After all the solids from the various settling tanks are collected, they are pumped into primary digester #1. The reason that the process is called anaerobic is because the entire process takes place in the absence of oxygen. The La Porte Wastewater Treatment Facility has three digesters. Two primary digesters and a secondary digester. The two primary digesters are heated to 95 degrees Fahrenheit and mixed, where the secondary digester is unheated and is not mixed.