For anyone who has experience with culturing algae, I am curious about the best way to harvest the cells.  I have tried a few different filtration devices but so far, the types of filters I use seem to clog once coated thinly with algae and so I cannot filter out the quantity I was hoping for.  I found a piece of nylon mesh cloth in the lab that is 10 micron but this only works for  Scenedesmus culture and not Neochloris (which is smaller than 10 micron).  Even though it catches the Scenedesmus cells, It is not extremely easy to get the cells out of the mesh and the water filters through VERY slowly.  My main problem is with the Neochloris because the cells are so small and tend to stay suspended in the medium even with no aeration.  The Scenedesmus sinks to the bottom when I stop aeration so it is much easier to suck out of the bottom of the tank.  I also did some separation with a centrifuge but this seems inefficient to me and for some reason the Neochloris didn't get forced to the bottom very easily.  The only type of centrifuge I have access to is the Lab type which involves separating everything into multiple test tubes which is incredibly inefficient for the 80L I want to harvest from.  Anyone have any tips or ideas?  What if I installed one of those fish tank pumps and filters but replaced the filter part with the mesh cloth?  I wonder if I could tweak it to only catch a small amount at a time so the culture could still grow while I continuously harvest a small amount through the filter.  Any help or suggestions would be appreciated.  Thanks,

Dave 

 Dave,

When we get to a production system, I feel your idea of harvesting algae while the culture continues to grow will be part of that process. 

One idea for harvesting is froth flotation - blow tiny bubles up a column and the bubbles will carry the algae cells to the top.  It is kind of like a protien skimmer for a salt water aquarium.

Conceptually, a centrifuge may work.  There is a medium size one being marketed as a DIY project.  If you have an equipment budget, it might work for you.

A third idea is a sand filter.  It will take a while to get started working, particularily for the smaller species.  One for a small home pool may be good for your volume of culture.

If you like any of these, I will try to dig up URLs. 

 in our closed loop londs,the best way is to harvest by flotation, stop agitation or aereation and the algae will begin to create clumps on the surface these can be collected by surface skimming thats how we do it the algae also accumulates along the  side walls or banks of the pond.

in  glass culture tanks or jars, the algae has a tendancy to adhere to the surface of the tanks this we call" leather" we simply scrape that off the glass  and viola youve got nice dense algae to work with...this will work with large poly tanks that are translucent as well. the algae go to the light.

 hope this helps

 marc

I like the froth flotation idea.  To get it to froth like you are explaining, I am figuring compressed air might be needed to shoot a jet of air up a tube. have you thought about what type of nozzle might be used?  For the tube, I figure a piece of PVC would work and ideally, the 3 or 4 inch tube would alow for harvesting more at a time.  Would the tube have to be smaller than that?  Could the bottom also be kept open so I could simply dip the tube into the tank and shoot air up the tube and scoop or strain the froth without having to take anything out of my tank?  I am not sure if you have tried this technique yet but either way If someone has input it might help me to figure this out faster.  

Ecogenics- when you scrape the algae from the side of the tanks, do you drain the growth medium so you can scrape and catch the algae?  If I just scrape the sides in the medium, it will  mix in with the water and I won't actually be able to catch much of the algae. How do you isolate the area you scrape from?

For the most part I am having trouble with Neochloris because it is so small.  They are tiny little cells and make it difficult to capture them.  For some reason they are extremely well suspended in their growth medium but don't float to the top.  They even stay suspended throughout the mixture when I stop aeration.  Usually when I stop aeration I only give it a day or two to settle; do I need to leave it for a longer time to see if they float to the top or sink to the bottom? 

