Filtration characteristics in dead-end ultrafiltration of wwtp-effluent

Abstract

With a growing need for water for many purposes on one side and an increasing degree of wastewater treatment producing water with an improved quality on the other side, the necessity of water reuse will grow, especially in highly industrialised countries with restricted natural water sources. Improvement of advanced treatment schemes for effluent will promote water reclamation and conservation in the near future. Membrane filtration will play an important role in such treatment schemes. The research presented in this dissertation focused on the application of ultrafiltration membranes for advanced treatment of wastewater treatment plant (wwtp) effluent. The filtration characteristics of wwtp-effluent are unknown and the mechanisms behind the decreasing fluxes are hardly understood. The objective of research was to determine the filtration characteristics of wwtp-effluent in dead-end ultrafiltration. The research was performed both on pilot-scale and lab-scale. Pilot-scale tests were done at four different wwtps for at least 6 months of research each. Two different techniques for pre-treatment of the wwtp-effluent were investigated, i.e. multi-media filtration and in-line coagulation (⤠2 mg Al3+/Fe3+ per liter). Although the optimal process conditions were different at the various wwtps, the permeate was in all tests free of particles and bacteria. The fluxes were found between 60 and > 100 l/m2h at a TMP of 0.3-0.6 bar. The variations in effluent composition at different wwtp's urge the need for extensive pilot-plant studies to find the optimal process configuration and pre-treatment technique. These tests would be less time-consuming when proper parameters exist, which can describe the filterability of the wwtp-effluent. Such experiments were done on pilot-scale, in which well-defined experiments were performed for the determination of the filterability of the wwtp-effluent and on the reversibility of the fouling layer. These experiments showed that pre-treatment of wwtp-effluent with multi-media filtration or coagulation had a minor influence on the filterability, indicating that filtration characteristics are determined by particles smaller than 5-10 μm. The reversibility of the fouling layer was improved by pre-treatment. Nevertheless, these experiments were time consuming and underlined the need for quick tests for the determination of the filtration characteristics. The Specific Ultrafiltration Resistance (SUR) is proposed as such a parameter. The SUR was developed in the current research and is determined in a lab-scale ultrafiltration set-up. Within 30 minutes a filtration curves is measured at a TMP of 0.5 bar, using a membrane module with a capillary membrane. The filterability of effluent was determined and the range for the SUR was for wwtp-effluent 5â¢1012 to 30â¢1012 m-2. Low SUR values relate to high filterable water, whereas high SUR values relate to less filterable water. The TMP affected the SUR, which was caused by compression of the fouling layer (compression coefficient is 0.61-0.75). The influence of the particle diameter from effluent constituents on the filterability was determined by fractionation of the effluent. Effluent constituents that passed a 0.2 μm filter, but were retained by a 0.1 μm filter determined the filterability for 39% to 57%. Pre-treatment of the effluent had hardly any influence on this fraction, which is five to twenty times larger than the pore diameter of the applied membrane. This shows that the formation of a fouling layer on top of the membrane is the main filtration mechanism. This was confirmed by analysis of the filtration data using the filtration laws. Because the filtration laws could not completely cover the filtration data, it is hypothesised that the occurring filtration mechanism is the formation of a fouling layer and the retention of smaller particles within this layer, thus responsible for an increased resistance of the fouling layer. The results of pilot-plant tests were combined with the results of SUR measurements. Pre-treatment and membrane cleaning of effluent with a low filterability showed only partly improvement of ultrafiltration performance. Effluent with a high filterability (i.e. low SUR) is a requirement for highly reversible fouling layers that will result in stable ultrafiltration performance with fluxes as high as 100 l/m2â¢h. The optimisation of the ultrafiltration process for treatment of wwtp-effluent should therefore include the influencing of the properties or removal of the 0.1-0.2 μm fraction.