环境水质学国家重点实验室知识库

        混凝超滤组合工艺以其 优异的性能逐渐应用 在 实际 饮用水处理 工程 中 但滤饼层引起的膜污染始终是 影响 该工艺的 瓶颈 。与混凝 沉淀 超滤工艺（长流程）相比， 往往认为 直接混凝 超滤（短流程）工艺 中 膜池内絮体粒径大、滤饼层疏松，膜污染程度更低。 然而， 核心 问题是短流程膜工艺中 滤饼层厚度、结构等 理化 性质 均 深刻影响着膜表面污染负荷 ，是否存在 临界沉淀时间 影响 混凝 超滤组合工艺的运行效能 。 基于此 以南水北调水（北京段）为原水， 系统研究 了 长 、短流程膜工艺 运行效能及调控机制。

       研究证实 混凝 超滤组合工艺 存在 “临界沉淀时间”，使得 长 流程 工艺运行效能优于短流程 （水温 20 30 o C ）。 以氯化铁为混凝剂 时 当沉淀时间小于 30 min长流程膜工艺运行 12 d 导致的 膜污染程度较 短流程膜工艺 严重，此时长流程膜工艺引起的跨膜压差（ TMP ）高达 19.2 kPa ，短流程膜工艺引起的跨膜压差为 7.05kPa 。 然而 随着沉淀时间增加，膜表面污染负荷降低， 当沉淀时间为 60 min 时，长流程膜工艺 导致的 膜污染程度优 于短流程膜工艺， 此时长流程膜工艺 TMP 降低至 5.8 kPa 。 与铁盐混凝剂相比， 铝盐混凝剂水解絮体疏松，引起的膜污染程度较低，但也存在 “临界沉淀时间”。 以氯化铝为混凝剂时， 运行 12 d 且沉淀时间分别为 0 、 10 、 30 、 60 min 时， TMP 分别增至 3.9、4.4、3.5、2.5、1.2 kPa。由于铁盐水解絮体密实且不均匀，膜表面滤饼层形貌及结构受沉淀时间影响较大，“临界沉淀时间”较铝盐混凝剂长。
       膜工艺运行过程中，水温随季节周期性变化。 原水 温度较低时（ 10 20 o C超滤膜孔收缩，致使到达膜表面污染物总量成为膜表面污染负荷的主要原因。沉淀时间越长，到达膜表面的污染物含量越少，膜表面污染负荷越低。因此，与长流程膜工艺相比，短流程膜工艺导致的膜污染程度始终 更严 重， 未 发现 “临界沉淀时间”。以氯化铁为混凝剂 且 沉淀时间 为 0 、 10 、 30 、 60 min 时，运行 12 d 后TMP 分别增至 13.5 、 11.5 、 10.0 、 7.2 kPa。以氯化铝为混凝剂， 水解絮体更疏松，膜污染程度较氯化铁时低， 沉淀时间 为 0 、 10 、 30 、 60 mi n 时，运行 12 d 后 TMP分别增至9.6 、 7.9 、 7. 7 、 7.6 kPa。
        混凝效能易受原水pH 影响， 进一步 研究 表明 水温较低时 1 0 20 o C 絮体粒径 和 z eta 电位 随原水 pH 变化 较大， 显著 影响混凝 超滤 组合工艺“临界沉淀时间” 。 以 氯化铁 为混凝剂且 pH 6 时，“临界沉淀时间” 为 10 min 运行 12 d 且沉淀时间 为 0 、 10 、 30 、 60 min 时， TMP 分别增至 17 .0 、 19.8 、 13.5 、 13.2 kPa 。而碱性 和 中性 条件下， 较大的絮体粒径 和 弱 静电吸附作用 使得膜表面污染负荷随沉淀时间降低，因而 长流程膜工艺 导致的膜污染程度 始终 低于 短流程膜工艺。 对铝盐混凝剂而言，无论酸性 、 中性 或碱性 条件 亦是长流程膜工艺 引起的 膜污染程度更低 。
       以上研究表明混凝 超滤组合工艺 运行过程中 膜表面污染负荷随沉淀时间发生变化 存在“临界沉淀时间”使得长流程膜工艺中膜污染程度低于短流程膜工艺膜污染程度，且 易 受混凝剂种类、水温、 pH 等 影响 。 该研究对饮用水厂膜工艺改造具有重要意义。

