Open pond systems are, by far, the most common method for cultivating algae. They are simple in design, low-maintenance, and inexpensive to operate. They’re “open” because the algae is exposed to open air during the growth stage. Open ponds can produce algae for nutritional products, biofuels, and chemicals. Open ponds are also preferred for activities like treating wastewater.
Closed systems include systems like photobioreactors (PBR) where algae is contained in some sort of transparent vessel and supplied with CO2. Closed systems reduce invasion by unwanted strains of algae and hungry zooplankton. They’re particularly valuable where growing conditions must be closely managed in ways that are impossible for open ponds, but they’re complex, artificial environments and are much more expensive to build and maintain.
We’ll talk more about those common production systems, but many more are in development, including hybrids of other systems. In the end, there is no one single way to grow algae at commercial scale, and this versatility is one of algae’s great strengths.
When the question of wide scale adoption of algae for food security or for wholesale replacement of fossil fuels comes up, low-tech open pond systems offer significant advantages over closed systems.
Advantages of Open Systems
- The simple structure of ponds and minimal equipment means low capital investments for construction, operation and maintenance.
- Maintenance for open pond systems
- Open ponds can be established on low-value land that’s not suitable or desirable for agriculture or development.
- Water requirements are low and can be satisfied by accepting wastewater from many municipal or industry sources.
- Minimal power requirements can typically be met using solar or wind.
- Algae grown in open ponds are high in oil content
- Open ponds represent the closest approximation of a natural environment for algae, so maintenance and support needs are minimal, while harvesting is relatively simple.
- Large ponds can produce significantly more algae than other systems at the same price point.
Basic Design
Open ponds for algae production are designed to maximize surface area and sun exposure, so they’re typically very shallow and broad; often spanning many acres. Raceway ponds are a popular configuration, where the water flows through an endless loop, similar to a racetrack. This perpetual flow prevents water from being trapped in corners, where fresh air and sun may be limited and result in production loss.
Paddlewheels or other devices are used to keep the water moving and to promote vertical circulation. Since algae are dependent on sunlight to grow, it’s important to maximize their exposure, but as algal mass increases, lower layers of water may become partially shaded. Steady rotation is necessary to bring all water to the surface and maximize growth.
Availability of CO2 is a fundamental limiter in the growth of algae, so adequate supplies must always be provided. The circulating/rotating mechanism may be able to mix in enough to meet needs, but supplemental CO2 can also be easily introduced through bubble aerators.
Challenges
An open environment trades ease of construction and operation with certain challenges that can reduce production levels compared to closed systems.
- In open systems, water is exposed directly to air, so some amount of evaporation is expected. The problem arises when the concentration of different compounds changes as water levels change, which could impact the growth rate of the algae.
- Temperature fluctuations can affect growth rates, and cool nighttime temperatures or summer heat waves are generally out of our control.
- In situations where monoculture (restricting growth to a single strain of algae) is important, open systems present a challenge, since cells or spores of competing strains can easily be carried in on the wind.
Greenhouses or other transparent/translucent barriers are increasingly used in conjunction with open ponds to create a hybrid option where large ponds still offer natural sunlight, but the ponds are protected from contaminants carried on the wind and from rainfall or wide temperature fluctuations. Using this kind of system, a wider variety of algae strains can be grown, and it allows those being grown to remain dominant. It also extends the growing season - potentially to year-round if the ponds are heated.
Nuts and Bolts
Open ponds can be constructed by placing simple open tanks and lining them to avoid any loss of product in the case of a leak. If the soil is suitable, you may choose to dig shallow depressions and build earthen banks built up around the perimeter to contain the water. This setup needs an impermeable liner so that water doesn’t seep into the soil. In other cases, ponds can be constructed of poured concrete.
Algae ponds are typically no deeper than 12” since sunlight doesn’t reach beyond that depth in adequate amounts to support growth.
The most common and most practical configuration for an open pond is the raceway style, often designed as a long, somewhat narrow oval. The oval or circular configuration is important to prevent formation of stagnant corners where the algae may not have access to adequate CO2 or sunlight past the surface layer. To promote mixing and movement, paddle wheels are used, sometimes one in the case of small ponds, sometimes two or more, depending on the size.
Paddlewheels alone may not move enough CO2 into the pond to maximize algae growth. In these cases, supplemental CO2 can be introduced using aerators.
