The sludge may also be buried or dumped as landfill, burned, or dried in sludge drying beds for use as fertilizer. Primary treatment removes about half of the suspended solids and bacteria in sewage, and about 30 percent of the organic wastes. Some cities use chemicals that coagulate some of the solids into particles of a size and weight that will settle, so that they can be separated in a settling tank. The use of chemicals makes it possible to remove 80 to 90 percent of the suspended solids. Today, large cities are usually required to put their wastes through both primary and secondary treatment because primary treatment alone removes so little organic material.
Secondary treatment uses aerobic, or oxygen-breathing, bacteria to decompose organic wastes.
The main object is to put the wastewater in contact with as many bacteria as possible while keeping it aerated so that the bacteria have an adequate supply of dissolved oxygen. One of the most common secondary treatments of this type is the activated-sludge method, so called because it uses sludge that is activated, or teeming with microorganisms. After going through primary treatment, the sewage is put into the activated-sludge tank, where it is aerated by pumps or blasts of compressed air. The compounds produced by the bacteria remain mostly suspended in the water and flow out with it into a secondary sedimentation tank. The sludge from the bottom of the tank is handled in much the same way as the sludge from the primary sedimentation tank, except that about a quarter of it is recirculated back into the activated-sludge tank.