         The coagulation ultrafiltration combin ed process is gradually applied in a ctual drinking water treatment project with its excellent performance but the membrane fouling caused by the cake layer is always the bottleneck affecting the process.Compared with the coagulation sedimentation ultrafiltration process (long process), it is often considered that the direct coagulation ultrafiltration process (short causes lower membrane fouling owing to the large floc size in the membrane cell and the loose cake layer degree is lower. However, the core problem is that
p hysicochemical properties such as thickness and structure of cake layer in the short process profoundly affect m embrane s urface c on tamination load , and whether there is “critical settling time” to affect the operation efficiency of the coagulation ultrafiltration combined process . Based on this, the water from South to North (Beijing section) was used as the raw water, and the operating efficiency and regulation mechanism of the lo ng and short process were systematically studied.
        It was confirmed that there was a “critical settling time” in the coagulation ultrafiltration combined process, which ma de the operation efficiency of long process better than that of short process (water temperature: 20 30 o C). W hen the settling time was less than 30 min, the membrane fo uling degree caused by the long process after 12
days was more seriou s than that caused by the short process with f erric chloride. At this time, the transmembrane p ressure difference (TMP) caused by the long process reached 19.2 kPa, and the TMP caused by the short process was 7.05 kPa. However, as the settling time increase d , the membrane surface pollution load decrease d . When the settling time was 60 min, the membrane fouling caused by the long process was better than the short process, and the TMP o f the long process reduced to 5.8 kPa. Compared with f erric chloride , alumin um chloride can hydrolyze flocs loosely and cause lower membrane fouling, b ut there was also a “critical sedimentation time” The TMP increased to 3.9, 4.4, 3.5, 2.5, and 1.2 kPa after 12 d ays when the settling time was 0,10, 30, and 60 minutes with alumin um chloride . Due to the denser and non uniform flocs hydrolyzed by ferric salts, the morphology and structure of cake layer on the membrane surface were greatly affected by the settling time, so it was more difficult to reach the “critical settling time” than aluminum salt coagulant 。

      During the operation of the membrane process the water temperature changed periodically with the season. When the water temperature was low er (10 20 o C), the pore size of the ultrafiltration membrane was reduced, so that the total amount of pollutants reaching the membrane surface bec a me the main ca use of the membrane surface pollution load. The longer the settling time, the less the amount of contaminant reaching the surface of the membrane, and the lower the surface contamination load on the membrane. Therefore, compared with the long process, the degree of membrane fouling caused by the short process was always more serious, and there is no “ critical settling time” When ferric chloride was used as a coagulant and the settling time was 0, 10, 30, 60 mi n, the TMP increased to 13.5, 11.5, 10.0, and 7.2 kPa after 12 days of operat ion, while the TMP increased to 13.5, 8.8, 8.5, and 8.3 kPa with aluminum chloride b ecause the hydrolyzed flocs were looser.
        Coagulation efficiency is susceptible to raw water pH further research showed that the floc size and zeta potential were greatly affected by the influent pH water temperature 10 20 o C which in turn affect ed the “ critical settling time of the coagulation membrane combin ed proces s. When ferric chloride was used as acoagulant and the pH was 6, the critical settling time ” was 10 min. When the settling  time was 0, 10, 30, 60 min after 12 days, the TMP increased to 17.0, 19.8, 13.5, 13.2 kPa, respectively. Under alkaline and neutral conditions, the larger floc size and the smaller electrostatic adsorption ma de the fouling load on the membrane surface decrease with the settling time, so the fouling degree caused by long process was always lower than that of short process. For aluminum salt coagulants, the de gree of membrane fouling caused by long process was lower, regardless of acidic, neutral or alkaline conditions.
        The results show ed that the fouling load of membrane surface varie d with the settling time during the operation of coagulation ultrafiltration combined process. There was a “critical settling time”time”, which ma de the degree of membrane fouling in the long process lower than that of the sh ort process, and was affected by the variety of
co agulants, water temperature, pH. This research is of great significance for the membrane process modification of drinking water plants.