Microalgae cultivation comes with two main measurement challenges: 1) accurately monitoring the growth phases and 2) determining the optimal time for harvesting. In particular, ensuring precise cell density measurements and consistent monitoring of pH and dissolved oxygen levels in photobioreactor systems at all times is critical.
Traditional Monitoring Techniques: Pros and Cons
Traditionally, cell density has been measured optically by instruments using a transmission/absorbance or light scattering technique. Algal cultivation is a relatively slow process and the time-based trends generated by these instruments give valuable insights into the growth stage of the organisms. pH and DO measurements provide essential information to feedback loops that control gas and nutrient additions.
Shortcomings of Conventional Sensors
However, the sensors used for these real-time monitoring tasks are often inadequate, especially in the face of fluctuating external light conditions and the adherent nature of algae.
External light from lamps providing energy for photosynthesis can cause significant errors in optical instrumentation measuring biomass. Due to their design, many optical instruments are affected by this incident light and report incorrect readings vs. offline samples. To mitigate external light effects, optical sensors designed to work in the NIR region of the spectrum should be used. “NIR only” sensors are not affected by the high levels of visible light necessary for algal growth.
Sensor Coatings: A Common Problem
Fouling is a major problem on the active surfaces of both optical and electrochemical sensors and can cause a great deal of measurement error and uncertainty. Optical sensors therefore require continual standardization against offline sample analysis to keep biomass readings close to actual values while electrochemical measurements like pH and DO often use multiple sensors of the same type to combat this issue, in the hope that some will last longer than others.
Introducing EXtract Series: The Next-Level Monitoring Solution
Enter the EXtract series. EXtract retractable housings from Exner allow optical, pH and DO probes to be withdrawn from the process to be de-fouled, re-sterilized and reinserted to ensure that measurement remains reliable and accurate. Retraction also allows faulty or expended sensors to be replaced if necessary, something that a closed system does not permit. Of course, in order to be successfully deployed, probes and sensors must fit the retractor footprint. EXtract housings operate with most 12mm diameter probes, including optical probes such as the EXcell 231, a high-precision NIR absorption sensor.
The EXcell 231: Precision in Microalgae Monitoring
Designed to observe and analyze production processes, especially in industries like biotech, food, and pharmaceuticals, the EXcell 231 is ideal for monitoring and trending algal growth. Unlike sensors that can be affected by external daylight conditions, the EXcell 231 offers optically based cell density measurements unaffected by ambient and incident light.
Addressing pH and DO Challenges with Advanced Sensors
pH probes are susceptible to failure in algal processes as their wetted reference junction connection can become compromised and blocked. Sensors such as the OPS71E are designed to be resistant to junction blockage. The OCOS22E DO sensor has a smooth membrane in contact with the process that can be easily cleaned of fouling.
Retractable housings can help in the prevention of algae sticking to the sensor active surfaces and causing inaccuracies in readings. Probes and sensors can be aseptically extracted from the process at will, cleaned of any coating and then returned to the process to continue measurement. This ability adds not only another layer of confidence to measurement results, it also extends the life of the sensors themselves along with the significant cost saving that can bring.
Integrated Cleaning Systems
Automated cleaning systems ensure that probes and sensors are regularly extracted for cleaning during both attended and unattended operation of the plant. EXtract retraction housings can be fitted with an actuator for automated operation and controlled by an EXmatic cleaning system. An EXmatic sensor cleaning system can independently operate up to two separate measurement points with up to three cleaning solutions each to make sure regular probe and sensor maintenance is carried out.
One of the most defining features of measurement points using both EXtract and EXmatic series units is the promise of long-term measurement stability. Given that algae remain “in process” for weeks, much longer than microbial fermentations, the need for a system that maintains consistent measurements over extended periods is extremely important.
The reliability of measurement in microalgae cultivation is greatly improved when EXtract and EXmatic systems are deployed with EXcell series optical sensors, OPS series pH probes, and OCOS DO sensors. This provides much better data for optimal cultivation outcomes.
Contact South Fork
Have questions? South Fork Instruments provides solutions designed to produce accurate, real-time measurements for biomass-related applications in the biotech, pharmaceutical, food and beverage industries. Contact us today to start a conversation.
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