WEEK #13

Pumps (not simultaneously) will extract the culture medium up to the filter/scraper module (see pictures). The medium will fall inside a shallow tub with a fine weave cloth (mesh density around 30 – 50 µm) at the bottom and a big funnel underneath that will recycle the filtered water directly into the pond of origin.

In order to select the destination pond, there is a servo-controlled flow divider valve at the end of the funnel so that the filtered water return back to where it came from.

The filtered water should be light green because it contains small “baby” spirulina too tinny to be stopped by the filter.

To facilitate the filtration, a scrapper positioned under the injector and activated by a lead screw will scrap the mesh continuously from side to side.

When the filtration process is done and the biomass left in the tub looks like a very thick paste, take out the biomass and store or press and dry it.

- Weighting the filtrate to get an estimation of the quantity of spirulina harvested. Filtered biomass contains 10 % dry matter (1 liter = 100 g dry spirulina) and 50 % residual culture medium.

- Sieving: Then the filtered culture passes through a sieve (mesh size about 200 µm) to remove any foreign matter such as insects, larvae, leaves and lumps of polysaccharide or muds from the bottom of the tank.

- Dewatering is accomplished by pressing the biomass enclosed in a piece of filtration cloth + a strong cotton cloth, either by hand or in any kind of press. The simplest is to apply pressure (0.15 kg/cm² is enough) by putting a heavy stone on the biomass bag.

The expelled "juice" comes out colorless. When it turns slightly green, we must stop (otherwise too much product will be lost). For the usual thickness of cake (about 2.5 cm after pressing), the pressing time is about 15 to 20 minutes.

Practically all the interstitial water (culture medium) is removed. The pH of the well pressed biomass is near 7 (neutrality).

- Cleaning: the pumps, the piping network and the filter/scrapper module should be purged with clean water after each harvest. Each pump is fitted with a purge valve that will let water flow into the network without leaking into the spirulina pounds. The flow divider at the end of the funnel has a 5^th valve solely dedicated to purge. The water will be released in an external water tank that can later be recycle for other needs.  

Filter/ Scrapper Module