No abstract

A cleaning protocol that effectively removes fouling from hollow fiber UF systems without excessive use of chemicals, product water or (long) down time is needed. Cross flushing with UF feed water has been reported to increase the net flux of hollow fiber systems by reducing the frequency of backwashing, the consumption of permeate and the system down time. In this study, the flux restoration achieved in a vertical and horizontal UF system employing an intermittent water and water/air cross flush were compared. The flux restoration in the vertical UF system was not improved by the addition of air to the water flush and a maximum flux restoration of 82% was achieved, irrespective of the presence of air. Similarly, in a horizontal ultrafiltration system, a maximum flux restoration of 82% was also achieved with a water flush (v = 1.63 m/s). However, the addition of air to the water flush decreased the flux restoration to 40% at the highest water/air ratio (33% air). Low flux restoration in the horizontal system was attributed to residual air in the module after cross flushing. Flushing with water alone (v = 1.63 m/s) yielded a wall shear stress of 16 Pa compared with 130 Pa and 279 Pa in the liquid film surrounding the air slugs in the horizontal and vertical UF system, respectively, with a water/air ratio of 2:1. Despite the high shear force on the cake layer accumulated when air was added to the system, the maximum flux restoration was 82% both with and without air. This was attributed to the fact that it was the filtration mechanism and not the shear force on the cake layer that limited flux restoration during cross flushing. To improve the flux restoration that can be achieved by the cross flushing process, the filtration mechanism must be manipulated to minimize blocking filtration and induce cake filtration from the beginning of each filtration cycle.

Filtration mechanisms occurring during ultrafiltration of coagulated and non-coagulated surface water were investigated using the general blocking filtration laws. The sharp decline in permeate flux in the first 10-20 minutes of filtration was largely due to a combination of the blocking filtration mechanisms. However, a distinction between “complete”, “standard” and “intermediate blocking” mechanisms was not possible as a large degree of overlap existed between the mechanisms and the transition from one type of blocking to another was very smooth. Coagulation appeared to retard blocking and consequently, the duration of the “blocking phase” was twice as long for coagulated water compared with non-coagulated water. The “transition phase”, where both blocking and cake filtration occurred simultaneously, was also extended in the case of coagulated water as continuous blocking of the membrane through the pores of the cake was observed. The modified fouling index-ultrafiltration (MFI-UF) was employed to examine the impact of coagulation on filter cake properties. Coagulation reduced the specific resistance of the filter cake by 50% at a TMP of 1.5 bar, and 35% at a TMP of 0.5 bar. Depth filtration was hardly evident at a TMP of 0.5 bar, but was very pronounced at a TMP of 1.5 bar as the specific resistance of the filter cake increased continuously, particularly in the absence of coagulant. The filter cake was sensitive to the applied TMP and the MFI-UF value increased by 130% for non-coagulated surface water and 55% for coagulated water due to compression of the cake, when the TMP increased from 0.5 to 1.5 bar.

No abstract

Submerged ultrafiltration membranes for use in wastewater treatment have been practiced for several decades and is becoming the standard for water reuse applications. In this paper, the characteristics of the unique and robust Integrated Permeate Channel (IPC ) membrane are presented for membrane bioreactor (MBR) applications. The design choices made during the 10-year development of the IPC membrane translate in key product attributes that the end users are looking for: a high capacity of clean permeate per footprint at the lowest cost of total ownership over the lifetime of the product.