Cultivation of Algae in Photobioreactor Algae cultivation can be achieved in two ways: open ponds and photobioreactors (PBR). A photobioreactor is a closed equipment which provides a controlled environment and enables high productivity of algae. As it is a closed system, all growth requirements of algae are introduced into the system and controlled according to the requirements. PBRs facilitate better control of culture environment such as carbon dioxide supply, water supply, optimal temperature, efficient exposure to light, culture density, pH levels, gas supply rate, mixing regime, etc., Photobioreactor - How it works
Working of a photobioreactor:
Flow description: 1. From the feeding vessel, the flow progresses to the diaphragm pump which moderates the flow of the algae into the actual tube. Built into the pump is the CO2 inlet valve. 2. The photobioreactor itself is used to promote biological growth by controlling environmental parameters including light. The tubes are made of acrylic and are designed to have light and dark intervals to enhance the growth rate. 3. The photobioreactor has a built-in cleaning system that internally cleans the tubes without stopping the production. 4. After the algae have completed the flow through the photobioreactor, it passes back to the feeding vessel. As it progresses through the hoses, the oxygen sensors determine how much oxygen has built up in the plant and this oxygen is released in the feeding vessel itself. It is also at this stage that the optical Cell Density sensor determines the harvesting rate.
5. When the algae are ready for harvesting, they pass through the connected filtering system. This filter collects the algae that are ready for processing, while the remaining algae passes back to the feeding vessel. 6. The flow continues. Advantages of Photobioreactors
Cultivation of algae is in controlled circumstances, hence potential for much higher productivity
Large surface-to-volume ratio. PBRs offer maximum efficiency in using light and therefore greatly improve productivity. Typically the culture density of algae produced is 10 to 20 times greater than bag culture in which algaeculture is done in bags - and can be even greater.
Better control of gas transfer.
Reduction in evaporation of growth medium.
More uniform temperature.
Better protection from outside contamination.
Space saving - Can be mounted vertically, horizontally or at an angle, indoors or outdoors.
Reduced Fouling - Recently available tube self cleaning mechanisms can dramatically reduce fouling. Covering ponds does offer some of the benefits that are offered by photobioreactors, but enclosed systems will still provide better control of temperature, light intensity, better control of gas transfer, and larger surface area-to-volume ratio. An enclosed PBR design will enhance commercial algal biomass production by keeping algae genetics pure and reducing the possibility of parasite infestation.
Disadvantages of Photobioreactors
Capital cost is very high. This is one of the most important bottlenecks that is hindering the progress of algae fuel industry.
Despite
higher
biomass
concentration
and
better
control
of
culture
parameters, dataaccumulated in the last two decades have shown that the productivity
and production cost in some enclosed photobioreactor systems are not much better than those achievable in open-pond cultures.
The technical difficulty in sterilizing these photobioreactors has hindered their application for algae culture for specific end-products such as high value pharmaceutical products. General Specifications of a Photobioreactor
Data in the table are for the Aquasearch-coupled production system for photosynthetic microbes from HR Biopetroleum. Source: HR Biopetroleum
Requirements
to
develop
a
high-performance
photobioreactor
for
algal
cultivation:
1. In order to attain high productivity, the volume of the non-illuminated parts of the reactor should be minimized.
2. In order to ensure a high efficiency of light use by the culture, the design must provide for the uniform illumination of the culture surface and the fast mass transfer of CO 2 and O2.
3. To prevent rapid fouling of light-transmitting surfaces of reactors, photobioreactors must be frequently shut down for their mechanical cleaning and sterilization
4. High rates of mass transfer must be attained by means that neither damage cultured cells nor suppress their growth.
5. For the industrial-scale production of biomass, the energy consumption required for mass transfer and the arrangement of the light-receiving surface of the algal suspension must be reduced to its minimum possible. PBRs are complex systems composed of several subsystems. The key systems are:
Light system
Optical transmission system
Air handling & gas exchange systems
Mixing system
Nutrient system
Instrumentation system
Electrical system Some of the key sub-components of the above system are:
Oxygen & CO2 sensors
Temperature sensor
pH sensor
Light sensor
Conductivity sensor
Recirculation pump
Harvest pump
CO2 injection valve
Substrate pump
Filtrate recirculation valve
Water inlet valve purge valve
Connectors and hoses
Oxygen release system
PLC control panel
Feeding tank Several of the subsystems of a PBR interact. For instance, the optical transmission system and gas exchange system interact via the mixing that takes place in the reaction area.
