For a Wastewater Treatment Plant to work to maximum efficiency following start- up or following chemical upset, there needs to be an established microbial community at a level required to remove organics within the hydraulic retention time. In order for this microbial community to be effectively retained the microbial community needs to form flocs with good settling properties.

If either of these events do not take place the start-up period can take a considerable time, causing delays and possible production hold-ups for industrial wastewater treatment plants.

Use of activated imported sludge can reduce the start-up / recovery time however it often requires a long acclimation phase and can introduce undesirable organisms to the system such as filamentous bacteria, that can cause foaming and poor sludge compaction.