Membrane fouling of raw coking wastewater in membrane distillation: Identification of fouling potential of hydrophilic and hydrophobic components

The flux decline of raw coking wastewater in membrane distillation (MD) is not serious as expected, yet little is known for such phenomenon. This study provides a systematic analysis of the fouling potential of hydrophilic substances (HIS) and hydrophobic fractions (HOA, HOB, HON) extracted from real coking wastewater. The results showed that HIS had a similar flux decline as compared to raw wastewater, whilst HOA and HOB demonstrated an obviously higher initial flux but a higher fouling rate as well. SEM, AFM and XPS analysis proved that a dense layer was formed for HIS, similar as for raw coking wastewater. However, the pores of HOA, HOB and HON fouled membranes remained open and little inorganic elements were presented. Surface charge results confirmed that hydrophobic fractions were negatively charged but the hydrophilic fraction was nearly neutral. Considering the negatively charged membrane surface, HIS was more readily to form a dense layer. With such a hydrophilic protective layer, the heat transfer resistance increased suddenly, leading to a lower initial flux but a lower fouling rate at the same time. This study provides a theoretical fundamental for understanding membrane fouling of high-strength organic wastewater, which is helpful for further development of MD technology. • The fouling potential of hydrophilic and hydrophobic components was investigated. • HIS fouling pattern is similar to raw-water, but HOA, HOB and HON are different. • HIS is prone to form a dense layer due to lower electrostatic repulsion with film. • HOA/HOB have higher initial flux but tend to cause pore narrowing via absorption. • A hydrophilic layer is preferentially formed on membrane for coking wastewater.