Wastewater treatment is done in a series of steps that can have increasing effectiveness and complexity depending on the resources available. The conventional sequence goes from primary, secondary, to tertiary treatment.
Primary treatment involves basic processes to remove suspended solid waste and reduce its biochemical oxygen demand (BOD) – the amount of oxygen microorganisms must consume to breakdown the organic material present in the wastewater. This, in turn, increases dissolved oxygen, which is good for aquatic organisms and food webs. Primary treatment can reduce BOD by 20 to 30 percent and suspended solids by up to 60 percent.1
Secondary treatment uses biological processes to catch the dissolved organic matter missed in primary treatment. Microbes consume the organic matter as food, converting it to carbon dioxide, water, and energy.2 While secondary treatment technologies vary, from the activated sludge process New York City deploys, to constructed wetland systems, the final phase of each involves an additional settling process to remove more suspended solids. Secondary treatment can remove up to 85 percent of BOD and total suspended solids.3
The highest level of wastewater treatment is tertiary treatment, which is any process that goes beyond the previous steps and can include using sophisticated technology to further remove contaminants or specific pollutants. Tertiary treatment is typically used to remove phosphorous or nitrogen, which cause eutrophication. In some cases, treatment plant operators add chlorine as a disinfectant before discharging the water. All in all, tertiary treatment can remove up to 99 percent of all impurities from sewage, but it is a very expensive process.4
Ideally the Wastewater Treatment indicator would showcase more advanced levels of treatment, but most countries lack the necessary data. The EPI’s indicator considers “at least primary treatment,” because reported values of overall treatment performance entail going through primary treatment first. Hopefully, layers of specificity can be added in the future as more refined data becomes available.
1, 2, 3, 4 Flörke, M., personal communication. December 5, 2013.