Batch Tests – To Study Substrate Characteristics:
Batch tests tell us how much gas can be expected from a material. It also tells us, to some extent, how fast the material will degrade and the metabolic activity of the microorganism. Batch tests are often called BMP (Biomechanical Methane Potential tests), and are the most convenient way to analyse a substrate for biogas production.
The test is quite simple: an inoculum is mixed with the sample substrate to test. This inoculum contains all the microorganisms necessary to degrade the substrate. The mixture is then monitored to see how much gas is produced, and how much time it takes.
Batch tests are best suited to evaluate the substrate characteristics and microorganism activity.
For example, it can be used to screen different substrates, or to study the effects of different pre-treatments on a substrate – as discussed in Lesson 1. s
Continuous Tests: To Stimulate the Process and Study Long-Term Effects:
Continuous tests simulate a full-scale process by performing experimental tests in a continuous mode. This means that substrate is continuously added to the digester, so you can analyse performance over a long period of time.
Gas production is not the only parameter analysed here. pH, alkalinity, gas composition, VFA and ammonium contents are also measured. Those parameters give you a better understanding of the process and indicate how various changes may affect its performance.
The set up for continuous tests is more complex than for batch tests. It also requires more equipment and human effort. This in turn creates cost and limits the number of tests that can be performed simultaneously.
Continuous fermentation tests are most suitable to evaluate and optimise the way a process operates and to study the long-term effects of substrates.
To summarise what we’re learned, let’s look at this table of pros and cons for both tests:
| Benefits | Downsides |
Batch test | Simple and cheap to perform, Many tests can be performed simultaneously, Easy to compare different types of substrates or evaluate the effect of different pre-treatments, co-digestions, additives etc | No information on long-term effects, Limited information on process dynamics |
Continuous test | Possible to evaluate the long-term effects of substrates; to investigate and optimise operational parameters | More expensive and labour intensive to perform, Can only compare a limited number of tests. |
Conclusion
While batch and continuous fermentation tests are key to unlock AD’s true capabilities, researchers currently face many challenges in using them. It is very difficult to accurately measure and compare results, and this mainly because of technical limitations.
Let’s take a closer look at those limitations in next lesson.
