Biogas Digester Mixing EquipmentBiogas plant designers will tell you that the main factors that influence the rates (and, therefore, the success) of these biological transformations in the AD reactor are hydraulic and solids residence time (HRT/SRT), temperature, pH and the presence of toxic materials.
But, that omits one other essential requirement which can be left to chance on some occasions, and that is getting the tank agitation, or more commonly called 'mixing". right.
Unmixed, low rate AD reactors will stratify into four zones (layers): Floating scum; liquid supernatant; actively digesting solids; and digested and inert solids. Mixing systems for high rate AD reactors are typically designed to achieve a specified percentage of active working volume, typically defined as the total volume of the reactor minus an allowance (usually 10%) for the floating scum layer, and the inerts layer at the bottom. Mixing systems try to minimize the sizes of these two capacity-stealing layers. Working volumes less than 85 percent are inefficient digester reactors.
More energy is needed to keep heavy solids in suspension than to homogenize the reactor contents. via www.biocycle.net
|Exhibition attendees looking at an in-tank propeller type biogas mixing system.|
Mixing the digester requires a significant amount of energy, which becomes a parasitic load on the output of energy which can be used on-farm or sold. One possibility to operate biogas plants more efficiently is to reduce the energy consumption by avoiding constant stirring. AD Operators and water companies can reduce the energy usage of their digester pumps and mixers by up to 50%, according to leading manufacturer, Landia. While most operators find this to be the answer to high energy use during mixing, some say they can experience more foaming during intermittent mixing.
An advantage of gas mixing can be that the high velocity gas-liquid jet causes mixing of the tank at a higher level than the hydraulic mixing. The gas bubbles reduce the density of the liquor in the digester. The result of this is that, low-density materials, such as plant fibers that often float to the surface, will be forced to sink into the tank. This can reduce the floating scum on the surface, and be a very welcome result.
Operating an AD plant with lowest possible electricity consumption and minimized OPEX is a key factor in optimizing the total cost over the lifetime of the biogas plant, so the subject of anaerobic digestion reactor mixing is an important one.