Secondary treatment is designed to substantially degrade the organic and
biological content of the sewage. Dissolved or suspended material is brought
into contact with oxygen in the presence of aerobic and anaerobic
microorganisms to break down and consume the organic matter. The effluent can
then move onto another sedimentation stage, including floc (clumps of solids)
removal. Wetlands perform these same functions through the processes of
aerobic and anaerobic bacterial decomposition.
TERTIARY TREATMENT—CHEMICAL TREATMENT
Disinfection occurs at this stage (e.g., chlorine, ozone, UV, bromine, hydrogen
peroxide, chloramines) to destroy any pathogens (i.e., disease-causing
microorganisms such as E. coli) whose presence in water indicates fecal
contamination and can cause diarrhea and other dysentery symptoms.
Soluble phosphates are removed by adding various precipitating chemicals at
this stage. Specific bacteria, called polyphosphate-accumulating organisms
(PAOs), can also be added at this stage to accumulate up to 20% of their body
weight in phosphorus in their cell structure. When the biomass enriched in these
bacteria is separated from the treated water, these biosolids have a high fertilizer
value. Nitrates and ammonia are removed at this stage through denitrifying
bacteria in an anaerobic microbial digester.