Open raceway vs closed PBR: comparison
- Open raceway:Simple, low cost, easy to scale. But exposed to airborne contamination (rotifers, competing algae), temperature fluctuations, evaporation losses, and atmospheric CO⊂2; concentration (0.04%) is insufficient at high densities.
- Closed PBR:Protected from environmental contamination. CO⊂2; can be enriched to 1–5% (25–125× atmospheric). Temperature is more stable. pH can be precisely controlled via CO⊂2; injection. Productivity: 1–4 g/L/day vs 0.1–0.3 g/L/day for open raceways.
Closed PBRs are not for everyone: they require more initial investment, regular cleaning of the closed vessel, and some engineering comfort. They suit growers who have mastered open culture, experienced contamination problems, and want to move to serious production.
Tubular photobioreactor
The most common PBR design: transparent tubing (glass or borosilicate-free acrylic) arranged in a loop with a pump and degassing vessel.
Basic design
- Transparent tubes: 4–6 cm internal diameter. Larger diameter reduces light penetration to the centre; smaller increases surface-area-to-volume ratio. For home use: 4–5 cm is optimal.
- Total tube length: 10–40 m of transparent tube per 10 L culture volume is typical. Tubes can be coiled vertically (saves floor space) or laid horizontally on a frame.
- Pump: a gentle peristaltic pump or low-shear centrifugal pump circulates culture at 0.1–0.3 m/s — sufficient mixing without mechanically stressing filaments.
- Degassing vessel: an open or partially open tank at one end of the loop where oxygen (photosynthetic by-product) is released and CO⊂2; is injected. Oxygen accumulation inhibits photosynthesis above ~300% air saturation.
CO⊂2; injection in tubular PBR
- Inject CO⊂2; into the degassing vessel or directly into the culture loop through a diffuser stone
- A pH controller with CO⊂2; solenoid valve maintains pH 9.5–10.0 automatically — when pH rises above setpoint, CO⊂2; valve opens; when pH drops to lower limit, valve closes
- CO⊂2; concentration in gas: 2–5% (mixed with air or pure CO⊂2; at appropriate dilution). Food-grade CO⊂2; cylinders from home brewing suppliers are the most accessible source.
Flat panel photobioreactor
A flat panel PBR is a thin (3–8 cm) transparent panel with aeration or liquid flow creating mixing. The thin light path means all culture volume receives light with minimal self-shading.
Basic design
- Panel material: clear acrylic (PMMA) or polycarbonate sheet, 5 mm thick minimum, bonded at the edges with silicone. Working volume: 10–30 L per panel in a 60×40 cm footprint.
- Mixing: air or CO⊂2;/air mixture bubbled through a distribution manifold at the panel base. Rising bubbles create turbulent mixing and transport oxygen out of solution simultaneously.
- Light path: 5–8 cm is ideal for spirulina at 2–5 g/L. Thinner panels require more frequent harvesting to prevent culture becoming too dense and self-shading.
- Multiple panels can be banked vertically, illuminated by LED panels on both sides, doubling the culture volume per floor area.
Artificial lighting in closed PBR
Closed PBRs indoors require artificial illumination:
- LED grow lights: 630–680 nm (red) and 450–470 nm (blue) are the photosynthetically active wavelengths for spirulina. Red–blue LED strips or grow panels at 100–200 µmol photons/m²/s are effective.
- Light:dark cycle: 16h light / 8h dark is optimal for indoor PBR. Continuous light marginally increases productivity but increases energy consumption 33% for modest gain.
- LED power consumption for a 20 L flat panel: approximately 30–60 W. Running 16h/day = 0.5–1 kWh/day. At typical UK electricity prices, energy cost for the lighting is £0.12–0.25/day — factor this against dried spirulina value (~£0.30–0.50/g commercial).
Cleaning and sterilisation
The main operational challenge in closed PBRs is biofilm formation on tube or panel walls, and periodic sterilisation:
- Monthly: drain culture, circulate 0.5–1% sodium hypochlorite (diluted bleach) for 30 minutes, then flush 5–10 times with clean water until no bleach odour remains. Acetic acid (5%) can be used for mineral scale (calcium/magnesium deposits from the medium).
- Weekly: check tubing for biofilm (green-brown deposits on inner wall). Mild buildup is manageable; thick biofilm indicates a cleaning cycle is overdue.
- Before restart after sterilisation: inoculate from a clean backup culture at 0.3–0.5 g/L density. Run at low density for the first week before harvesting.
Who should build a closed PBR
- You have been growing spirulina successfully in an open system for at least 6–12 months
- You’ve experienced repeated contamination events (rotifers, competing algae) that disrupted your production
- You want >50 g dry spirulina per week from a compact indoor setup
- You have basic DIY skills for bonding acrylic panels, plumbing tubing, and wiring a pH controller
- Budget: a basic flat panel PBR (20 L, LED lighting, air pump, CO⊂2; controller) costs £200–400 to build; commercial units start at £1,000–3,000
