Wastewater treatment is the physical, chemical or biological process that reduces the concentrations of organic waste through settling or organic matter conversion — nitrogen through the conversion of organic nitrogen to nitrogen gas, and phosphorus through assimilation and sedimentation and through biological and chemical processes. Some wastewater requires pretreatment filtering to remove objects such as tree branches, dead animals, automobile parts from wrecks, guns and “fatbergs,” (giant blobs of baby wipes and grease that coagulate and interfere with subsequent treatment processes).
All wastewater treatment harnesses the natural abilities of microbial communities to digest waste, typically aiming to create an optimal environment that can both speed up and control the process. Degrees of waste treatment describe the levels of treatment provided by a given system and its processes and technologies. These degrees of treatment include:
- Primary treatment. Mechanical screening, active screening or settling of solids, and some reduction of organic matter (BOD) but typically no reduction on nutrients.
- Secondary treatment. Biological treatment for removal of BOD (including soluble BOD), additional clarification, and some reduction of nutrients.
- Secondary treatment with extended aeration; nitrification of effluent.
- Tertiary treatment. These are polishing treatments that can be used to remove a variety of pollutants left over after secondary treatments. It includes the use of filters for chemical removal of phosphorus with clarification, aeration, and nutrient removal through biological or chemical processes; denitrification is also possible through carbon-amended filters. Most common tertiary treatments are disinfection techniques such as chlorination, reverse osmosis, UV radiation or ozone treatment to remove bacteria and viruses. Tertiary treatment can result in effluent qualities of 5 mg/L total suspended solids, 5 mg/L BOD, 3 mg/L total nitrogen, and less than 1 mg/L total phosphorus (Evans et al., 1978; Ragsdale, 2007). While this is an advanced level of treatment, it is not sufficient for ocean or groundwater disposal without risking impacts to sensitive habitats or organisms. For example, a recent study looking at TSS thresholds in corals showed that adverse effects occurred as low as 3.2 mg/L in adult corals, by causing bleaching and mortality (Tuttle & Donahue, 2020).
- Quaternary treatment. Advanced oxidation and or ultra-low phosphorus reduction (<0.2 mg/L); term is not commonly used (Ragsdale, 2007).
Some systems may offer only minimal treatment before releasing effluent, while newer state-of-the art treatment plants are experimenting with resource recovery methodologies that create clean water that meet national standards while providing energy and fertilizer.