Flux enhancement using flow destabilization in capillary membrane ultrafiltration

[摘要] ENGLISH ABSTRACT:The aim of the thesis was to investigate the use of flow destabilization methods, combined withpermeate backflushing (BIF) or on their own, on flux recovery and maintenance in capillary UFmembrane systems under cross-flow (XF) and dead-end (DE) operating conditions. Varioushydraulic and mechanical methods have been used to remove the accumulated cake layer andimprove steady state process flux. Permeate backflushing (B/F) is the most widely used but thedrawbacks are loss of product and extensive down-time. In a pilot plant study for ultrafiltrationof surface waters containing high NOM, turbidity and cation loads, the use of flowdestabilization, or feed flow reversal (FFR) combined with cross-flow B/F was able to improvethe normalised flux by 10.7 ± 3.4 %, compared with 3.2 ± 1.6 % improvement for BIF withoutFFR. When a second B/F included FFR, the flux improvement was 7.0 ± 2.0 % compared with4.3 ± 2.5 % for a B/F without FFR. The hypothesis was proposed that the flow destabilizationcaused slight lifting of the oriented cake layer, while the cross-flow B/F was able to sweep thelifted cake out of the lumen. If the flow destabilization may be effected by a simple but effectiveand low-cost method, and if this flow destabilization may be combined with reverse flow forshort durations, the lift-and-sweep approach will be the ideal method of maintaining processflux and increasing membrane life. Such a flow destabilization method, now named reversepressurepulsing (RIP), was developed. The method involves circulation of feed water in arecycle loop for 2 s to gain momentum, followed by closure of a fast-action valve upstream ofthe modules. The momentum of the water in the concentrate loop carries it into an air-filled feedaccumulator, while concentrate and reverse-flow permeate (which also lifts the fouling layer) aredischarged to the atmosphere using the recycle pump for 15 s. When the valve opens again, theair in the accumulator forces the water under pressure through the membrane lumens, causing apressure pulse and flow perturbations that lift, shift and break up the fouling layer. During 3 suchlift-and-sweep events, the cake is lifted and the debris is swept out of the lumen. Experimentalresults for uninterrupted dead-end filtration at a UF pilot plant using RIP only on a severelyfouled membrane, indicated that the RIP increased the flux by 18.4 % and decreased the dP by8.2 % over a 7.2 h period. The method is effective in removing the cake layer intermittently andno long-term flux decline occurred for a period of 555 h since the previous chemical cleaning.

[发布机构] Stellenbosch University