How to Harvest Algae From a Healthy Culture

5 min read · Updated 15 October 2026

Harvesting algae means separating the cells from the water. Spirulina can simply be strained through a fine cloth, while Chlorella is too small for a cloth and needs settling, flocculation or a centrifuge. Whichever you use, harvest only part of a healthy culture, return the clean medium to the jar, and use what you collect while it is fresh.

Choosing a method

Method	Works for	Equipment	Notes
Straining	Spirulina	Fine cloth or 30 to 50 µm mesh	Fast and cheap; see harvesting Spirulina
Settling	Chlorella and others	Tall container, patience	Slow (hours to a day) but needs nothing; cooling and darkness help
Flocculation	Chlorella and others	pH shift or a flocculant such as chitosan or alum	Faster settling; residues may matter for feed use
Centrifuge	All	Centrifuge	Fast and clean, but costs more

Harvesting Chlorella without a centrifuge

Because Chlorella cells are only 2 to 10 micrometres, they pass through ordinary filters. At home, the usual approach is settling:

Switch off aeration and light for 12 to 24 hours, preferably in a cool place.
Cells slowly sink; the top becomes clearer and the bottom darker.
Siphon off the clear top water, which can be reused.
Collect the concentrated green layer at the bottom and use it.

For faster results, some growers raise the pH briefly with a small amount of sodium hydroxide or lime to make cells clump. This works but requires care and neutralising before reuse. At small scale, settling is simpler and safer.

How much to harvest

A healthy semi-continuous culture can lose a third to a half of its volume every day or two, if you replace it with fresh medium. Harvesting more than that slows regrowth. For one-off use, a quarter of the jar is plenty. See how to split and scale up for refilling and reserve rules.

What to do with the harvest

Feed it fresh to Daphnia, rotifers, shrimp or fish. See culturing live food.
Use it as a starter for a new jar.
Freeze in small portions for later feeding.
Dry it in thin layers at low temperature for a backup stock.

Hygiene

Harvest is a point where contamination can enter. Use clean tools, rinse them between jars, and return the medium to the culture only if the culture looks healthy. See preventing contamination.

Estimating how much you will get

The amount you can harvest depends on culture density and volume. As rough hobby numbers, a dense Chlorella culture holds around 0.5 to 1.5 grams of dry biomass per litre, and a dense Spirulina culture 0.3 to 0.8 grams per litre. A 5 litre jar harvested by a third every two days therefore yields something in the order of a gram of dry biomass per harvest. That is not a lot in weight terms, but because a few millilitres of concentrate are enough to feed a Daphnia colony or tint a fry tub, it is a lot in practice.

Jar volume	Harvest one third	Approx. dry biomass (dense culture)
1 L	330 mL	0.15 to 0.5 g
5 L	1.7 L	0.8 to 2.5 g
20 L	6.7 L	3 to 10 g

Settling Chlorella in detail

Gravity settling is the simplest method but takes planning. Cells sink very slowly, so you help them along:

Use a tall, narrow container such as a measuring jug or a cut-off bottle; a deeper column settles in a cleaner layer.
Stop aeration and light. Turbulence and light-driven movement keep cells in suspension.
Cool it. A cooler water column has less convection; a cellar or fridge-cool room works.
Wait 12 to 24 hours. Check whether the top third is noticeably clearer.
Remove the supernatant gently with a tube or siphon, taking care not to disturb the concentrate.

Settled concentrate typically reaches 5 to 10 times the original density. If you need more, use a second round or a small centrifuge.

Flocculation: how it works and what to know

Algal cells carry a slight negative charge that keeps them apart. Flocculants neutralise this charge so cells clump into larger flocs that settle fast. Options include:

pH shift. Raising pH to about 10 to 11 with lime or sodium hydroxide makes many algae flocculate. The pH must be brought back before reuse of the water.
Chitosan. A natural biopolymer that is effective at low doses and is used in water treatment.
Metal salts. Aluminium or iron salts work well, but leave metal residues, and are less suitable for feed.

For hobby use, settling or a fine cloth is easier and avoids chemical residues. Use flocculants only if you have a clear need and have checked the residue and the intended use.

Using concentrates

A thick concentrate of Chlorella can be stored in the fridge in a sealed container for several days. Shake before use, and dilute in clean water before feeding. For animals that are sensitive to chemical changes, rinse the concentrate once by letting it settle again in clean water. See feeding rotifers with Chlorella for dosing.

Common questions

Why is my Chlorella not settling?

The cells are very small and slow to sink, and the culture may be warm or aerated. Give it more time in a cool, still place.

Can I use a coffee filter?

For Spirulina, yes, though it clogs. For Chlorella, cells pass through it.

Do I have to harvest?

If you are feeding animals or splitting, yes. A culture that is never harvested eventually declines.

Is a centrifuge worth buying?

For routine production, yes, because it saves time. For occasional use, settling is fine.

Can I harvest at night?

Yes. Timing is a convenience. Harvest when you can and when the culture is dense.

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