Filtration of the water with low suspended solids

Pressure filtration

Dead-end filtration is commonly for tertiary filtration and surface water filtrations. Fig. 1 illustrates a dead-end filtrationusing a hollow fiber membrane bundle in a pressure vessel. Feed water is fed to membrane modules at a constant flow rate with a closed concentrate outlet. Flux is set typically at 20-50 LMH for secondary effluent recycle and at 40-80 LMH for surface water filtration. All feed water permeates through the membrane while all the particles contained in the feed water deposit on membrane. Cake layer gradually builds up causing TMP rise. Once the TMP reaches a preset threshold level, backwash cycle starts with an optional air scouring to enhance cake layer detachment.

Typically the threshold TMP is set at 100-300 kPa, but the lower the threshold is, the better the membrane permeability recovery is due to the less cake layer compaction. Therefore, the threshold TMP tends to set at low side in the range.Filtration time and backwash time are variable depending on the feed water quality, but filtration time ranges 10-30 min and backwash time 0.25-1 minutes in general. Backwash flow rate ranges from 1x to 2x of the normal permeate flow rate in typical applications. The commercial membranes fall into this category are Siemens MEMCOR® XP and CP, GE ZeeWeed® 1500, Asahi’s Microza® UNA and UNS, etc. Norit XIGA™ can run at either dead end mode or partial crossflow modes depending on the feed water quality.

FiltrationMode2Fig. 1. Dead end filtration with a HF bundle in a pressure vessel. The permeate suction can be done through both ends of the fiber. The exact module configuration and operational sequence vary depending on manufacturer.

Vacuum filtration

Alternatively, vacuum can be used to obtain permeate. In this case, pressure vessel is not required and the membrane is submerged directly in the tank as shown in Fig. 2. Depending on manufacturer, hollow fiber can be mounted either vertically (MEMCOR® XS and CS, Siemens) or horizontally (ZeeWeed 1000, GE Power and Water). In this filtration mode, feed water supply and concentrate purge can be performed either continuously or intermittently.

  • If the system runs with continuous feed and purge, the suspended solids level membranes face is high at steady state depending on the concentration factor in membrane tank. By timing the filtration and backwashing cycles of the membrane modules in a tank, output of the tank can be maintained relatively constant.
  • If the system runs with intermittent feeding and purging, the suspended solids level membranes face is fluctuating depending on the fill and purge cycles, which provides better filtration environment during low solids condition. However, lost time in filtration during fill and draw cycles is inevitable.

Flux is set typically at 20-45 LMH for secondary effluent recycle and at 30-60 LMH for surface water filtration. No air scouring is performed during the filtration in most commercial processes with exceptions. Once vacuum pressure reaches a threshold (50-80 kPa) or cycle time (10-30min) ends, filtration stops and backwash cycle starts using the stored permeate. Air scouring is simultaneously performed to enhance cake layer removal from the membrane.

FiltrationMode
Fig. 2. Dead end filtration with immersed membrane, where vacuum is used to obtain permeate

Effect of flux on the permeate volume obtained in one filtration cycle

In both pressure filtration and vacuum filtration, cumulative filtrate volume obtained in one filtration cycle is highly affected by the operating flux. Although the amount of the cake layer formed is proportional to the cumulative filtrate volume in a dead-end filtration mode, the chance of cake layer compaction is lower at lower flux as discussed here due to the lower dynamic pressure drop across the cake layer. Fig. 3 is a conceptual graph showing the effect of flux on the duration of a filtration cycle.

For instance, when secondary effluent from sequencing batch reactor (SBR) process was filtered by MF membrane at dead end modes, TMP reached 20 kPa in 90 minutes at 30 LMH. But, when the flux was raised three fold to 90 LMH, the same TMP was reached in 5 minutes. As a result, the volume of filtrate obtained in one filtration cycle was six fold more at lower flux condition (Parameshwaran, 2001). Though lower operating flux allows less membrane fouling per permeate volume, it requires more capital costs to have more membrane modules. Therefore, system must be optimized considering capital costs as well as operating costs.

EffectOfFluxFig. 3. Effect of flux on the filtration duration and cumulative filtrate volume