The soluble organic matter that escapes primary treatment is eliminated through secondary treatment.
Secondary therapy often eliminates more suspended solids, typically by biological mechanisms in which organic impurities are ingested as food by bacteria and then converted into carbon dioxide, water and electricity.
The removal at the treatment plant of soluble organic matter helps to protect the dissolved oxygen content of the receiving stream, rivers, or lakes.
By making use of the bacteria in it, according to the EPA, the secondary stage of treatment removes about 85 percent of the organic matter in sewage. The trickling filter and the activated sludge process are the principal secondary treatment techniques used in secondary treatment.
It flows or is pumped to a facility using one of these procedures after effluent leaves the sedimentation tank in the primary stage. Instead of trickling filters, people tend to use the activated sludge process, since the activated sludge process speeds up the bacteria’s work.
After the sewage in the primary stage leaves the settling tank, it is pumped into an aeration tank. The bacteria break down the organic matter into harmless byproducts during this time.
With additional billions of bacteria and other small organisms, the sludge is now activated and can be used again by returning it to the aeration tank to be mixed with air and new sewage.
The partially treated sewage flows from the tank to another sedimentation tank for excess bacteria to be removed. The waste water from the sedimentation tank is usually disinfected with chlorine before being discharged in order to complete secondary treatment.
Many states now also require the removal of excess chlorine by a process called dichlorination, according to the EPA, prior to discharge into surface waters.
New pollutant removal methods, including advanced waste treatment, filtration, carbon adsorption, distillation and reverse osmosis, are being developed.