Biocarriers (or media) that attaches microorganisms on them can reduce membrane fouling in MBR when they are added to aeration tank. Two different mechanisms have been identified: direct membrane scouring by biocarriers and the reduction of SMP in aqueous phase.
When hollow fiber modules (Mitsubishi Rayon, Japan) were mounted vertically with and without an iron net that guards the membrane bundle from the floating sponge cubes (13mm x 13mm x 13mm) coated with activated carbon as shown in Fig. 1, the TMP of unguarded membrane bundle increased much slower than its counterpart as shown in Fig. 2. It was found that the unguarded membranes (Type B) suffered much less from cake layer formation than the guarded membranes (Type A) (Lee, 2006).
Similar biocarrier effects were observed in other experiments performed with porous flexible biocarriers (Yang, 2006). In this experiment, particle size distribution curve slightly moved to smaller direction as shown in Fig. 3, but the direct membrane scouring by biocarriers appeared enough to overcome the negative impact of the smaller particle sizes. Similar fouling mitigation effect was also observed with immersed flat sheet ceramic membranes, where plastic media (AnoxKaldnesTM, K1 carrier) were allowed to contact with the membrane (Jin, 2013).
On the contrary, when polyethylene based hollow fiber membranes (Mitsubish Rayon, Japan) were horizontally mounted and the biocarriers with cylindrical geometry (3 mm length x 3 mm diameter) were added directly to the membrane tank, positive impact on membrane fouling was observed only when biocarrier dosage was low, e.g. 1 v/v %. At a higher biocarrier dose (5 v/v%), TMP increased faster than control. The faster membrane fouling at a biocarrier dose of 5 v/v% was attributed to the smaller particle sizes than those in the control reactor without biocarrier (Wei, 2006). It was postulated that large flocs broke down to smaller particles due to the the collisions with rigid biocarriers.
Fig. 1. Experimental setup to compare direct membrane scouring effect by floating sponge media, where only A type setup has an iron net that prevent biocarriers from directly contacting with membranes (Lee, 2002)
Fig. 2. TMP increasing rates and the membrane pictures after experiment with and without iron net that prevents membranes from direct contact with floating biocarrier (Lee, 2002; Lee, 2006).
Fig. 3. Comparison of particle size distributions with and without soft flexible biocarrier (MBR-No biocarrier, HMBR – With biocarrier) (Yang, 2006)
It has been suggested by some researchers that combining biofilm reactor with MBR (BF-MBR) as shown in Fig. 4 is synergistic, where the strainers in between the aeration tank and the membrane tank prevent floating biocarriers from proceeding to the membrane tank. Following benefits were suggested relative to the MBR without biocarriers.
- More biomass can be retained in aeration tank so that aeration tank size can be reduced.
- By maintaining a lower suspended solids in liquid phase (or low MLSS) while maintaining a higher total microbial population, membrane fouling can be reduced.
- Due to the lower MLSS, oxygen transfer efficiency (OTE) can increase and membrane fouling can decrease.
However, the following still need to be answered by further research.
- As discussed here, biocarriers negatively impact OTE according to the literatures available in public domain. If this is the case, more air flow is required to supply same amount of dissolved oxygen. Since volumetric oxygen dissolution has a high limit due to the coalescence of bubbles at high air flow rate, increasing organic loading (or reducing HRT) of aeration tank won’t be easy.
- Further research is required to answer at which condition biocarriers are beneficial for reducing membrane fouling. Though majority of literature indicate less membrane fouling with biocarriers, some suggest opposite as discussed earlier this page.
- By retaining more biosolids in the system using biocarriers, excess biosolids production may decrease. But, the economics may get hurt due to the lowered OTE and increased oxygen demand to destroy (or oxidize) more biosolids. Overall economics must be reviewed for BF-MBR options.
Fig. 4. Process diagram of biofilm – membrane bioreactor, BF-MBR (Leiknes,2006).
When powdered activated carbon (PAC) was added to aeration tank and was allowed to directly contact with membrane in a MBR treating oily refinery wastewater, significant retardation of TMP increase (or membrane fouling) was observed (Conner, 2011). Due to the adsorption of foulants by PAC, membranes were kept clean as shown in Fig. 5. However, membrane life was expected to decrease by up to 40% due to the abrasion of membranes by PAC. Fig. 6 shows the damaged membrane surface after cleaning. As a consequence, the MBR combined with GAC (granular activated carbon) was developed, where GAC particles are packed in a column and do not directly contact with membrane.
In other study (Kurita, 2011), granular media (or biocarrier) shown in Fig. 7 were added to the MBR equipped with flat sheet membrane from Toray and allowed to contact with membrane. Result showed granular media reduced membrane fouling significantly at a same scouring air flow. It was estimated that around 50% air could be saved if similar membrane fouling rate is targeted as control MBR.
Fig. 5. Used membranes with and without PAC in the MBR treating oily refinery wastewater (Conner, 2011).
a) New (125x) b) 30 days (125x) c) 30 days (500x)
Fig. 6. Membrane surface damage due to the abrasion by PAC (Conner, 2011).
Fig. 7. Cylinder shaped polyethyleneglycol granular plastic media sized 4 mm diameter and 4 mm length with a specific gravity of 1.01-1.05 (Kurita, 2011).
Table 1. Type of biocarrier (or media) used (Brentwood Industries, 2011)
|PVC Structured Sheet Media||• Easy to install
• Low initial cost
• No material losses
• May foul if rag removal is inadequate
• Easy to install
|• Prone to redworm blooms
• May foul if rag removal is inadequate
|• Excellent mixing
• High Surface Area
• Media losses (washout or abrasion)
© Seong Hoon Yoon