Dewatering algae 1

[tuanjim]

HI,

I'm involved in the early R&D stages of identifying the best available technology for dewatering algae and am wondering whether anyone out there has had experience in this field at all and would be able to offer up useful advice? Following initial screening tests with various coagulants and flocculants we have found a chemical mixture which enables good floc formation and background clarity but the chemical costs associated with getting to this step are prohibitively high. The system that we are operating with is a marine/salty medium containing a very small algae species (around 5 microns), at high algae/biomass densities (0.1 - 0.3 % TSS) and SG's very close to one. At this stage I am leaning towards DAF type technology or possibly some type of hollow-fibre type filtration (which would eliminate chemical costs). Centrifugation is too expensive as is gravity sedimentation at this stage.
The algae is our product rather than waste by the way, and my goal is to produce as high a TSS loading by the end of processing (5 - 25%). Our budget is very tight which makes this a tough one.
I new to such biological systems so any ideas or suggestions that you guys have out there would be hugely appreciated!!

 

[cub3bead]

Interesting application.

If you use a membrane are you interested in a submerged type? Kubota makes a flat sheet membrane for MBRs that may fit your situation besides the hollow-fiber types.

 

[kenvlach]

I suggest filter pressing -- either a plate-and-frame or belt type. These are usually used for wastewater treatment, but food processors also use to separate liquid & pulp. Look for a mfr. or used equipment vendor that will let you rent one (& apply rent toward purchase if suitable) so can trial. Suggest a 1-micron polyproylene filter cloth if using plate & frame. Some of the newer ones have hollow passages for warm air drying of the filter cake.

http://www.metchem.com

http://www.lanco-corp.com/catalog/by_cat_array.asp?strID=13

http://www.arnoldeqp.com/filterpresses.htm

http://www.usfilter.com/en/Product+Lines/Dewatering_Systems/Dewatering_Systems_Products/

[also vacuum-aided option]

 

[tuanjim]

Thanks guys.
Kenvlach - we would be looking at processing close to 34 MGD..Is it possible that a filter press system could handle those loads economically could the low permeability of the algae once a cake has formed?

 

[kenvlach]

Hi,
I'm only familar with the smaller filter presses, but some large sewage treatment plants do filter pressing, so must be large units. However, you may need to pre-concentrate, e.g., centrifuge or belt thickener or clarifier.

To avoid the low permeability problem, use a suitable flocculant. Another method is to blend in a slurry of diatomaceous earth (DE) as a filtering aid.

Suggest contacting US Filter Dewatering Systems -- they are now part of Siemens and operate worldwide, e.g., water recycling in the Middle East.

http://www.usfilter.com/en/Product+Lines/Dewatering_Systems/

 

[Rob99]

Hi Tuanjim...

To recap my understanding, the need is to harvest algae at about 2400 gpm and 1000 to 3000 ppm concentration.

There are several ways to go about this with the selection being a function of end product purity required versus what kind of cost can be economically justified. A multistep concentration is going to be required.

Option 1...

Dissolved air flotation should have about 99% recovery with the float likely being in the 3 to 4% TSS range. Feed the float to a recessed chamber filter press.

Chemical conditioning is going to be required for the DAF and I suggest looking a metal salt coagulant(ex: ferric sulfate, ferric chloride, aluminum sulfate or aluminum chloride) followed by an organic polymer. The filter press operation will be greatly improved if a diatomaceous earth precoat will not adversely affect product quality. Polymer conditioning of filter press feed could improved dewatering rate, which will translate into smaller filter press equipment. Depending again on what your product requirements are, you can play some tricks with filter press feed such as lowering the pH to about 3.5 with acid, which will lyse the cells. This will put the cell contents into the filtrate stream and the particulates into the solids cake. Don't go cheap with a filter press or labor or other cost factors can skyrocket.

Option 2...

Use a continuously cleaning, mixed media filter train (TSS recovery about 99%)to receive the 2400 gpm feed. The filter backwash feeds into a DAF and filter press train as described in Option 1. By preconcentrating the TSS, the DAF equipment will be smaller and lower capital.

Option 3...

Substitute a plate clarifier for the mixed media filter in Option 2. Keep it covered so it is not exposed to sunlight.

Things not likely to work technically and/or economically....

Centrifuge
Screens
Rotary Vacuum Filter
Sand Filter

Good luck!