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Scaling up spirulina production.

A healthy 10-litre culture producing 20–30 g dry weight per week is rewarding — but at 100 litres, you can produce 200–300 g per week: a meaningful personal supplement supply. Scaling introduces challenges that don’t exist at small volumes: CO⊂2; limitation, mixing dead zones, and contamination propagation. This guide covers how to scale safely.

When to scale up

Scale only from a stable, productive culture. Prerequisites before increasing volume:

  • Culture has been productive for at least 4–6 weeks at current volume — colour deep blue-green, no pH crashes, no recurring contamination
  • You have passed a recent microscope check confirming no rotifers, competing algae, or non-Arthrospira filaments
  • You have spare culture to inoculate the new volume (target inoculation density: 0.3–0.5 g/L in the new vessel before first harvest)

Never scale sick culture. A contamination problem that is manageable at 10 L will propagate rapidly at 100 L and result in a complete loss.

Container options at scale

20–50 L: intermediate scale

  • Large aquarium tanks (60–120 cm):Standard 60×30×40 cm tank = 72 L; fill to 50 L. Glass allows visual inspection on all sides; easy to illuminate. Downside: heavy, fixed position.
  • Food-grade rectangular storage totes:Translucent HDPE or polypropylene containers at 40–80 L. Lightweight and stackable. Confirm food-grade certification — containers must not leach plasticisers into alkaline spirulina medium (pH 9.5–10.5).
  • Raceway pond (DIY):A shallow, wide vessel with a paddle wheel for circulation. Even at 40 L, a raceway offers better light distribution than a deep tank — surface area matters more than volume for photosynthesis.

50–200 L: serious home production

  • Open raceway ponds: 1–2 m² surface area, 20–30 cm deep. PVC-lined shallow beds or dedicated aquaculture raceways. These are outdoor-appropriate and achieve 0.3–0.5 g/L/day volumetric productivity in good summer conditions.
  • Food-grade IBC totes (1000 L) are used by semi-commercial producers — heavy mixing requirement; not suitable without a dedicated paddle or airlift system.

The CO⊂2; limitation problem

At small volumes, bicarbonate in the medium provides adequate carbon. At 50–100 L, photosynthetically active cultures exhaust dissolved bicarbonate between weekly replenishments:

  • Signs: pH rises above 10.8 despite bicarbonate additions; culture colour shifting slightly yellow-green; growth rate declining despite adequate nitrogen
  • CO⊂2; injection:A CO⊂2; cylinder with a pH controller (available from aquarium suppliers) injects CO⊂2; when pH rises above 10.5. This maintains carbon supply and pH simultaneously. At 50–100 L, CO⊂2; injection doubles productivity compared to bicarbonate-only.
  • Without CO⊂2; injection: switch to twice-weekly bicarbonate additions at 1–1.5 g/L rather than weekly at 2 g/L — smaller, more frequent additions prevent pH spikes.

Mixing at scale

Spirulina requires continuous or near-continuous mixing to:

  • Prevent self-shading (dense upper layer blocks light from lower culture)
  • Distribute CO⊂2; and nutrients evenly
  • Prevent settling and gas vacuole loss in low-flow zones

At 10 L, a small aquarium air pump is adequate. At 50–100 L, options are:

  • Airlift system:A vertical tube with air injection at the base creates a rising current that circulates the whole culture. Simple, no moving parts, low cost. Suitable for tubular photobioreactors.
  • Paddle wheel:Standard for open raceways. A slow-rotating paddle (5–20 rpm) creates a gentle circular flow that perfectly matches spirulina’s needs. DIY paddle wheels are easy to build with a small motor, shaft, and flat HDPE blades. Velocity target: 20–30 cm/second at the surface.
  • Submersible pump:A small fountain pump creates flow in rectangular tanks. Avoid pumps that create shear stress (high-speed propellers at close proximity to culture can physically damage Arthrospira filaments).

Harvest efficiency at scale

Harvesting 50–100 L cultures by hand filtration is time-consuming. Options:

  • Gravity filtration:Elevate the culture vessel; drain through a 20–30 µm filter cloth into a collection vessel. Gravity does the work; only squeezing the filter cake requires manual effort.
  • Siphon filtration:Siphon culture through a filter cloth draped inside a bucket — straightforward for 50 L volumes.
  • Drum filter:A rotating cylindrical filter with a scraper. Used commercially. DIY versions are feasible at 100–200 L with basic engineering skills.

Harvest 20–30% of culture volume every 3–5 days at optimal density (2–4 g/L). At 100 L and 3 g/L density, harvesting 25 L yields approximately 75 g wet paste — producing 12–15 g dry spirulina after pressing and drying.

Contamination management at scale

  • Monthly microscope checks are non-negotiable at large volumes — a rotifer population at 100 L will consume the culture faster than at 10 L
  • Maintain a frozen backup at every scale change — before adding culture to a new large vessel, freeze 500 mL of starter as insurance
  • Never combine cultures from different sources in the same vessel without microscope confirmation of both cultures’ purity — contamination in one culture will contaminate all combined volume
  • Cover open raceway ponds with fine insect mesh — flying insects, particularly in summer, introduce rotifers and competing organisms

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