We are happy to announce that Zhenyu Zhao has successfully defended his PhD These. His research was about microalgae. 

Microalgae have been drawing a lot of attention due to their strong photosynthesis, efficient CO₂ fixation, fast growth rate and by not requiring any arable land for growth. Their plentiful multi-bioactive substances can make microalgae into competitive resources for production of biofuel, food and medicine. In addition, microalgae can play a role in removal of pollutants and heavy metals from waste streams by removing e.g. phosphates and nitrates, or in capturing CO₂ from industrial flue gases. A major contributor to the cost and energy demand of microalgae production is the harvesting of the microalgal biomass owing to their small cell size (about 1 to 30 µm) and growth in highly diluted conditions (0.5-5 g dry matter/L). Besides, large-scale industrial production of microalgal biomass is restricted by the high water requirements, causing increased water footprint and wastes of nutritive salts and potentially valuable extracellular polysaccharides (EPS). 

Membrane technology with a low energy demand and 100% microalgae harvesting efficiency offers an alternative solution to deal with the drawbacks from conventional methods, like centrifugation (i.e. a high energy input of >8 kWh/m³) and flocculation-based sedimentation (i.e. a low harvesting efficiency and slow process). However, membrane fouling is still problematic, resulting in low permeances and frequent membrane cleanings. 

This study covers different approaches to investigate a set of solutions and their combinations for fouling control during the microalgae harvesting process. It consists of nine parts: (1) optimization of the magnetically induced membrane vibration (MMV) system; (2) investigation of the synergy between flocculation and membrane filtration; (3) development of patterned membranes for microalgae harvesting; (4) unravelling the anti-fouling mechanism of patterned, negatively charged membranes; (5) assessment of the synergy between membrane surface charge, patterned membranes and shear-enhanced dynamic filtration; (6) economic analysis of microalgae harvesting using flocculation combined with patterned membrane filtration; (7) combination of membrane surface charge, patterned membranes, shear-enhanced dynamic filtration, and flocculation for energy-saving microalgae harvesting; (8) development of a pH-responsive patterned membrane for microalgal biofilm formation and harvesting; and (9) optimization of negatively charged membranes for EPS recovery.

To manifest the feasibility of the MMV system for microalgae harvesting, the MMV system parameters were first optimized in Chapter 2 using the response surface methodology (RSM) to find the optimal frequency and amplitude for energy-saving microalgae harvesting. Intermittent filtration was found to significantly reduce energy consumption during the harvesting process. The results also showed that the MMV system did not damage the microalgal cells, and demanded a low energy input (0.2 kWh/m3) with an operational flux of 40 L/m² h.

To lower the vibration frequency in the MMV system, and decrease the flocculant dose, chitosan-based flocculation was used prior to the membrane filtration in Chapter 3. The synergy between membrane filtration and flocculation was investigated and showed a higher membrane filtration performance and harvesting efficiency than without flocculation in both dead-end filtration and filtration with vibration. The filtration under sub-optimal flocculation conditions also showed significant potential. Introducing such flocculation into the membrane filtration process prominently reduced both energy input and required membrane area.

To prove the feasibility of patterned membranes for microalgae harvesting, membranes with a wave pattern on the membrane surface were proposed for the first time in Chapter 4 to alleviate microalgal fouling and increase the membrane permeance in a cross-flow system. Patterned membranes showed higher permeances and critical pressures than the corresponding flat membranes. Larger patterns gave higher membrane permeances and less fouling. Computational fluid dynamics (CFD) simulation showed a higher velocity and wall shear on the pattern apexes.

To investigate the effect of pattern shape on membrane performance in microalgae harvesting, and to unravel the fouling mitigation mechanism of negatively charged, patterned membranes, the flow behavior near the patterned membrane surface, as well as the interaction energy between membrane and microalgae were investigated using CFD simulation and the improved extended “Derjaguin, Landau, Verwey, Overbeek” (XDLVO) theory in Chapter 5. A wave pattern was best for microalgae harvesting. XDLVO analysis showed that sPSf blend patterned membranes realized the lowest interaction energy and highest energy barrier for microalgal attachment. CFD simulation showed a higher velocity and wall shear on the pattern apexes.

After revealing the advantages of the MMV system and patterned membranes for microalgae harvesting, membrane surface charge, patterned membranes and shear-enhanced dynamic filtration were for the first time combined in Chapter 6 to further improve the membrane performance in microalgae harvesting. Membrane vibration could mitigate membrane fouling both for the patterned and flat membranes. The vibration system can easily achieve turbulent flow at frequencies higher than 7 Hz. The synergy between membrane vibration, surface pattern and charge thus resulted in a clearly enhanced membrane performance.

After manifesting the advantages of patterned membranes and flocculation prior to membrane filtration, membrane surface patterning and flocculation prior to filtration were combined to improve membrane performance in microalgae harvesting in a cross-flow system in Chapter 7. Patterned membranes showed a lower filtration resistance than flat membranes. Increasing cross-flow velocity could increase membrane permeance in most cases. The highest stable membrane permeance (110±17 L/m² h bar) and the lowest filtration resistance were achieved when combing patterned membrane filtration with flocculation at optimized chitosan dosage. A low energy consumption (0.28 kWh/kg) and a very low harvesting cost (0.16 €/kg) were achieved under these conditions.

The synergies between membrane surface patterning and vibrating filtration as well as between surface patterning and flocculation prior to membrane filtration were unraveled in Chapters 6 and 7, respectively. In Chapter 8, membrane surface charge, patterning, shear-enhanced dynamic filtration and flocculation prior to filtration were for the first time combined to ultimately improve membrane performance in microalgae harvesting. A recorded low energy consumption (0.33 Wh/m³) was finally achieved under these conditions. Interaction forces, calculated based on permeate drag and inertial lift forces, revealed that increasing particle radius through flocculation prior to membrane filtration can significantly prevent microalgae attaching on the membrane surface.

After all aforementioned investigations, a high operational flux was achieved from 40 L/m² h to 95 L/m² h, and a considerably low energy consumption was realized from 200 Wh/m³ to 0.33 Wh/m³ by using negatively charged polysulfone (PSf) membrane in an MMV filtration system with flocculation prior to filtration. Besides, novel cationic cellulose nanoparticles were used as flocculant, achieving high flocculation efficiency without pH adjustment.

To further decrease the costs of microalgae cultivation and harvesting, a win-win strategy for high-density microalgal cultivation and low-cost harvesting through biofilm cultivation on a pH-responsive, charge-switchable, patterned membrane was investigated in Chapter 9 using a polyethylenimine (PEI)-crosslinked polyvinylidene fluoride membrane. Membranes with a higher pattern height resulted in faster biofilm development and increased final biomass accumulation. Low-energy membrane vibration was applied to enhance cell detachment. pH=7 was found to be the optimal pH for enhanced microalgal biofilm formation on PEI-crosslinked patterned membranes, and a switch to pH=10 was best for microalgal harvesting.

After cultivating and harvesting mciroalgal biomass, microalgal metabolite was further separated and purified using membrane technology. To separate and purify EPS from the spent medium of Arthrospira platensis, a negatively charged PSf membrane was prepared in Chapter 10 by blending PSf with sulfonated PSf, and was optimized using the RSM. The EPS retention of the optimized membrane was higher than 96%.