In this post I will talk to you about five must-know parameters for biodigesters design. These are essential to know before going any further with biodigesters
1 - Useful Volume
2 - Dry Matter of the substrate
3 - Organic Matter of the substrate
4 - Dry Matter of the digestate
5 - Hydraulic Retention Time
6 - Organic Loading Rate
From now on, every time you read a paper about biodigesters, evaluate the functionality or compare the operation between biodigesters, compare these parameters. They tell you a lot about if a biodigester is working or not or why a biodigester produces more biogas than another.
1 - Useful Volume: The whole biodigestion chamber represents the total volume. Inside the chamber there is a space for the gas (20 - 30 % of the total volume) and the rest is the space occupied by the digestate. The digestate space is the useful volume. In the image below, you can see that the useful volume is limited by the horizontal tube on the right. It is impossible that the digestate reaches a higher volume because it will leak through the horizontal tube.
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| Total and Useful volume in a household biodigester. |
2 - Dry matter of the substrate: The substrate is the feed to be used in the biodigester. Every substrate contains water, everyone, even the straw. The dry matter is the difference between the total weight of the substrate and the weight of the water. The dry matter is represented as percentage of the total weight. This parameter is determined in a laboratory or you can use a literature value.
3 - Organic matter of the substrate: The dry matter of a substrate is composed by minerals and organic matter. The organic matter is represented as percentage of the total weight or the dry matter weight. This is also determined in a laboratory or you can use a literature value.
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5 - Hydraulic Retention Time: This parameter is determined with a very simple formula. By analysing the formula below, the Hydraulic Retention Time means the time the substrate is left in the biodigester for fermentation. This parameter is usually set up between 30 and 100 days. Yes, it is a large range; however, this value will also vary depending on the dry matter of the digestate and the Organic Loading Rate. We will see this in a future post exercise.
6 - Organic Loading Rate: This is the amount of organic matter to be fed to the biodigester per cubic meter of useful biodigester volume per day. The higher this parameter, the more biogas you will produce, but if it is too high it can inhibit methanogenic activity. In a small-scale context I suggest not to be more than 1.0. After using this OLR you can try with higher amounts and see if the system is still working alright.
After knowing these parameters, we are ready to see the continuous system.
