Mixing tanks and other collection devices, such as pits, are used primarily to pretreat manure that is in a slurry form, having a solids concentration of 3-10%. This is generally found on farms which use a scraping system or a floor flushing system to move manure out of the animal buildings.

These tanks or pits, which can be installed above or below ground, require less land than lagoons, and are used mainly to warm the manure prior to the digestion process. Even in summer months, incoming manure is often colder than desired; therefore, water is heated inside the mixing tank to approximately 113 degrees Fahrenheit to heat the manure and activate the bacteria. Once the proper consistency is reached, the liquid manure is ready to be pumped into the digester.

Mixing tanks can also be used for separating solids and liquids. In some operations, solids are settled out in the tank to avoid clogged valves and settling of manure while in the digester. However, this separation also results in loss of some biogas production. Solids left in the mix for the digestion process may be dried, bagged and sold as fertilizer.

A volume equal to two days of manure collection is recommended for this collection or mixing tanks, typical for dairy operations. Once the mixer completes this pretreatment process, the manure is ready to be pumped into the digester.

In many poultry egg-laying operations, scrapers move down rows of cages to remove manure. The waste is then augered into a tank where it's mixed with 1 to 11/2 parts water. A pump aerates the slurry bringing the feathers to the surface. Feathers must be swept off the tank surface and into another auger to remove them from the process. The slurry then goes to a settling tank to allow sand and all other heavy matter to settle, leaving it ready for the digester.