1, indicating that heterotrophic microorganisms are less sensitive to Cr(VI) than nitrifiers. Activated sludge floc size and structure characterization showed that Cr(VI) concentrations higher than 1mgl-1 reduced the filaments abundance, causing the appearance of pin-point flocs and free-dispersed bacteria. Additionally, the variability of protozoa and rotifers was reduced. As a result of disperse growth, effluent quality deteriorated, since significant amounts of suspended solids escaped with the effluent. Termination of Cr(VI) addition led to a partial recovery of the nitrification process (up to 57% recovery). Similar recovery signs were not observed for activated sludge floc size and structure. Finally, shock loading to the control plant with 5mgl-1 Cr(VI) for 2 days resulted in a significant inhibition of the nitrification process and a reduction in filamentous microorganisms abundance. r 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Toxicity; Wastewater; Nitrification; COD removal; Sludge settling

1. Introduction