I wonder if there is a substance I can add to the tanks to change the specific gravity of the growth medium and force the cells to float or sink without affecting the cells themselves...that way I could keep growing them in the same medium

Dave 

Do you have any literature on the froth floatation or the centrifuge you are referring to? If you have the URLs for anything you mentioned, it couldn't hurt to check them out. Thanks

some algae are heavier and will fall to the bottom. but the upwelling vortex we  create bring them up to the surface where we can  capture them. for collecting on the banks we wait till late afternoon  in the hot days, a good amount of water evaporates and accumulates on the roof of the system due to condensation this loweres the level of the water somewhat and leaves a ring of algae accumulated on the sides this we gather with a wetdry vacuum that  has a nozzle we modified to suck up the algae on the sides  without damaging the liner and we use a skimmer net to capture the large blobs of algae that float up to the surface evry day. then, after we harvest the algae we simply tap the system's poly cover and shake the frame a little and  the water that condensed on the cover pours, like rain, back down into the pond so nature helps us by lowering the water level in the daytime on hot days. people visiting comment how its like a rain forest in the system. and thats exactly our goal, to replicate nature to the greatest extent. with a living ecosystem comprised of mutually symbiotic polycultures of  litoral emergent plants ,fish ,and a mono culture of algae in the winter ,we use the vacuum attachment to suck the algae off the sides and the little bit of water that is  in the algae pudding is strained off after collection through a five mesh screen that is streched on a frame and the algae is drawn off with a squeegee and then dried by a solar convection drying system or a large flat. metal surfaceapproximate description of how we do it there is a little more to it but that is proprietary. for subsurface harvesting we use a pump and run the algae through a series of screens in a housing. mounted at the surface and from there to the convestive  heater or hot plate depending on the season.

marc

Grtfulbuster:

  froth floatation or the centrifuge

Froth floatation is used in the mining industry using wetting agents and flocculents and... to get out minerals and coal outta ore. Froth isnt what you want because your not making antisoaps and there will be issues with the return water. What you people are really talking about is called a  DAF, a widely used industrial process. Tiny little air bubbles drag all the solids to the top to be skimmed. This is the lowest economy way of doing large scale clarification. Flocculate/Modifiers if needed (but that causes effluent issues).

IMEO, virtually all of the scaled algal systems will utilize DAF technologies because of the inherent issue of less than 1% solids algae water. IMEO, there is no way a centrifuge could be used on unconcentrated algae water. Algae water as grown is much too low % of solids for a centrifuge to have any value. After the DAF, then a centrifuge might have value depending on final usage. Energy harvesting is very low margin production and there is no way centrifuging 99+% water will be thermodynamically correct, much less economic.

 

 Marc,

Spirulina does tend to clump.  Many other species do not. 

 Grtful,

You don't want to use a jet - you want tiny bubbles.  Although the air is somewhat compressed, it does not need much pressure.  For a trial setup, you can use an aquarium air pump and a bubble stone.  The tube does not need to be that large.  The trick on making it easy to use is to put a tee in the pipe an inch or so above the water.  Then continue the pipe up another 6" or so.  Hang the tee over the edge of your culture tank and you can let it run on its own.

Yes, you would be hanging the tube in your tank and pumping the air up it. 

The researchers found that the PH had to be high for the process to work.

A couple of URLs (the first is froth flotation):

http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1057831 

http://www.simplecentrifuge.com/ 

http://www.green-trust.org/TBB/2007/06/cleaning-vegetable-oil-with-centrifuge.html 

Agree with Froggy ( for a change)  - DAF - dissolved air floatation - is the way the commercial plants will have to go.

May be a bit expensive for your lab set up. You may need to try using filter paper, drying the harvested algae and then washing off the paper with a solvent. If you are going to extract the oils then Hexane can be used :-

Hexane Solvent Method

Algal oil can be extracted using chemicals. Benzene and ether have been used, but a popular chemical for solvent extraction is hexane, which is relatively inexpensive. The downside to using solvents for oil extraction are the inherent dangers involved in working with the chemicals. Benzene is classified as a carcinogen. Chemical solvents also present the problem of being an explosion hazard.

Hexane solvent extraction can be used in isolation or it can be used along with the oil press/expeller method. After the oil has been extracted using an expeller, the remaining pulp can be mixed with cyclo-hexane to extract the remaining oil content. The oil dissolves in the cyclohexane, and the pulp is filtered out from the solution. The oil and cyclohexane are separated by means of distillation. These two stages (cold press & hexane solvent) together will be able to derived more than 95% of the total oil present in the algae.

