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
