Equilibrium management (Continuous System)
In a Continuous Rotifer System rotifers are harvested daily and the culture continues growing in the same tank. Continuous rotifer systems are equilibrium systems, and it can take some time to find the right balance. However, these systems can produce tremendous numbers of rotifers in a very small space. With proper protocols for cleaning-in-place and transfer of the culture to new vessels, continuous cultures can remain extremely stable for years. A good rotifer system with a good microalgal feed can be harvested at a rate of 50% per day, although 30%-45% is more practical.
Continuous rotifer systems are chemostats. That is to say, they are steady-state or semi-steady-state bioreactors with a single limiting factor (feed) that is added at a constant rate. Optimally, feed rate per hour and harvest rate per day are fixed at constant rates and all other variables (notably rotifer count) float to equilibrium.
Note: It is common for culturists to try to adjust feed and harvest rates as rotifer counts change. Sometimes these adjustments are necessary. However, this kind of tinkering invites chaos because there are no fixed parameters to anchor the equilibrium point of the system.
Because Net Biomass Conversion is constant, increasing the harvest rate does not increase the harvest. Instead, the culture will thin, the ratio of feed to rotifer will increase, and the culture will grow faster. In a couple of days the culture establishes a new equilibrium at a lower density, higher harvest rate and an identical net harvest. In order to increase (or decrease) the rotifer harvest, the feed rate must be increased (or decreased).
- In Continuous Systems at equilibrium, the harvest rate only determines rotifer age. Feed rate determines the biomass of the rotifer harvest.
The primary benefit of a continuous system is that can provide a consistent and reliable supply of rotifers with minimal monitoring and labor. Continuous rotifer systems are usually fairly automated, with a continuous-feed peristaltic pump and water exchange. Because they usually have a high density of rotifers they often need supplemental Dissolved Oxygen (DO). It is somewhat common for continuous systems to include automated DO monitoring and supplementation and sometimes pH and temperature monitoring as well. However, daily logs* with manual entries are often sufficient. In higher pH systems (pH>7.8) a common automated input is the addition of ClorAm-X or other ammonia detoxifiers.
* An example of the rotifer production log used at RMI is included in the protocol section