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Flocculation of high lipid content algae

Natural vs. Synthetic Flocculents

October 25, 2010, by Ed Laurent, President, Water & Oil Technologies, Inc.

Synthetic polymers used in water/wastewater treatment, and in some instances algae flocculation, are made from monomers obtained from the fractional distillation of oilfield hydrocarbons at oil refineries. The problem with these monomers, is that some are carcinogenic. Acrylamide is such a monomer, and when combined with acrylic acid monomer in a polymerization process to make polyacrylamide liquid and dry polymers, the reaction is never 100% complete. So, you have unreacted monomer attached, like a barnacle to a ship, riding attached to the repeating backbone of the polymer that is carcinogenic. Free acrylamide monomer has a strong affinity to remain soluble in water, thus affecting rivers, lakes, and streams, and that which lives in them.

In algae production, if synthetic polymers are used to floc the high lipid content algae, and water is recycled back to your algae cultivation area, you are adding residual acrylamide monomer to your growing algae process. The carcinogenic affects of Synthetic Polymeric Flocculents limit the use of algal biomass in the by-product market to only fuel production; but more importantly, what is it doing to the cultivated algae?

Natural Polymeric Flocculants do not contain toxic monomers in their base composition. Some are even approved for pharmaceutical use. Natural Polymeric Flocculants can capture 100% of the high lipid algae for mechanical removal, while leaving nitrates and phosphates to be re-cycled back for use in either an open pond or closed loop bioreactor algae cultivation system, thus saving on water and nutrient costs as well.

Many natural polymeric flocculants, such as starches and alginates (seaweed), have been used in food products for many centuries. Stradivarius even coated his violins with a natural polymeric flocculent to preserve the wood and affect the resonance tone of the violin, when played.

If you have ever caught a fish, then attempted to hold the fish bare-handed while removing the fishhook from its mouth, the slippery-slimy texture on your hand best describes how a natural or synthetic polymeric flocculent feels, when touched. This slipper-slimy substance in polymers is composed of intrinsic viscosity (electrical charge) and molecular weight. The electrical charge can be positive (cationic), negative (anionic), or have no charge (nonionic). The molecular weight is made up of a repeating backbone that is linked together like railroad cars, by chemical bonds. It is this chemical bonding that allows the repeating backbone or molecular weight to vary, just as the weight of each human varies.

The combined charge and weight, along with the monomer that is the base building block of the polymeric material creates the slippery-slimy texture which, when used to adhere to algae via flocculation, allows the high lipid content algae biomass to be held together. This allows the water phase to be released and separated from the cultivated algae, be it freshwater or saltwater (and available for re-use in the algae cultivation process). The high lipid bearing algal biomass is then ready to have the algal cells, containing the high lipid algae oil, ruptured, and separated from the algal biomass.

Using natural flocculents, algae can be caused to sink or float based on a strain’s molecular weight as well as other variables.

The resulting water from flocculation can be adjusted to either be clear or retain an algal culture to recycle back to the source.

Selecting the correct flocculant to use is like viewing a paint chart, with all the different colors to choose from. Flocculants, in comparison, are similar in their various combination of charge and molecular weight combinations to achieve flocculation of algae. Each genus and species of algae require different combinations of charge and molecular weight to achieve the phase separation desired.

People frequently get confused when trying to understand or comprehend the difference between a flocculent and a coagulant. By definition, flocculation is a process when colloids come out of suspension in the form of a floc or flakes. Whereas, coagulation is a change of state that takes place, consisting of the alteration of a soluble substance  (e.g., a protein) into an insoluble form, or of the flocculation, or separation of colloidal, or suspended matter.

Now here’s the real mind twist: coagulants can perform like flocculants, but most flocculants do not perform like coagulants. Both flocculants and coagulants have charge and molecular weight characteristics; however, if you have high electrical charge, then you have corresponding low molecular weight, and if you have low electrical charge, then you can have high molecular weight.

Each flocculant or coagulant is selected from the paint chart for use, after trial and error bench testing, or field-testing where they are to be used for treatment, such as in the harvesting of algae.

No, you cannot find these answers in a computer, unless the physical testing results conducted by one skilled in the art has entered the data for you to review. There are no shortcuts to natural or synthetic polymer testing. It is tedious work to non-skilled artists, and requires a great deal of patience and accuracy. Testing polymeric material is like a surgeon doing surgery, it might take five minutes, or it might take five hours.

Flocculants are organic compounds. Synthetics work in a pH range of 4.5 – 10, whereas natural polymeric flocculants are not pH dependent due to their iso-electric points that are usually between a pH range of 6.0 – 7.0. So acid and caustic costs for pH adjustment, to achieve flocculation, are not needed when using Natural Polymeric Flocculants. This factor, at times, can be a considerable cost savings, in itself.

Coagulants are pH dependent for performance. Coagulants contain heavy metals and are not part of the “Go Green” movement. Aluminum sulfate (alum), PAC (poly-aluminum chloride), sodium aluminate, the iron salts: ferric chloride, ferric sulfate, ferrous chloride, ferrous sulfate are some of the main coagulants used in water treatment.

Ed Laurent

My first algae treatment was back in the 1960s when I helped my grandfather apply copper sulfate, a toxic chemistry, to kill the algal bloom on his farm pond. It didn’t do much to the algae but sure killed the fish. I’ve learned a lot since then. In the last few years, with the world looking to replace in ground hydrocarbons with above-ground produced high lipid algae, I have switched my work from algae blooms to high lipid bearing algae, either freshwater or saltwater species, either open pond, or closed loop bioreactor. The surge in worldwide algae development is truly creating an unlimited opportunity for the use of Natural Polymeric Flocculants in the harvesting of any volume of algae you desire to achieve.

Ed Laurent

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