The aeration basins form the heart of the Activated Sludge process. Here a mixture of Primary Clarifier effluent and Return Activated Sludge is kept under aeration for 4 to 8 hours. Air is supplied by large centrifugal blowers. The aeration provides oxygen to the microbes, such as rotifers, stalked ciliates, suctoria, and nematodes in the Return Activated Sludge, which feed on the organic matter in the Primary Clarifier effluent. The microbes convert this matter into water, carbon dioxide and other gasses, and additional cell mass. In so doing, the microbes convert difficult-to-remove disolved and colloidal organic matter into easily removed sludge.