What is a WWTP?
WWTP: Wastewater Treatment Plant
A WWTP (wastewater treatment plant) is a facility that collects sewage and industrial wastewater and treats it, in stages, until the effluent is clean enough to be discharged to a river, lake or ocean, or reused. In the United States, publicly owned plants are called POTWs (publicly owned treatment works) and discharge under an NPDES permit that sets limits on what leaves the plant. Klarifi tracks 11,783 publicly owned plants across 50 states, treating a combined 14,938 million gallons per day for about 144,666,621 residents.
How a wastewater treatment plant works
Preliminary treatment removes grit, rags and debris with screens and grit chambers. Primary treatment settles out heavier solids in clarifiers. Secondary treatment uses biology, most often activated sludge, oxidation ditches, trickling filters or lagoons, to consume dissolved organic matter. Many plants add nutrient removal for nitrogen and phosphorus and disinfect the effluent with chlorine or ultraviolet light before discharge.
The solids removed along the way, called biosolids, are thickened, digested (often anaerobically, producing biogas) and dewatered before land application, landfill or incineration.
Types of wastewater treatment plants
Mechanical plants (activated sludge, sequencing batch reactors, membrane bioreactors) serve most cities. Lagoon systems, which rely on large ponds and long retention times, serve thousands of small towns: Klarifi’s aerial imagery review finds lagoons at 5,662 US plants. 1,548 plants have biogas or anaerobic digestion infrastructure on site.
Plants are sized by design flow in million gallons per day (MGD). A small-town lagoon may treat 0.1 MGD; the largest US plants treat several hundred MGD.
WWTP, POTW, WRF, WRRF: what the acronyms mean
WWTP and STP (sewage treatment plant) describe the facility. POTW is the regulatory term for a publicly owned one under the Clean Water Act. WRF (water reclamation facility) and WRRF (water resource recovery facility) are newer names that emphasise reuse and energy recovery. WPCP (water pollution control plant) is common in the north-east.
Find wastewater treatment plants by state
The Klarifi facility directory lists every publicly owned plant in each state with its average flow, NPDES permit number and expiry, treatment technology, three-year violation count and the capital improvement projects planned in its city.