Work with caution.

 

Slippery:

   May be a bit expensive for your lab set up.

No this isnt expensive to experiment with. You just need a decent air pump, some air stones/bubble device and a way to manipulate that air stream pressure. You need to be able to adjust the air pressure to produce the optimal bubbles size and pressure/flow. You need some pretty good pressure, your normal fish pump is only theoretically correct and not a real comparison to a pump like this. Which solids and % concentration will differ with different DAF air pressure, its likely that each algal spp will have different optimized pressure. The more 'floc's or others' that needed to be introduced into the DAF, the worse (expense and effluent poisoning).

Commercial production would have something like... One pond for primary growing/inoculation, one pond for bulk growth, one pond for your DAF ---> solids moves into processing. Balance the DAF effluent with the bulk growth water so they can exchange (or some other usage that has value)and you got something.

 

Slippery:

  Hexane solvent extraction

The idea that a bunch of backyard myth dreaming algae economy growers are going to start extracting using materials like Hexane, Benzene, even toluene scares the hell outta me. There are plenty of 'state for pay labs' that will do this work for you and give you real results for not alot of $. And because you dont know WTF you are doing... how do you know if you are doing it right?  

Also, there is no way on earth that Hexane sorts of extractions will ever be used for commercial productions of biooil, please dont make me do the math. So if this is true, then why even bother with them in your garage.

This is not for the back yard experimentation and should completely be left off the table unless you are a chemical engineer. Lets move on...

our observations are that the majority of the algae we grow clump(though not as large as spiro.) or form concentrated films on the surface of the water, and beyond a doubt, if the tanks are transparent or translucent, the algaes form "leather" on the surface of the sidewalls that can be easily harvested. these observations are exactly why we set up our culture lab so we can find out first hand what the behaviours  of algae are, how user friendly are they and and rates of growth. so far we see that other than nutrient requirements and water chemistry, algae IS algae... evryone who buys our algae gets dense cultures and a good bit of leather. this constitutes much more algae in concentrated form than anyone else offers, and it comes in 50 ml. tubes instead of 15 ml, like other labs offer,.also the survivability of our algae is amazing, we had a shipment to papua new guinea , possibly the most primitive place on earth held by customs and after the trip and confinement the algae survived, this customs delay has happened a couple of times when shipping overseas.to china and south america. and to boznia. but 99 % of the algae get to thier destinations without delay. were glad to see..

 Marc

froggy:

Also, there is no way on earth that Hexane sorts of extractions will ever be used for commercial productions of biooil, please dont make me do the math. So if this is true, then why even bother with them in your garage.

 

 

Hexane is commonly used in vege-oil production, so apparently the math has already been done.  However, the ecotoxic effects of hexane mean that it will be very hard to build new plants to EPA standards.  

 

And on your second point - many techniques which will never be feasbile in commerical applications are used quite rightly in research (see centrifuges for preliminary small scale studies of algal cultures).  No-one will get anywhere if they have to do everything at the start as they expect it to be done at the end. 

 The focus of my current research is on extraction so I am not worried about finding the ultimate scalable technique to produce and harvest algae.  Of course I want to figure out a good way to harvest but I am not trying to spend all my time figuring that out.   By asking this question I was just hoping to get some tips about how I might best be able to harvest the algae I have been growing without having to create a whole new experiment.  

Thanks for the links to those articles, Liberty.  I might actually try the froth floatation as that paper covers the process pretty well.  I am only going to try it on one of my smaller tanks just in case I kill the algae with the HCl.  I wonder if that will help kill some of the bacteria that is living amongst my algae as that would be a good thing.  Apparently once you balance the pH with NaOH the algae are fine and able to continue growth.    

For now, I just harvested what sank to the bottom of my tanks by sucking it from the bottom and currently have a couple liters of dark green liquid.  I figure I will let that settle for a day and decant the liquid from the top while keeping the algae that settles to the bottom.  After that I haven't decided what I will do to further concentrate and dry the algae.  I want it dried so I can use the mass of algae to compare to the mass of lipids I extract, so I might just spread it out on a pan and put it in our greenhouse to evaporate.  Do you ever dry your algae, Marc?

