When feedstock is pumped into the digester it is important to ensure the solids remain in suspension. If this isn’t done it can lead to many process issues, with the result being poor biogas production. The most popular ways to do this is through digester mixing.
What is Digester Mixing?
In a digester, there are no moving parts. Therefore, solids will naturally settle and inhibit biogas production. Because of this digester mixing is commonly used. Digester mixing is when the medium inside the digester is mixed or stirred to ensure solids stay in suspension. This can be done through the use of mechanical mixing using propellers, or through the use of pumps or gas injection systems that keep recirculating the fluid.
Types of Digester Systems
Continuously Stirred (CSTR)
A continuously stirred digester is the most common in the AD world. As the name suggests the digester is continuously mixed. The liquid coming in displaces the liquid going out and this ensures the process stays balanced.
Plug Flow
A plug flow digester means that the contents are that much thicker that the solids continue to stay in suspension with little to no use of additional mixers. However, the liquid is displaced by incoming feedstock.
Different Methods of Digester Mixing?
Gas Injection
Gas injection systems can often be grouped into four common types:
- Lances
- Floor mounted diffusers
- Draft tubes
- Bubble guns
These systems are designed to introduce gas into the tank from different directions to allow the sludge to effectively move around and avoid causing the floating layer on the top of the fluid, as this is what inhibits the production of biogas.
Mechanical – Propeller
Propeller mixers are one of the most common types of mixers in an AD system. They’re efficient, easy to maintain and cost effective. They come in many variations. Such as wall/side mounted or even floor mounted. Due to their nature they are not removing or adding anything to the process but simply stirring everything round in the digester.
Mechanical – Pumping
Another way of digester mixing is to recirculate the fluid by means of pumping. This involves using a pump to draw the liquid out of the digester at a certain point and then pressurising it and pumping it back into the digester. The pressure that the fluid is being pumped back into is high enough to move the solids in the tank.
References
Different Types of Anaerobic Digester Mixing Systems (phsenesacinc.com)
