助凝剂投加量及pH对BF-UF工艺膜污染的影响

Effects of coagulant-aid dosage and solution pH on membrane fouling during ballasted flocculation and ultrafiltration process

以聚丙烯酰胺(PAM)为助凝剂,在加载絮凝(BF)与超滤(UF)组合工艺净水过程中,系统考察了PAM投加量和原水pH对膜通量衰减及污染可逆性等的影响,并结合加载絮体形态特征与膜污染的相关性,进一步探究了膜污染的生成机理。结果表明,PAM投加量对加载絮体形态、膜滤效能和膜污染的影响显著,且PAM投加量在不足或较大的情况下均会产生不利影响,应以0.3 mg·L-1为宜;在中性条件下的膜污染程度低于偏碱性条件下,而酸性条件下的膜污染程度最为严重,此时膜表面累积形成滤饼层与膜本身之间的静电作用主导膜污染;对于平均粒径大于0.75 mm、分形维数小于1.35的加载絮体,其粒径较大、结构较为松散时形成的滤饼层并不能有效缓解膜污染。从优化运行的角度来看,减少加载絮凝水样中溶解态污染物和未被捕获微砂颗粒的含量以及不含微砂的凝聚体的占比,成为延缓膜污染和稳定出水水质的关键。

During a combined water-purification process of ballasted flocculation(BF)and ultrafiltration(UF),the effects of polyacrylamide(PAM,as coagulant aid)dosage and raw-water pH on both ultrafiltration membrane flux decline and fouling reversibility were systematically investigated.Then,the generation mechanism of membrane fouling was explored according to relevance analysis of ballasted floc morphological characteristics and resultant fouling.The results showed that the PAM dosage had a significant effect on ballasted floc morphology,membrane filtration efficiency and fouling properties;and especially,negative influence would occur at PAM dosages both lower and higher than 0.3 mg·L-1.Membrane fouling occurring at neutral pH of raw water was lighter than that at alkaline pHs,while the heaviest membrane fouling occurred at all acidic pH values,possibly due to distinct electrostatic interactions between ultrafiltration membrane itself and cake layer formed on membrane surface.For ballasted flocs with average size larger than 0.75 mm and fractal dimension lower than 1.35,cake layer formed from these flocs with larger and more loosely-structure failed to effectively alleviate membrane fouling.From a perspective of optimal operation,it is key for weakening membrane fouling and producing stable purified-water quality to reduce concentrations of dissolved pollutant and non-captured micro-sand particle,as well as percentage of non-ballasted floc,in treated water by ballasted flocculation.