Development of an Open-Source Sensing System for the Measurement of Dissolved CO2 Throughout a Microalgal Photobioreactor

In order to optimise growing conditions for microalgae, it is important to be able to determine the concentration of dissolved CO₂ within the algal media. Typically, pH measurements are taken in order to indirectly determine the dissolved CO₂ concentration, as some CO₂ reacts with H₂O to produce carbonic acid (H₂CO). However, this is an unreliable method since the alkalinity of the media will determine its buffering capacity and therefore its ability to resist changes in pH. This means that without knowledge of the alkalinity of the solution, dissolved CO₂ concentration cannot be determined from pH alone. Additionally, the presence of additional acidic or basic chemicals in the media will impact the pH and therefore impact the calculated dissolved CO₂ concentration, especially problematic in scenarios such as wastewater treatment where chemicals such as ammonia are present.

  As a result, a more direct measurement is preferable through membrane diffusion of CO₂. This is where a gas-permeable, liquid-impermeable membrane separates the solution from an infrared gas analyser (IRGA) in air. The dissolved CO₂ can diffuse through the membrane, reaching an equilibrium with the air on the other side. Then, the CO₂ concentration can be measured using the IRGA and the dissolved CO₂ concentration can be calculated using Henry's Law.

  While commercial direct measurement dissolved CO₂ sensors exist, these are typically costly and require the use of proprietary software, making them unaffordable for small companies and researchers, and difficult to integrate with industrial systems. Therefore, a low-cost and open-source sensor system was developed alongside Northumbrian Water Limited, capable of continuously monitoring dissolved CO₂ concentration within a microalgal photobioreactor (PBR). This is a microcontroller and microprocessor-based system, taking advantage of Arduino and Raspberry Pi architecture respectively. A K33 CO₂ gas sensor was used, separated from the water in the PBR by a gas-permeable polydimethylsiloxane (PDMS) membrane, allowing a passive equilibrium to be reached with the dissolved CO₂ in the media. This sensor was used to quantify CO₂ uptake by the microalgae, with the possibility of using this data to compare the efficiency of different PBR bubbling regimes. Due to the modular and open-source nature of the system, these sensors can be readily deployed in an array to measure the spatial distribution of dissolved CO₂ throughout a PBR and easily integrated into a company's Supervisory Control and Data Acquisition (SCADA) system.