Bioreactors are essentially vessels or systems designed to provide a controlled environment for the growth, metabolism, and production of desired substances by living organisms or cells. The choice of bioreactor type depends on multiple factors such as the scale of operation, the nature of the organisms or cells being cultivated, the process requirements, and the available resources.

Small glass bioreactors offer several advantages, particularly in laboratory-scale research and development. Their transparency allows for direct visual observation of the process occurring inside. This visual access is invaluable for monitoring cell growth, morphology changes, and the formation of aggregates or biofilms. Researchers can make real-time adjustments and take samples for analysis without disrupting the process.

The small size of these glass bioreactors also makes them suitable for experiments where sample volume is limited or when a high level of control and precision is needed. They can be easily integrated with various sensors and monitoring devices to measure parameters like pH, dissolved oxygen, and temperature. Moreover, the inert nature of glass helps prevent unwanted chemical reactions and ensures a clean and sterile environment for the biological processes.

Fermenters, on the other hand, are commonly used in industrial-scale applications for the production of a wide range of substances such as alcohols, antibiotics, and enzymes. They are designed to handle large volumes of substrate and organisms, and are equipped with systems for efficient aeration, agitation, and heat transfer.

The design of a fermenter takes into account the specific needs of the microorganisms or cells being cultivated. For example, in aerobic fermentations, a robust aeration system is necessary to supply sufficient oxygen. The agitation mechanism ensures uniform distribution of nutrients and oxygen throughout the vessel, preventing the formation of dead zones and maximizing productivity.

Stirred bioreactors are a popular choice in many applications due to their ability to provide thorough mixing. The stirrer helps to create a homogeneous environment within the reactor, reducing concentration gradients and ensuring that all cells have access to the same conditions. The type and speed of the stirrer can be adjusted based on the viscosity of the medium, the size and density of the cells, and the nature of the metabolic processes.

In stirred bioreactors, the selection of the impeller or stirrer design is critical. Different designs such as Rushton turbines, pitched blade impellers, or magnetic stir bars offer different flow patterns and mixing efficiencies. Computational fluid dynamics (CFD) simulations are often employed to optimize the stirrer configuration and predict the flow behavior inside the reactor, enabling better process design and scale-up.

The scalability of bioreactor systems is an important consideration when moving from laboratory-scale experiments to industrial production. Small glass bioreactors provide valuable data and insights that can be used to inform the design and operation of larger fermenters or stirred bioreactors. However, scaling up is not simply a matter of increasing the size; changes in fluid dynamics, mass transfer, and heat transfer need to be carefully accounted for to maintain process performance and efficiency.

Another aspect to consider is the control and monitoring systems associated with bioreactors. Advanced control algorithms and sensor technologies allow for real-time adjustment of parameters such as pH, temperature, and dissolved oxygen to maintain optimal conditions for cell growth and product formation. Automation and remote monitoring capabilities not only improve process control but also reduce the risk of human error and increase operational safety.

In conclusion, bioreactors come in various forms and sizes, each with its unique features and applications. The selection of the appropriate bioreactor depends on a careful assessment of the process requirements, scale of operation, and available resources. 

Whether it's a small glass bioreactor for research purposes or a large-scale stirred fermenter for industrial production, these systems are at the core of modern biotechnology, enabling the efficient and controlled production of valuable biological products and the advancement of scientific knowledge. Continued innovation and improvement in bioreactor design and technology will undoubtedly drive further progress in the field of biotechnology and related industries.