Closed PBRs have advantages over open ponds for microalgal growth in terms of culture conditions and growth parameters such as pH, temperature, mixing, CO2, and O2 levels, evaporation and contaminations can be easily avoided, and increasing cell intensities with less land area requirement (Perez-Garcia et al. 2011; Singh & Sharma 2012). Despite these advantages, PBRs have several disadvantages, including overheating, scaling difficulties, and increasing construction costs (Singh & Sharma 2012). Table 2 compares open and closed large-scale microalgae growing methods.
Comparison between open and closed algal cultivation systems
| Culture systems for microalgae . | Open systems (raceway ponds) . | Closed systems (PBRs) . |
|---|---|---|
| Contamination control | Hard | Simple |
| Contamination risk | Exceptionally high | Less |
| Space required | More | Less |
| Water losses | Exceptionally large | None |
| CO2 losses | More | None |
| Temperature | Highly variable | Required cooling |
| Temperature control | Difficult | More uniform temperature |
| Sterility | None | Achievable |
| Process control | Difficult | Easy |
| Mixing | Uniform | Very poor |
| Operation regime | Batch or semi-continuous | Batch or semi-continuous |
| Weather dependence | Insignificant, because closed layouts allow productivity even when the weather is severe | Absolute, production impossible during rain |
| Area/volume ratio | Low (5–10/m) | Large (20–200/m) |
| Efficiency of treatment process | Low, time-consuming, lower mass per volume | Comparatively higher mass per volume |
| Gas transfer control | Less | Large |
| Algal species | Restricted | Flexible |
| Biomass quality | Variable | Reproducible |
| Shear | Less | Large |
| Population density | Less | Large |
| Harvesting efficiency | Less | Large |
| Harvesting cost | High | Lower |
| Light utilization efficiency | Poor | Good |
| Most costly parameters | Mixing | O2 and temperature control |
| Energy requirement | High | Lower |
| Scale-up | Difficult | Difficult |
| Capital costs | Low | High |
| Operation costs | Low | High |
| Cleaning | None | Required |
| Culture systems for microalgae . | Open systems (raceway ponds) . | Closed systems (PBRs) . |
|---|---|---|
| Contamination control | Hard | Simple |
| Contamination risk | Exceptionally high | Less |
| Space required | More | Less |
| Water losses | Exceptionally large | None |
| CO2 losses | More | None |
| Temperature | Highly variable | Required cooling |
| Temperature control | Difficult | More uniform temperature |
| Sterility | None | Achievable |
| Process control | Difficult | Easy |
| Mixing | Uniform | Very poor |
| Operation regime | Batch or semi-continuous | Batch or semi-continuous |
| Weather dependence | Insignificant, because closed layouts allow productivity even when the weather is severe | Absolute, production impossible during rain |
| Area/volume ratio | Low (5–10/m) | Large (20–200/m) |
| Efficiency of treatment process | Low, time-consuming, lower mass per volume | Comparatively higher mass per volume |
| Gas transfer control | Less | Large |
| Algal species | Restricted | Flexible |
| Biomass quality | Variable | Reproducible |
| Shear | Less | Large |
| Population density | Less | Large |
| Harvesting efficiency | Less | Large |
| Harvesting cost | High | Lower |
| Light utilization efficiency | Poor | Good |
| Most costly parameters | Mixing | O2 and temperature control |
| Energy requirement | High | Lower |
| Scale-up | Difficult | Difficult |
| Capital costs | Low | High |
| Operation costs | Low | High |
| Cleaning | None | Required |