I want to thank Joe for his reply and very detailed write up.
"Here's a very quick and crude schematic with some description that might be help:
The main "bioreactor" is as follows: acrylic tube (2 3/4" OD, 1/8" wall, 14" tall from USPlastic) which holds ~ 1 L of culture. Glued to the inside wall of that tube is a small diameter (~1/2") that sticks above the liquid, but the end of that tube ends ~1/2" above the bottom of the reactor. A piece of flexible tubing that carries the medium (see below) passes through the reactor's wall (your #8) and bends into the small acrylic tube. This way, as medium is dripped (pumped) into the reactor, it must flow into the bottom of the reactor, which keeps it from instantly flowing out of the reactor. As medium in pumped into the reactor it flows out via a tube near the top that extends into the refugium (not visible in photo, but shown in drawing). The reactor was inoculated with Tetraselmis (Microalgae Disk Type: AA-TET, Florida Aqua Farms) and allowed to grow in reactor filled with f/2 medium for a few days before medium flow was turned on.
The output from the bioreactor drips into a acrylic tube inside the refugium (on left in drawing) that has a hole near the bottom. This insures the algae are fed to the bottom of the refugium, and not dripped on the surface where they might be quickly swept out of the refugium. It may not be needed.
A piece of 1/8" stainless steel tubing is connected to an aquarium air pump via plastic air hose. I have a sterile gas filter in-line (between your #5 and #6), but it is probably not needed, but you could use a piece of large dia tube stuffed with cotton. The stainless steel tube that also passes through the side of the reactor (your #7), is open at the bottom, so creates large bubbles (you don't want or need a sparging stone, just big bubbles and just a slow train of them). The air provides CO2 and oxygen, but much more importantly provides mixing to keep the algae well suspended. The air flow rate is controlled by a standard aquarium air valve (your #1). A similar piece of SS tubing is also in the tube within the refugium (on left in diagram, but air tube not shown), and controlled by valve #3. Your #4 is just the input to the 3-way air valve from the aquarium air pump, and #2 is not used.
There is a 5 L Pyrex bottle with f/2 medium ("Micro Algae Grow Mass Pack without Silicate" from Florida Aqua Farms). A piece of norprene L/S 13 tubing (Cole-Parmer, but any very small ID tubing could be used) starting at the bottom of the medium bottle passer through a fixed speed Cole-Parmer peristaltic pump and into the bioreactor as described above (#8). The f/2 medium is prepared as described by Florida Aqua Farms (or where every you buy it), and it is brought to a boil to pseudo sterilize it. While still near boiling, it is pored into the medium bottle to "sterilize" it too (and why the bottle needs to be made of Pyrex so it does not shatter). The pump that feed medium to the reactor is not on constantly, as it would have to have an extremely slow flow rate. Instead, the pump is turned on a few times (~5) for a short period of time (17 min in my case) so as to attain a desired daily flow rate (see below).
The light is provided by a strip of LEDs of around 18 W (strip I use is no longer made, but there are many out there now) on a 12 hr on/off cycle. You wan LED that have wavelengths that grow algae well (I'm just using some leftover 8K lights, but it's not critical.
The medium flow rate is rather important. The term dilution rate is just the medium flow rate (say in mL/day) divided by the volume of the bioreactor (in mL). The specific growth rate of algae is their growth rate per unit volume (say in g/mL/day) divided by their biomass concentration (in g/mL). Both the dilution rate and specific growth rate have units of 1/day then, and eventually, the specific growth rate of the algae will match that set by the dilution rate. Algae don't grow much faster than 1 1/d, so if the dilution rate is much higher than that, then the algae will just be washed out of the reactor and you will just end up pumping in pure medium, which you don't want. Consequently, I've been running the bioreactor at a dilution rate of 0.1 1/day. For my 1 L (or 1000 mL) reactor, that means I pump medium in at 100 mL per day. If you have a big aquarium/refugium (my total volume is around 60-70 gal) you can use a higher flow rate of medium, but the bioreactor needs to be bigger as well. For instance, if you wanted to pump in 1 L of culture per day, then your bioreactor would need to be 10 L in volume, so as to get the same dilution rate of 0.1 1/day. Search google on chemostats, and you'll find tons of information on them. Also important, the higher the dilution rate, less of the medium that gets consumed. At a dilution rate of 0.1 1/d, I measured very low inorganic P leaving the bioreactor. That is, most of the P in the medium had been consumed by the algae. If I ran the dilution rate closer to 1 1/d or higher, the more of the inorganic P (and associate N) would not be consume and would also be pumped into the aquarium, which would not be desirable.
This algal chemostat has been running continuously since Feb 2017, and when I check the culture under a microscope, it is pretty much 100% Tetraselmis, which I find some what surprising. Tetraselmis must be pretty good at out competing other algae, because the setup is not very sterile. As long as the bioreactor does not get taken over by an algae that loves growing on the bioreactor walls, I will keep it going. If it does get taken over by wall growth at some point, I'll just sterilize the bioreactor (i.e., drop it in some boiling water) and start again.
I thinks that's most of the details. Feel free to pass this information around.
cheers,
joe"
