Application of powdered activated carbon in membrane filtration processes for advanced treatment of municipal wastewater

Anthropogenic microcontaminants such as microplastics, heavy metals, pharmaceuticals, and industrial chemicals are increasingly harming the aquatic environment and therefore appear more and more frequently in the scientific literature in the field of advanced wastewater treatment. Dissolved organic microcontaminants, generally referred to as micropollutants, are ubiquitous in wastewater and are not effectively addressed by existing conventional treatment processes. Their elimination requires additional treatment steps, which will also be legally required for large wastewater treatment plants in the near future. Nowadays, the hybrid process combining powdered activated carbon (PAC) with membrane filtration has emerged as a possible advanced wastewater treatment method for the elimination of micropollutants. In this study, fundamental research was conducted to gain a deeper understanding of the factors influencing the process. Batch studies using synthetic model water have shown that commercial PAC tends to form agglomerates with increasing PAC stock concentration (meaning the highly concentrated suspension of PAC dosed in the process to adjust the desired PAC dose). Since highly concentrated PAC stocks are used in practice, various measures were investigated to reduce agglomerate formation. Neither mechanical stirring, the use of ultrasound, adjusting the pH value or the ionic strength, nor the addition of inorganic or organic dispersants proved to be effective. The only effective measure to reduce agglomerates was to dilute the PAC stock. Bench-scale studies with flat sheet membranes and synthetic model water have shown that PAC agglomerates can negatively affect the hybrid process and slow down the adsorption kinetics. Lab-scale experiments with an inside-out ultrafiltration membrane as well as an outside-in microfiltration membrane showed an additional negative influence of PAC agglomerates caused by an unfavourable distribution of the PAC on the membrane surface, leading to an inefficient utilisation of the PAC adsorption capacity. Based on these findings and possible optimisation options, the hybrid process was finally scaled up to technical scale and tested in pilot studies with a real secondary effluent matrix. For both the inside-out and the outside-in hybrid process, the negative influence of PAC agglomerates on the adsorption process by an undesirable transport of PAC in combination with the inaccessibility of the PAC could be demonstrated. Due to the usually higher crossflow velocities in inside-out capillaries, this process is significantly more affected than the outside-in membrane process. Further investigations showed that a commonly practiced coagulant dosage upstream of the membrane stage additionally intensifies the unfavourable distribution of PAC in the membrane system and that the PAC-coagulation-membrane process requires the consideration of further constraints. In conclusion, the following recommendations can be made for users of the hybrid process: A key factor to reduce PAC agglomerates is the installation of an intermediate tank to pre-dilute the highly concentrated PAC stock before dosing. For a hybrid process that runs in a stable manner without the need for continuous coagulation, the process combination of floc precoating at the beginning of the filtration cycle followed by single dosing of PAC should be used. This can be advantageous due to the so-called “filter effect”, where the capacity of the activated carbon is better utilised compared to continuous dosing. If longer coagulation phases are required for fouling reduction, intermittent dosing with alternating PAC and coagulant dosing at three-minute intervals should be applied.