On the topic of Hexane, I have used it to extract algal oils and it seems to work pretty well.  When it comes to working in a lab, I don't see why hexane would be a problem since it is just one of the MANY solvents that can be used for the same end goal.  The issue of scaling up hexane  is not about whether it is possible or affordable, (since hexane is used EXTENSIVELY in the food industry) but whether it is wise.  Hexane is a byproduct of gasoline production and comes from crude oil.  Wouldn't it be pretty dumb if our algae oil relied on crude oil for extraction? 

Dave 

Grtfulbuster:

 The focus of my current research is on extraction so I am not worried about finding the ultimate scalable technique to produce and harvest algae.  Of course I want to figure out a good way to harvest but I am not trying to spend all my time figuring that out.   By asking this question I was just hoping to get some tips about how I might best be able to harvest the algae I have been growing without having to create a whole new experiment.  

...

On the topic of Hexane, I have used it to extract algal oils and it seems to work pretty well.  When it comes to working in a lab, I don't see why hexane would be a problem since it is just one of the MANY solvents that can be used for the same end goal.  The issue of scaling up hexane  is not about whether it is possible or affordable, (since hexane is used EXTENSIVELY in the food industry) but whether it is wise.  Hexane is a byproduct of gasoline production and comes from crude oil.  Wouldn't it be pretty dumb if our algae oil relied on crude oil for extraction? 

Like I said, If you have some experience with Chem's and have a real lab with a real hood and a real amount of knowledge on what you are doing... sure go nutz. I agree, the lab is for shortcuts and let the engineers figure it out later.

As to the cost of hexane to extract... one has to remember that industrial food has more value per lb than energy. 1 gal of gaz =$3 and weights 7lbs with ~ 0% water weight. A tomato costs $1/lb with a 65% water weight. A pepper even more. A box of frosty miniwheats even more.

Also, Im thinking ahead when hexane/other solvents arnt as cheap as it is now because current crude refining pricing. Chem's like Hexane have gone up 3x in the past few years.

dave,

 yes we dry  our algae often for extraction tests weve described how in many previous postings.

CAUTION. HEXANE IS VERY VERY DANGEROUS.. USE IN A VERY WELL VENTILATED AREA IT IS A CARCINOGENIC ANDS DOESNT TAKE MUCH TO KILL YOU EVENTUALLY. NOT A PLEASANT THING TO LOOK FOREWARD TO.. ive gotten sick from it because my  laminar flow fume extraction got clogged by a birds nest and i didnt know it.. really, you dont want to mess with it unless youre properly protected..

 marc

 

When it comes to Hexane extraction the cost of the hexane is a very minor one. The main cost is the energy required to recover the solvent from the Liquid (Oil) and Solid (Meal) streams after the extractor and then to recover hexane from vapor and water streams.

Hexane is not really "consumed" during the extraction it is basically almost all recovered and reused. Commercial Extraction plants are regulated by the EPA and their Hexane usage is one of the main things the EPA look at. When you add up all losses such as residual hexane in oil (almost untraceale), residual hexane in the solid meal (a couple of hundred ppm max), emissions to atmosphere, emissions to waste water and any leakage etc.within the plant, the total usage will be <1kg of Hexane for every 1000kg of seed (this is the UK number but US EPA is at least as tight with their controls) they look at a mass balance typically over the year or quarter etc. and they will be given access to records of Hexane delivered to site so it is difficult to hide how much has been "lost".

Hexane Extraction plants in the US are typically sized to extract anything from 2000 tons per day upwards to anything around 6000 tons per day based upon soy as the feed stock.

 

 

 

fuzznag:

the energy required to recover the solvent from the Liquid (Oil) and Solid (Meal) streams after the extractor and then to recover hexane from vapor and water streams. 

