Batch reactors are tanks used to contain chemical or biological reactions. The term "batch" distinguishes this type of reactor from continuous reactors — in a batch reactor, all the necessary ingredients are placed in the tank and the chemical reaction or fermentation is allowed to take place, whereas in a continuous reactor ingredients are continuously fed into the tank. Batch reactors are used for processes such as wastewater treatment, pharmaceutical production, and fermentation of various products.
The simplest type of batch reactor consists of only a tank with holes in the top for input of the reactants and removal of the product, although typically some type of stirring mechanism must be included as well. A jacket that fits around the tank may be used for temperature control as well. Batch reactors might also require some type of aeration system.
Adding ingredients to a batch reactor is known as "charging" and is the first step in carrying out a batch reaction. The user of the reactor calculates how much of each ingredient is needed to produce the desired product, based on chemical reaction formulas or microbial growth formulas. Similar calculations allow the user to determine how long the reaction will take. Once the reactor is charged, it can be mostly be left alone while the reaction takes place. The user may need to monitor temperature and aeration within the reactor, depending on the type of reaction.
One common type of batch reactor is a sequential batch reactor or sequencing batch reactor, both abbreviated SBR. SBR reactors are typically used at wastewater treatment plants for the activated sludge process, a procedure that uses a wide range of bacteria to remove contaminants from the water. The bacterial cultures grow in a thick sludge, which is where the procedure gets its name.
For sequencing batch reactors, a reactor containing activated sludge is first filled with wastewater. The sludge and water are aerated to add oxygen and to enable the components to be mixed together, both of which help the bacteria to feed on contaminants in the wastewater. Once the bacteria have consumed most of the nutrients in the water, they begin to die off and aeration is stopped so that the sludge can settle to the bottom of the reactor. After the sludge settles, the treated wastewater is removed from the reactor and sent on to the next stage in the treatment process.
Fed-batch reactors are a combination of batch and continuous reactors. In a fed-batch reactor, the limiting substrate — the ingredient that runs out first as the reaction takes place — is continuously fed into the reactor. The other ingredient is added at the beginning of the reaction and none is added as the reaction progresses.