There are eight aeration tanks at Delaware No. 1. Mechanical surface aerators are used to mix the activated sludge, the primary wastewater, and the pure oxygen. The nearby compressor building can also produce oxygen from the ambient air. In the aeration tanks, the bacteria have formed clumps as they grow and consume solids. Here in the final sedimentation tanks, these clumps are allowed to settle to the bottom forming secondary sludge.
Secondary sludge is more liquid than primary sludge.
It is composed of small particles. The rest is removed for further processing. Delaware No. 1 WPCF has eight final sedimentation tanks. The final stage of treatment takes place in the chlorine contact tanks. To kill any remaining bacteria, the water is mixed with a solution of sodium hypochlorite.