Table 2 Comprehensive comparison of common harvesting processes in microalgal biomass production
Harvesting processes | Factors influencing feasibility | Recovery | Advantages | Disadvantages |
|---|---|---|---|---|
Centrifugation | ||||
 Disc stack centrifuges | • Cell settling characteristics • Centrifugal force • Type of centrifuges | High | • Rapid and reliable • Suitable for almost all microalgal species • No chemicals | • High capital investment • Energy intensive • Cell damage by shear stress and high gravitational force |
 Scroll centrifuges | High | |||
 Hydrocyclones | High | |||
Filtration | ||||
 Microfiltration | • Cell size • Flow rate • Transmembrane pressure difference • Turbulent flow | Low | • Less cell disruption • No chemicals • Simplicity of operating and functioning | • Fouling • High cost in filter membrane replacement and pumping • Suitable for large volume cells • Effective for low volume cultures • Low permeability and selectivity of membranes |
 Macrofiltration | Low | |||
 Ultrafiltration | High | |||
 Dead end filtration | High | |||
 Vacuum filtration | High | |||
 Pressure filtration | High | |||
 Tangential flow filtration | High | |||
Flocculation | ||||
 Chemical flocculation | • Selection of cationic flocculants • Charge density • Electronegativity and solubility | High | • Low cost and high efficiency • Simple and fast • No energy input | • Presence of metal salt residues • pH dependent • Recycling of medium is limited |
 Electro-flocculation | • Selection of electrode materials • Current density • Electrolysis time • Composition of the microalgal suspension | High | • Non-species specific • No residual anions • Low chemical usage • Low power consumption | • Need for electrode replacement • Residual metals in algal biomass • pH changes • Temperature increase of algal suspensions and cell damage |
 Bio-flocculation | • Selection of bio-flocculants | General | • No chemicals or specific culture conditions are needed | • Highly species-dependent process • Unclear mechanism • Long flocculation period • Possibility of biological contamination |
 Auto-flocculation | • Changes in nitrogen, pH and dissolved oxygen | High | • No chemicals • Neutralizing negative charge | • pH dependent • Unclear mechanism • Unstable |
Other processes | ||||
 Flotation | • Type of collector • pH and ionic strength • Type of bubble formation • Air tank pressure • Hydraulic retention time • Particle floating rate | High | • Short operation time • Low space requirement • Large scale harvesting • High flexibility with low initial cost | • Flocculant or surfactant required • Energy intensive • Ozoflotation is expensive |
 Gravity sedimentation | • Cell settling characteristics • Cytoplasmic density | Low | • Simple and low cost | • Time-consuming • Not reliable and effective |