Hexane is not really "consumed"

This is interesting in that how is a backyard lab gonna know what % weight is Hexane/solvent and what isnt? They are going to have to test this on their particular feedstock to get an accurate mass balance. Again, why even bother with this because you are opening a can of worms without being able to read the label and know what the can holds, unless you have a GC and other analytical testing equipment (and if you have those, Im not worried about you killing me with Hexane (as much)). This is what testing labs are for, and they will do it right, and its a 3rd person.

Im curious how they typically recover the Hexane, distillation? And how the heck they recover from the mash?

fuzznag:

When you add up all losses such as residual hexane in oil (almost untraceale), residual hexane in the solid meal (a couple of hundred ppm max), emissions to atmosphere, emissions to waste water and any leakage etc.within the plant, the total usage will be <1kg of Hexane for every 1000kg of seed ... 

Hexane Extraction plants in the US are typically sized to extract anything from 2000 tons per day upwards to anything around 6000 tons per day based upon soy as the feed stock.

Fun with math...

Lets say there is .5kg of hexane lost /1000kg of seed to outside the recovery system.

.5 x 2.2 = 1.1lb of Hexane for every (1000 x 2.2 = 2200lbs) so lets round this up to 1lb lost per ton of soy feedstock.

2000t/day plant = 1 tons of Hexane outside the system/day in the form of 'pollution' /day.

Soy is ~ 20% oil. 2000t/day x  20% = 400t of oil/day x 2000lbs/t   / 7lbs per gal of diesel = 150000 gal of biod/day plant.

400lbs of oil/1lb of hexane / 7lbs/gal = For every 57 gal of biod, there is 1lb of hexane pollution.  

froggy:

This is interesting in that how is a backyard lab gonna know what % weight is Hexane/solvent and what isnt? They are going to have to test this on their particular feedstock to get an accurate mass balance. Again, why even bother with this because you are opening a can of worms without being able to read the label and know what the can holds, unless you have a GC and other analytical testing equipment (and if you have those, Im not worried about you killing me with Hexane (as much)). This is what testing labs are for, and they will do it right, and its a 3rd person.

Firstly I would not suggest that any back yard producer try hexane extraction, I was simply pointing out how the cost of hexane is a small value in the overall scheme of things which is not what you suggested in your post when you mentioned the costs involved in hexane extraction. You implied that hexane cost was the big thing and my answer was to suggest otherwise. Hexane is actually easier to remove from the oil than methanol is mainly due to its lower boiling point.

Im curious how they typically recover the Hexane, distillation? And how the heck they recover from the mash?

To recover solvent from the oil they start with a 2 or 3 stage rising film evapoartor set followed by vacuum stripping. Typical residual hexane at this point will be a couple of ppm but more often than not at untraceable levels. The residual solids (meal) are conveyed out of the extractor into a desolventizer which is basically a vertical vessel with multiple steam heated trays with a drive shaft down the centre that has sweep arms at each level to stop the meal sticking on the trays. Residual hexane in meal is typically down to a couple of hundred ppm maximum (depending upon final use of the meal it can be further processed).

Fun with math...

Lets say there is .5kg of hexane lost /1000kg of seed to outside the recovery system.

.5 x 2.2 = 1.1lb of Hexane for every (1000 x 2.2 = 2200lbs) so lets round this up to 1lb lost per ton of soy feedstock.

2000t/day plant = 1 tons of Hexane outside the system/day in the form of 'pollution' /day.

Soy is ~ 20% oil. 2000t/day x  20% = 400t of oil/day x 2000lbs/t   / 7lbs per gal of diesel = 150000 gal of biod/day plant.

400lbs of oil/1lb of hexane / 7lbs/gal = For every 57 gal of biod, there is 1lb of hexane pollution.  

Yep, those numbers look pretty good (1lb per ton), according to your math the consumption of hexane is only 1 lb to make 57 gal of biodiesel hence the reason for the post hexane is a minor cost in the overall scheme of things.

 The fact that tons and tons of hexane are discharged from solvent extraction plants all over the world all day everyday is another issue.

 Froggy,

If Grtful were building a production plant, your $500 - $5000 pump might be an idea.  He has 80 L. of algae culture he wants to get the algae cells out of.   He can start with an aquarium pump.  If that doesn't work well enough, he can use his lab air.